TOKYO - A negligible amount of radioactive steam released at a nuclear plant in northern Japan escaped outside the compound, but there is no fear of harm to the environment, operator Tokyo Electric Power Co. said Friday.

An increase in the level of tritium was detected during an air sampling outside the plant on Sunday, and the operator later found that steam containing radioactive material was leaking from the Fukushima Daiichi Nuclear Power Station, about 240 kilometers (150 miles) northeast of Tokyo, the company said.

The amount of radiation, however, was negligible and would not harm the environment, it said.

The plant shut down the boiler which was leaking the steam on Friday, the company said.

The plant is currently investigating the cause of the leak, it said.

Tritium is a radioactive isotope of the element hydrogen and is produced in nuclear reactors.

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