SHIZUOKA – Radioactive coolant water was found leaking from a valve at a nuclear power plant in Shizuoka Prefecture early Saturday — just one day after it resumed operations.
The leak was discovered at the No. 2 nuclear reactor at Chubu Electric Power Co.’s Hamaoka Nuclear Power Plant in the city of Hamaoka.
A fine mist of coolant water was discovered leaking from a welded spot on a valve for the reactor’s low-pressure core injection system during a regular patrol of the facility at around 2:20 a.m. Steps were taken to manually shut down the 840,000-kw boiling-water reactor. The process was completed at 4:34 a.m.
No radioactive material has been released outside the reactor, Chubu Electric officials said.
The valve where the leak occurred is part of the reactor’s emergency core cooling system. The ECCS system is one of the most important protection measures to ensure the safety of nuclear reactors as it is designed to reduce the temperature of the reactor’s core by flooding it with water in an emergency, according to the officials.
It is estimated that roughly 600cc of coolant water per minute had leaked from the valve. Judging from the liquid on the floor, some 20 liters had probably been released, the firm said.
The company began closing the inner valve at 6:41 a.m. and the leak was reportedly stopped at 7:14 a.m. Officials have been unable to pinpoint exactly what time the leak began.
The coolant water registered a radioactive density of 103 becquerels per cu. cm. Radioactivity in the vicinity of the valve was 0.08 millisieverts per hour, it added. The total amount of radioactive exposure suffered by the 15 Chubu Electric personnel who worked to contain the problem came to 0.12 millisieverts, according to Chubu Electric.
The Hamaoka plant was also affected by accidents in November, including an incident when steam leaked through a pipe in a high-pressure core-injection system of its ECCS. The pipe had ruptured due to a ductile fraction caused by extreme pressure — probably explosive hydrogen combustion — on Nov.
7. A leak of radioactive coolant water was identified three days later.
The No. 2 reactor was temporarily shut down along with the No. 1 reactor after these incidents since the two facilities share the same structure and construction. The pipes of the two reactors were exchanged to reduce hydrogen build-up.
Saturday’s accident occurred as the No. 2 reactor restarted operations on Friday. It was to start transmitting power Saturday, Chubu Electric said.
Company officials said tests and inspections at the No. 2 reactor ahead of the resumption of operations found nothing wrong with the piping.
“This leak is not an accident that directly affects the safe operation of the nuclear power plant,” said Junichi Ishihara, head of the utility’s operations group, at a news conference at the firm’s headquarters in Nagoya on Saturday morning.
“It is very regrettable that the water leak occurred, since we had been conducting thorough inspections since we suspended operations,” Ishihara said. “While we do not think this is a minor problem, we cannot reduce the occurrence of small problems to zero.”
But antinuclear groups criticized the latest incident, saying it was unacceptable for such an accident to occur so soon after operations resumed after an inspection.
They also said that nuclear safety authorities should share the responsibility for allowing Chubu Electric to resume operations at the plant.
Some experts pointed out that the latest leak is proof that the reactor is too old. The No. 2 reactor began commercial operations in 1978, two years after the No. 1 reactor.
Later in the day, Chubu Electric Power President Fumio Kawaguchi issued a statement apologizing for the incident.
” The trouble occurred after we conducted a full inspection. We will do our utmost to discover the cause (of the leak) and take appropriate measures,” the statement said.
Chubu Electric was criticized for its delay in notifying authorities of the November accidents. It said it reported the latest leak to the central and prefectural governments, as well as the five neighboring municipalities, between 3:07 a.m. and 3:20 a.m.
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