熔鹽式反應器為一液態核燃料系統，在運轉過程中可以線上添加燃料及移除分裂產物，視需求調整系統組成。但由於液態燃料的特性，使得在反應器物理模擬上有兩點困難需要考量，第一點是每個計算階段間的線上再處理過程，必須自動化調整組成，第二點則是考量液態燃料流動現象，必須利用分區燃耗及隨計算階段的組成混合來近似運轉情形。本研究針對一個俄羅斯設計的簡易自然對流熔鹽式反應器，透過中子物理(TRITON)及熱流(Fluent)疊代找到熔鹽式反應器收斂平衡時的溫度分布，以及建立爐心多分區自動化計算程式來模擬其長時間之燃耗情形。
有鑑於穩定的爐心運轉情況，其參數在互相影響後會趨至收斂不再變動，因此藉由中子及計算流體力學分析(CFD)，建立中子物理及熱流手動疊代程序。經由TRITON計算出多分區的功率密度分布，帶回Fluent計算得到分區平均體積溫度，再輸回TRITON改變其材料溫度，數次疊代後得到收斂穩態平衡，包含爐心溫度分布、流體速度、功率分布等。此外，藉由疊代模擬，也證實了此簡易自然對流熔鹽式反應器運轉過程中，在沒有外力驅動下，其燃料流動型態為自然對流型態。
為了模擬多分區爐心燃耗情形，必須將單一分區的自動化計算程式做修改。在爐心燃耗計算上，是以分區個別做到燃耗動作，但是因液態燃料是流動混合的，因此必須在每個階段燃耗結束後，先透過原子密度的計算，做到各個分區燃料組成的混合。接著以全爐心作為考量，完成移除分裂產物及透過控制有效增質因數來決定燃料的添加量，藉由上述過程來近似模擬多分區爐心的燃料循環組成。最後針對本研究之簡易自然對流熔鹽式反應器，將疊代過程中所得到的溫度分布及全爐心均溫當作比較條件，進行5年的爐心燃耗計算，除了分析比較燃耗結果外，也針對所需核醫藥物之核種Mo-99，分析其隨計算階段之產量活度。

Molten salt reactor (MSR) is one of the generation IV reactor which fuel is liquid phase state of molten salt fluorides. MSRs are distinguished by the circulation of fluid fuel in and out of reactor cores, which provides unique advantages for innovative applications, such as fuel addition, fission products removal. However, these features complicate neutronics analyses because of online reprocessing and fuel mixing. The goal of this research is to establish the Neutronics and Thermal-Hydraulics coupled calculation procedures, and to take fuel depletion, circulation and reprocessing into consideration in stepwise neutronics simulations.
The properties of system will converge in the steady state after a long-timed operation. With iterated neutronics and CFD simulations, the behavior of fluid dynamics, including velocity, power and temperature distributions for full core were known. The power and temperature distributions of the system eventually converged as iterations proceed. The circulation of molten salt is driven by buoyancy and gravity forces due to the change of fluid density at different temperatures. Under the prescribed condition, the feasibility of natural circulation in fuel cycle is supported
An automatic calculation procedure was developed to analyze MSR operations with online reprocessing. Because of significant variations in temperature and energy distributions over the system, the whole molten loop was divided into several zones. The fuel composition of every zones should be mixed after depletion. After mixing the fuel, the fuel composition was adjusted by online reprocessing so that the k-effective in stepwise calculation was limited in control. Based on the converged temperature distribution of fuel in equilibrium, a fuel depletion analysis considering fuel circulation and reprocessing was performed to simulate a scenario of five years continuous operation of system.

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