A newly revealed document shows the Nuclear Safety Commission backed a claim by utilities in a 1992 report that no safety measures were needed to deal with a long-term complete power loss at nuclear plants, illustrating they minimized risk assessments together.
A text written by Tokyo Electric Power Co. was included in the report to revise safety measures compiled by the commission's working group, and in the end no steps were introduced to counter the loss of key electricity-run reactor cooling functions.
Loss of power was a main cause of the disaster at the Fukushima No. 1 nuclear plant. Three of its six reactors suffered meltdowns after the plant lost most of its power following the magnitude 9.0 earthquake and tsunami of March 11, 2011.
At the time, the working group was discussing revising a guideline the commission had endorsed in 1990 that ruled out the need to consider a long period of power loss.
According to newly unveiled materials, the commission's secretariat, at that time part of the Science and Technology Agency, asked utilities in a document dated October 1992 to "write a composition" on why there was no need to consider measures to counter station blackouts lasting longer than 30 minutes.
Tepco responded that "sufficient safety will be secured" as long as nuclear plants are controlled properly, touching on the reliability of emergency power sources at that time.
Kansai Electric Power Co. also said the risks of power loss are extremely low and measures to deal with such risks need not be included in the guideline.
The commission adopted Tepco's claim in its guideline and no safety measures for a long power outage were included.
Haruki Madarame, who chaired the commission, apologized over the issue, saying it was inappropriate to have power companies play a part in creating the original guideline draft.