Kashiwazaki Kariwa inspections continue
15 November 2007
Tokyo Electric Power Company (Tepco) continues to carefully inspect the quake-hit Kashiwazaki Kariwa nuclear power plant. Regular updates have revealed that a fuel assembly was found to be misalinged while a stuck control rod has been unstuck.
During in-core inspections of the fifth of the power plant's seven nuclear power reactors, Tepco had trouble removing one of the reactor's 764 fuel assemblies. The crane used to lift the 5m-long metal structures sounded an overload alarm and one fuel assembly was left to remain in the reactor. An underwater inspection showed no visible damage or deformation, but nevertheless it was misaligned, along with its support structure. Tepco was additionally assured there had been no damage to the nuclear fuel by the concentrations of iodine in the reactor's coolant water, which were in the normal range.
Tepco said that one possibility is that the assembly had not been loaded correctly during the most recent refuelling and had then been shaken from position by the earthquake of 16 July. The assembly is to be moved to the used nuclear fuel cooling pool for a thorough examination and investigation into the misalignment.
Another in-core anomaly found during inspections following the earthquake was a stuck control rod, discovered at unit 7 in October. This was removed in early November for disassembly and analyses of the labyrinth seal and ratch mechanisms. No abnormalities were found that could illuminate the cause of the malfunction. Tepco said it would also inspect neighbouring fuel assemblies and guides as well.
Separately, a report submitted to the Ministry of Ecomony, Trade and Industry on four notable events during the earthquake placed none of them on the International Nuclear Event Scale (INES).
Leakage of mildly radioactive water into an uncontrolled area of unit 6 was graded at level '0-', as was the spilling of similar water onto the refuelling floors of all seven reactors. A transformer fire in a non-nuclear part of the plant and damage to the reactor building ceiling crane of unit 6 were both listed as 'out of scale'.
Further information
Tokyo Electric Power Company (Tepco)
WNA's Nuclear Power in Japan information paper
WNA's Nuclear Power Plants and Earthquakes information paper
WNA's Earthquakes and Nuclear Safety microsite
WNN Event Reports: The International Nuclear Event Scale (INES) Rating System
WNN: Control rod stuck in Kashiwazaki Kariwa unit
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WNN: IAEA team arrives in Japan
WNN: Tepco lists earthquake effects
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WNN: Kashiwazaki Kariwa nuclear units shut down on earthquake