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.
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