Oregon officials ordered the U.S. Army to quit destroying rockets armed with nerve agent at the Umatilla Chemical Depot after a rocket exploded during processing Wednesday, the third such blast in six weeks.

No one has been hurt in any of the incidents, which happened inside explosive-containment rooms designed to withstand blasts and hold in nerve agent. No workers are in those rooms while machines automatically drain sarin, a nerve agent, from the rockets, chop them up and send the pieces to an incinerator.

The blast Wednesday occurred as a rocket, drained of most of its sarin, was chopped into pieces by an automated metal blade. The blade was cutting through a portion of the rocket containing propellant when ignition occurred, said Mary Binder, an Army spokeswoman.

Improved firefighting systems stopped the morning fire within about three seconds, Binder said. That's much faster than the 10 minutes it took to extinguish an April 25 fire. That fire, and one on April 7, both seem to have caused more damage to processing equipment than Wednesday's fire, Binder said, though workers still were assessing Wednesday's blaze.

Although weapons incinerators in other states have had similar fires, no facility has had as many as Umatilla. The depot near Hermiston had its first blaze in November, two months after starting up.

"We are concerned," said Sue Oliver, an official with the Oregon Department of Environmental Quality. "With three of these events in a short amount of time, we want to make sure that Washington Demilitarization Company and the Army have done everything possible to detect the cause."

Washington Demilitarization runs the incinerator for the Army, which stores chemical weapons at eight U.S. sites, including Umatilla. Those weapons are being destroyed under international treaty. Umatilla is starting with the sarin-tipped M55 rockets, the most hazardous in the Army's arsenal. Umatilla initially stored more than 220,000 arms. Workers had destroyed about 15,000 sarin rockets when Wednesday's fire began, Binder said.

But the Umatilla plant is barred from processing more rockets until it finishes "thorough investigations" into the fires' causes, according to the Wednesday DEQ letter that halted rocket processing. The facility also must show it has taken steps to prevent and limit damage from such fires, the letter said. Neither Binder nor Oliver could estimate how long that will take.

Processing had stopped for more than a week after each April fire. This pause, however, probably will be longer. The DEQ did not require investigations to be completed after those fires, and workers are studying the causes.

"At this point, we haven't ruled out anything," Binder said. "But from what we've seen . . . the people, procedures and design worked as planned" within the incinerator plant.

Investigators are focusing on the rockets, especially their explosive propellant. All three explosions happened as a metal blade made the fifth of seven cuts through rockets that have had most of their sarin drained out, Binder said. At that point, the blade is cutting the propellant.

Binder said the rockets in April's blasts shared propellant from the same production lot. She said crews had not yet determined the propellant lot for Wednesday's fire. A recent fire at an Arkansas plant involved propellant that was from a different lot but was made around the same time, she said.

Workers are preparing to sample propellant from other arms at Umatilla as they look for possible causes to the fires, Binder said.

If the propellant is the problem, Oliver said, it's possible that some fires are inevitable. But the DEQ wants the Army to determine the cause and reduce risks.

Andy Dworkin: 503-221-8239; andydworkin@news.oregonian.com