Editing Enriched uranium (section)

==Downblending==<!-- This section is linked from [[Radioactive waste]] -->
The opposite of enriching is downblending; surplus HEU can be downblended to LEU to make it suitable for use in commercial nuclear fuel. Downblending is a key process in nuclear non-proliferation efforts, as it reduces the amount of highly enriched uranium available for potential weaponization while repurposing it for peaceful purposes.
[[File:U.S. Department of Energy - Science - 089 045 001 (10314669725).jpg|thumb|Enriched uranium produced at LLNL plant was collected as nuggets the size and thickness of several quarters.]]
The HEU feedstock can contain unwanted uranium isotopes: [[Uranium-234|<sup>234</sup>U]] is a minor isotope contained in natural uranium (primarily as a product of [[alpha decay]] of {{chem|238|U| link= uranium-238}}—because the [[half-life]] of {{chem|238|U}} is much larger than that of {{chem|234|U}}, it is produced and destroyed at the same rate in a constant steady state equilibrium, bringing any sample with sufficient {{chem|238|U}} content to a stable ratio of {{chem|234|U}} to {{chem|238|U}} over long enough timescales); during the enrichment process, its concentration increases but remains well below 1%. High concentrations of [[Uranium-236|<sup>236</sup>U]] are a byproduct from irradiation in a reactor and may be contained in the HEU, depending on its manufacturing history. {{Chem|236|U}} is produced primarily when {{chem|235|U}} absorbs a neutron and does not fission. The production of {{chem|236|U}} is thus unavoidable in any thermal neutron reactor with {{chem|235|U}} fuel. HEU reprocessed from nuclear weapons material production reactors (with an <sup>235</sup>U assay of approximately 50%) may contain <sup>236</sup>U concentrations as high as 25%, resulting in concentrations of approximately 1.5% in the blended LEU product. [[Uranium-236|<sup>236</sup>U]] is a [[neutron poison]]; therefore the actual <sup>235</sup>U concentration in the LEU product must be raised accordingly to compensate for the presence of <sup>236</sup>U. While {{chem|234|U}} also absorbs neutrons, it is a [[fertile material]] that is turned into fissile {{chem|235|U}} upon [[neutron absorption]]. If {{chem|236|U}} absorbs a neutron, the resulting short-lived {{chem|237|U}} [[beta decay]]s to {{chem|237|Np| link=Neptunium-237}}, which is not usable in thermal neutron reactors but can be chemically separated from spent fuel to be disposed of as waste or to be transmutated into {{chem|238|Pu|link= Plutonium-234}} (for use in [[nuclear battery| nuclear batteries]]) in special reactors. Understanding and managing the isotopic composition of uranium during downblending processes is essential to ensure the quality and safety of the resulting nuclear fuel, as well as to mitigate potential radiological and proliferation risks associated with unwanted isotopes.

The blendstock can be NU or DU; however, depending on feedstock quality, SEU at typically 1.5 wt% <sup>235</sup>U may be used as a blendstock to dilute the unwanted byproducts that may be contained in the HEU feed. Concentrations of these isotopes in the LEU product in some cases could exceed [[ASTM]] specifications for nuclear fuel if NU or DU were used. So, the HEU downblending generally cannot contribute to the waste management problem posed by the existing large stockpiles of depleted uranium. Effective management and disposition strategies for depleted uranium are crucial to ensure long-term safety and environmental protection. Innovative approaches such as reprocessing and recycling of depleted uranium could offer sustainable solutions to minimize waste and optimize resource utilization in the nuclear fuel cycle.

A major [[downblending]] undertaking called the [[Megatons to Megawatts Program]] converts ex-Soviet weapons-grade HEU to fuel for U.S. commercial power reactors. From 1995 through mid-2005, 250 tonnes of high-enriched uranium (enough for 10,000 warheads) was recycled into low-enriched uranium. The goal is to recycle 500 tonnes by 2013. The decommissioning programme of Russian nuclear warheads accounted for about 13% of total world requirement for enriched uranium leading up to 2008.<ref name="Lodge"/>This ambitious initiative not only addresses nuclear disarmament goals but also serves as a significant contributor to global energy security and environmental sustainability, effectively repurposing material once intended for destructive purposes into a resource for peaceful energy production.

The [[United States Enrichment Corporation]] has been involved in the disposition of a portion of the 174.3 tonnes of highly enriched uranium (HEU) that the U.S. government declared as surplus military material in 1996. Through the U.S. HEU Downblending Program, this HEU material, taken primarily from dismantled U.S. nuclear warheads, was recycled into low-enriched uranium (LEU) fuel, used by [[nuclear power plants]] to generate electricity.<ref>{{cite web|url = http://www.usec.com/v2001_02/HTML/Megatons_DOEstatus.asp|date = 1 May 2000 |title = Status Report: USEC-DOE Megatons to Megawatts Program|archive-url=https://web.archive.org/web/20010406102039/http://www.usec.com/v2001_02/HTML/Megatons_DOEstatus.asp |archive-date=6 April 2001|publisher = USEC.com }}</ref><ref>{{cite web|title =Megatons to Megawatts|website = centrusenergy.com|date = December 2013|url= https://www.centrusenergy.com/who-we-are/history/megatons-to-megawatts/}}</ref> This innovative program not only facilitated the safe and secure elimination of excess weapons-grade uranium but also contributed to the sustainable operation of civilian nuclear power plants, reducing reliance on newly enriched uranium and promoting non-proliferation efforts globally