燃料包壳铬涂层中氧和氢扩散行为研究

The diffusion behavior of oxygen and hydrogen in Chromium coating on fuel cladding

在压水反应堆中,锆合金包壳与水发生腐蚀化学反应,会对包壳的机械性能产生不利影响,从而限制燃料元件的使用寿期。为了减缓包壳氧化速率,防止氢气燃爆风险的发生,人们提出了事故容错燃料的概念设计。金属铬具有优异的抗腐蚀氧化性能,作为包壳涂层已备受核电领域的广泛关注。目前,铬涂层耐高温腐蚀氧化行为微观机理研究尚不清楚,亟须开展相关研究。本文采用第一性原理方法从电子尺度探索了铬晶体中氧和氢的扩散机理。研究表明,氧的最稳定占位是八面体间隙,氢更倾向于占据四面体间隙。氢的溶解度远低于氧的溶解度。氧在间隙的溶解能为负值,表明氧与近邻铬之间存在较强的相互吸引。进一步采用弹性带方法计算氧和氢的反应扩散路径和迁移能垒。氧沿着四面体间隙至八面体间隙的反应路径扩散,迁移能垒为0.18 eV,而氢更容易沿着四面体间隙至第一近邻的四面体间隙的反应路径扩散,迁移能垒为0.79 eV。同时,氧沿着四面体间隙至第一近邻八面体间隙的反应路径扩散,迁移能垒为0.65 eV。这表明氧倾向于沿着四面体间隙至八面体间隙的反应路径扩散。氢在铬中的迁移能垒为0.17 eV。进一步结合阿伦尼乌斯扩散方程,拟合出温度与扩散系数的变化关系,为涂层高温腐蚀性能实验研究提供理论支持。

[Background]In a pressurized water reactor,the corrosion chemical reaction between zirconium alloy cladding and water will adversely affect the mechanical properties of the cladding,thus limiting the service life of the fuel elements.In order to slow down the oxidation rate of the cladding and prevent the potential risk of hydrogen explosion,a conceptual design of accident tolerant fuel was proposed.Chromium metal has excellent corrosion and oxidation resistance,and has been widely used as cladding coatings in the field of nuclear power.At present,the micro-mechanism of corrosion and oxidation resistance of chromium coating at high temperature is not clear,so it is urgent to carry out relevant research.[Purpose]This study aims to investigate the diffusion behavior of oxygen and hydrogen in coating on fuel cladding.[Methods]The diffusion mechanism of oxygen and hydrogen in chromium crystals was investigated on the electronic scale by using the first principles method.The Arrhenius diffusion equation was employed to obtain the diffusion coefficients of O and H at different temperatures.In addition,the reaction-diffusion paths and migration energy barriers of oxygen and hydrogen were calculated by elastic band method.[Results]Simulation results show that oxygen occupies the most stable position in the octahedral interstitial site(OIS),and hydrogen tends to occupy the tetrahedral interstitial site(TIS).The oxygen atoms diffuses from the reaction path TIS to TIS with the diffusion energy barrier 0.79 eV whilst the oxygen atoms diffusion along the TIS to OIS reaction path has the diffusion energy barrier 0.65 eV.It suggests that there is a preferential diffusion pathway from TIS to OIS for oxygen atom due to its lower diffusion energy barrier.Notably,hydrogen demonstrates comparable diffusion energy barriers(0.17 eV)when moving along the reaction path from TIS to TIS and from TIS to OIS,respectively.The diffusion coefficients of oxygen and hydrogen increase linearly with the increase of tempterature,respectively.And for,the diffusion coefficients of both oxygen atom and hydrogen atoms along the TIS to OIS reaction path are higher than that of TIS to TIS reaction path at the different temperatures.[Conclusions]The solubility of hydrogen is much lower than that of oxygen.The negative dissolution energy of oxygen in the interstitial site indicates that there is a strong mutual attraction between oxygen and the first nearest neighbor chromium.A fitted relationship established between temperature and diffusion coefficient in this study provides theoretical support for investigation coating corrosion properties at elevated temperatures.