Reactor Lab
Choose the parts. Follow the neutrons. Explain how a controlled chain reaction can power a generator.
Your objective
Explain how a reactor maintains a controlled, sustained chain reaction, then explain how its energy can produce electricity.
This is an on-screen concept model. You do not need to know how to assemble it before starting.
First time? Watch a working example
We place the components in the model for you. One click starts a short sequence of five symbolic generations. Watch the dots and the “Steady generations” counter.
You should see the same number of events in successive generations: this is a sustained chain reaction.
Ready to choose your own materials?
Use the component menu, select a material and press Place in model. Feedback explains your choice.
You can return here and use the guided example whenever you need help.
Student procedure
- Observe a steady chain. Press Start guided example. Let the five steps finish. What stays the same from one generation to the next?
- Compare the three model states. In Build & investigate, choose Raised, then Start / restart test, then Play 5 steps. Repeat with Lowered. Which chain grows and which dies out? These buttons select simplified teaching states.
- Investigate a component. Choose a component and a different material; press Place in model. Read the feedback. Start a fresh test and compare. Change one choice at a time.
- Connect electricity. Run the guided example again to supply model heat. Open Make electricity. Tick the steam and shaft connections, choose a conducting coil, add a magnetic field and press Test generator. Use Animate energy transfer to see the motion.
- Explain your result. Open Explain & reflect. Complete the three questions and compare your notebook entries.
How do I know I have succeeded?
You can identify each component’s role, recognise a steady chain as a chain reaction, explain what the control rods change, and trace energy from the core to an induced voltage in the generator. The four mission badges show your progress.
If nothing seems to happen
Start / restart test prepares the first generation. It does not run all the steps. Press Next generation once or Play 5 steps to observe the change.
Lowered absorbing rods make the chain die out; that is an expected observation. For a working steady example, return here and select Start guided example.
Changing a material resets the current test. Start a fresh test after each change. A growing-chain demonstration pauses itself when its comparison is complete.
Reactor test dashboard
Start a test to compare the indicators.
How to read these indicators
k describes neutron replacement from one generation to the next: k < 1 means a declining chain, k = 1 a sustained chain, and k > 1 a growing chain. These are called subcritical, critical and supercritical; “critical” here means steady.
This game assigns illustrative k values to its three teaching states. The dots are symbolic fission events, so rounding near zero need not reproduce the exact ratio. Relative power follows fission rate. The temperature index follows the heat model and changes more slowly. The radiation index represents a source reduced by shielding, with a residual contribution after shutdown. None is calibrated to a real plant or a safety limit.
Neutron story
Symbolic generations · not real timeDots represent fission events, not individual atoms or exact neutron numbers. Both a steady chain and a growing chain are chain reactions.
Turn heat into electricity
Connect the steam turbine and its shaft. Choose a conducting coil and include a magnetic field.
Conceptual pressurised-water layout: the primary coolant transfers energy through the heat exchanger; it does not mix with the secondary water. The condenser transfers unused thermal energy to the surroundings.
Connect the energy pathway
A rotating coil in a magnetic field
Rotation changes the magnetic flux through the coil and induces a voltage. In this closed circuit, current flows. No magnetic field, no conductor or no rotation → no generated current. This small coil model uses slip-ring connections; station generators may rotate the magnetic field instead.
Explain your choices
Complete these three checks to earn your final mission badge.
Investigation notebook
Tests are recorded here so you can compare one change at a time.
| Test | Rods | Outcome | Prediction |
|---|
Discussion prompts
Why can a reactor still be hot after the chain reaction stops?
What is the difference between slowing a neutron and absorbing it?
Why does shielding protect the surroundings without being the main control for the chain reaction?
Describe each energy transfer between the fuel and the lights.
Fuel → fission
A neutron can cause a suitable heavy nucleus to split, releasing energy and more neutrons. Some of those neutrons can cause further fissions.
Uranium dioxide is a ceramic fuel material used in fuel rods. Coal provides energy by combustion, not by sustaining this nuclear chain.
Moderator ≠ control rods
In this thermal-reactor model, water slows neutrons so they are more likely to cause further fission. This is moderation.
Boron-containing control rods absorb neutrons. Their role is to reduce the number available for further fission.
Coolant ≠ shielding
Coolant carries thermal energy away from the core. Shielding reduces radiation reaching the surroundings.
Water serves as both moderator and coolant in a pressurised-water reactor. They are separate choices here so you can practise the two different roles.
Three chain behaviours
Dying out: fewer fissions follow each generation.
Sustained: approximately the same number follows each generation. This is still a chain reaction.
Growing: more fissions follow each generation. A power reactor aims for controlled, steady output.
From thermal to electrical
Energy released in the fuel heats the coolant. A heat exchanger transfers energy to a separate water loop. Steam drives a turbine; its shaft drives a generator.
Relative motion between a conductor and a magnetic field can induce a voltage. Some thermal energy is transferred to the surroundings, so not all becomes electricity.
About this model
This is a conceptual game, not an engineering or operating simulator. Parts, neutron paths, heat levels and time are illustrative. It omits dimensions, material quantities, pressures, operating temperatures and reactor kinetics.
Moving rods selects a qualitative teaching outcome. Real reactor behaviour depends on many coupled effects. Residual heat remains after fission is stopped; cooling is still needed.
Sources & classroom use
Built for conceptual discussion at IGCSE level; this is not a core practical.
US Department of Energy — How a nuclear reactor works · NRC — Pressurised-water reactors · NRC — How nuclear power makes electricity
Progress lasts for this open session. Download the notebook to keep your evidence.
One neutron. Then what?
Objective: follow neutron capture, fission and the start of a chain reaction. Choose a scene, press Play, or use Next stage to inspect each event.
One possible fission channel
¹₀n + ²³⁵₉₂U → ²³⁶₉₂U* → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy
The asterisk means an excited nucleus. Nucleon number: 1 + 235 = 141 + 92 + 3 = 236. Charge number: 92 = 56 + 36.
This is one possible outcome. Fission can produce other fragment pairs and neutron numbers. The particle drawings, timing and selected outcomes are schematic, not probability or scale predictions. These fragments can subsequently decay.
What should I notice?
Moderator: collisions transfer some neutron kinetic energy to nuclei in the material. A slower neutron is generally more likely to cause U-235 fission in a thermal reactor.
Absorber: captures the neutron, removing it from the chain. Absorbing a neutron in U-235 can lead to fission, but capture without fission is also possible.
Chain reaction: emitted neutrons can reach other uranium nuclei. Some instead escape or are absorbed. Fast neutrons can also cause U-235 fission; “fast” does not mean “impossible”.
Where does the released energy come from?
The products are more tightly bound overall. Their total rest mass is smaller than the total initial rest mass, including the incoming neutron. The difference corresponds to the energy released: E = Δmc². Much appears as fragment kinetic energy; collisions transfer it to the surroundings as thermal energy. Neutrons and radiation also carry energy.
A nucleus’s binding-energy mass defect compares its mass with its separated nucleons. The reaction mass difference compares all reactants with all products. Binding energy is not a separate substance being turned into missing nucleons; total mass–energy is conserved.
Sources: OpenStax: Fission · Binding energy · DOE: Reactor concepts