Departure from Nucleate Boiling Ratio (DNBR) Calculator
Determine critical heat flux (CHF) via Tong W-3 correlation, actual local heat flux, and DNB thermal-hydraulic design safety margin.
PWR Subchannel Core Conditions
Critical Heat Flux & Safety Margin
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Departure from Nucleate Boiling (DNB) Mechanics
Under normal high-pressure PWR conditions ($2250\text{ psia}$), heat is transferred from Zircaloy fuel cladding to coolant via highly efficient subcooled nucleate boiling. However, if local heat flux exceeds the Critical Heat Flux ($q''_{chf}$), steam bubbles coalesce into an insulating vapor blanket. This causes the heat transfer coefficient to plummet by an order of magnitude, triggering a sudden temperature excursion that can melt fuel cladding.
NRC 95/95 Design Criterion (10 CFR 50.34)
To ensure cladding integrity during normal operations and anticipated operational occurrences (AOOs), reactor core thermal-hydraulic safety analysis requires that the Departure from Nucleate Boiling Ratio ($\text{DNBR} = q''_{chf} / q''_{actual}$) maintain at least a $95\%$ probability at a $95\%$ confidence level that DNB will not occur. This translates to an allowable design limit of $\text{DNBR} \ge 1.30$.