A spokesman said he could not say when the company would restart the plants, which supply 40 percent of the power to the Tokyo area - a region of more than 37 million people.

Demand for power in Tokyo peaks in summer when households and offices switch on air conditioners to cope with stifling heat and humidity, adding to demand from heavy industry such as steel and car plants as well as the city's famous montage of neon signs.

Whether those air conditioners or signs will have to be switched off to save power remains to be seen.

"We are conducting the necessary safety and maintenance checks, as well as asking for the understanding of the local people...it is still too early for us to be talking about when or which nuclear reactor we can restart," the spokesman said.

Company President Tsunehisa Katsumata said in a statement: "We are facing an unprecedented situation due to a series of scandals, and we view this as a very serious situation.

"We will do everything within our power to recover the trust of the public and of those people living in the area."

TEPCO is facing opposition to restarting the plants from suspicious local residents who are distrustful of the nuclear industry after a series of scandals.

The plants are located far from Tokyo but supply power only to the Tokyo region, meaning locals would not suffer from power shortages if the plants stayed shut.

RACE AGAINST THE CLOCK

TEPCO has turned to thermal power plants, which use fossil fuel, to fill the supply shortage caused by the closure of the nuclear plants, but the company has said about 10 of its nuclear reactors must be back on line if it is to meet summer demand.

The company will be racing the clock.

Katsumata said last month that Tokyo, as well as Yokohama, Japan's second biggest city, could face shortages this summer.

"Without the restart of nuclear reactors, there will be a major power supply shortage in the summer," he said.

TEPCO's choices are limited.

One of its 17 reactors will remain shut for one year after a government order issued in late October as punishment for what was considered the most serious offence.

At this plant, TEPCO admitted that personnel had manipulated the air pressure of reactor containers to pass safety tests. In other instances, the company did not disclose the existence of cracks in the shrouds that cover the reactors.

Of the remaining 16 units, four will be closed beyond July due to scheduled maintenance checks.

Theoretically, this leaves just 12 units that TEPCO can begin restarting to cover summer demand.

The last major power outage to hit its service area occurred in July 1987 when some of its facilities were unable to keep up with a sudden surge in electricity demand. The power failure affected 2.8 million homes and lasted for a few hours.

There have only been minor power failures since then.

RISING FUEL COST

The suspension of reactor operations, which forced the company to rely on thermal plants, has added to TEPCO's fuel procurement costs at a time of high crude prices.

TEPCO, already the world's largest buyer of liquefied natural gas, has stepped up purchases of crude oil and fuel oil.

Thermal plants are more expensive to operate than nuclear reactors, and the company has said daily generating costs rise by about 100 million yen ($830,000) if a one million kilowatt nuclear reactor is closed and replaced by a thermal plant.

TEPCO has said that extra fuel procurement costs amounted to 140 billion yen ($1.2 billion) in the year ended March.

Public confidence in Japan's nuclear industry has been badly bruised by a series of scandals to hit the industry, including Japan's worst nuclear accident which occurred in September 1999.

In this incident, two workers at a uranium processing plant at Tokaimura, northeast of Tokyo, died and hundreds of residents, plant workers and emergency personnel were exposed to radiation.

Japan, the world's second biggest economy, has virtually no domestic sources of crude oil or coal and relies on nuclear power for more than 30 percent of its power needs.