Nuclear Chain Reaction Simulation
Nuclear fission sits at the heart of nuclear energy generation, yet the processes inside a reactor core are usually hidden from view. This interactive simulation is designed to reveal those invisible dynamics while making them engaging and intuitive. It offers a simplified look at how neutrons move, how fuel nuclei split, and how control rods shape the balance between energy production and stability. By adjusting the sliders, you can explore how different factors influence the behaviour of a nuclear fission chain reaction.
Unfortunately the simulation is not compatible with mobile devices. For the best experience please view this page on a desktop.
- Active nuclei
- Spent nuclei
- Neutrons
- Control rods
Quick Guide to using the Nuclear Chain Reaction Simulation
START – Sets the simulation going with the selected parameters by introducing a small burst of neutrons at the centre of the simulation.
PAUSE – Freezes the simulation in the current state, allowing you time to study the behaviours occurring at any moment.
SLOW MOTION – Sets the simulation speed to 25%. Can be toggled on and off during the simulation.
NUCLEI – Sets the initial number of fissile nuclei (light blue dots) within the simulation. These are distributed randomly so the simulation will be different every single time, even if identical start conditions are chosen. Once a nuclei has undergone fission, it will be ‘spent’ (dark blue dots), meaning it can’t undergo another reaction.
CONTROL RODS – Sets the number of control rods (red bars) which are inserted into the simulation space. Control rods are inserted from the top and are spaced equally across the width of the simulation. Control rods absorb any neutron they come into contact with.
ROD INSERTION – Sets the depth to which the control rods are inserted from the top, where 0.0 corresponds to no insertion while 1.0 corresponds to full insertion.
NEUTRON SPEED – Sets the speed at which the neutrons (yellow dots) travel. Neutrons exist for 10 seconds or until they cause a fission event.
FISSION PROBABILITY – Sets the likelihood that a nuclei undergoes a fission reaction following a collision with a neutron.
NEUTRONS PER FISSION – Sets the number of neutrons ejected by a nuclei following a fission reaction.
ENERGY OVER TIME GRAPH – Tracks the number ‘spent’ nuclei as an indicator of energy release over time.
NEUTRONS OVER TIME GRAPH – Tracks the number of neutrons in the simulation.
How does a fission chain reaction work?
A nuclear reactor sustains a controlled chain reaction by managing how neutrons move, interact, and multiply inside a dense lattice of fuel. The simulation you’ve been exploring visualises simplified versions of these processes, but each one corresponds to a real physical mechanism that determines whether a reactor is safely producing power, shutting down, or becoming dangerously unstable.
FUEL AND FISSION EVENTS – In an operating reactor, the fuel consists of heavy nuclei such as uranium‑235 or plutonium‑239. When a neutron is absorbed by one of these nuclei, the nucleus becomes unstable and splits into two smaller fragments. This fission event releases a large amount of energy as heat and typically emits two or three fast neutrons. Those neutrons can go on to trigger further fissions, creating a branching chain reaction.
NEUTRON BEHAVIOUR – Freshly emitted neutrons are extremely fast, but most reactors use a moderator such as water, graphite, or heavy water, to slow them down. Slow (thermal) neutrons are far more likely to cause fission in uranium‑235. Neutrons scatter many times, diffuse through the core, and may be absorbed by fuel, structural materials, coolant, or control rods. Their population at any moment determines the reactor’s power level.
CONTROL RODS AND ABSORPTION – Control rods are made of materials such as boron, hafnium, or cadmium that strongly absorb neutrons. Inserting them deeper into the core removes more neutrons from the chain reaction, reducing the number available to cause fission. Withdrawing them has the opposite effect. This is the primary mechanism operators use to regulate power.
NEUTRON FLUX AND POWER DISTRIBUTION – Neutron flux, which is the number of neutrons passing through a region per second, varies across the core. High‑flux regions produce more fissions and therefore more heat. In real reactors, flux is shaped by fuel arrangement, coolant flow, control rod position, and the geometry of the core. Monitoring flux is essential for ensuring the reactor remains within safe operating limits.
CRITICALITY AND K-EFFECTIVE – The key measure of reactor behaviour is the effective multiplication factor, k‑effective. It represents the ratio of neutrons in one generation to the next:
k < 1 (subcritical): the chain reaction dies out.
k = 1 (critical): the reaction is self‑sustaining at a steady power level.
k > 1 (supercritical): the reaction grows, increasing power.
Operators continuously adjust control rods, coolant flow, and sometimes soluble neutron absorbers (like boric acid in pressurised water reactors) to keep k extremely close to 1.
ENERGY PRODUCTION AND HEAT REMOVAL – The energy released by fission appears as heat in the fuel. Coolant, usually water, removes this heat and transfers it to a steam cycle that drives turbines. Maintaining proper cooling is as important as controlling the chain reaction itself, because fuel must remain below temperature limits to avoid damage.