Understanding the Impact of Nuclear Environment on the Hydrothermal Corrosion in SIC

Abstract

SiC/SiC fiber composites are potential candidates for advanced cladding materials to improve the accident tolerance of commercial light-water nuclear reactor fuel. To evaluate the fiber composites’ viability, understanding the kinetics of their corrosion in Light Water Reactor (LWR) conditions is critical. SiC corrosion results in the formation of silica, which can then be rapidly dissolved in the LWR environment. LWR conditions are demanding on materials because they are subjected to irradiation, high pressure, high temperature, and aqueous chemistry. Experiments were performed under prototypical LWR conditions (Pressure, temperature, flow rate) to understand the corrosion and silica dissolution characteristics of high purity chemical vapor deposited (CVD) SiC. Sensitivity studies are performed to develop a comprehensive model for silica dissolution considering the impact from irradiated microstructure (through Si/ proton, Co60 gamma, and neutron pre-irradiation), flow rate, electrical resistivity, surface roughness, surface wettability, and CRUD (metallic oxide impurities) deposition. The corrosion rate in the irradiated microstructures was found to be an order of magnitude higher compared to the unirradiated microstructure under boiling water reactor (BWR) conditions. Electrochemical and spectroscopic studies revealed that the enhanced corrosion in irradiated samples was the result of an increased surface reaction potential that can be associated with the structural defects and the electronically excited states produced by irradiation. Surface roughness effects on hydrothermal corrosion also accelerated the corrosion rate significantly at high mass flow rates relevant to LWR operating conditions. Based on the experimental results, the existing semi-empirical SiC hydrothermal corrosion kinetic models are updated to include the effects of irradiation, resistivity, flow rate, and pH. Further, the experimental results suggest that the CRUD deposition on the CVD SiC would reduce corrosion significantly. Enhanced CRUD formation was observed under gamma irradiation and was correlated to the reduced zeta potential and the contact angle of the surface. Further adhesion properties responsible for CRUD deposition in SiC are investigated to evaluate the likelihood of CRUD deposition in LWR conditions.

The silica dissolution rate of nuclear grade Hi-Nicalon type S fibers and fibers manufactured with Rapid Laser chemical vapor deposition (R-LCVD) with varying surface chemistries were also obtained through experiments performed in static autoclave simulating PWR conditions. The hydrothermal corrosion behavior of stoichiometric R-LCVD fibers was observed to be comparable to the nuclear grade Hi-Nicalon Type S fibers. The results show that the impact of stoichiometry was much higher than the particular manufacturing technique, though the higher surface roughness in R-LCVD fibers significantly affected the corrosion kinetics. Thermal pre-treatment of R-LCVD fibers leads to a drastic reduction in the corrosion of SiC fibers and was correlated to the increased grain size on the fiber surface when exposed to high temperatures. The effect of pre-ion irradiation on the hydrothermal corrosion behavior of SiC fibers was found to exhibit a complex relationship based on the stoichiometric composition of the fibers.

Finally, the Radiation Chemistry Analysis Loop (RADICAL) code that models the complete coolant loop chemistry, radical, and species transport in LWRs, is modified to include SiC/SiC cladding corrosion and silica transport based on experimentally determined silica formation and dissolution rates. Sensitivity analysis is further carried out on several parameters in RADICAL to inform the industry on the extent of spatial inventory of silica deposition in typical BWR and pressurized water reactor (PWR) primary loops. RADICAL modeling suggests that silica deposition in PWR components and CVD SiC thickness loss is not of great concern even when the effect of irradiation damage on SiC corrosion is considered. However, for BWRs, significant silica deposition on components and CVD SiC thickness loss is expected unless the fuel rod is covered entirely in stable CRUD within the first few months of operation. As such, a feasibility study on different protective metallic coatings applied on the SiC/SiC fiber composite was conducted to reduce the thickness loss. Out of different coatings tested, plasma spray coated and vacuum annealed FeCrAL with blended FeCrAl, Cr coating served as a stable protective barrier against SiC dissolution in hydrothermal conditions.