One of the contractors charged with cleaning up the nation's most contaminated nuclear site has agreed to pay an $18.5 million civil and criminal penalty related to a time card fraud scheme.

U.S. Attorney Michael Ormsby of the Eastern Washington district says the penalty is the largest ever assessed to a contractor at south-central Washington's Hanford nuclear reservation, and perhaps the largest ever from his office.

CH2M Hill Hanford Group Inc. held a contract from 1999-2008 to clean out underground waste tanks at Hanford. The company is a subsidiary of Denver-based CH2M Hill Companies Ltd.

Eight people have pleaded guilty in the time card scheme. Under the settlement agreement, CH2M Hill agreed to pay as much as $580,000 additionally for independent monitoring to ensure the company takes appropriate corrective actions.