Frontier physics
Nuclear reactions and fission
Nuclear reactions conserve charge and nucleon number; fission of heavy nuclei releases energy and neutrons that may sustain a chain reaction.
Nuclear reactions conserve charge and nucleon number; fission of heavy nuclei releases energy and neutrons that may sustain a chain reaction.
Definition: Core idea
For a reaction a+A→b+B, total charge and mass number are conserved; positive Q denotes released rest-mass energy. Fission such as U-235+n→lighter fragments+2–3 n releases about 200 MeV per event, mostly as fragment kinetic energy. In a reactor model, k_eff<1 is subcritical, =1 critical, and >1 supercritical.
Model and interpretation
For a reaction a+A→b+B, total charge and mass number are conserved; positive Q denotes released rest-mass energy. Fission such as U-235+n→lighter fragments+2–3 n releases about 200 MeV per event, mostly as fragment kinetic energy. In a reactor model, k_eff<1 is subcritical, =1 critical, and >1 supercritical.
Example: Quantitative example
For U-235+n→Ba-141+Kr-92+3n, check conservation of mass number and charge.
Solution
Mass number: 235+1=141+92+3=236. Charge: 92=56+36. Balancing nuclei is necessary; the Q-value still requires precise masses.
At research level, fission is described probabilistically, not as one fixed product channel. Cross sections depend on neutron energy; fission fragments span distributions in mass, charge, and excitation, and may subsequently emit delayed neutrons and gamma rays. Reactor kinetics therefore couples the neutron spectrum and transport to thermal hydraulics. A one-group model using k_eff is useful for classifying criticality, but it cannot by itself predict power distributions or transient response. Marie Curie's experimental work established foundations for radioactivity and nuclear research; Enrico Fermi advanced neutron physics and chain-reaction theory, helping shape its quantitative modern approach.
The mean neutrons per fission do not fully describe kinetics: neutron emission timing and the delayed fraction create distinct feedback timescales. Safety analyses also track reactivity, temperature coefficients, and moderator feedback. These quantities are inferred from measurements and uncertainty-bearing models; they are not universal constants of every fuel configuration.
Quick check
What does k_eff=1 mean in a neutron-multiplication model?
Which statement best describes “Nuclear reactions and fission”?
References
- Frank Close (2015). Nuclear Physics: A Very Short Introduction