In a worst-case scenario, the central government would have requested the evacuation of Tokyo and everyone within a 250-kilometer radius of the damaged Fukushima No. 1 nuclear power plant.
最悪のシナリオにおける避難範囲
In a worst-case scenario, the central government would have requested the evacuation of Tokyo and everyone within a 250-kilometer radius of the damaged Fukushima No. 1 nuclear power plant.
Goshi Hosono, minister in charge of the nuclear disaster, on Jan. 6 unveiled the emergency plan, which was personally drawn up two weeks after the Great East Japan Earthquake by Shunsuke Kondo, chairman of the Japan Atomic Energy Commission.
The plan would have ordered mandatory evacuations of everyone within a 170-km radius of the plant. Evacuations would have been voluntary for those living between 170 km and 250 km from the plant, including the Japanese capital.
After the March 11 earthquake, Hosono, then special adviser to Prime Minister Naoto Kan, asked Kondo to produce reports on possible scenarios at Kan's behest.
Hosono said the government had not disclosed the report, which had been submitted on March 25, out of concern over public reaction.
"We had refrained from publicizing it because we were afraid that it could cause the public to grow excessively worried," he said.
He also said the government took into consideration the possibility that the No. 1 reactor at the plant would explode, even though chances were slim at that time.
The worst-case scenario imagined the melting of 1,535 fuel assemblies, an equivalent of fuel used for two reactors, kept in a spent fuel storage pool at the No. 4 reactor.
Although the No. 4 reactor had been shut down for maintenance before March 11, the roof of the building housing the reactor was blown off by a hydrogen explosion on March 15, leaving the spent fuel storage pool exposed to the atmosphere.
The building housing the No. 1 reactor had already been destroyed by a hydrogen explosion on March 12, after a blackout and failure of emergency power sources to cool the reactors.
The scenario envisioned that if another hydrogen explosion took place at the No. 1 reactor, workers would be forced to flee and suspend recovery operations, resulting in an enormous amount of radioactive material released from the pool in two weeks.
The report also predicted the extent of soil contamination of areas required to evacuate in light of standards set after the 1986 Chernobyl accident in Ukraine.
The scenario said areas within a 170-km radius of the plant would have been contaminated with 1,480 kilobecquerels per square meter, a level that requires mandatory evacuation.
Areas where the government would have requested voluntary evacuations were predicted to have 555 kilobecquerels per square meter, extending to a 250-km radius, which included Tokyo and surrounding areas.
If the release of cesium was limited to an equivalent of one reactor, the mandatory evacuation zone would have been a radius of 110 km and recommended evacuation a radius of 200 km.
The report said it would have taken several decades for radiation levels to decrease naturally in the mandatory and voluntary evacuation zones.
The report also said high radiation levels could have extended beyond the 250-km radius, and people in those areas would have also been advised to relocate.
Blocking radiation by covering the damaged reactors and the pool with 1,100 tons of materials mixed with sand and water each was considered a last resort to contain the crisis, according to the report.