Many nuclear reactors in Japan remain shut after the March 11 earthquake and tsunami [Reuters]

A Japanese power firm said it would halt operations at a nuclear reactor because of a technical failure, placing further strain on the country's power supply.

Kansai Electric Power Co said it would manually shut down reactor Number 1 at its Ooi plant in central Japan on Saturday because of a temporary pressure drop in a standby tank.

The tank contains boric acid solution that can be pumped in to slow nuclear fission in case of emergency.

Pressure in the tank had already returned to the correct level, but the company decided to shut down the reactor "to give the top priority to safety and find out the cause", a company spokesman said.

The company did not yet know when the reactor would resume operations, but there had been no radiation leak, he said.

The stalled reactor is one of four at the Ooi plant and has a capacity of 1.18 million kilowatts. It provides four per cent of the power generated by the company, according to Jiji Press news agency.

Power capacity

Japan is already sweltering in the summer heat as it seeks to save electricity and avoid blackouts, with many nuclear reactors remaining shut after the March 11 earthquake and tsunami.

Public sentiment flared up against nuclear power after the tsunami tore into the Fukushima Daiichi plant, triggering reactor meltdowns, radiation leaks and the evacuation of tens of thousands of people.

Utilities not directly affected by the twin disasters have held back from restarting reactors that were under maintenance at the time or entered regular check-ups after the disaster. As a result, only 19 of Japan's 54 reactors are now operating.

The Ooi shutdown - to be completed on Saturday evening - will bring the number to 18, with further reactors soon due to shut down for regular checks.