RICHLAND, Wash. - The Department of Energy has given the go ahead to fabricating key elements of Hanford's waste treatment plant, despite serious concerns being raised by engineers. A labor union that represents workers at the plant has asked for a work stoppage and filed a formal grievance.

The federal government is building the $12 billion plant to treat massive amounts of radioactive sludge. Among the most important components of that process are vessels that mix up the sludge in a pre-treatment facility. The union representing Hanford workers says small scale tests revealed problems with those mixers. They're designed to work for decades but the union says the tests showed they could develop holes and leak much sooner.

In a review last month, the Department of Energy said federal contractor Bechtel underestimated the risks. Nonetheless, the agency gave the green light for the company to fabricate the mixers and the vessels. The union has asked for a work stoppage and filed a formal grievance stating its concerns.

Jen Stutsman, a spokeswoman for the Department of Energy, issued this statement: "We will not allow the vessels to be installed until the results from large scale testing have shown that the vessel designs will safely and effectively handle the waste at the site."

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