Keywords: core design, computer codes, fuel cycle, spectral calculations, validation, KARATE-440 code, nuclear power plants, NPP, nuclear energy, VVER reactors
General features and validation of the recent KARATE-440 code system
In the last few years several projects aiming at the introduction of new VVER-440 fuel types and resulting in more economic fuel cycles were initiated: increased average enrichment, modification of the lattice pitch and fuel diameter, profiled enrichment, application of burnable absorber, modification of the absorber assembly coupler part. The first version of the KARATE-440 code system was elaborated in the KFKI Atomic Energy Research Institute (KFKI-AEKI) in the mid-1990s for the core design calculations of VVER-440 type reactors. The above fuel modifications and the upgraded regimes requiring more accurate calculations have necessitated the further development and validation of the code system. Owing to the new fuel types (e.g., burnable poison), greater attention has had to be paid also to the spectral calculations of the assemblies, especially to the space-dependent thermalisation in a large heterogeneous system. The fast algorithm applied to solve this problem with satisfactory accuracy is also detailed. The first part of the paper outlines the general features of KARATE, while in the next part the validation results are presented in relation to zero reactor measurements, mathematical benchmark problems and the operational data of the Paks (Hungary) Nuclear Power Plant (NPP).