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Former Toshiba Engineer: Dai-1 Meant for 8.0 Quake

Shiro Oguro, a former Toshiba nuclear design engineer, speaking at the Foreign Corresponds’ Club of Japan in Tokyo Wednesday night said that stress criteria he was given to design equipment used in the Dai-1 Fukushima Nuclear Plant were rated to withstand an 8.0 magnitude earthquake.

He said he received such design specifications when working on designing “the pumps and heat-exchange systems involved in the structural safety of reactors numbers 1, 2, 3, 5 and 6 used in the Dai-1 Fukushima Plant.” The earthquake that hit the area last Friday afternoon had a magnitude of 9.0.

Oguro is one of a group of engineers and scientists who support the Citizens’ Nuclear Information Center, and anti-nuclear group based in Tokyo, which arranged the press conference. Oguro said that the basic thinking behind these design conditions at the time was that an earthquake having a magnitude of 8 or greater would not occur.

Though we now know the servere threat tsunami pose for nuclear plants, that wasn’t the case back when Ogura was working on the Fukushima Dai-1 Nuclear Plant design, he added.

Photo: TEPCO/Reuters

Helicopters and Cannons Spray Water on Japan's Unstable Nuclear Plant

Mar 17

Special Report: Fukushima and the Future of Nuclear Power

Editor's Note: John Boyd is an IEEE Spectrum contributor reporting from Kawasaki, Japan. This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency. For more details on how Fukushima Dai-1's nuclear reactors work and what has gone wrong so far, see our explainer.

Helicopters and water cannons: those are the latest tools employed in the desperate attempt to stabilize Japan's damaged power plant on day seven of the nuclear emergency.

On Thursday morning, steam flowed from the spent fuel storage pool in the No. 3 reactor building of the Fukushima Dai-1 nuclear power plant, and international authorities argued over the severity of the situation in the No. 4 reactor's storage pool.

Japan's Ministry of Defense, after talking with Prime Minister Naoto Kan and the government task force set up to deal with the nuclear crisis, sent two CH-47 Chinook helicopters (capable of carrying 7000 kilos of water) to drop seawater over the stricken No. 3 reactor building. The helicopter floors were fitted with lead plates to protect the crews, who also wore protective clothing.

The No. 3 reactor building was damaged by a hydrogen explosion on Monday morning. The openings made by the explosions should have enabled at least some water to enter the buildings from above. The water was intended to help cool down the overheating spent fuel rods: The still-radioactive rods continue to generate heat long after they're taken out of operation, so they're constantly cooled by water circulating through the storage pools. Although it's not clear exactly what went wrong, it appears that the storage pool in the No. 3 building has heated up and some of the water may have evaporated, which could have exposed the spent fuel rods to the air. If the rods have been exposed their nuclear fuel will heat up further and could begin to melt, increasing the likelihood of dangerous radiation.

(Yesterday afternoon it seemed that reactor building No. 4 would become the plant's top priority following alarming congressional testimony from Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission. Jaczko stunned listeners by declaring that there was little or no water remaining in the storage pool in building No. 4 reactor, leaving the spent fuel rods exposed and leaking radiation. But the plant's owner, Tokyo Electric Power Company (TEPCO), has rebutted Jaczko's claims, and a Self-Defense Force pilot flying over the site on Thursday reported seeing water in the spent fuel storage pool of reactor building 4.)

Despite the real danger posed by the spent fuel rods and the radiation they're emitting, one expert told Spectrum there was no chance of recriticality, the re-ignition of a self-sustaining nuclear reaction, occurring. "The fissionable material remaining inside the spent fuel pellets is maybe about 1 percent," said Kazuaki Matsui, executive managing director of the Institute of Applied Energy, an independent organization in Tokyo. "Fissionable material contains maybe 3 or 4 percent of uranium-235. But we're talking here about spent fuel, otherwise it wouldn't have been removed."

NHK, Japan's national broadcaster, positioned a TV camera 33 kilometers southwest of the plant, and broadcast the helicopter operation live. At 9:48 a.m. local time a helicopter flying from the west dropped the first load of water almost directly over the No. 3 reactor, though a significant amount spread away from before reaching the target. A second helicopter flying from the east then dumped its water over reactors No. 4 and No. 3. Two more passes were made before the operation was suspended around 10 a.m.

No steam was seen escaping from building No. 3 immediately after the last helicopter pass, but later views showed small amounts of steam being released. NHK reported a TEPCO official saying that there had been little change in the radiation level measured after the operation ended.

Minister of Defense Toshimi Kitazawa told reporters at a press conference following the aerial drop that operations had been suspended for the time being, but "more aerial drops will be conducted as necessary." He said an inspection was taking place to determine how effective the aerial drop had been.

Kitazawa also announced that the National Police Agency had special water cannon trucks and crews ready to move in to begin spraying water directly into the reactors once plant operators deemed it was safe to approach the plant. The crews would wear protective clothing provided by the Self-Defense Force. The water cannons are typically used to control riots, and can reportedly hit a target with a powerful stream of water from a distance of 100 meters. That operation began on Thursday afternoon (local time), but according to NHK the water failed to reach its target. The operation was soon suspended due to high radiation levels.

The next assault involved what Kitazawa described as "eleven special water-spraying trucks," which he said came from various military bases around Japan. The Self-Defense Force put these trucks into action on Thursday evening (local time), and sprayed a total of 30 tons of water over reactor building No. 3 over the course of 30 minutes. Officials told NHK that the water did reach the building's interior, but they couldn't immediately say whether it had reached the spent fuel storage pool.

In a separate press conference, an official from Japan's Nuclear and Industrial Safety Agency said that workers were trying to reconnect the Fukushima Dai-1 plant to outside power lines on Thursday. Power was cut off when the earthquake struck last Friday afternoon, so the plant's operators have been running the emergency cooling systems on generators and batteries.

Image: DigitalGlobe / Getty Images

Explainer: What Went Wrong in Japan's Nuclear Reactors

Special Report: Fukushima and the Future of Nuclear Power

Editor's Note: This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency. This explainer was last updated on May 13.

The eyes of the world have been riveted on Japan's Fukushima Dai-1 nuclear power plant and its workers' desperate efforts to stabilize the nuclear reactors. Explosions first occurred in the buildings housing reactors No. 1 and No. 3 in the days following the March 11 earthquake and tsunami, and troubling problems next arose in reactor building No. 2. Fires at the building housing reactor No. 4, which was shut down at the time of the earthquake, raised a new set of concerns regarding spent nuclear fuel.

As the first week of the nuclear crisis came to a close, the spent fuel had become the primary safety concern. The second week showed signs of progress, as the plant operators worked to reconnect the plant to the electricity grid and get cooling systems working again. But an accident that exposed three workers to radioactive water highlighted the continued danger, and made it clear that the process of stabilizing and cleaning up the plant will be a long slog. In the third week, further discoveries of highly radioactive water outside the reactor buildings showed that the contamination was spreading.

On March 30 the Tokyo Electric Power Co. (TEPCO) confirmed the obvious, and announced that at least four of the plant's reactors would never go back into service. On April 17, TEPCO finally announced a "roadmap to restoration," which the company said would stabilize the plant and end most radioactive emissions within nine months.

Let's step back from the news cycle for a moment, though, and look at both how the Fukushima Dai-1 plant is supposed to work, and what went wrong following the earthquake on Friday March 11.

How a Boiling Water Reactor Works

Fukushima Dai-1 uses six boiling water reactors to produce electricity for TEPCO. At the time of the earthquake, three reactors were active and three were down for routine maintenance.

Let's start at the heart of a boiling water reactor where the nuclear fuel dwells. In most of Fukushima Dai-1's reactors, the radioactive element uranium is the source of the nuclear fission reaction: when one atom of the uranium isotope U-235 breaks down into smaller parts, it produces both energy and neutrons. When a large enough quantity of uranium fuel is gathered together it starts a self-sustaining chain reaction, in which emitted neutrons smack into other uranium atoms and cause them to split in turn. The energy from the fission reaction is used to boil water into steam, which drives turbines to produce electricity.

Pellets of uranium fuel are contained in long, narrow fuel rods made of an alloy of zirconium. There are thousands of these fuel rods inside a reactor's innermost chamber, which is called the pressure vessel. Water inside the pressure vessel keeps the fuel rods from overheating, and also creates the steam for the turbines.

The pressure chamber is encased in a protective steel shell called the primary containment vessel. Ringing the base of that containment vessel is a doughnut-shaped structure called the torus, which serves a safety function: If pressure rises too high in the pressure vessel, operators can vent steam into the torus through a series of relief valves. (The torus will be important when we explain what went wrong in Fukushima's No. 2 reactor building.)

The primary containment vessel and the torus are in turn encased by the secondary containment building, a large box of steel and concrete. This protective building also houses a storage pool where spent nuclear fuel is kept in cool, circulating water. The water keeps the still-radioactive spent fuel from overheating and melting, and also prevents radiation from reaching the atmosphere. The storage pool is above the primary containment vessel because the spent fuel assemblies are removed from the top of the reactor, and transferred via water canals to the pool to keep them cool throughout the process. The storage pools at Fukushima Dai-1 are reportedly about 14 meters deep; the 4-meter tall spent fuel assemblies sit at the bottom of the pool. 

