Comparison of the updated solutions of the 6th dynamic AER benchmark - main steam line break in a NPP with VVER-440

The 6th dynamic AER Benchmark is used for the systematic validation of coupled 3D neutron kinetic/thermal hydraulic system codes. It was defined at the 10th AER-Symposium. In this benchmark, a hypothetical double ended break of one main steam line at full power in a VVER-440 plant is investigated. The main thermal hydraulic features are the consideration of incomplete coolant mixing in the lower and upper plenum of 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 re-criticality temperature was defined.
First solutions of the benchmark were compared during the 11th AER-Symposium. Due to the considerable spreading of these first results, the participants continued the work on the calculation of the benchmark. Updated solutions were received from VTT Processes Espoo (HEXTRAN/SMABRE), Kurchatov Institute Moscow (BIPR8/ATHLET), NRI Rez (RELAP5-3D) and Forschungszentrum Rossendorf (DYN3D/ATHLET). The solution of KFKI AEKI Budapest (KIKO3D/ATLET) remained unchanged.
The paper gives an overview on the behaviour of the main thermal hydraulic and neutron kinetic parameters in the provided solutions. The differences in the updated solution in comparison to the previous ones are described. Improvements in the modelling of the transient led to a better agreement of a part of the results while for another part the deviations rose up. The sensitivity of the core power behaviour on the secondary side modelling is discussed in detail.