ALBUQUERQUE, N.M. (AP) — The lack of space at the federal government's only underground nuclear waste repository is among several challenges identified Friday by a group of scientists and other experts who are looking at the viability of disposing tons of weapons-grade plutonium at the desert location.

The National Academies of Sciences, Engineering, and Medicine released a preliminary report on the U.S. government's plan, which calls for diluting 34 metric tons of plutonium and shipping it to the Waste Isolation Pilot Plant in southern New Mexico.

The purpose of the work would be to satisfy a nonproliferation agreement with Russia.

Another challenge, the scientists say, would be getting officials in that country to approve of the dilution of the materials.

The pact between the two countries was initially based on a proposal for turning the surplus plutonium into fuel that could be used for commercial nuclear reactors. That project, beset by years of delays and cost overruns, was cancelled earlier this year.

The review of the plan that calls for shipping the plutonium to New Mexico was requested by Congress. A final report from the National Academies is expected next summer.

The U.S. Energy Department's Office of Environmental Management has demonstrated that diluting the plutonium is possible by working with a separate batch of material. However, citing a lack of information, the scientists did not study the agency's ability to scale up that process to handle the 34 metric tons that are part of the nonproliferation agreement.

If the plan were to be approved, the Energy Department has estimated that it would take 31 years to dilute and dispose of all 34 metric tons. The work would involve four sites around the U.S. — the Pantex Plant in West Texas, the Savannah River Site in South Carolina, Los Alamos National Laboratory in northern New Mexico and the Waste Isolation Pilot Plant.

The panel of scientists found that the agency doesn't have a well-developed plan for reaching out to those host sites and stressed that public trust would have to be developed and maintained over the life of the project.