Conversion and enrichment

Hana's next task is commercial rather than technical. TVA's fuel buyers want a clear explanation for the board of why the cost of a reload batch changed so much over three years. "Most of the answer is in three markets," Amara says. "Uranium, conversion and enrichment. Fuel buyers do not think in metric tons of rock or in machines. They think in kilograms of uranium, kilograms of conversion and units of separative work. Learn to think the way they do."

Refining and conversion

Uranium ore concentrate from a mill is not pure enough to use directly. A refinery dissolves it and purifies it to nuclear grade, removing impurities that absorb neutrons or would harm the fuel. What happens next depends on the reactor. For natural uranium reactors such as CANDU, the purified uranium is converted to uranium dioxide powder, ready to be pressed into pellets. For reactors that need enriched fuel, it is converted instead to uranium hexafluoride, UF₆, a compound that becomes a gas at modest temperatures and is the feed material for all commercial enrichment plants.

The only uranium conversion plant in the United States is Honeywell's Metropolis Works in Illinois, operated by ConverDyn, which converts concentrate to uranium hexafluoride. Other conversion capacity is in Canada, France, Russia and China. Conversion is a small part of the cost of fuel, but supply has been tight at times, which is why fuel buyers watch it closely.

Uranium hexafluoride is handled in large steel cylinders designed and certified for the purpose. It is a solid at room temperature and pressure and is heated to become a liquid or gas for transfer. Its main hazard is chemical rather than radiological: if released, it reacts with moisture in the air to form hydrogen fluoride, which is highly toxic and corrosive, and uranyl fluoride particles. Plants that handle it are designed and regulated as chemical as well as nuclear facilities, with containment, detection and emergency plans for releases.

What enrichment does

Natural uranium is about 99.3 percent uranium-238 and 0.71 percent uranium-235. Enrichment increases the fraction of uranium-235. Commercial power reactor fuel is low-enriched uranium, defined internationally as containing less than 20 percent uranium-235, and in practice today's light water reactor fuel is enriched to between about three and five percent. Some advanced reactor designs plan to use high-assay low-enriched uranium, called HALEU, enriched to between five and just under twenty percent. Material above twenty percent is classed as highly enriched uranium and is subject to the strictest controls.

Almost all commercial enrichment today uses the gas centrifuge, in which uranium hexafluoride gas spins at high speed and the slightly heavier molecules containing uranium-238 move outward, so that gas drawn from near the axis is slightly richer in uranium-235. A single machine changes the composition only a little, so plants link very large numbers of machines together. The technology is among the most sensitive in the nuclear field because the same principle could in principle be used to reach high enrichments, and its details are controlled by national export rules and international agreements. This course treats enrichment as an industrial service bought by the kilogram, which is how utilities and regulators see it.

The only operating commercial enrichment plant in the United States is the Urenco USA plant at Eunice, New Mexico, licensed by the NRC. Other plants are in Europe, Russia and China. After 2022 the United States moved to reduce reliance on Russian enrichment, and in 2024 Congress passed a law banning imports of Russian enriched uranium, with waivers possible through 2027. The Department of Energy has also funded programs to build domestic HALEU supply for advanced reactors.

Separative work: the unit of enrichment

Enrichment is sold in separative work units, written SWU and pronounced like 'swoo'. A SWU is a measure of the effort needed to separate a quantity of uranium into a richer product and a leaner tails stream; it is to enrichment what kilowatt-hours are to electricity. The SWU needed depends on the product assay, the feed assay and the tails assay, through a standard function called the value function. Buyers use it to compare offers and to choose the best tails assay: a lower tails assay saves natural uranium but needs more separative work, and the economic optimum shifts as the prices of uranium and SWU change.

In the equations, F is the kilograms of natural uranium feed needed for each kilogram of product, x is the assay as a fraction of uranium-235, and the subscripts p, f and t mean product, feed and tails. The first equation is simply a mass balance on uranium-235. The second gives the separative work per kilogram of product. These are the formulas in every fuel buyer's spreadsheet and in regulators' economic analyzes, and they say nothing about how the work is done in a plant.

# Front-end fuel cost per kilogram of low-enriched uranium (LEU).
# The separative work (SWU) value function is the standard economic accounting tool.
# Prices are illustrative.
import math
def V(x):                       # value function of an assay x (a fraction)
    return (2 * x - 1) * math.log(x / (1 - x))
xf, xp, xt = 0.00711, 0.045, 0.0025     # feed, product and tails assays (commercial LEU)
F = (xp - xt) / (xf - xt)              # kg natural uranium feed per kg of product
T = F - 1                              # kg tails per kg of product
SWU = V(xp) + T * V(xt) - F * V(xf)    # kg-SWU per kg of product
price_U3O8_lb, conv, swu_price, fab = 80.0, 15.0, 150.0, 400.0
u_cost = F * price_U3O8_lb * 2.20462 * 1.1792   # dollars of uranium per kg product
total = u_cost + F * conv + SWU * swu_price + fab
print(f"feed {F:.2f} kg U and {SWU:.2f} kg-SWU per kg of product at {xp*100:.1f} %")
print(f"uranium {u_cost:6.0f}  conversion {F*conv:5.0f}  enrichment {SWU*swu_price:5.0f}  fabrication {fab:4.0f}")
print(f"total about {total:,.0f} dollars per kg of fuel uranium")
for t in (0.0020, 0.0030):
    F2 = (xp - t) / (xf - t); S2 = V(xp) + (F2 - 1) * V(t) - F2 * V(xf)
    print(f"tails {t*100:.2f} %: feed {F2:.2f} kg, {S2:.2f} kg-SWU")

