The Theoretical Study Of The Water Absorbed And Dissociated Behavior On The Perfect And Defect ThO₂ Surfaces

As a new generation of nuclear fuels,ThO₂ has an important prospect for application in the light water reactors and high temperature gas cooled reactors.In term of safety in the management and the storage of these materials,investigations of the absorbed and dissociated behavior on the ThO₂ surfaces of water molecules,which is widespread in surroundings,is extremely essential.In this work,we adapt the DFT+U methord and slab models to systematically investigate the configuration and electronic structure of the ThO₂?111?,?110?and?100?perfect surfaces as well as the defect?111?surfaces with three different color centers.In addition,we also explore the reaction mechanism for absorption and dissociation of the water molecule on these surfaces.As our result shows,the thermal stability order of the low-index perfect surfaces is?111?>?110?>?100?.As to the defected ThO₂?111?surfaces with different types of color centers,which are denoted by Fs⁰?Fs⁺ and Fs²⁺,the defects will be formed more easily with the color center charged more positively.In the result of the calculation of the band structures,we can get that the surface electrons will be located in the defect sites,and the formation of the oxygen defects will not lead to the obvious occupation of the 5f-orbital of the thorium.The water molecules all exist two types of the absorbed structures on the ThO₂?111?,?110?and?100?perfect surfaces,the molecular and dissociated ones and the surface hydrogen bonds take the roles to stable the absorbed structures.In the thermal point of view,the dissociated structures will be dominant than the moleculaer ones,especially for the?110?and?100?surfaces.As the result of the dynamic study of the dissociation processes shows,H₂O will all be dissociated into one absorbed hydroxyl and one absorbed H atom easily on three perfect surfaces.In addition the migrations on the?111?and?110?surfaces need a higher energy-barrier?0.8 eV?,and on the?100?surface the value is 0.4 eV.Hence,the?100?surface will be hydroxylated more easily.Like the stoichiometric surfaces,on three different defected ThO₂?111?surfaces,the water molecules also exist two types of absorptions,and the dissociated ones are more favorable than the molecular ones.The difference on the defected surfaces with the perfect ones is that the water molecules will be dissociated completely,then hydrogen will be released.As the dynamic study of the pathways shows,the type of the ThO₂?111?color centers will obviously influence the water dissociated behavior.Only on the Fs+-type defected surface,the water molecule will be dissociated completely and easily,with the one on the Fs2+ surfaces following.On the Fs⁰ surface,as there are more electrons in the vacancy,the intermediate structure with the H atom filling the hole will be stabilized more than others,and that leads to the complete dissociation becomes more difficult.