What Went Wrong

When the 9.0-magnitude earthquake struck offshore on Friday March 11 the Fukushima Dai-1 plant on Japan's northeast coast was not badly damaged, and its emergency shutdown procedures went into effect. The first step went fine: To stop the nuclear fission chain reaction, control rods with neutron-absorbing properties were inserted among the fuel rods. 

But even though the fission reaction came to a halt, the danger wasn't over. Radioactive byproducts of past fission reactions continued to generate heat inside the pressure vessel even though the reactor was no longer active, so cooling systems were supposed to kick in to circulate cold water and remove steam. But the tsunami that swiftly followed the earthquake swamped the coastal facility and damaged the generators and power systems that ran Fukushima Dai-1's cooling mechanisms. That's when things started to go bad.

Reactor No. 1

The plant's operators rushed in new generators and turned on battery-powered backup systems, but apparently this gear couldn't keep the recently active reactors from heating up. It happened first in reactor No. 1 on March 12, when high temperatures inside the pressure vessel evaporated too much of the water inside the chamber. As the water level dropped, the zirconium alloy fuel rods reacted with the steam and other gases to produce hydrogen gas.

As pressures in the inner chamber reached dangerously high levels, operators decided to vent steam (containing some radioactive elements) first into the primary containment vessel, and then into the secondary containment building. But the volatile hydrogen gas appears to have reacted with oxygen in the secondary containment structure, causing an explosion that ripped the roof off the building. This explosion released some radioactive material; it's not clear whether it damaged the primary containment vessel.

Extremely high temperatures inside the reactor in those first few days are believed to have melted parts of the zirconium alloy fuel rods and some of the uranium pellets themselves. That's a serious concern, because melted uranium can drip down and pool at the bottom of the pressure chamber. If enough of it gathers there, it can eat through the chamber's steel floor and drip down into the primary containment vessel. Over time, it can even eat through the thick concrete floor of the containment structure. That worst-case scenario is commonly referred to as a "meltdown." There is also a danger of the fuel collecting and momentarily re-igniting a self-sustaining chain reaction.

Following the hydrogen explosion in reactor building No. 1, plant operators pumped seawater through the reactor in an effort to keep it cool and avert any further blasts. The corrosive salt water has rendered the reactors unfit for future use. On March 25 TEPCO officials began to switch the pumping system back over to fresh water, due to concerns that the salt water would corrode the pumping equipment and cause it to malfunction. 

On Mach 29 TEPCO officials announced that radioactive water had been found outside reactor building No. 1. Scroll down for more information on that threat.

By April 5, water temperatures inside reactor No. 1 had begun to fall, raising hopes that the reactor was on its way to a stable "cold shutdown." But another problem quickly arose: On April 6, TEPCO announced that hydrogen gas was again building up inside the primary containment vessel, increasing the chances that it would react with oxygen and cause another explosion. To prevent another accident, TEPCO workers began injecting nitrogen gas into the containment vessel to thin the concentrations of hydrogen. Periodic nitrogen injections have continued through April and May.

At the beginning of May, workers entered the reactor building for the first time since the earthquake to install air-filtering equipment that could remove radioactive elements and make working conditions safer. Despite this procedure, radiation levels have remained very high in some parts of the No. 1 reactor building. Workers have repeatedly entered the building since then to assess conditions and install monitoring equipment. 

On May 12, TEPCO confirmed that fuel rod melting has indeed occurred in reactor No. 1. The announcement was based on the company's analysis of water gauge readings, which showed that the water level in the pressure chamber was below the normal location of the fuel rods, suggesting the the fuel rods had been partially or completely exposed. This led a TEPCO official to announce that the fuel rods had probably melted, at least partially, and the molten fuel had "fallen to the bottom of the reactor. So it can be said the No. 1 reactor is a state of meltdown.”

The TEPCO official went on to say that the melted fuel may have damaged the bottom of the pressure chamber, allowing radioactive water to leak out into the primary containment vessel. It's possible that leaks in the primary containment vessel have allowed that contaminated water to reach other parts of the building.

The good news is that the melted fuel is thought to have been cooled by water at the bottom of the pressure chamber, as temperatures in the chamber have been relatively low and stable. And there is no sign that the melted fuel has reignited in a nuclear chain reaction.

Reactor No. 3

A chain of events similar to those that caused the explosion in building No. 1 tore the roof off reactor building No. 3 on the morning of March 14. In that building, operators had already resorted to pumping seawater into the pressure chamber to cool it, but they weren't able to prevent the rise in temperature, the pressure buildup, or the hydrogen explosion (pictured below).

TEPCO officials initially said that the No. 3 primary containment vessel wasn't damaged. But on March 16 white steam began to rise from building No. 3, raising fears that the primary vessel had, in fact, cracked due to the explosion. If the steam was leaking from the primary containment vessel, it was likely to be contaminated with radiation. As of yet there has been no firm answer on the status of the primary containment vessel in reactor No. 3, but damage is suspected.

On the morning of March 17, new problems arose at the No. 3 reactor building, this time at the spent fuel pool. Word came that the pool had heated up, causing some of its water to evaporate away and possibly exposing the spent fuel rods to the air. That exposure would cause the rods to heat further and could cause the nuclear fuel within to begin melting, which would increase the amount of radiation emitted. Since the water circulation system intended to keep the storage pool cool wasn't working, TEPCO called in the big (water) guns.

On March 17, two helicopters flew over the building and dumped water on building No. 3. Later in the day police trucks used water cannons to send jets of water into the building, with limited success. Finally the Japanese military sent in its own water-spraying trucks, which reportedly blasted 30 tons of water into building No. 3 over the course of 30 minutes. On March 18 the military trucks repeated the water-spraying operation, blasting 45 tons of water into building No. 3. TEPCO said that the white steam that billowed up during the military's mission indicated that the water reached the spent fuel pool.

Throughout the crisis, spikes of radiation made the situation dangerous for workers in the plant's shielded control rooms, and made it difficult for outside personnel to approach the site. Water-spraying trucks have continued to hose down building No. 3's reactor and spent fuel pool in a series of missions through March and April, but operations were occasionally disrupted when radiation levels temporarily increased or mysterious bursts of smoke emanated from the building.

In the second week of the crisis TEPCO got to work reconnecting the plant to the electricity grid, which would allow its cooling systems to go back into operation. On March 22 the company announced that it was ready to restore power to building No. 3, and would soon have lights and instruments working in the other buildings as well. In the midst of this promising news, however, a terrible incident occurred in building No. 3.

On March 24, three subcontractors were laying electrical cables in the turbine building behind the No. 3 reactor when they waded into water and heard their radiation alarms go off. It soon became clear that they had waded into highly radioactive water, which seeped through the protective clothing of two of the men. The workers were taken to the hospital to be treated for radiation exposure (pictured), and were discharged on March 28.

Officials from Japan's Nuclear and Industrial Safety Agency said that the radioactive water indicates a problem with the No. 3 reactor. The officials said the primarily containment vessel may be leaking, but other experts said it's more likely that a pipe or a valve in the reactor's water circulating system is cracked. Following the accident with the workers, more radioactive water has been found outside many of the reactor buildings. Scroll down for more information on that development.

TEPCO officials have been particularly concerned about the No. 3 reactor because it's the only one of the plant's six reactors that uses a mix of uranium and plutonium fuel. This "mixed oxide fuel" can produce dangerously radioactive materials.

On March 28 TEPCO officials announced that plutonium had been detected in five soil samples taken around the plant, but declared that the levels were very low and didn't pose a threat to human health. The composition of the plutonium suggested that it came from a nuclear reactor. However, the plutonium didn't necessarily come from reactor No. 3--the reactors that use only uranium fuel also produce some plutonium as a byproduct of the nuclear fission reaction.

Reactor No. 2

The explosion in the No. 2 reactor building, which occurred on the morning of March 15, was initially viewed as more serious than the prior two explosions because it was the first blast that clearly involved a primary containment vessel.

The incident occurred while operators were trying, with limited success, to pump seawater into the pressure chamber. According to reports, the vents intended to release steam and relieve pressure were stuck closed, and the high pressure inside the chamber prevented the injection of seawater. As the water level in the chamber stayed obstinately low, the fuel rods were reportedly fully exposed to the air for six and a half hours. Commenting on the crisis in the No. 2 reactor shortly after the blast, TEPCO said it "could not deny the possibility that the fuel rods were melting." Later, international nuclear authorities estimated that about 33 percent of the fuel in the No. 2 reactor had melted.

The blast in reactor building No. 2 is thought to have occurred in the torus, when operators were venting steam into the structure to relieve pressure in the pressure chamber. It's thought that hydrogen exploded within the torus, damaging the primary containment vessel. The problems here are twofold: the melting of the uranium fuel pellets in reactor No. 2 contaminated the pressure chamber's steam and water with radioactive material. And the damage to the primary containment vessel allowed that contaminated water to spread beyond the pressure chamber.

On March 17, TEPCO workers began efforts to reconnect the plant to the electrical grid; since the earthquake the power station had been relying on backup generators and batteries. On March 21 TEPCO restored some power to reactor building No. 2, and workers continue to work on making the building's cooling systems operational again.

On March 27 TEPCO officials announced that they'd discovered highly radioactive water outside reactor building No. 2.

