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Main accidents and breakdowns in nuclear power facilities in FY2011

Year Date Facility Overview Report Classification
2011 April 1 Onagawa 1 Damage to field system circuit of emergency diesel generator (A). Law
  April 7 Tomaru 1 & 2 As the Hokkaido-Honshu HVDC link (600 MW) automatically shut down due to the earthquake Off-shore of Miyagi Prefecture, reactor heat output was reduced. Because of this, the axial direction neutron flux output deviation of Units 1 and 3 differed from the standard value and deviated from operational limits.  
  April 7 Higasidori 1 The Miyagi Prefecture off-shore earthquake caused a loss of external power source and the plant deviated from operational limits.  
  April 7 Onagawa 2 Startup transformer heat radiator oil leak.  
  April 7 Onagawa 3 Automatic full open operation of the suction valve on the high pressure core spray system pressure suppression chamber was not possible.  
  April 12 Kashiwazaki-Kariwa 5 Reduction in output due to lubricating oil filter changeover on the reactor coolant recirculation system MG (motor and generator) set.  
  May 2 Tsuruga 2 A rise in the concentration of radioactivity in the primary coolant, which appeared to be caused by radioactivity leaking from the fuel. The reactor was shut down and an inspection confirmed that one fuel assembly was leaking.  
  May 14 Hamaoka 5 A rise in conductivity occurred, indicating that seawater had entered the main condenser during the time the reactor was being shut down. Of the roughly 21,000 main condenser (A) SG tubes, in an area roughly 14 m wide and 70 cm deep, 43 SG tubes were damaged and 2 deformed. The endgap of the recirculation piping of the electrically driven water supply pump, which is installed inside the main condenser (A) was severed and had fallen off. The circulation water pump and the water supply pump stopped, but it is thought that 10 tons or more of seawater flowed into the reactor core.  
  May 17 Tomari 1 Fractures (one 6 mm, one 9 mm long) found in two locations in welds on reactor vessel B loop discharge nozzles.  
  June 27 Shika 2 When inspecting 25 hafnium flat tube control rods, no fractures were found on the tie rods, but fractures were discovered on the welds of the handle and sheath of one rod and on the guide rollers of 11 rods.  
  July 7 Tokai 2 A worker involved in removal and handling operations for the control rod drive mechanism reactor coordinates 18-23 in the area in front of the pedestal in the reactor containment was exposed to radioactivity exceeding the scheduled dose (total of 3.30 mSv per day).   
  July 22 Rokkasho Reprocessing Plant Inability to start up two safety steam system boilers in the pre-processing building. Law
  August 3 Onagawa 2 & 3 Fastening bolts on the spent fuel storage racks in the spent fuel pool were found to the loose.  
  August 18 Takahama 4 Fractures due to stress corrosion cracking were found on one SG tube high temperature side tube plate in each of two (B and C) out of three steam generators. Law
  August 30 Ohi 2 Deviation from operating limits due to fall in reading of a thermometer in the containment vessel.  
  August 30 Fukushima Daini 2 Cessation of the high pressure reactor core spray system component cooling seawater system. Law
  September 6 Tomari 3 Deviation from operating limits due to inoperable central control room emergency circulation system.  
  September 13 Tokai Reprocessing Facility During changeover work on external electricity access, the ventilation blower, dissolution off-gas blower, shearing off-gas blower for the high-level radioactive liquid waste storage tank in the separation and refining plant in the facility ceased working, making it impossible to maintain negative pressure inside the storage tank. Law

September 13 Kashiwazaki-Kariwa 7 Detection of radioactive iodine in sampling from main exhaust pipe.  
  September 26 Tokai 2 Missing inner part of main steam relief valve (D).  
  September 27 Tsuruga 1 Seawater leak from seawater return pipe on emergency diesel generator (A).  
  October 4 Genkai 4 Automatic shutdown of reactor due to reduction in degree of vacuum in condenser. Flaw in the procedure manual for replacement of the torque switch for the steam master valve on turbine equipment shaft seals. Law
  October 8 Kashiwazaki-Kariwa 7 When implementing operational tests on each of the scram pilot valves, one out of the 103 scram pilot valves (core position 58-31/54-11) did not function correctly.  
  October 17 Fukusima Daini 4 Unscheduled exposure 1.58 mSv during decontamination work in the containment.  
  October 19 Tsuruga 2 Internal exposure (inhalation of Cobalt-60) by worker. Measurement by whole body counter showed a reading of approximately 58,500 counts/sec.  
  October 26 Tokai 2 Water was found to be leaking from one location (core coordinates 30-51) of the control rod drive mechanism housing flange in the lower part of the reactor pressure vessel.  
  October 28 Tokai Reprocessing Facility Through-holes due to corrosion were found in the external duct linked to the main exhaust pipe leading from the separation and refining plant. Law
  November 4 Kashiwazaki-Kariwa 7 Defect in a valve on an emergency diesel generator.  
  November 17 Shika 1 Damage found to a cable in the control panel of the diesel generator for the high-pressure core spray system.  
  November 28 Onagawa 3 Corrosion found on discharge check valve of control rod drive water pump (B).  
  December 1 Fukushima Daiichi 1to 6 Wrongful use of alarm pocket dosimeters by workers of contracted companies.  
  December 1 Kashiwazaki-Kariwa 2 Shutdown of high-pressure core spray system diesel generator during regular inspection.  
  December 7 Mihama 2 Manual shutdown of reactor due to increase in A-pressurizer spray valve gland leak-off flow rate.  
  December 16 Genkai 3 Breakage of C charging pump main shaft. Law
2012 January 13 Tokai Fire in cooling tower on rooftop of solidification treatment building.  
  January 13 Onagawa 3 During implementation of repacement work for new rotor blades on a low-pressure turbine (B), a fissure was discovered on one location and damage on another location on the base of one rotor blade on the 15th stage rotor blades. After a detailed inspection, fissures were discovered on a further two rotor blades.   
  January 16 Onagawa 3 Fractures found on a steam turbine cylinder, shell plate and discharge outlet.  
  January 19 Kashiwazaki-Kariwa 5 Trouble occurred during a confirmation test on the movement of control rods during a regular inspection when control rods (14-27) were supposed to be inserted one notch from full withdrawal, but actually entered 11 notches.  
  January 27 Shimane 2 Neutron source range monitoring system inoperable.  
  March 3 Tokai 2 A water leak was found in the neighborhood of the weld on the residual heat removal system (C) low-pressure core injection system injection valve differential pressure detection pipe.  
  March 27 Fukushima Daini 3 & 4 Contamination by radioactive substances confirmed in a non-controlled area of a service building. Law
  March 29 Takahama 3 Fracture due to stress corrosion cracking found in tube plate on high temperature side of one SG tube in C-steam generator out of the three steam generators. Law
  March 30 Hamaoka 5 40 corroded pores were confirmed in the condensate tank lining material during an inspection in connection with the influx of seawater into the condenser system, which resulted in damage to the main condenser SG tubes on May 14, 2011 in the course of cold shutdown operations after the reactor had been shut down.   Law

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