The Energy Department manages cleanup at the highly contaminated south-central Washington site, which was created in the 1940s as part of the top-secret Manhattan Project. Cleanup costs are expected to total $50 billion to $60 billion.

In notifying Fluor Hanford of its intent to issue a fine, the department cited a series of violations that occurred at the Plutonium Finishing Plant over a two-year period between September 2003 and July 2005. The notice also cited several recent and more significant criticality safety issues, "which are representative of long-standing criticality safety deficiencies dating back to 1996," the department said in the statement.

"We want our contractors to identify and address safety issues before they become more serious problems," John Shaw, the Energy Department's assistant secretary for environment, safety and health, said in a statement. "Our goal is to have work conducted in a manner that protects workers, the public and the environment."

Beginning in 1949, the Plutonium Finishing Plant was the last step in converting plutonium nitrate solutions into pure plutonium "buttons" about the size of hockey pucks, which were sent to other Energy Department sites to make atomic bombs. The work stopped in 1989 at the end of the Cold War.

Early last year, workers completed a project to stabilize and package the last remaining 4.4 tons of plutonium - a project that was considered one of three critical cleanup problems at Hanford. Other key cleanup targets are underground tanks containing highly radioactive waste and corroding spent fuel rods from the nuclear reactors.

Work is now focused on dismantling and tearing apart the plutonium plant's contaminated equipment, which will be packaged and sent to a nuclear waste repository in New Mexico. The deadline for the plant to be demolished is 2016 under the Tri-Party Agreement, the cleanup pact signed by the state, Energy Department and the Environmental Protection Agency.

The notice of intent to fine also cited an event at the K West Basin in November 2004, when several workers received low-level radiological exposure. The workers were conducting work outside the scope of the planned work activity and moved contaminated tools that had not had a radiological survey, the department said.

The K East and K West basins are two pools of water designed to hold spent nuclear fuel. The pools have been prone to leaks, and cleaning them up has proven more difficult than originally thought.

Fluor Hanford could have been fined $275,000 for the violations, but the Energy Department mitigated between 25 percent and 75 percent of three of the four violations in recognition of the steps Fluor Hanford had already taken to correct the problems.

"We take the enforcement action very seriously and we are aggressively taking action to address the concerns," said Geoff Tyree, spokesman for Fluor Hanford. "Also, we're pleased to see the Office of Enforcement acknowledges the steps we have already taken to address some of these issues.

Cleanup at the Hanford site is expected to continue until 2035.