Radioactive Water Leakages

Following the accident that exposed three workers to radioactive water on March 24, TEPCO officials began to look for more radioactive water--and they found it. Pools of contaminated water were found in the turbine buildings behind reactor buildings 1, 2, and 3. The most dangerously radioactive water is in the buildings around reactor No. 2. The discovery complicated repair work on all of the buildings' cooling systems, and also set off a frantic effort to determine the source of the water and to prevent it from spreading. 

The high level of radiation in the water is due to the rapid decay of radioactive atoms with short half lives. This shows that the water came from the reactor systems rather than the pools containing spent fuel (in the pools, this decay would have already taken place). Since the plant's operators need to keep pumping water into the reactors to keep them cool, the radioactive water will continue to accumulate.

The weekend after the workers' accident, TEPCO scrambled to drain water storage tanks to make room for the radioactive water. But by March 28 the situation had gotten worse. TEPCO officials announced that radioactive water had been found in concrete tunnels that house cables and pipes alongside the reactor buildings.

On Monday April 4 TEPCO began dumping 11 500 tons of water contaminated with low levels of radioactive iodine into the Pacific Ocean to make room in the storage tanks for highly radioactive water from the turbine buildings. The waste water released had about 100 times the legal limit for radiation, while the water that TEPCO was so desperate to store had about 10 000 times the limit.

The deliberate discharge of radioactive water wasn't the only source of ocean pollution. TEPCO also scrambled to deal with a leak that poured radioactive water into the sea near reactor No. 2 (pictured below). After several days of unsuccessful attempts, TEPCO finally plugged the leak on April 6 by injecting 6000 liters of liquid glass into the ground near the leak.

Reactors No. 4, 5, and 6

These three reactors were offline at the time of the earthquake, but they still became a source of concern. Fires broke out in reactor building No. 4 on March 15 and March 16, and TEPCO officials worried that fires were possible in the other two buildings as well.

In these three buildings, spent fuel is stored in water-filled tanks, which keep them cool. In reactor building No. 4, the water temperature reportedly rose from 40 degrees Celsius to 84 degrees Celsius. It's likely that the fuel rods overheated, causing the zirconium alloy cladding to partially melt and react with water or steam. That would have produced volatile hydrogen gas, which could have sparked a blast. According to reports, the actual substance burning in building No. 4 was lubricating oil used in machinery near the storage pool.

The fires in building No. 4 soon went out, but for many days concerns remained that the spent fuel in all three buildings was too hot. The smoke from the fire in building No. 4 was thought to have drastically--but temporarily--increased radiation levels around the reactor, so operators were very keen to prevent more blazes.

While Japanese emergency response teams have not focused on building No. 4 since the fires abated, reports from nuclear engineers and officials in the United States have suggested that the building should be a high priority. On March 17, the head of the U.S. Nuclear Regulatory Commission told a congressional committee that building No. 4's storage pool had lost all its water, leaving its spent fuel exposed to the air. On March 18 the Los Angeles Times reported that the No. 4 pool had either been cracked or breached during the earthquake, causing water to drain away. However, TEPCO officials have contradicted these statements.

Water-spraying trucks hosed down building No. 4 periodically beginning on March 20. On March 22, TEPCO announced that the building was reconnected to the grid, and power had been turned on. A photo from the No. 4 building's spent fuel pool is below.

The storage pools in buildings No. 5 and No. 6 continued to warm up for about a week after the earthquake, but as of March 22 they had returned to near-normal temperatures. They're not currently considered a threat.

Citizens' Health Concerns

The Japanese government evacuated all residents living within 20 kilometers from Fukushima Dai-1 early in the crisis, and advised people living between 20 and 30 kilometers of the plant to stay indoors. Later the government issued a voluntary evacuation advisory for all those living within 30 kilometers of the plant.

But by early April it had become clear that radioactive materials would continue to leak from the plant for some time, making the situation more dangerous for residents who would receive a low but steady dose. On April 11 the Japanese government expanded the evacuation zone, ordering residents of many towns in the 20 to 30 km zone (and some even farther out) to leave. Strong aftershocks have also raised fears that the already crippled plants could be further damaged.

On April 12, the Japanese government officially acknowledged the severity of the Fukushima Dai-1 incident by raising its rating on the International Atomic Energy Agency's scale of disaster. The Fukushima incident is now rated 7, the same rating Chernobyl got, because it involved a major release of radiation with widespread health or environmental affects. But the amount of radiation released at Fukushima is still far less than that emitted by the Chernobyl accident--the highest estimates for Fukushima's emissions to date are about 5 to 10 percent of Chernobyl's.

One week after the earthquake, the Japanese Ministry of Health, Labour and Welfare announced that radiation levels exceeding the legal limit had been found in milk and vegetables produced in the Fukushima region. Shipments of agricultural products from the region were quickly banned. In the second week, the government announced that radioactive substances had been detected in the tap water in Tokyo. On March 24, levels of iodine-131 in the tap water were declared unsafe for infants, causing the government to distribute bottled water to families. The next day the government said that the amount of radioactive iodine in Tokyo tap water was again within safe limits.

How did those radioactive substances get into cow's milk and Tokyo's tap water? It started with the steam that plant operators vented in the days after the earthquake in an (unsuccessful) attempt to reduce pressure in the reactor buildings and prevent explosions. That steam carried small amounts of radioactive substances. The subsequent explosions and the steam that rose up when firefighters sprayed water on the reactor buildings also brought radioactive substances into the air. Officials think the radiation levels in Tokyo's tap water spiked following a rainfall that brought radioactive substances down from the clouds.

Even before the deliberate release of radioactive water into the ocean, Japan's Nuclear and Industrial Safety Agency has reported high levels of radioactive iodine-131 in seawater samples taken 1.6 kilometers away from the coastal power plant's drainage pipes. Higher levels of iodine-131 were detected in seawater about 300 meters from the plant on March 30: The iodine-131 level was 3350 times greater than the government safety limit. Elevated levels of cesium-137 have also been detected in seawater.

When TEPCO announced on April 4 that it would begin dumping low-level radioactive waste water into the ocean, Japan's Nuclear and Industrial Safety Agency announced that it did not consider the ocean contamination to be a health threat, because no fishing is currently permitted within 20 kilometers of the plant. However, The New York Times reports that marine biologists are worried that radioactive elements will accumulate in big fish as they eat smaller, contaminated fish.

Looking Forward

For more than a month after the earthquake and tsunami, TEPCO seemed to be reeling from one urgent crisis to the next. Finally, on April 17, the company announced a "roadmap to restoration," which outlines the steps necessary to stabilize the plant and stop most radioactive emissions. TEPCO hopes to bring all the plant's reactors to a "cold shutdown" within 9 months, meaning that the water inside the reactor would be below the boiling temperature of 100 degrees Celsius. But some experts are skeptical that TEPCO will be able to stick to its proposed timeline.

The plan calls for the installation of new cooling systems for the reactors, as the plant's existing cooling systems may be damaged beyond repair. It proposes the installation of a temporary covering over the damaged reactor buildings to prevent further radiation emissions. And TEPCO is also developing a water decontamination system with the French nuclear company Areva and the U.S. waste management company Kurion.

On May 10, Japanese prime minister Naoto Kan suggested that the country should rethink its national energy plan, and should place more emphasis on renewable energy and conservation. "We need to start from scratch," Kan said in a press conference. "We need to make nuclear energy safer and do more to promote renewable energy." His statement suggested that the government will drop its existing energy policy, which calls for the construction of 14 more nuclear reactors before 2030.

Kan had previously requested that the Chubu Electric Power Co. suspend operation at its Hamaoka nuclear power plant in Shizuoka prefecture, an area southwest of Tokyo that is thought to be very vulnerable to earthquakes. On May 10, the company agreed to close the coastal plant "until further measures to prevent tsunami are completed."

IMAGES: DigitalGlobe/Getty Images; Nuclear Energy Institute; TEPCO/Reuters; NTV Japan; Kyodo/Reuters

Fire and Steam at Japan's Damaged Nuclear Plant Spread Radiation Worries

Special Report: Fukushima and the Future of Nuclear Power

Editor's Note: This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency. John Boyd is an IEEE Spectrum contributor reporting from Kawasaki, Japan.

Day six of Japan's nuclear emergency brought few signs of relief to the troubled nation. People woke up Wednesday morning to the news that there had just been a second fire in the No. 4 reactor building of the Fukushima Dai-1 nuclear power plant, which was damaged by the earthquake and tsunami that struck northeastern Japan on March 11.

But this was only one of a series of events that occurred throughout the day. A billow of steam raised fears that two of the structures that contain radioactive materials inside the reactor buildings may have been damaged. And fluctuating radiation levels at the nuclear plant made it difficult for workers to combat the problems.

Fresh Flames

The new fire was discovered by a Tokyo Electric Power Company (TEPCO) worker at 5:45 a.m. local time. The blaze broke out in the same location where a fire occurred yesterday: the storage pool where spent fuel from the No. 4 reactor is kept. TEPCO reported that no flames could be seen thirty minutes after the fire was initially sighted.

The No. 4 reactor was shut down at the time of the earthquake for inspection and maintenance. The fires at the No. 4 building have raised fears that similar incidents could occur at the No. 5 and No. 6 reactor buildings, which were also shut down for maintenance when the earthquake struck.

No one has been able to say with certainty what caused yesterday's fire in the No. 4 building, or the new fire today. TV footage of the damage caused yesterday showed two huge holes in the walls of the reactor building, as well as substantial damage to the roof.

