RICHLAND, Wash. -

 The state of Washington has formally triggered the dispute resolution phase in its dispute over cleanup of the Hanford Nuclear Reservation.

The move came Wednesday, after the federal government last week rejected Washington state's plan to clean up the former nuclear weapons production site.

The state announced its proposed amendments to a federal court consent decree at a news conference on March 31, and that plan was rejected last Friday.

The state sent a letter to the Justice Department on Wednesday, triggering a 40-day process of negotiation.

If the state and federal governments cannot reach agreement, the state can then ask a federal court to implement the state's cleanup plan.

Hanford for decades made plutonium for nuclear weapons and now has the nation's biggest collection of nuclear waste.