Nuclear Waste Heat Calculator
Estimate the decay heat power still being generated in a shutdown nuclear reactor or spent fuel assembly using the ANS-5.1 standard approximation. Used for designing cooling systems, spent-fuel pool sizing, and emergency core cooling analysis.
Last updated: September 2026
Decay Heat
20.2 MW
Formula below · 2 sources (nrc.gov, Wikipedia) · Updated Sep 2026
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About this calculator
After a reactor shuts down, fission product decay continues to generate significant heat — initially about 6–7% of full thermal power. This calculator uses the Untermyer–Weills correlation, a conservative fit to the ANS decay-heat data: P/P₀ = 0.1 × [(t + 10)^−0.2 − (t + T + 10)^−0.2 + 0.87 (t + T + 2×10⁷)^−0.2 − 0.87 (t + 2×10⁷)^−0.2], where P₀ is the operating thermal power, t is time since shutdown and T is the operating time, both in seconds (the calculator converts your hours and months). It gives about 6% at shutdown, 1.6% after 1 hour, 0.66% after 1 day and about 0.2% after a month. The simpler Way–Wigner form 0.066 × [t^−0.2 − (t + T)^−0.2] (also in seconds) gives somewhat lower values; licensing analyses use the full ANS-5.1 standard with uncertainty margins. Proper cooling must be maintained throughout the decay period.
How to use
Consider a 3,000 MWth reactor that operated for 12 months before shutdown, and we want decay heat 1 hour later. In seconds: t = 3,600 s and T = 12 × 30.44 × 86,400 = 3.156×10⁷ s. P/P₀ = 0.1 × [3,610^−0.2 − (3.156×10⁷ + 3,610)^−0.2 + 0.87 × (5.156×10⁷)^−0.2 − 0.87 × (2.0004×10⁷)^−0.2] = 0.1 × [0.19363 − 0.03165 + 0.02480 − 0.03015] = 0.01566, so P ≈ 3,000 × 0.0157 = 47 MWth — enough to boil away tonnes of water an hour if cooling is lost. The default (24 hours after an 18-month cycle) gives 20.2 MW, 0.67% of full power.
Frequently asked questions
Why does a nuclear reactor still produce heat after it is shut down?
Inserting control rods stops the fission chain reaction almost instantly, but it does not stop the radioactive decay of the hundreds of fission product isotopes already present in the fuel. These unstable isotopes continue emitting beta particles and gamma rays, converting their decay energy directly into heat. This decay heat starts at roughly 6–7% of full power immediately after shutdown and decreases over days, weeks, and years as shorter-lived isotopes decay away. The 2011 Fukushima accident is a stark reminder of what happens when decay-heat cooling is lost.
How long does spent nuclear fuel need active cooling after removal from a reactor?
Freshly discharged spent fuel generates enough decay heat to require active water cooling in a spent-fuel pool for at least 5 years, and typically more than 10 years before it can be safely transferred to dry cask storage. After about 5 years the heat output drops to a level where passive air cooling in a sealed metal cask is sufficient. The exact timeline depends on initial fuel enrichment, discharge burnup, and the specific reactor's power history — all factors this calculator can help quantify.
What is the ANS-5.1 decay heat standard and when should it be used?
ANS-5.1 (American Nuclear Society Standard 5.1) provides the industry-consensus method for calculating fission product decay heat in light-water reactor fuel. It defines analytical fits to measured decay heat data as a function of cooling time and irradiation history. It is used for licensing calculations, thermal-hydraulic safety analyses, spent-fuel pool design, and dry cask storage assessments. The Way-Wigner power-law approximation used in this calculator is a simplified version suitable for scoping estimates; for regulatory submittals, the full ANS-5.1 summation method with uncertainty bounds should be applied.
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