Some experts surmise that the water level in the storage pool dropped after circulation failed due to the string of aftershocks that have followed the main earthquake. With circulation in the storage pool halted, the radioactivity in the spent fuel would have raised the temperature of the water. This would have caused water to evaporate, eventually exposing the spent fuel rods to the air.

Masashi Goto, a former Toshiba Corporation design engineer of nuclear containment vessels of the kind used in the Dai-1 Plant, said another possibility was "sloshing": the water may have sloshed out of the storage pool due to the earthquake's shaking. Goto said this kind of splashing happened in 2007: "This is what happened during the Kashiwazaki (Nuclear) Plant accident after the earthquake struck it and sloshed water outside the pool."

Goto noted that spent fuel rods continue to generate heat long after they are taken out of operation; that's why they must be submerged in a storage pool container filled with water that is constantly circulated to maintain a safe temperature. NHK, the Japan national broadcaster, reported that 783 fuel rods are held in the No. 4 building's storage pool.

Regardless of whether water circulation stopped or water sloshed out, the exposure of the spent fuel likely started the fire. As the exposed fuel rods heated up, their zirconium casings may have partially melted, causing a chemical reaction between the zirconium and the water or steam. The reaction may have produced volatile hydrogen gas, which may have been sparked to produce a blast.

But Kazuaki Matsui, executive managing director of the Institute of Applied Energy, an independent research organization in Tokyo, told Spectrum that a hydrogen blast would not cause a fire. "Hydrogen burns without flames, so it seems some kind of burnable materials were around, possible brought into the room when maintenance was being conducted," he surmised. The New York Times offered a possible explanation today, reporting that the actual substance burning in the building was lubricating oil from machinery near the storage pool.

To add to the problem, TEPCO informed the government that water temperatures in the spent fuel storage pools in the No. 5 and No. 6 reactor buildings "were higher than normal." In a Wednesday press conference, Chief Cabinet Secretary Yukio Edano said TEPCO had made preparations to deal with the situation.

Matsui told Spectrum that since the No. 5 and No. 6 reactors were shut down for maintenance before the earthquake, he didn't think "there was anything to worry about as long as the situation is monitored."

A Cloud Rises

Meanwhile, white steam was reported billowing out of the No. 3 reactor building earlier in the morning. The No. 3 reactor building was damaged in an explosion on Monday.

An NHK helicopter positioned 30 kilometers from the plant filmed the emissions that continued to rise from the plant at 10:15 a.m., local time. Edano told reporters during his press conference that the emissions appeared to be steam leaking from the No. 3 reactor's containment vessel, which probably accounted for the rise in radiation levels recorded at 10:45 a.m. Edano said workers trying to inject water into the No. 3 reactor system had to evacuate temporarily, but they returned to the plant after one hour when radiation levels fell.

"We don't know for sure what is causing the steam," says nuclear expert Matsui. "But it seems the earlier hydrogen explosion has damaged a valve or something like that [controlling the] container vessel."

The primary containment vessel is a solid structure of steel and concrete that surrounds the pressure vessel, where the actual nuclear reactions that produce power take place. If the containment vessel in the No. 3 reactor was damaged by Monday's explosion, it increases the possibility that radioactive material will escape the reactor and contaminate the site.

But to add to the confusion, the Nuclear and Industrial Safety Agency said it had measured high radiation levels near the No. 2 reactor. As of now, the source of the leaking radiation is not clear. The No. 2 reactor has caused considerable worry, as a blast on Tuesday at that reactor's building seems to have occurred inside the containment vessel. Officials said Tuesday that the No. 2 containment vessel may be cracked.

TEPCO informed the government on Wednesday that it was having trouble injecting water into the No. 3 reactor because radiations levels had risen to 300 to 400 millisieverts per hour in the vicinity, making it dangerous for the plant workers to stay on site. The company was considering several options to cope with the situation, including calling in a Self Defense Force helicopter to dump water over the No. 3 reactor building (its roof was blown off in Monday's explosion); calling in fire fighters to spray water over the building; and have a ground crew ready to inject water once radiation levels became low enough.

Around 4 p.m. local time a Self Defense Force Boeing CH-47 Chinook helicopter flew over the stricken plant to monitor the radiation and gauge the possibility of a water dump. But the helicopter operation was later aborted. Broadcaster NHK reported that radiation levels above the plant were too high to allow Self-Defense Force personnel to safely carry out the mission. In a 6 p.m. press conference Chief Cabinet Secretary Edano said a TEPCO ground crew was making preparations to deal with the problem reactor from the ground. NHK announced around 7 p.m that a ground crew had moved in with a mobile pump and had begun spraying water over the reactor area.

The Public Worries

While all this was going on a 6.0 magnitude earthquake struck just off the coast of Chiba Prefecture around 1 p.m. local time on Wednesday, rattling nearby Tokyo. The Japan Metrological Agency gave no tsunami warning, but the earthquake has put many Japanese in the capital on edge. Tokyo residents are already dealing with rolling power black-outs, bare shelves in supermarkets due to panic buying, and limited train services. International schools are closed because worried families are taking their children out of country, and a growing number of both foreigners and Japanese are also escaping abroad.

In an attempt to raise the spirits of the people, Emperor Akihito gave a televised address to the nation urging the country to unite, and letting the Japanese people know that he was praying for the nation.

The Institute of Applied Energy's Kazuaki Matsui says even as the crisis continues to unfold, he has begun to look ahead to the future of the nuclear industry in Japan. "TEPCO planned to add two larger 1.36 gigawatt reactors to the site, but this has been postponed," he said. "To say I'm worried about the future of the industry is being optimistic."

Image: DigitalGlobe/Getty Images

Timeline: The Japanese Nuclear Emergency

Special Report: Fukushima and the Future of Nuclear Power

Editor's Note: This timeline is no longer kept up to date. For the latest coverage on the aftermath of the earthquake, view our interactive timeline.

Since March 11th's record earthquake, IEEE Spectrum has continued to report on Japan's ongoing nuclear emergency. In the following timeline, we've compiled the recent happenings from information released by the Tokyo Electric Power Company (TEPCO) and the International Atomic Energy Agency (IAEA), as well as stories published by IEEE Spectrum. We will continue to update this post as we learn more.

Friday, 11 March 2011

2:46 p.m. A 9.0-magnitude earthquake, 130 kilometers off the coast of Miyagi Prefecture in northeastern Japan, shakes the country. A tsunami with waves as high as 10 meters follows within the hour.

-- Tokyo Electric Power Company (TEPCO) reports that turbines and reactors in three units of the Fukushima Dai-1 Nuclear Power Station, which includes six reactor units in total, automatically shut down.

--Japanese authorities report a fire at the Onagawa nuclear power plant, later extinguished.

3:41 p.m. Emergency diesel generators at Fukushima Dai-1 shut down, a result of the tsunami, forcing cooling pumps to start using battery backup. TEPCO uses mobile electric generators to help power to the cooling system.

6:00 p.m. TEPCO announces increase in reactor containment vessel pressure in Fukushima Dai-1 reactor No. 1. Officials decide to vent the reactor to lower pressure.

7:00 p.m. Japanese government issues a state of emergency, and officials evacuate thousands of people living within a 3-kilometer radius of the plant.

10:00 p.m. TEPCO reports blackout for about 2.97 million households.

Saturday, 12 March 2011

7:00 a.m. Government announces that officials will evacuate residents within 10 km radius of Fukushima Dai-1 and Fukushima Dai-2 Nuclear Power Stations.

9:00 a.m. Preparations for air venting from Fukushima Dai-1 reactor No. 1 begin.

3:36 p.m. Blast and white smoke come from Fukushima Dai-1, reactor No.1--blowing off the roof and the upper part of the outer walls of the structure containing the reactor. TEPCO says that the primary containment vessel remains intact. Four plant safety workers are injured.

--TEPCO begins venting steam to reduce the growing pressure inside reactor No. 1. Radiation density around the structure rises to the level of 1015 microsieverts. By 6:48 p.m. it falls to 70.5 microsieverts.

7:11 p.m. Japanese government evacuates local residents within 20 km of plants. Japanese authorities prepare to distribute iodine to residents.

8:00 p.m. TEPCO announces radiation exposure of one employee, who was working inside Fukushima Dai-1 reactor building, exceeded 100 microsieverts (106.3 microsieverts).

8:20 p.m. TEPCO begins injecting sea water mixed with boron to cool reactor No. 1.

--Japan's Nuclear and Industrial Safety Agency (NISA) detects caesium-137 and iodine-131 near Fukushima Dai-1 plant.

11:00 p.m. TEPCO announces that they have been injecting sea water into the reactor core of Fukushima Dai-1's reactor No. 1 and then boric acid to absorb radiation. Current reactor water level is lower than normal in reactor No. 2.

Sunday, 13 March 2011

9:20 a.m. TEPCO completes air venting in reactor No. 3 to reduce internal pressure, then begins injecting water containing boric acid into the reactor via fire pump, given failure of high pressure injection system.

--At Fukushima Dai-1, one worker is exposed to higher-than-normal radiation, and four injured and one dies in crane accident.

Monday, 14 March 2011

6:15 a.m. TEPCO reports that Fukushima Dai-1 reactor No. 3's pressure increases to 530 kilopascals.

