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Saturday, 4 October 2014

TEPCO doubles tsunami height in damage estimate for Fukushima plant

October 04, 2014
By TSUYOSHI NAGANO/ Staff Writer
A tsunami of 26 meters would inundate the already-stricken Fukushima No. 1 power plant, causing a huge amount of radioactive substances to spill into the sea, according to Tokyo Electric Power Co.’s updated estimates.
The crippled plant’s operator told a Nuclear Regulation Authority commission Oct. 3 that it raised its projected tsunami height to 26.3 meters--nearly double its previous estimate--and increased the scale of the largest potential earthquake by 1.5 times.
According to the new estimate, if such a towering tsunami struck the facility, hundreds of trillions of becquerels of cesium-137 could be released into the ocean from the basements of reactor buildings.
Currently, nuclear plant operators across Japan are making efforts to receive permission to resume operations of reactors that have remained or were put offline after the nuclear crisis started in March 2011.
Other utilities have been raising the scale of the highest possible tsunami for individual plants to meet stricter safety standards introduced following the Fukushima disaster. But TEPCO had not revised its height estimate for tsunami because the new safety standards do not apply to the Fukushima No. 1 plant reactors, which are scheduled to be decommissioned.
However, due to the unique risk factors at the plant, including destroyed reactor buildings and accumulating contaminated water, the NRA demanded that TEPCO review its tsunami forecast and countermeasures.
TEPCO’s latest estimate assumed a scenario in which a tsunami of 26.3 meters triggered by an earthquake of 900 gals struck the coast at the north end of the facility. A gal is a unit of gravitational acceleration. TEPCO’s previous report assumed a strongest possible quake of 600 gals.
Some of the plant’s reactors are at the southern part of the plant, where the elevation is 10 meters above sea level. Reactors in the north lie on land 13 meters above sea level.
Tsunami of 15.5 meters struck the southern coastal area following the magnitude-9.0 earthquake in 2011.
Projecting a succession of smaller tsunami at a maximum height of 14 meters may inundate the nuclear power plant, TEPCO erected 14-meter-tall temporary levees at some areas in the south following the disaster.
However, the latest estimate shows that the highest potential tsunami exceeds the height of the land and levees. Such a deluge would thus swamp the reactor buildings, where highly contaminated water has accumulated, causing the release of radioactive substances.
In the new forecast, TEPCO said storage tanks for radioactive water would not be affected by such a tsunami because they are situated on higher ground. It also said damaged reactor buildings could withstand the potential strongest quake of 900 gals.
To minimize the impact of the estimated 26.3-meter tsunami, TEPCO said it will reduce the vast quantity of radioactive water accumulating on site instead of raising the height of the levees to block tsunami.
According to TEPCO officials, the amount of tainted water estimated to spill into the ocean could be reduced to 30 percent by filling in trenches near reactors, where a large quantity has accumulated.
The NRA is not expected to demand TEPCO raise the height of levees, as there is no equipment around the reactor buildings that could cause critical damage in the event of inundation. However, the nuclear watchdog plans to check the appropriateness of TEPCO’s latest estimate and proposed countermeasures.
Source: Asahi Shimbun
http://ajw.asahi.com/article/0311disaster/fukushima/AJ201410040044
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