- 學位論文
可燃毒物對高溫氣冷式反應器之爐心特性影響分析
Influence of neutron burnable poison on neutronic characteristics in HTTR high temperature engineering test reactor core
摘要
高溫試驗反應器 (High Temperature engineering Test Reactor，簡稱 HTTR) 為第四代超高溫氣冷式反應器 (Very High Temperature gas-cooled Reactor，簡稱 VHTR) 的前身，用以測試此類型反應器的特性。HTTR 使用石墨作為緩速劑，氦氣作為冷卻劑，熱功率達 30 MW。HTTR的爐心由六角柱型的燃料柱與反射體所組成，根據燃料柱位置的不同，各別包含 31/33 根燃料棒，每根燃料棒由 14 個燃料單元堆疊而成，約莫 13,000 顆多層燃料球 (TRISO particle) 均勻分布其中。為了進一步強化多層燃料球的結構安全性，於燃料球核心與碳化矽之間增加一層碳化鋯材料，稱為 3S (safety, security, safeguard) TRISO particle。該層碳化鋯的主要功能為吸收氧原子，抑制一氧化碳的形成，降低燃料內部壓力，以減少結構破裂導致放射性物質從多層燃料球外洩的可能性。此論文將探討 3S 結構用於低濃縮度鈾 LEU 與 PuMA 兩種燃料的爐心中子特性，從有效中子增殖因數、中子能譜、徑向與軸向之中子通量、溫度係數、控制棒效能與燃耗進行特性分析。結果顯示 3S-TRISO particle 設計對中子特性無顯著影響。為了抑制起爐時的過溢反應度，並使運轉過程中的 keff 更加平穩，針對可燃毒物的種類與分布方式對中子特性帶來的影響進行分析，可燃毒物核種選擇 B 與 Gd-157，並與未加可燃毒物時的情形比較，可燃毒物分布方式分為原始設計的可燃毒物棒與將可燃毒物均勻散佈於 kernel 處兩種。計算結果顯示使用 Gd-157 分布於可燃毒物棒中與 B 散佈於 kernel 處較原始設計 B 分布於可燃毒物棒中更適合 LEU 燃料，前者僅需原始可燃毒物用量的一半 (2×10-4 atom/b-cm)，還可以延長燃料運轉週期，後者更僅需原始可燃毒物用量的 0.4 倍 (此時 B 中的 B-10 與 B-11 原子密度分別為 6.62×10-5 與 2.66×10-4 atom/b-cm)，大幅降低可燃毒物的成本，且兩者的溫度係數皆維持負值。PuMA 燃料方面，由於 B 與 Gd-157 主要吸收熱中子，兩者分布於可燃毒物棒中對中子能譜偏硬的 PuMA 來說皆不適合，而當 Gd-157 以 5 倍原子密度散佈於 kernel 處時 (此時 Gd-157 的原子密度為 8.28×10-4 atom/b-cm)，其吸收中子的機會上升，不僅抑低 PuMA 起爐時的過溢反應度，且運轉過程中 keff 隨時間的變化更加平緩，也使 HTTR 採用 PuMA 燃料的概念成為可能。
並列摘要
High temperature engineering test reactor (HTTR) is one of the most promising reactors belonging to GEN-IV high temperature gas-cooled reactor (HTGR). The 30 MWt reactor adopts graphite as moderator and helium as coolant. The HTTR core contains hexagonally prismatic fuel and graphite blocks. The fuel block consists of 31 or 33 fuel rods (depending on locations), each of which is made up of 14 stacking fuel compacts. Every fuel compact holds approximately 13,000 tiny tri-isotropic (TRISO) particles randomly distributed in a graphite matrix. The purpose of this dissertation was to assess the influence of using 3S (safety, security, safeguard) TRISO particles on the neutronics characteristics of LEU and PuMA fuel in HTTR. 3S-TRISO particles have an extra 10-μm-thick ZrC layer meant to reduce the possibility of fuel failure owing to the increase of internal pressure because of CO production. The result showed that 3S-TRISO particles influenced little on neutronics characteristics. Furthermore, in order to suppress the excessive reativity in the beginging of fuel cycle and flatten the effective multiplication factor (keff) during the operation period, the dissertation changed the allocation (rod and dispersion in the kernel) and material (Non-bp, B and Gd-157) of burnable poison (bp) and examined the neutronics characteristics. The neutronics characteristics such as neutron spectrum, spatial flux distribution, effective multiplication factor, temperature coefficient, and control rod worth were examined in depth after these changes. In the end, the dissertation concluded that the optimal options of bp in LEU fuel model were Gd-157 rods and B dispersing in the kernel, both of which used less amount of bp, suppressed the excessive reactivity in the beginning and flattened the keff. Moreover, the former model prolonged the operation period and lowered temperature coefficient. In terms of PuMA, the optimal model was Gd-157 dispersing in the kernel, which required only three tenths the amount of original HTTR did. The result saw the future of HTTR employing PuMA fuel.