This is an old revision of this page, as edited by DV8 2XL (talk | contribs) at 04:13, 4 October 2005 (→Downblending). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.
Revision as of 04:13, 4 October 2005 by DV8 2XL (talk | contribs) (→Downblending)(diff) ← Previous revision | Latest revision (diff) | Newer revision → (diff)Enriched uranium is uranium whose uranium-235 content has been increased through the process of isotope separation. Natural uranium consists mostly of the U-238 isotope, with about 0.7 percent by weight as U-235, the only isotope existing in nature in any appreciable amount that is fissionable by thermal neutrons.
The ability to enrich uranium is one of the key factors in nuclear weapons proliferation.
During the Manhattan Project enriched uranium was given the codename oralloy, a shortened version of Oak Ridge alloy, after the plant where the uranium was enriched. The term oralloy is still occasionally used to refer to enriched uranium.
The U-238 remaining after enrichment is known as depleted uranium (DU), and is considerably less radioactive than even natural uranium, though still extremely dense and is useful for armor and armor penetrating weapons.
For information on how uranium is enriched see isotope separation.
Highly enriched uranium (HEU)
Highly enriched uranium (HEU) has a greater than 20% concentration of U-235.
The fissile uranium in nuclear weapons usually contains 85% or more of U-235 (weapons-grade or weapon-grade), though for a crude inefficient weapon 20% is sufficient (this is called weapons-usable or weapon-usable; some argue that even less is sufficient, but that then the critical mass rapidly increases). The presence of too much of the U-238 isotope inhibits the runaway nuclear chain reaction that is responsible for the weapon's power.
HEU is also used in nuclear submarine reactors, where it contains at least 50% U-235, but typically exceeds 90%.
Low-enriched uranium (LEU)
Low-enriched uranium (LEU) has a lower than 20% concentration of U-235.
For use in commercial light water reactors (LWR), the most prevalent power reactors in the world, uranium is enriched to 3 to 5 percent U-235. It is not a direct bomb risk. LEU used in research reactors is usually enriched 12% to 19.75% U-235, the later concentration being used to replace HEU fuels when converting to LEU.
Slightly enriched uranium (SEU)
Slightly enriched uranium (SEU) between 0.9% to 2% enrichment.
This new grade is being used to replace Natural uranium (NU) fuel in some heavy water reactors like the CANDU. Costs are lowered because less uranium and fewer bundles are needed to fuel the reactor. This in turn reduces the quantity of used fuel and its subsequent waste management costs. Recovered uranium (RU) is a variation of SEU. It is based on a fuel cycle involving used fuel recovered from light water reactors (LWR)
Downblending
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:U-234 is a minor isotope contained in natural uranium; during the enrichment process, its concentration increases even more than that of U-235. High concentrations of U-234 may cause excessive worker radiation exposures during fuel fabrication; U-236 is 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 U-235 assay of approx. 50%) may contain U-236 concentrations as high as 25%, resulting in concentrations of approx. 1.5% in the blended LEU product. U-236 is a neutron poison; therefore the actual U-235 concentration in the LEU product must be raised accordingly to compensate for the presence of U-236.
The blendstock can be NU, or DU, however depending on feedstock quality, SEU at typically 1.5 wt-% U-235 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.
Also a more detailed economic analysis of downblending actually performed in Russia suggests that blendstock enrichment even consumes 20% more Separative Work Units (SWU) than can be recovered. This means that no recovery takes place at all, and the whole process is a SWU sink.
External links
- Uranium enrichment (PDF, 651 KB)