Kinetic modeling of IG-110 oxidation in inert atmosphere with low oxygen concentration for innovative high-temperature gas-cooled reactor applications

ABSTRACT The oxidation behavior of IG-110, a graphite core component, was investigated at temperatures ranging from 400 to 1000°C in a 10 ppm Ar/O2 flow to simulate the oxidation process between the graphite core component and helium coolant with low O2 concentrations employed in advanced High-Temperature Gas-cooled Reactors (HTGRs). The results reveal that IG-110 undergoes significant mass loss at temperatures above 700°C, resulting in total mass changes of −1.5%, −5.3%, and −9.0% at 700, 800, and 1000°C, respectively, during a 10-hour oxidation period. No significant mass loss is observed below 600°C. To understand the oxidation mechanism of IG-110 under low O2 concentrations, we propose a kinetic model as the current chemical kinetic-controlled model does not fully explain the oxidation behavior observed in our research. Our analysis shows lower estimated reaction rates compared to studies at higher O2 concentrations; the activation energy values exhibit good agreement. The proposed kinetic model sheds light on the oxidation mechanism of IG-110 under 10 ppm Ar/O2 flow. This study provides new insights into the oxidation behavior of graphite core components in HTGRs and highlights the importance of controlling the O2 concentration in the helium coolant to prevent severe degradation of SiC-matrix fuel compacts.


Introduction
While the Fukushima Daiichi nuclear power plant accident has raised concerns about the future contribution of nuclear power in global energy supply [1], nuclear power still holds significant potential to contribute to the mitigation of global climate change and air pollution.High temperature gas-cooled reactors (HTGRs) can play a central role in addressing the climate crisis by offering a reliable source of carbonfree energy.HTGRs offer excellent safety features eliminating the risk of core melting events.HTGRs generate electricity and simultaneously produce hydrogen for industrial applications [2].Therefore, HTGRs present the most promising solution to the pressing challenges humanity is facing today: the need for alternative clean energy resources and nuclear reactor safety.
For the further application of HTGRs in practical power generation, some improvements are necessary in reactor core; higher core power density has been strongly desired in recent years [3].To meet this requirement, a sleeveless fuel compact design with a SiC-matrix has been proposed as an innovative technology for a blocktype core of HTGR [4][5][6][7][8][9][10].It has recently gained attention as a result of its promising possibility for higher-power-density operation in the future.This design enables direct cooling of the fuel compact by helium coolant and offers better heat-removal efficiency.Furthermore, the SiC fuel matrix has excellent antioxidation properties due to its inherent high-temperature stability of SiC and the formation of a protective layer of SiO 2 .SiC is known to have a high melting point and excellent chemical stability, making it highly resistant to corrosion and oxidation in high-temperature environments, thus enhancing the safety of the proposed design during air ingress accident.
However, the proposed SiC-matrix fuel compact requires an improvement in the helium coolant system to ensure fuel safety [5].The helium coolant contains impurities, including oxygen, that can lead to severe corrosion of the SiC fuel compact.Under normal operation of HTGR, the helium coolant contains O 2 less than 0.04 ppm as impurity [11].This value is strictly maintained; helium is carefully purified to remove the impurity species and supply pure coolant to the core all the time.Although the current HTGR design effectively manages this aspect, the proposed design presents a challenge as the SiC fuel matrix experiences severe corrosion.The corrosive behavior, known as active oxidation, is more likely to occur at lower oxygen concentrations during high-temperature CONTACT Yosuke Nishimura nishimura-y@g.ecc.u-tokyo.ac.jpThe Department of Nuclear Engineering and Management, Graduate School of Engineering, The University of Tokyo, 7-3-1, Hongo, Bunkyo-Ku, Tokyo 113-8654, Japan oxidation.In HTGRs, fuel temperatures can potentially rise during transient processes and can exceed 1400°C even under normal operating conditions [6,12].These observations suggest that fuel temperature may undergo unexpected changes, making it challenging to control active oxidation solely based on temperature.Therefore, regulating the oxygen concentration in the helium coolant offers a much simpler and much easier approach compared to temperature control.
The concept of intentionally raising the O 2 content in the helium coolant appears reasonable; nevertheless, another concern needs to be addressed.The oxidation reaction between graphite core component and oxygen contained in helium coolant is possible (Figure 1).Increasing the oxygen content level in the coolant may cause oxidation between the graphite core components and the oxygen contained in the helium coolant, leading to mechanical degradation of the graphite structures.To ensure the material integrity of core components in the proposed coolant system, it is crucial to investigate the corrosion behavior of IG-110, a nuclear grade graphite used as a core structural material, under conditions of HTGR normal operation.The fundamental understanding of the targeted topic remains largely unclear.Previous studies have primarily focused on air oxidation due to safety concerns surrounding the air ingress accident, which is the most critical event caused by the guillotine-type rupture of the main coolant pipe [13].This event leads to significant degradation of the elements and structures of graphite fuel, making it the most concerning issue [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30] (Table 1).However, no previous study examining the corrosive kinetics of IG-110 with low O 2 has been found.
