WASHINGTON, June 6 (Reuters) - A special uranium fuel

planned for use in next-generation U.S. nuclear reactors poses

security risks because it can be used without further enrichment

for use in nuclear weapons, scientists said in an article

published on Thursday.

The fuel, called high-assay low-enriched uranium, or HALEU,

is enriched to levels of up to 20%, compared with about 5% for

the fuel that powers most existing reactors. Until recently it

was made in commercial amounts only in Russia, but the United

States wants to produce it to fuel a new wave of reactors.

President Joe Biden's administration believes nuclear power

that is virtually emissions-free is essential in the fight

against climate change. Biden's Inflation Reduction Act provided

$700 million for a HALEU availability program including

purchasing the fuel to create a supply chain for some planned

small modular reactors and other planned high-tech reactors.

Uranium is a radioactive element that exists naturally. To

make nuclear fuel, raw uranium undergoes processes that result

in a material with an increased concentration of the isotope

uranium-235.

"This material is directly usable for making nuclear weapons

without any further enrichment or reprocessing," said Scott

Kemp, one of five authors of the peer-reviewed article in the

journal Science. "In other words, the new reactors pose an

unprecedented nuclear-security risk," said Kemp, a professor at

the Massachusetts Institute of Technology and a former science

adviser on arms control at the State Department.

A bomb similar in power to the one the U.S. dropped on

Hiroshima, Japan in 1945 could be made from 2,200 pounds (1,000

kg) or less of 19.75% enriched HALEU, the article said.

"Designing such a weapon would not be without its challenges,

but there do not appear to be any convincing reasons why it

could not be done," it said.

The authors said if enrichment is limited to 10% to 12%, the

supply chain would be far safer with only modest costs.

The authors said HALEU is a domestic risk as it is not

required to have the protections normally required for

weapons-usable material. U.S. use of the fuel could also set a

precedent for other countries building the reactors where

proliferation standards are not as strict.

"Were HALEU to become a standard reactor fuel without

appropriate restrictions determined by an interagency security

review, other countries would be able to obtain,

produce, and process weapons-usable HALEU with impunity,

eliminating the sharp distinction between peaceful and

nonpeaceful nuclear programs," said the article, also written by

Edwin Lyman at the Union of Concerned Scientists nonprofit

group.

The U.S. Department of Energy estimates that more than 40

metric tonnes of HALEU could be needed before the end of the

decade, with additional amounts required each year, to deploy

advanced reactors to support the Biden administration's goal of

100% clean electricity by 2035.

The DOE did not immediately respond to a request for

comment.

TerraPower, a company backed by Bill Gates that has received

funding from the Energy Department, hopes to build its Natrium

nuclear plant in Wyoming by 2030 to run on HALEU. TerraPower in

late 2022 delayed Natrium's launch date by at least two years to

2030 due to a lack of HALEU.

TerraPower did not immediately respond to a request for

comment. The plant is expected to start construction on the

non-nuclear side but needs federal permits to build the nuclear

work.

Centrus Energy ( LEU ) a U.S. company that has begun making

small amounts of HALEU in Ohio and is working with TerraPower to

establish commercial production capabilities for the 2030 start,

did not immediately respond to a request for comment.