WASHINGTON (Reuters) - Two Democratic lawmakers urged the U.S. Energy Department to move faster with a research program on converting nuclear reactors in the Navy’s submarines and carriers from bomb-grade uranium to a safer fuel, a letter viewed by Reuters showed.

Last year, Congress approved $5 million in defense policy and spending bills for the Energy Department’s office of Naval Reactors to research whether naval reactors could convert to using low-enriched uranium.

The fuel, enriched to under 20 percent uranium, is far safer than what the Navy currently uses: highly enriched uranium, or HEU, which is enriched to above 90 percent and was originally produced for use in nuclear bombs.

But the National Nuclear Security Administration, part of the Energy Department, has not transferred the money from its Defense Nuclear Nonproliferation (DNN) office to the Naval Reactor office, a move necessary for work to start, said the letter dated June 26.

Democratic U.S. Representatives Jim Langevin and Rick Larsen, members of the House of Representatives’ Committee on Armed Services, have pushed for the conversion of Navy ships to low-enriched uranium, saying that doing so would reduce the risk of the fuel getting into the hands of militants. In addition, if the United States converts its fleet, it could be used as an example to prevent other countries that want to use HEU in their fleets from doing so.

“We are at a minimum disappointed with this development,” the lawmakers said in the letter addressed to the National Nuclear Security Administration’s head, Frank Klotz. “We urge DNN to ensure this funding is allocated as Congress intended.”

BWXT Nuclear Operations Group Inc, a subsidiary of BWX Technologies Inc, manufactures nuclear reactor components for the Navy.

The National Nuclear Security Administration did not immediately respond to a request for comment on the letter.