The only underground nuclear waste repository in the US is running out of room and will reach its capacity in less than a decade, federal auditors have found.

Key points: The repository is carved out of an ancient salt formation about 800m below the desert in New Mexico

The repository is carved out of an ancient salt formation about 800m below the desert in New Mexico It's already too small to accommodate the waste already in the US' inventory

It's already too small to accommodate the waste already in the US' inventory Auditors say there are no plans in place to properly expand the facility before it reaches capacity in the next decade

The Waste Isolation Pilot Plant (WIPP) in New Mexico does not have enough space for radioactive tools, clothing and other debris left over from decades of bomb-making and research, much less tons of weapons-grade plutonium that the nation has agreed to eliminate as part of a pact with Russia, the US Government Accountability Office said.

It added the Energy Department had no plans for securing regulatory approvals to expand WIPP over the next 10 years.

"DOE modelling that is needed to begin the regulatory approval process is not expected to be ready until 2024," the auditors said in their report released on Tuesday.

Energy Department officials contend there is enough time to design and build addition storage before existing operations are significantly affected.

A Senate committee requested the review from auditors amid concerns about ballooning costs and delays in the US effort to dispose of 34 metric tons of its plutonium.

Citing the delays and other reasons, Russia last fall suspended its commitment to get rid of its own excess plutonium.

The US has not made a final decision about how to proceed.

However, the Energy Department has agreed with auditors about the need to expand disposal space at the repository and devise guidance for defence sites and federal laboratories to better estimate how much radioactive waste must be shipped to New Mexico as the US cleans up Cold War-era contamination.

WIPP was 'never meant to be one and only'

The WIPP facility, pictured in 2006, needs to be expanded to accommodate the Government's current inventory. ( Facebook: WIPP )

The New Mexico repository was carved out of an ancient salt formation about 800 metres below the desert, with the idea that shifting salt would eventually entomb the radioactive tools, clothing, gloves and other debris.

Don Hancock, director of the nuclear waste safety program at the Southwest Research and Information Centre in Albuquerque, said he was pleased the auditors acknowledged the space limitations and hoped the report would spur a public discussion about how to handle the surplus plutonium and waste from bomb-making and nuclear research.

"The Waste Isolation Pilot Plant, it was never supposed to be the one and only," Mr Hancock said.

"So it's past time to start the discussion of what other disposal sites we're going to have."

Federal auditors say another two disposal vaults would have to be carved out to accommodate the waste already in the Government's inventory.

More space would be needed for the weapons-grade plutonium.

AP