"Legal challenges are inevitable," said Grant Nelson, government affairs director for the Association of Washington Business. The measure is scheduled to take effect in 30 days.

Initiative 297 blocks the U.S. Department of Energy from sending more waste to the Hanford nuclear site until all the existing waste there is cleaned up.

By a more than 2-to-1 margin, voters overwhelmingly approved the initiative. With 97 percent of precincts reporting statewide early Wednesday, 69 percent of voters approved, with just 31 percent voting against it.

"It's clear that the rule of the people of the state of Washington is that Hanford needs to be cleaned up before more waste can be dumped there," said Gerald Pollet, executive director of Heart of America Northwest, a Seattle-based Hanford watchdog group and the initiative's sponsor.

The 586-square-mile reservation in south-central Washington, which was created in World War II as part of the top-secret Manhattan Project to build the atomic bomb, remains the most contaminated site in the nation.

Supporters called the initiative a no-brainer: Don't add more waste until the existing waste is cleaned up. Opponents feared that barring waste shipments to Hanford could backfire if other states take similar steps to ban Hanford waste.

The Energy Department took no official position on the initiative. Agency spokeswoman Colleen French said the Energy Department would be studying the initiative and evaluating its options over the next 30 days.

Opponents have said the initiative is likely to end up in court because they believe it is illegal on several fronts: It pre-empts the federal government's nuclear waste and interstate commerce policies, imposes a tax on the federal government and addresses more than one issue, which would violate the state constitution.

The bigger concerns are the potential for delaying cleanup and jeopardizing annual federal funding for cleanup, which now stands at about $2 billion, Nelson said.

"I think it's safe to say the federal government will not want to put its limited available resources toward a project that is now clouded," he said.

But Pollet said the initiative will stand up in court, and supporters will mount a vigorous defense.

"The voters will be outraged by anyone who takes this initiative to court to make this a radioactive waste dump," he said.

At issue are the federal government's plans for disposing of waste from World War II and Cold War nuclear weapons production nationwide.

The Energy Department chose Hanford to dispose of some mildly radioactive waste and mixed low-level waste, which is laced with chemicals.

The site also would serve as a packaging center for some transuranic waste - plutonium-contaminated rags, tools and other discarded items - before it is shipped elsewhere for long-term disposal. Transuranic waste is highly radioactive and can take thousands of years or more to decay to safe levels.

A citizens' petition sent the initiative to the Legislature early this year. Lawmakers declined to act on it, sending the measure to the ballot. The roughly $1 million cost of the initiative was largely funded by Heart of America Northwest.

Energy Department officials have said the site's most dangerous waste will be shipped out-of-state anyway. Of the 405 million curies of radioactivity at Hanford, about 374 million curies will be sent to other states for long-term disposal.

Hanford already is home to 53 million gallons of highly radioactive liquid, sludge and saltcake stored in 177 underground tanks. The Energy Department aims to bury much of that waste in a nuclear waste repository in Nevada. Another 75,000 55-gallon drums of transuranic, radioactive and hazardous waste also are buried at Hanford.

In 2003, Washington state filed suit to block waste shipments from entering the state, fearing Hanford would become a radioactive waste dump. The Energy Department voluntarily suspended the shipments after the lawsuit was filed, but the case remains in U.S. District Court.