JACKSON (AP) - A federal judge has ruled against a Wyoming-based watchdog group in its efforts to block the U.S. Department of Energy from upgrading an aging nuclear reactor in Idaho that could be used to produce radioactive plutonium-238.

U.S. District Judge B. Lynn Winmill has denied a request from Keep Yellowstone Nuclear Free. The group had asked the judge to block the U.S. Department of Energy from operating the reactor at the Idaho National Lab until the agency prepared a detailed environmental analysis.

The Energy Department is considering using the Advanced Test Reactor at the lab to produce plutonium-238 for power generation in space craft and for classified national security projects. The material is more radioactive than plutonium-239, which is commonly used in atomic weapons.

The group argued in its lawsuit that "neglect, antiquated equipment, poor design, and many years of what the Energy Department has termed 'budget austerity"' had left the 40-year-old reactor unsafe.

In particular, the group questioned the reactor's cooling system and its ability to withstand an earthquake.

The federal government rejected all the group's claims in its response to the lawsuit.

Winmill stated in his ruling that the Energy Department was engaged in ongoing work to make the reactor viable "indefinitely," and therefore it did not require an environmental study.

Federal agencies are required to undertake environmental studies on new, major actions.

Lawyer Mark Sullivan represented Keep Yellowstone Nuclear Free in the lawsuit. He said the judge affirmed many of his group's positions and affirmed that members had standing to bring the suit, even though he ultimately ruled in favor of Energy Department.

Sullivan said his clients, including Wilson resident and KYNF Executive Director Mary Woollen and the Idaho-based Environmental Defense Institute, are considering whether to appeal.

Wyoming Gov. Dave Freudenthal said in 2005 that he opposed production of plutonium-238 at the Idaho reactor.

"My concerns revolve around the fact that the DOE wants to use old equipment in a potentially dangerous place to produce an extremely toxic substance," Freudenthal said.

He said he was concerned that the federal government had no specific plans for storing radioactive waste produced in the process.