--In reactor No. 2 in Fukushima Dai-1 core coolant levels decreasing and sea water pumping begins.

9:05 a.m. Pressure decreases in reactor Fukushima Dai-1 No. 3 to 450 kilopascals.

11:01 a.m.  Hydrogen explosion from Fukushima Dai-1 reactor No. 3 injures seven workers. TEPCO believes that the reactor containment vessel remains intact. 

1:25 p.m. TEPCO confirms that Fukushima Dai-1 reactor No. 2's cooling system has failed.

--IAEA reports 230 000 units of stable iodine distributed to evacuation centers. 

5:17 p.m.  TEPCO reports that water level in the Fukushima Dai-1 reactor No. 2 remains above fuel rods and that the company has restarted water injection.

Tuesday, 15 March 2011

6:00 a.m. TEPCO initiates rolling blackouts, given reduced power supply. The company reports an explosive sound in 5th floor of Fukushima Dai-1 reactor No. 4.

6:14 a.m. Blast from No. 2 reactor building of Fukushima Dai-1 nuclear power plant.

9:38 a.m. A fire breaks out in the Fukushima Dai-1 No. 4 reactor building. IAEA reports that Japanese authorities believe that spent fuel storage pool is burning, releasing radiation into atmosphere.

10:22 a.m. Radiation readings taken near the reactors show that emissions range from 30 millisieverts per hour to 400 millisieverts per hour--higher than the microsievert numbers reported when steam was vented. Officials ask Japanese residents up to 30 km from the Fukushima Dai-1 to stay indoors. 

10:30 p.m. One of many aftershocks from Friday's earthquake, a 6.0-magnitude earthquake strikes Shizuoka Prefecture southwest of Tokyo.

Wednesday, 16 March 2011

5:45 a.m. TEPCO employee discovers another fire in reactor No. 4 of Fukushima Dai-1.

6:15 a.m. TEPCO reports that, upon inspection, no fire is present in Fukushima Dai-1 reactor No. 4, but will keep under surveillance. 

8:30 a.m. TEPCO reports "fog like steam" from Fukushima Dai-1 reactor No. 3

4:00 p.m. Temperature in spent fuel pool of Fukushima Dai-1 reactor No. 4 rises. Self Defense Force considers spraying water from helicopters but attempt cancelled.

--Japan's Nuclear Safety Commission recommends that local authorities instruct evacuees leaving the 20-kilometre area to ingest stable iodine, available at evacuation centers.

Thursday, 17 March 2011

9:48 A Self Defense Force helicopter flying from the west drops the first load of water almost directly over the Fukushima Dai-1 No. 3 reactor. A second helicopter flying from the east then dumps its water over reactors No. 4 and No. 3. Two more passes are made before the operation is suspended around 10 a.m. 

--TEPCO patrols reactor No. 4 building to inspect dry casks--longer term storage for spent fuel--and notices nothing abnormal visually. Prepares for later detailed inspection. Also monitors common spent fuel pool located in a separate building from reactor units to confirm that water levels in pool are secure.

10:00 a.m. Radiation levels measured by TEPCO read  400mSv/h at the inland side of the No. 3 reactor building and 100mSv/h at the inland side of the No. 4 reactor building.

2:00 p.m. Self Defense Force fire engines spray water on reactor No. 3.

5:30 p.m. IAEA reports that engineers have begun to lay an external grid power line cable to No. 2 reactor.

--IAEA reports that operators able to start one of the diesel generators to reactor No. 6.

7:00 p.m. Riot police's high-pressure water cannon trucks and Self-Defense 
Force's fire engines spray more water on reactor No. 3.

Friday, 18 March 2011

2:00 p.m. TEPCO reports that Self Defense Forces spray reactor No. 3 with water from trucks.

--TEPCO reports that water level in spent fuel pool in reactor No. 4 is secure.

Saturday, 19 March 2011

12:30 a.m. Tokyo Fire Department's Hyper Rescue sprays more water on spent fuel pools in reactor No. 3

5:00 a.m. TEPCO reports that they have started the Residual Heat Removal System pump in reactor No. 5 to ensure that spent fuel is cooling in pool.

--TEPCO reports that it has made three holes in the roofs of reactors No. 5 and 6 to prevent hydrogen gas from accumulating within the buildings

10:14 p.m. TEPCO starts another diesel generator in reactor No. 6.

Sunday, 20 March 2011

--TEPCO reports temperatures have gone down in the spent fuel storage pools of the No. 5 and No. 6 reactors.

8:00 a.m.  Radiation levels had been measured at 500 meters from the No. 3 reactor building: At 8 a. m. this morning the level was 2625 microsieverts compared to 3443 microsieverts measured at 2 p.m. yesterday, as reported by Hidehiko Nishiyama, deputy director general of the Nuclear and Industrial Safety Agency.

8:21 a.m. Self-Defense Forces uses fire engines to spray water on reactor No. 4.

3:46 p.m. TEPCO finishes laying cable to and starts providing power to reactor No. 2. IAEA reports that off-site electrical power has been connected to an auxiliary transformer and distribution panels.

5:20 p.m. TEPCO has injected some 40 tons of seawater into reactor No. 2 

9:30 p.m. Fire Rescue Task Forces of Tokyo Fire Department sprays more water on reactor No. 3.

Monday, 21 March 2011

6:30 a.m. Self-Defense Force and U.S. Army fire engines spray more water into reactor No. 4, finishing at 8:40 a.m. 

--According to representative from Nuclear and Industrial Safety Agencyradiation measured 500 meters from the plant at 6:30 a.m. Sunday morning was 2670 microsieverts per hour, 3346 microsieverts when measured at 2:50 p.m. and 2346 microsieverts at 6:30 a.m. Monday.

9 a.m. TEPCO reports that it has a 450-volt cable connected to the plant and is readying to supply a central control room for the No. 1 and No. 2 reactors. 

11:36 a.m. TEPCO reports that power from emergency diesel generators partially restored to reactors No. 5 and 6 through transmission line.

3:55 p.m.  TEPCO employee spots light gray smoke rising from the southeast side of the rooftop of the reactor No. 3 building.

--TEPCO reports that radioactive materials were detected [PDF] from the seawater around the discharge canal (south) of Fukushima Daiichi Nuclear Power Station. According to IAEA, testing samples included iodine-131, cesium-134, and cesium-137. Japan Agency for Marine-Earth Science and Technology plans to measure radioactivity around the plant from March 22 to March 23.

Tuesday, 22 March 2011

--TEPCO reports that the color of smoke from reactor No. 3 changed to somewhat white and is slowly dissipating.

3:10 p.m. Tokyo Fire Department's Hyper Rescue sprays more water into reactor No. 3.

4:00 p.m. TEPCO employees inject 18 tons of seawater into reactor No. 2.

5:17 p.m. Concrete pumping vehicle sprays more water into reactor No. 4, at rate of 50 tons per hour.

10:45 p.m. By this time, TEPCO reports that electricity supply from external source restored to reactors No. 1 to 6. Lights turn on in the mail operation room and reactor No. 3 main control room.

Wednesday, 23 March 2011

2:30 a.m. TEPCO reports injection of seawater into the reactor No. 1 from feed water system.

10:00 a.m. TEPCO sprays more water on reactor No. 4 building using the concrete pumping vehicle.

11:00 a.m. TEPCO injects more seawater into spent fuel pool in reactor No. 3 building.

4:20 p.m. TEPCO employees observe light gray smoke coming from reactor No. 3 building.

Thursday, 24 March 2011

--IAEA publishes slides with overview of each reactor's status.

--Three workers exposed to high radiation levels at the plant after stepping into water contaminated with radiation, and received between 173 and 180 millisieverts. 

2:36 p.m. Concrete pump truck starts to spray reactor No. 4's spent fuel pool with around 150 tons of water using concrete pump truck.

3:37 p.m. Power supply restored to common spent fuel's cooling system.

4:14 p.m. Temporary pump for Residual Heat Removal repaired for reactors No. 5 and 6.

7:30 p.m. Workers continue to inject seawater into the reactor pressure vessels of Units 1, 2 and 3 and are preparing to inject pure water.

8:30 p.m. Off-site electricity and lighting in the reactor No. 1's main control room recovered.

9:00 p.m. 120 tons of seawater injected into reactor No. 3's spent fuel pool via the cooling and purification line.

Friday, 25 March 2011

--Japan's Chief Cabinet Secretary holds a press conference later on Friday to advise residents living between 20 and 30 kilometers from Fukushima Dai-1 to consider voluntary evacuation.

6:05 a.m., TEPCO injects seawater into reactor No. 4's spent fuel pool using Fuel Pool Cooling and Filtering system.

10:30 a.m. TEPCO begins injecting seawater into the spent fuel pool of reactor No. 2, using Fuel Pool Cooling and Filtering system 

1:28 p.m. Kawasaki City Fire Department sprays water on reactor No. 4's spent fuel pool.

3:37 p.m. TEPCO pumps fresh water (instead of sea water) into reactor No. 1 pressure vessel.

7:05 p.m. Concrete pumping vehicle sprays water onto reactor No. 4's spent fuel pool.

Saturday, 26 March 2011

8:50 a.m.  TEPCO tests radiation levels in reactor No. 2 basement's water levels. Detailed results, here [PDF].

