Drag a control rod and watch a chain reaction respond in real time.
Nuclear Reactor simulator
Drag the rod up or down, or tap for an emergency scram
This simulation needs a modern browser with canvas support. The measurements and formula below still describe the physics.
Live measurements
Controls
Graph
Formula
ρrod = (x / 100 − 0.5) × 2 × W
- ρrod Rod reactivity
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Observations
What's happening
Real-world examples
Slow startup
Withdrawing the rod a little adds a small positive reactivity, and power climbs on a period of many seconds, plenty of time for an operator to react.
Prompt critical
Push reactivity past 1 $ and the chain reaction can sustain itself on prompt neutrons alone, so power rises in a fraction of a second, the line no reactor operator is meant to cross.
Negative feedback
As the core warms, a negative temperature coefficient quietly removes reactivity, the main reason most power reactors settle into a new steady state instead of running away.
Positive void coefficient
Some early reactor designs added reactivity as they heated up at low power, a self-reinforcing loop implicated in the Chernobyl accident.
Uses of nuclear reactor
- Understanding why nuclear reactors take seconds, not milliseconds, to change power safely
- Seeing why reactor operators watch the period, not just the power level
- Explaining why a negative temperature coefficient is a core safety feature
- Introducing why the Chernobyl accident involved a positive void coefficient at low power
nuclear reactor pitfalls
- Confusing reactivity with power: reactivity is what makes power change, not the power level itself
- Assuming a control rod acts instantly on power rather than on the rate of change
- Thinking every reactor design is automatically self-stabilizing
nuclear reactor questions (8)
What is reactivity in a nuclear reactor?
Reactivity measures how far a reactor sits from critical, not how much power it makes. It compares one generation of fissions to the generation before it. At zero reactivity the reactor is critical, so power holds steady. Positive reactivity makes power rise; negative reactivity makes it fall. Reactivity is expressed in dollars, where 1 $ equals the delayed neutron fraction, the exact amount needed to reach prompt critical.
What does prompt critical mean?
A reactor becomes prompt critical at 1 $ of reactivity. At that point, prompt neutrons alone can sustain the chain reaction, without waiting for delayed neutrons at all. Below 1 $, delayed neutrons set the pace instead, so the reactor responds over seconds and stays controllable by rods. At or past 1 $, power rises on the prompt neutron lifetime, a fraction of a second, too fast for a mechanical rod to catch. Because of that, real reactor designs keep a large margin below 1 $.
What is a reactor trip or scram?
A reactor trip, also called a scram, is an automatic safety shutdown. Instruments detect power or temperature crossing a preset limit, and the control rods drive fully into the core within seconds. Scram rods carry a shutdown margin far larger than the operating rod worth, so the reaction stops even if every other effect is pushing the other way. This simulator inserts 5 dollars of negative reactivity on a trip, which beats any slider combination on the panel.
How do control rods control a chain reaction?
Control rods absorb neutrons. Inserting one removes reactivity; withdrawing it adds reactivity. Because reactivity sets the rate power changes rather than the power level itself, a rod move does not snap power to a new value. Instead, it sends power onto a new trajectory, rising or falling over a period set by how far from critical the reactivity sits. Small, slow rod movements are therefore how operators keep that period long and controllable.
Can a nuclear reactor explode like a bomb?
No. A power reactor cannot detonate like a weapon, because its fuel is far too dilute and its geometry cannot hold a supercritical mass together under explosive compression. What can happen instead, and did happen historically, is a power excursion. Reactivity pushed past prompt critical drives a very fast, damaging rise in power and heat, which can destroy the reactor through steam explosions or fire. That excursion, not a nuclear detonation, is the failure mode this simulator makes intuitive.
Why did the reactor trip in this simulator?
A trip fires automatically once reactor power exceeds 200% of its rated value, or once core temperature passes 80 degrees C, whichever comes first. Both limits stand in for the many real instruments a reactor protection system actually watches. Once tripped, the rod is driven fully in regardless of where its slider sits. Tap the reactor to restart it with your settings intact, or press Reset to return every slider to its default.
What control rod position makes the reactor critical?
In this simulator the rod is worth nothing at 50 percent, so that is the critical position: withdraw past it and reactivity goes positive, insert below it and reactivity goes negative. The canvas marks the height with a dashed critical line. Rod reactivity in dollars is (position / 100 minus 0.5) times 2 times the rod worth, so at 0.9 dollars of worth, 70 percent buys 0.36 dollars. Type a position into the formula panel to solve it the other way round and the reactor follows.
Why does the reactor period read 999 seconds?
At exactly critical the period is infinite, because power is not changing at all and never reaches a factor of e. A number that large is not useful on a readout, so the meter pegs at 999 seconds, the same way a real period meter pins at the top of its scale near critical. Treat a pegged reading as "no measurable drift" rather than a literal 999 seconds. Move the rod off the critical line and the period drops to a real value.