Two huge 21-foot-deep holes in central Hanford filled with a jungle of steel rebar are ants' nests of activity as hundreds of construction workers build the lower levels of two plants designed to encase radioactive wastes in glass.

Soon, those workers will emerge from the holes to begin erecting the upper floors of the $5.6 billion complex that is powering the Tri-Cities' latest economic boom.

Project contractor Bechtel Hanford had been waiting for approval from the Department of Energy to begin building the radioactive waste glassification complex above the ground's surface. That green light was recently switched on, allowing Bechtel to tackle that work when it is ready.

That means that by January or February, hiring for the vitrification project is expected to increase, with Bechtel reaching peak employment of about 4,300 in 18 to 24 months.

Currently, the project employs about 2,000 engineers, scientists and white-collar workers, plus about 850 construction workers.

Construction hiring had recently stalled, accompanied by about 100 layoffs because engineering could not keep up with the ground work. But that process will begin reversing, and white-collar workers will be laid off as engineering and design work is finished.

The two holes where work is under way will hold the subterranean levels of the two melter plants that will begin the process of turning millions of gallons of radioactive wastes now stored in 177 aging underground tanks into glass logs, which will later be buried.

Another large building next door will hold a pretreatment building that will separate the wastes into high-level and low-level wastes before sending them to the appropriate melting plant.

Bechtel is supposed to start turning the first wastes into glass by 2007.

The two glassification plants are designed so the melters -- which will mix and melt glass and wastes together -- will be on the ground floors to make maintenance easy. The 21-foot-deep basements provide room to place containers beneath the melters to receive the molten wastes.

The complex is massive, with the footprint of all three buildings covering more than 7 acres.

The pretreatment building will cover 116,640 square feet and be 119 feet tall. The low-level treatment plant will cover 79,200 square feet and be 58 feet above ground, while the high-level plant will cover 121,000 square feet and be 65 feet above ground.

Concrete basement walls are now in place at the low-level waste plant. But temperature problems last summer caused that building's concrete to be poured at separate times, so the walls are not as solidly connected as they should be.

Next week, Bechtel plans to drill about 3,000 holes in the low-level building's concrete and insert rebar to properly connect the wall segments, said Joe Dougherty, Bechtel's site manager.

The pretreatment building doesn't need a basement, and so far it consists of only a foundation.

The three buildings are being built in parallel, with Bechtel shifting workers from spot to spot to keep momentum going.

"It's like Chinese checkers," Dougherty said.

"Three-dimensional Chinese checkers," added Ken Hollenbach, Bechtel project superintendent.

The game is even more complicated because not all decisions about the complex have been made.

The biggest is that Bechtel and DOE expect to sign a revised construction and testing contract by January that will lock the complex's design into installing two high-level waste melters and two low-level waste melters.

But the state wants at least three low-level melters installed to handle some additional wastes. That's because two high-level and two low-level melters can glassify only 19 million gallons of wastes by the 2028 legal deadline to glassify all 53 million gallons.

If the number of melters goes up, DOE and Bechtel will have to renegotiate the revised contract, as well as figure out if the additional equipment will fit into the buildings.