"We've gained about 400 people since last summer," said Clay Ramsey, the National Nuclear Security Administration's MOX federal project director.
The $4.87 billion project, scheduled to open in 2016, will dispose of surplus plutonium from dismantled nuclear bombs by blending it with uranium to make commercial reactor fuel.
Workers have been assembling sections of concrete and rebar since construction on the heavily fortified building began in 2007.
"We expect to start installing the roof later this summer over on the east side, and we'll work our way across on the west side," Ramsey said.
The facility will have three levels on the fuel-manufacturing side and five levels on the chemical-processing side.
Work has also been completed on a 57,000-square-foot administration building nearby and on a warehouse-type building where equipment to be used in the plant is being assembled and tested.
The project's work force, Ramsey added, will continue growing as the building moves closer to finishing and occupancy.
"We would expect it to peak in the 2012 time frame in the 2,200 to 2,300 range," he said. "We hope we have enough parking."
Another recent milestone, Ramsey added, was the completion of 3 million work hours without lost-time injuries. "We're very proud of that," he said.
Notable changes in the project in the past year include the Energy Department's decision to locate the plutonium-disassembly facilities away from the MOX plant, rather than on adjoining land, as the original plan suggested. That function will now be consolidated in Savannah River Site's K area, where a secure plutonium-storage facility is already maintained in an old reactor complex.
The plant will be capable of processing 3.5 metric tons of plutonium each year by blending small amounts of the material with larger volumes of uranium oxide -- a standard ingredient for commercial reactor fuels. The objective is to guarantee that the material can never again be available for nuclear weapons.
Because it will make commercial reactor fuels, the project also requires licensing from the U.S. Nuclear Regulatory Commission, which is monitoring the construction.
"They have filed a license application, but that process doesn't get going into full gear until they are closer to completion of construction," said NRC spokesman Roger Hannah.
"But once it is determined the construction meets all the standards, we look at their application for an operating license," Hannah said.