Decay calculations play an important role in reactor physics simulations. In particular, the isotopic decay of the burned fuel during refueling is important for predicting the startup reactivity of the following burnup cycle. In addition, there is a growing interest in high-fidelity simulations where the mesh in the burnup region can involve millions of regions. However, existing models repeatedly solve the same Bateman equations for each region, which is a waste of calculational resources. RMC is a Monte Carlo neutron transport code developed for advanced reactor physics analysis including criticality calculations and burnup calculations. This paper presents a decay calculation method named the Decay Chain Method (DCM) to optimize the RMC code for large-scale decay calculations. Unlike traditional methods, the Decay Chain Method solves the Bateman equations one decay chain at a time rather than one region at a time. The decay calculation in the burnup mode then treats the decay steps as zero power burnup steps with some optimized calculational methods to further reduce the calculational time. These methods were evaluated for a single pin example and for a Virtual Environment for Reactor Applications (VERA) full-core example. The calculations for the single pin example verify the accuracy of the decay step treatment in the burnup mode and show the improved efficiency. The single pin is divided into 1 to 1,000,000 decay regions to study the efficiency differences between the Transmutation Trajectory Analysis (TTA) and DCM methods. Both methods have a linear complexity with respect to the number of regions but DCM costs just one-sixtieth of the TTA time. In the simplified VERA full core example, the DCM method reduces the decay calculation time to 0.32 minutes from 75.26 minutes while the accuracy keeps unchanged.

中文翻译：

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使用衰减链方法优化 RMC 代码进行大规模衰减计算

衰变计算在反应堆物理模拟中起着重要作用。特别是，加燃料过程中燃烧燃料的同位素衰变对于预测后续燃耗循环的启动反应性很重要。此外，人们对高保真模拟越来越感兴趣，因为燃耗区域中的网格可能涉及数百万个区域。然而，现有模型对每个区域重复求解相同的 Bateman 方程，这是一种计算资源的浪费。RMC 是蒙特卡罗中子输运代码，为高级反应堆物理分析开发，包括临界计算和燃耗计算。本文提出了一种称为衰减链法 (DCM) 的衰减计算方法，以优化用于大规模衰减计算的 RMC 代码。与传统方法不同，衰变链方法一次求解一个衰变链而不是一次一个区域的 Bateman 方程。燃耗模式中的衰减计算然后将衰减步骤视为零功率燃耗步骤，并通过一些优化的计算方法进一步减少计算时间。这些方法针对单个引脚示例和反应堆应用虚拟环境 (VERA) 全核示例进行了评估。单引脚示例的计算验证了燃耗模式下衰减步骤处理的准确性，并显示了改进的效率。单个引脚分为 1 到 1,000,000 个衰减区域，以研究嬗变轨迹分析 (TTA) 和 DCM 方法之间的效率差异。两种方法都具有相对于区域数量的线性复杂度，但 DCM 仅花费 TTA 时间的六分之一。在简化的 VERA 全核示例中，DCM 方法将衰减计算时间从 75.26 分钟减少到 0.32 分钟，同时精度保持不变。