Multiscale neutronics/thermal-hydraulics coupling with COBAYA4 code for pin-by-pin PWR transient analysis

Simulation of a hypothetical MSLB core transient in VVER-1000 with several stuck rods

A hypothetical main steam line break transient in a VVER-1000 core with multiple equipment faults was simulated with the coupled COBAYA4/CTF and COBAYA3/FLICA4 nodal core models. The objective was to test recent versions of the models developed in the EU NURISP and NURESAFE projects and to analyze the thermal-hydraulic conditions in the hot assembly at the pin-cell level. The accident scenario was specified as an aggravated variant of the OECD/NEA VVER-1000 MSLB benchmark with eight peripheral control rod clusters stuck out of the core after scram, all of them in the overcooled sector. Coarse-mesh models and a realistic cross-section library were used to compute the full-core behavior with pre-calculated MSLB boundary conditions and to identify the hot assembly. Then a sub-channel thermal-hydraulic analysis of the hot assembly was performed with the CTF code, using time-dependent assembly boundary conditions from the coarse-mesh vessel and core simulation. The results show that the predicted values of the safety parameters do not exceed the safety limits.