10:10 a.m. TEPCO begins pumping fresh water with boric acid into reactor No. 2

--TEPCO reports results of radioactive material analysis in seawater around discharge canal of plant to Nuclear and Industrial Safety Agency (NISA) and Fukushima prefecture. Also reports results of nuclide analysis of radioactive materials contained in the air.

4:46 p.m. The light in the main control room of reactor No. 2 turns on.

Sunday, 27 March 2011

--TEPCO releases more results of radioactive material analysis in seawater and air.

3:30 p.m. TEPCO workers find water in the trenches of reactor buildings No. 1 to 3. The radioactive emission at the surface of the water was 0.4 mSv/h for reactor No. 1 and over 1000 mSv/h for No. 2. Could not access No. 3 trench due to rubble.

4:55 p.m. More water sprayed on reactor No. 4 using concrete pump.

Monday, 28 March 2011

--TEPCO releases updated results of analysis of radioactive materials in air surrounding plant.

--TEPCO detects plutonium in soil surrounding the plant. Company says density of detected plutonium (around .27 and .26 becquerel per kg of dry soil from Pu-239 and Pu-240 in two out of five sample sites, and .54 Bq/kg from Pu-238 from one site) is equivalent to the fallout observed in Japan from atmospheric nuclear tests and poses "no major impact on human health." IAEA notes: 

Traces of plutonium are not uncommon in soil because they were deposited worldwide during the atmospheric nuclear testing era. However, the isotopic composition of the plutonium found at Fukushima Daiichi suggests the material came from the reactor site, according to TEPCO officials. Still, the quantity of plutonium found does not exceed background levels tracked by Japan's Ministry of Education, Culture, Sports, Science and Technology over the past 30 years.

Tuesday, 29 March 2011

8:20 a.m. TEPCO switches to injection of fresh water from using fire engine to temporary electrical pump in reactor No. 1

11:50 a.m. Lights in the main control room of reactor No. 4 building turn on. Now all of the control room lights are on.

12:03 p.m. Water sprays three workers while they're taking off the flange of the pipe to remove the residual heat in the seawater system. TEPCO reports that no radioactive material "adhered to their bodies."

2:17 p.m. Concrete pump starts injection of fresh water (instead of seawater) into reactor No. 3.

4:30 p.m. Motor-driven pump starts injecting fresh water into reactor No. 2 instead of seawater.

Wednesday, 30 March 2011

9:25 a.m. TEPCO delivers freshwater to reactor No. 2's spent fuel pool using a temporary motor driven pump and then (after that pump malfunctions) using a fire fighting pump.

2:04 p.m. More water sprayed on reactor No. 4 spent fuel pools using a concrete pumping vehicle.

5:56 p.m. TEPCO employee spots smoke coming from power panel in reactor No. 1 building. The power panel supplies electricity to the water pump motor. Fire department investigates. Smoke stops after TEPCO cuts power to panel.

--Media continue to question if there's been a breach of a pressure vessel in any of the Fukushima reactors. Richard Lahey, former chair of nuclear engineering at Rensselaer Polytechnic Institute, clarifies previous statements in an interview with IEEE Spectrum. He believes that all of the cores have melted, and that reactor No. 2's core may have melted through, but the data make it difficult to come to any firm conclusions. Japanese officials say that there is not evidence of a big breach.

Thursday, 31 March 2011

--TEPCO establishes "Fukushima Nuclear Influence Response Division," sending employees to help evacuees at emergency evacuation sites.

--Nuclear Industry and Safety Agency (NISA) says radiation in the seawater sampled on Wednesday 330 meters south of plant has risen to 4385 times maximum safety levels, a rise from the previous high of 3355 times that was detected on Tuesday.

--President Sarkozy of France, as chair of the G20 and G8, flies to Japan to show unity with the nation and to find out what additional help these countries might offer.

Friday, 1 April

--TEPCO reprimanded for sending teams of workers into the Fukushima plant without supplying the proper safety equipment. Some work groups entered with only one worker having been issued a dosimeter .

 Sunday, 3 April

--TEPCO says that it has detected levels of radioactive iodine in seawater samples taken near the plant that are 5 million times the maximum concentration allowed by law. Company officials said that samples taken on 2 April showed the figure at 7.5 million times the limit.

Monday, 4 April

--TEPCO announces plans to release more than 11 500 tons of what it’s calling "low-level radioactive waste water" into the Pacific Ocean. The water, which was contaminated with radiation due to still-mysterious leaks, is being dumped to make room in the plant’s storage tanks for more highly radioactive water, which has pooled in the basements of several of the reactors’ turbine buildings.

--As TEPCO undertakes its planned release of contaminated water into the sea, it tried, without success several schemes to stem the flow of highly radioactive water from an as yet undiscovered source that is pouring out of a 20-centimeter-long crack in a concrete pit near the seawater intake area for reactor No. 2.

 Tuesday, 5 April

--TEPCO has made some headway in its efforts to stop the leak from the crack near the reactor No. 2 intake area. Workers injected liquid glass (sodium silicate) into the bedrock near the concrete pit and said that the water flow has since decreased by half. Where the water still flowing from reactor buildings will go instead is still an open question. 

Wednesday, 6 April

5:38 a.m. TEPCO confirms that water is no longer spilling from the crack in the concrete lateral of the pit near the seawater intake for reactor No. 2. But a TEPCO official cautioned that the stoppage of water running through the crack does not mean that the water isn’t escaping via another route.

--TEPCO injects 6000 cubic meters of nitrogen gas into reactor No. 1 in an effort to thin the concentration of hydrogen and prevent an explosion like the ones that damaged buildings housing reactors in the first few days following the 11 March earthquake and tsunami. The utility is also considering injecting nitrogen into the No. 2 and No. 3 reactors.

Thursday, 7 April

12:32 a.m. A 7.1 magnitude earthquake triggered a tsunami warning in the vicinity of the Fukushima nuclear plant. TEPCO says there was no additional damage, and its efforts at cleaning up and controlling the plant would continue as planned.

--TEPCO reports that the highly contaminated water that had been flowing through the now stoppered crack in a concrete pit near reactor No. 2’s seawater intake is pooling inside the vertical shaft of the trench carrying cables and pipes for the Unit 2 turbine building.  The water level has risen 5 centimeters inside the shaft; there is about 1 meter of shaft above the water’s surface.

 Friday, 8 April

--Though Fukushima was basically unscathed by the earthquake that struck Japan on 7 April, NISA reports that the quake caused trouble at several other nuclear facilities in the tregion. The Higashidori Nuclear Power Plant and the Rokkasho spent fuel reprocessing plant temporarily lost their off-site power feeds and  had to rely on emergency diesel generators to keep spent fuel storage pools cool. Radioactive water spilled from a spent fuel pool at the Onagawa power plant during the quake. 

Monday, 11 April

--Fukushima lost power after a 7.0-magnitude quake. During the outage, water injections into reactors 1, 2, and 3 were halted for about 50 minutes. A tsunami warning caused workers to evacuate the facility without manually switching on diesel-powered back-up systems. When the workers returned, monitoring systems showed no signs of radiation increases, and no other significant problems were found.

--The Japanese government says it will order residents of some towns in the areas just beyond the original 20-kilometer evacuation zone around Fukushima to leave. The new evacuation orders will apply to municipalities in the 20-to-30-km zone and beyond.

Tuesday, 12 April

--The Japanese government raises the severity rating of the Fukushima disaster to a 7 on the International Atomic Energy Agency's severity scale--the highest rating possible. The change puts the incident on par with the 1986 accident at Chernobyl.

Monday, 18 April

--TEPCO officials outline a plan to achieve a cold shutdown of Fukushima within nine months. The utility’s first-phase efforts will include removing  60 000 tons of contaminated water, installing so called stable cooling circulation systems, and halting the release of radioactive dust and water to the air and sea. TEPCO says that once all this has been achieved (supposedly in 3 months), it will spend the ensuing 6 months trying to further stabilize the reactors so it can reliably keep the cooling water at 100 degrees Celsius.

Tuesday, 26 April

--TEPCO sends a remote controlled robot to check the No. 1 reactor for water leaks. Finding none, nor a significant rise in radiation levels, the company says it will try to increase the amount of cooling water it is injecting into the reactor from 6 metric tons a day to about 14 metric tons. The aim is to eventually fill the suppression pool until the water covers the reactor’s pressure vessel and can cool it down from the outside.

Tuesday, 3 May

--TEPCO reports that workers will enter the damaged No. 1 reactor building to install filtering equipment for removing 95 percent of the radioactive dust from the air inside the building. The aim is to create a working environment safe enough for workers to put in place a cooling system to replace the one that was damaged by the tsunami that followed the 11 March earthquake.

--A TEPCO official told IEEE Spectrum that the company also plans to construct a temporary wall or dyke to protect the No. 3 and 4 reactors, which are considered the most vulnerable to another tsunami. The 1- to 2-meter-high wall, which would be built on ground some 10 meters above sea level, would stretch for about 500 meters. He added that the design was still under consideration.

Monday, 9 May

--TEPCO workers and government safety officials enter the No. 1 reactor building to carry out a survey of conditions inside. A TEPCO official  radiation levels in the building ranged are as high as 700 millisieverts per hour—nearly three times the 250-millisievert annual exposure limit for workers prescribed by law.

