Analysis of primary bleed and feed strategies for selected SBLOCA sequences in a German Konvoi PWR using ASTEC V2.0

Abstract

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.