Press Release

Nuclear and Industry Safety Agency

On August 9, 2004, the Nuclear and Industrial Safety Agency (NISA) received a report from the Kansai Electric Power Co., Inc. (KEPCO) stating that the Mihama Power Station Unit 3 (PWR, rated electric power: 826 MWe) had automatically shut down. The details are as follows:

1. The report from Kansai Electric Power Co., Inc.

(1) Conditions at the time of the accident

The Mihama Power Station Unit 3 had been in operation at the rated thermal power. At 15:22 on August 9, 2004, the fire alarm was activated and at 15:28 the mismatch trip signal alarm to the 3A steam generator was activated*, followed by the reactor’s automatic shutdown .

An investigation by the resident nuclear safety inspectors confirmed that the mouth of the rupture was located at the condensate water piping near the ceiling of the second floor of the turbine building (the condensate water piping between the fourth low-pressure feed-water heater and the deaerator).

Four workers at the facility were killed and seven others were burned by steam from the rupture of the condensate water piping. They were on the second floor of the turbine building preparing for the coming periodical inspections scheduled for August 14.

The unit is a PWR-type reactor and no radioactive material was included in the leaked secondary steam.

There were no abnormal indications from any of the on-site or field monitors and there was no release of radioactive material into the environment.

The reactor was safely transferred to a condition of cold shut down at 19:05 on August 10.

*: The trip signal is activated when the flow rate of water fed to the steam generator is less than the steam flow rate under the condition of a low water level in the steam generator. 

(2) Investigation of the Cause

The causes of the accident are now under investigation. At present, the following facts are known.

The location of the rupture was in the downstream side from the flow measurement point, an area where erosion on the inner surface of the piping is liable to occur due to turbulent flows. KEPCO had not checked the area in spite of KEPCO’s internal policy that the portion concerned should have its residual life controlled and be checked systematically.

(Piping Specifications)

Material:  carbon steel
Outer Diameter:     about 560 mm
Thickness:            about 10 mm
Maximum Operating Temperature:       about 195 deg. C
Maximum Operating Pressure:            about 1.27 MPa (about 13.0 atg)

(Conditions at the rupture area at the time of the accident)

Position:                between the fourth feedwater heater and the deaerator
Temperature:          about 140 deg. C
Pressure:               about 0.93 MPa (about 9.5 atg)
Flow Rate:             about 1,700 cubic meters per hour

(3) Response by other plants

Investigation of previous inspection records is in progress to check the degree of thinning of piping at other plants of KEPCO.
Safety checks will be immediately conducted for the main pipes which have never been checked.

2. Response by NISA

(1) On-site investigation by Minister of METI

The Minister of METI (accompanied by the Director-General of NISA) visited Mihama Power Station on August 10 to study the situation and to exchange opinions with local people.

(2) Establishment of the local response headquarters

The METI local response headquarters headed by the Deputy Director-General of NISA was established on August 9 as the base for site investigation and information collection.

(3) Establishment of the Investigative Committee on Mihama Power Station Unit 3 Secondary System Pipe Rupture Accident

An investigative committee was established on August 10 in the Nuclear Reactor Safety Subcommittee of the Nuclear and Industrial Safety Subcommittee, under the Advisory Committee for Natural Resources and Energy, in order to investigate the pipe rupture accident. The Committee dispatched two experts to the site and started a special investigation on the same day. The first investigative committee meeting will be convened on August 11, 2004.

(4) Instructions given to electric power companies other than KEPCO to conduct an investigation, on August 10, 2004

NISA instructed four PWR-owning electric power companies (Hokkaido Electric Power Co., Inc.; Shikoku Electric Power Co., Inc.; Kyushu Electric Power Co., Inc.; and the Japan Atomic Power Company) to check for the presence of possible thinning in un-inspected portions and to report the results to NISA as soon as possible.      

   - Results of Provisional Evaluation of INES -

Contacts:
Messrs. Maeda and Amano
Nuclear Incident Response Office
Nuclear Emergency Preparedness Division
NISA/METI

Phone: +81-3-3501-1511 (Ext: 4911)
Direct Phone: +81-3-3501-1637