A reactor is a heat engine like any other: it can never convert more of its thermal power to electricity than the Carnot limit allows, set by the hot steam temperature and the cold heat-sink temperature — around one-third for typical light-water-reactor conditions, so close to two-thirds of the thermal power leaves as waste heat through the condenser and cooling tower. Set the thermal power, the hot and cold temperatures, the coolant flow, and schedule a SCRAM at a chosen hour after some number of operating days, then watch the reactor run — and watch what happens after SCRAM, when the chain reaction stops but decay heat from radioactive fission products keeps heating the core, and must be actively cooled or core temperature climbs toward damage. Track the spent fuel piling up along the way. It all runs on your device.
Thermal power in, electricity out — and where the rest of the heat goes.
Electrical power—
Thermal power—
Efficiency—
Carnot ceiling—
Core temp—
Safety—
Spent fuel—
Capacity factor—
Power output
Energy split — electricity vs waste heat
Spent fuel accumulated
Core temperature after SCRAM — vs the meltdown line
Reactor & cooling loop
About two-thirds of a reactor's heat becomes waste, not electricity. A reactor is a heat engine: its ceiling on turning thermal power into electricity is the Carnot limit — one minus the cold-sink temperature over the hot-steam temperature, both in kelvin — and real turbine-generator losses push the actual efficiency further below that ceiling still, near one-third overall for a typical light-water reactor. That leaves close to two-thirds of the thermal power to leave as waste heat through the condenser and cooling tower. And efficiency isn't the only number that keeps working after you stop generating: once the reactor SCRAMs, fission stops but decay heat from radioactive fission products keeps the core hot, and if the coolant flow can't keep up, core temperature climbs toward the fuel-damage threshold — which is why the cooling systems have to keep running long after shutdown.
Reading the simulation
What the reactor scene, the energy-split bar and the safety plot are telling you — then what SCRAM does to the core.
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The reactor & cooling loop
The core heats coolant to the hot temperature, driving the turbine and generator before the spent steam condenses against the cold sink and cooling tower. The gauge beside the scene fills toward installed capacity as the electrical output tracks the Carnot ceiling those two temperatures set.
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The energy-split bar and the ceiling
The energy-split bar breaks the thermal power down into three slices: the share the Carnot limit forbids outright, the further loss from real turbine-and-generator inefficiency, and what's left as net electricity — the smallest slice, and the only one that reaches the grid. Everything else in the bar leaves as waste heat through the condenser and cooling tower.
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SCRAM and decay heat
Schedule a SCRAM at your chosen hour and the chain reaction stops instantly, but the safety plot keeps tracking core temperature as decay heat from fission products fades against your coolant flow. The safety readout reports whether the core stays normal, drifts into a warning margin, or crosses the meltdown threshold, while spent fuel keeps accumulating from the energy already produced.
How It Works
A hard ceiling on conversion, a heat balance that sets live output, and a decay-heat cooldown that doesn't stop at shutdown.
Inside the containment, the reactor core heats water; its control rods drop to shut the chain reaction down (a SCRAM). The heat drives a turbine and generator — but the Carnot limit means only about a third reaches the grid, while about two-thirds leaves as waste heat through the cooling tower.
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The Carnot limit sets a hard ceiling
Any heat engine's efficiency is capped by the Carnot limit: one minus the cold-sink temperature over the hot-steam temperature, both measured in kelvin. For a typical light-water reactor's steam conditions that ceiling sits near 33%, and the turbine-generator system only reaches roughly 72% of that ceiling in practice — a limit set by thermodynamics, not by how well the plant is built.
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Thermal power and temperatures set live output
Electrical power is thermal power · Carnot efficiency · generator factor, and whatever thermal power isn't converted leaves as waste heat through the condenser and cooling tower — typically close to two-thirds of the total. Raising the hot-steam temperature or lowering the cold-sink temperature widens the Carnot ceiling and lifts the electrical output.
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After SCRAM, decay heat has to go somewhere
Stopping the chain reaction doesn't stop the heat: radioactive fission products built up during operation keep releasing decay heat that fades over hours to days, following a curve set by how long the reactor ran beforehand. Coolant flow has to carry that heat away or core temperature climbs toward the fuel-damage threshold, and the spent fuel produced by the energy already generated remains as the reactor's lasting material waste.
What is the Carnot limit on a reactor's efficiency?
The Carnot limit is the theoretical ceiling on how much of a heat engine's thermal energy can become work, set purely by its hot and cold operating temperatures in kelvin: one minus the cold-sink temperature over the hot-steam temperature. For a typical light-water reactor's steam conditions that ceiling sits near one-third, and turbine and generator losses push the real electrical efficiency further below it still. No amount of engineering can beat the temperature-set ceiling itself — only narrow the gap between real efficiency and it.
Why does a reactor reject about two-thirds of its heat as waste heat?
A reactor is a heat engine, and every heat engine's electrical efficiency is capped by the Carnot limit — the fraction it can't convert to work is forbidden by the second law of thermodynamics itself, not by poor design. Combine that Carnot-forbidden share with the turbine-generator's own losses and only around a third of the thermal power reaches the grid as electricity; the rest leaves through the condenser and cooling tower as low-grade waste heat, warming the cooling water or the air rather than doing useful work.
What is a SCRAM?
A SCRAM is a reactor's emergency shutdown. Safety systems — or an operator — release the neutron-absorbing control rods so they drop all the way into the core, soaking up the neutrons that sustain the fission chain reaction and halting it within seconds. It can fire automatically the instant a sensor reads an unsafe condition (a power spike, a loss of coolant flow, an earthquake) or be thrown by hand. Crucially, a SCRAM only stops the chain reaction: the radioactive fission products already built up in the fuel keep releasing decay heat afterward, which is why cooling has to continue long after the reactor trips.
Why does a reactor need cooling after it's shut down?
Shutting down a reactor — a SCRAM — stops the fission chain reaction almost instantly, but the fuel is still full of radioactive fission products from the fission that already happened, and their ongoing decay keeps releasing heat. This decay heat starts at a few percent of the reactor's full thermal power and fades over hours to days, but it doesn't vanish the moment the reactor trips. If the coolant flow can't carry that heat away fast enough, the core temperature keeps climbing toward the point where fuel cladding is damaged — which is why cooling systems have to keep running long after the chain reaction itself has stopped.
What happens to the spent nuclear fuel?
Fuel is removed from a reactor once its usable uranium is spent, but it stays extremely radioactive and thermally hot from the same decay-heat process that keeps a shut-down core warm. Spent fuel is generally cooled in water-filled pools for years before it can be moved to dry-cask storage, where it remains for the long term as the reactor's principal material waste — with total mass tracked by the amount of thermal energy the fuel has actually released.