808s Online : A Nuclear Fuel Cycle Blog reported in 2020 that enrichment is often a black box, but understanding it is key to fine-tuning any model of global uranium requirements. Here’s part one of an overview of enrichment technology.
Summary
Reactors require fuel with a higher-than-naturally-occurring concentration of the uranium-235 isotope. U-235 concentration is increased via enrichment. The balance between uranium requirements and enrichment work (SWU) is moderated by the residual U-235 in enrichment waste material (the “tails assay”). Predictions (or knowledge) of the future price of uranium and enrichment determine a utility’s choice of tails assay. If a guess is not precise, it can add additional cost and increase or decrease the amount of uranium they buy.
Introduction to Enrichment
Long before Chicago Pile-1 went critical in 1942, atomic scientists knew that the ratio of uranium isotopes from in nature would make nuclear chain reactions difficult. Uranium-238 forms the 99.3% majority of all uranium on Earth, but its atomic properties make it difficult to split in a nuclear reactor (or in a bomb). Uranium-235, 0.7% of all natural uranium, is chemically identical to uranium-238 (same number of protons, different number of neutrons), but its atomic structure makes it easier to fission (split) when hit by a neutron. [There are reactors that can use natural/un-enriched uranium to make power, including both the current generation of CANDU reactors and future “fast” reactors, but these make up a small percentage of global demand and I don’t want to spend a ton of time discussing them (sorry, Canadian readers!)]
To build an atomic bomb (power reactors came later), uranium had to be “enriched” in uranium-235 content, that is, the relative proportion of U-235 to U-238 must be increased. Uranium concentrates (U3O8, as it is sold), is converted into uranium hexafluoride gas (UF6), and fed into centrifuges to perform enrichment.
There’s a microscopic amount of U-234 out there in nature, but this is the short summary, after all. The general theory is simple: by using energy in the form of centrifuges spinning, natural uranium is divided into product (with more U-235) and tails (with less U-235). Product goes on to become reactor fuel, while tails are treated as waste. The tails assay, as it is called, is the U-235 content in the tails (e.g. 0.22%). Product material is generally produced between 3% and 5% U-235.
At 0.22% U-235 tails assay: 8.5 kilograms of natural UF6 at 0.7% U-235 go into the centrifuge cascade, 1 kilogram of EUP at 4.95% U-235 and 7.5 kilograms of tails at 0.22% U-235 come out.It’s worth noting that any uranium can be fed into centrifuges, including material that is already enriched to enrich it more, or depleted tails material to capture more of its U-235 content (so-called “SWU for U”). Just as natural uranium can be enriched to 5%, depleted uranium can be enriched back to 0.7% (such that it mimics natural uranium).
Measuring Enrichment Work
Enrichment is measured in separative work units, known by its acronym, SWU [“swoo”]. The exact SWU calculation for enrichment is tedious, but it depends on two factors:
- The quantity of enriched uranium to be produced (mass)
- The difference in U-235 content between the product and tails
- The number of uranium-235 atoms are conserved in the enrichment process; it’s just a question of how they are divided between product and tails. The greater the difference (i.e. the lower the tails assay), the more enrichment work (SWU) is required. There are two options, then, for how to produce the same amount of enriched uranium:
Use more SWU. Less uranium is needed, and tails have less U-235.
Use fewer SWU. More uranium is needed and tails have more U-235.
Much more information on this site
Managed & Built by I2M Consulting, LLC | © 2010-
