The leakage of about 300 tons of highly radioactive water from a surface tank at the crippled Fukushima No. 1 nuclear power plant may have gone unnoticed for more than a month before it was discovered on Aug. 19, according to a spike in workers' beta-ray exposure levels.
A representative of Tokyo Electric Power Co., the plant operator, told a meeting of a working subcommittee of the Nuclear Regulation Authority on Aug. 27 that the leak had likely already begun by July. He said that inference is based on a study of beta-ray doses in workers who each spent about 2.5 hours a day at a radio relay station, some 20 meters from the storage tank where the leak occurred.
He said the worker dosage readings began to rise some time around mid-July, adding that TEPCO had yet to investigate the data taken before July.
TEPCO officials said they believe the leak initially involved only tiny amounts of contaminated water but gradually gained strength. They said rainfall of up to 30 millimeters was recorded on some days in mid-July and later, and the rainwater likely spread the radioactive water and helped it seep into the ground.
The NRA on Aug. 27 called on TEPCO not only to investigate the cause of the leak, but also dig wells in an area surrounding the leak site to gauge the spread of radioactive substances. TEPCO has admitted that the leaking water may have reached the ocean.
The leaking tank, which utilizes steel sheets connected by bolts, entered service in October 2011. It used to hold highly radioactive water, one liter of which contained 136,000 becquerels of radioactive cesium in addition to 200 million becquerels of beta-ray source materials, which include radioactive strontium. Of the 1,000 surface tanks in use at the Fukushima plant, about 350 are of a similar design.
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