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Press Release
August 10, 2004
The Kansai Electric Power Co., Inc.

A Report on the Conditions of the Victims of the Event at Mihama Unit 3 and on Investigations of the Automatic Reactor Trip


Details of the Event and Its Victims
The event took place in the turbine building when 221 persons in total were present inside the building (Later, the number of people who were inside the building at the moment of the accident was corrected to 104. 221 was the total number of people who entered the building during that day), preparing for the 21st periodical outage of the Mihama Nuclear Power Station Unit 3. The work included curing the floor, partitioning work areas and carrying tools. Under this circumstance, a fracture occurred to the condensate piping at a position close to the ceiling of the second level of the turbine building, causing ejection of high temperature water which vaporized and filled the second level of the building with steam. This immediately triggered a fire alarm (15:22) and caused variation of plant parameters; thus, emergency load down was soon initiated (15:26). Shift operators discovered workers falling down inside the building (15:27) and this was reported via the main control room to the Manager of General Manager Office, who then called ambulances (1st call at 15:30). At the moment, employees of the Kansai Electric Power Co., Inc. (Kansai Electric) and its cooperating companies carried the victims one by one out of the turbine building. Subsequently, Kansai Electric employees accompanied by the fire fighters checked the building to assure that nobody was left inside and completed confirmation at 19:00.

Of the eleven persons injured, eight were taken to Tsuruga City Hospital by ambulances. (Of the eight, four were pronounced dead, two are under medical treatment at Tsuruga City Hospital, one was transferred to Fukui Prefectural Hospital by helicopter and is now under medical treatment there, and one was transferred to University of Fukui Hospital by an ambulance and is now under medical treatment there.) The remaining three were taken to NHO (National Hospital Organization) Fukui National Hospital. (One of the three was transferred to University of Fukui Hospital by helicopter and is now under medical treatment there.)

Conditions of the Turbine Building
On-the-spot inspections are planned to be performed by the police and other authorities on August 10. (The time of the day is unconfirmed.)

Conditions of the Plant
The fractured piping has been isolated by upstream and downstream valves and the leakage has been stopped. A cooling-down operation has been undertaken since 23:30 on August 9 and the main coolant temperature was reduced to approximately 168 degrees state (at the pressure of 2.7 MPa) by 8:00 on August 10. The reactor will be in a cold shutdown condition (main coolant temperature at 93 degrees centigrade or less) by 16:00 on August 10.

State of the Inspection on Inoperable Two Valves
To place the turbine-driven auxiliary feedwater pump into a stand-by condition after it had stopped, the plant operators attempted to open the outlet flow regulating valves of this pump to the degree of approximately 60 percent. However, two out of the three valves (Valves A and C) were unable to be opened. Subsequently, from 01:20 on August 10, the valve-opening procedure was resumed in the presence of Kansai Electric staff and the valves responded normally at this time. While deciding to investigate the cause of the failure to open the valves in a separate occasion, the operators opened the two valves to the normal stand-by degree of opening (approximately 60 percent) at 01:53 in order to secure feedwater from the turbine-driven auxiliary feedwater pumps as a backup means to compensate the function of the motor-driven auxiliary feedwater pumps. Investigations are now underway on the causes of the initial inoperability of the valves and cause-and-effect relations in the event.

Piping with the Fracture
In our guideline for the maintenance of the secondary system piping, the affected piping falls under the category of "the major systems subject to inspection (whose remaining age must be under control and which must be inspected according to schedule)". The fractured region of the piping, in fact, has not been inspected in the past because the region was not registered to the management system, even though the bending portions at both ends of the affected region had been inspected for their integrities. In April, 2003, Kansai Electric's cooperating company checked the list of the management system and found the affected region of the piping being unregistered. Therefore, the relevant region was planned to be inspected during the 21st periodical outage scheduled from August 14, 2004.

Corresponding Measures at Kansai Electric's Other Plants:
In connection with the event at Mihama Unit 3, Kansai Electric is planning to conduct inspections at its other nuclear power plants in the following manner:

  • When the wall thickness of the fractured section was checked, significant thinning was identified. The company is now investigating the inspection records to elucidate the thinning status in other plants.
  • It has been confirmed that piping regions which correspond generally to the position of fracture and leakage at the Mihama Unit 3 (i.e. at the outlet of lowpressure feedwater heater) have been inspected in the past and evaluated properly in the following units of other plants: Mihama 1 and 2; Takahama 1 and 2, that are of the same type; Takahama 3 and 4 as well as Ohii 1 and 2, that are of a different type but have an orifice* at the similar location. (The orifices of Ohi 3 and 4 are not installed at similar locations.)
  • In addition, inspection records of other plants will be checked for major valves in "the major systems subject to inspection" in the maintenance guideline and major regions such as downstream location to an orifice, all of which are within the feedwater system and condensate system, other than locations similar to the affected position at Mihama Unit 3. Should any locations be identified with no inspection record, the company will urgently carry out safety inspections on relevant regions.
  • During the above-mentioned investigation and inspection processes, inspectors who need to approach any uninspected area will ensure safety by all means such as wearing fireproof outfit.

    * An orifice is a component used to narrow the fluid channel to reduce the flow or to produce differential pressure required for water flow measurement.

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