Simulated Ex-Service SFR Fuel Cladding for Characterization of Degradation Under DGR Groundwater Conditions

Sodium fast reactors (SFRs) use metallic fuel, bonded to the fuel cladding material with liquid sodium metal. HT9, a ferritic–martensitic stainless steel, has been used as cladding material in test SFRs, where fuel-cladding chemical interaction (FCCI) was shown to be the life-limiting factor. In this work, FCCI has been induced in HT9 via diffusion couple testing at 650°C to simulate the ex-service cladding condition, representative of SFR operating conditions and the expected fission products. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) were used to characterize the thickness and chemical composition of the diffusion layer between non-heat-treated HT9 and Nd in the first round of trials, as Nd is the most abundant lanthanide fission product. Vickers microhardness testing was performed on post-diffusion couple samples to compare the hardness of observed regions to post-irradiation examination (PIE) of HT9 fuel cladding that had undergone FCCI in test SFRs. Analogous testing will be carried out with heat-treated HT9 and a fabricated lanthanide alloy in subsequent trials. The resulting HT9–lanthanide coupons will be used for further cladding degradation experiments incorporating simulated deep geological repository (DGR) groundwater in a worst-case scenario DGR flooding event.