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

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