Media and government reports often refer to "highly-enriched" and "weapons-grade" uranium, sometimes interchangeably. But in fact the terms have different meanings that shouldn't be confused: "weapons-grade" refers to a level of enriched uranium that is capable of being used in an atomic bomb; "highly-enriched" refers only to the fact that uranium has been enriched, not necessarily enough to be used in a bomb.
Alexander Melishkivili, a policy analyst at the Center for Nonproliferation Studies, says that interchanging the two descriptions can be highly misleading, and can exaggerate unnecessarily the threat posed by reported incidents of uranium trafficking . "The threat is there," says Melishkivili, "but it's not helpful to fan the flames of hysteria."
Uranium straight out of the ground is composed almost entirely of a relatively heavy isotope called U-238. Nuclear fission requires the presence of a highly reactive isotope called U-235, which appears in negligible quantities in raw uranium. Enrichment involves depleting the concentration of U-238 isotopes, and dramatically increasing the concentration of U-235 isotopes. An enrichment level of 5-20% is necessary to fuel a power plant; enrichment of at least 85-90% is what's needed to fuel a nuclear weapon.
An enrichment plant requires a complicated network of centrifuge processing to transform uranium from its raw to an enriched state. Much of the technology is subject to multilateral export controls. But once up and running, the same technology that increases the concentration of U-235 isotopes to a level high enough to fuel a power plant can also be used to increase the concentration to a level suitable for use in nuclear weapons. It's a question of how many centrifuges are spinning and how long they spin–in other words, of intent.
Next time you encounter a uranium salesman, or more likely a reference to it in the media, here's a handy reference guide:
- Raw uranium is composed of 0.7% U-235.
- Low-enriched uranium, used in power plants, is enriched to less than 20% U-235.
- Highly-enriched uranium, used in research reactors for medicine, engineering and other specialized civilian applications, is enriched to between 20 and 90 percent U-235.
- Weapons-grade uranium, used in bombs and in some civilian reactors, is enriched to over 90% U-235.
News stories occasionally report that a smuggler was caught with "enough enriched uranium for a dirty bomb," or radiological dispersal device. But these use conventional weapons technologies to spread radioactive material over a large area, and do not use uranium.
Instead, the radioactive ingredient in a dirty bomb would more likely be cesium-137 or strontium-90. Smuggling incidents involving these materials are not recorded on the IAEA list of "proliferation-significant" events.