高溫氣冷式反應器HTR-10之模擬計算與燃耗特性分析

Computer simulation and burnup characteristics analysis of the HTR-10 high temperature gas cooled reactor

吳尚謙

指導教授 ： 梁正宏 ； 裴晉哲

國立清華大學/原子科學院/核子工程與科學研究所/碩士(2013年)

摘要

高溫氣冷式反應器HTR-10 (10MW High Temperature Gas Cooled Test Reactor) 為北京清華研究院主導設計建造之第四代核反應器，擁有球床式爐心及球型燃料元件的設計。該反應器的最大特點為可進行不停爐的連續裝卸燃料 (online-refueling) ，此運轉方式的優點為能大量減少大修時間、有效使用天然鈾、以及獲致高燃耗的用過燃料。但這種動態的運轉，在模擬計算與燃耗特性分析上是一大挑戰，因此有必須藉助於建立適宜的近似模型。本論文的目的即在於建立一個適宜的燃耗模型，以可有效地模擬不停爐的連續裝卸燃料，並搭配使用MCNP5/X以及ENDF/B-VII截面資料庫來進行相關的計算工作。
在本論文中，首先茲將文獻中提出的模型作修正，並針對爐心物理特性進行一系列的探討，包括：不同緩速劑對燃料原子比下的中子增殖因數與中子能譜等。
之後在燃耗計算中，本論文建立了可用來近似不停爐的連續裝卸燃料的燃耗模型，其中，茲將爐心底部的圓錐狀簡化為圓柱狀，並使爐心以軸向分為五個燃料層，用一層一層取代實際一顆一顆地裝卸燃料，並假設裝填進爐心的燃料層只含全新燃料球。如此一來，燃料球在爐心中所佔比例會隨運轉時間上升，最後以中子增殖因數作為何時裝卸燃料的指標，即當中子增殖因數介於1.005 ~ 1.01之間時進行裝卸燃料。結果顯示，本一模擬計算的結果與文獻上的結果十分相近，證實了本一模型的適用性。此外，本論文亦針對中子增殖因數、功率分佈變化、燃耗分佈變化、爐心平均功率及部分錒系元素總量等參數隨運轉時間變化進行一連串的探討。結果發現，爐心功率分佈會隨著燃料的裝卸，而由底部尖峰變為頂部尖峰，而且經過五次裝卸燃料後，爐心會漸漸進入一平衡循環 (equilibrium cycle) ，爐心各項參數也自此變成定值。
最後，為了使模型更貼近實際的運轉型式，例如將模型的軸向拓展為細分更多層的燃料，本論文還建立了燃耗模型的自動化程序，如此一來，可有效減少在進一步修正模型時所需花費的人工處理時間與精力。此一自動化程序分為三個階段建立，每一階段的子程式在建立完成後，皆有一組實際驗證結果以確認該子程式的可靠度。

關鍵字

高溫氣冷式反應器 ； 不停爐裝卸燃料 ； 燃耗計算 ； 自動化程序

並列摘要

HTR-10 is an experimental type generation IV reactor built in China, which belongs to the pebble bed design and contain spherical shape fuel elements. In particular, HTR-10 possesses the highly desirable feature of online-refueling in fuel management, which greatly reduces outage times, leads to a more efficient use of natural uranium, and results in higher burnup in spent fuels. However, such an online-refueling presents a great challenge for computer simulation and bunrup characteristics analysis. In this study, an appropriate burnup model was thus proposed to solve the problem. All the associated computations were performed by MCNP5/X Monte Carlo computer code together, with the ENDF/B-VII cross-section library. First, the model proposed in the literature was revised in this study, and the associated neutronics characteristics such as the variation of effective multiplication factor and neutron spectrum with moderator-to-fuel ratio was investigated.
For burnup computations, an appropriate burnup model was proposed to closely simulate the online-refueling. In particular, this model simplified the bottom of the core from the conical-shaped to cylindrical-shaped, and the core was equally divided into five layers in the axial direction from bottom to top. At the time of refueling, the bottom layer was discharged from the core and discarded while a new layer containing only fresh fuel pebbles was added to the top layer of the core. Hence, the ratio of the fuel pebbles to total pebbles increased with greater operation time. This study further proposes that each fuel cycle attempts to initiate the refueling process for next fuel cycle whenever the effective multiplication factor (keff) lies between 1.005 to 1.01. The fuel cycle tends to reach an equilibrium cycle once the core has been refueled five times. Notably, the axial power distribution tends to change from a bottom-peaked to a top-peaked phenomenon as the fuel cycle number increases. In essence, the axial power distribution is nearly un-changed once the reactor core reaches an equilibrium cycle. This phenomenon can be also verified by the corresponding axial burnup distribution, average burnup, and mass of special nuclides as a function of operation time.
Finally, an automatic process was established in this study in order to reduce the artificial process in preparing any necessary revision of the burnup model. For example, the core can be equally divided into more than five layers in the axial direction from bottom to top. Also notice that, three subroutines were built in the automatic process, in which there existed some verification-used results in order to comfirm the reliability of each subroutine.

參考文獻

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