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Fuel and enrichment
Natural uranium is 0.7% U-235. Depending on the reactor, it is used as is or concentrated up to 5%, and in the new designs up to 20%.
Uranium out of the mine is 0.7% uranium-235, the isotope that fissions with slow neutrons, and 99.3% uranium-238. Heavy water reactors such as the CANDU and Atucha can run on that natural uranium because heavy water barely absorbs neutrons; in Argentina, Embalse and Atucha II do just that, while Atucha I switched in 2001 to slightly enriched uranium, at 0.85%. Light water reactors, PWR and BWR, need the U-235 concentrated to 3 to 5%: that is enrichment.
Enrichment is done in centrifuges that separate uranium hexafluoride by the small mass difference between isotopes. The uranium is then converted into ceramic uranium dioxide and pressed into one-centimeter pellets, which are stacked inside four-meter zircaloy cladding tubes. Hundreds of rods make up a fuel assembly, and 150 to 200 assemblies make up the core of a large PWR.
A fuel assembly spends three to six years in the core. When it comes out, 95% is still uranium, 1% is plutonium and 4% is fission products, which are what make it intensely radioactive. Fast reactors and several SMRs use HALEU, uranium enriched between 5 and 20%, or mixed-oxide fuel with plutonium.
Quick facts
| Natural uranium | 0.7% U-235: CANDU, Atucha II, Magnox · Atucha I runs 0.85% since 2001 |
|---|---|
| Low enrichment | 3 to 5%: PWR, BWR, VVER |
| HALEU | 5 to 20%: fast reactors, HTGR and several SMRs |
| UO₂ pellet | ≈ 1 cm in diameter, ≈ 7 g of uranium |
| Time in the core | 3 to 6 years |