Russia’s state-owned uranium supplier Tenex has notified U.S. customers they have 60 days to secure a waiver exempting them from a law recently signed by President Joe Biden that bars imports of the company’s nuclear fuel.

The force majeure notice seen by Bloomberg was sent to utilities including Constellation Energy Corp., the largest U.S. nuclear plant operator, Duke Energy Corp. and Dominion Energy Inc., according to a person familiar with the matter who wasn’t authorized to speak publicly. Constellation declined to comment and Duke and Dominion didn’t respond to a request seeking comment.

The May 14 letter underscores the uncertainty facing the U.S. nuclear power industry following this week’s enactment of the ban on Russian uranium imports. Utilities can receive waivers allowing import of Russian enriched uranium until 2028 if the U.S. determines no alternative source can be found or if such shipments are determined to be of national interest. Russia supplies about a quarter of the uranium used in U.S. reactors, making it the nation’s top supplier of the fuel.

Tenex said in its letter that utilities that opt to suspend production would see it resumed after a waiver was granted, though a new delivery schedule could need to be negotiated. Such a move means utilities that don’t receive a waiver within 60 days could effectively lose their place in line for reactor fuel shipments that can take three to four months to complete. 

Tenex, which is owned by Russian nuclear corporation Rosatom, has said in intends honor all their contractual commitments in the U.S., but it has no control over what action the Kremlin might take. 

Analysts have said it’s possible Russia responds by preemptively halting reactor fuel shipments to the U.S. prior to the waiver period expiring in 2028. Such a move could cause the spot price of enrichment services to jump by as much as about 20 per cent, according to nuclear fuel market research firm UxC.