The National Nuclear Security Administration (NNSA) on Thursday announced a new interagency agreement to continue providing the federally owned Tennessee Valley Authority with low-enriched uranium that makes it possible to produce tritium for U.S. nuclear weapons.

Under the agreement, the semiautonomous Department of Energy nuclear weapons agency will downblend highly enriched uranium into low-enriched uranium that will be burned — along with tritium-producing burnable absorber rods — in the Tennessee Valley Authority’s (TVA) Watts Bar Nuclear Plant reactor Unit 1 in Rhea County, Tenn.

To continue transferring uranium to TVA, Secretary of Energy Rick Perry had to issue a secretarial determination that it remains in the national interest to tap into the agency’s enriched uranium supply.

“As support for the tritium program requires the transfer of enriched uranium to the Federal partner responsible for preserving the unencumbered LEU [low-enriched uranium] to be used as fuel in a reactor used to produce tritium, the transfers of LEU promote national security,” Perry wrote in the Aug. 21 determination.

Tritium, a radioactive hydrogen isotope, increases the explosive power of nuclear weapons. It decays rapidly, so the NNSA must periodically make more of the material to refresh the tritium reservoirs of existing weapons. Without the refresh, warheads would lose much of their destructive power.

The Energy Depatment must use its own supply of enriched uranium to irradiate tritium because the open market does not provide any so-called unobligated highly enriched uranium — “material that is free from peaceful use restrictions.”

“The Interagency Agreement is expected to be signed in the coming weeks and will allow for down-blending to begin in 2019 and continue through 2025,” an NNSA spokesperson said by email Thursday. “The agreement will continue through 2040 as TVA manages the LEU derived from down-blending. NNSA plans to down-blend 20 metric tons of highly enriched uranium (HEU) that is expected to result in about 10 reactor reloads worth of low enriched uranium (LEU) for a pressurized water reactor such as TVA’s Watts Bar.

“There is no predecessor interagency agreement with TVA for down-blending. NNSA has a separate, continuing interagency agreement with TVA for tritium production,” the spokesperson said.

The NNSA is set by 2019 to choose between two options for future production of uranium for U.S. defense programs: a large centrifuge that could cost $7.5 billion to $14 billion, and a small centrifuge that could cost $3.8 billion to $8.3 billion, according to a Government Accountability Office report published in February. Without a new domestic enrichment capability, the NNSA believes it could run out of usable uranium by 2027.

The DoE branch reimbuses TVA for tritium production, and pays the federally owned corporation a fee for each burnable rod it irradiates in the process. An NNSA spokesperson did not immediately reply to a request for comment about the terms of the interagency agreement.

Currently, TVA tritium production is limited to Watts Bar Unit 1. The Nuclear Regulatory Commission is considering amending TVA’s license to allow for tritium production in Watts Bar Unit 2, the civilian power-plant regulator said in a June Federal Register notice. TVA applied for the license amendment in December, about a year-and-a-half after Unit 2 went critical.

Tritium produced by TVA is shipped to the Savannah River Site’s Tritium Extraction Facility for processing into a form usable in nuclear weapons. The work is handled by Department of Energy contractor Savannah River Nuclear Solutions, which is led by Fluor [FLR].

Savannah River Nuclear Solutions has been on the job as the site’s management and operations prime since 2008, and the company’s contract is set to run out July 31, 2019, after a last-minute one-year, $1-billion extension in June. This week, the Department of Energy released a draft solicitation for a successor contract worth an estimated $15 billion over 10 years, with options, including a roughly $7-billion, five-year base.

The draft solicitation includes the NNSA-funded tritium work, plus a line item valued at some $520 million for construction of new tritium extraction facilities.