Hanford's most dangerous tank of radioactive waste has been partially emptied and the waste diluted - progress officials hope means the end to years of troubles that have brought it national scrutiny.

"This alleviates one of the most hazardous problems in the tank farm and proves we can retrieve waste to send to a (treatment) plant," said Fran DeLozier, president of Lockheed Martin Hanford Corp.

In the past, the tank's high concentration of radioactive waste has proved unpredictable for scientists and researchers working on the problem.

Tank SY-101 won notoriety for periodically burping a potentially flammable gas mixture every few months during the late 1980s and early 1990s. Hanford officials tried to fix the problem by installing a mixer, but instead the gas became trapped in a growing crust that rose like a soufflé above the liquid.

The crust eventually grew to 10 feet thick and came within about 2 feet of the top of the tank's double wall, where it would have entered the single-shell top of the tank. That increased the threat of a leak. And the rising crust compressed the air in the top of the tank, concentrating the gases and increasing the potential for combustion, Hanford critics charged.

Months of planning culminated Saturday at daylight when Lockheed and the Department of Energy Office of River Protection began to pump liquid waste from the underground tank.

"There was a lot of uncertainty," DeLozier said.

The waste settles into three layers - sludge under liquid under the crust. Those layers had to remain somewhat mixed during the pumping to prevent the tank from burping hydrogen gas. High concentrations of organic compounds combined with water and decomposing radioactive elements in the tank to generate the gas.

Hanford officials and technical specialists also didn't know how the crust would react as radioactive liquid was pumped out, and water was added to dilute the waste.

The added water caused gas to be released, and officials had to be careful to keep the gas at acceptable levels.

"It was very closely controlled and monitored," she said. "Anytime radioactive material is transferred, you have to be extremely careful to prevent leaks."

About 23 hours later, workers had removed about 90,000 gallons of the approximately 1.1 million gallons in the tank. An equal amount of water was added to the tank, but levels still dropped as 2 feet of gas trapped in the crust was released.

"It's the single most complicated technological piece of work (Lockheed Martin Hanford Corp.) has done, and we've done it practically flawlessly," DeLozier said.

The waste that was removed also has been diluted with water and is being stored temporarily in a nearby double-shelled tank. In the second week of January, it's scheduled to be piped seven miles across the site from the West Tank Farm to the East Tank Farm. That cross-site transfer line already has been used three times.

Eventually, the waste from the tank should be prepared at a proposed waste glassification plant for permanent burial. The original waste is a byproduct of the process used to extract plutonium from uranium irradiated at Hanford's now-defunct nuclear reactors.

The pumping and diluting process will be repeated two more times to remove about 300,000 more gallons of waste from the 38-foot tank.

The already diluted waste should be somewhat easier to work with, however, and Hanford officials know what to expect now that they've pumped the tank once, DeLozier said.

There has been no danger of a flammable concentration of the gases in the tank since the mixing pump was installed in July 1993, she said. Before then, volatile gas built up at the bottom of the tank and then would erupt through the wastes in the tank about every three months.

The pump mixed the waste every three days so small amounts of the gas were released continuously. But it led to other problems.

Only the sludge and liquid waste were mixed, while waste at the top of the tank crystallized and formed a hard crust with gas trapped inside.

It fluffed up somewhat like the meringue atop a pie, DeLozier said.

In May, workers punched a hole through the crust, and the tank level dropped immediately.

But "over time, that hole would have closed up, and we would have had the same situation again," she said.

"That's why this is such an accomplishment," said Tony Valero, project manager for tank waste storage for the Washington State Department of Ecology. "The burping issue has been put to rest, and the crust issue has been put to rest."

Hanford still must solve the problem of its 149 single-shell waste tanks that have far exceeded their design life, and many have leaked. That waste also will be prepared for permanent storage at proposed waste glassification facilities.