Abstract

Recent efforts in MPACT have focused on improving the performance of the 2D/1D subplane implementation to help target computational performance goals. This paper builds on previous efforts that targeted the use of subgrid treatments to improve the accuracy of control rod representation, presenting three additional applications of subgrid treatments with the goal of reducing the computational burden of simulations. These subgrid applications include treatment of spacer grids, thermal feedback, and axial reflector material representation. With these approaches, a single method of characteristics (MOC) plane can contain several different materials axially that are represented explicitly via subgrids on the coarse mesh finite difference (CMFD) mesh but are axially homogenized on the MOC mesh. This allows for a substantial reduction in the number of MOC planes needed in the calculation through the introduction of an approximate treatment, particularly with regard to the self-shielded cross sections and MOC-informed radial current coupling coefficients in CMFD.

Several test problems ranging from single rod to quarter core are used to assess the solution accuracy and performance of these various subgrid representations. Overall, the accuracy of the approximations seems very reasonable, with extremely small differences in eigenvalue observed and maximum pin power errors in the 0.5% to 1.0% range. Several cases show substantial value in the compromise between accuracy and computational performance. Others highlight the new computational hurdles that future research will aim to resolve.

摘要

MPACT 最近的工作重点是提高 2D/1D 子平面实现的性能，以帮助实现计算性能目标。本文建立在以前的工作的基础上，这些工作旨在使用子网格处理来提高控制棒表示的准确性，提出了子网格处理的三个附加应用，目的是减少模拟的计算负担。这些子网格应用包括间隔网格的处理、热反馈和轴向反射器材料表示。通过这些方法，单一的特征法 (MOC) 平面可以包含轴向上的几种不同材料，这些材料通过粗网格有限差分 (CMFD) 网格上的子网格明确表示，但在 MOC 网格上轴向均质化。

从单杆到四分之一核心的几个测试问题用于评估这些各种子网格表示的解决方案的准确性和性能。总体而言，近似值的准确性似乎非常合理，观察到的特征值差异极小，最大引脚功率误差在 0.5% 到 1.0% 的范围内。几个案例在准确性和计算性能之间的折衷中显示出重要价值。其他人则强调了未来研究将要解决的新计算障碍。