Study Of The Irradiation Effect And Corrosion Of Domestically-made Nickel-based Alloy

With the rapid development of the economy and the expansion of population, thedemand for energy is also growing. Low carbon economy and green energy is the trend ofworld development in the future, and thus build a safe and efficient nuclear reactor is the bestway to solve the current energy and environment issues. On one hand, China has abundantthorium resources which can provide the nuclear fuel for long-term; on the other hand,compared with pressurized water reactor,thorium base molten salt reactor (TMSR), one of thefourth generation nuclear fission reactor usually has some advantages, such as higher heattransfer efficiency, online processing of nuclear fuel, inherent safety and free of water coolingand so on. Molten salt itself has good heat resistance and specific strong corrosive. While themolten salt pipe pile and shell undergo the irradiation of neutrons, alpha ion, and so on, thechoice of core container and construction materials for molten salt reactor is a major problem.Last century, Ni-based alloy (Hastelloy-N) was chosen as the reactor structure and thecircuit material in the United States because of its good corrosion resistance and antiirradiation properties. Therefore, it is very important to study the performance of alloy underhigh temperature corrosion and complex irradiation environment for a long time.Theperformance of alloy materials was determined by the microstructure of material. In themolten salt reactor, the changes of the microscopic structure of alloys were caused by theirradiation of various ions and corrosions of molten saltfor a long time, thus threaten thesafeoperation of the reactor.In this paper, the corrosion experiments were finished in the independentbuilding tubularheater and box furnace with a cooling water circulation system. Both the corrosion behavior and ion radiation damageof Ni-based were observed in order to explore the mechanism ofalloy damage. The corrosion behavior ofthe alloy in FLiNaK (LiF, NaF, KF:46.5-11.5-42mol%) at different temperatures was studied by SEM, gravity measurement, PIXE and othermeans. The results show that corrosion weightlessness increased with the increase ofcorrosion temperature (600℃to900℃), but the increased rate declines. Corrosion depthalso increases with the corrosion temperature increased. The enrichment of Mo and loss of Crappeared after the corrosion. While the grain boundary structure significantly widened andapparent on the surface. The grain size after the first refine coarsening occurred with thetemperature of corrosion increased. Precipitates, which was Mo ingredients appeared in thealloy surface for high temperature (800℃,900℃) corrosion, especially in the grainboundary. In addition, the corrosion was significantly increased when there were impurities(water, oxygen, etc.) in molten salt.In the study of radiation damage, Ni-based alloys were irradiated by proton, xenon ionswith different energies, respectively. Alloys were irradiated by Xe+with energies of7MeVand2MeV at room temperature with the dose range from0.5dpa to8dpa. Slow positroncoincidence Doppler broadening spectrum (CDB) combined withSEM, TEMandnanoindentation techniques to characterize the changes of mechanical properties andmicrostructure of irradiated alloys. The nanoindentation results showed that the hardness ofalloys increased after irradiation. The hardness increased with the irradiation dose increases,while the rate of hardening becomes saturatedgradually. Compared the same irradiation dosewith different damage rate, greater irradiation hardening was occurred at the lower dose rate.CDB results showed that the S parameter has a slight increase with the irradiation dose increases, indicating that the vacancy-type defects were produced after irradiation. However,the Sparameter for the most heavily irradiated samplewas just2.2%, suggesting that a fewlarge-vacancyclusters exist after irradiation, mostly dominated by monovacancy. TEM resultsindicated that the size of the defects increased, while their number density decreased as thedose increased. In addition, alloys were irradiated by proton with energies of70keV and3MeV, respectively.After irradiation; SEM/EDS were carried out to analysis the morphologyand elements segregationof the alloy. Results showed that low energy (70keV) protonirradiations with implantation dose to8.0×1017ion/cm2make alloy surface smooth, while theirradiation zone appeared visible tumor and cracking for high energy (3MeV) protonirradiation with implantation dose to1.3×1018ion/cm2.