TOKYO - Japan on Monday announced a tiny radiation leak at a nuclear reactor on its west coast, while another power company is expected to close a nuclear plant in central Japan due to its vulnerability to a major quake.

Electricity wholesaler Japan Atomic Power said it had stopped the leak after a minute amount of radiation was found in gas that escaped from the Tsuruga nuclear plant on Sunday. It said there had been no impact on the environment.

Japan Atomic said it was investigating the cause of the leak, which is bound to increase public unease over the troubled nuclear industry in the wake of the massive earthquake and tsunami that crippled the Fukushima nuclear facility in March.

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Japanese media reports said the country's third biggest electricity producer, Chubu Electric Power Co , was expected later in the day to decide to temporarily shut one of its nuclear plants on the coast of central Japan until it can build a tsunami wall and take other steps to improve safety.

That could take two years, raising the risk of a shortage of electricity after the Fukushima nuclear plant was hit by the March 11 earthquake and tsunami in the northeast.

Prime Minister Naoto Kan has urged Chubu to close the Hamaoka plant, about 200 km (120 miles) southwest of Tokyo, signalling a potential shift in energy policy following the worst nuclear crisis in 25 years.

Chubu Electric Power Co shares tumbled as much as 14 percent on the request. Chubu was due to hold a board meeting at 0630 GMT at which it was expected to make a final decision to close the plant, Japanese media said.

A board meeting on Saturday had failed to reach a consensus, reflecting worries over whether the company would be able to get alternative supplies of energy as well as cope with a sharp jump in its fuel costs.

The amount of radiation at Tsuruga was estimated at about one-four hundred thousandth of the annual legal limit, a Japan Atomic spokesman said, and there were no changes in readings from radiation monitoring devices placed around the plant.  

The government is under pressure to review its energy policy, of which atomic power is a major part, after the March 11 quake and tsunami wrecked the Fukushima nuclear power plant run by Tokyo Electric Power .

Nearly 26,000 people were killed or are unaccounted for following the natural disaster which triggered the world's biggest nuclear crisis since Chernobyl in 1986. The plant is still leaking radiation.

Government experts put the chance of a magnitude 8.0 quake hitting the Hamaoka area in the next 30 years at 87 percent, which has raised questions over why it was built there in the first place.

Shares of Chubu were down 11 percent at 1,570 yen in afternoon trade, after falling as low as 1,521 yen. Chubu's tumble helped push Tokyo's electric and gas subindex down 2.7 percent.

"This news is triggering uncertainty not just about Chubu Electric but the whole utility sector," said Yoshinori Nagano, a senior strategist at Daiwa Asset Management.

"Investors are concerned that on the back of this news other reactors currently under inspection may not resume operations soon."

Kan, under fire for his response to the crisis in northeast Japan after the March quake, said the government would try to prevent the halt of the Hamaoka reactors from causing power supply problems.

Chubu has said it can meet peak demand of 25,600 MW even if Hamaoka shuts. But relying on thermal plants to fill the power gap would push up costs by 700 million yen ($8.7 million) a day -- or about 256 billion yen a year, double its projected profit of 130 billion yen in the year to March 2012.

However, an unusually hot summer would raise the risk of Chubu not having enough capacity to meet peak demand, which could cause problems for Toyota Motor Corp and other major manufacturers with factories in the region.

Chubu generates a relatively small proportion of its power from nuclear plants, accounting for 14 percent of the firm's total electricity generated in 2009/10. That was below a 30 percent average nationwide, while gas thermal power held a 47 percent share.