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Main accidents and breakdowns in nuclear power facilities in FY2010

Year Date Facility Overview Report Classification
2010 April 2 Onagawa 1 Damage to a pipe joint in the standby liquid control system accumulator.  
  April 2 Shika 1 Primary loop recirculation system flow restrictor device alarm activated.  
  April 12 Hamaoka 5 In a confirmation test following re-modelling of the control rod block monitor, when the withdrawal of control rod numbers 34-03 above the fully withdrawn position (100 steps) was implemented, the "coupling" display should normally have lit up when the control rod was over-withdrawn to the 102nd step. However, at step 101, the "control rod withdrawal prevention" alarm light came on and the control rods stopped. It has been confirmed that the interlock had failed.  
  April 19 Mihama 2 Rise in the concentration of radioactivity in the primary coolant due to leaks from two fuel assemblies.  
  April 20 Mihama 5 Fractures in the welding on the moisture separator and reheater (B).  
  April 25 Mihama 2 Rise in reading on the containment exhaust gas monitor when extracting air from inside the containment. The cause was found to be coolant leaking from the primary coolant pump B.  
  April 27 Ikata 1 Damaged piping in the seawater coolant for the emergency diesel generator (B). Law
  May 20 Onagawa 1 Discovery of a fracture, 30 mm long, 5.2 mm deep, in one location when 6 locations on the recirculating system piping welds were inspected.  
  May 31 Fukushima Daiichi 1 When measuring condensate system pipe wall thickness, a value (7.6 mm) less than the minimum required thickness (8.3 mm) was detected.  
  June 1 Fukushima Daini 1 When confirming the opening and closing of the isolation valve inside steam piping in the reactor core isolation cooling (RCIC) system during a regular inspection, a problem was experience with the valve not opening fully. The reactor was shut down manually for an inspection. Law
  June 2 Shika 2 Alarm activated to indicate abnormality in the detector monitoring the degree of opening of the main steam stop valve (D).  
  June 4 Hamaoka 4 Of the main turbine shaft vibrations, a slight rising trend was confirmed in the shaft vibration values in the bearings (No.7 or No.8 bearings) of the low-pressure turbine (C).  
  June 11 Ikata 1 Damage to the seawater coolant piping of the component cooling water heat exchanger (A). Law
  June 15 Tsuruga 1 It was discovered in an inspection of a welded ferrule on the recirculation pump casing that there was a weld line on the pressure-containing section of the recirculation pump that was not shown in the blueprints and was also missing from the items on the in-service inspection.  
  June 17 Fukushima Daiichi 2 As the "generator field circut breaker trip alarm" was activated, the generator protection device was also activated causing the generator to shut down, and the turbine and nuclear reactor also automatically shut down. As the changeover to the plant's power source system did not occur and the plant's power system stopped, the emergency diesel generators were activated and a transient phenomenon occurred where the water level in the reactor was greatly reduced.  
  June 17 Shika 1 Fracture in the welding of the recirculation pump (B) outlet valve on the leak disposal system piping.  
  June 24 Shika 1 Trouble occurred when inserting one control rod after removal of all fuel from the reactor.  
  June 25 Tokai Daini In a regular inspection of two seawater pumps (B and D) of the residual heat removal system, the value of the seawater flowmeter was found to be below the standard value. The reactor was shut down manually when it proved to be impossible to specify the cause of the low reading after dismantling and checking the pumps. Law
  July 7 Fukushima Daini 1 An inspection tool fell into the nuclear reactor during inspection work on the Local Power Range Monitor.  
  July 7 Fukushima Daiichi It was confirmed that the pressure reading on the device (pressure guage) system monitoring the cask sealed gas inside one of four medium-sized dry casks in the cask storage building at Fukushima Daiichi Nuclear Power Station was downscaling. Downscaling means the reading dropped to the lowest level.  
2010 July 11 Fukushima Daini 1 A foreign object was found on the baffle plate in the vicinity of jet pump No.1 in the nuclear reactor.  
  July 16 Fukushima Daini 1 While replacing valves on the residual heat removal system instrumentation system line valves, indicating patterns were confirmed on four of the valve casings when penetrant tests were performed on the welds.  
  July 28 Fukushima Daiichi 3 & 4 Gamma-emitting substances (Silver-110m, Cobalt-60, Manganese-54) were detected on sampling filters taken from the main exhaust stack between July 28 and August 4.  
  July 30 Rokkasho Reprocessing Plant High-level radioactive liquid waste leaked into the protective pipe on the thermometer on the high-level liquid waste concentration tank in the separation building. This was due to corrosion in the lower part of the concentration tank. Law
  August 4 Fukushima Daiichi 1 Confirmation of damage and missing parts in the support structure for the downstream piping for the return valve in the high-pressure injection system startup test return piping. Confirmation also of eight cases of damage and missing parts in the support structure in other locations  
  August 12 Fukushima Daiichi 1 Water containing radioactive substances was confirmed to be dripping from the vicinity of the lower part of the high-pressure turbine casing to the vicinity of the feedwater heater area on the 1st floor of the turbine building (controlled area) and the reactor was shut down manually to investigate this.  
