本文研究针对中国第四代核电站球床模块式高温气冷堆(HTR-PM)正常运 行平衡堆芯状态开展放射性积存量分析的先进方法。HTR-PM 是采用石墨慢化和 氦气冷却的球床式反应堆，通过燃料球连续自上而下循环来控制反应性。平衡堆 芯状态是指 HTR 从初始装载新燃料状态经多年连续换料和燃耗达到稳定状态。在 平衡堆芯状态下，功率分布、中子通量以及球床卸料和装料燃料数目等均达到平 衡，整个堆芯包含约 420,000 个随机排布的球形燃料元件。论文研究了 HTR-PM 堆芯在 15 次燃料球通过后的功率和热通量分布，使用开源蒙特卡罗(MC)输运 燃耗耦合程序 OpenMC 和点燃耗计算程序(NUIT)进行了综合的源项估算和屏蔽 优化。论文提出四种保真度不同的方法，其中最精细的是考虑 420,000 个单独随 机生成的燃料球的方法。本文首先开展源项计算方法比较研究，以评估保真度对 源项结果的影响，然后对 HTR-PM 堆芯和燃料管道等多种工程场景进行了屏蔽优 化研究。本文利用 OpenMC 实现三种提高保真度的方法:平均功率法、流道平均 功率法和分层功率分布法，精细到燃料球的方法则利用 NUIT，设计了“随机通 量分布算法(RFDA)”，为 420,000 个燃料球逐个随机生成通量历史并进行燃料计 算。RFDA 还可以与 OpenMC 集成进行输运燃耗耦合计算，提供精细化的堆芯源 项。模拟结果表明，OpenMC 和 NUIT-RFDA 的结果与参考数据吻合，裂变产物、 锕系元素和总放射性源项的百分比差异均低于 5.0%。在源项分析的基础上，本文 利用 OpenMC 固定源模拟开展管道屏蔽分析，以观察单个燃料球在传输过程中的 剂量率。使用燃料的放射性核素源项配置辐射源，并使用 OpenMC 内置的剂量转 换因子进行剂量率计算。结果表明，在没有屏蔽材料的情况下，管道内平均浓度 球床导致的距离管道 30 厘米处管道内平均浓度球床的计算剂量率为 20,254.86 mSv/h，标准偏差约为±1.60%，与参考值(20,900 mSv/h，相对误差为±3.08%) 吻合。综上，本论文研究了使用点燃耗分析程序 NUIT 及输运燃耗耦合程序 OpenMC 来计算 HTR-PM 源项的方法，提出的 RFDA 算法可实现精细到燃料球的 高保真度堆芯源项分析。本论文对同一堆芯状态开展不同源项估算方法比较分析， 结果印证了精确放射性源项和辐射屏蔽分析的重要性，对推动 HTR-PM 技术发展 具有重要意义。

This thesis presents advanced methodologies for estimating the radioactive source term inventory of the Chinese fourth-generation nuclear power plant (NPP) design, the High-Temperature Reactor Pebble-bed Module (HTR-PM), during normal operation under equilibrium core conditions. The HTR-PM operates as a pebble-bed, graphite- moderated, and helium-cooled reactor, where reactivity is controlled through the continuous circulation of pebbles from top to bottom. From the initial core, which contains fresh fuel pebbles and graphite pebbles together [1], HTR-PM requires years of continuous refueling and burnup to reach equilibrium core state, characterized by a balance of power distribution, neutron flux, and equal exchange of pebbles between discharge and loading. In this state, 420,000 unique spherical fuel elements are randomly positioned in the core.This thesis investigates the power and thermal neutron flux distributions within the HTR-PM core after 15 fuel pebble passes by employing an open-source Monte Carlo (MC) transport-coupled depletion tool OpenMC and a point-deterministic depletion tool (NUIT) for comprehensive source-term estimation and shielding optimization. Four distinct approaches were developed, with the pebble-wise approach being the most sophisticated, involving the investigation of 420,000 individual and randomly generated pebble states. A comparative study was led to assess the impact of fidelity in the source term results. Subsequently a shielding simulation study is conducted for various engineering scenarios of the HTR-PM core and fuel transfer pipeline to provide potential optimizations regarding to the HTR-PM core shielding.The thesis provides four distinct approaches to source term estimation of HTR-PM equilibrium core under normal operation; three fidelity-increasing approaches in OpenMC: average power, pass-averaged power, and layer-wise power distribution. The pebble-wise approach utilizes the NUIT in conjunction with a custom-developed algorithm called "Randomized Flux Distribution Algorithm" (RFDA) to generate random flux histories for each pebble and deplete the 420,000 pebble states. RFDA can also be integrated with OpenMC for transport-coupled depletion, providing a refined pebble-wise source term. Simulation results demonstrate the consistency and accuracy of OpenMC and NUIT-RFDA results with reference data, with percentage differences in fission product inventory, actinides, and total radioactive source term inventory all below 5.0%. The thesis also develops an OpenMC pipeline shielding model using the obtained source term for fixed-source simulations to observe the dose rate of a single pebble during transfer. Dose rate conversion is performed using OpenMC's built-in dose conversion factors, and the total source term is used to identify the radiation source of the study. Calculated dose rate at 30 cm from the pipe due to an average concentration pebble inside the pipeline without shielding is 20,254.86 mSv/h, with a standard deviation of ±1.60%.The reference value is 20,900 mSv/h, resulting in a relative error of ±3.08%.In summary, the study concludes that HTR-PM source term estimation can be achieved using point-deterministic tools like NUIT or transport-coupled depletion tools like OpenMC for simplified cases. The unique pebble-wise approach with the custom RFDA algorithm provides the highest fidelity for pebble-wise analysis. This study is not only presenting a comparative study of different source-term estimation tools under the same parameters but also contributes to the science and technology of HTR-PM by emphasizing the importance of accurate radioactive source term estimation and effectiveradiation shielding.