Study On Irradiation Optimization And Screening Of Structural Materials For Molten Salt Reactor

The molten salt reactors(MSRs),one of the advanced reactor designs of“Generation IV” nuclear power plants,offer many advantages in comparison with other current nuclear reactors,namely increased safety,reliability,comprehensive hightemperature utilization.However,high temperature,high neutron dose and strong fluoride molten salt corrosion pose severe challenges in MSR to its structure materials.The nickel-based alloy UNS N10003(Hastelloy N alloy and GH3535 alloy)has been developed for use with liquid fluoride salts,and it is considered the most promising candidate structural material for the MSR.Raising the operating temperature of MSRs is one of the most economical methods to improve efficiency in their comprehensive high-temperature applications,such as hydrogen production.However,due to its unsatisfied high-temperature mechanical properties,UNS N10003 alloy is difficult to fulfill the temperature requirements of higher-temperature MSRs operated over 700 °C.Therefore,based on potential improvements of higher-temperature MSRs,screening suitable materials from existing commercial alloys has become one of the effective ways.Apart from the high temperature and corrosive coolant in the MSR,neutron irradiation also degrades the mechanical performance of the structural materials during the operation.Therefore,it is necessary to carry out a systematic study on evaluating the irradiation resistance properties of candidate alloys for higher-temperature MSRs.In this thesis,nickel-based alloy Inconel 617,Alloy 800 H were used as the research object,where UNS N10003 alloy was chosen as reference material to make comparison.These two alloys possess excellent high-temperature strength.Additionally,with the development of nickel cladding,both alloys have the potential to meet the corrosion resistance requirements.In order to reveal the irradiation resistance of these two alloys and study the evolution behaviour of irradiation-induced defects,two aspects of helium-induced damage and displacement damage were studied by characterizing the changes of microstructure and mechanical properties before and after irradiation.the high-temperature-He-ion irradiation study show that the different alloy concentrations lead to distinct characteristics of helium bubbles and dislocation loops by affecting the diffusion of point defects.The resistance to irradiation induced hardening and swelling of Inconel 617 and alloy 800 H alloys are both inferior to UNS N1003 alloy.With irradiation temperature increasing,the formation of precipitate clusters in Inconel 617 alloy and bubble-loop complexes are responsibility for the special evolution of irradiation defects.As for the displacement damage study,due to the high number density of dislocation loops,the resistance to irradiation hardening is inferior to that of other two alloys.Hence,the influence mechanism of alloying elements(Ni,Mo and Cr)in Nickel-based alloy on the evolution of irradiation-induced defects is revealed,which provides a basis for the selection of alloys for higher temperature molten salt reactor and development of new alloys from the perspective of irradiation resistance.Further irradiation results show that the edge-on Frank loop variation have distinct evolution behaviour with the increasing ion fluence.Hence,the quantitative characterization of loops is more accurate by measuring two Frank loop variations.Moreover,the yield strength of irradiated induced shallow damaged layer is obtained by analyzing the indentation curve of nanoindentation,and a new method to evaluate the irradiation induced mechanical degradation of alloy is established.New processing technologies,such as large plastic deformation and additive manufacturing,provide a potential method to improve the irradiation resistance of alloy.In order to reveal irradiation resistance of nickel-based alloy manufactured by these technologies,the evolution behaviour of He-induced damage in these alloys was studied.The alloy processed by large plastic deformation possess better resistance to helium swelling and inferior resistance to irradiation induced hardening.Additionally,the SLM alloy has better resistance to irradiation induced hardening,but it needs to optimize the process parameters to suppress helium swelling.Therefore,the influence mechanism of alloy structure on evolution behaviour of helium-induced defect in nickel-base alloy is revealed,which provides a reference for the improvement of new processing technology from the perspective of irradiation.