  August 21 Kashiwazaki-Kariwa 1 The degree of vacuum of the condenser declines and a variation in power output of max. 13,000 kW occurred. It was confirmed that the water box communication valve was in the fully open position, whereas the condenser (B) should normally be in a totally closed state  
  August 23 Fukushima Daiichi 3 Subcontractor workers were polluted and suffered internal exposure when carrying out a dismantling inspection of a valve in the reactor building pressure suppression chamber area (controlled area).  
  August 26 Onagawa 3 When implementing the dismantling inspection of scram outlet valves in 137 control rod drive hydraulic system hydraulic control units, the valve shaft slide on two of the units were confirmed to be damaged.  
  August 26 Monju While being hoisted out of the reactor, an in-vessel transfer machine was deformed when dropped back into the reactor. Law
  September 2 Fukushim Daiichi 5 When implementing the monthly regular inspection of the reactor core isolation cooling system, a turbine in the same system automatically stopped. It was confirmed that the signal cable controlling the steam governing valve, which governs the amount of steam being provided to the turbine, was disconnected.  
  September 7 Hamaoka 5 Reheat cracks found in the moisture separator & reheater outlet pipe welds.  
  September 10 Kashiwazaki-Kariwa 7 Rise in the reading of the high-sensitivity off-gas monitor on the off-gas system. Leak confirmed from one fuel assembly.  
  September 14 Fukushima Daiichi 6 When conducting a confirmation test for leaks in one gas-side system in the flammable gas concentration control system, it was found that the test pressure could not be maintained.  
  September 25 Fukushima Daini 3 Power output fell to 380,000 kW due to the occurrence of an incident in which a large amount of seaweed etc. was taken into the intake screening device on the seawater intake for the turbine condenser coolant seawater.  
  September 27 Onagawa 3 Damage to the accumulator of one water pressure control unit of the control rod drive mechanism.  
  October 1 Tomari 2 A fluctuation of the concentration of boric acid occurred due to a disorder in the temperature control of the extraction line nonregenerative heat exchanger observation outlet.  
  October 4 Onagawa 1 An alarm was activated in the central control room indicating inability to operate devices for containment radiation detection for the containment atmosphere monitoring system monitor.  
  October 14 Hamaoka 4 Temporary operational aberration due to faulty operation of the startup range monitor.  
  November 1 Kashiwazaki-Kariwa 7 Fractures were discovered in the sheaths and tie-rods of 28 hafnium flat tube type control rods.  
  November 1 Mihama 2 During a regular inspection of the reactor containment air recirculation fan, when one of the air recirculation fans was started up the cooling water intake valve and discharge valve of the D-containment air handling device cooling coil should have opened automatically upon receipt of the startup signal, but a problem occurred in the form of the discharge valve not opening.  
  November 2 Fukushima Daiichi 5 During work to reduce power output in order to adjust the control rod pattern, the water level in the reactor core rose and fell rapidly, causing the turbine to go into automatic shutdown followed by automatic shutdown of the reactor. Law
  November 4 Kashiwazaki-Kariwa 5 During a manual startup of diesel generators for a regular inspection of the high-pressure reactor core spray system, the alarm "HPCS diesel generator equipment failure" occurred in the central control room and the alarm "generator overvoltage" occurred at the control panel at the location.  
  November 25 Fukushima Daini 4 A fracture was discovered in one location in the base metal that had separated from the weld line of the inside of the middle shell region of the reactor core shroud.  
  December 1 Kashiwazaki-Kariwa 3 During work to restore the control rod drive mechanism, the control rod drift alarm occurred when the worker at the site operated the drive water pressure system valve for control rods 38-59. Although they should have been in the fully withdrawn position, it is thought that trouble had occurred when returning the control rods to their original position after inserting them by one notch. It is assumed that air had entered the piping of the control rod drive system. Law
  December 1 Shika 1 Due to discovery of damage to the mechanical seal on a recirculation pump, the reactor was manually shut down to replace the mechanical seal.  
  December 7 Shika 1 A worker was sprayed with water during work to replace the mechanical seal on a recirculation pump.  
  December 8 Hamaoka 3 During the implementation of an inspection of a steam dryer, a fracture was confirmed in the proximity of a weld on the steam dryer.  
  December 10 Genkai 3 Occurrence of a rise in the concentration of iodine in the primary coolant. A leak in one fuel assembly (uranium fuel) was confirmed.  
  December 11 Global Nuclear Fuel Japan The temperature adjustment device on a gadolinia sintering furnance (B) in operation failed to work correctly. During work to restore the device, heating ceased due to activation of the interlock for the heating limit, and operation of the sintering furnace was stopped manually. Law
  December 13 Shika 1 During reactor startup, when the operation for the withdrawal of one control rod was implemented, the "control rod drift" alarm was activated, and the problem that the rod was withdrawn 3 notches beyond the scheduled withdrawal position of 2 notches occurred. The reactor was manually shut down for inspection.  
