Japanese authorities have confirmed there was an explosion at the Fukushima No. 1 nuclear power plant Saturday afternoon but it did not occur at its troubled No. 1 reactor, top government spokesman Yukio Edano said.

The chief Cabinet secretary also told an urgent press conference that the operator, Tokyo Electric Power Co., has confirmed there is no damage to the steel container housing the reactor.

Edano said the 3:36 p.m. explosion resulted in the roof and the walls of the building housing the reactor’s container being blown away.

The authorities expanded an evacuation area for all local residents from a 10-kilometer radius of the Fukushima No. 1 and No. 2 plants to a 20-km radius.

Officials of Japan’s nuclear safety agency also said after examination that they believe there has been no serious damage to the container of the No. 1 reactor, judging from the latest radiation data monitored around the facility.

The incident came after the plant lost its cooling functions after it was jolted by a magnitude 8.8 earthquake Friday and radioactive substances of cesium and iodine were detected near the facility Saturday.

The detection of the materials, which are created following atomic fission, led Japan’s nuclear safety agency to admit the reactor has been partially melting—the first such case in Japan.

A partial core meltdown also occurred in a major nuclear accident at Three Mile Island in the United States in 1979. About 45 percent of nuclear fuel was melted in the incident, causing radioactive materials to be released.

According to the Fukushima prefectural government, the hourly radiation from the Fukushima plant reached 1,015 micro sievert in its premises before the explosion, an amount equivalent to that allowable for ordinary people in one year.

Four workers—two from the company and two others from another firm—were injured in the explosion, according to Tokyo Electric Power. The four were working to deal with problems caused by a powerful earthquake that hit northeastern Japan on Friday, it said.

The company said the injuries the four have suffered are not life-threatening and that they are conscious.

The operator of the quake-hit nuclear plants in Fukushima Prefecture, successfully released pressure in the container of housing one of its reactors to prevent a nuclear meltdown, the Nuclear and Industrial Safety Agency said.

Even before Tokyo Electric Power succeeded in reducing the pressure, which would involve the release of steam that would likely include radioactive materials, radiation had risen to an unusually high level in and near the No. 1 nuclear plant.

Work to depressurize the containers, aimed at preventing the plants from sustaining damage and losing their critical containment function, has been conducted under an unprecedented government order.

The agency said the core at the No. 1 reactor of the No. 1 plant may be partially melting, and the work to depressurize the container was necessary to prevent the container from sustaining damage and losing its critical containment function.

The agency said that as a result of reducing the container’s pressure radioactive levels at the plant went up. The depressurizing work involves the release of steam including radioactive materials.

But the agency denied that the radiation amount will pose an immediate threat to the health of nearby residents, as wind is currently blowing toward the sea in the northeastern Japan prefecture on the Pacific coast.

At the No. 1 plant, the amount of radiation reached around 1,000 times the normal level in the control room of the No. 1 reactor, and 70 times the normal level near the main gate of the plant.

It was the first time an external radioactive leak had been confirmed since the disaster.