Reactor Decay Heat After Shutdown

Reactor Setup

Operating time before shutdown 180 d

Thermal power P₀ 1000 MW

Coolant flow after SCRAM 100%

Live Readouts

● CRITICAL — steady power

Decay heat

— MW

Core temp

315 °C

Time since SCRAM

00:00:00

Heat fraction

— % P₀

How it works

Even after a reactor is shut down (SCRAM — control rods fully inserted, fission chain reaction stopped), the fuel keeps generating heat from the radioactive decay of accumulated fission products (mostly ¹³¹I, ¹³⁷Cs, ¹⁴⁰Ba/La and dozens of other isotopes). This "decay heat" is what a loss-of-coolant accident cannot switch off — it is why cooling must continue for hours to days after shutdown.

The simulator uses the classic Way–Wigner approximation for decay heat as a fraction of the pre-shutdown power P₀:

f(t) = 0.0622 · [ t^-0.2 − (t + T_op)^-0.2 ]

t     = time since SCRAM (s)
T_op  = reactor operating time before SCRAM (s)
f(t)  = decay heat / P₀   (valid ~10 s to ~100 days)

At the instant of shutdown f ≈ 6–7% of P₀ — for a 1000 MW reactor that is 60–70 MW of heat with nowhere to go but the coolant. The core temperature follows a simple lumped energy balance:

C_th · dT/dt = Q_decay(t) − Q_cooling(flow)

Q_cooling scales with the coolant-flow slider; if it falls
below what Q_decay(t) demands, core temperature climbs
toward the ~1200 °C zirconium-cladding damage threshold.

Operating time before shutdown — longer operation builds up more fission-product inventory, raising the decay-heat curve.
Thermal power P₀ — sets the reactor's rated power and its steady-state operating temperature.
Coolant flow after SCRAM — the safety-critical control: pumps, natural circulation or emergency cooling removing decay heat. Drop it too low and the core overheats — exactly the mechanism behind a meltdown from loss of coolant.
SCRAM button — stops the fission chain reaction; only decay heat remains, decaying roughly as t^-0.2 over the following hours and days.

Cool (<350 °C) Hot (~700 °C) Danger (>1200 °C)

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