Severe Accident Management Measures for a Generic German PWR. Part II: Small-break loss-of-coolant accident

This series of two papers focuses on the analysis of severe accident management measures for a generic German PWR of type Konvoi. A nuclear power plant model based on the severe accident code ATHLET-CD was developed in order to assess the code applicability for simulation of accident scenarios with core degradation. It was applied for investigation of two main groups of accident scenarios: station blackout and small-break loss-of-coolant accidents.
Part I presented the analyses of a station blackout accident. Part II is focused on the analysis of the plant response in case of a hypothetical small-break loss-of-coolant accident (SBLOCA with 50 cm² leak in cold leg of pressurizer loop). Due to failure of components of the emergency core cooling system and assumed unavailability of several preventive and mitigative accident management measures (AMM), the accident develops into a severe accident scenario with core melt and reactor pressure vessel failure. An alternative AMM, which is not implemented in German PWRs, is investigated in the current paper: late coolant injection to the primary circuit with a mobile equipment (bleed and feed with low pressure pump). The analysis is performed on the basis of ATHLET-CD simulations (code version Mod 3.0 Cycle A). Two injection points are investigated: injection to cold leg and injection to hot leg. As in SBLOCA scenarios the primary pressure might remain high that injection by low pressure systems is not effective, the influence of additional primary side depressurization (PSD) is investigated.
The simulations show, that mobile pump injection significantly reduces the amount of the released fission products and hydrogen, if it is started within 75 min after the core exit temperature (CET) exceeded 400 °C. Without PSD, melting of the fuel can be prevented if the mobile pump is started immediately when CET > 650 °C, but release of fission products can only be prevented if early pump injection is combined with PSD. Cases with a delay of mobile pump injection in combination with a delay of PSD (e.g. due to temporary malfunction) are also investigated: until 50 min after CET > 400 °C a lower release of fission products and longer grace times to prevent melt relocation to the lower head are observed (compared to immediate PSD at CET > 400 °C).