YAKIMA — The contractor handling construction of a nearly $6 billion waste-treatment plant at the Hanford nuclear reservation announced plans yesterday to lay off 700 workers as the plant's design is reviewed to determine whether it could withstand a severe earthquake.

Bechtel National had announced layoffs of about 300 workers in the past two weeks. An additional 350 workers were laid off yesterday, reducing the total number of construction workers at the site by almost half.

The company employed about 1,400 construction workers as of March 1.

Another 350 employees not handling construction work also were to receive 60-day layoff notices. Bechtel employs about 2,400 such employees at the site.

"We will periodically review our staffing plan, but I am hopeful that no further reductions in the waste-treatment-plant work force will be necessary," Project Director Jim Henschel told Bechtel employees.

The plant is being built to treat millions of gallons of radioactive waste left from Cold War-era nuclear-weapons production.

The layoffs come as Bechtel and the U.S. Department of Energy, which manages the Hanford site near Richland, review the plant's design after a new study found that the impact a severe earthquake could have on the plant was 38 percent greater than previously estimated.

In 2002, the Defense Nuclear Facilities Safety Board ruled that the Energy Department had failed to adequately investigate the impact a severe earthquake might have on the plant.

The agency had gathered seismic data from the 586-square-mile Hanford reservation to determine the impact such a quake might have on the plant, but it did not conduct a seismic investigation of the plant site.

The Energy Department and Bechtel have stressed that the chances of a severe earthquake at the site are slim. However, the new seismic data could have a significant impact on the cost and schedule of the project.

Design of the plant is about 70 percent complete, and construction is about 35 percent complete. Bechtel had planned to lay off about 800 engineers this year as work shifted more toward construction, but those plans are on hold while engineers review the plant's design, spokesman John Britton said.

In addition, costs of building the one-of-a-kind plant have ballooned since the original estimate of $4.35 billion before the contract was awarded in 2002.

The current estimate is close to $5.8 billion, an increase of more than 30 percent. Last year, the U.S. Army Corps of Engineers concluded there is a significant risk of additional cost increases.

That review came even before the seismic issues were raised. Under the proposed 2006 federal budget, funding for the project falls about $65 million to about $625 million.

The Energy Department cited the unresolved seismic issues as one reason for the budget cut.

The impact of the new seismic data on the final cost of the building and the construction schedule should be released in about two weeks, Energy Department spokesman Erik Olds said.

The current schedule requires the plant to be operating in 2011.

For 40 years, the Hanford reservation made plutonium for the nation's nuclear-weapons arsenal. Today, work there centers on a $50 billion to $60 billion cleanup, to be finished by 2035.

Much of the cleanup involves treating 53 million gallons of highly radioactive waste stewing in 177 aging underground tanks less than 10 miles from the Columbia River. The waste-treatment plant will use a process called vitrification to turn some of the waste into glass logs for permanent disposal in a nuclear-waste repository.