Abstract

The Transient Multilevel (TML) scheme in the MPACT code has reduced the computational burden for three-dimensional, full-core, time-dependent reactor simulations with pin-resolved detail. However, the total computational cost is still large for practical applications. In this paper, we present a new method that uses a one-group coarse mesh finite difference (1GCMFD) approach to further accelerate the TML scheme. Since multigroup coarse mesh finite difference (MGCMFD) calculations in TML dominate the simulation run time, the 1GCMFD method developed here is shown to reduce the overall computational time by as much as 50% for some large-scale applications. The 1GCMFD method accelerated the calculation primarily by accelerating the convergence of the source for MGCMFD calculations through 1G/MGCMFD iteration. A new 1GCMFD level was implemented in the TML scheme (TML-4) assuming that the energy distribution of the scalar flux shape varies more slowly than the energy-integrated amplitude of the scalar flux. Various numerical cases were used to investigate the practicality of the 1G/MGCMFD iteration and TML-4 scheme. Numerical results show that using the 1G/MGCMFD iteration with a dynamic iteration strategy alone does better capture the evolution of the amplitude function when the scalar flux distribution in energy space varies rapidly and thus provides more accurate results. However, TML-4 is more efficient in capturing the variation of the energy-integrated amplitude when cross-section changes are small and feedback dominates the change of the reactivity. For smaller problems, the 1G/MGCMFD iteration and the new TML-4 scheme can reduce the run time of the coarse mesh finite difference (CMFD) solver by 50% and the total run time by at least 16%. For large-scale, full-core problems, the run time of the CMFD solver can be reduced by 78% and the total run time by as much as 47%.

摘要

MPACT 代码中的瞬态多级 (TML) 方案减少了具有引脚解析细节的三维、全核、瞬态反应堆模拟的计算负担。然而，实际应用的总计算成本仍然很大。在本文中，我们提出了一种新方法，该方法使用一组粗网格有限差分 (1GCMFD) 方法来进一步加速 TML 方案。由于 TML 中的多组粗网格有限差分 (MGCMFD) 计算在模拟运行时间中占主导地位，因此此处开发的 1GCMFD 方法可以将某些大规模应用的总体计算时间减少多达 50%。1GCMFD 方法主要通过通过 1G/MGCMFD 迭代加速 MGCMFD 计算源的收敛来加速计算。在 TML 方案 (TML-4) 中实施了新的 1GCMFD 级别，假设标量通量形状的能量分布比标量通量的能量积分幅度变化得更慢。使用各种数值案例来研究 1G/MGCMFD 迭代和 TML-4 方案的实用性。数值结果表明，当能量空间中的标量通量分布快速变化时，单独使用带有动态迭代策略的 1G/MGCMFD 迭代可以更好地捕捉幅度函数的演化，从而提供更准确的结果。然而，当横截面变化很小并且反馈主导反应性变化时，TML-4 在捕获能量积分幅度的变化方面更有效。对于较小的问题，1G/MGCMFD 迭代和新的 TML-4 方案可以将粗网格有限差分 (CMFD) 求解器的运行时间减少 50%，总运行时间至少减少 16%。对于大规模、全核的问题，CMFD求解器的运行时间可减少78%，总运行时间最多可减少47%。