TOKYO, FEB. 18 -- The emergency shutdown of a Japanese nuclear power plant earlier this month rates at the bottom -- or least dangerous -- end of the international scale of nuclear incidents, the government said today.

The Ministry of International Trade and Industry said the Feb. 9 incident at the Mihama No. 2 plant, on a cape jutting into the Sea of Japan about 220 miles west of Tokyo, would be rated as a 1 or 2 on the International Energy Association's scale of nuclear accidents.

In comparison, the Three Mile Island accident in the United States rated a 5 and the Chernobyl disaster in the Soviet Union was a 7 on the same scale, the agency said. The 1987 shutdown at VEPCO's North Anna plant in Virginia, which had some similarities to the Mihama incident, rated a 3 on the scale, the energy agency said.

The Japanese government said today that it would convene a committee to investigate the Mihama incident, and the government ministry announced new guidelines for nuclear plant operators requiring closer checks of operating conditions and set new rules for mandatory shutdown of reactors.

The government and Kansai Electric Co., the Osaka-based utility that owns Mihama, have issued frequent assurances that the incident was not dangerous.

The incident began when a metal tube about as big around as a thumb split open in the steam generator of the Mihama plant, a pressurized water reactor similar to many in the United States. Operators noticed an increase in radioactivity, but did not shut down the plant. After about 50 minutes, the plant's emergency core cooling system automatically shut off the nuclear reaction and flooded the plant's nuclear fuel rods.

The government said a "small" amount of radioactive material was released into the atmosphere and that emission tests around the site have found no dangerous levels of radioactivity.

Because the emergency shutdown system at Mihama worked as designed and there was no significant risk of core damage, the incident would be rated no higher than a 2 on the International Energy Association's scale, said Ryuko Fujii, a trade ministry official.

Fujii would not predict how long the Mihama No. 2 plant would be shut down.