TOKYO—Another Japanese nuclear reactor was taken off line for maintenance on Monday, leaving the country with only one of its 54 reactors operational following last year's devastating earthquake and tsunami.

The last reactor is expected to be shut down by early May, raising the possibility of power shortages across the nation as demand increases in the hot summer months.

The No. 6 reactor at the Kashiwazaki-Kariwa complex was taken off line early Monday by the Tokyo Electric Power Co. The utility also runs the plant in Fukushima, northeast of Tokyo, that suffered meltdowns, explosions and radiation leaks after the March 11 quake and tsunami.

Japanese reactors are taken off line every 13 months for regular checks. With concerns over nuclear safety high following the Fukushima crisis, none of the reactors that have been shut down for checks, and none that were already off line at the time of the disaster, have been allowed to restart.

The last reactor, on the northern island of Hokkaido, will be shut down in May. The timing for when any reactors will be restarted remains unclear.

Before the crisis, Japan depended on nuclear power for one-third of its electricity. Japan's government wants to restart reactors as soon as "stress tests" prove they are safe, but faces strong public opposition. Local leaders, fearing a political backlash, are reluctant to give their approval.

Authorities have required all reactors to undergo the stress tests and make necessary modifications to improve safety. The stress tests, similar to those used in France and elsewhere in Europe, are designed to assess how well the plants can withstand earthquakes, tsunamis, storms, loss of power and other crises.

Prime Minister Yoshihiko Noda has promised to reduce Japan's reliance on nuclear power over time and plans to lay out a new energy policy by the summer.

In the meantime, Japan has temporarily turned to oil and coal generation plants to make up for the shortfall, and businesses have been required to reduce electricity use to help with conservation efforts.

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