A single fission is a one-off. But each split releases spare neutrons — and if those neutrons go on to trigger more fissions, which release yet more neutrons, the reaction sustains itself. Controlled, this is a power station; uncontrolled, it’s a bomb. The difference comes down to one thing: how much fuel you have.
📘 What you need to know
Each fission produces two daughter nuclei and at least one neutron (usually two or three)
A chain reaction happens when the neutrons from one fission go on to cause further fissions
The neutrons must be slow thermal neutrons — fast neutrons rebound off the nucleus instead of being absorbed
Only one neutron per fission needs to trigger the next fission for a steady, controlled chain reaction
Critical mass is the minimum mass of fuel needed to sustain a steady chain reaction
Subcritical (too little fuel) → the reaction dies out; supercritical (too much) → a runaway reaction and explosion
At critical mass, the rate of neutron loss equals the rate of neutron creation by fission
How a Chain Reaction Builds
Every fission of uranium-235 releases two or three neutrons. Here’s the crucial idea: each of those neutrons can be absorbed by another uranium nucleus and trigger a fresh fission, which itself releases more neutrons, which trigger more fissions again. This self-propagating sequence is a chain reaction, and it’s what allows a tiny trigger to unlock the energy of a huge number of nuclei.
Each fission releases neutrons that trigger further fissions, so the number of reactions grows rapidly — 1, then 2, then 4, and so on. This runaway growth is a chain reaction.
1 fission
→ releases neutrons →
2–3 more fissions
→ each releases more →
chain reaction
Thermal Neutrons Keep It Going
For the chain to continue, the neutrons must actually be absorbed by the next nucleus. That only happens if they are slow — thermal neutrons, with low energy and speed. A neutron carrying too much energy simply bounces off the uranium-235 nucleus without being captured, and no fission occurs. This is why the neutrons released in fission (which are fast) have to be slowed down before they can sustain the reaction.
Critical Mass
A chain reaction can only sustain itself if enough neutrons stay in the fuel to keep triggering fissions. Some neutrons inevitably escape from the surface of the fuel without hitting anything. Whether the reaction grows, holds steady, or dies out depends on the mass of fuel present.
The critical mass is the minimum mass of fuel needed to maintain a steady chain reaction — the point where the rate of neutron loss exactly equals the rate at which new neutrons are created by fission. There are three cases:
Too little fuel (subcritical) and too many neutrons escape, so the reaction dies. At critical mass, loss equals creation and the reaction is steady. Too much fuel (supercritical) and the reaction runs away.
Subcritical mass (less than critical): more neutrons are lost than created, so the reaction eventually stops.
Critical mass (exactly critical): neutron loss equals creation, giving a steady, self-sustaining reaction — what a reactor aims for.
Supercritical mass (more than critical): more neutrons are created than lost, so the reaction runs away, leading to an explosion.
Critical mass condition
rate of neutron loss = rate of neutron creation by fission
Quick recap: fission releases spare neutrons that trigger more fissions — a chain reaction. Slow thermal neutrons keep it going, and only one per fission needs to for a steady reaction. Critical mass is the minimum fuel for a steady chain; below it dies out, above it runs away.
🧭 Which mass gives which outcome?
Compare neutron loss to neutron creation in the fuel
Loss > creation (too little fuel) → subcritical → reaction dies out
Loss < creation (too much fuel) → supercritical → runaway reaction
For a reactor, aim for critical: exactly one neutron from each fission goes on to cause the next
WE 1
Explain how a chain reaction is sustained in a sample of uranium-235, and why the neutrons involved must be slowed down.
How the chain is sustained
each U-235 fission releases two or three neutrons
these neutrons are absorbed by other U-235 nuclei, causing further fissions
each of those releases more neutrons, and so the reaction continues
→ a self-sustaining chain reactionWhy the neutrons must be slowed
only slow (thermal) neutrons are readily absorbed by U-235
a fast neutron would rebound off the nucleus without being captured
→ slowing the neutrons keeps the chain going
WE 2
Define critical mass, and describe what happens to the chain reaction if the mass of fuel is (a) below and (b) above the critical mass.
Definition
critical mass = the minimum mass of fuel needed to maintain a steady chain reaction
at this mass, neutron loss equals neutron creation
Part (a) — below critical (subcritical)
more neutrons escape than are created
→ the reaction eventually dies outPart (b) — above critical (supercritical)
more neutrons are created than escape
→ a runaway reaction, leading to an explosion
💡 Top tips
Each fission needs only one of its neutrons to trigger the next fission for a steady chain reaction
Slow = thermal. The neutrons must be slowed so they’re absorbed rather than bouncing off
Critical mass is a minimum: below it the reaction dies, above it runs away, at it stays steady
Link mass to neutrons: more fuel means fewer neutrons escape the surface, so more go on to cause fission
⚠ Common mistakes
Saying all the neutrons from each fission must trigger new ones — only one is needed for a steady chain
Forgetting the neutrons must be slowed — fast neutrons rebound and don’t sustain the reaction
Muddling the mass cases: subcritical dies out, supercritical runs away — don’t swap them
Thinking critical mass is a fixed universal number — it depends on the fuel and its arrangement
Up next: Operation of a Nuclear Reactor. You now understand the chain reaction — next we see how a reactor keeps it exactly critical and safe, using control rods, a moderator, heat exchangers, and shielding.
Want this to actually click before the exam?
Book a free meeting and let’s work through the tricky bits together.
