Verification of the current coupling collision probability method with orthogonal flux expansion for the assembly calculations

Abstract

The operation of nuclear reactors requires detailed knowledge of important safety parameters, such as the spatial power distribution, control rod worth, margin to departure from nucleate boiling (DNB), fuel pin burnup etc. To obtain a detailed analysis of all of the safety parameters requires a full core pin-by-pin coupled neutronics and thermal-hydraulics simulations which are too computationally expensive even for modern high-performance computer clusters. Therefore, the industrial standard approach in design and safety calculations are coupled neutronics and thermal-hydraulics codes for the steady state and transient simulations. In these codes, the neutronics calculations are typically performed at a nodal level using the diffusion approximation and assembly-homogenised sets of cross-sections while the thermal hydraulics relies on a channel model with fuel assembly sized channels. However, for determining safety limits, which are based on local pin-based parameters, the knowledge of the power and temperature distribution on a nodal level is not sufficient. Therefore, novel new approaches are required to resolve this multiscale and multiphysics problem to resolve the power distribution within the zones of interest. Pin-wise calculations, in this case, are performed by applying a transport solver using the heterogeneous fuel assembly geometry on an unstructured mesh with boundary conditions extracted from the 3D full core nodal diffusion solution. This combined nodal-transport approach will provide the detailed power distribution on the pin-level and perform coupled multiphysics simulations within reasonable simulation time limits, which is important for industry.To follow this strategy, a transport solver is required which can be used for the flux reconstruction on the pin level. Current coupling collision probability (CCCP) method seems to be a good choice for the development of such a solver.In this study, the developed transport solver utilising CCCP method with orthogonal flux expansion is tested and verified on the set of the benchmark problems. The results of simulations are compared with the results of Monte Carlo and deterministic code. The expansion of the flux by orthogonal polynomials allows us to avoid discretisation of the calculation regions while keeping the accuracy of the calculations to an acceptable level. The results of the calculations demonstrate good agreement with the results of Monte Carlo calculations. The comparison of the new method with the flat flux (today's industry standard approach) approximation demonstrates either an improved quality of the result for identical cell discretisation or reduced computational time to achieve the identical solution.