5.2.6 · D5Nuclear & Radiochemistry
Question bank — Fission — chain reaction, critical mass, reactors (thermal vs fast)
True or false — justify
Each line: statement, then the reasoning. Say "true/false because…" before revealing.
Fission of always releases exactly 3 neutrons.
False — it releases 2–3 on average (typically 2.4), and the split is random; sometimes 2, sometimes 3, occasionally more, varying with the fragment pair.
A larger lump of fissile material has a smaller surface-to-volume ratio.
True — surface grows as but volume as , so surface/volume shrinks as grows; that's exactly why bigger lumps leak proportionally fewer neutrons.
If each fission emits 2.5 neutrons, then must be about 2.5.
False — counts only neutrons that cause new fission; most of the 2.5 leak out or are absorbed without fissioning, so can easily be below 1.
At the reactor's power output is constant in time.
True — each generation produces exactly as many fissions as the last, so the fission rate (and thus power) holds steady; this is the normal operating point.
Doubling the mass of a critical assembly doubles the power output the instant you do it.
False — pushing past critical makes , so power grows exponentially (as over generations), not linearly; mass and steady power aren't proportional.
A moderator increases the energy of neutrons.
False — a moderator slows neutrons down via elastic collisions with light nuclei, dropping them from ~MeV to ~0.025 eV where fissions readily.
Control rods work by physically blocking neutrons like a wall.
False — they work by absorbing neutrons (Cd, B have large capture cross-sections); the neutrons are swallowed, not deflected.
A fast breeder reactor needs no moderator.
True — it deliberately keeps neutrons fast so spare neutrons can convert to ; moderating them would waste them to parasitic capture.
Fission and fusion release energy for the same underlying reason.
True in spirit — both climb toward the iron binding-energy peak (Binding Energy & Mass Defect); fission of heavy nuclei climbs from the right, fusion of light nuclei climbs from the left.
Critical mass is a fixed property of alone.
False — it depends on shape, density, purity, and whether a neutron reflector surrounds it; a reflector bounces neutrons back and can cut critical mass to a fraction of the bare value.
Spot the error
Each line gives a flawed statement; the reveal names the mistake and the fix.
"Fission conserves mass, so reactants and products weigh the same."
The mass number () is conserved, but actual mass is not — a tiny vanishes and becomes ~200 MeV via (Einstein Mass-Energy Equivalence).
"The energy comes from breaking the strong-force bonds, like breaking chemical bonds."
Breaking bonds costs energy; the energy is released because the fragments are more tightly bound than the parent — they fall to a lower energy state, releasing the difference.
"A sphere is used for a bomb core because spheres are easiest to machine."
A sphere gives the minimum surface area per volume, hence least neutron leakage and smallest critical mass — it's a physics choice, not a machining one.
"Thermal reactors use slow neutrons because slow neutrons carry more energy to split the nucleus."
The neutron doesn't supply the fission energy; slow neutrons are used because 's capture-and-fission cross-section is far larger at thermal speeds (Neutron Cross-section).
" is useless in a reactor."
In a breeder it's fertile: absorbs a fast neutron and beta-decays twice into fissile , so it becomes fuel.
"Heavy water () is a good moderator because it's heavy and stops neutrons fast."
It moderates well because deuterium is light enough to slow neutrons by collision yet rarely absorbs them; its low neutron-absorption is the real advantage, not its weight.
"Increasing above 1 forever is how a reactor makes steady power."
A power reactor holds ; a sustained is runaway growth. Operators raise slightly only to increase power, then return to .
"A neutron reflector adds neutrons to the core."
It adds no new neutrons — it redirects escaping ones back in, reducing leakage and thereby raising .
Why questions
Say your answer aloud, then reveal for the real reasoning.
Why does a chain reaction need at least one surviving neutron per fission, not just neutrons emitted?
Because emitted neutrons that leak or are absorbed can't continue the chain; sustaining requires the count of fission-causing survivors to hold at one per generation ().
Why do fast fission neutrons collide better with hydrogen than with uranium to slow down?
A neutron transfers the most energy when it hits a nucleus of nearly its own mass (billiard-ball collision); hydrogen's proton is ~equal mass, uranium is ~235× heavier and barely recoils.
Why does enrichment let a thermal reactor stay subcritical-safe yet operational?
Raising the fraction to ~3–5% ensures enough fissile targets that can reach 1 with moderation, but the low fraction plus water's negative feedback keeps it far from bomb-grade behaviour.
Why is a sphere's critical mass smaller than a cube's of the same material?
The sphere has less surface per unit volume, so proportionally fewer neutrons leak, letting a smaller mass reach .
Why do reactors use delayed neutrons for control even though they're a tiny fraction?
A small fraction of neutrons is emitted seconds later by fragment decay; this delay stretches the effective generation time from microseconds to seconds, making slow enough for mechanical control rods to respond.
Why does breeding require fast neutrons rather than thermal ones?
Fast operation leaves more spare neutrons free (less parasitic capture), and those extras convert fertile into fissile Pu — moderating would waste the surplus.
Why can't you simply use pure as reactor fuel?
is fertile, not fissile — it rarely fissions and instead captures neutrons, so it can't sustain a chain on its own.
Edge cases
The scenarios the topic quietly assumes you can handle.
What happens to if a subcritical assembly is suddenly compressed to higher density (same mass)?
Higher density packs nuclei closer, shortening neutron travel to the next fission and reducing leakage — rises and it can go supercritical (the implosion-bomb principle).
Two subcritical hemispheres are slammed together — what changes?
Merging into one ball lowers surface-to-volume ratio, cutting leakage so jumps above 1 → supercritical (gun-type assembly).
If all control rods are fully inserted, what is ?
Well below 1 — the strong absorbers swallow neutrons so the chain dies; this is the shutdown (scram) state.
What limits the chain when a fuel lump is exactly at critical size but you double the surrounding reflector?
The extra reflector returns more leaked neutrons, so the same mass now runs supercritical — reflectors effectively lower the critical mass.
At the zero-mass limit (a single fissile atom), can a chain reaction exist?
No — one atom yields neutrons that have no further targets nearby; a chain needs enough neighbouring nuclei that survivors keep finding fuel, i.e. above critical size.
What happens to the fission products after they form — are they stable?
No — fragments are neutron-rich and radioactive, undergoing chains of beta decay, which is why spent fuel stays hot and hazardous (Nuclear Reactor Safety & Waste).
If a reactor's coolant boils away in a thermal design using water as both coolant and moderator, what happens to ?
Losing water removes moderation, so neutrons stay fast and fission less efficiently — in such designs typically drops (a self-limiting negative void feedback), a key safety feature.
Recall One-line summary to lock in
Q: What single quantity is every fission trap secretly about? A: The multiplication factor — whether neutrons that actually cause fission stay at exactly one per generation.