TOKYO: A second nuclear reactor has begun working in Japan, officials said on Thursday, the day after its operator was ordered to examine a possible active tectonic fault directly under the plant.

Kansai Electric Power Co (KEPCO), which runs the Oi power plant in the nation’s industrial heartland, said it switched on Unit No 4 late on Wednesday, following the restart of Unit No 3 reactor earlier this month.

On Wednesday Japan’s nuclear watchdog ordered KEPCO to probe claims that the plant lay on one of the many fault lines that riddle the earthquake-prone country.

The restart comes after tens of thousands of people rallied in Tokyo on Monday demanding an end to nuclear power, the latest in a series of anti-atomic gatherings following the tsunami-sparked disaster at Fukushima last year.

The restarts at Oi ended two months in which Japan was nuclear-free after reactors across the country were shuttered in the aftermath of the Fukushima meltdowns.

In mid-June, Prime Minister Yoshihiko Noda gave the green light to restart two reactors amid concerns about looming power shortages this summer.

The government has asked business and households to cut back on their power usage by as much as 15 percent from summer levels two years ago.

With the Oi restarts, KEPCO is expected to be able to meet demand for power, even as electricity usage peaks with the increased use of air conditioners over Japan’s hot and humid summer, a KEPCO spokesman said.