Oregon officials probably will let Umatilla's chemical weapons incinerator resume work Thursday, three weeks after a series of fires spurred regulators to stop the process.

Since April, three rockets have exploded and burned while being chopped to pieces at the U.S. Army's Umatilla Chemical Agent Disposal Facility west of Hermiston.

The blasts hurt no one and caused a few thousand dollars in equipment damage. They happened in reinforced, unmanned rooms where machines automatically drain sarin nerve agent from the rockets before chopping them into pieces that burn in a high-temperature furnace.

The plant was designed to withstand occasional rocket fires. However, engineers had estimated just one fire out of Umatilla's roughly 105,000 rockets, said Doug Hamrick, project manager with Washington Group International, the contractor that runs the plant. On May 18, after the third blast in six weeks, the Oregon Department of Environmental Quality suspended processing.

The DEQ wanted to check the plant, and Army officials were properly investigating the cause of the fires, their effect on safety and ways to limit future blasts, said Dennis Murphey, DEQ chemical demilitarization program administrator.

No root cause has been found. But DEQ workers have seen data suggesting the fires do not threaten people or the environment, and they have some confidence in work under way to prevent or manage future fires. Murphey said his staff has many reports to read, so he may not issue a "restart letter" until Thursday.

"That's my best estimate," Murphey said, although "obviously, something could come up" as the staff reviews reports.

Umatilla officials said they are ready to restart because they think it is safer than to wait, even if more fires occur.

"I have to expect this will continue. It would not be prudent for me to expect this will go away," said Don Barclay, the Army's manager for the project. "The key is, are we protecting our workers? Are we protecting the public? And we are."

The depot's chemical weapons are so deadly that both plant and DEQ officials say it's safer to burn the aging rockets than to store them.

"It's not like fine wine," Hamrick said. "It's not going to improve with age."

Workers are addressing the fires in several ways, according to Hamrick and Barclay. Engineers confirmed that the machinery and reinforced rooms used to process rockets can withstand more fires. Analysts calculated that more fires do not increase safety risks for workers or the public.

Crews took steps to limit fires, including adding more spray nozzles to cool the rocket-chopping blade and extinguish flames. Plant engineers are considering whether other steps could limit fires, such as cutting the rockets more slowly or in different places, Hamrick said.

Efforts to find the fires' cause continue. Umatilla has had four rockets catch fire since November, all while a blade chopped through their explosive propellant. The four other U.S. chemical arms incinerators have had fires at the same point. That has made the propellant the focus of investigations.

Starting Monday, workers in a secure room will remove the propellant and motor sections from nine sarin-armed rockets at Umatilla. After making sure the sections are not contaminated with nerve agent, they will be sent to an Army lab in New Jersey for testing.

Scientists there will look for chemical or physical changes that could make fires more likely. That includes testing whether a lot of propellant made in October 1962 -- involved in Umatilla's three recent fires -- is especially prone to explode. Those tests should take roughly six weeks.