Umatilla's chemical weapons incinerator resumed destroying rockets armed with the nerve agent sarin Thursday afternoon.

Oregon's Department of Environmental Quality had shut down the incinerator May 18, after a rocket exploded and burned as a machine chopped it into pieces -- the third such blast since April. The fires at the U.S. Army's Umatilla Chemical Agent Disposal Facility caused no injuries and little damage.

There are no workers in the room where the rockets are destroyed, which is designed to withstand occasional fires. But the series of blasts spurred DEQ's action and a nationwide U.S. Army investigation into fires at Umatilla and similar, less-frequent blasts at other arms incinerators.

Investigators have not found the fires' cause but think age may make explosive propellant in the rockets less stable. All three of Umatilla's fires involved propellant made in October 1962. On Wednesday, workers at the Army depot west of Hermiston finished removing propellant sections from nine rockets. Those will be shipped to New Jersey next week for testing by Army scientists, looking for clues to the blasts.

The DEQ decided it was safe to resume processing rockets without knowing what caused the fires and assumed more blasts are likely. Engineering reports indicated that the processing rooms could safely withstand more, and larger, blasts. Plant officials worked to limit fires by strengthening systems that cool the rocket-chopping blade and douse blazes.

Only time will tell whether those changes make fires less likely or less damaging, said Dennis Murphey, administrator of DEQ's Chemical Demilitarization Program.

Don Barclay, the Army's project manager, said workers are restarting gradually. They will first destroy 49 rockets from October 1962 already in the plant, then move on to rockets made at other times. The plant hopes to soon start high-speed processing tests required under the weapons furnace's DEQ permit, he said.