Governments and oth­ers promoting the use of high-assay low-enriched uranium (HALEU) for nuclear power have not considered the po­tential terrorism risk that widespread adoption of this fuel creates, nuclear scientists have warned.

HALEU is a nuclear reactor fuel enriched with uranium-235 to between 5 and 20%. At 20% uranium-235 and above, the mix­ture is called highly-en­riched uranium (HEU) and it is internationally recognised that it can be employed in nuclear weapons.

Historically, HALEU use has been limited to research reactors, where it is used in small quantities, while commercial reactors typically use fuels with low enrichments, in the range of 3 to 5% uranium-235, which cannot sustain an explosive chain reaction.

However, new advanced reactors are being designed to run on HALEU – most favouring 19.75% uranium-235 HALEU – in the hope that these reactors will be smaller, more flexible and less expensive.

In the US, the Department of Energy (DOE) and US Department of Defense are providing funds for more than 10 reactor concepts, while the UK’s Civil Nuclear Roadmap, announced on 11 January, promised up to £300 million of investment specifically to develop HALEU fuel production.

However, in a policy forum in Science, experts in nuclear science and global security highlight that in many of the designs, the amount of HALEU needed is ‘hundreds to thousands of kilograms’, which may mean that a single reactor contains enough HALEU to make a nuclear weapon.

The authors said that estimates indicate that quantities ranging from several hundred kilograms to about a tonne of 19.75% HALEU could produce explosive yields similar to or greater than that of the Little Boy bomb dropped on Hiroshima.

If this is the case, they said, commercial­ising HALEU fuels without ensuring that the material is ‘appropriately protected against diversion by national governments or theft by terrorists would pose a serious threat to security’.

‘The time has come to review policies governing the use of this material,’ the authors write. ‘We recommend that the US Congress direct the DOE’s National Nuclear Security Administration to com­mission a fresh review of HALEU prolif­eration and security risks by US weapons laboratory experts.’

They also suggested that, according to the informa­tion available, a reasonable balance of the risks and benefits could be struck if enrichment of uranium-235 was restricted to 12% or less.