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Tsunami alone not primary cause

Fukushima nuke disaster investigative panel rejects TEPCO tsunami claims



The final report released by the Diet's Fukushima nuclear disaster investigative panel has concluded that factors other than the tsunami may have triggered the loss of power at the plant, which aggravated the unprecedented disaster.

In its 641-page report released on July 5, the panel said there is no denying that the Fukushima No. 1 nuclear plant's No. 1 reactor was damaged by the earthquake that struck northeastern Japan on March 11, 2011.

"The primary cause of the disaster should not be attributed to the tsunami alone," the final report said. The report also stated that there is a possibility that the loss of backup power at the plant "may not have been triggered by the tsunami," rejecting the views previously presented by plant operator Tokyo Electric Power Co. (TEPCO) and the government's disaster investigation committee. Because experts' opinions are divided over the issue, calls may arise for further verification.

The panel analyzed the tsunamis' arrival time at the Fukushima No. 1 plant and concluded that the second tsunami wave reached the backup power at the plant at least two minutes later than the tsunami arrival time claimed by TEPCO. The utility has earlier reported that the second wave reached the plant at 3:35 p.m. on March 11, 2011, but the panel said the actual arrival time was observed from 1.5 kilometers off the coast.

As one of the two emergency power sources for the No. 1 reactor is believed to have already lost function between 3:35 p.m. and 3:36 p.m. -- an assumption based on interviews with the plant's workers -- the final report concluded that backup power was lost before the second tsunami hit. There are also doubts about whether the damage to the No. 2 and No. 3 reactors is attributable to the tsunami, the report said, calling for further investigation.

While determining that there was no major quake-induced pipe ruptures in the plant's other important safety equipment, the report said the possibility of coolant water having leaked from minute cracks in the No. 1 reactor cannot be ruled out. The report, meanwhile, did not delve into the operations of the nuclear plant, which was elaborated earlier in the report released by the government's disaster investigation committee.

Takashi Sawada, director at the Atomic Energy Society of Japan, was dismissive of the investigative panel's report.

"From an engineering point of view, the report's judgment is insufficient. If pipes were even only slightly damaged, allowing coolant water to leak, the temperatures and pressure inside the reactor containment vessel would be abnormally high. However, the measurement data released by TEPCO does not indicate anything like this between the time the quake occurred and the tsunami arrived. Further verification is necessary," he said.

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