ALVARADO, Texas (AP) _ A munitions plant blew up, leveling a concrete storage bunker, sending a fireball into the sky and spewing chunks of concrete and light explosive charges for hundreds of yards.

Smaller explosions followed the main blast just before midnight, and Johnson County Sheriff Eddy Boggs said today that officials planned to let the explosions continue before trying to enter the plant.

''We're just going to let the fire burn itself out and the explosions run their course,'' he said.

The Goex ammunitions plant, which makes explosives for both domestic and military use, was closed at the time, and no one was injured, he said. The plant is in a secluded area about four miles east of Alvarado.

He said the plant, a satellite of a larger operation at nearby Cleburne, made Class A explosives, including charges, hand grenade fuses, and plastic explosives.

''No one knows what set it off,'' the sheriff said.

He said the blast appeared to have occurred in a partially buried concrete storage bunker.

Firefighters from several communities were on the scene and had grass fires that sprang up under control in less than an hour, authorities said.

Residents of nearby towns said they felt the explosion and could see the fire from the blast. Homeowners miles around reported the blast blew out their windows.

Alvarado is 25 miles south of Fort Worth.

Alvarado police officer Eddy Puckett was on patrol when the bunker blew up.

''It was like daylight. There was a fireball in the sky,'' he said. ''After the flash there was a mushroom cloud. Then the boom hit.''