TOKYO — In a fresh blow to public confidence, a reactor in southern Japan went into automatic shutdown on Tuesday because of problems with its cooling system, clouding the outlook for an imminent restart of the country’s idled nuclear plants.

Kyushu Electric, the operator of the reactor at the Genkai nuclear power plant, characterized the incident as minor and said there was no risk of a radiation leak. A problem with the condenser unit that turns steam back into cooling water appeared to have caused the halt, but the reactor stopped safely and was undergoing checks, the utility said.

“At no point was the plant under any danger, and the reactor has been brought to a stable shutdown,” said Eiji Yamamoto, a spokesman for Kyushu Electric. “There has been no effect on radiation levels outside the plant.”

Still, the shutdown came as the government was renewing a push to restart reactors that were idled after the nuclear accident at Fukushima Daiichi in March. Kyushu Electric said that inspection work had been carried out on a valve of the condenser in question on Tuesday, raising the possibility that human error had triggered the shutdown.