Water containing a small amount of radioactivity has leaked from a nuclear power generation unit in Japan, owner Kansai Electric Power said on Tuesday, adding to a long line of problems in the tarnished industry.

Kansai Electric Power Co Inc, Japan's second-biggest utility, said 3.4 tonnes of water had leaked from the 1,175-megawatt No.1 generating unit at its Ohi power station, but none had made it to the environment and it would stop generating electricity from the affected unit by around 11 p.m. (1400 GMT)

The problem at the Ohi plant, in Fukui prefecture on the Sea of Japan coast around 320 km (200 miles) west of Tokyo, follows years of scandals in Japan's nuclear industry involving cover-ups and fudged safety records that have tarnished public faith in the sector.

Only this year, Kansai Electric restarted commercial operations at another nuclear power unit following Japan's worst-ever nuclear plant accident more than two years ago, in which five workers were killed after being sprayed with steam and hot water from a broken pipe.

Kansai said it had discovered the problem at its Ohi plant on Monday evening and the leak was stopped that evening.

It said it would begin inspection of the affected unit after manually shutting down the plant around 12:30 a.m. on Wednesday (1530 GMT Tuesday).

A Kansai spokesman said the shutdown will not have an immediate impact on the firm's power supply in September. He said Kansai still had supply capacity of 32,000 megawatts after the shutdown of the unit, an 8.5 percent surplus over expected peak demand.

The spokesman did not know how long the shutdown would last.

The power output loss will be offset mainly by firing thermal power plants, the spokesman said, but the company did not know how much its fuel purchases would increase as a result.

The extended shutdown of the No.1 unit could hurt the firm's profits for the business year to next March.

In July, Kansai forecast its nuclear power plants to operate at an average 80.5 percent of their capacity in 2007/08. Every 1 percentage point fluctuation in the nuclear run rate would affect costs such as fuel by 6.4 billion yen ($55.28 million), Kansai said.

In July, there was a minor radiation leak to the environment at the world's biggest nuclear power station, Tokyo Electric Power Co.'s Kashiwazaki-Kariwa plant, after a major earthquake.

TEPCO, which services the Tokyo area, was forced to indefinitely shut down the plant, causing it to struggle to provide enough power to Tokyo during the sweltering, humid summer months when electricity consumption soars.