Investigation Of Helium Behavior In Nickel-based Alloys Of Molten Salt Reactor By Using Ion-Beam Technique

Molten Salt Reactor (MSR) can operate at high temperature, and due to its highneutron fluxes and highly corrosive condition, nickel-based alloy was selected as thecandidate structure material because of its good corrosion resistance and irradiationresistance abilities. But helium embrittlement is a problem in nickel-based alloyswhich caused by neutron irradiation. ORNL had invented Hastelloy-N alloy andapplied it in MSRE successfully. GH3535alloy was invented in our country’s TMSRproject, its mechanical properties and molten salt corrosion resistance performancehad already studied, but its irradiation resistance ability is still unknown. Despite theresearch of helium behavior in pure metal and alloy has investigated a lot, butstudying all kinds of properties of nickel-based alloy after being irradiated by heliumions will help us understand how helium embrittlement generate and would contributeto modify the alloy.Based on ion beam technique, this thesis mainly investigated the helium behaviorin nickel-based GH3535and Hastelloy-N alloys. We studied the changes ofmicrostrain and microstructure and what was the connection of the hardness andmicrostructure of nickel-based alloy after helium irradiation. We also investigated thechange of helium concentration in the alloy surface and the comprehensive effect ofnickel-based alloy by helium irradiation and corrosion.After GH3535alloy being irradiated by70keV helium ions and annealed atdifferent temperature subsequently. The results showed that with increasingirradiation temperature, the microstrain first increased and then decreased. Smallbubbles and dislocations were found at the annealing temperature of773K and873K,in the case of the973K and1073K annealing, no bubble was observed. The result wasthe evidence that the existence of helium bubbles would cause lattice distortion whichwas the reason why microstrain increased.After Hastelloy-N alloy being irradiated by helium ions at different energy anddose, we investigated the hardness and found the added hardness was in directproportion to irradiation dose. With the energy of helium ions increasing, the damage depth increased while the average damage decreased at the same dose, so the hardnessmeasured by NI decreased. After873K annealing, the hardness decreased a little butstill kept the increasing discipline with the dose. The microstructure result showed1nm~7nm black dots appeared after high dose irradiation which would cause hardnessincrement by stopping dislocation line from gliding. And some small black dotsdisappeared observed by TEM which was the evidence that the decreasing hardnesswas due to small defects annihilation. Small Helium-Vacancy cluster would aggregateand form bigger cluster, but the helium concentration decreased after873K annealing.We concluded that although the increasing of helium-vacancy cluster size wouldcontribute to hardness, but the number of helium-vacancy cluster decreased mightcause the reduction of hardness.In order to investigate the irradiation resistance ability of corroded Hastelloy-Nalloy. We used Hastelloy-N alloy corroded in FLiNaK salt for100h at the temperatureof973K, the surface and especially grainboundary of Hastelloy-N appeared manyholes. The corrosion thickness was about20μm and significant depletion of Cr and Ferose from this depth. When the corroded samples was irradiated by70keV heliumions subsequently, more holes was observed in the surface, but with the doseincreasing to a specific high, some holes disappeared and the surface became moreflatness.