The operator of Japan's ageing Hamaoka nuclear plant, located near a tectonic faultline southwest of Tokyo, has begun shutting down one of its two running reactors.
Chubu Electric Power started installing control rods into the number four reactor of the power plant early today, the first procedure in the operation, said Kazuhide Enoo, an official at the plant.
"We plan to stop electricity generation in the morning, and then the reactor is scheduled to be non-critical around noon today," Enoo said.
"So far procedures went smoothly as no problems were found," he said, adding that the reactor was expected to be in "cold shutdown" status "within a day at the earliest."
Prime Minister Naoto Kan last week called for the closure of the plant, eight weeks after a massive quake and tsunami damaged the Fukushima Daiichi nuclear plant northeast of Tokyo, sparking the world's worst atomic crisis since Chernobyl in 1986.
Seismologists have long warned that a major quake is overdue in the Tokai region southwest of Tokyo where the Hamaoka plant is located. It is only 200 kilometres from the capital, Tokyo.
The Hamaoka plant has five reactor units, but only two are currently running _ numbers four and five. Reactors one and two, built in the 1970s, were stopped in 2009, and three is undergoing maintenance.
The firm also plans to begin shutting down the number five reactor on Saturday.
Standard and Poors on Thursday lowered its ratings on Chubu Electric to `A+' from `AA-', leaving the outlook on the firm's long-term corporate credit rating negative.
"Chubu Electric's decision to suspend operations at the Hamaoka nuclear power plant will cause profitability and cash flow protection to deteriorate for at least two years," the ratings agency said.
"We expect downward pressure on the ratings will continue, particularly in measures of its financial profile," it said.
The Hamaoka plant accounts for almost 12 per cent of the output of Chubu Electric, which services part of Japan's industrial heartland, including many Toyota auto factories.
Kan has explained the plant should stay shut while a higher sea wall is built and other measures are taken to guard it against a major quake and tsunami. Local media said the suspension would last about two years.
Japan, the world's number three economy, endures 20 per cent of all major earthquakes and generates about 30 per cent of its power from nuclear plants.
The record March tremor and wave which battered Japan's northeast coast caused 11 of Japan's 55 nuclear reactors to automatically shut down, while triggering a major crisis at the Fukushima plant.