HERMISTON -- Workers on Wednesday fed a rocket armed with 10 pounds of sarin nerve agent into machinery designed to burn it clean -- a long-practiced debut almost derailed by the careless press of a button.

The M-55 rocket was the first of 220,000-plus chemical weapons to be destroyed in the $395 million incinerator complex at the Army's Umatilla Chemical Depot. When workers hand-loaded the missile on the plant's conveyor line at 9:25 a.m., Umatilla became the fourth of five planned U.S. chemical weapons incinerators.

"Today, there's one less rocket that can harm the public," Don Barclay, the Army's site project manager, told about 30 area residents who gathered at an open house celebration in Hermiston.

But Wednesday's startup -- held up for years by legal and logistic concerns -- was delayed after a worker inadvertently pushed a button that prevented the rocket from dropping through a trapdoor into the incinerator.

"I was hoping for a flawless run today," Barclay said. "But I've operated plants since 1984. And rarely have I seen them operate flawlessly."

The process begins after the nerve agent is drained, and a large blade chops the rocket seven times. The resulting eight pieces should pass, in ones or twos, through a trapdoor into the furnace, which burns away the nerve agent. But about 10:30 a.m., after the blade had severed the rocket's conical head, trouble hit: The nose cone lay on the trap door. The door refused to budge.

Plant managers downplayed the glitch as a routine delay, as crews struggled to find the source of the problem. Eventually, they wired the stuck door to another power source and got the gate to open. The rocket finally finished processing at 3 p.m., 51/2 hours after it started. It should take less than a half-hour.

Only after that, when safety-suited maintenance workers entered the blast-proof rocket processing room, did they find the source of the problem. The night before, a worker accidentally hit a big, red emergency button, shutting the trapdoor, said Rick Kelley, spokesman for the plant's contractor, Washington Demilitarization Group.

"Apparently, somebody inadvertently hit the stop last night in that room," Kelley said. "The system worked, exactly the way it was designed. Somebody hit it (the button). We just didn't reset it."

Army leaders had said they were happy even when the plant seemed to have mechanical problems.

"We expect these types of things to happen, dealing with machines," said Lt. Col. David "Doc" Holliday, the depot's commander. The plant holds 819 miles of wire and 18,000 instruments, and with so many complex machines, he said, "you expect to have delays. The process we're using is safe. The munitions are secure. And we're taking our time."

Barclay said the delay did accentuate some tension in the plant's control room, but workers responded quickly and methodically to track and solve the problem.

The start of chemical arms incineration actually delayed efforts to uncover the problem, Barclay added. Before Wednesday, maintenance crews could have walked right into the blast-resistant processing room and spotted the problem in five minutes, he said. But once nerve agent entered the room, maintenance workers must enter with full protective suits and masks, for limited time periods. That will be the case until the plant finishes its work, which Army officials say could take six years.

Barclay said the stuck trapdoor never threatened safety because the rocket was locked away from people behind 30-inch, steel-and-concrete walls. "While it's unexpected and unusual, it doesn't rise up to the level of something bad happening," he said.

Workers planned to spend Wednesday night reviewing data from the first rocket's destruction, Barclay said. If all looks good, the plant will process one or two rockets today, and perhaps two each on Saturday and Sunday.

After the rocket was processed, many area residents praised the move -- though some worry that the incinerator may release unhealthy levels of pollution.

Paul Cook, one of the two workers to handle the rocket Wednesday, said he backs the weapons' destruction: "I'd rather have them gone than sticking around here another 50 years."

Researcher Kathleen Blythe contributed to this report.

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