Nuclear Waste Heat Calculator
Estimate an upper-bound decay heat of spent nuclear fuel from its burnup, cooling time (up to 15 years) and mass. Used to sanity-check pool, cask and transport heat loads.
Last updated: September 2026
Formula below · 2 sources (nrc.gov, Wikipedia) · Updated Sep 2026
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About this calculator
Spent fuel keeps producing decay heat from fission products and actinides. This calculator applies the Way–Wigner correlation per tonne of heavy metal: P(t) = P₀ × 0.066 × [t^−0.2 − (t + T)^−0.2], with t the cooling time and T the irradiation time in seconds and P₀ the fuel's specific power while it was in the reactor. P₀ is derived from the burnup assuming a typical 3-year (1,095-day) irradiation: P₀ = burnup / 1,095 (MW per tonne). Result (kW) = P × 1,000 × fuel mass (tonnes). Compared with detailed ORIGEN results for PWR fuel this simple fit overestimates decay heat by roughly 1.5–2.5× between about 1 and 15 years of cooling (for 45,000 MWd/t at 5 years it gives about 5.5 kW/t versus about 2.3 kW/t), which is the conservative direction for cooling and storage. Beyond about 15 years actinides such as Am-241 dominate and the fit is no longer conservative, so the calculator stops there. Licensing work uses ANS-5.1 or ORIGEN with the actual power history.
How to use
Example: a 0.5-tonne PWR assembly discharged at 45,000 MWd/tU after 5 years of cooling. P₀ = 45,000 / 1,095 = 41.1 MW/t. t = 5 × 3.156×10⁷ = 1.578×10⁸ s, T = 9.46×10⁷ s. P/P₀ = 0.066 × [(1.578×10⁸)^−0.2 − (2.524×10⁸)^−0.2] = 0.066 × (0.02294 − 0.02088) = 1.36×10⁻⁴. Heat = 41.1 MW/t × 1.36×10⁻⁴ × 1,000 × 0.5 = 2.8 kW (a conservative figure; detailed inventories give about 1.2 kW).
Frequently asked questions
Why does spent nuclear fuel need active cooling after the reactor shuts down?
Even after a reactor is shut down, the fission products accumulated in the fuel continue to undergo radioactive decay, releasing energy as heat. Immediately after shutdown this decay heat equals roughly 6–7% of the reactor's full operating power — for a 3,000 MW thermal plant, that is 180–210 MW of residual heat. Without cooling, fuel temperatures would rise rapidly, potentially melting the cladding and releasing radioactive gases. This is precisely what occurred at Fukushima Daiichi in 2011 when tsunami damage disrupted cooling water flow. Decay heat drops sharply over hours and days but remains significant for years, requiring continuous heat removal from spent fuel pools.
What is fuel burnup and how does it affect decay heat?
Fuel burnup measures how much energy has been extracted from nuclear fuel, expressed in megawatt-days per tonne of uranium (MWd/tU). Higher burnup means the fuel has undergone more fission events, producing a greater inventory of radioactive fission products and transuranic elements. This directly increases the decay heat of discharged fuel — the calculator's (BU / 33000)^0.4 term captures this relationship. Modern light-water reactor fuels typically achieve burnups of 40,000–60,000 MWd/tU. Higher burnup is economically attractive because it reduces refueling frequency, but it also increases the thermal and radiological challenges for interim storage and final disposal.
How long must spent nuclear fuel be cooled before it can be placed in dry cask storage?
Regulatory and engineering requirements typically mandate a minimum of 5–10 years of wet storage in a spent fuel pool before transfer to dry cask storage. During this time, decay heat and radiation levels drop to levels that passive air-cooling systems in dry casks can safely manage — generally below about 40 kW per assembly. The exact waiting period depends on fuel burnup, assembly design, and the thermal capacity of the specific cask model. Some high-burnup fuels require longer cooling times. Dry cask storage is considered a safe and proven interim solution, with casks designed for at least 100 years of service while permanent geological repositories are developed.