Five loose bolts discovered on the bottom of a storage tank are believed to be the cause of the leak of 300 tons of radioactive water at the Fukushima No. 1 nuclear power plant, Tokyo Electric Power Co. said Sept. 20.
TEPCO is currently disassembling the leaky storage tank to inspect its interior. The plant operator said the loose bolts were from bottom panels near the eastern edge of the storage tank.
The bolts are designed to be fastened to set an impermeable rubber seal that blocks an opening along a juncture of steel panels. A loose bolt can leave such an opening unblocked.
"The leak likely took place there, but we will study the matter further," said Masayuki Ono, acting general manager of TEPCO's Nuclear Power and Plant Siting Division.
TEPCO said sealing sections to block openings along junctions were found bulging in eight areas. Packing sections below the sealants were also protruding in several areas. The utility said it will study if those problems arose because the tank was disassembled and relocated before the radioactive water leak occurred.
TEPCO also said it monitored radiation levels in earth samples taken from a hole drilled beside the tank and measured a maximum beta-ray level of 1.7 millisieverts per hour at a depth of 30 centimeters below the ground surface.
Radioactive water may have penetrated the soil through cracks formed in the concrete-covered ground surface. TEPCO said it will investigate once the tank dismantlement is completed.
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