Oregon state officials yesterday ordered a halt to the processing of nerve-gas-filled rockets at the Army's Umatilla Chemical Depot because of a troubling series of fires that broke out in the facility during the past month.

The three fires, the latest of which briefly flared yesterday for about three seconds, occurred in a sealed-off processing room.

None injured any workers or leaked nerve gas into the broader plant or outside, according to the state and a spokeswoman for the depot in Eastern Oregon.

The fires fed on rocket fuel rather than the sarin-nerve agent that, for the most part, already had been drained out.

But the frequency of the fires and the intensity of the burns prompted state environmental regulators to order a shutdown that could last for days, weeks or possibly months as Army officials and contractors try to figure out what is happening.

"A fire is a contingency that is of great concern when you are processing chemical weapons," said Dennis Murphy, an Oregon Department of Environmental Quality official who monitors the Umatilla Chemical Depot.

"And though so far the [processing] room has done an excellent job, it wasn't designed to do that over and over," he said.

The first two fires, which occurred April 7 and April 23, both lasted for more than a minute and burned with great intensity before an automated water system helped snuff them out, Murphy said.

Depot officials say they are working hard to try to determine the cause of the fires.

"We strive for 100 percent [safety]," said Mary Binder, a depot spokeswoman. "And, when we don't reach that, we need to stop and take a look at what we're doing. ... We want to do it right."

The Umatilla fires are happening with a greater frequency than those at other facilities processing nerve-gas rockets. In Utah, for example, only one fire has been reported since operations began in 1996, Murphy said.

The Umatilla chemical incinerator complex near Hermiston is a $2.5 billion project designed to destroy more than 200,000 rockets, bombs, mines and other items that hold nerve agents and mustard gas.

Rockets are drained of nerve agents, then burned along with the chemicals in four separate furnaces.

Since the complex opened last summer, the Army has voluntarily shut it down on several occasions to troubleshoot problems, including the two April fires.

This is the first time since startup the state has stepped in to order a shutdown.

Murphy said no one is sure what's triggering the fires. One theory is that the depot is now processing a batch of rockets that have deteriorated and become more unstable.

Since April, the depot has improved its remote-controlled watering system in the containment room, and that system was credited with quickly snuffing out yesterday's fire.

Hal Bernton: 206-464-2581 or hbernton@seattletimes.com