The opposite of enriching is downblending; surplus HEU can be downblended to LEU to make it suitable for use in commercial nuclear fuel.
The HEU feedstock can contain unwanted uranium isotopes: 234U is a minor isotope contained in natural uranium; during the enrichment process, its concentration increases but remains well below 1%. High concentrations of 236U are a byproduct from irradiation in a reactor and may be contained in the HEU, depending on its manufacturing history. HEU reprocessed from nuclear weapons material production reactors (with an 235U assay of approx. 50%) may contain 236U concentrations as high as 25%, resulting in concentrations of approximately 1.5% in the blended LEU product. 236U is a neutron poison; therefore the actual 235U concentration in the LEU product must be raised accordingly to compensate for the presence of 236U.
The blendstock can be NU, or DU, however depending on feedstock quality, SEU at typically 1.5 wt% 235U may used as a blendstock to dilute the unwanted byproducts that may 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.
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.
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.
- A uranium downblending calculator designed by the WISE Uranium Project
Read more about this topic: Enriched Uranium