Drawing No. EH–NR–006 // Nuclear Engineering & Radiation

Nuclear Reactor Decay Heat Calculator

Reviewed August 2026

Estimate residual thermal power after reactor shutdown, explore how operating history changes the result, and follow the decay curve from seconds to 100 days using the Way–Wigner approximation.

Inputs

Representative reactor presets

Representative nominal thermal ratings. Confirm the applicable project value before use.

Reactor power before shutdown, P₀ MW(th)

Operating time before shutdown, T_op hours

8760 h ≈ 1 year of continuous full-power operation

Time after shutdown, t

Results

Decay heat power —

Fraction of P₀ —

Energy produced in the next hour —

Thermal output—

1 MW boilers—

Approx. homes at 2 kW—

Decay heat curve

The horizontal axis spans 10 seconds to 100 days. The selected operating point is highlighted. A logarithmic view makes both the rapid early fall and the long tail visible.

Post-shutdown milestones

Approximate decay heat at standard milestones for the selected reactor power and operating history.

Time	Power	% of P₀

Why decay heat matters

A reactor SCRAM stops the chain reaction, but radioactive fission products continue to decay and release heat. The initial residual power is only a small fraction of full reactor power, yet for a large reactor it still represents many megawatts. Continuous heat removal remains necessary to prevent fuel, coolant, and containment temperatures from rising.

Emergency and shutdown cooling systems are therefore designed to remove heat after fission power has ended. Depending on the reactor design, these functions may be provided by residual heat removal systems, isolation condensers, passive containment cooling, steam-driven injection, natural circulation, or other engineered and passive heat sinks.

Model boundary: this page estimates heat generation only. It does not calculate coolant inventory, boiling, heat-transfer limits, fuel temperature, pressure, or whether a particular cooling system is adequate.