TOKYO (REUTERS) - Japan has rebooted extra power plants, including a long-dormant nuclear reactor, and taken other steps to avoid a power crisis as temperatures soar and demand for cooling surges, especially in Tokyo, where the Olympics begin on Friday (July 23).

With the world's eyes on Tokyo as it hosts the Summer Games amid worries over risks posed by the coronavirus pandemic, Japan wants to avoid a recurrence of the electricity crisis suffered during the winter, when power companies urged customers to ration usage to prevent blackouts.

Earlier this month, Kansai Electric Power restarted another reactor, the fifth to be restarted in Japan since January, including a 44-year-old unit that had been shut for ten years.

Japan has nine reactors operating, the highest number since the Fukushima atomic disaster led to the shutdown of Japan's nuclear industry.

A gas-fired plant and a coal plant have also been brought online ahead of schedule.

The increased generating capacity gave the industry ministry confidence that electricity supplies would be sufficient.

"We are not worried about power supply during the summer as the restarts of extra power plants have boosted capacity," Yuri Ito, deputy director at the industry ministry's office of electricity supply policy, told Reuters.

Japan's power grid has faced periods of intense strain in the decade since the Fukushima disaster, and the government has introduced more competition into the sector and more renewables into the mix.

But Japan still relies heavily on fossil fuels, especially liquefied natural gas (LNG) and coal, which have to be imported.

Adding to pressures on the sector, many old oil and coal power plants are being shut down in Japan as liberalisation forces cost cuts, and pressure to reduce CO2 emissions makes them unviable.

The industry ministry warned in May of the risk of power crunches during peak summer demand and told utilities to secure adequate generation capacity along with LNG stockpiles.

LNG inventories were around 2 million tonnes by the end of June, about 100,000 tonnes above the average of the past four years, Ito said.

As summer progressed, some tightening in supply was evident as spot electricity prices for the Tokyo area have doubled this month.

On Tuesday, the country's weather bureau issued heat-stroke alerts for a fourth consecutive day. By midday, the temperature in Tokyo was 34 degrees Celsius, and there are forecasts for a hotter than usual summer.

Japan's grid monitor (OCCTO) estimates excess generation capacity in the Tokyo area at 5 per cent for July and 3.9 per cent for August.

Excess capacity below 3 per cent could lead to supply shortages and possibly even blackouts.

OCCTO has started a new monitoring programme using LNG inventory data from major generators to gauge how much electricity supply could be available at peak demand periods, and help to anticipate fuel shortages.

"It's still a trial phase, but we want to improve its accuracy and clarity before the more serious winter season," an OCCTO official said.