--Chubu Electric Power Co. says it will suspend operation of its Hamaoka nuclear power plant until construction of a concrete seawall to protect the facility against tsunami is completed. The plant will likely be off line for 3 years, making the 3.5 gigawatts supplied by its three active reactors (roughly 10 percent of the region’s power supply) unavailable. Chubu had been providing make-up power for the areas normally served by the Fukushima plant, but will now have to suspend that service.

Thursday, 12 May

--TEPCO reports that nuclear fuel rods in the No. 1 reactor have suffered at least a partial meltdown. Apparently, the water level in the reactor vessel fell below the fuel rods, leaving them at least partially exposed. TEPCO suspects that the molten fuel has damaged the bottom of the reactor’s pressure vessel. For now, TEPCO has halted plans to flood the reactor's containment vessel—a prerequisite to installing a new cooling system for the damaged reactor.  

Sunday, 15 May

--The Chubu Electric Power Co. completes the cold shutdown of its Hamaoka nuclear plant.


Tuesday, 17 May

--TEPCO reveals its revised plan for bringing Fukushima to a cold shut down. Despite having discovered problems such as a partial meltdown and a suspected pressure vessel breach inside reactor No. 1, TEPCO still expects to shut the plant down by January. But the company has changed how it plans to carry it out. For example, instead of pumping freshwater into the damaged reactors, the water already pooled in the reactor buildings will be decontaminated and used for cooling.

--TEPCO releases the most detailed description to date of what happened in the hours following the 11 March earthquake.


Note: All times are local. Some times are approximate (based on time of TEPCO press releases) and subject to change as news develops. 

Image: TEPCO

In Japan Earthquake, Early Warnings Helped

Last week's devastating 9.0-magnitude earthquake off the coast of northeastern Japan highlighted both the advantages and the limitations of earthquake early warnings. Four years ago, Japan rolled out the world's most sophisticated earthquake early-warning system. The EEW system can detect the approximate source and magnitude of an earthquake and send out public alerts via TV, radio, and cellphone--all in less than a minute of a quake's start. It also transmits signals that can automatically shut down computers, stop elevators at the nearest floor, and halt factory production lines. [In the photo, an early-warning text message reads: "An earthquake at Fukushima-oki has begun. Please be prepared for strong tremors. (Japan Meteorological Agency)".]

The EEW system, which is operated by the Japan Meteorological Agency, relies on a network of about 1000 seismic stations scattered around the country. It exploits the fact that an earthquake generates different kinds of energy that radiate out from the epicenter at different speeds. Here's how I described it in a June 2007 article when the early warning system was just about to come online:

Nondestructive P waves (P for primary) propagate out from a ground rupture at about 6 or 7 kilometers per second, while the much stronger S (secondary) waves that cause most of the damage travel only at about 3 or 4 km/s. The farther you are from the epicenter, then, the greater the difference between the P and S waves' arrivals, and the longer it takes the strong shaking to reach you. The seismic station closest to the epicenter picks up the P waves first and relays a warning to a central data center, which then quickly estimates the likely epicenter and magnitude. In a matter of seconds, the data center issues an alert to locations nationwide.

According to Masumi Yamada, an assistant professor in the Earthquake Hazards Division at Kyoto University, the 11 March earthquake began at 2:46 p.m. about 150 kilometers off the coast of Miyagi prefecture, and 31 seconds later, residents of the Tohoku region received the first warning via cellphone, TV, and radio. That gave people a lead time of between 10 to 30 seconds before they felt the first tremors [see map].

However, Yamada reports, "The EEW system did not work for aftershocks during the first 3 hours after the mainshock, since many earthquakes were occurring simultaneously, causing problems for the system." Indeed, the 11 March quake has been notable for the number and magnitude of its aftershocks. As of noon on 15 March (local time), the Japan Meteorological Agency had recorded 3 aftershocks above magnitude 7.0 and 45 greater than 6.0.

Click on image for a larger view.

And an earthquake alert alone won't prevent a poorly designed house or bridge from collapsing. Nor can it protect those closest to the epicenter, because the S wave will arrive before an alert can even be issued.

Much of the damage since Friday didn't come directly from the earthquake but rather from the accompanying tsunami, which swept away entire villages along the northeastern coast of Honshu, Japan's main island. The tsunami also flooded backup diesel generators at the Fukushima Dai-1 nuclear plant, thereby preventing cool-down measures. Japan has a tsunami early-warning system, but in this case, there was only about 15 minutes between the first tremors and the tsunami's arrival.

Further from the epicenter, residents of Tokyo received alerts about a minute before they felt the first tremor. Hiromichi Fujisawa, chief corporate scientist at Hitachi Central Research Laboratory (and a former member of IEEE Spectrum's editorial advisory board), said even a lead time of 10 seconds can be helpful. It's enough to allow people "to protect ourselves physically and psychologically," Fujisawa says.

Japan's earthquake early-warning system has inspired a number of other countries to consider similar measures. Two years ago, the U.S. Geological Survey received US $29.4 million in stimulus funding to upgrade the country's seismic sensors. Part of that money is now being used to develop and deploy a prototype early-warning network in California, which will issue test alerts to emergency response teams, utilities, and transportation agencies.

Even with the heads up on 11 March, Hitachi's Fujisawa says, the quake felt surprisingly intense. In fact, he and others immediately assumed that it was the long-anticipated Tokai earthquake that seismologists have warned could devastate the greater Tokyo area. Of course, it wasn't the Big One for Tokyo—the epicenter was some 300 kilometers to the north and east - but it was still big enough to rile people in the capital, despite the fact that they experience tremors on a regular basis.

"Usually the trembling decays in, say, 2 or 3 minutes," Fujisawa reports. "But it was not like that at all. A big amplitude [tremor] was followed by a series of bigger amplitude. It continued for longer than five minutes It was really scary."

— Additional reporting by Eliza Strickland

This is part of our ongoing coverage of Japan's earthquake and nuclear emergency.

Photo:  Nathan Bailey

MAP: Masumi Yamada

Another Large Quake Rattles Tokyo

Special Report: Fukushima and the Future of Nuclear Power

Editor's Note: This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency.

A 6.0-magnitude earthquake struck Shizuoka Prefecture southwest of Tokyo around 10:30 Tuesday night, Japan local time. The quake rocked buildings for about 10 seconds in the Tokyo area. Shinkansen bullet trains in the Shizuoka area have stopped running.

According to NHK, Japan's national broadcaster, Chubu Electric Company is reporting that its Hamaoka Nuclear Power Plant has not been unduly affected and the plant did not go into emergency shutdown. However, the power company is reporting that many of its electric utility poles have been knocked over.

The Japan Meteorological Agency said that while there may be slight increases in sea levels, there is no risk of a tsunami or significant tidal damage.

There have been hundreds of aftershocks following the devastating 9.0-magnitude earthquake last Friday. The Meteorological Agency notes that there have been 45 aftershocks with a magnitude of 6.0 or greater.

Image: Japan Meteorological Agency

John Boyd is an IEEE Spectrum contributor reporting from Kawasaki, Japan. This is part of IEEE Spectrum's ongoing coverage of Japan's earthquake and nuclear emergency.

Latest Blasts at Japan's Crippled Nuclear Plant Raise Fears and Radiation Levels


Special Report: Fukushima and the Future of Nuclear Power

Editor's Note: This is part of our ongoing news coverage of Japan's earthquake and nuclear emergency. John Boyd is an IEEE Spectrum contributor reporting from Kawasaki, Japan.

The news from Japan only grows worse day by day. At 6:10 Tuesday morning, local time, a blast was heard inside the No. 2 reactor building of the stricken Fukushima Dai-1 nuclear power plant in earthquake-ravaged northeast Japan. Then at 9:38 a blast was heard and a fire broke out in the No. 4 reactor building, Tokyo Electric Power Company (TEPCO) officials later reported. As a result of these two incidents, radiation levels near the plant have risen dramatically.

In a televised live address to the nation, Prime Minister Naoto Kan warned that "substantial amounts" of radiation were leaking from the damaged Dai-1 plant and said TEPCO workers were making every effort "to prevent further explosions." He asked the Japanese people to remain calm.

The government had already instructed people living within 20 kilometers of the plant to evacuate. On Tuesday Chief Cabinet Secretary Yukio Edano told reporters that because dangerous levels of radiation had been detected close to the reactor buildings, people living between 20 kilometers and 30 kilometers from the plant "should stay indoors and shut all the windows." He called this a precautionary measure, and added that laundry left outside to dry should not be brought indoors and air conditioners used for heating should be turned off.

Radiation readings taken near the reactors at 10:22 on Tuesday morning showed that emissions ranged from 30 millisieverts per hour to 400 millisieverts per hour. These figures are far higher than the microsievert numbers that have been reported over the past several days when steam was vented from the endangered reactors to reduce pressure. For comparison's sake, the BBC reports that the average nuclear employee's radiation limit is 20 millisieverts per year. Workers not directly involved in emergency operations have been evacuated due to the level of danger. But Edano stressed that radiation levels decreased the farther away one was from the plant.

The blast heard in the No. 2 reactor building occurred near the unit's suppression pool, which helps maintain the proper level of pressure in the reactor, and also supplies water to the emergency core cooling system. Edano added that TEPCO is continuing to inject seawater into the system.

