The fission matrix method for criticality search in Heat-Pipe Micro Reactors

In this work the Fission Matrix (FM) method is proposed to conduct the neutronics analysis of a Heat-Pipe Micro Reactor (HPMR). The aim of this model is to support the real-time modelling of heat-pipe-based microreactor designs, which have recently gained significant industrial interest. A database of FMs is generated through different criticality calculations, each of them considering a different core configuration (temperature profile and control drums position). The database is then interpolated to generate, in principle, the results for any arbitrary core layout. In this study the accuracy decrease of the FM method in the presence of a temperature difference between the core and the reflector is highlighted and addressed. The accuracy of the method is assessed through comparison with reference Serpent 2 simulations. Different core temperature profiles and control drum positions are tested. Finally, leveraging the computational efficiency and high accuracy of the FM, a criticality search algorithm is developed. Overall, the results show very good agreement with the corresponding Serpent 2 reference cases, while requiring only a very short computational time.