Cracked pipes in nuclear reactors owned by three power utilities must be repaired or replaced before the reactors are put back into operation, the Nuclear and Industrial Safety Agency said Wednesday.
The agency, which is supervised by the Ministry of Economy, Trade and Industry, issued a report saying that the present method of examining reactors for fractures needs to be improved so the precise depth of the cracks can be gauged.
It said it will take time for the agency to confirm the credibility of data about reactor defects.
Fractures were found in pipes at five reactors belonging to Tokyo Electric Power Co., one belonging to Tohoku Electric Power Co. and three run by Chubu Electric Power Co., according to agency officials.
The agency considers data obtained under the current method to be unreliable. It cited the case of a 12-mm-deep crack found in a pipe at the No. 1 reactor of the Onagawa Power Station of Tohoku Electric Power that an ultrasound test by the utility showed to be only 2 mm deep.
The three utilities are currently using transverse waves in their ultrasound tests, but it would be more accurate to use longitudinal waves, the officials said.
The agency said the utilities will use longitudinal wave testing from now on, which should reduce the margin of error to 3 mm. It plans to compile an evaluation of the accuracy of the data by May.
At the same time, however, the agency drew up a policy basically allowing reactors to be restarted without repairing minor fractures, provided the cracks can be measured precisely and safety can be guaranteed for five years even if the fissures widen, the officials said.