According to TEPCO's explanation, the valve to vent steam and reduce pressure inside the reactor had closed. The closed valve caused high pressure that prevented workers from injecting more seawater to bring temperatures down. Residual heat in the reactor boiled away some of the water that was present and caused water levels in the reactor to drop. According to Japan broadcaster NHK, the fuel rods were fully exposed for six and a half hours beginning on Monday evening. Engineers were trying to open the valve when the blast occurred. TEPCO said "it could not deny the possibility that the fuel rods were melting."

If the fuel rods melt, some of the uranium and plutonium pellets may sink down and collect at the bottom of the reactor. The fear is that loose, radioactive fuel could begin to eat through the containment vessel. Experts also worry that the blast in the No. 2 reactor building could have damaged the containment vessel, but reports on that possibility have been mixed.

On Tuesday the company said it was again trying to fill the reactor vessel with seawater to cover the fuel rods, but according to the company some portion of the rods are still exposed.

A fire, meanwhile, broke out in the No. 4 reactor building. That reactor had been shut down for routine maintenance when Friday's earthquake occurred. Edano said TEPCO suspects that spent fuel stored on the reactor building's fourth floor reheated and caused a hydrogen blast that started the fire. After the fire was extinguished engineers found that the temperature of the cooling pool holding the spent fuel had risen to 84 degrees Celsius, twice the usual temperature.

UPDATE: In a news conference on Tuesday afternoon, local time, Edano told reporters that the No. 1 and No. 3 reactors in the Dai-1 Fukushima nuclear plant were now getting stable supplies of water, and that water was being injected into the No. 2 reactor. He confirmed that the fire in the No. 4 reactor building had been extinguished. He also said that radiation monitoring near the reactors, which had shown radiation levels reaching as high as 400 millisieverts per hour on Tuesday morning, were now down to just below 600 microsieverts per hour. Japanese government officials have also stated that the prevailing winds over the last few days have pushed radiation out to sea, rather than over populated areas. Nevertheless, the situation remains in flux, and residents' fears are far from assuaged.

Image: DigitalGlobe/Getty Images

The Spent Fuel Danger in Japan's Nuclear Disaster

Special Report: Fukushima and the Future of Nuclear Power

UPDATE 3/16: For the latest news, read Timeline: The Japanese Nuclear Emergency.

Editor's Note: Post updated 17 March 2011. This is part of our ongoing news coverage of Japan's earthquake and nuclear emergency

With news that a third reactor at Japan's Fukushima Daiichi nuclear power plant exploded early Tuesday morning--the aftermath of Friday's quake and tsunami--experts continue to debate the possible outcomes of a core meltdown and leak of radioactive materials from any of the three reactors. In a larger discussion regarding the disaster among representatives from the watchdog group, the Union of Concerned Scientists, experts questioned whether spent fuel, stored on-site in the Japanese plants, might also pose a threat. Though hard to determine the danger in Japan as the disaster is ongoing, the discussion pointed to vulnerabilities in that country's and the United States' plants.

In Mark-I reactors, such as the Fukushima Daiichi plant's, spent fuel is located in cooling pools in the reactor's "attic," explains David Lochbaum, director of the Union of Concerned Scientists' nuclear safety program. For exploding units, which have blown off their roofs and walls, the problem is clear:  "If there's any radiation released, it's got to get to the environment." He compares that scenario with radiation released from fuel in the core, protected by the containment building which "would reduce the amount of radioactivity that reaches the public."

Chaim Braun, a consulting professor at Stanford's Center for International Security and Cooperation, notes that though the spent fuel pools are in fact exposed in the tops of the reactor buildings which no longer have roofs, the key is the water level in these pools. The pools must continuously have water pumped into them and If the water level goes down, he says, "the gamma radiation from the fission products contained in the fuel is not attenuated by the water cover and might escape into the atmosphere." He notes that these pools usually need about five meters of water level above the spent fuel to block the radiation from reaching the atmosphere.

"If the water level in the pool recedes down to the point where the spent fuel assemblies are partially exposed," Braun adds, "then partial melting of the spent fuel might occur thus releasing some of the fission products from the spent fuel into the external environment."

Because the spent fuel's radiation heats the water surrounding it, constant circulation of that water is also required. Tokyo Electric Power Company continues to monitor the temperatures in these spent fuel pools, especially given Tuesday's and Wednesday's fires in the reactor No. 4 building. The International Atomic Agency has published these rising temperatures in the pools of reactors 4, 5, and 6 (see update from 16 March 22:00 UTC). 

Fortunately, the total amount of spent fuel on-site in the Japanese plants is relatively small explains, Edwin Lyman, a physicist in the UCS Global Security Program. Before Friday's disasters, Japan had routinely transported some of the spent fuel to other parts of the country and other countries entirely, such as the United Kingdom and France, for processing. "The good news, if there is any, is that, from the numbers I've seen, the inventory of spent fuel in these pools was relatively small and they were well below capacity," Lyman says.

Describing hypotheticals, Lochbaum explains that if there was widespread release of spent fuel [radiation], the casualties for a worst case scenario might be similar to the worst case scenario for leakage from the core. The difference, he explains, is that the core would release a lot of short-lived radioisotopes, causing more fatalities in the first year, while a spent fuel accident might mean that primary casualties would come from cancer "down the road."

Ellen Vancko the UCS Nuclear Energy and Climate Change Project Manager expects that an early influence of this tragedy on the United States' nuclear policy will be for officials to take a harder look at operating U.S. plants. "It would be naive to think that the events in Japan will have no impact on the nuclear industry in this country," she says.

Lochbaum notes that the United States, which has 23 plants of the Mark-1 design, hasn't been nearly as fastidious with its spent fuel supply as Japan. He points out that the UCS has advocated for programs that would force plant operators to transfer spent fuel from pools inside the plants into dry casts stored on-site. He also believes other designs such as Mark-3 reactors, which store their spent fuel pools at ground level, might be less vulnerable. "If something were to happen and one of our pools released radioactive contents into the air, there'd be a lot of people in harm's way," Lochbaum says, "and it [would be] shame on us for allowing that to happen."

Images: Wikimedia / Theanphibian / Nuclear Energy Institute; Top: TEPCO/Nuclear Energy Institute

Why the Japan Earthquake Didn't Take Down the Country's Internet

The news from Japan is still pretty grim. Four days after a 9.0-magnitude earthquake shook the country, engineers are trying to prevent further explosions at a damaged nuclear power plant, and Japan’s largest electric utility has introduced rolling blackouts. But here's a piece of good news for one of the most wired societies on the planet: For the most part, the Internet is working.

The fiber optic network of undersea cables that connect Japan to the rest of the world was damaged when the earthquake struck beneath the Pacific seafloor, about 200 kilometers from Japan's northeast coast. The Wall Street Journal reports that many telecom operators have battled service disruptions, and anecdotal reports from Japan residents (including IEEE Spectrum commenters) suggest that some people have experienced slow Internet speeds, especially when accessing international sites.

But the situation could have been far worse. TeleGeography, a company that keeps tabs on Internet traffic around the world, told IEEE Spectrum that the undersea cable network experienced "limited" damage due to the earthquake. While more than a dozen undersea cable networks land in Japan, most of the landing stations are in areas that weren't too damaged by the quake. Companies whose cables were impacted have mostly been able to reroute traffic through intact cable lines to avoid major service problems.

From TeleGeography:

Most of Japan's cable landing stations are well to the South of Tokyo, or on the other side of the sheltered inlet that becomes Tokyo Bay.  We're not aware of disruptions to any of the many cables that land here.  All of the cable systems that have reported outages also operate cables that land to the South of Tokyo, so no system appears to have suffered a complete outage....

All of the outages appear to be on cable segments that land in the Ajigaura or Kitaibaraki landing stations, approximately halfway between Tokyo and Sendai.

Some of the damage reports are already in. According to TeleGeography:

* The Hong Kong-based cable-network operator PacNet has reported damage to two segments of its East Asia Crossing undersea cable, which connects Japan to other parts of Asia.

* Japan's NTT Communications Corporation has reported damage to some segments of its PC-1 submarine cable system, which connects Japan and the United States.

* Korea Telecom has also reported that a segment of the Japan-US Cable Network is damaged.

* Chunghwa of Taiwan has reported damage to segments of the Asia Pacific Cable Network 2.

UPDATE: The monitoring company Keynote Systems has more info on how telecoms have coped with the cable problems. Over the weekend, Keynote told us that they'd detected few large-scale problems with internet service. Now the company has provided more details of the types of glitches that have occurred since the earthquake, and the steps telecoms have taken to deal with them. From a Keynote statement:

We captured some peering issues (delays for traffic transiting from one major carrier to another) on Saturday night, 9 pm Pacific. In the graphic below we can see that traffic from Sprint to NTT had 50% packet loss and latency of almost half a second:

The bottom chart shows a 4-hour span today. The lack of troublesome red numbers suggests that telecoms have found short-term fixes to their problems.

Keynote also passed along messages from Japan's NTT Communications Corporation, the country's primary Internet backbone provider. NTT announced today that it will send out submarine cable repair ships within the next 24 to 48 hours to work on busted cables just offshore from the landing station. NTT also warned of the potential for more problems:

It is possible that we may experience an increase in latency and packet loss during periods of peak utilization, specifically 12:00 to 15:00 UTC. Our engineers and operations staff continue to work towards restoring additional capacity on our cable systems and return them to full functionality.

Images: TeleGeography; Keynote Systems


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