It provided plutonium for World War Two atomic bombs and for the US Cold War arsenal.

The three-judge appellate panel invalidated the 2004 voter-approved measure, saying it infringes on federal rules that apply to radioactive wastes and the US Department of Energy's ability to dispose of that waste.

The Washington Department of Ecology had appealed the case after a lower court struck down the new law.

Washington State Gov. Christine Gregoire said in a statement that she was disappointed by the court's decision, but pledged to work to clear the Hanford site.

Since 1989, the US Department of Energy, the US Environmental Protection Agency and the state Department of Ecology have conducted the nation's largest environmental cleanup at Hanford, where radioactive and chemically hazardous waste and spent nuclear fuel have contaminated 80 square miles (207 square km) of groundwater.

It is expected to be complete by 2035. But in 2004 the Energy Department said it wanted to bring mixed radioactive waste from other cleanup sites into Hanford disposal facilities.

In response, voters passed the Cleanup Priority Act to prevent new radioactive and hazardous waste from coming to Hanford until the decontamination is finished.

Subsequently, the Department of Energy agreed to hold off on bringing in new waste until it conducts a new environmental analysis, which is expected in 2009.