RICHLAND – Workers at the nation’s most contaminated nuclear site made one final sweep Thursday to remove the last of the radioactive sludge from a leaky pool built in the 1950s to hold spent fuel from nuclear reactors.

Emptying the K East Basin has been one of the cleanup priorities at south-central Washington’s Hanford nuclear reservation, and sludge removal has proven one of the more difficult tasks. Much work remains at the basin, but the sludge project concluded on schedule Thursday as the federal government’s top manager at the site marked his last day on the job.

“This milestone is representative of so many of the challenges at Hanford. They seem impossible at times. They rarely go as originally planned,” said Keith Klein, retiring manager of the Department of Energy’s Richland Operations office.

“It sounds so simple to the folks out there who don’t understand this … but you figured it out and you did it,” Klein told workers gathered to mark the project’s completion. “For a last day on the site, this is just icing on the cake for me.”

The federal government created Hanford in the 1940s as part of the top-secret Manhattan Project to build the atomic bomb. Over 40 years, nine nuclear reactors were built alongside the Columbia River to support weapons production during World War II and the Cold War.

Workers built the K East and K West basins in the 1950s to hold spent fuel from the reactors. Attached to different reactors, the 125-foot-long, 20-foot-deep pools can hold an estimated 1.3 million gallons of water.

In 2004, workers completed the removal of 2,100 tons of spent nuclear fuel left in both basins after weapons production stopped – one of three Hanford cleanup tasks identified as urgent to protect public safety and the environment. Both basins sit just 400 yards from the river, and the K East basin previously leaked millions of gallons of contaminated water into the soil.

But left behind was a mess of radioactive sludge, an estimated 37 cubic yards in the K East basin alone. Cleaning up the mix of dirt, sand and corroded fuel proved more difficult than originally planned.

Workers began vacuuming the sludge in October 2004, deploying new technologies as sludge jammed filters and pumps stopped working. They completed the bulk of the work in October 2006. Final vacuuming finished in May, with one last sweep through the troublesome basin Thursday, the current deadline for removing the sludge.

The deadline was pushed back several times in the past few years as workers encountered technical difficulties.