Abstract
Accurate reactor core steady state safety analysis requires coupling between thermal-hydraulics and three dimensional multigroup pin by pin neutronics. Concerning the neutronics modeling, the Nodal Expansion Method (NEM) code is developed at North Carolina State University in the framework of high fidelity multiphysics coupling with CTF. NEM includes a simplified third-order Spherical Harmonic (SP3) solver. In this work, the solver has been improved by incorporating higher order scattering matrix library. The boundary conditions were corrected with one dimensionalP3theory and a consistent coupling coupling between zeroth- and second-order flux moments was established. Two methods for generating second order discontinuity factors (DFs) has ben developed, one based on the Generalized Equivalence Theory (GET) and one based on Parial Current Equivalence Theory (PCET). DFs were generated with three lattice sizes: single pin, 2 pins and assembly level. These developments were tested using the C5G7 benchmark. The results of the SP3 solver improvement, by usingP2andP3scattering cross sections, show a 50% decrease in the eigenvalue (keff) prediction error compared to the reference transport solution. The GET DFs are applied in the C5G7 core pin by pin calculation and are compared with PCET DFs. The results show that PCET have a better performance in global results (eigenvalue). Concerning the different lattice sizes studies, the results show that DFs generated in smalll colorsets can improve local solutions. However, in order to reveal strong global trends, DFs should be generated in a larger corloset representative of the whole core. For the core calculations, DFs generated with the three colorsets together with an additional mixed type DFs were tested. For the mixed type, DFs generated from assembly size lattice were used for the internal interfaces and DFs generated from 2 pins size lattice were used for the assemblies boundary interfaces. These mixed DFs outperformed all the other configurations indicating that they manage to accomplish a satisfying compromise between global and local trends.
Highlights
Accurate reactor core steady state safety analysis requires coupling between thermal-hydraulics and three dimensional multigroup pin by pin neutronics [1,2]
This paper summarizes some recent developments for the Nodal Expansion Method (NEM) SP3 solver enhancement
The method generates discontinuity factors (DFs) based on both Partial Current Equivalence Theory (PCET) and Generalized Equivalence Theory (GET) to eliminate the error introduced by pin-wise homogenization
Summary
Accurate reactor core steady state safety analysis requires coupling between thermal-hydraulics and three dimensional multigroup pin by pin neutronics [1,2]. NEM has a diffusion based neutronic solver This solver was recently extended to solve SP3 equations in order to achieve a pin cell resolution of highly heterogeneous reactor cores [6]. This creates an improved solver modeling (NEM Version 2) that we test and verify in the C5GC7 benchmark core. The method generates DFs based on both Partial Current Equivalence Theory (PCET) and Generalized Equivalence Theory (GET) to eliminate the error introduced by pin-wise homogenization In this approach, side-dependent zeroth and second order DFs are generated for each type of pin cell neighbored by other types of pin cells within different lattice size (colorset) calculations.
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