Development and Testing of an Advanced Neutron-Absorbing Gadolinium Alloy for Spent Nuclear Fuel Storage

Localized attack of a two-phase metal, scanning electrochemical microscopy studies of NiCrMoGd alloys

“…The gadolinium forms a Ni 5 Gd (gadolinide) eutectic secondary phase. As expected, this gadolinide phase has inferior corrosion properties to that of the NiCrMo base material [6,7]. Gadolinium is a reactive metal but appears to be somewhat more passive when alloyed with Ni.…”

“…Electrochemical testing in solutions used in previous studies of NiCrMoGd corrosion resistance [6,7] were too aggressive for observing the dissolution and passivation of the surface with the SECM. It was found that iodidecontaining solutions provided ample reactivity and served as a redox mediator for detection of breakdown and dissolution sites.…”

“…The composition of the M322 sample was: Cr, 14.93%; Mo, 14.71%; Gd, 2.38%; Fe, 0.025%; C, 0.006%; Si, 0.013%; and S, <0.001% with the remaining percentage being Ni. The samples were cut orthogonal to the rolling direction to form cylinders used in corrosion testing experiments discussed elsewhere [6,7]. These cylinders were crosssectioned into shorter cylinders and mounted using epo-quick epoxy (Buehler), such that a disk of material approximately 6 mm in diameter was exposed to solution.…”

“…Boron and gadolinium have been added to austenitic steel [5] and NiCrMo alloys [6,7], respectively, to create alloys for application in high-level nuclear waste storage. These elements are not soluble in the base metal matrix and form secondary phases that are potential sites for corrosion.…”

“…Gadolinium is a reactive metal but appears to be somewhat more passive when alloyed with Ni. Electrochemical and exposure tests were performed using ASTM methods in simple chloride solutions and in simulated groundwater solutions at various temperatures [6,7]. The gadolinide phase was found to dissolve rapidly in neutral to acidic solutions at potentials above the open circuit potential.…”