Fission, Chain Reactions, and Nuclear Reactors
Nuclear Fission
Fission mechanism
In fission, a heavy nucleus such as uranium-235 absorbs a neutron, becomes unstable, and splits into two medium-mass fragments plus several neutrons and energy. The products are typically more tightly bound per nucleon than the parent.
Why fission releases energy
Heavy nuclei lie to the right of the binding-energy peak, so splitting them into mid-mass fragments increases total binding energy and releases energy.
Chain reactions
Each fission event can produce additional neutrons that may trigger more fissions. A self-sustaining chain reaction requires, on average, at least one neutron from each event to cause another fission.
Reactor concepts
| Term | Meaning |
|---|---|
| Subcritical | Chain reaction dies out |
| Critical | Chain reaction is steady |
| Supercritical | Chain reaction grows |
| Moderator | Slows neutrons to increase fission probability |
| Control rods | Absorb neutrons to regulate power |
What role does a moderator play in many reactors?
Slower neutrons are often more effective at inducing fission in certain fuels.
Correct answer: Slows neutrons down
What condition defines a critical reactor state?
At criticality the chain reaction is steady.
Correct answer: one neutron on average causes one more fission
Which component is used to absorb neutrons and control reactor power?
Control rods absorb neutrons and regulate the chain reaction.
Correct answer: Control rods
Why are fission fragments usually more stable than the original heavy nucleus?
Their position closer to the binding-energy peak makes them more tightly bound.
Correct answer: they have higher binding energy per nucleon