The U.S. Energy Department has waived a ban on licenses for the export of weapons-grade uranium for making medical isotopes, a move that critics said raises proliferation risks and undermines companies that are converting to safer materials.

Highly enriched uranium (HEU) is commonly used to make medical isotopes, and for this purpose large quantities are not needed. However, if enough of it were accumulated it could find its way into a bomb.

U.S. Secretary of Energy Dan Brouillette has said in a letter to the top members of the U.S. House of Representatives energy committee that his department had determined that the global supplies of a substance that is used to make medical isotopes but does not involve highly enriched uranium are not sufficient to meet the needs of American patients.

In the letter, which was dated Jan. 2, Brouillette waived for two years a ban on licenses issued by the U.S. Nuclear Regulatory Commission that had been set to begin on Jan. 3.

The substance is known as non-HEU-based Mo-99.

A company in Belgium called Institute for Radioelements (IRE) wants the HEU to make the isotopes.

Radioisotopes are used for a variety of medical purposes such as imaging, diagnosis and treatment of cancer.

Alan Kuperman, a University of Texas professor and founding coordinator of the Nuclear Proliferation Prevention Project, said the move undermines companies’ converting to low enriched uranium, a material that does not pose proliferation risks, and startups that are making the medical materials without HEU.

“It’s irresponsible and unnecessary,” Kuperman said. He said the Energy Department was hurting U.S. policy, “akin to putting the soccer ball in your own net.”

The National Nuclear Security Administration, part of the Department of Energy, did not immediately respond to a request for comment.

While the amount of HEU that IRE asked for in a recent request is less than 11 pounds (5 kilograms), exports of any amount of the material is risky, especially as alternatives are available, the nonproliferation experts said.

IRE did not immediately respond to a request for comment.

Belgium has had security problems at nuclear facilities. In 2014, a perpetrator drained lubricant at a reactor, shutting the power plant and causing $200 million in damages. In 2016 police said bombers who blew themselves up in Brussels had secretly filmed movements of the head of Belgium’s nuclear research program.

Experts said the risk is low that HEU would make it into the hands of militants seeking to make crude weapons, but the United States should not ship any since alternatives exist. “The more of this material out there in circulation, the more dangerous it is,” said Miles Pomper, a fellow at the James Martin Center for Nonproliferation Studies.

Proliferation fears have recently been fueled even more by tensions between the U.S. and Iran.

Tehran said Sunday it is going ahead with unrestricted uranium enrichment as a fifth step toward leaving a major nuclear deal with world powers aimed at curbing Iran’s nuclear activities. The Iranian government said it is no longer abiding by uranium enrichment limitations under the 2015 agreement but it will continue to cooperate with the U.N. nuclear watchdog, the International Atomic Energy Agency.

“By taking the fifth step in reducing its commitment, the Islamic Republic of Iran eliminates the last key operational restriction it faced under the JCPOA, which is the limitation imposed on the number of centrifuges,” the government said, according to Iran’s Press TV.

JCPOA stands for the Joint Comprehensive Plan of Action, the nuclear deal Iran forged with major powers such as Britain, China, Russia and the United States, in 2015.

The government also said it is ready to implement its commitments under the deal if sanctions imposed by the United States are lifted.

The agreement placed limits on Iran’s nuclear program in exchange for relief from economic sanctions.

In 2018, the United States unilaterally pulled out of the deal and imposed stringent sanctions on Iran, including banning Iranian oil exports, which constitute the country’s major revenue source.

Beginning last May, Iran had been taking steps every 60 days to reduce its compliance with the deal, in a bid to receive an economic relief package from Europe that would counter the U.S. sanctions.

Iran in July began enriching uranium beyond the 3.67 percent purity allowed under the deal, but as of late last year, it was enriching uranium at around 4.5 percent, which is equivalent to that for fuel used at nuclear power plants.