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.

This work deals with “Full-Core” VVER-440 extended calculation benchmark which was proposed on the 24th Symposium of AER in October 2014 [2]. This benchmark is based on calculation benchmark defined by ŠKODA JS a.s. on the 21st Symposium of AER in 2011 [1]. This benchmark differs from the first “Full-Core” VVER-440 benchmark in use of control rods from group No. 6. Reason why these benchmarks exist is problematic validation of power distribution predicted by macro-code on the pin by pin level against experimental data. This new benchmark is also a 2D calculation benchmark based on the VVER-440 reactor core cold state geometry with taking into account the geometry of explicit radial reflector. Loading pattern for this core is very similar to the first pattern of the Mochovce NPP. This core is filled with fuel assemblies with enrichment of 1.6%w 235U, 2.4%w 235U and 4.25%w 235U. The main task of this benchmark is to test the pin by pin power distribution in fuel assemblies predicted by macro-codes that are used for neutron-physics calculations especially for VVER reactors. The reference solution has been calculated by MCNP6 code using Monte Carlo method and the results have been published in the AER community. The results of reference calculation were presented on the 27th Symposium of AER in 2017 [3]. In this paper is presented comparison of available macro-codes results for this calculation benchmark.

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.