本論文旨在研究中子可燃毒物（Burnable Absorbers, BAs）燃料組件佈局組合對於 NuScale 小型模組化反應器（Small Modular Reactors, SMRs）燃料營運特性之影響。 NuScale 反應器是由美國 NPC 公司所生產之壓水式小型模組化反應器（Pressurized Water Small Modular Reactors, PWSMRs），係全美國第一座通過美國核能管制委員會 （Nuclear Regulatory Commission, NRC）認證之 SMR。PWSMRs 具有較小的規模，與 大型反應器的中子物理特性不同，在燃料營運上必須講求更為經濟性的燃料佈局。本論 文研究透過 OpenMC 進行幾何模型建立，並且使用 ENDF/B-VII.1 中子截面資料庫進 行計算，整體爐心溫度設定 560K 且為均勻分佈。 本研究之反應器模型針對 2%、5%、8% 的中子毒物釓（Gd），採用兩種摻雜方式 進行模擬。第一種方式將釓以粉末形式均勻分散於燃料丸內部，以實現中子可燃毒物的 均勻分佈；第二種方式則將釓均勻塗佈於燃料丸表面，形成中子毒物的表面集中分佈。 此外，亦考量兩種類型的中子可燃毒物燃料棒配置方式（Type-A 與 Type-B），並設計八 種中子可燃毒物燃料組件爐心配置模型進行分析，以評估中子可燃毒物爐心分佈對爐心 燃料性能之影響。並針對上述模擬結果取較佳的配置進行改良加入 1% B4C，得出額外 三種爐心配置模式 Core_A (中央十字區域加入 B₄C )、Core_B (以 25 pFA 為基礎，局 部添加 B₄C)、Core_C (中央配置 2% Gd₂O₃、外圍配置 B₄C)。結果顯示，將可燃毒物均 勻摻雜於燃料丸，具備較佳的中子吸收效果。官方建議上限為 8% 的毒物設計，在實際 運轉週期內無法完全消耗，而當濃度調降至 2% 時，則 1200 天內完全消耗。八種燃料 組件配置模型之結果顯示，起爐初期採用 37 束組件的配置會導致 keff 低於臨界值，其 餘配置則可維持超臨界狀態，並於運轉中期出現 keff 回升趨勢。其中 25 束組件之配置， 展現出最長的臨界運轉時間。若採用 1% 濃度的 B₄C 作為輔助毒物以抑制額外功率峰 值，則以 Core_C 之爐心功率分佈最為平坦，可有效提升爐心功率均勻性與運轉安全性。 關鍵字：小型模組化反應器、NuScale、可燃毒物、燃料組件

This study aims to investigate the impact of various burnable absorbers (BAs) fuel assembly configurations on the fuel operational characteristics of the NuScale small modular reactor (SMR). The NuScale reactor, developed by the U.S.-based NuScale Power Corporation, is a pressurized water small modular reactor (PWSMR) and the first SMR design certified by the Nuclear Regulatory Commission (NRC) in the United States. Due to their smaller scale, PWSMRs exhibit different neutronic behavior compared to large-scale reactors, necessitating more economical fuel arrangement strategies to achieve optimal performance. In this study, the reactor model was constructed using OpenMC, and the ENDF/B-VII.1 nuclear data library was employed for neutron cross-section calculations. The core temperature was uniformly set to 560 K throughout the simulations. The reactor model in this study simulates three gadolinium (Gd) concentrations—2%, 5%, and 8%—using two different doping methods for burnable absorbers. The first method involves homogeneously dispersing gadolinium in powder form within the fuel pellets to achieve a uniform distribution of burnable absorbers. The second method applies gadolinium uniformly coating on the surface of the fuel pellets, resulting in a surface-concentrated distribution. Additionally, two types of burnable absorber fuel rod configurations, referred to as Type-A and Type-B, are considered. A total of eight core configurations featuring different burnable absorber fuel assembly arrangements were designed and analyzed to evaluate the influence of spatial distribution of burnable absorbers on core fuel performance. Based on the results of these simulations, the optimal configurations were further modified by Incorporating 1% B₄C to suppress localized power peaking, three additional core configurations were developed: Core_A, featuring B₄C in the central cross-shaped region; Core_B, based on the 25 pFA layout with localized B₄C additions; and Core_C, combining 2% Gd₂O₃ in the central region with B₄C v placed along the periphery.Simulation results indicate that burnable absorbers homogeneously doping within the fuel pellets yield superior neutron absorption performance. While the official design limit for gadolinium content is 8%, this concentration cannot be fully depleted over a standard operating cycle. In contrast, reducing the concentration to 2% allows for complete depletion within 1200 days. Among the eight evaluated fuel assembly configuration models, the initial loading with 37 BA-containing assemblies results in a multiplication factor (keff) below criticality during startup. All other configurations maintain a supercritical state and exhibit a rising trend in keff during the mid-cycle period. Notably, the configuration employing 25 BA-containing assemblies demonstrates the longest critical operation duration. When 1% B₄C is introduced as a supplemental burnable absorber to suppress localized power peaks, the Core_C configuration achieves the flattest power distribution across the core, effectively enhancing power uniformity and operational safety. Keywords: Small Modular Reactor, NuScale, Burnable Absorbers, Fuel Assembly