High temperature oxidation of SiC under helium with low-pressure oxygen. Part 3: β-SiC–SiC/PyC/SiC

Initial assessment of environmental effects on SiC/SiC composites in helium-cooled nuclear systems

“…The kinetics is governed by the linear-parabolic law, with the rate constants of the linear region being correlated with the O 2 partial pressure. 61 This behavior contrasts with the report cited above 58 according to which oxidation is not sensitive to P O2 in the range of 0.2 -2 Pa. The difference of behavior may possibly be caused by the quality of SiC fibers used (Nicalon first generation in one report 61 versus Hi-Nicalon in another 58 ).…”

“…61 This behavior contrasts with the report cited above 58 according to which oxidation is not sensitive to P O2 in the range of 0.2 -2 Pa. The difference of behavior may possibly be caused by the quality of SiC fibers used (Nicalon first generation in one report 61 versus Hi-Nicalon in another 58 ). Gasification rates of the same Nicalon-based composite in helium containing 1 % and 0.1 % hydrogen were 6 to 26 times slower than the rates in oxygen, based on the linear kinetics approximation.…”

“…The conclusion of these studies 3,24,58,59 is that in order to withstand high temperatures for prolonged durations that may occur in severe accident scenarios, the SiC/SiC composites must be protected by thicker layers of β-SiC. Substitution of He (with 2 Pa O 2 ) by N 2 (with 0.5 Pa O 2 ) did not cause important differences in oxidation behavior of β-SiC (Fig.…”

“…It was assumed that these conditions simulate the environment of gas-cooled fast reactors (GFR). 58 Weight changes versus temperature at the two oxygen concentrations showed the existence of three domains in the zone of active oxidation (Fig. 14).…”

“…However, the coating remains essential for closing the porosity and preventing direct contact of oxidizing gas with the body of the composites at temperatures below about 1500-1900 o C. The partial pressure of oxygen (0.2 -2 Pa) did not have a strong impact on the mass loss rate. 58 Unfortunately, the fibers used were second generation Hi-Nicalon; no information is available about the behavior of third generation SiC fibers in similar conditions. The β-SiC coated SiC/PyC/SiC composite is a candidate for the cladding gas cooled fast reactor (GFR) and its chemical resistance in other environments that may develop in case of severe accident conditions was recently studied.…”