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.
Here's the story of Japan's nuclear emergency from the first jolt to what was known midday Sunday:
After Japan was struck by its biggest-ever recorded earthquake on 11 March, and then was hit by a devastating tsunami an hour later, the country now faces a third potential catastrophe in the form of a nuclear meltdown.
The epicenter of the massive earthquake (whose magnitude has been upgraded to 9.0, from the previous 8.8) was some 130 kilometers off the coast of Miyagi Prefecture in northeastern Japan. The earthquake occurred at 2:46 Friday afternoon and caused damage and disruption along much of the coast of eastern Honshu, the country’s largest island. Tokyo, located some 300 km south of the worst-hit areas, also felt the impact: all train and plane services were halted, stranding millions of commuters. The temblor was soon followed by a tsunami with waves as high as 10 meters that swamped coastal areas in northeastern Japan, where most of the 1000 deaths initially reported occurred. A Kyodo News report on Saturday said 1800 were feared dead or missing, while another report said all contact had been lost with some 10,000 people in the town of Minami Sanriku in Miyagi Prefecture.
As if this double whammy weren’t enough to deal with, the country is now facing a third calamity from two malfunctioning nuclear power plants.
Despite being an earthquake-prone country, Japan has 55 nuclear reactors in operation. Nuclear plants generate some 30 percent of Japan’s electricity, and with a significant number of reactors automatically shut down, the government is warning that power cuts are likely. Four nuclear plants are located along the coast near to the earthquake’s epicenter.
Yukio Edano, the government’s chief cabinet secretary, urged everyone to help by conserving electricity.
With a number of nuclear plants located in the northeast, Prime Minister Naoto Kan was quick to establish an emergency group and issued a statement at 5:00 p.m. Friday saying, “We have no reports of any radioactive materials … affecting the surrounding areas.”
But the situation changed almost immediately after that announcement when reactors in the hardest-hit areas began shutting down automatically and when Tokyo Electric Power Company (TEPCO) reported that pressure inside the No. 1 boiling water reactor (rated at 460 megawatts) in its Fukushima Dai-1 plant (which houses six reactors) had risen when the main power and then the diesel engines powering its emergency cooling system failed. According to Japan’s Nuclear and Industrial Safety Agency, radiation 1000 times normal levels was detected inside the TEPCO Dai-1 plant’s control room, and levels 8 times normal were found outside the plant.
The government issued a state of emergency at 7:03 Friday evening, and thousands of people living within a 3-kilometer radius of the plant were evacuated. The evacuation area was later increased to 10 km and then 20 km, after reports that radiation had been leaked into the atmosphere.
The news arm of the World Nuclear Association, citing TEPCO as its source, said pressure within the reactor had reached 840 kilopascals, about twice the normal level. To reduce this pressure, TEPCO has been releasing filtered reactor steam into the atmosphere, resulting in some radiation leakage.
TEPCO later notified the government that the water-cooling system in its Fukushima Dai-2 plant, located 12 km north of the Dai-1 plant, had failed, and all four of its reactors had been shut down automatically. The company added it was bringing in mobile electric generators to restore power to the system. Residents living within 10 km were evacuated.
Both these plants house boiling water reactors to generate electricity. They use water (free of minerals) as a coolant and produce heat through nuclear fission. The heat boils the water to produce steam, which is used to drive the turbines for generating the electricity. The steam is then condensed into water and returned back to the reactor to continue the process. Even when the reactors are shut down, the cores continue to generate heat for a number of days, requiring the continual pumping of water through the system to cool them.
At 3:36 on 12 March, an explosion occurred in a reactor of the Fukushima Dai-1 plant, blowing off the roof and the upper part of the outer walls of the structure containing the reactor. Edano told reporters that the water level in the reactor dropped when the pumping system failed, causing steam to be generated. The steam filled the space outside the reactor and the inner walls of the outer structure. This in turn generated hydrogen that mixed with the steam and caused the explosion.
“Because there was no oxygen in the container, there was no explosion in the container, so there was no damage,” said Edano. “And there was no great amount of radiation leaked.”
Earlier, he said TEPCO had begun venting steam from the container to reduce the growing pressure inside. As a result, the density of radiation around the structure rose to the level of 1015 microsieverts. “After the explosion, the pressure inside the container fell and remains at a very low level,” says Edano. At the same time, the density of radiation also fell to 860 microsieverts, and by 6:48 p.m. it had fallen further to 70.5 microsieverts.
TEPCO has now decided to flood the container with seawater to bring down the temperature. Boric acid will also be added to prevent nuclear criticality, a self-sustaining nuclear chain reaction. Experts consider this operation a dire measure that will render the reactor unusable and result in its decommission. After the government consulted with other experts and TEPCO, it gave its consent to what is “an unprecedented step,” says Edano. There is speculation, given the need to use seawater, that the water pipes feeding the system must have been damaged by the earthquake.
Several nuclear experts speaking on national television said that if the light-water reactors were to go into meltdown, they wouldn’t explode or affect people outside the 10-km safety zones that have been set up around both plants. (The evacuation zone around Dai-1 has since been increased to 20 km.) Professor Masaki Saito, of the Department of Nuclear Engineering at the Tokyo Institute of Technology, said there was no possibility of a disaster like the one that happened in 1986 at the Chernobyl nuclear power plant in Ukraine.
In the latest development of a fast-changing situation, NHK, Japan’s national broadcasting organization, reported midday Sunday that the cooling system controlling Fukushima Dai-1 plant’s No. 3 boiling water reactor (rated at 784 megawatts) has failed. TEPCO said the pressure had passed 800 kilopascals, forcing the company to vent filtered steam into the atmosphere, mirroring the emergency measures taken yesterday when the same situation arose with the No. 1 reactor.