  December 20 Hamaoka 4 Fractures were discovered in three places in the proximity of the welds on the suspension rod and support ring of a steam dryer.  
  December 26 Fukushima Daini 4 A problem of decrease in the water level in the spent fuel pool occurred.  
  December 27 Onagawa 3 Rise in the concentration of radioactive substances in the off-gas system and confirmation of a leak in one of the fuel assemblies. Bulging was found in the cladding of one of the fuels rods in the relevant fuel assembly.  
  December 28 Monju Damage found in seven places in the cylinder liner of an emergency deisel generator. Law
2011 January 9 Tomari 3 One worker was confirmed as having body contamination by radioactive substances when implementing a change of filter element on the 3B-coolant demineralizer intake filter in the reactor auxilliary building, and this was confirmed to include internal exposure of radioactive substances by whole body counter.  
  January 18 Shimane 1 Regarding 69 weld locations on piping, etc, in the primary recirculation system, when implementing an inspection for damage by ultrasonic test, fractures were discovered on January 18 in one location on the weld joint on the downstream piping of the A-primary recirculation pump discharge valve, and on February 17 in one location on the weld joint on the upstream piping of the A-primary recirculation pump intake valve.  
  January 21 Shika 2 Manual shutdown of reactor due to reduction in the amount of condensation in the containment inner heat exchanger.  Law
  January 21 Hamaoka Occurrence of control rod drift due to poor assembly of the control rod drive water pressure system accumulator.  
  January 27 Fukushima Daiichi 1 Power output manipulation due to restart of reactor water clean-up system.  
  January 31 Kashiwazaki-Kariwa 1 Departure from operation limits due to symptoms of failure in the operation of the recirculation flow control valve on the flammable gas concentration control system.  
  February 2 Fukushima Daini 3 Departure from operation limits due to inspection of equipment related to emergency deisel generators (B).  
  February 3 Shika 2 Defect found when starting power source for the drive of the control rod drive mechanism during equipment checking work while plant was in shutdown.  
  February 8 Mitsubishi Nuclear Fuel Uranium dioxide powder leaked from extraction device in the conversion and processing room of the conversion factory. The leak amounted to 5.5 x 10^5 Bq. Four workers were exposed to a maximum of 4.98 mSv. Law
  February 14 Ikata 3 As it was found that there was a delay when performing the release operation of the reactor trip breaker, the circuit was replaced.  
  February 17 Tomari 3 Damage on the solenoid valve terminal block for the cooling water flowrate changeover valve on the control air compressor.  
  February 21 Kashiwazaki-Kariwa 6 Contamination by radioactive substances on the floor of the reactor building pump room.  
  February 25 Genkai 2 Pipe wall thinning exceeding the standard was found on the inner surface of five SG tubes in the component cooling water heat exchanger.  
  February 28 Shika 1 Reactor shut down manually due to replacement of shaft seal on the reactor coolant recirculation pump (B)  
  March 7 Ikata 3 The reading of the central control room radioactivity measurement monitor temporarily rose to 60 ÁSv/h (alarm set to activate at 2.6 ÁSv/h), and the central control room ventilation system isolation was operated.  
  March 8 Higashidori 1 Damage on the pressure guage of the control rod drive mechanism pressure control unit.  
  March 11 Fukushima Daiichi Great East Japan Earthquake: Units 1, 2 and 3 go into automatic shutdown. Later, due to loss of all alternating current power, reactor core meltdowns and hydrogen explosions occur and massive amounts of radioactive substances were released into the environment. Law
  March 11 Fukushima Daini Great East Japan Earthquake: Units 1, 2, 3 and 4 go into automatic shutdown. Function of pressure suppression chamber temporarily lost. Reactor core damage was closely avoided due to maintenance of one external power source. Law
  March 11 Tokai 2 Great East Japan Earthquake: Large vibrations in turbine cause reactor to shut down automatically. Reactor core damage closely avoided due to function of emergency diesel generators. Law
  March 11 Onagawa Great East Japan Earthquake: Units 1, 2 and 3 go into automatic shutdown. A fire occurred in the power panel and the emergency diesel generators stopped. Reactor core damage was closely avoided due to ability to secure one external power source. Law
  March 11 Higashidori Great East Japan Earthquake: Down for regular inspection. Due to loss of external power source, power was supplied by emergency diesel generators.  
  March 11 Rokkasho Reprocessing Plant Great East Japan Earthquake: Due to loss of external power source, power was supplied by emergency diesel generators.  
  March 31 Hamaoka 4 Defect in one reactor pressure signal used to calculate reactor heat output. A declaration of departure from operational limits according to the Nuclear Facility Safety Regulations was declared as it was judged that there was a possibility of an error occurring in the reactor heat output.  
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