⚛️Fission Reactor Simulator - Chain Reaction, Six-Factor Formula, Reactor Kinetics, Decay Heat

How to use Nuclear Fission Reactor Simulator

1. 1In Chain reaction set the fuel radius, fission share and rod depth, or use the presets for subcritical, critical and supercritical
2. 2In Fission energy enter a fuel mass and type to compare the coal, oil and gas needed for the same energy
3. 3In Six-factor formula choose enrichment, moderator, moderator-to-uranium ratio and fuel form to get k∞, k_eff and critical radius
4. 4In Point kinetics apply a reactivity step and watch the prompt jump and stable period; add a scram, temperature feedback or remove delayed neutrons
5. 5In Decay heat and PWR plant see the heat left after shutdown and how primary and secondary conditions set the plant efficiency

FAQ

What do k_eff below, equal to and above 1 mean?

k_eff is the number of neutrons in one generation per neutron in the previous one. Below 1 each generation is smaller and the chain dies out (subcritical); at exactly 1 the population is steady, which is how a reactor runs at constant power (critical); above 1 it grows geometrically (supercritical). To raise power a reactor is taken just above 1 and brought back to 1 at the new level.

Why do delayed neutrons make a reactor controllable?

Prompt neutrons have a generation time of about 20 μs, so with prompt neutrons alone k = 1.001 would multiply power by e every 20 ms, far too fast for any mechanical control. About 0.65% of neutrons come from the decay of fission products, on average a dozen or so seconds later, and they stretch the period of power changes to tens of seconds or more. As long as the reactivity stays below the delayed fraction β (one dollar), the reactor remains controllable.

What is prompt criticality and why must it be avoided?

When the reactivity ρ reaches β, prompt neutrons alone can sustain growth and the period falls to milliseconds. Neither control rods nor operators can respond in time, so only inherent negative feedback such as fuel heating can limit the power. Reactor designs limit the worth and withdrawal speed of individual rods, and operating rules forbid approaching this state.

Why does a reactor still need cooling after shutdown?

The chain reaction stops, but fission products in the core keep decaying and releasing heat: about 6–7% of full power right after shutdown, about 1% after an hour and still about 0.5% after a day. For a 3000 MW thermal core that is more than ten megawatts a day later. If cooling is lost the fuel keeps heating, so decay-heat removal and emergency cooling matter just as much as shutting down.

Why is a nuclear plant only about 33% efficient?

A PWR keeps its primary water liquid at 15.5 MPa, reaching about 320 °C at most, and the secondary steam must be colder than the cold leg, typically saturated steam at about 280 °C and 7 MPa. Heat-engine efficiency is limited by those temperatures: an ideal Rankine cycle gives about 37%, and turbine losses bring it to about 32–34%. The remaining two-thirds of the heat goes through the condenser to the sea, a river or cooling towers.