A new model which describes the dynamics of a vertically stratified flow correctly within the limits of a single-pressure two-fluid model has been developed. The model is based on the modification of finite-differences of convective terms and pressure gradients taking into account a distinct interface.

We propose to use the vapor quality as a criterion for the onset of dryout. The choice of the criterion is based on the analysis of experimental and theoretical studies. To determine the boundary vapor quality we used the correlation xcr = 1.26·G0.2, which was found from experimental data fit.

A review of articles has shown that for today it is impossible to predict correct superheat value. Therefore the superheat value was determined as a parameter of the model from the experimental data of a particular simulated experiment. Thus a boiling up regime was selected. The model described in this paper allows us to calculate the boiling up of sodium under the superheat conditions as well as problems of the evolution of the vapor volume.

The verification of the models was done by using the SOCRAT-BN code [1]. SOCRAT-BN is a coupled code which consists of modules for calculation of damage and melting of a reactor’s core, thermohydraulic processes and neutron physics.

The models of vertical stratification, dryout and slug boiling of superheated sodium are described in details in this paper. Also we present the results of verification for the models within analytic tests and experimental data.

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