| Molten salt reactor (MSR) is one of the six most promising Generation IVreactors in the International symposium on fourth generation nuclear reactor. Also itis an important part of the future advanced nuclear fission belonging to the strategicleading science and technology projects of Chinese academy of sciences. It hasincomparable advantages: fission fuel sustainable utilization, excellent heat transfercharacteristics, producing less long-lived wastes, inherent safety and so on.Thorium-based Molten Salt Reactor technology is an important way to supplylong-term nuclear energy for the future. It is well known that the primary coolant ofmolten salt reactor is a kind of molten salt with severe corrosion. Hastelloy N alloywas developed for high temperature structural materials, such as the core container ofmolten salt reactor, molten salt return piping and the heat exchanger. The structuralmaterials are directly contact with liquid fuel salt. The invalidation problems ofirradiated Hastelloy N alloy caused by high temperature, intense irradiation andsevere corrosion are the key issues for the service life of structural materials of MSR.In this study, we adopted a variety of traditional research methods, such aspositron annihilation lifetime spectroscopy, scanning electron microscopy,weight-loss method and so on. As the development of the third generationsynchrotron radiation source, synchrotron radiation X-ray analytic techniquesgradually grew up with some advantages, such as samples of low damage analysis,high detection sensitivity and high spatial resolution. It is well-known that thecorrosion resistances of Hastelloy N alloy is related to its microstructure, theinvestigation of the microstructure changes under neutron irradiation in the reactor onHastelloy N alloy can evaluate the invalid problems of alloy in the reactor, predictingthe service life of materials. Direct neutron irradiation testing is vitally important, butthe experiment condition is restricted due to some unfavorable factors, for instance,long cycle, high cost, harsh experimental condition and so on. Since the1960s,simulation irradiation researches have turned up by using the other particles, such aselectrons, protons and ions instead of neutrons to investigate the related performanceand mechanism of the alloy, in recent years, this method got more and more attentionfrom people with short cycle, high efficiency, low cost and easily implementedexperiment scheme. Therefore, this paper presents some investigation results on thecorrosion of Hastelloy N alloy after4.5MeV He+ion irradiation performed in moltenfluoride salt at700℃for500h to simulate the corrosion irradiation in reactors. Hastelloy N samples were irradiated by4.5MeV He+ions. The irradiation doseis0He+cm2、1×1015He+cm2、1×1016He+cm2、5×1016He+cm2，thecorresponding peak damage is0,0.05,0.5,2.5dpa calculated by SRIM2008code.The results showed that with dose accumulation I1constantly decreased, and1wasabout100ps from beginning to end. The helium irradiation produced many vacancytype defects. The instability in crystal lattice resulted in that the vacancies incessantlymigrated and gathered, which brought about the decrease of I1unceasingly. Withincreasing irradiation dose, I2increased, which implied that the new generatedvacancies increased constantly, which led I2to increase. The generated vacancieswould form voids. The decrease of2suggested the formation of interstitial clusterswhich kept the trend of decrease of2. The nano indentation experiments show thatwith the increasing irradiation dose the hardness of the alloy surface increases, and thehardness decrease with the increase of depth.Hastelloy N alloy irradiated by4.5MeV He+ions was performed in moltenfluoride salts at700℃for500h. Then the samples were analyzed using synchrotronradiation microbeam hard X-ray fluorescence analysis at BL15U1beamline station ofShanghai Synchrotron Radiation Facility. The results show that with the doseincreasing from0,1×1015He+cm2,1×1016He+cm2,5×1016He+cm2, significantdepletion of Cr rose from the depth of24μm,34μm,43μm and55μm, which showsthat the irradiation has a serious impact on the corrosion resistance of alloy. That isbecause that grain boundaries and dislocations act as quick paths for Cr elementdiffusion, the dislocations of Hastelloy N alloy increased with increasing irradiationdose, so that the Cr element diffusing out of matrix became fast, and then thecorrosion resistance became weak. If in the process of corrosion mixed with too muchoxygen and water vapor, and the corrosion will be further intensified. The element Crin the matrix suffered from the most serious corrosion, and other element Ni and Modidn‘t drain easily, which illustrated that the corrosion of ally was mainly due to theloss of element Cr.Finally, we summarized the corrosion mechanism of Ni-based alloys under hightemperature molten salt, and expounded the molten salt corrosion mechanism of thecomposition, temperature and other influence factors. Furthermore, we studied theinfluence mechanism of the diffusion of the element Cr caused by the He+ionirradiation. |
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