The Army plans to destroy 10 decades-old chemical weapon warheads found between 2009 and 2012 at a Schofield Barracks firing range — the last of such weaponry discovered during clearance work for a Stryker vehicle "Battle Area Complex" training area.

As part of the $2.5 million effort, the Army brought out a portable Explosive Destruction System from Aberdeen Proving Ground in Maryland to destroy nine old phosgene rounds and one with chloropicrin and other agents.

The weapons are believed to be World War I/World War II-era, and their destruction follows a $7 million effort in 2008 to neutralize 71 chemical rounds in what was then the largest concentration of unexploded, or "dud," chemical weapons ever found in the United States.

The Army at the time said it had no explanation for the large number of chemical weapons found at Schofield.

"The Army no longer uses these types of munitions," Col. Richard Fromm, commander of U.S. Army Garrison-Hawaii, said of the additional weapons to be destroyed. "However, as a matter of safety and environmental stewardship, we have a responsibility to ensure that when we find these types of historic munitions, we also safely destroy them."

According to the Federation of American Scientists, phosgene, a choking agent, was used for the first time in 1915, and it accounted for 80 percent of all chemical fatalities during World War I.

One round at Schofield was found to have CNS, a tearing agent cocktail of chloroacetophenone, chloropicrin and chloroform that would cause a victim to gasp for air, the Compendium of Chemical Warfare Agents said.

Five of the rounds are 75 mm, two are 155 mm and three are known as Stokes mortars, officials said.

The Army provided a media tour Thursday of the temporary site set up on the backside of Schofield near a firing range.

Extensive measures are being taken to safely handle and destroy the chemical weapons, including having a ventilation system in the Quonset tent that pulls air in from the outside and sends it through a filter before being released, officials said.

A joint team of about 20 from the Recovered Chemical Materiel Directorate of the U.S. Army Chemical Materials Activity and the Army’s Edgewood Chemical Biological Center is destroying the munitions.

Cameras inside the Quonset tent provide a feed to nearby trailers so personnel can monitor what’s happening with the munitions.

At the center of the operation is an 18,000-pound steel containment chamber with a bank vault-like door that’s 9 inches thick. The chemical rounds are placed within a much smaller cylinder with three-quarter-inch-thick steel that also contains shaped charges, and the package goes into the bigger chamber for detonation, said site manager Rob Snyder.

The shaped charges cut like a knife into the chemical rounds and destroy any explosives, and what’s left gets washed for an hour or more with a pumped-in reagent that neutralizes the chemicals in the weapon, Snyder said.

The actual destruction of the shells is expected to occur over two weeks starting April 27, officials said.

All waste from the operation will be shipped to the mainland, the Army said.

The United States stopped producing chemical weapons in 1969, said Karen Jolley, a spokes­woman for the Recovered Chemical Materiel Directorate. In 1997 the decision was made to destroy existing stockpiles of such weapons, she said.