The onset of subcooled nucleate boiling in nuclear fuel bundles

Abstract

One of the requirements for the thermalhydraulic analysis of water cooled nuclear reactors is the prediction of two-phase pressure drop of the coolant in the fuel channel. Many existing pressure drop prediction methods employ two-phase multiplier correlations based on adiabatic gas-liquid flow. The paper shows that the use of such multiplier correlations with the thermodynamic equilibrium quality results in unsatisfactory predictions of the


pressure profile in heated bundles. In order to compute a realistic vapour weight quality, a reliable onset of Nucleate Boiling correlation valid for bundles must be used. Existing correlations for the Onset of Nucleate Boiling (ONB) or the Onset of Significant Void (OSV) do not adequately pred Let the location in a reactor channel where boiling starts to affect the pressure profile. A reactor channel contains many subchannels of varying enthalpy and consequently boiling may occur in one subchannel and not in another at the same cross-sectional average conditions. Therefore, a new correlation was derived from some recently obtained 37-element CANDU-geometry simulated bundle pressure drop data. The emphasis of this paper is on the derivation of this Onset of Nucleate Boiling correlation. The paper also highlights the pressure profile predictions that may be obtained using the proposed ONB correlation.