If you’ve played 7-card stud poker, you know that your hand can be both high and low at the same time (I won a nice pot that way once). So it appears with uranium, enriched in the fissionable isotope U-235 to a level called HALEU: high-assay low-enrichment uranium.
In 1979, the US Nuclear Regulatory Commission adopted a limit of 20% U-235 in uranium as being fissionable in weapons. Now a number of governments and industry groups are calling for HALEU up to 19.75% U-235 to be used in peaceful reactors. The high enrichment would make small power reactors feasible despite their inefficiency in using neutrons to push the chain reaction along. The thinking is that further enrichment is technically very challenging if any government or other actor wants to make fission weapons.
However, the 20% limit is not magical. Even back in 1984, Carson Mark at Los Alamos testified to the US Congress as a weapons expert that weapons can be made with enrichments even as low as 10%. At 19.75% U-235 it would only take a ton of uranium – the amount in an 18”-diameter ball - to make a Hiroshima-level explosion! Five illustrious long-time experts in security urge that the limit be set at 12% enrichment. Shout it loud and clear!
This has been an outreach activity of the Las Cruces Academy, viewable at GreatSchools.org.
Ref.: R. Scott Kemp et al., Science 384: 1071-1073 (June 7, 2024)
