Development of Yttrium Stabilized Zirconia (YSZ) diffusion barrier coatings for mitigation of Fuel-Cladding Chemical Interactions

Fuel-Cladding Chemical Interactions (FCCIs) in a nuclear reactor occurs due to thermal and radiation enhanced inter-diffusion between the cladding and fuel materials. This can have the detrimental effects of reducing the effective cladding wall thickness and the formation of low melting point eutectic compounds. Deposition of thin diffusion barrier coatings in 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 Yttrium Stabilized Zirconia (YSZ) as the diffusion barrier coating. The deposition parameters, including the nanofluid solvent, additive, particle size, current, and voltage were optimized using test flat substrates of T91 ferritic-martensitic steel. A post deposition sintering step was also conducted and optimized to improve the bonding and mechanical integrity of the coating. Diffusion characteristics of the coatings were investigated by diffusion couple experiments using cerium as a fuel fission product responsible for solid state FCCI. These diffusion couple studies performed at 575 °C for 100 h showed that the YSZ coatings significantly reduced the solid state inter-diffusion between cerium and steel. A heat transfer model was developed to simulate the changes in temperature profile inside the fuel cladding by addition of YSZ coating. It was found that even though the temperature can increase in the coated cladding, the temperature falls below the melting point of uranium and eutectic temperature in Fe-U phase diagram. Using a co-axial configuration in conjunction with the EPD process, YSZ was successfully deposited uniformly on the inner surfaces of 12″ length sections of cladding with 4 mm inner diameter. Such a coating is extremely hard to make by conventional coating technologies like thermal spray or vapor deposition.