feed 9.22 kg U and 6.87 kg-SWU per kg of product at 4.5 %
uranium   1917  conversion   138  enrichment  1031  fabrication  400
total about 3,486 dollars per kg of fuel uranium
tails 0.20 %: feed 8.41 kg, 7.69 kg-SWU
tails 0.30 %: feed 10.22 kg, 6.23 kg-SWU

With the illustrative prices in the program, uranium is about half the front-end cost, enrichment about a third, and conversion and fabrication the rest. Lowering the tails assay from 0.25 to 0.20 percent saves almost a kilogram of natural uranium per kilogram of product but costs almost one more SWU. When uranium is expensive relative to enrichment, buyers ask the enricher to strip the tails lower; when enrichment is expensive, they accept higher tails. This trade is one reason why the price of uranium and the price of SWU move together over the long run.

Depleted uranium

Enrichment leaves behind large amounts of depleted uranium, usually stored as uranium hexafluoride in steel cylinders at the enrichment sites. Worldwide stocks amount to well over a million metric tons. Some of it may be re-enriched in future when uranium prices are high, some is converted to a stable oxide for long-term storage or disposal, and small amounts are used for shielding and counterweights. Its radioactivity is low, but its chemical form and sheer volume make it a significant long-term management task, and the deconversion plants that turn it into oxide are an often-overlooked part of the fuel cycle.

Regulation, safety and safeguards at front-end plants

Conversion and enrichment plants are licensed nuclear facilities with safety cases built around criticality safety, chemical hazards, fire and radiological protection. Criticality safety, keeping enriched material in amounts and shapes that cannot sustain a chain reaction, governs the design of containers, piping and storage arrays and the procedures for moving material. Enrichment plants are also among the most heavily safeguarded facilities in the world. International inspectors verify that a plant produces only the declared enrichment levels, using measurements, seals, cameras and unannounced inspections, as lesson 12 describes. A commercial enrichment plant that produced more than its declared assay would be detected, and the international community treats any such deviation very seriously.

The market for nuclear fuel services

Each step of the front end is a separate commercial market. Utilities buy uranium concentrate from mining companies or traders, pay a converter to turn it into UF₆, pay an enricher for separative work and pay a fabricator to make fuel. Most of this is done under long-term contracts lasting five to ten years, because security of supply matters more to a reactor operator than the lowest possible price. A smaller spot market handles short-term purchases. Prices are published by independent market reporting firms, and are quoted in dollars per pound of U₃O₈ for uranium, dollars per kilogram of uranium for conversion and dollars per SWU for enrichment.

The markets for conversion and enrichment are concentrated: only a handful of companies in a few countries provide these services. This concentration has made supply security a policy concern, particularly since 2022, when Russia's invasion of Ukraine raised questions about the reliance of Western utilities on Russian conversion and enrichment, which supplied a large share of world capacity. Governments and companies in North America and Europe have since announced expansions of conversion and enrichment capacity, and several advanced reactor developers have sought domestic sources of HALEU.

The United States relies heavily on imports for uranium, conversion and enrichment. Its only commercial enrichment plant, Urenco USA in Eunice, New Mexico, is owned by a European consortium. Congress has funded programs to build a domestic HALEU supply chain, and the DOE has signed contracts with several companies for HALEU and LEU enrichment services. The aim is to reduce dependence on foreign suppliers while keeping all enrichment within licensed, safeguarded facilities.

Regulation of conversion and enrichment

Conversion and enrichment plants are licensed nuclear facilities. Their main hazards are chemical: UF₆ reacts with moisture to form hydrogen fluoride, which is highly toxic and corrosive. Plants are designed to contain UF₆ in the event of a cylinder or process leak, and they have emergency plans coordinated with local authorities. Criticality safety is another requirement, met by limits on enrichment, mass and geometry. Enrichment plants are under IAEA safeguards, as lesson 12 explains, which verify that they produce only the declared enrichment and do not divert material. The transport of UF₆ in cylinders is regulated under international transport rules based on the IAEA's transport regulations, with cylinders designed and tested to withstand accident conditions.

Depleted uranium, the tails of the enrichment process, accumulates in large quantities, stored as UF₆ in steel cylinders at enrichment sites. Because UF₆ is chemically hazardous, the preferred long-term form is an oxide, which is stable and much less reactive. Deconversion plants turn depleted UF₆ into uranium oxide and recover the fluorine as hydrofluoric acid for industrial use. Small amounts of depleted uranium are used as shielding, counterweights and in MOX fuel, but most is stored for possible future use or disposal as low-level waste.