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TEPCO's analysis of disaster lacks credibility

July 13, 2012

TEPCO's nuclear disaster analysis unreliable: panel



A government panel investigating the ongoing nuclear crisis at a Fukushima nuclear power plant has concluded in its final report, slated for release later this month, that plant operator Tokyo Electric Power Co. (TEPCO)'s analysis of the disaster lacks credibility, the Mainichi Shimbun has learned.

The Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company reconstructed what was happening at the stricken reactors at Fukushima No. 1 Nuclear Power Plant following the Great East Japan Earthquake and tsunami that set off the disaster there, and found that its analysis results differed from those released by the utility, sources close to the case said.

TEPCO released computer analyses on the progression of the disaster in May and November of last year. That June, the Ministry of Economy, Trade and Industry's Nuclear and Industrial Safety Agency (NISA) also released analyses run by the Japan Nuclear Energy Safety Organization (JNES).

According to TEPCO's November results, the core of the No. 1 reactor became exposed at around 6:10 p.m., approximately two-and-a-half hours after tsunami struck the plant on March 11, 2011. The analysis concluded that molten nuclear fuel melted through the reactor's pressure vessel at around 1:50 a.m. on March 12.

The government panel independently reconstructed what happened at the No. 1, 2 and 3 reactors following the onset of the crisis using available data such as reactor temperatures, reactor pressure and water levels, and radiation concentration in the containment vessels.

The committee's results showed that the pressure vessel of the No. 1 reactor suffered damage at around 11 p.m. on March 11, 2011, some two to three hours earlier than TEPCO's estimate. A similar difference was found between the panel's results and TEPCO's results for the No. 3 reactor. A lack of data prevented a comparison of the No. 2 reactor figures.

The difference may have been due to the utility and the government overestimating the amount of water pumped into the reactors -- information used in the analyses -- reflecting in the figures the mistaken understanding that the reactors were cooled more quickly than they were.

The investigation committee pointed out that TEPCO had not been thorough in confirming preconditions that would greatly affect the final results of its analyses.

One insider criticized the utility, saying, "That it didn't more closely scrutinize the data in its November analysis last year (by which information had become available) is an act of negligence."

Sources say the panel's final report, set to be released on July 23, will criticize analyses by both TEPCO and the national government as lacking in credibility.

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