The report states that there is only a slim possibility of any serious situation such as another hydrogen explosion and reactor core meltdown even if a worst-case scenario is assumed. It then says that radiation levels are highly unlikely to increase outside a no-entry zone within a radius of 20 kilometers from the plant even if the spent nuclear fuel pool cooling system stops functioning for a long time.
The report shows that the plant now meets one of the important preconditions for lifting the designation of emergency evacuation preparation zones in some areas 20-30 kilometers away from the crippled plant.
However, NSC, which is under the umbrella of the Cabinet Office, has pointed out that the areas must be decontaminated and kept under surveillance as other key requirements for lifting the designation. It then urged the government to proceed with such work while consulting with local governments concerned.
NISA, which is part of the Ministry of Economy, Trade and Industry, compiled the report based on a report that the plant operator Tokyo Electric Power Co. (TEPCO) filed with it on Aug. 3, which declares that the injection of water into the crippled reactors is stable.
NISA’s report states another hydrogen explosion is highly unlikely, noting that nitrogen is being injected into the power station’s No. 1 to 3 reactors in order to prevent such an explosion and that newly generated hydrogen is released into the air with steam.
Even if a hydrogen explosion takes place, it will have little effect on radiation doses in areas 20 kilometers away from the No. 1 reactor.
Moreover, it describes TEPCO’s readiness to reactivate the water injection function within 30 minutes to three hours even if it accidentally stops as “an appropriate emergency response.”
As water injection into the reactors was suspended for up to about 14 hours shortly after the accident in March, NISA analyzed what would happen if water injection stopped for 15 hours.
On the assumption that most of the fuel has already melted down, the report says the annual radiation dose in an area 20 kilometers from the plant is estimated at 0.65 millisieverts.
NSC, which closely reviewed the report, has concluded that there is little chance that the situation with the plant will worsen to a level that would require further evacuation orders.
Questions remain about NSC’s approval of the NISA report, which concludes that there is only a slim possibility of another hydrogen explosion and meltdown, as the crisis at the nuclear power station was triggered by factors beyond the scope of experts’ assumptions.
When asked about his interpretation of what the report describes as “a worst-case scenario,” NSC Chairman Haruki Madarame said, “There’s no limit if you start assuming serious situations. The commission approved the report based on commonsensical standards.”
It also remains unclear how NSC decided to endorse the report. NISA instructed TEPCO on Aug. 2 to file an additional report by the following day about measures that the utility will take if the injection system, which is a key to the stabilization of the crippled reactors, develops trouble.
TEPCO kept the deadline, and NISA filed its report on Aug. 4 with NSC, which approved it later in the day.
Sources involved in the case have revealed that the relevant parties consulted with each other over the report for about a month behind closed doors, but neither NSC nor NISA has disclosed the details of what they discussed.
“What we discussed was highly technical, and we thought the public would hardly understand it if we disclosed the details,” says Vice Chairman Yutaka Kukita, former professor at Nagoya University’s graduate school.
However, the situation in which important matters that could affect citizens’ lives are discussed behind closed door should be rectified at an early date. (By Ei Okada, Tokyo Science and Environment News Department)