Simulation and integrated uncertainty analysis of the OECD/ NEA V1000CT2 vessel mixing problem with RELAP/ SCDAPSIM mod3.5

The paper presents validation results for multi-1D thermal-hydraulic models with RELAP/SCDAPSIM mod3.5 used for pseudo-3D reactor vessel simulation. The main objective is to test the capabilities and limitations of such calculation models to represent the vessel mixing phenomena in a VVER-1000. The test case is from Phase 2 of the OECD/NEA VVER-1000 coolant transient benchmark (V1000CT-2) and includes the calculation of an asymmetric vessel thermal-hydraulic transient caused by a steam generator isolation leading to heat-up of the disturbed loop. The considered 3D thermal-hydraulic transient in the reactor vessel is relevant to the initial part of VVER-1000 main steam line break scenarios from hot full power. The task is to solve a reactor vessel boundary condition problem with given initial and boundary thermal-hydraulic conditions and to calculate the fuel assembly inlet temperatures in the final state, the formation of the disturbed sector at the core inlet, the angular shift of the loop flows in the final state and the reactor vessel outlet parameters during the transient. Plant data from flow mixing experiments conducted during the commissioning of Kozloduy-6 are available for comparison. Two calculation models with different detail of vessel discretization and cross-flow junctions were tested: a 12-azimuth sector vessel model without radial discretization and a 24-sector model with radial discretization. The predicted integral parameters are in generally good agreement with the reference plant data. The preliminary results from the 24-sector model show some improvement in the predicted peripheral azimuthal and radial transition zones of the disturbed core inlet sector compared to the 12-sector ones but indicate a need for further mesh refinement studies. An integrated uncertainty analysis for input parameters used as vessel boundary conditions was carried out with RELAP/SCDAPSIM for the 12-sector model simulation. The benchmark calculation results illustrate the applicability of such models to asymmetric vessel transients involving thermal mixing and azimuthal flow rotation.