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.
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."
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