Bechtel, which is building a $12.2 billion waste-treatment plant at the federal government’s decommissioned Hanford nuclear weapons site in Washington state, has been buying critically important parts without subjecting them to the required quality assurance regime, according to a new report by the Energy Department’s inspector general.

The department’s inspector general, Gregory H. Friedman, said a sample of 235 design documents over a three-year period showed that Bechtel had failed to run more than a third of its design changes in equipment purchased from subcontractors past Bechtel’s own environmental and nuclear safety group as required.

Friedman said that the shortcoming could cause delays and cost increases for the Hanford cleanup plan, which has already experienced years of delay and billions of dollars of cost overruns. He also faulted the Energy Department for failing to properly oversee Bechtel’s progress.

“Bechtel determined that there was a systemic problem and a breakdown in controls over the review of design changes,” the inspector general’s report said, adding that the company has taken steps to correct the problem.

Bechtel’s project director Peggy McCullough said in a statement that “the audit identified two matters that we are already addressing.” She said that “none of the reviewed change documents resulted in a safety basis impact.”

The Hanford site, nearly half the size of Rhode Island and adjacent to the Columbia River, has 56 million gallons of highly radioactive waste in underground tanks, a leftover of the nuclear bomb production there that lasted from World War II until the late 1980s.

The Bechtel-designed waste-treatment plant, due to start operating in 2019, would vitrify the radioactive and chemically hazardous waste, turning it into glass. That requires the fabrication of unique pieces of equipment designed by Bechtel but in many instances manufactured by other companies.

The inspector general singled out the example of a low-activity waste-melter lid that did not meet design specifications. Bechtel was unable to provide evidence that the supplier had made necessary repairs to the lid or that Bechtel had reexamined the repair to ensure that it met requirements, the report said. The purpose of the lid is to contain harmful byproducts — nitrogen oxide gases — produced during the vitrification process.

McCullough did not address that issue.

The long-running Hanford cleanup project has been plagued by controversy and setbacks. In February, the Energy Department reported that seven of 177 underground storage tanks were leaking. Six of those were single-shell storage tanks that the Washington State Department of Ecology said were “unfit for use and decades past their design life.” The seventh was a double-shell tank previously assumed to be secure, the state agency said.

In August 2009, then-Energy Secretary Steven Chu visited the site, early in his tenure. The new energy secretary, Ernest Moniz, made a visit during his first month in office.

“In America’s time of need, the Hanford Site in Washington was critical to the production of plutonium,” Moniz said in a Sept. 24 blog post. “Today, we remain strongly committed to the American people and the communities around Hanford to clean up the effects of this activity.”

Moniz acknowledged unresolved “technical issues” but proposed that the department push ahead with a new “framework” with a “phased approach.” His new plan would turn the “low-activity” radioactive waste from storage tanks into glass while the department continues to solve problems with higher-activity radioactive waste. Earlier, the department planned to send all the waste to a pretreatment facility; construction of that facility has been suspended because of technical issues.

“It is critical that we move forward as expeditiously as possible to begin immobilizing the tank waste, and it is critically important to me that the Department continue to work closely with the state of Washington as we continue to advance this essential cleanup project,” he wrote.

Moniz noted that cleanup progress made so far at the 586-square-mile site included the demolition of 741 contaminated buildings, the moving of 2,300 tons of spent fuel from reactors to secure dry casks and the “cocooning” of six of nine nuclear reactors in steel and cement.