In the event of a loss of integrity of the main coolant line, a large mass and energy release from the primary circuit to the containment is to be expected. The temporal evolution of such depressurization is mainly governed by the critical flow, whose correct prediction requires, in first place, a correct description of the different friction terms. Within this work, selected friction models of the CESAR module of the Accident Source Term Evaluation Code (ASTEC) V2.1 integral code are validated against data from the Moby Dick test facility. Simulations are launched using two different numerical schemes: on the one hand, the classical five equation (drift flux) approach, with one momentum conservation equation for an average fluid plus one algebraic equation on the drift between the gas and the liquid; on the other hand, the recently implemented six equation approach, where two differential equations are used to obtain the phase velocities. The main findings are listed hereafter: The use of five equations provides an adequate description of the pressure loss as long as the mass fluxes remain below 1.24 kg/cm2 s and the gas mass fractions below 5.93 × 10 − 4. Beyond those conditions, the hypotheses of the drift flux model are exceeded and the use of an additional momentum equation is required. The use of an additional momentum equation leads to a better agreement with the experimental data for a wider range of mass fluxes and gas mass fractions. However, the qualitative prediction for high gas mass fractions still shows some deviations due to the decrease of the regular friction term at the end of the test section.

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The severe accidents at Fukushima have shown that a further development of Severe Accident Management Guidelines (SAMGs) is necessary. Within this work, the severe accident code ASTEC V2.0 is used to assess the impact of selected SAM measures on the in-vessel progression of Small Break (SBLOCA) scenarios in a generic German Konvoi PWR. The progression of reference SBLOCA sequences with and without Station Blackout (SBO) is firstly analyzed. Calculations show that melting and vessel failure can be delayed if the secondary side is filled before the plant is struck by the SBO. Based on these results, a systematic evaluation of primary side depressurization and core reflooding as primary SAM measures is carried out. Simulations yield the following results: • Primary Side Depressurization must be initiated before the Core Exit Temperature (CET) > 400°C or, if not possible, with a maximum delay of 20 min to delay core melting and vessel failure. • Core reflooding must be launched immediately after CET > 650°C with at least 7.50-20 kg/s in order to mitigate the accident without major core damage. • If an external injection is used for such aim, the deployment order of a medium-high pressure head (>20 bars) mobile pump must be issued 1 h after the entrance in SBO. • Vessel failure cannot be prevented if more than 20 corium tons are present in the lower plenum for more than 1 h regardless of the injection rate. The performed investigations clarify ASTEC V2.0 capabilities to describe the in-vessel progression of a severe accident in PWRs and contribute to extend the technical basis for the further improvement of SAMGs in German Konvoi PWR.

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