Japan The Japan Atomic Power Company (JAPC) has said it plans to replace the heat exchangers at the Tsuruga nuclear power plant, which was shut down on 12 July following a loss of coolant accident in unit 2 (See NEI Aug, p4).
A detailed report of the incident has now been developed by investigators.
The coolant leak was first detected at 6.05 am when fire alarms in the passage outside steam generator compartments C and D were activated. At the same time another alarm showed a high level in the containment sump followed by an increase in charging flow and an increase in the reading of the containment vessel particulate radiation monitor. These are symptoms of a coolant leak inside containment so a controlled shutdown was started at 6.24 am. The reactor was tripped manually at 6.48 am when the electrical output had been reduced to 81 MW.
When cold shutdown conditions had been reached, radiation levels inside containment began to fall. Engineers entered containment at 18.45 and found a leak at the outlet of one of the regenerative heat exc hangers of the chemical and volume control system. They valved out the heat exchanger to stop the leak and removed insulation from the pipework where they found a crack in an elbow joint. Some 51 m3 of water had leaked from the crack and the water had overflowed the sump and flooded the floor to a level of about 5cm. It was later pumped out to the liquid waste disposal system using the installed pumps. No radiation was released from containment and the accident was rated 1 on the INES scale.
Inspection of the outer surface of the elbow found a longitudinal (running in the direction of the flow) crack about 44 mm long, on the outer surface of the pipe. After the elbow had been removed, a 99 mm crack was found in the corresponding position on the inner surface. The elbow was sent to a laboratory. Visual observations, dye penetrant and ultrasonic tests showed three longitudinal cracks of 151 mm, 100mm and 72 mm on the inner surface and a 26 mm crack crossing the weld on the upstream side. The 151 mm crack had penetrated the wall for a length of 47 mm. In addition there were five circumferential cracks in the welds.
The fracture surfaces were typical of fatigue cracks with striations (so called beach marks) indicating that the cracks had grown from the inside surface of the elbow. There was no evidence of stress corrosion cracking or other corrosion problems. Also a similar elbow which was cut out and sent to the laboratory for comparison had no cracks. On-site tests carried out on other parts of the heat exchanger did not show any signs of cracking.
As thermal stresses may be the cause, JAPC will carry out checks at site to see if the heat exchanger supports were free to expand as designed and check heat exchanger performance.
Tsuruga 2 is a 4-loop Mitsubishi PWR rated at 1160 MWe. It went into service in June 1986 and has operated well with a lifetime cumulative load factor of 78.6%.