Those tanks, now rusty from four decades of exposure to the weather, were used to hold the nerve agent before it was transferred to small storage containers or munitions. About 250,000 gallons of VX are stored at the Newport Chemical Depot about 30 miles north of Terre Haute.

When the tank removal is complete next April, only a few pieces of equipment from the VX production plant will remain, said Karen Drewen, a spokeswoman for the U.S. Army’s Non-Stockpile Chemical Materiel Project at Aberdeen Proving Ground in Maryland.

She said the six 56,000-gallon tanks and two 32,000-gallon tanks were initially rinsed out with a liquid solution in 1969 after President Nixon ordered a moratorium on chemical weapons production.

“Some of them still show trace amounts of residue, and to be on the safe side before we cut them up for recycling, we’re going to heat them to drive out any chemical agent still left inside,” Drewen said.

The tanks will be wrapped with electrical resistance heaters that will slowly heat each one to 1,000 degrees Fahrenheit over a seven- to 10-hour period to vaporize residue, which will be collected by filters. Those filters will then be sent to a secondary waste treatment center.

Workers began wrapping one of the 56,000-gallon tanks this week with the electrical resistance blankets, but have not yet heated it, she said.

Once the tanks are decontaminated and cut up with welding torches, the resulting 407 tons of scrap steel will be shipped to a recycling facility in Rock Island, Ill.

Under the Chemical Weapons Convention international treaty, the destruction of the VX production plant, and all other U.S. facilities, must be completed by April 2007.

The sprawling plant has taken contractors years to dismantle, including the demolition of large scrubber towers that filtered VX from plant emissions during the 1960s.

VX is so toxic that a single droplet can kill a healthy human.

Drewen said the plant is now about 86 percent dismantled. Besides the eight large storage tanks, only a few smaller storage tanks and two buildings remain at the site.

As the plant’s demolition nears its end, the Army’s project to chemically neutralize the depot’s VX stockpile is only at its beginning.

Since neutralization began in May, about 8,200 gallons of the depot’s more than 250,000-gallon stockpile have been destroyed, said depot spokeswoman Terry Arthur.

The project has been plagued by equipment failures Army officials said they had anticipated. In the most recent incident, gaskets failed in one of the complex’s two chemical reactors Oct. 29, spilling about 500 gallons of a caustic wastewater called hydrolysate in a sealed area.

After workers replaced the gaskets in one of the reactors, it returned to operation Nov. 19. Work on the second reactor is expected to be completed this weekend, with both reactors back in operation by Dec. 12, Arthur said.