Orlik (Czechia)

Abstract In order to increase the efficiency of fuel utilization at Dukovany NPP, the design of FA was changed by shroud removal and replacement with a structure called “Karkas”. Optimization of PK-3+ type FAs with different average enrichments was performed in order to find out those enrichment profiles with minimized non-uniform energy generation in FA (during burn-up). In addition, it was assumed that such a radial enrichment profile in FA could be achieved by making a change in the location of the fuel pin with a Gd2O3 burnable absorber – from the 2nd row to the 3rd row of pins from the edge of the fuel assembly on the fuel assembly diagonal. The aim of this study was to achieve a full quadruplicate cycle, every 15 months (approx. 450 days) at 1475 MWt nominal power. Preliminary results indicate that combination of PK-3+ and Gd-2M+ fuel assemblies does not show any unusual phenomena from the point of view of reactor physics. The proposed strategy is based on B1C33 cycle implemented at Dukovany NPP that is designed to be 395 FPDs. Already in the first “transient” cycle (34th) loaded with 60 fresh PK-3+ FAs and 12 Gd-2M++ CAs, the reached length at EOR is 424 FPDs, which means stretch-out 26 effective days. Averaged, the transition cycle stretch-out length is 23.5 effective days. For steady cycles, this average value is 19.2 effective days.

Abstract Geometry models for Monte Carlo transport codes have been using standard constructive solid geometry (CSG). The standard approach is using analytical equations for defining surfaces from which spatial cells are constructed. However, this approach can be quite time consuming and possibly error prone for complex models. Monte Carlo transport codes are continuously developed, one of the paths is using CAD-based mesh geometry. MCNP6 features unstructured meshes (UM) created with Abaqus/CAE as geometry description. Attila4MC package for creation of UM geometry from CAD model can be used for MCNP6 models. VVER-1000 fuel assembly model in UM geometry was created for TVSA-T.mod.2 fuel type. Basic validation of the model was performed, initially for criticality calculations. In the future, the model will be used for criticality safety analyses, preparation of boundary conditions for diffusion codes and radiation shielding analyzes of spent fuel transport and storage facilities.