Army officials and contractors are investigating the cause of a Monday night accident at the Anniston Chemical Demilitarization Facility in which a secondary furnace broke in two during a test run.

Officials were testing systems at the Bynum incinerator when bolts suspending an "afterburner" furnace broke.

No chemical agent was in the plant, as workers are updating systems to destroy VX nerve agent. The last of Anniston's sarin nerve agent was destroyed in March.

No workers were injured in the incident, which happened around 10 p.m. Monday, according to Army spokesman Mike Abrams.

Tim Garrett, the Army's project site manager at the incinerator, said that during the changeover for VX destruction, workers had disassembled the deactivation furnace afterburner -- three sections of 7-foot-diameter steel cylinders, stacked and suspended from a steel-beam superstructure.

Workers had removed and replaced three insulating layers inside the steel -- one of insulating board and two layers of different types of brick.

After the sarin campaign, workers had noticed what Garrett called "problems" with 16 of the 72 carbon-steel bolts holding the lower two sections to the top one. They'd replaced all 72 bolts with a higher-grade version, which engineers expected would stand up under 3 million pounds or more of weight.

Instead, as workers heated the furnace for a test Monday night, the bolts broke under about 134,000 pounds -- the weight of the steel, insulation and brick. Garrett said no one yet knows why.

"There was a tremendous force put on those bolts," Garrett said.

Garrett could not name the supplier of the bolts Tuesday afternoon. He said the bolts, which are zinc-coated carbon-steel ¾-inch diameter by 2 ¾ inch long, were clearly stamped with the higher-than-required grade.

The two afterburner sections that separated -- about 34 feet tall -- now are wedged into the surrounding steel superstructure, suspended five feet above the ground.

Garrett said the Army was looking Tuesday for a contractor to lift the massive steel afterburner back into place. Officials then will try to determine why the bolts failed, make whatever changes are needed in the design, and replace the insulation if necessary.

During the accident, some of the bricks tumbled out of the afterburner. Wooden scaffolding near the afterburner smoldered after the accident, but there was no explosion and no large-scale fire, Abrams said. Anniston Army Depot firefighters extinguished the scaffolding.

The deactivation furnace is used to destroy explosives and M55 rockets. Exhaust from that furnace is fed into its afterburner, where the exhaust is heated to 2,000 degrees Fahrenheit before it is treated in the plant’s pollution-abatement system. The inside of the afterburner was at about 1,800 degrees at the time of Monday’'s accident. The outside of the steel cylinders normally heats to between 100 and 120 degrees, Garrett said.

Abrams said workers had been scheduled to resume destroying weapons in July. He and Garrett said officials did not know how Monday's accident will affect the schedule.

Meanwhile, the Army announced Monday that it will not meet the April 29, 2012, deadline to destroy the United States' entire chemical weapons stockpile, as stipulated under the terms of the Chemical Weapons Convention.

The treaty originally set a 10-year deadline, ending in 2007. The United States recently had requested a one-time, five-year extension to the 2012 date.

The Army pointed to delays in obtaining permits to destroy the weapons, slower-than-expected destruction rates, safety improvements, deteriorating weapons, community preparedness requirements and other unforeseen problems as contributing to the missed deadline.

Abrams said Tuesday that officials believe Anniston's incinerator still is on track to have the local stockpile of VX nerve agent and mustard blister agent destroyed by late 2010.