Modeling Of Hydrogen Isotopes Retention In Self-Damaged Metallic Tungsten

Tokamak is one of the most promising devices for achieving the nuclear fusion energy,and its fuel is deuterium(D)and tritium(T).For safety and economic reasons,it is necessary to limit the T inventory in plasma facing materials(PFMs)of a tokamak below a certain amount because it is radioactive and expensive,e.g.International Tokamak Experimental Reactor(ITER)requires T inventory below 700 g.Therefore,it is very important to control the T inventory in PFMs.Tungsten(W)is recognized as primary candidate for PFMs due to its excellent physical properties,such as high melting temperature,good thermal conductivity,low sputtering and low T retention rates.Tungsten will suffer strong irradiation by intense particle and energetic neutron fluxes,which can give rise to serious radiation damage,thus aggravating the T retention.However,there is great uncertainty of PFMs performance due to radiation damage.Prediction and control of T retention in W under neutron irradiation is one of critical topics for fusion energy.The aim of this thesis is to develop physical models and numerically study the fuel retention in W to understand the main factors influencing fuel retention in selfdamaged W.The whole thesis is organized as follows:In Chapter 1,the background of this specific topic and its current progress are presented.Then,the physical models and numerical methods of Hydrogen Isotope Inventory Process Code(HIIPC)are introduced in Chapter 2.The main contributions of this thesis are represented,respectively,in chapter 3,4 and 5.In Chapter 3,the hydrogen isotope(HI)exchange module of HIIPC code is developed.The simulation has been done on HI retention and exchange in self-damaged W.First,the roles of irradiation-induced defect in retention are analyzed,and the effects of D fluence and material temperature on retention are discussed.It is found that irradiation damage depends strongly on the fluence and energy of W ions,that both high fluence and energy can aggravate irradiation damage,eventually enhancing D retention.The simulation verifies that the irradiation damage can be a dominant factor for D retention.It is also found that D fluence,material temperature and de-trapping energy affect the D retention strongly.As the D fluence increases,the D retention increases first,then varies gradually,and turns constant,finally;the high material temperature and small de-trapping are conductive to reduce D retention.Besides,simulation of implanting H exchange D is also undertaken.Results show that implantation of H reduces the D retention in self-damaged W.However,the efficiency of D exchanged by H relies on the detrapping energy,due to the trapped D being frozen in a high trapping energy trap,thus restricting the HI exchange.The simulation results are well consistent with experiments.In Chapter 4,a surface module for the HIIPC,which simulates the effects of surface processes on D retention in self-damaged W under low-energy D irradiation,is further developed.Results show that both surface adsorption and absorption processes play an important role in the D retention.The surface adsorption leads directly to surface retention,which in turn plays a blocking effect for the sticking of incident D particles,thus affecting the adsorption and absorption processes.Owing to the blocking effect for preventing D particles into bulk substrate,an outgassing flux from the surface is formed,which is linked to the particle recycling.It is found that before surface retention saturation,material temperature and flux strongly affect particle recycling.Surface absorption acts as a source for bulk D retention,increasing the material temperature,bulk D retention first increases then decreases.By considering the different ratios of direct implantation D,bulk D retention variation with material temperature is well explained.Moreover,influence of surface-energy barrier on bulk D retention is also studied.The total D inventory under small barrier obtained from simulation shows a good agreement with experiments,while high barrier can suppress D absorption,thus reducing the D retention.In Chapter 5,a Vacancy-Type Defect Evolution(VTDE)code is then developed,based on the developed HIIPC with the surface module,and the effects of annealing on vacancy-type defect(Vn=1,2,3,…)evolution and D retention in self-damaged W are investigated.Results show that annealing reduces the mono-vacancy(V1)concentration,leading to the formation of vacancy-type defects cluster(Vn?2).The temperature has a crucial role in V1 annealing,which affects Vn clustering processes,and the clustering will further influence the Vn concentration and distributions.Increasing annealing temperature,concentration of Vn remarkably decreases,which is attributed to the loss result from Vi and di-vacancy(V2)captured with other Vn or annihilated by the grain boundary and dislocation.Comparing of effects of different grain sizes on Vn are made,showing small grain size reduces Vn concentration.It is also found that Vn concentration is strongly affected by cluster size n,high annealing temperature,Vn with larger cluster size n can be stable,which can act as steady trap sites for D species.By simulating the annealing impact on D retention in self-damaged W,it reveals that different types of defects linking to annealing differ greatly.Before annealing,V1 defect dominates D retention,while vacancy-type and dislocation-type defects dominate D retention.The simulation well explains experiment.By further analyzing D amount retained in different types of defects and by comparing the simulated results with those in experiments,it can be predicted that contribution of irradiation-induced dislocation-type defect to D retention is about 69-71%,in which vacancy-type defects account for 29-31%.In Chapter 6,the conclusions and innovation points are first summarized,and the outlook of the PhD project are subsequently discussed.