TOKYO (AP) - A nuclear reactor at a plant in northern Japan was shut down Sunday after an unusually high level of radiation was detected in its cooling water.

Radiation did not leak into the environment, and the level of radiation, which was about 300 times the normal level, was still within safety limits, said Ichiro Kudo, spokesman for Tokyo Electric Power Co., the plant's utility company.

The No. 6 reactor, one of seven reactors at the Kashiwazaki-Kariwa plant, was being shut down manually, Kudo said.

A similar high level of radiation was detected momentarily last August. The plant, 130 miles northwest of Tokyo, will be checked for a possible fuel leak as a precautionary measure, he said.

Japan's worst nuclear accident, on Sept. 30 at a fuel-processing plant in Tokaimura, 70 miles northeast of Tokyo, took the lives of two workers.