Vahid Firouzdor

Development of Diffusion Barrier Coatings for Mitigation of Fuel-Cladding Chemical Interactions

Fuel Cladding Chemical Interactions (FCCI) in a nuclear reactor occur due to thermal and radiation enhanced inter-diffusion between the cladding and fuel materials, and can have the detrimental effects of reducing the effective cladding wall thickness and the formation of low melting point eutectic compounds. Deposition of diffusion barrier coatings of a thin oxide on the inner surface of the cladding can potentially reduce or delay the onset of FCCI. This study examines the feasibility of using nanofluid-based electrophoretic deposition (EPD) process to deposit coatings of titanium oxide, yttria-stabilized zirconia (YSZ) and vanadium oxide. The deposition parameters, including the nanofluid composition, current, and voltage were optimized for each coating material using test flat substrates of T91 ferritic-martensitic steel. Diffusion characteristics of the coatings were investigated by diffusion couple experiments using the fuel surrogate cerium. These diffusion couple studies performed in the temperature range of 560°C and 585°C showed that the oxide coatings significantly reduce the solid state inter-diffusion between cerium to steel.

Stress Corrosion Cracking of Austenitic Alloys in Supercritical Water

Tapered uniaxial tensile samples under constant load and bent ring samples under constant strain were used to investigate the stress corrosion cracking (SCC) of Alloy 600 and 690 in supercritical water. To verify the effectiveness of the tapered tensile sample design, the SCC of two 304 stainless steels with high and low carbon content were tested in supercritical water at 400°C, 25MPa and 10–15ppb dissolved oxygen. Both testing methods are effective to initiate SCC in 304SS, Alloy 600 and 690. Based on the bent ring SCC testing, the SCC resistance of Alloy 690 in supercritical water is lower than Alloy 600. This unexpected lower SCC resistance is probably due to the much lower tensile ductility (less than 10%) of the tested Alloy 690 as compared with the standard 40–50% ductility and the severe plastic deformation and strain in the bent ring samples. The large TiN particles and their inhomogeneous distribution in the alloy may promote the SCC initiation in this specific Alloy 690 in the bent ring samples. SCC is more likely to initiate from the Electrical Discharge Machined (EDM) or end mill machined surfaces than from ground and polished surfaces.