A new module for the AZtlan Nodal HEXagonal (AZNHEX) code, which is part of the AZTLAN Platform, was recently developed based on the Simplified Spherical Harmonics (SPL) scheme to deal with the challenges presented in small fast reactor cores, such as the China Experimental Fast Reactor (CEFR), with high leakage and significant scattering effects. For the verification and validation process, we generated nodal homogenized macroscopic cross-sections (XS) through a full heterogeneous core model using the stochastic code SERPENT and subsequently, these XS were employed in AZNHEX. To verify the SPL implementation, several mesh sensitivity exercises were performed demonstrating that the SPL module was implemented successfully. Furthermore, to validate the code with this new implementation, we modeled some exercises contained in the CEFR benchmark with AZNHEX and compared the results with the experimental data available. The final results show a great improvement compared with the original diffusion solver reducing the deviations significantly from experimental data. In conclusion, it is shown and discussed the relevance of improved numerical models (transport approximations instead of diffusion) for the deterministic calculations of small fast reactors.
On the basis of the neutron transport equation, several neutron diffusion codes were developed in the past due to its simplified form and the limitations on computational power. Numerical schemes were developed to solve the diffusion equation with acceptable performance but still with problems in the regions where diffusion theory is not physically accurate. Nowadays, computational power has significantly increased and thus, new numerical schemes are being implemented to deal directly with neutron transport equation. The Simplified Spherical Harmonics approximation can be found among such methods. One of the interesting features of SPL is that, depending on the order of approximation, several diffusion-like equations are derived and thus, the same solvers that were developed to deal with diffusion equation can be used to solve the SPL equations with adaptations on the equation's coefficients. In Mexico, the AZNHEX code is being developed. AZNHEX is a neutron diffusion code for hexagonal geometry which employs the Raviart-Thomas-Nédélec of index zero for the space discretization of the scalar neutron flux and the multigroup theory for energy discretization. In this paper, a novel implementation of SPL in the AZNHEX code is presented. Due to the canonical representation of the solution of SPL equations, no changes in source code are necessary, instead, a pre-processor for input file is developed, and new coefficients and artificial energy mesh structure are considered on the implementation, which has been verified through the improvement of the numerical results of a practical example presented here.
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