High temperature oxidation behaviors of bulk SiC with low partial pressures of air and water vapor in argon
Graphical abstract
Introduction
SiC, which is stable and effective against irradiation, has been used as structural components in high temperature gas reactors (HTGRs), plasma-facing components in fusion reactors, and nuclear fuel claddings in light water reactors (LWRs) [[1], [2], [3]]. This includes the tristructural-isotropic (TRISO) fuel particles developed for the HTGRs [[3], [4], [5]]. Despite these capabilities, after the Fukushima nuclear plant accident, higher requirements for safety, durability, and efficiency of nuclear plants necessitate a comprehensive understanding of SiC behaviors in accidental conditions. Particularly, the TRISO fuel particles will be exposed to severe conditions with over 105 Pa partial pressures of water vapor and high temperatures in a loss of coolant accident (LOCA) scenario [6].
The structure of a TRISO fuel particle consists of a UO2 kernel with several different layers [4,5]. A porous buffer layer is next to the kernel followed by an inner pyrolytic carbon (IPyC) layer, a fully dense SiC shell, and finally an outer pyrolytic carbon layer (OPyC). Each layer plays different roles, including collecting released gases, relieving thermal stress, and binding the TRISO particles to the graphite matrix. Among the layers, the SiC layer serves as the first containment barrier to withhold radioactive fission products and to endure high burn-ups, owing to its good thermal conductivity as well as high mechanical and environmental stabilities [4,5,7]. However, this layer can be damaged by air and/or water vapor ingress in the LOCA, resulting in degradation of SiC with volatile species.
Numerous studies have been carried out on the oxidation of SiC in air and/or water vapor environments at various temperatures and partial pressures [[8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. For the air oxidation, two different oxidation regimes have been observed: passive and active [[8], [9], [10], [11]]. The oxidation behaviors vary with the oxidant, temperature, total pressure, and partial pressure of the oxidant(s). The slow oxidation rate of SiC is due to the formation of a very thin, adherent protective layer of SiOx (with SiO2 being dominant and sometimes Si2O3 [17]), which results in a net mass increase by passive oxidation where [[8], [9], [10], [11]]
However, at sufficiently high temperatures, a volatile and nonpassivating oxide layer is formed. This is active oxidation of SiC and results in a mass loss according to [[8], [9], [10], [11]]SiC(s) + O2(g) → SiO(g) + CO(g)
For a given temperature, the active oxidation occurs when the partial pressure of oxygen is less than a critical pressure in the bulk gas. The passive oxidation takes place in the opposite case. With the water vapor ingress, two reactions (3) and (4) occur as [14]SiC(s) + 2H2O(g) → SiO2(s) + CH4(g) at T < 1127 °CSiC(s) + 3H2O(g) → SiO2(s) + 3H2(g) + CO(g) at T > 1127 °C
SiO2 formed in these reactions simultaneously volatilizes by forming a silicon hydroxide or silicon oxyhydroxide species under atmospheric pressures [13]. Possible reactions are [14]:SiO2(s) + H2O(g) → SiO(OH)2(g)SiO2(s) + 2H2O(g) → Si(OH)4(g)2SiO2(s) + 3H2O(g) → Si2O(OH)6(g)
The dominance of these reactions depends on the total pressure and gas velocity [13,20]. Since previous studies were mostly for turbine applications, oxidation conditions were mainly focused on high temperature, high total pressure, various mixed gas, and high gas flow rate. These parameters have significant differences compared to those of the nuclear environments, although basic mechanisms for SiC oxidations should be similar [[8], [9], [10], [11], [12], [13], [14],20]. SiC oxidation behaviors under water vapor ingress were reported for nuclear applications [[15], [16], [17], [18], [19]]. However, LWR fuel cladding was the target. Only pure water vapor or water vapor with an inert gas at very high total pressures was considered [[15], [16], [17], [18], [19]]. As mentioned above, the SiC layer in the TRISO fuel particle is embedded in the graphite matrix and inside the OPyC layer. The large volume of the matrix graphite and the OPyC layer should consume the oxidants to a large degree, which will result in low partial pressures for the SiC layer. The partial pressure of the water vapor that the TRISO particles can be exposed to during the HTGR accidents is more than an order of magnitude lower than that in the LWR accidents.
Recently, SiC oxidation under water vapor was studied using surrogate TRISO fuel particles [21,22]. A thinning of 2.5 μm and crack and pore formations were observed at the SiC layer after 1500–1700 °C, 1 atm water vapor oxidation [21]. Also, a parabolic oxidation behavior was reported at 1000–1400 °C in an Ar-38 vol% H2O atmosphere, and the corresponding layer thinning deteriorated the fracture strength of the SiC layer [22]. However, the SiC layer oxidation in the LOCA scenario is expected to experience low partial pressures of both air and water vapor up to 1600 °C [[23], [24], [25]]. A detailed understanding of the SiC oxidation under these conditions is still lacking. Oxidation properties such as oxidation rate, activation energy, and kinetic equation, and microstructure changes including oxide, crack, and pore formations in such accidental conditions are urgently needed to predict and analyze the fuel element performance and HTGR safety, which can also facilitate the design of new HTGR fuel systems.
In the present study, oxidation behaviors of SiC under water vapor + air ingress conditions were investigated for the first time using bulk SiC specimens. The microstructural features after oxidation in Ar-20 vol% H2O-10 vol% air at high temperatures up to 1400 °C were characterized and analyzed. The kinetics and activation energies were derived by evaluating the thickness of the oxide layer and gaseous species. Furthermore, a modified analytical model was presented to estimate the oxide layer thickness and the SiC mass loss with the best fitting with the experimental data.
Section snippets
Experimental procedures
Commercial monolithic -SiC fabricated by a chemical vapor deposition (CVD) process (Prematech Advanced Ceramic. Ltd.) was used for the high temperature oxidation tests. The samples were prepared into a size of 5 mm 5 mm 5 mm and finely polished using diamond-grid disks up to 6 μm to remove preexisting oxides on the sample surfaces. The oxidation experiments were performed in a horizontal tube furnace (1739-29 HT Furnace, CM Furnace Inc., Bloomfield, NJ, USA) with a high purity alumina
Effect of water + air ingress on oxidation kinetics
The oxide thickness measured after the oxidation tests at different temperatures is shown in Fig. 1 as a function of time. At 1000 °C, the oxide layer is barely noticeable owing to the slow kinetics even though the SiO2 layer forms on the SiC surfaces by reactions (1) and (3). The thickness is less than 80 nm. At 1100 °C, the oxidized layer thickness slowly increases with the temperature, from 10.2 nm to 26.6 nm. At ≥1200 °C, an apparent growth of the oxide layer is found, at which reactions
Conclusions
In this study, the oxidation behaviors of CVD SiC in Ar-20 vol% H2O-10 vol% air at 1000–1400 °C have been studied for the first time in consideration of the LOCA conditions for HTGRs, which includes oxidation kinetics, microstructures, and a new prediction model. Parabolic and passivation oxidation behaviors are observed in the considered atmosphere with the average activation energy at 228.4 kJ/mol. The oxidation of SiC in the Ar-H2O-air atmosphere results in SiO2 formation and
Data availability
The raw data required to reproduce these findings are available to download from https://data.lib.vt.edu/.
CRediT authorship contribution statement
Yi Je Cho: Conceptualization, Methodology, Software, Investigation, Validation, Writing - original draft, Visualization. Kathy Lu: Conceptualization, Validation, Writing - review & editing, Supervision, Project administration, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported by the Office of Nuclear Energy of Department of Energy (grant no. DE-NE0008808).
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