Incineration of nerve gas rockets at the Umatilla Chemical Depot is on hold while workers study the cause of fires erupting in containment rooms where weapons are dismantled.

Three rockets have ignited while being sheared into pieces by automated machinery in sealed rooms walled with reinforced concrete. Monday, in the most recent incident, a fire burned for about ten minutes. Sprinklers and other suppression systems quenched each of the fires with no injuries or outside leak of deadly nerve agents, according to representatives of the U.S. Army and the private contractor for the facility.

But the fires should not have happened in the first place and have triggered an investigation into the likely causes.

Officials said incineration won't resume until investigators are done. The delay will depend on what investigators find and whether officials call for changes in equipment or procedures, said Army spokeswoman Mary Binder.

The depot, about six miles from Hermiston, is destroying tens of thousands of M55 rockets filled with GB, also known as sarin, a lethal nerve agent. Incineration of rockets and chemical agents began in September in a new facility that took years to build. Less than 2 percent of the total volume of nerve agents stockpiled at the depot has been destroyed.

Robotic equipment carries out the crucial stages of the work in containment rooms, which are built to withstand fires and explosions without leaking. Walls are made of reinforced concrete more than 28 inches thick, said Rick Kelley, a spokesman for Washington Group International, systems contractor at the facility.

At one station in the containment room, machines drill a series of holes in a rocket's casing and then pump the liquid nerve agent into a sealed storage tank. Batches of 500 gallons are piped into an incinerator for disposal.

At another station, an automated machine with a heavy V-shaped blade chops the rockets into eight pieces after they have been drained. The fires this month occurred in different containment rooms, but both erupted during the fifth cut at the shearing station. This is the first cut into the propellant section of the rockets, which holds solid fuel made of nitrocellulose and nitroglycerin, Binder said.

After the first fire, in November, officials tentatively assigned blame to friction from the chopping blade for sparking ignition. Washington Demilitarization Co., a subcontractor at the depot, said a nozzle spraying water on the blade to keep it cool was hit and dislocated by a stray piece of the rocket.

But after repeated fires, officials on Thursday said investigators are unsure of the cause. "Sometimes in these type incidents we are not able to determine the exact cause," Binder said. She said the Army remains confident in the overall process, however.

Six similar fires have flared during the shearing: three at Umatilla and one each at the incinerators in Utah, Alabama, and on Johnston Atoll southwest of Hawaii, Binder said.

Kelley said the incidents have proved the effectiveness of fire suppression systems at the depot. Water sprinklers deluged the burning rocket, the ventilation system cut off oxygen, and the fire died within 15 minutes, he said. At the adjacent station, a rocket being drained of nerve agent did not catch fire.

Damage is still being assessed. Kelley and Binder said it appears to be less than was caused by an April 7 fire. Workers then had to replace a fire sensor and some other minor parts. They managed to get the system operational in just over a week after that fire.

Kelley said investigators are going to take more time to compare the fires and whether the rockets might provide clues. In the two most recent fires, he said, the rockets both came from batches manufactured in 1962.

The Umatilla depot holds 7.4 million pounds of chemical weapons, about 12 percent of the nation's remaining stockpile. The weapons are scheduled to be destroyed by 2010 at an estimated cost of $2.4 billion.

Joe Rojas-Burke: 503-412-7073; joerojas@news.oregonian.com