To address these research gaps, this paper presents a study on the oxidation behavior of IG-110, a nuclear grade graphite used as the core structural material, at low concentrations of O 2 in the helium (argon is used instead for practical reasons) coolant.Nuclear grade graphite is specifically produced for use in nuclear reactors.This type of graphite has higher purity compared to natural graphite to prevent absorption of thermal neutrons [31] and the creation of undesirable radioactive substances.Nuclear grade graphite must also possess acceptable thermal and mechanical properties both before and after exposure to fast neutron irradiation for incore applications.Some historical grades are no longer manufactured, while new grades such as NBG-17, NBG-25, PCEA, PPEA, IG-110, and IG-430 are currently being developed.Among these grades, IG-110 is most commonly used in HTGRs.The objective of this study is to establish a fundamental understanding of the oxidation behavior of IG-110 under low O 2 concentrations, which is currently not well understood.The goal is to ensure the material integrity of core components within the proposed coolant system.

Experimental
In the present study, oxidation kinetics of IG-110 (a fine quality-nuclear grade graphite manufactured by Toyo Tanso Co., bulk type, approx.50 mg), used in High-Temperature engineering Test Reactors [32] (HTTR), was investigated using Thermogravimetry Differential Thermal Analysis (TG-DTA), Bruker, model TG-DTA2020SA.The thermomechanical properties of IG-110 used in the present experiments are provided in Table 2.The real-time monitoring capability of TG-DTA allowed us to measure the mass change with increasing temperature accurately.The data sampling time was 1 sec for each measurement.The experimental setup involved placing the test specimens in an alumina dish in an electric-resistance furnace, while a standard sample made of alumina powder was placed on a high-accuracy electronic balance for precise mass change measurements as 0.1 μg.The appearance of the IG-110 test specimen is provided in Figure 2.
To prevent undesired oxidation prior to oxygen gas injection, a careful vacuuming process was carried out before each measurement.Sequently, argon gas was introduced as substitute inert gas of helium coolant at a flow rate of 50 mL/min [34].The temperature was then elevated from room temperature (27°C) to the targeted temperatures at a constant rate of 20°C/min under argon gas atmospheric pressure.Following the injection of an Ar/O 2 mixed gas the oxidation process was observed for 10 hours at a constant temperature.TG-DTA measurements were conducted within the temperature range of 400 ~ 1000°C, using Ar/O 2 mixed gases with O 2 concentrations ranging from 1 to 100 ppm.The gas evolved from the test specimens during heating was carried away with Ar (or Ar/O 2 ) flow.

Results and discussion
Figure 3 shows the mass changes of IG-110 test specimens in response to oxidation in a 10 ppm Ar/O 2 flow for 10 h at temperatures ranging from 400°C to 1000°C.Mass changes were measured in weight percent.The results reveal that there are almost no mass changes below 600°C.At 400°C, the mass slightly increases by 0.015 mg in the first 2 h and then decreases by 0.03 mg.The reaction rate for mass decrease is estimated to be 2.11 × 10 −12 kg/s.At 600°C, the mass increases by 0.15 mg in the first 7 h and then decreases by 0.01 mg; the reaction rate is estimated to      regime transition temperatures reported by different researchers varied widely, in most cases this kinetic model can be applied to each graphite material.
