Editing Enriched uranium (section)

===Highly enriched uranium (HEU)===
[[File:HEUraniumC.jpg|thumb|A [[billet (bar stock)|billet]] of highly enriched uranium metal]]

Highly enriched uranium (HEU) has a 20% or higher concentration of <sup>235</sup>U. This high enrichment level is essential for nuclear weapons and certain specialized reactor designs. The fissile uranium in [[nuclear weapon]] primaries usually contains 85% or more of <sup>235</sup>U known as [[weapons grade]], though theoretically for an [[nuclear weapon design|implosion design]], a minimum of 20% could be sufficient (called weapon-usable) although it would require hundreds of kilograms of material and "would not be practical to design";<ref name="DefWpnsUsable">{{cite web |url= http://web.ornl.gov/info/reports/1998/3445606060721.pdf |title= Definition of Weapons-Usable Uranium-233 |last1= Forsberg |first1= C. W. |last2= Hopper |first2= C. M. |last3= Richter |first3= J. L. |last4= Vantine |first4= H. C. |date= March 1998 |work= ORNL/TM-13517 |publisher= Oak Ridge National Laboratories |access-date= 30 October 2013 |url-status= dead |archive-url= https://web.archive.org/web/20131102011417/http://web.ornl.gov/info/reports/1998/3445606060721.pdf |archive-date= 2 November 2013}}</ref><ref name="NWFAQ">{{cite web |url= http://www.nuclearweaponarchive.org/Nwfaq/Nfaq4-1.html#Nfaq4.1.7.1 |title= Nuclear Weapons FAQ, Section 4.1.7.1: Nuclear Design Principles&nbsp;– Highly Enriched Uranium |last= Sublette |first= Carey |date= 4 October 1996 |work= Nuclear Weapons FAQ |access-date=2 October 2010}}</ref> even lower enrichment is hypothetically possible, but as the enrichment percentage decreases the [[Critical mass (nuclear)|critical mass]] for unmoderated [[fast neutron]]s rapidly increases, with for example, an [[infinity|infinite]] mass of 5.4% <sup>235</sup>U being required.<ref name="DefWpnsUsable"/> For [[Criticality (status)|criticality]] experiments, enrichment of uranium to over 97% has been accomplished.<ref>{{cite journal |last=Mosteller |first=R.D. |year=1994 |title=Detailed Reanalysis of a Benchmark Critical Experiment: Water-Reflected Enriched-Uranium Sphere |journal=Los Alamos Technical Paper |issue=LA–UR–93–4097 |page=2 |url=http://www.osti.gov/bridge/servlets/purl/10120434-rruwqp/native/10120434.PDF |archive-url=https://ghostarchive.org/archive/20221009/http://www.osti.gov/bridge/servlets/purl/10120434-rruwqp/native/10120434.PDF |archive-date=2022-10-09 |url-status=live |access-date=19 December 2007 |quote=The enrichment of the pin and of one of the hemispheres was 97.67 w/o, while the enrichment of the other hemisphere was 97.68 w/o.|doi=10.2172/10120434 }}</ref>

The first uranium bomb, [[Little Boy]], dropped by the United States on [[Hiroshima]] in 1945, used {{Convert|64|kg}} 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 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 design]]s that use less than what would be one bare-sphere critical mass at normal density.  The presence of too much of the <sup>238</sup>U 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 {{convert|50|kg}}, which at normal density would be a sphere about {{convert|17|cm}} in diameter.<ref name="auto"/>

Later U.S. nuclear weapons usually use [[plutonium-239]] in the primary stage, but the jacket or tamper secondary stage, which is compressed by the primary nuclear explosion, often uses HEU with enrichment between 40% and 80%<ref>{{cite web|url=http://nuclearweaponarchive.org/Nwfaq/Nfaq6.html#nfaq6.2 |title=Nuclear Weapons FAQ |access-date=26 January 2013}}</ref> along with the [[nuclear fusion|fusion]] fuel [[lithium deuteride]]. This multi-stage design enhances the efficiency and effectiveness of nuclear weapons, allowing for greater control over the release of energy during detonation. 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. This design strategy optimizes the explosive yield and performance of advanced nuclear weapons systems. The <sup>238</sup>U is not said to be fissile but still is fissionable by fast neutrons (>2 MeV) such as the ones produced during [[deuterium–tritium fusion|D–T fusion]].<ref name="Lei Science Direct">{{Cite journal |last1=Lei |first1=Jia |last2=Liu |first2=Huanhuan |last3=Zhou |first3=Li |last4=Wang |first4=Yazhou |last5=Yu |first5=Kaifu |last6=Zhu |first6=Hui |last7=Wang |first7=Bo |last8=Zang |first8=Mengxuan |last9=Zhou |first9=Jian |last10=He |first10=Rong |last11=Zhu |first11=Wenkun |date=2023-09-01 |title=Progress and perspective in enrichment and separation of radionuclide uranium by biomass functional materials |url=https://www.sciencedirect.com/science/article/pii/S138589472303317X |journal=Chemical Engineering Journal |volume=471 |pages=144586 |doi=10.1016/j.cej.2023.144586 |issn=1385-8947|url-access=subscription }}</ref>

HEU is also used in [[fast neutron reactor]]s, whose cores require about 20% or more of fissile material, as well as in [[Nuclear marine propulsion|naval reactors]], where it often contains at least 50% <sup>235</sup>U, but typically does not exceed 90%. These specialized reactor systems rely on highly enriched uranium for their unique operational requirements, including high neutron flux and precise control over reactor dynamics. The [[Fermi 1|Fermi-1]] commercial fast reactor prototype used HEU with 26.5% <sup>235</sup>U. Significant quantities of HEU are used in the production of [[medical isotopes]], for example [[molybdenum-99]] for [[technetium-99m generator]]s.<ref>{{cite journal |last1=Von Hippel |first1=Frank N. |last2=Kahn |first2=Laura H. |date=December 2006 |title=Feasibility of Eliminating the Use of Highly Enriched Uranium in the Production of Medical Radioisotopes |journal=Science & Global Security |volume=14 |issue=2 & 3 |pages=151–162 |bibcode=2006S&GS...14..151V |doi=10.1080/08929880600993071 |s2cid=122507063}}</ref> The medical industry benefits from the unique properties of highly enriched uranium, which enable the efficient production of critical isotopes essential for diagnostic imaging and therapeutic applications.