Two workers at the Umatilla Chemical Depot have been exposed to nerve agent but were not injured, an Oregon Department of Environmental Quality official said Thursday.

The leak occurred Monday in a corridor outside a sealed area for handling equipment contaminated with GB, the deadly nerve agent also called sarin. One worker was in the corridor when a monitor detected the agent and sounded an alarm, then another worker entered the corridor.

Normal operations halted as workers put on protective masks and waited for an all-clear, said Sue Oliver, a DEQ official in Hermiston.

The two workers showed no symptoms of GB poisoning, said Mary Binder, a U.S. Army spokeswoman. An examination and blood tests revealed no signs of exposure, Binder said, adding that they returned to work. Blood tests repeated Tuesday were negative.

Investigators are trying to find out how the nerve agent migrated into the corridor. Workers wearing protective suits inside the sealed area were using a sampling device that draws air into a hose leading to a monitor in the corridor.

Binder said investigators are examining that equipment to see whether it malfunctioned and leaked the nerve agent. Officials have stopped use of the area until they figure out what went wrong.

Last fall, shortly after depot operations began, two workers were exposed, but not injured, when they walked into a contaminated area wearing inadequate protective gear.

The depot had resumed limited dismantling of nerve gas rockets last week while continuing to investigate the causes of fires in heavily shielded containment rooms. Since operations began in September, three rockets have burst into flames while automated equipment sheared them into pieces inside containment rooms. The fires were quenched with no injuries or outside leak of nerve agents.

Officials resumed dismantling work on May 5 with limits and additional safety measures, but they stopped Monday to service equipment. Binder said destruction of rockets could start again before the end of the week.

The depot, about six miles from Hermiston, is destroying tens of thousands of M55 rockets filled with GB, along with other chemical weapons.

The Umatilla depot holds 7.4 million pounds of chemical agents, about 12 percent of the nation's original 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