The U.S. nuclear regulator is considering long-term shipments of weapons-grade uranium to a medical research reactor in security-challenged Belgium, something critics say would set back global anti-proliferation efforts.

With a final decision still months away, the Belgian Nuclear Research Center is seeking permission from the U.S. Nuclear Regulatory Commission to receive 144 kgs of highly enriched uranium, or HEU, fuel in a series of shipments over 10 years.

The United States has supplied the reactor, which produces radioisotopes for fighting cancer, with HEU for decades. But the long-term nature of the latest request is unprecedented; previous agreements have been for periods of one to three years.