Nitroglycerine + high pressure = boom.

That's the U.S. Army's working equation on what caused several rockets to catch fire in chemical weapons incinerators, including three recent fires at the Umatilla Chemical Depot near Hermiston. Those fires caused only minor damage but halted rocket-destruction work for weeks this spring while crews looked for a source of the fires.

The investigation centers on solid explosive propellant in the M55 rockets, especially a lot made in October 1962. All three Umatilla blasts occurred as a steel blade chopped through the propellant section of rockets from that lot. Machines chop the rockets into pieces and feed them into a furnace after draining sarin nerve gas from them.

Umatilla workers sent October 1962-lot motor sections from nine rockets to Army explosives experts in New Jersey for tests. Those tests showed that nitroglycerine had slowly leaked from the propellant in all nine cases, Army engineers said Thursday. The propellant is mostly a mix of two explosives, nitrocellulose and nitroglycerine, with a stabilizer meant to hinder the eventual chemical breakdown of the explosives.

Liquid nitroglycerine leaching from the propellant can become trapped between the propellant and the membrane surrounding it, Army spokeswoman Mary Binder said. If the blade chopping the rocket happens to hit a blister of that pressure-sensitive explosive, it could explode and set the rocket on fire.

"This has yet to be confirmed as the root cause," Binder said. Crews in New Jersey are doing more tests to confirm that theory.

They also are checking to see whether the roving nitroglycerine poses other threats. Tests so far indicate that "the rockets are safe in storage, transport and handling," the Army said. A final report is expected in August.

Umatilla has destroyed roughly a quarter of the 91,400 sarin-armed M55 rockets the Army stored at the depot. Since the incinerators restarted in June, several hundred rockets from the October 1962 lot have been destroyed without any fires, Binder said.

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