Stable operation of the crippled Fukushima No. 1 Nuclear Power Plant’s system to purify contaminated water will determine whether or not the current crisis can be brought under control and is one of the conditions for lifting emergency evacuation preparatory zones some 20 to 30 kilometers from the plant.
“If temperatures in nuclear reactors drop below 100 degrees Celsius to reach a state of cold shutdown, vapor generation including radioactive substances will be restrained, and we can expect a further decline in the dosage rate in the air around the plant,” said Tadashi Narabayashi, a professor of reactor engineering at Hokkaido University.
Tokyo Electric Power Co. (TEPCO) estimates that radiation doses at the plant’s boundary would reach a maximum 1.7 millisieverts on the premise that the entire amount of radiation monitored from June 20-28 had been released from the plant’s nuclear reactor buildings.
TEPCO and the government aim to lower the dose of radiation to less than one millisievert by early next year following the completion of step 2 (three to six months from mid-July) under a road map to bring the crisis under control.
As of Aug. 1, temperatures at the bottom of the pressure vessel stood at 93 Celsius at the No. 1 reactor, 121 Celsius at the No. 2 reactor and 106 Celsius at the No. 3 reactor. In order to reduce vapor from the reactor cores, the amount of water pumped into the No. 2 and No. 3 nuclear reactors has to be increased to bring temperatures there to less than 100 Celsius.
But TEPCO is having difficulty in stably operating the plant’s cleanup system. And contaminated water is rising, forcing the utility to restrict water injections to stop contaminated water from increasing further.
“To ensure stable operations of the plant’s cleanup system, it is essential to pump water into the reactors without worrying about an increase in contaminated water. If the temperatures are reduced to less than 100 Celsius, the radiation dose along the plant border may be cut to the target of one millisievert a year,” said a senior official of the Nuclear and Industrial Safety Agency.
Haruki Madarame, chairman of the Nuclear Safety Commission of Japan, said at a news conference on Aug. 7 that stable operations of the water purification system are the biggest challenge. He said the amount of radioactive materials spewing from the nuclear reactors will help determine if the emergency evacuation preparatory zones will be lifted.
The government will enter into talks as early as later this month with local governments concerned on removing the emergency evacuation zones.
Narabayashi said, “A cold shutdown should be realized as soon as possible by giving priority to Fukushima evacuees. Multiple systems should be studied instead of the current single route to prevent the cleanup system’s operating rate from falling.”
TEPCO on Aug. 3 submitted to the Nuclear and Industrial Safety Agency a plan to install a cesium absorption unit called SARRY made by Toshiba Corp. and other parties to back up the current cleanup equipment.
TEPCO will launch this double track program as early as mid-August.