高温气冷堆准多孔介质对流传热传质耦合化学反应现象和机理研究

High Temperature Gas-cooled Reactor (HTGR) is an advanced nuclear energy system. It has significant meanings for solving the energy and environmental problems. After reactor core moisture ingress accident, the convective heat and mass transfer characteristics of coolant, water and steam in the pebble bed and fuel pebble graphite pores, and also the steam corrosion of the reactor core graphite both need to be further investigated. This project aims to reveal the convective heat and mass transfer characteristics in the reactor core pebble bed and nuclear grade graphite pores and also their coupling mechanisms with the chemical reactions. Theoretical associated with numerical and experimental methods are adopted to investigate the wall channeling effect, momentum, thermal and species dispersions and their anisotropic mechanisms of the cross flow over reactor core pebble bed; momentum, thermal and species dispersions and their relevance with the micro-pore structures in graphite pores; the influence mechanisms of temperature, reactant adsorption, product desorption, multi-component effects on the graphite oxidation. Then determine theoretical porous media models for describing heat and mass transfer for the reactor pebble bed and nuclear grade graphite pores, and also the graphite oxidation model. Establish a pebble-bed-scale and pore-scale multi-phase and multi-species heat and mass transfer porous media model for the HTGR reactor core, which can also couple the chemical reactions. Realize the analysis of the phenomena of temperature non-uniformity, random porosity, local hot spot, and water ingress accidents. Reveal the intrinsic connection between the micro-scale transport mechanisms in different dimensions and the macro-scale corrosion phenomena.

高温气冷堆是先进的核能系统，对于解决能源和环境问题具有重要意义。发生堆芯进水事故后，冷却剂、水和水蒸汽在堆芯球床及燃料球石墨孔隙内的对流传热传质特性，以及水蒸汽对堆芯石墨的腐蚀机理仍需深入研究。本项目以揭示堆芯球床及核级石墨孔隙内对流传热传质特性及其与化学反应耦合机理为目标，采用理论分析结合数值计算和实验的方法研究横掠堆芯球床壁面漏流、动量、热量、组分弥散及其各向异性机理，核级石墨孔隙内动量、热量、组分弥散及其与微观孔隙结构的内在联系，温度、反应物吸附、产物脱附、多组分气氛等对石墨氧化的影响机制，确定描述堆芯球床及核级石墨孔隙内热质传递的多孔介质理论模型以及石墨氧化模型，进而建立球床及孔隙尺度下多相多组分热质传递耦合化学反应的高温气冷堆堆芯准多孔介质模型，实现对温度不均匀性、随机孔隙率、局部热点、进水事故等现象的分析，揭示不同尺度多孔介质内微观传递机理与宏观腐蚀现象的内在联系与规律。

高温气冷堆燃料球采用石墨材料为基体，堆芯进水和进气是其最严重事故，会影响球形燃料原件寿命及放射性产物的释放。石墨的氧化过程由化学反应机理和不同尺度的扩散传质耦合控制。本项目对核级石墨氧化机理、石墨微观多孔介质内扩散、宏观球床多孔介质对流传热传质及混合特性、典型球床氧化腐蚀特性等方面进行了深入研究。最终将这些研究成果综合应用于高温气冷堆堆芯球床的进水及进气事故后的模拟，得到了进水及进气事故后高温气冷堆燃料元件的具体腐蚀量及其分布，用于高温气冷堆安全分析。.首先，开展了对多种新型国产核级石墨及日本IG110核级石墨的氧化实验，得到表观动力学参数及四步总包反应动力学参数。首次将四步总包反应引入高温气冷堆核级石墨氧化腐蚀分析。.然后，针对石墨微观空隙内传热传质，采用扫描电镜、拉曼光谱、压汞分析等方法对石墨氧化前后的微观形貌演化、孔径分布变化和纳米级微晶体进行表征。发现粘结剂总是先于石墨颗粒氧化。针对石墨微观空隙内扩散传质，提出了更精确的计算有效扩散系数的二元孔径分布修正公式。.其次，针对宏观尺度下对球床堆芯的对流传热传质过程中的质量混合和温度混合效应进行了数值研究，得到了球床正常运行工况下质量混合和温度混合的Pe数，分析了影响混合效应的三大机理—分子扩散、湍流扩散和质量弥散效应。.再次，将多步反应机理和石墨内部扩散传质过程耦合起来，分别研究单个燃料元件、典型球床单元体心立方单元（BCC）和面心立方单元（FCC）的流动换热和氧化腐蚀特性，得到了燃料元件表面及内部的氧化腐蚀分布、氧化深度和反应不均匀度等参数，然后给出了典型球床单元对流氧化腐蚀过程的多孔介质结构参数。.最后将典型球床单元氧化腐蚀过程的多孔介质结构参数及宏观球床混合特性应用于整体球床多孔介质模型，计算得到了双管断裂进气事故和一根传热管断裂进水事故工况自然循环下堆芯的石墨氧化量、氧化分布、组分分布等，分析了高温气冷堆燃料元件的安全性。