Comparison of the Results of the 6. Dynamic AER Benchmark - Main Steam Line Break in a NPP with VVER-440

The sixth dynamic AER benchmark is used for the continuation of a systematic validation of coupled 3D neutron kinetic/thermohydraulic system codes. In this benchmark, a hypothetical double ended break of one main steam line (asymmetric MSLB) at full power in a VVER-440 plant is investigated. The main thermohydraulic features are the consideration of incomplete coolant mixing inside the reactor pressure vessel and an asymmetric operation of the feed water system. For the tuning of the different nuclear cross section data used by the participants, an isothermal recriticality temperature was defined.

Solutions were received from VTT Energy Espoo (HEXTRAN/SMABRE), Kurchatov Institute Moscow (BIPR8/ATHLET), KFKI AEKI Budapest (KIKO3D/ATHLET), NRI Rez (RELAP-3D) and Forschungszentrum Rossendorf (DYN3D/ATHLET). This paper gives an overview on the behaviour of the main thermohydraulic and neutron kinetic parameters in the provided solutions. In all calculations, the overcooling after leak opening leads to a power rise and the activation of the reactor scram. The continuing overcooling after reactor scram results in a second power rise in all calculations. This power rise is eventually stopped by the injected highly-borated water from the high pressure injection system. Differences in the gradient of overcooling and different moderator temperature coefficients (MTC) in the used nuclear data are responsible for a spreading of the time of scram activation over several seconds in the various calculations. Differences in the thermohydraulic behaviour of the secondary side (modeling of the steamgenerator and the critical flow) together with the mentioned different MTC cause significant differences in the value of the second power peak.