Departure from Nucleate Boiling (DNB) is a limiting phenomenon in the operation of water reactors. DNB occurrence may damage the rod clad and thus the confinement of fission products in the fuel rod is not anymore ensured. DNB must therefore be avoided not only in all operating states but also in case of occurrence of most frequent transient conditions. For accident analyses, DNB must be limited to a few percentage of rods. As DNB cannot be modelled using first principles, and as transient and accident analyses contain a lot of hypotheses and unknown variables, DNB analyses remain subject to significant variations. However a comprehensive knowledge of physical (mechanistic) model is not necessary to achieve reliable and satisfactory calculations, providing the identification of unknown parameters, and their impact, and the evaluation of uncertainties, are accurately taken into account. This paper will present some deficiencies in DNB analyses that may challenge the safety of the operation of a nuclear reactor.
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