information about Fukushima published in English in Japanese media info publiée en anglais dans la presse japonaise
9 Avril 2013
April 9, 2013
The Yomiuri Shimbun
http://the-japan-news.com/news/article/0000116890
Tokyo Electric Power Co. may have underestimated by as much as 50 times the amount of radioactive substances contained in water that leaked from an underground storage pool at its crippled Fukushima No. 1 nuclear power plant, according to experts.
The power company has estimated that the leaked water contains a total of 710 billion becquerels of radioactive substances.
However, TEPCO used the concentration level of radioactive substances contained in water trapped between a three-layered waterproof sheet that lines the pool as the basis for its calculation.
If the utility used the concentration of radioactive substances contained in water inside the pool as the basis of its calculation, the estimated figure would be about 50 times larger--about 35 trillion becquerels. This suggests environmental impact of the leak could also be much bigger than TEPCO's estimate.
TEPCO announced the water leakage Friday. According to the utility, about 120 tons of contaminated water has seeped from the storage pool, the largest water leak since the government announced that the power plant achieved a state of cold shutdown in December 2011.
According to TEPCO, the concentration of radioactive substances contained in water trapped between the second and third layers of the protective waterproof sheet was about 6,000 becquerels per square centimeter.
Based on this figure, the power company estimated that about 710 billion becquerels of radioactive substances was contained in the 120 tons of leaked water.
However, if the calculation was based on the concentration level of radioactive substances contained in water inside the pool--about 290,000 becquerels--the total amount leaked would be about 35 trillion becquerels.
Prof. Hideo Yamazaki of Kinki University, who specializes in environmental analysis, said: "I cannot understand why TEPCO used a lower figure as the basis for this calculation. Such calculations should be conducted strictly from the viewpoint of ensuring safety."
A TEPCO spokesman said the water trapped between the second and third layers of the waterproof sheet were chosen because the contaminated water is believed to have leaked from there. The spokesman stressed that neither calculation method is wrong.
However, TEPCO said it is unclear why the concentration of radioactive substances in water from the waterproof sheet was lower than that of water inside the pool. If the decline of the concentration level was caused by an inflow of water from outside the pool, the total amount of water leaked through the sheet could be bigger than the initial estimate of 120 tons.
The water became contaminated when it was poured on damaged reactor cores at the nuclear plant to keep them from overheating.
Prof. Masanori Aritomi of Tokyo Institute of Technology, who specializes in nuclear reactor engineering, said: "There's a common understanding that such calculations should be based on the initial concentration of radioactive substances. I'm afraid TEPCO is underestimating the seriousness of this incident."
Construction checks 'insufficient'
The underground storage pool was exempted from the pre-operation checking required by the law on nuclear material and reactors when it was built by TEPCO late last year, according to sources.
Safety inspectors stationed at the Fukushima No. 1 nuclear power plant inspected construction work on the pool, but their inspections were nothing more than a formality. Strict checking, such as confirming whether there were any errors in the construction work, was not conducted, the sources said.
According to the Nuclear Regulation Authority's Secretariat, the now-defunct Nuclear and Industrial Safety Agency decided the storage pool would not be subject to the regulations of the Law on the Regulations of Nuclear Source Material, Nuclear Fuel Material and Reactors, as with other makeshift facilities built at the plant to speed up measures to deal with water contaminated with radioactive materials.
The agency hosted a hearing to ask the opinions of experts on the safety of the storage pool. At the hearing, TEPCO explained that leakage would be prevented by the three-layered waterproof sheet and a leakage detector, and the experts did not raise serious objections to the plan, the secretariat said.
The NRA, which took over the role of the agency, ordered safety inspectors to examine some aspects of the storage pool's construction in late last year. However, the inspectors merely confirmed the progress of the construction.
As the NRA was busy dealing with the transfer of operations from the agency, the NRA did not have time to assess the agency's evaluation of the pool's construction, the secretariat said.
However, the NRA said it believes the leakage from the storage pool has undermined the overall credibility of measures for dealing with the contaminated water. The NRA plans to discuss measures on dealing with contaminated water at regular meetings of NRA members and meetings of experts tasked with evaluating work at the Fukushima No. 1 nuclear plant.
The NRA plans to promptly draw up its plan to move contaminated water from the underground storage pool to tanks above ground, the secretariat said.
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