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All 14 Flashcards — Fission and chain reactions
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Question
What is nuclear fission?
Answer
A **large nucleus splits** into two smaller nuclei, releasing **energy** and a few spare **neutrons**.
Question
What is induced fission?
Answer
Fission **triggered** by a nucleus **absorbing a neutron**, which makes it unstable so it splits (not happening on its own).
Question
What is a chain reaction?
Answer
Each fission releases neutrons that go on to cause **more** fissions — one fission triggers the next.
Question
What does 'self-sustaining' mean for a chain reaction?
Answer
The chain **keeps itself going** without any extra neutrons being added from outside.
Question
How many neutrons does one fission typically release?
Answer
About **2 or 3** (plus two daughter nuclei and a lot of energy).
Question
Condition for a STEADY (critical) chain reaction?
Answer
On average **exactly one** neutron per fission goes on to cause the **next** fission.
Question
If N neutrons are released per fission, how many are lost or absorbed when steady?
Answer
**N − 1.** One continues the chain; the rest must be lost or absorbed.
Question
What happens if fewer than N − 1 neutrons are lost per fission?
Answer
More than one continues, so the rate **grows** — the reaction is **supercritical**.
Question
What happens if more than N − 1 neutrons are lost per fission?
Answer
Fewer than one continues, so the reaction **dies out** — it is **subcritical**.
Question
Why does each fission release energy?
Answer
The products are slightly **lighter** than the original — that tiny **mass defect** becomes energy via **E = mc²**.
Question
Subcritical, critical, supercritical — what do they mean?
Answer
Subcritical = dying out; **critical = steady**; supercritical = growing. Set by how many neutrons continue per fission.
Question
What is spontaneous fission?
Answer
A heavy nucleus splitting **on its own**, without absorbing a neutron; like induced fission it releases energy.
Question
How do you find the energy released in a fission reaction from masses?
Answer
Δm = (mass of U + neutron) − (fragments + all neutrons out); E = Δm × 931.5 MeV.
Question
About how much energy does one U-235 fission release?
Answer
About **200 MeV** (3.2 × 10⁻¹¹ J).
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