YAKIMA, Wash. (AP) — The company hired to design and build a massive plant at the nation's most contaminated nuclear site should no longer have authority over its design, according to an internal U.S. Department of Energy memo released Tuesday.

The memo raises more questions about a waste treatment plant project long viewed as critical to ridding the Pacific Northwest of pollutants left from decades of weapons production for the nation's nuclear arsenal, but one that has endured countless technical problems, delays and skyrocketing costs.

It also raises more concerns about the contractor at the center of the mix, Bechtel National Inc., which has come under fire in recent months from critics who say the company has suppressed employee concerns related to the plant's safety and retaliated against whistleblowers.

The memo noted 34 instances where Bechtel National provided information that was incorrect, technically unfeasible or failed to provide the best value to the government, among other things, while designing the $12.3 billion plant at south-central Washington's Hanford nuclear reservation.

"The behavior and performance of Bechtel Engineering places unnecessarily high risk that the WTP design will not be effectively completed," Gary Brunson, the Energy Department's engineering director assigned to the project, said of the waste treatment plant in a memo to top Energy Department managers Thursday.

The federal government created Hanford in the 1940s as part of the top-secret Manhattan Project to build the atomic bomb, and the site continued to produce plutonium for nuclear weapons through the Cold War. Today, it is the nation's most contaminated nuclear site, with cleanup expected to last decades.

"This memo details exhaustive and disturbing evidence of why Bechtel should be terminated from this project and subject to an independent investigation," Tom Carpenter, executive director of Hanford Challenge, a watchdog group that monitors cleanup efforts, said in a statement.

Bechtel project director Frank Russo said the findings highlight how the project — and the demands on the contractor — have changed in the past decade, rather than failures by the company.

Russo said the company and the Energy Department have hired hundreds of consultants who are experts in their fields to review calculations and design decisions for a project where the available information is constantly changing — including the cost and scope.

"If we ignore new information, we can finish it on price and we can finish it on schedule, and it would look like an ideal project that would make some people happy," he said. "But if we review new information as it becomes available, it's not failure.

"The reality is that this project has a huge element of science and a huge element of research involved before (the Energy Department) decides it has enough information to make a decision," he said. "And when that happens, there will always be groups of people who believe it's a wrong decision."

Long considered the cornerstone of Hanford cleanup, the waste treatment plant is being built to convert millions of gallons of highly radioactive waste into a stable glass form. The waste is currently stored in underground tanks, many of which have leaked into the groundwater, threatening the neighboring Columbia River.

The plant's unresolved issues include inadequate mixing of the waste, which could lead to a buildup of flammable gases or a small risk of creating a nuclear reaction inside the plant, and whether the rate of corrosion in piping and vessels will hold the radioactive waste. The issues are of particular concern because workers will be barred from entering highly radioactive areas of the plant — and repairing potential problems — once it is operating under the current design.

Design of the plant is 85 percent complete, and construction is more than 50 percent complete.

The plant had been scheduled to begin operating in 2019, but the Energy Department has said a new estimate on the plant's start date and cost will be pushed off for a least a year while it embarks on additional tests to resolve those problems.

The Energy Department said in a statement Tuesday that it was reviewing Brunson's memo and continues to work with Bechtel to address ongoing technical issues, though it continues to be frustrated with the lack of progress.

"Addressing these challenges effectively will require both additional work by the contractor, as well as improved oversight by the department," the statement said. "It's also important to note that the successful completion of this important project depends on employees continuing to be able to freely raise concerns."

The federal government spends roughly $2 billion each year for Hanford cleanup, $690 million of which goes toward the plant.