高溫氣冷式反應器(High Temperature Gas-cooled Reactor, HTGR)因具被動性安全及能利用產生的高熱來製氫等兩項特點，而成為眾多第四代核反應器中最具希望突破重圍的反應器。核能級石墨因具較高的熱導率、較大的比熱容及較具等向性的熱膨脹係數使其成為高溫氣冷式反應器的結構材料。在HTGR正常運轉期間，石墨的工作溫度約在500-1000 °C，若此時發生Hot Gas Duct破管事件，反應器洩壓後會引發空氣進氣事故。在反應器底部的石墨組件會與空氣產生化學反應進而引發爐心崩塌， 使燃料直接接觸反應器容器底部並與氧氣反應，進而引發輻射物質外釋的疑慮。
本氧化腐蝕實驗由兩部分組成，第一部分的主要目的為探討核能級石墨IG-110、IG-430及NBG-18在700-1100°C因氧化效應產生的孔洞顯微結構變化與氧化速率變化之差異來源。利用高溫爐產生的高溫(700-1100 °C)並通以0.35L/min、2L/min之乾燥空氣來模擬核能級石墨在HTGR發生進氣事故時的高溫氧化效應。氧化前的試片以場發射槍掃描式電子顯微鏡(FEG-SEM)輔光學顯微鏡(Optical Microscopy)觀察材料未氧化表面形貌並統計填充物顆粒(Filler Particle)的尺寸及形狀，並以水銀測孔儀(Mercury Porosimetry)分析未氧化的孔洞分布情形。利用比表面積分析儀(BET Surface Area Analyzer)分析材料的Total Surface Area(TSA)，並使用X光繞射儀(X-Ray Diffraction Analysis)分析材料之晶體常數。利用石墨材料修正因子(Fe)修正因材料填充物形狀、尺寸不同所造成的Active Surface Area(ASA)差異。最後可計算出材料的ASA，並作為氧化效應實驗前的評估。對照實驗結果，Active Sites理論與實驗結果相符。氧化後的試片以FEG-SEM觀察表面形貌改變，並以水銀測孔儀定量因氧化效應而改變的孔洞分布。進而探討石油焦炭基(Petroleum-Based)的IG-110與瀝青焦炭基(Pitch-Based)的IG-430在進氣事故中的氧化環境(700-1100°C)下，所造成之孔洞的顯微結構變化及氧化機制，以及比較兩種使用相同焦炭但尺寸差異極大的核能級石墨(IG-430、NBG-18)在中溫(700-1000°C)及高溫環境(1000-1100°C)之氧化速率差異之原因。第二部分主要使用真實尺寸之石墨護套(Graphite Sleeve)與以石墨為基材製成之燃料丸(Fuel Compact)進行高溫腐蝕實驗，藉以評估發生進氣事故之石墨組件腐蝕情況。此實驗利用高溫爐建立之高溫1000°C環境，並通以2L/min的乾燥空氣來評估以IG-110製成之石墨護套的削薄現象(The Thinning of Graphite Sleeve)及以IG-110、IG-430製成之燃料丸之質量損失情況。
最後，第一部分的實驗結果顯示不同焦炭來源的核能級石墨(IG-110、IG-430)在中溫(700-900°C)的氧化速率有明顯差異，並可由材料的氧化速率差異和孔洞分布說明Active Sites集中在孔洞內部；高溫(1000-1100°C)區間，氧化速率的高低是由材料表面孔隙率來決定。第二部分的實驗結果顯示在1000°C、2L/min乾燥空氣的環境下，石墨護套在5小時內即被燒穿，使放置在內的燃料丸暴露在空氣中。而在相同環境下，燃料丸在1小時內會損失22%的質量，使均勻散佈在內的TRISO Fuel Particles暴露在空氣中，進而引發輻射物質外釋的疑慮。

This work consists of three main parts. The first part describes IG-110, IG-430, and NBG-18 grade nuclear graphite based on the size and shape of the filler particles and how the forming method affects the pore distribution. The second part presents an experimental investigation of nuclear graphite oxidation in air at temperatures ranging from 700°C to 1100°C and correlates the results to the theory of graphite active sites. Mercury porosimetry is used to quantify the phenomenon of pore structure developments at various temperatures. X-ray diffraction analysis of selected graphite is conducted to determine the crystallographic parameters. The results of mercury porosimetry and scanning electron microscopy images are correlated to the theory of graphite active sites, demonstrating the relationship between pore distribution and active sites. The third part of the study presents two experiments. The first experiment investigates the size effect of samples with the same aspect ratio and the second examines the size of fuel pellets and graphite sleeves to evaluate the degradation of graphite components in an air-ingress scenario.

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