CARLSBAD, N.M. (AP) State environmental regulators have cleared the way for work to continue on a multimillion-dollar ventilation shaft at the federal government's underground nuclear waste repository in southern New Mexico.

Ventilation has been an issue since 2014, when a radiation release contaminated parts of the Waste Isolation Pilot Plant and forced an expensive, nearly three-year closure, delayed the federal government's cleanup program and prompted policy changes at national laboratories and defense-related sites across the U.S.

The state Environment Department last week approved a permit modification requested by the U.S. Department of Energy to build and use the utility shaft. Temporary authorization previously was granted but work stopped in November 2020 after state officials opted not to renew the authorization, citing a rise in COVID-19 infections among workers at the repository.

Estimated to cost about $100 million, the shaft will be a key part of the repository's revamped ventilation system. With more airflow, officials have said more employees can be in the underground space working on mining and waste operations simultaneously.

The project is expected to be completed in 2025 and would triple the available air flow, the Carlsbad Current-Argus reported.

In the notice of approval, New Mexico Environment Secretary James Kenney said more airflow was needed for the repository to achieve its mission.

Donavan Mager, a spokesman for Nuclear Waste Partnership _ the contractor that manages the facility _ said it was unclear when construction would resume.

Subcontractor Harrison Western-Shaft Sinkers was awarded a $75 million contract to build the shaft in 2019. When complete, it will reach a depth of about 2,275 feet (693 meters) and will include two access drifts to connect the shaft with the rest of the WIPP underground.

The project saw intense criticism from environmental and government watchdog groups that argued it was part of a larger plan to expand the repository beyond its presently permitted mission, which allows for disposal of 6.2 million cubic feet of waste. Officials have said it would take congressional action to increase the volume of waste permitted at WIPP.

The DOE is seeking a separate permit change to allow the mining and use of two additional disposal rooms to replace space that was lost to due contamination in 2014.

Still, in public comments submitted to state, opponents voiced concerns that the project could lead to further expansions of the facility.

"We object to NMED's refusal to explain the true purpose of the shaft in their required notification documents for the public," wrote Virginia Necochea, executive director at the New Mexico Environmental Law Center. "In order to provide informed public comments, the public must be able to review DOE's entire plan."

The project was supported by members of the Carlsbad City Council, the Carlsbad Chamber of Commerce and local business owners. They defended WIPP, a major employer in the community, and argued that the shaft was needed to improve worker safety.

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