Nuclear Fuel Cycle Calculator
Estimate uranium mass and separative work units (SWU) needed to fuel a nuclear power reactor over one cycle. Used by nuclear engineers and fuel cycle analysts to plan procurement and enrichment costs.
Last updated: September 2026
Formula below · 2 sources (nrc.gov, Wikipedia) · Updated Sep 2026
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About this calculator
A light-water reactor's annual natural-uranium need follows from three steps. (1) Thermal energy per year: E (MWd) = electric power (MWe) ÷ net efficiency × capacity factor × 365.25 days. (2) Enriched fuel loaded: E ÷ discharge burnup (MWd per tonne of heavy metal). (3) Natural-uranium feed from the enrichment mass balance: feed / product = (x_p − x_w) / (x_f − x_w), with x_f = 0.711% U-235 in natural uranium, x_p the product enrichment and x_w the tails assay. The calculator reports the feed as tonnes of U₃O₈ (× 1.1792 kg U₃O₈ per kg U). Lower tails assay or higher burnup reduce the requirement; higher enrichment (at the same burnup) raises it. Separative work (SWU) is a separate quantity computed from the value function V(x) = (2x − 1)·ln(x/(1 − x)); it is not part of this result.
How to use
Suppose a 1,000 MWe reactor at 33% efficiency runs at a 90% capacity factor, discharging fuel at 45,000 MWd/tonne, enriched to 4.5% U-235 with a 0.3% tails assay. Step 1: thermal energy = 1,000 / 0.33 × 0.90 × 365.25 = 996,136 MWd. Step 2: fuel = 996,136 / 45,000 = 22.1 tonnes of enriched uranium. Step 3: feed factor = (4.5 − 0.3) / (0.711 − 0.3) = 10.22, so 226.2 t of natural uranium = 266.8 t U₃O₈. With the defaults (4.2% enrichment, 0.25% tails) the requirement is 223.7 t U₃O₈ per year.
Frequently asked questions
What is discharge burnup and how does it affect uranium requirements?
Discharge burnup measures the total energy extracted from each tonne of fuel, expressed in megawatt-days per tonne (MWd/tonne). Higher burnup means more energy is wrung from each fuel assembly before it is removed, so fewer tonnes of enriched uranium are needed annually. Modern LWR fuels achieve 45,000–60,000 MWd/tonne. Increasing burnup generally requires slightly higher enrichment, creating a trade-off that this calculator helps you optimize.
What is a tails assay and why does it matter in uranium enrichment?
The tails assay is the residual U-235 concentration (%) in the depleted uranium 'waste' stream leaving the enrichment plant. A lower tails assay (e.g., 0.2%) extracts more U-235 but consumes more separative work; a higher assay (e.g., 0.35%) uses less SWU but wastes more natural uranium. The economically optimal tails assay depends on the relative prices of natural uranium and enrichment services. Utilities routinely adjust their tails assay contracts as commodity prices shift.
How are separative work units (SWU) used to price uranium enrichment?
Separative work units are the standard commercial measure of the effort required to enrich uranium. Enrichment plants sell capacity in SWU, typically priced in USD per SWU. The total enrichment cost for a fuel reload equals the SWU requirement multiplied by the prevailing SWU price, plus natural uranium feed costs. Because SWU and uranium feed are partial substitutes, buyers can choose a tails assay that minimizes total fuel cost given current market prices for each input.