The fissile uranium in
nuclear weapon primaries usually contains 85% or more of 235U known as
weapons-grade, though theoretically for an
implosion design, a minimum of 20% could be sufficient (called weapon(s)-usable) although it would require hundreds of kilograms of material and "would not be practical to design";
[4][5] even lower enrichment is hypothetically possible, but as the enrichment percentage decreases the
critical mass for unmoderated
fast neutrons rapidly increases, with for example, an
infinite mass of 5.4% 235U being required.
[4] For criticality experiments, enrichment of uranium to over 97% has been accomplished.
[6]
The very first uranium bomb,
Little Boy dropped by the
United States on
Hiroshima in 1945, used 64 kilograms of 80% enriched uranium. Wrapping the weapon's fissile core in a
neutron reflector (which is standard on all nuclear explosives) can dramatically reduce the
critical mass. Because the core was surrounded by a good neutron reflector, at explosion it comprised almost 2.5 critical masses. Neutron reflectors, compressing the fissile core via implosion, fusion boosting, and "tamping", which slows the expansion of the fissioning core with inertia, allow
nuclear weapon designs that use less than what would be one bare-sphere critical mass at normal density. The presence of too much of the 238U isotope inhibits the runaway
nuclear chain reaction that is responsible for the weapon's power. The critical mass for 85% highly enriched uranium is about 50 kilograms (110 lb), which at normal density would be a sphere about 17 centimetres (6.7 in) in diameter.
Later US nuclear weapons usually use
plutonium-239 in the primary stage, but the secondary stage which is compressed by the primary nuclear explosion often uses HEU with enrichment between 40% and 80%
[7] along with the
fusion fuel
lithium deuteride. For the secondary of a large nuclear weapon, the higher critical mass of less-enriched uranium can be an advantage as it allows the core at explosion time to contain a larger amount of fuel. The 238U is not
fissile but still fissionable by
fusion neutrons.
HEU is also used in
fast neutron reactors, whose cores require about 20% or more of fissile material, as well as in
naval reactors, where it often contains at least 50% 235U, but typically does not exceed 90%. The Fermi-1 commercial fast reactor prototype used HEU with 26.5% 235U. Significant quantities of HEU are used in the production of
medical isotopes, for example
molybdenum-99 for
technetium-99m generators.
[8]