Enrichment and the SWU

Separating uranium-235 from uranium-238 is not a matter of purifying a substance, but of doing thermodynamic work against the entropy of a mixture. That work is measured in separative work units, SWU: a quantity that depends on how much uranium is fed in, what concentration it is meant to reach, and how much U-235 one is willing to leave in the depleted tails. The poorer the tails are left, the less natural uranium is needed but the more SWU are required; producing one kilogram of 5% uranium takes about 7.9 SWU if the tails are left at 0.25% U-235, or 8.9 SWU if they are depleted down to 0.20%. It is, at bottom, the same trade-off as in mining: more separative work in exchange for less raw material.

That work is now done almost exclusively with gas centrifuges: cylinders spinning at 50,000–70,000 revolutions per minute, using about 50 kWh per SWU, designed to spin without stopping for 25 years, because stopping and restarting them is riskier than leaving them running. Gaseous diffusion, the method that dominated the 20th century, pushed UF₆ through porous membranes thousands of times and needed about 2,500 kWh per SWU, fifty times more energy; the last US diffusion plant closed in 2013, and the method is now commercially obsolete.

World enrichment capacity runs at about 68 million SWU a year, and it is concentrated in few hands: Rosatom controls about 29.1 million (43% of the total), Urenco 17.3 million, CNNC 13.8 million and Orano 7.5 million. That concentration, with Russia in the lead, is why several Western countries are now financing new enrichment capacity of their own.

The enrichment level depends on the reactor. Light-water PWRs and BWRs need low-enriched uranium, 3 to 5% U-235; CANDUs and Atucha, with heavy water, need no enrichment at all, though Atucha I has used a little anyway since 2001, at 0.85%, to stretch its fuel burnup. Newer designs, fast reactors, HTGRs and most SMRs, use HALEU, high-assay low-enriched uranium, between 5 and 20%. Argentina has had its own plant since the late 1970s: the Pilcaniyeu Technological Complex, built by INVAP for CNEA near Bariloche, which enriched by gaseous diffusion at pilot scale in the early 1980s, was dismantled in the 1990s, and had its capabilities recovered and upgraded between 2006 and 2015, though the WNA notes commercial production was halted again in 2018 and the plant has never been used for commercial or export purposes. It now places the country among the thirteen in the world that master enrichment technology, with the declared goal of supplying the CAREM.