Boiling heat transfer follows a boiling curve: as wall superheat ΔTsat = Twall − Tsat rises, heat flux climbs through nucleate boiling — until the surface hits the critical heat flux (CHF), the point where the vapor generation rate outpaces liquid rewetting and a insulating vapor film blankets the rod. This "boiling crisis" is called departure from nucleate boiling (DNB); wall temperature then jumps hundreds of degrees in milliseconds because film boiling is a far worse heat-transfer mode — the mechanism that limits how hard a PWR core can be run.
Zuber pool-boiling CHF (upward-facing surface):
q"_CHF,0 = 0.131 · h_fg · √ρ_v · [σ·g·(ρ_l − ρ_v)]^(1/4)
Flow correction (higher flow & subcooling delay CHF):
q"_CHF = q"_CHF,0 · (1 + 0.6√(G/1000)) · (1 + 0.015·ΔT_sub)
Safety margin:
DNBR = q"_CHF / q"_actual (plants trip near DNBR ≈ 1.3)
- Heat flux — the power the fuel rod is delivering to the coolant right now.
- Pressure — a PWR runs near 15.5 MPa; CHF falls as pressure approaches water's 22.06 MPa critical point because ρ_l and ρ_v converge and h_fg → 0.
- Mass flux (G) — faster coolant flow strips bubbles off the wall before they coalesce into a film, raising CHF.
- Subcooling — coolant colder than saturation absorbs more sensible heat before flashing to vapor, also raising CHF.
- When DNBR drops below 1, the rod surface is shown transitioning into film boiling: bubble nucleation stops looking like discrete bubbles and becomes a continuous vapor blanket, and the rod glows hotter as clad temperature runs away.
Real-world relevance: DNBR is exactly the safety parameter reactor protection systems monitor in real time in a pressurized-water reactor — operators and the automatic trip system keep it above a licensed limit (commonly ~1.3, from correlations like W-3 or WRB-1) at every point along every fuel rod in the core.