TOKYO — High levels of radiation were detected at the Fukushima No. 1 nuclear power plant Tuesday morning after a fire broke out near a pool in the No. 4 reactor where spent nuclear fuel is temporarily kept, Tokyo Electric Power Co. said.

TEPCO said radiation measuring 400 millisieverts (400,000 microsieverts) per hour was detected at 10:22 a.m. following the fire, which broke out at 9:38 a.m.

“There is no doubt (these radiation levels) may pose health risks to humans,” Chief Cabinet Secretary Yukio Edano told a news conference.

Earlier in the day, an explosion occurred at the No. 2 reactor at 6:14 a.m., leading to lower pressure in the suppression pool in the lower part of the reactor containment vessel.

Experts fear that a massive amount of radioactive material has leaked from the reactors after the series of accidents that may have damaged nuclear fuel rods.

A minute amount of radioactive material has been detected in Ibaraki and Tochigi prefectures and Tokyo on Tuesday, local governments said.

At a news conference Tuesday, Prime Minister Naoto Kan urged the roughly 136,000 residents within a 20- to 30-kilometer radius of the Fukushima power plant to stay indoors.

According to TEPCO and other sources, high levels of radiation were detected at multiple locations near the plant _ 30 millisieverts (30,000 microsieverts) per hour between the No. 2 and No. 3 reactors, 400 millisieverts (400,000 microsieverts) around the No. 3 reactor, and 100 millisieverts (100,000 microsieverts) near the No. 4 reactor.

“The levels are completely different from the microsievert figures we had announced previously,” Edano said. “These figures may cause health damage.”

Four-hundred millisieverts per hour can increase incidence of cancer among those exposed. The figure also is 400 times legal radiation limits citizens are normally allowed to be exposed to, except for medical purposes.

In the wake of the fire at the No. 4 reactor, TEPCO informed the central and the Fukushima prefectural governments about the incident.

It also sought cooperation from the Self-Defense Forces and U.S. forces to extinguish the fire.

However, it was confirmed later that the fire went out by itself.

TEPCO said the No. 4 reactor was out of operation for regular checks when the magnitude-9 earthquake hit the Tohoku region Friday.

However, the earthquake knocked out electricity to the reactor needed to circulate cooling water in the pool that temporarily stores spent nuclear fuel. As a result, residual heat from nuclear fuel rods raised the water temperature in the pool from the ordinary level of about 40 C to 85 C, TEPCO said.

“Lower water level in the pool exposed tubes (encasing the fuel rods), which reacted with steam, likely generating hydrogen and causing an explosion,” said Tetsuji Imanaka, assistant professor at Kyoto University Research Reactor Institute.

A total of 783 spent nuclear fuel rods were stored in the pool. At the Fukushima No. 1 nuclear power plant, three reactors _ Nos. 4, 5 and 6 _ were out of service for regular inspections. About 300 to 500 spent nuclear fuel rods are also kept at the Nos. 1, 2 and 3 reactors.

The explosion at the No. 2 reactor is believed to have damaged the suppression pool. The blast was heard at 6:14 a.m. at the No. 2 reactor, whose cooling functions had already been deteriorating.

Pressure inside its suppression pool dropped from the normal level of three atmospheres to one atmosphere, according to TEPCO.

TEPCO said there is a possibility the suppression pool may have been partially damaged and radioactive material may have leaked outside.

At 7:50 a.m., 1{ hours after the explosion, radiation of 1,941 microsieverts per hour was observed at the main gate of the No. 1 plant.

Forty minutes later, the level shot up to 8,217 microsieverts per hour, which is more than eight times the exposure limit considered to be healthy for one year.

On Tuesday morning, wind at 1.5 meters per second was blowing from the northeast near the nuclear power plant.

The main gate is on the northeast side of the No. 2 reactor, TEPCO said.

The University of Tokyo’s research institute in Tokaimura, Ibaraki Prefecture, located about 100 kilometers south of the power plant, detected radiation of more than 5 microsieverts per hour, the legal limit set under the Law on Special Measures Concerning Nuclear Emergency Preparedness.

According to TEPCO, the pressure suppression pool is designed to release and lower steam pressure if it rises in the reactor containment vessel.

The pressure inside the containment vessel remained unchanged at 7.3 atmospheres, according to TEPCO. According to the Economy, Trade and Industry Ministry’s Nuclear and Industrial Safety Agency, the suppression pool contains steam and water with radioactive material.

• MCT