KERENA is an innovative boiling water reactor concept equipped with several passive safety systems. For the experimental verification of performance of the systems and for codes validation, the Integral Test Stand Karlstein (INKA) was built in Karlstein, Germany. The emergency condenser (EC) system transfers heat from the reactor pressure vessel (RPV) to the core flooding pool in case of water level decrease in the RPV. EC is composed of a large number of slightly inclined tubes. During accident conditions, steam enters into the tubes and condenses due to the contact of the tubes with cold water at the secondary side. The condensed water flows then back to the RPV due to gravity. In this paper two approaches for modeling of condensation in slightly inclined tubes are compared and verified against experiments. The first approach is based on the flow regime map. Depending on the regime, heat transfer coefficient is calculated according to specific semi-empirical correlation. The second approach uses a general, fully-empirical correlation. Due to the modularity of models, both of them may be used in the future in systems incorporating condensation in horizontal or slightly inclined tubes. Depending on his preferences, the modeller may choose one-equation based approach or more sophisticated model composed of several exchangeable semi-empirical correlations.
Evaluation of the two-phase water mixture level in the case of the sudden depressurization of the Reactor Pressure Vessel resulting from an accident scenario is an important aspect in the reactor safety analysis. This paper discusses results of simulations of the water dynamics and heat transfer during the process of an abrupt depressurization of a vessel filled up to a certain level with saturated liquid water and with the rest of the vessel occupied by steam under saturation conditions. During the pressure decrease e.g. due to a break in the steam pipeline, the liquid water evaporates abruptly leading to strong transients in the vessel. These transients and the sudden emergence of void in the area occupied by liquid at the beginning, result in the elevation of the two-phase mixture. This work presents several approaches for modelling of the void fraction, the level swell and the collapse level. The first approach was based on the churn turbulent drift-flux correlation and an explicit analytic equation for the averge void fraction as a function of dimendsionless superficial vapor velocity. The second and the third aproaches were based on dimensionless analysis and purely empirical corelations. The models were verified against independent experimental data. The models represent the Reactor Pressure Vessel of the Integral Test Facility Karlstein (INKA) -a dedicated test facility for experimental investigation of KERENA -a new medium size Boiling Water Reactor design of Framatome. The comparison of the simulations results against the reference data shows a good agreement.
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