YAKIMA, Wash. (AP) — State and federal officials announced Tuesday that construction on a waste treatment plant at the nation's most contaminated nuclear site can begin to be ramped up again, nearly seven months after a new cost estimate and construction schedule for the plant were delayed to address technical problems and safety concerns.

The plant has long been considered the cornerstone of cleanup at south-central Washington's Hanford nuclear reservation. Design of the $12.3 billion plant is 85 percent complete, while construction is more than 50 percent complete.

However, the U.S. Department of Energy, which manages cleanup at the highly contaminated site, halted construction on significant portions of the plant amid technical issues and concerns about its eventual safe operation.

Gov. Chris Gregoire and Energy Secretary Steven Chu said in a statement Tuesday that the state and federal government have worked closely to resolve those issues in recent months and that, based on information gathered from leading scientific experts, the Energy Department is confident that construction can be ramped back up again.

They also said the federal agency is evaluating options to feed waste directly into two parts of the facility that deal with either low-level or high-level radioactive waste, rather than feeding it into a so-called pretreatment facility. The move could increase flexibility and reliability of the project overall and could enable the plant to begin operating before the pretreatment facility is completed, they said.

Mixing the waste in the pretreatment facility has posed significant technical challenges and contributed to delays of the plant, which had been scheduled to begin operating in 2019.

"We are pleased to be able to announce these important developments with the project, and we look forward to continued progress on this important project," the statement said. "The state of Washington and DOE remain committed to the safe treatment of the tank waste and the completion of the cleanup at Hanford."

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

The cornerstone of that cleanup is the plant, which will convert millions of gallons of highly radioactive waste into glasslike logs for permanent storage. That waste is currently stored in 177 aging, underground tanks, many of which have leaked into the groundwater, threatening the neighboring Columbia River.

Several workers have raised concerns about erosion and corrosion in tanks and piping, leading to whistleblower complaints of retribution and federal investigations of activities at the site.

Last March, the Defense Nuclear Facilities Safety Board ruled that the Energy Department lacked necessary information to resolve some problems and establish a complete safety plan.