(3) INES evaluation (final)

  On November 11, 2004, "the INES Evaluation Subcommittee" (Chairman: Professor Haruki Madarame, Graduate School of Engineering, University of Tokyo) of "the Nuclear and Industrial Safety Subcommittee" of "the Advisory Committee for Natural Resources and Energy" was held at the Ministry of Economy, Trade and Industry (METI) to carry out the evaluation of the following events and failures occurred at the nuclear facilities.
  The result of the evaluation is as shown in the attachment.
  The Subcommittee has been established to evaluate events and failures occurred at the METI licensed nuclear facilities by using INES from professional and technical points of view.

Occurrence Date	Nuclear Facility	Event	Final INES Level
July 16, 2004	Ohi Power Station Unit-1, Kansai Electric Power Co.	Water ooze from a crack in the vicinity of the root of the backplate weld portion of the refueling water storage tank return pipe	Level 0-

1.	Power Plant:
	Ohi Power Station Unit-1 (PWR, rated electric power 1,175MWe), Kansai Electric Power Company
2.	Date of Occurrence
	July 16, 2004
3.	Event
	Water ooze from a crack in the vicinity of the root of the backplate weld portion of the refueling water storage tank return pipe
4.	Event Description
	The Ohi Power Station Unit-1 has been under the periodical inspection since June 4, 2004. Because transformation event of the refueling water storage tank (hereafter "the tank" is used) occurred on June 10, 2004, repair work of the concerned tank was performed and inspection for preparing pressure-resisting test of the tank has been performed. On July 14, 2004, a small amount of water ooze from the vicinity of the support backplate weld of the concerned tank return pipe (3 inch diameter) was found. Stress corrosion crack under the influence from sea salt particles generated and developed at the location of ooze recognized, and a small crack resulted in a wall-through. As a result of liquid penetrant test in the vicinity of the backplate weld portions of four locations except the location of ooze recognized, penetrant indication film was recognized.
	Visual inspection of the vicinity of the backplate weld portion of the return pipe support of the tank was performed. As a result of the inspection, a crack of about 5 mm in maximum length was recognized in the vicinity of the root of the backplate weld portion concerned. Liquid penetrant test of similar locations (63 locations) was performed. As a result of the test, penetrant indication film was recognized in the vicinity of the backplate weld portions of five locations, including the backplate where ooze was recognized. SUMP observation of these five indication portions was performed. As a result of the observation, a treelike intergranular crack was recognized. The tank was installed in the outside without coating in 1974. Afterwards, the tank was coated in 1981. In the period between 1974 and 1981, the tank was outside of the building, which is susceptible to influences from sea salt particles. The backplate shape was also discontinuous. Therefore, sea salt particles were easy to stay under the circumstances.
	It is estimated that the chlorine-typed stress corrosion crack generated at the location of ooze recognized and other four baclplate locations. Afterwards, the stress corrosion crack developed, and resulted in a wall-through. Furthermore, in addition to aging progress of the coating due to liquid contact, heat influence was added, when the tank return pipe support, which was installed to the backplate concerned, was cut and welded to repair the tank. This led to separation of the coating film of the concerned portion, and resulted in water ooze with the tank water filling.
	There was neither release of radioactive material to environment nor unexpected personnel exposure.
5.	Evaluation Results and Their Basis
	(1) Criterion-1:-
	(Basis: There was no radiological impact on the outside of the power plant.)
	(2) Criterion-2:-
	(Basis: There was no radiological impact on the inside of the power plant.)
	(3) Criterion-3:Level 0-
	(Basis: Water ooze occurred through the crack in the vicinity of the root of the backplate weld portion during the periodical inspection. Leakage rate was extreamly small. It had no possibility to challenge safety requirement of the reactor immediately. Therefore, the evaluation is judged to be as the level 0-.)
	(4) Final Evaluation Result: Level 0-