YAKIMA, Wash. (AP) - A federal judge has prevented a state initiative - dealing with cleanup at the Hanford nuclear site - from taking effect.

Washington’s voters last month overwhelmingly approved Initiative 297, which would bar the U.S. Department of Energy from sending more radioactive waste to south-central Washington’s Hanford nuclear site until all existing waste there is cleaned up.

U.S. District Judge Alan McDonald ruled Thursday that there is a probability the federal government will succeed in establishing that the initiative is invalid, and a possibility the federal government would suffer irreparable injury with regard to onsite cleanup activities at Hanford if the initiative immediately became law.

It had been scheduled to take effect Thursday.

Federal government lawyers won a temporary restraining order after telling the judge there were too many uncertainties about how the state would interpret the new initiative.

Attorneys for the state argued the order was unnecessary. They offered assurances that state officials were still reviewing the initiative and would not begin to implement it in the next 60 days.

For now, I-297 will not be applied or enforced with respect to activities at the nuclear reservation “except to the extent that it prohibits the import of mixed waste to Hanford,” McDonald said.

Shipments of mixed waste - slightly radioactive waste laced with dangerous chemicals - have already been halted as the result of another lawsuit.

U.S. Justice Department lawyers ultimately hope to invalidate the initiative on the grounds that it violates federal laws governing interstate commerce and nuclear waste. Hanford, a federal site, is immune from state regulation, the government has argued.

Before the judge ruled, some cleanup was halted Thursday at the Hanford site because of the initiative, said Cynthia Morris, a Justice Department lawyer.

How many workers were idled and which projects were halted remained unclear.

The judge said it is in the public interest to “continue current onsite clean-up activities at Hanford,” unimpeded by the initiative.

A spokesman for the Energy Department said the agency was pleased with the decision but recognized the order is temporary.

“We remain dedicated to achieving the cleanup of Hanford, and this decision will allow our employees and contractors to continue working without fear of civil and criminal liability,” spokesman Joe Davis said.

At issue are the federal government’s plans for disposing of waste from 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 before it is shipped elsewhere for long-term disposal. Transuranic waste is highly radioactive and can take thousands of years to decay to safe levels.

In 2003, Washington state filed suit to block transuranic waste from entering the state, fearing Hanford would become a radioactive waste dump. The Energy Department voluntarily suspended the shipments of transuranic and mixed waste after the lawsuit was filed, but the case remains in federal court.

While those shipments remain on hold, I-297 also places other restrictions on cleanup at the site.

In requesting the stay, federal officials said they only wanted to continue cleanup under existing regulations until the court rules on their claims regarding the initiative.

A hearing to discuss a preliminary injunction was set for Dec. 13.

Sheryl Hutchison, a state Department of Ecology spokeswoman, said the ruling was not unexpected.

“We’re satisfied that shipments will not be coming in, and over the next 10 days we will prepare a vigorous defense,” she said.

Gerald Pollet, executive director of Hanford watchdog group Heart of America Northwest, which sponsored the initiative, also was pleased that no waste would enter the site.

More than 10,000 people work at the 586-square-mile reservation, which was created in World War II as part of the top-secret Manhattan Project to build the atomic bomb. It remains the most contaminated site in the nation, with cleanup costs expected to total $50 billion to $60 billion.