Kansai Electric Power Company’s Mihama No.2 nuclear reactor is being manually shut as a safety precaution.
Japan's second-largest power utility said the problem at the 500 megawatt reactor in western Japan had not affected pressure inside the reactor vessel or the operation of the unit. "We're checking the cause of the trouble," a company spokesman said.
Mihama No. 2, which was due to be taken down for maintenance on December 18, is Japan's fourth-oldest nuclear reactor and one of only nine still operating after the Fukushima crisis in March stirred fears over atomic power safety.
A valve connected to the reactor's pressuriser, which maintains pressure inside the reactor vessel to prevent water from boiling, was leaking greater amounts of water than usual, leading the utility to decide to shut the reactor. There had been no radiation leaks, the spokesman said.
A spokesman at the Nuclear and Industrial Safety Agency, Japan's nuclear watchdog, said it had been notified of the incident. "As we understand it, this is not due to a failure in the pressuriser," he said.
More than four-fifths of Japan's 54 commercial nuclear reactors are offline, some knocked out by the March 11 earthquake and tsunami that triggered the world's worst nuclear disaster in a quarter century at Tokyo Electric Power Co's Fukushima Daiichi plant.
Most have gone down for routine maintenance, however, and are unable to restart as fears about nuclear safety in the wake of the Fukushima crisis leave local communities reluctant to let them go back into operation.
A nuclear reactor at Kyushu Electric Power Company's Genkai plant in southern Japan was restarted last month despite public opposition, however, after it automatically shut down the month before due to a cooling system problem.