TOKYO >> Japan’s chief cabinet secretary says a hydrogen explosion has occurred at Unit 3 of Japan’s stricken Fukushima Dai-ichi nuclear plant. The blast was similar to an earlier one at a different unit of the facility.

Yukio Edano says people within a 12-mile radius were ordered inside following Monday’s explosion. AP journalists felt the explosion 30 miles away.

Edano says the reactor’s inner containment vessel holding nuclear rods is intact, allaying some fears of the risk to the environment and public.

The No. 3 Unit reactor had been under emergency watch for a possible explosion as pressure built up there following a hydrogen blast Saturday in the facility’s Unit 1.

TV footage showed a massive column of smoke belching from the Fukushima Dai-ichi nuclear plant’s No. 3 unit, about 125 miles north of Tokyo. Japanese officials said they believe it was a hydrogen explosion similar to an earlier one at a different unit in the facility. The problems at the plant stem from failed cooling systems caused by damage from Friday’s earthquake and tsunami.

More than 180,000 people have evacuated the area, and up to 160 may have been exposed to radiation.