Prime Minister Yoshihiko Noda on Dec. 16 declared that the stricken reactors at Tokyo Electric Power Co.’s Fukushima No. 1 nuclear power plant have entered the state of “cold shutdown” and that it has been confirmed that the nuclear crisis has “been resolved”(Shusoku ni itatta.) As far as Tepco and the goverment are concerned, “Step 2” of their “road map” to bring the nuclear crisis under control has been accomplished one month earlier than originally scheduled. After the completion of Step 2, work that will eventually lead to removal of molten nuclear fuel and decommissioning of the stricken reactors is supposed to start. But the prime minister’s declaration that the crisis has been resolved will not be accepted by many people, especially those in Fukushima Prefecture.
Workers who have struggled continuously since March 11 to stabilize the stricken reactors deserve the nation’s praise. But Mr. Noda’s announcement is political grandstanding designed to give an impression that the plant’s four crippled reactors — three of which suffered meltdowns — have been completely brought under control. Apparently he wanted to make the announcement before year’s end.
Mr. Noda’s announcement could have the negative effect of turning the public’s attention away from the reactors, which would be most unwelcome as their condition should be under continuous scrutiny. Tepco and the government must continue to give priority to accurately measuring the ongoing risks and keep the public fully informed of them. The prime minister’s declaration that the nuclear crisis has “been resolved” should not be used as an excuse by the government and Tepco to shy away from that duty.
The severity level of the Fukushima nuclear crisis is the maximum 7 on the International Nuclear Event Scale — the same level designated for the 1986 Chernobyl disaster — the world’s worst nuclear accident. In the Fukushima fiasco, four reactors were damaged by the March 11 earthquake and tsunami, three suffered full meltdowns, several hydrogen explosions occurred and a tremendous amount of radioactive material has been released. In view of this, the Fukushima nuclear crisis is not only an extremely grave disaster but the first of its kind in the history of nuclear power because it involves multiple reactors. It must be remembered that it will take decades before the crisis is truly resolved.
Although Prime Minister Noda announced that the stricken reactors have entered a state of cold shutdown, it must be emphasized that Tepco and the government’s definition of “cold shutdown” is very different from the nuclear power industry’s traditional definition. The prime minister announced that “cold shutdown” had been achieved because the temperature at the bottom of the pressure vessels and inside the containment vessels have fallen below 100 C, the radiation level of radioactive substances currently released from the reactors has come down to 0.1 millisieverts per year inside the plant compound, which exceeds the goal of one millisievert per year, and the “safety” of the external system that has been set up to cool the reactors has been confirmed.
The term “cold shutdown” is traditionally used to describe a reactor in normal working order that has reached a state of sub-criticality. If the nuclear fission process is stopped in such a reactor, the temperature inside the reactor falls below 100 C and the nuclear fuel is cooled by the reactor’s own internal cooling system in a stable manner, then a reactor can be said to be in the state of cold shutdown. In this state, no amount of radioactive substances is released from a reactor. In addition, a reactor in a true state of cold shutdown can be easily restarted.
The conditions of the stricken reactors at Fukushima No. 1 are completely different from a reactor that is in a true state of cold shutdown. An external cooling system that has been cobbled together uses about 4 km of rubber hosing and was not built to earthquake safety standards so it cannot be considered to be a stable system. Furthermore, while the containment vessels of reactors in true states of cold shutdown can be opened and their fuel rods removed, the melted nuclear fuel in the Nos. 1, 2 and 3 reactors cannot be removed and must be continually kept cool by the external makeshift cooling system. Finally, the reactors could easily suffer additional damage if another strong earthquake strikes, and the pools containing spent nuclear fuel remain in a vulnerable state.
Another major problem is that Mr. Noda made his announcement despite Tepco and the government not knowing the real conditions of the reactors. It is believed that most of the No. 1 reactor’s nuclear fuel and about 60 percent of the Nos. 2 and 3 reactors’ nuclear fuel melted through the bottoms of the pressure vessels and fell to the bottom of the containment vessels, even boring into their concrete floors. In this situation, it is impossible to accurately know the temperature of fuel by the method used by Tepco and the government. More importantly, neither Tepco nor the government knows what the conditions of the damaged pressure and containment vessels are truly like.
The Fukushima No. 1 power plant is facing other problems as well. Several hundred tons of ground water are seeping into the basements of the reactor buildings on a daily basis and becoming contaminated. Tepco must prevent leakage of this contaminated water into the sea, but available tanks to store such water will be filled to capacity in a short time. Tepco and the government must do their utmost to prevent additional leakage of radioactive substances.
Tepco’s middle- and long-range scenario includes such risks as spontaneous restart of a fission process, new hydrogen explosions, corrosion of the pools containing spent nuclear fuel, leakage of contaminated water or mud, and another strong earthquake and tsunami. Clearly, the nuclear crisis remains far from resolved and Tepco and the government must continue to make their utmost efforts to bring the situation at the Fukushima plant truly under control as quickly as possible and ensure that enough workers remain at the site to cope with any dangerous developments.