Impact of Temperature, Exposure Times and Microstructure on Tellurium Diffusion and Embrittlement in Alloy 617 and SS316

Tellurium is a known by-product of nuclear fission that can impact mechanical properties and cause embrittlement of structural alloys used in molten salt reactors; including both Fe-based and Ni-based alloys. This can cause issues, as the current anti-corrosion tactic of passivation erodes under the chloride salt conditions of MSRs, leaving the underlying alloy exposed. The purpose of the current study addresses Te diffusion routes and embrittlement mechanisms for a Ni-based alloy, Alloy 617, and the industry standard Fe-based stainless steel, SS 316. Samples of Alloy 617 and SS316 are exposed to moderately high temperatures (700-900oC) in a Te-rich environment for extended periods of time (1 h - 25 h). Te-exposed and control samples are characterized using mechanical testing, optical metallography, and Scanning Electron Microscopy with Energy Dispersive Spectroscopy of polished cross sections and SEM of fracture surfaces. Results have shown that Te diffusion follows 4 distinct pathways include grain boundary diffusion, diffusion along preferred crystallographic planes of the FCC Ni-rich alloy, diffusion along non-preferred crystallographic planes, and formation of a Te-rich crust on the alloy surface. Embrittlement occurs in the form of grain boundary decohesion as the grain boundaries become decorated with Te. Determination of the diffusion mechanism, sequence, and rates along the different microstructural pathways will provide the necessary materials data for determining strategies for alloy design, grain boundary engineering, and ideal prior processing to mitigate Te diffusion and embrittlement. The results from this study can guide MSR design policy with respect to structural alloy exposure limits.