Good to Know — Rules of Thumb (Nuclear)
Field-tested approximations and sanity checks for nuclear engineers and health physicists. Estimates for reasonableness checks only — design and licensing always follow the applicable code, standard, and QA program.
Reactor physics
- Six factors: k = η·f·p·ε·P_FNL·P_TNL. Each factor has a physical meaning (reproduction, utilization, resonance escape, fast fission, non-leakage) — if k misbehaves, identify which factor moved before reaching for the control rods.
- Reactivity units: 1 pcm = 10⁻⁵ Δk/k; 1% Δk/k = 1,000 pcm. β_eff ≈ 650 pcm (PWR) — prompt critical is ~$1.00 of reactivity, and the reactor period collapses toward e-folding in fractions of a second there. Respect the dollar.
- Xenon: peaks ~10–11 h after shutdown from full power; the xenon dead time after a scram is ~24–48 h before restart is practical (the "xenon precluded startup"). Load-follow xenon oscillations have ~15–30 h periods — PWRs damp them; some BWR/RBMK designs don't.
- Samarium-149: builds to equilibrium over ~20 days at power and stays after shutdown (stable) — the permanent ~5,000 pcm poison you design around.
- Temperature coefficients: PWR MTC −10 to −70 pcm/°C (must stay negative — tech specs enforce it); fuel (Doppler) coefficient −1 to −3 pcm/°C, prompt. If MTC approaches zero at hot full power, you're in tech-spec action-statement territory.
- Decay heat: ~6–7% of rated power at scram, ~1% at 1 hour, ~0.5% at 1 day, ~0.2% at 1 week (Wigner-Way/ANS-5.1 — the exam form: P/P₀ ≈ 0.066×[t^−0.2 − (t+T)^−0.2]). Loss of decay-heat removal is the accident that matters most.
Thermal-hydraulics
- DNBR: minimum ~1.17–1.3 depending on correlation (W-3, WRB-1) — the safety-limit guardrail for PWRs. CPR for BWRs ~1.06–1.1. Below the limit = fuel damage assumed.
- Pressurizer: PWR primary ~2,250 psia, T_avg ~580°F; saturation margin is the operator's religion — subcooling margin < ~15–30°F gets attention fast.
- Natural circulation: works at ~5–10% of forced flow per loop in a PWR — enough for decay heat, not for power. Single-phase → two-phase → reflux as inventory drops (the TMI lesson in one sentence).
- BWR water level: collapsed vs. two-phase level confusion has caused events — know which instrument reads which.
Shielding and dose
- Tenth-value layers (Co-60): lead ~4 cm, concrete ~20–25 cm, water ~60 cm, steel ~7 cm. HVL = TVL/3.32. Dose through n TVLs = D₀/10ⁿ — the mental-math workhorse.
- Inverse square: doubling distance quarters dose rate — the first ALARA tool, before shielding or time.
- Point source: Γ(Co-60) ≈ 1.32 R·m²/h·Ci; Γ(Cs-137) ≈ 0.33. Dose rate = Γ·A/d².
- ALARA: 10 CFR 20 occupational limit 5 rem/yr TEDE, but the design target is a fraction of that — ALARA is optimization, not a limit. Public: 100 mrem/yr.
- DAC-hours: 2,000 DAC-hours = 1 ALI = 5 rem CEDE (for stochastic ALIs). Airborne radioactivity control starts here.
Materials and integrity
- Embrittlement: reactor-vessel RT_NDT shifts upward with fluence — PTS screening criteria (270°F plate/300°F weld for PWRs) are the end-of-life guardrail. Surveillance capsules measure it; projections license it.
- ASME XI: inspection intervals (10-year ISI), flaw acceptance standards (IWB-3500) — a flaw is acceptable only if analysis says so, not because it's small.
- Stress corrosion cracking: the BWR's nemesis (IGSCC of sensitized stainless) — water chemistry (hydrogen water chemistry, noble metals) is the mitigation.
- Fatigue: cumulative usage factor < 1.0 (Miner's rule) — transients are budgeted; unanalyzed transients spend the budget unknowingly.
Safety analysis and licensing
- Defense in depth: five levels (prevent → control → protect → manage → mitigate offsite). Every design change should be checkable against all five.
- Single-failure criterion: safety systems perform with any single active failure — the deterministic design rule (IEEE 603).
- PRA numbers: core-damage frequency ~10⁻⁴–10⁻⁵/yr for operating PWRs; large early release ~10⁻⁵–10⁻⁶. Use PRA to find vulnerabilities, not to argue the plant is safe enough.
- Tech specs: LCOs, applicability, actions, completion times, surveillance requirements — the operator's contract. Know the difference between an LCO not met (enter actions) and a safety limit violated (shut down).
- 10 CFR 50.59: the change-control screen — does the change require a license amendment? The most misapplied screen in the industry; when in doubt, the conservative answer is documented.
Operations
- EOP philosophy: symptom-based, not event-based — the operator responds to what the plant is doing, not to the diagnosed event (the TMI lesson institutionalized).
- Fission-product barriers: fuel, RCS, containment — the three barriers; barrier-status trees drive the severe-accident guidelines.
- Conservative decision-making: when uncertain, take the conservative action and sort it out later — formalized in INPO principles and NRC expectations alike.
Niche credentials worth knowing
- NRRPT (National Registry of Radiation Protection Technologists): the technician-level radiation-protection credential — the RP tech's professional mark, below the CHP.
- INPO accreditation: not a personal credential but the industry's training-accreditation system — NRC licenses assume it.
Cross-discipline sanity checks
- Civil/structural: seismic design basis (SSE/OBE, now GMRS) — the containment's design pressure/temperature are the structural engineer's load cases.
- Electrical: Class 1E (IEEE 308/603) — safety-related power; station blackout coping (72 h for new designs) sizes the batteries and alternate AC.
- I&C: setpoint methodology (ISA 67.04) — analytical limit → allowable value → trip setpoint, each with its uncertainty stack.
- Fire protection: Appendix R / NFPA 805 — safe-shutdown trains must survive the fire; separation and suppression are licensing basis, not housekeeping.
Original ETS reference writing, verified against public sources on 2026-09-30. Thin sourcing is flagged inline. Not legal advice — confirm current editions with the issuing body.