With the boom of explosives and dusty clouds of debris rising from their bases, two towers crashed to the ground this weekend near the Columbia River at Hanford's N Reactor complex.

"I'm glad to say all went exactly as planned," said Kurt Kehler, project manager for Department of Energy contractor Washington Closure Hanford.

The explosives made quick work of demolishing two towers and some heavy steam-generating equipment used at N Reactor.

Only rubble remains of the 200-foot-tall tower used to vent filtered exhaust from N Reactor and of the stack that rose 70 feet above a nearby powerhouse.

Hanford workers had yet to tear siding off of the powerhouse Monday where a third internal explosion was detonated Saturday. But they expect to find that heavy equipment higher up in the building had dropped to lower levels where what was left of it would be easier to remove.

Washington Closure started preparing for the demolition seven months ago with subcontractor Controlled Demolition Inc., which has used explosives at other Hanford reactor sites and at DOE nuclear facilities in Colorado, Idaho and Ohio.

N Reactor was the last of Hanford's nine plutonium-production reactors to operate during the Cold War, shutting down after the Chernobyl accident in 1986. In addition to producing plutonium for the nation's nuclear weapons program, it also used six steam generators to produce electricity.

DOE and Washington Closure Hanford are working to "cocoon" the reactor by a legal deadline at the end of 2012.

It will be torn down to a little more than its radioactive core plus the adjacent turbine hall, which shares a wall, then sealed up for the next 75 years to allow its radioactivity to cool before anymore work is done to dismantle it.

But to get to the reactor and its turbine hall, Washington Closure has had to tear down the other buildings in the complex. It has about 55 structures down and about 20 more to go, Kehler said.

"These are old, decrepit, decaying facilities," said Chris Smith, deputy federal project director for DOE.

Removing the buildings also clears the way for DOE to clean up contaminated soil in the N Reactor area.

The Washington State Department of Ecology expects some surface soil will need to be excavated because of low levels of radioactive contamination, said John Price, environmental restoration project manager for the state's Nuclear Waste Program.

N Reactor had a more modern design than other Hanford plutonium-production reactors, circulating cooling water through a closed loop to reduce the amount of contaminated water released from the reactor. However, the system still was occasionally flushed out, creating additional soil contamination,

"We're very happy to see the structures come down so contaminated materials can be cleaned up," Price said. The state also worked with DOE to make sure the demolitions would be conducted safely and the air quality monitored.

The largest tower to come down Saturday was the 116-N stack, the tallest structure in the N Reactor complex. In preparation for the explosion, workers sealed off an underground tunnel that connected the tower to the reactor. Then they chipped away at one side of its base to weaken it and drilled holes for explosives in a technique similar to notching a tree, Smith said.

It fell after a blaze of flames at its base to land in a puff of dust on the ground.

The downed stack "is pretty flat," Kehler said. "It looks like a paved road."

The second tower was made of metal and gunite, a concrete-like material, and perched on top of the 184-N Powerhouse, which provided auxiliary power when the reactor was not producing its own. It crashed to the ground top first, its base then toppling back into the building.

An explosion inside the building seven minutes later was planned to drop a steel water tank on the roof used in the boiler operation and one just below it in the building to about 20 feet above the floor. The tanks had been cut in four pieces before the explosives were placed.

The explosions, done with gelatin dynamite, were scheduled for Saturday, a time picked when few Hanford employees would be on site. The Benton County Sheriff's Office patrolled the nearby Columbia River during the explosion.

Exploding the towers and equipment within the powerhouse was no less expensive than reaching up with heavy equipment to dismantle them piece by piece, according to DOE and its contractor. But they thought explosive demolition would be safer for workers, reducing their exposure to industrial risks and the use of heavy equipment.

"You have to love it when a plan comes together," Kehler said.