Fission generations and effective multiplication
Fission can emit neutrons that induce further fissions, while capture without fission and leakage remove neutrons from the chain.
Key knowledge
Fission can emit neutrons that induce further fissions, while capture without fission and leakage remove neutrons from the chain. In a fixed linear generation model the effective multiplication factor k is the ratio of successive expected neutron generations: N_g=N₀k^g. A chain is subcritical for k<1, critical for k=1 and supercritical for k>1. Criticality balances production and losses; it is not a universal material-mass threshold independent of geometry and environment.
Values and equations
k=N_(g+1)/N_g for N_g>0; N_g=N₀k^g; k<1 subcritical, k=1 critical, k>1 supercritical
- k
- Effective constant generation multiplication factor
- N_g,N_(g+1),N₀
- Expected neutron generation populations and initial value
- g
- Nonnegative generation count
Conditions and scope
- A specified stationary generation shape/composition and constant k≥0, no external feeding in the displayed recurrence; the value depends on isotope, energy distribution, absorption, leakage and the surrounding material/geometry.
- A generation-count model does not set the time evolution, reactor power, delayed-neutron dynamics, temperature feedback or an actual criticality safety margin. Prompt and delayed emission require their physical timescales; no reactor design, operating limit or risk assessment is certified.
Sources · 2
References
Sources retain their recorded scope. A source link alone does not establish verification of every statement.
Evidence and review
Introduction to Water Cooled Reactor Theory: TCS 70 (2019): Section 4.1.1, printed pages 25–27, equation 20 and critical/subcritical/supercritical table; section 4.1.4, printed page 32, equation 37: generation ratio and constant-factor recurrence. The bounded generation relation is used; the document's simplified continuous-time/delayed-period approximations and device procedures are not adopted. REGDOC-2.4.3: physical criticality factors: Section 2.3.3 opening: effective multiplication depends on mass, distribution, nuclear properties and associated materials; section 13.6.1–13.6.2: chain classes, neutron capture/scatter/leakage and energy dependence. No regulatory limit or professional practice is imported as a reference-completion obligation.
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Entry details
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