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Major Incidents at Nuclear Facilities April 2005- March 2006
Date	Company Facility	Incident Description	Comment
28 April 05	KEPCO Mihama-1	While conducting a visual inspection during periodic inspection of the auxiliary building exhaust stack, it was discovered that two drain pipes on the base of the exhaust stack had become detached and that there were cracks in the base plate.	This exhaust stack is located beside the reactor containment vessel. The cracks were caused by fatigue from repeated vibration.
12 May 05	Shikoku Ikata-3	An abnormal noise was noticed during inspection of a chiller used for air conditioning. Flaws were found in the suction portion of the impellers and in the seal ring.	.
3 July 05	TEPCO Kashiwazaki-Kariwa-5	While preparing to shut down the reactor for periodic inspection, a deterioration of condensor vaccum caused the reactor to trip automatically.	When switching operation of turbine gland sealing steam from the steam evaporator side to the auxiliary boiler side, the auxiliary boiler steam supply valve opened only 5%. The fault was not noticed and the operation was continued.
6 July 05	Chugoku Shimane-1	During adjustment operation, the degree of closure of the drywell vacuum break valve was displayed incorrectly. The reactor was shut down manually.	Caused by a defect in the microswitch.
10 Aug. 05	JAPCO Tokai-2	During start-up operation, the outlet valve stem broke in the motor-driven feedwater pump.	The valve stem had become brittle due to cracks along the crystal grain boundary, a feature of grain boundary type stress corrosion cracking. The stem broke when a load was applied.
22 Aug. 05	TEPCO Fukushima I-5	During periodic inspection, it was discovered that the necessary flow rate for the reactor core spray system was not secured. The reactor was shut down manually in order to check the flow rate control valve.	Discovered that the flow rate control valve stem was broken.
29 Sep. 05	KEPCO Mihama-1	While operating at 50% power for inspection and maintenance, an alarm indicated an increase of the safety valve outlet temperature of pressurizer-B. The reactor was shut down manually to check for a possible leak from the valve sheet.	.
9 Oct. 05	TEPCO Fukushima I-2	The recirculation pump stopped automatically following an alarm related to problems with control of the pump. The reactor was operated at reduced power until it was shut down manually on October 10th.	The inverter stopped automatically due to a bad connection in the fuse holder.
1 Nov. 05	TEPCO Fukushima II-2	During periodic inspection, it was discovered that the metal mesh in a sea water strainer in the residual heat removal component cooling system had ruptured.	The damage was caused by vibration and wear and tear.
6 Jan. 06	Hokkaido Tomari-1	During periodic inspection, 6 cracks were found near the welded portion of reinforcement metal in an emergency exhaust stack within the controlled area of the reactor building. 5 of these cracks penetrated right through the walls of the stack.	7 cracks were also found near the welded portion of reinforcement metal in the main exhaust pipe. 6 of these cracks penetrated right through the walls of the stack. The cracks were caused by fatigue due to vibration.
9 Jan. 06	TEPCO Fukushima I-6	During periodic inspection, cracks were found in sheaths and tie rods of 9 hafnium blade type control rods. The sheath of one of the control rods was significantly damaged.	The cracks were induced by neutron irradiation. One control rod had failed to insert when the reactor was shut down on 21 December 2005.
13 Jan. 06	Kyushu Sendai-1	During periodic inspection, damage was found in 13 tubes in the steam generator.	Stress corrosion cracks were found in the tube expansion zone of the hot leg side.
26 Jan. 06	Hokuriku Shika-2	During start up for trial operations, one valve failed to close fully when testing opening and closing of the steam supply isolation valves in the reactor core isolation cooling system. The reactor was shut down manually.	The electromagnetic contactor was stuck in the Ågvalve-openÅh position. Part of the contact point was slightly deposited due to prolonged "chattering" (repeated rapid opening and closing).
11 Mar. 06	TEPCO Fukushima I-3	During periodic inspection cracks were found in sheaths and tie rods of 5 hafnium blade type control rods. The sheath of one of the control rods was significantly damaged.	The cracks were initiated by neutron irradiation.
14 Mar. 06	TEPCO Fukushima I-2	A recirculation pump stopped automatically causing a power reduction. The reactor was shut down for inspection when damage was discovered in the electric circuit in the inverter.	.

Note: Only incidents that were reportable under the Reactor Regulation Law have been included. A much longer list, including other incidents that CNIC judged to be significant, was included in our Japanese newsletter No. 385.

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