During this graphite oxidation, the mass linearly decreases with Arrhenius-type rection rate: Here, w, k, and t mean mass [kg], reaction rate [kg/s], and oxidation time [s], respectively.The present results indicate that slight mass gain occurs at 400-600°C for certain hours, which is a deviation from the typical mass loss observed during graphite oxidation.This phenomenon can be explained by the following two possible mechanisms: The first model can be explained by a two-step oxidation mechanism of graphite strictures [39].Under relatively low-temperatures, graphite oxidation occurs via a two-step process; vacancies are initially saturated by stable O groups, such as ether (C-O-C) and carbonyl (C=O).The etching is activated by a second step of additional O 2 adsorption at either group, forming larger O groups, such as lactone (C-O-C=O) and anhydride (O=C-O-C=O), that may desorb as CO 2 .It has been reported that the partial pressure of O 2 is an important factor that affects the mechanisms of oxidation [40].The possible candidates for intermediate structures of oxidized graphite are shown in Figure 5.At above 600°C, the formation of these stable structures (graphene oxide) thus contributes to the slight mass gain phenomenon observed in the present study.The second possible mechanism can be described by physical model; under elevated temperatures in a 10 ppm low O 2 concentration flow, internal pores begin to open; however, insufficient O 2 diffuses quickly into the internal structure of graphite, leading to slow outward release of generated CO 2 until it is released from the surface.During this process, CO 2 gases temporarily stay inside between graphite multilayer etch pits [41] by van der Waals force, resulting in a slight mass gain at the initial stage of oxidation.The test condition was in a 10 ppm Ar/O 2 flow, which did not provide enough O 2 to the graphite.Therefore, the observed unstable behavior occurs prior to corrosion with a mass loss reaction.At 600°C, the formation of pass to the internal pores is accelerated, resulting in a larger mass gain than that observed at 400°C.The estimated reaction rates at these temperatures correspond to the apparent mass loss behavior based on E q 2, including the decomposition of the intermediate product of the graphite-CO 2 gas.
At 700°C, there is no mass change until 3 h, after which mass loss occurs with a calculated reaction rate of 2.71 × 10 −11 kg/s.Above 800°C, there is a significant mass loss corresponding to the typical linear decrease (E q. 2), with estimated reaction rates of 7.26 × 10 −11 kg/s at 800°C and 1.21 × 10 −10 kg/s at 1000°C, respectively.
Figure 6 shows the relationship between the calculated reaction rates and the corresponding temperatures on a logarithmic scale versus reciprocal temperatures (Arrhenius plots).The reaction rate increases at higher temperatures, exhibiting the typical Arrhenius behavior observed between 600°C and 800°C.The activation energy for this chemical kinetic controlled regime is estimated to be 210 kJ/mol, which is in good agreement with previously reported data (Table 1).
The regime transition mentioned in the passage refers to the change in the controlling mechanism of the oxidation reaction.At temperatures up to 800°C, the oxidation reaction is controlled by chemical kinetics, while at temperatures over 1000°C, it is controlled by mass transfer.Therefore, the regime transition from Regime 1 (chemical kinetic control) to Regime 3 (mass transfer control) is likely to occur at 800°C, as the reaction rate starts to increase rapidly beyond this temperature.
The measured reaction rates in this study are lower than previously reported data [18][19][20][21][22][23][24][25][26][27][28][29], which can be attributed to the insufficient supply of O 2 to the graphite specimen in the 10 ppm Ar/O 2 flow.In low O 2 concentration environments, the chances of reaction with O 2 become lower, resulting in lower rate constants (A) and lower power to the order of the partial pressure of oxygen (P O2 n ) in the Arrhenius-type reaction rates [29].Therefore, the absolute values of the reaction rates measured in this study are lower than those reported in previous studies.
Here, A, P O2 , n, M (B) , E a , R, and T are rate constant, partial pressure of oxygen, order of rate, multiplication factor of graphite burn-off, activation energy, gas constant, and absolute temperature, respectively.The study found that at temperatures between 400°C and 600°C, the oxidation behavior of the graphite specimen is not dominated by chemical kinetics, but rather by mass transfer limitations due to insufficient O 2 diffusion into the internal pores of the specimen.This results in slower oxidation rates and a shift in the dominant regime to higher temperatures.This finding is important because it suggests that the graphite material used in HTGRs may be less susceptible to severe corrosion at lower temperatures than previously thought, thus increasing its potential for use in HTGRs.
To investigate the effect of O 2 concentration on oxidation kinetics, additional tests were performed at 600°C using varying O 2 contents.In this study, the effect of oxygen concentration on the oxidation kinetics of IG-110 graphite was investigated through experiments conducted at different temperatures and partial oxygen pressures.Furthermore, the thermal and oxidation behavior of SiC/graphite components in HTGR normal operation previously studied [42] will be introduced to make a comparison with the present study.To develop a numerical model using STAR-CCM+ commercial software, a CFD simulation of thermal balance analysis, of inner graphite, SiC tube, and outer graphite during Argon gas injection were used.The article reported that the experimental and numerical results show a good comparison, indicating that the temperature of the outer graphite does not exceed 500°C in both cases, while the temperature of the SiC tube (representing a compact SiC fuel compact) is forced to keep at 1100°C.Additionally, the mass change of IG-110 graphite during the heating experiment was only −0.00912% for a 60 min duration.These findings suggest that the graphite core components surrounding the SiC fuel compacts never suffer severe corrosion with 10 ppm O 2 , corresponding to the present study.The results have shown that the proposed helium coolant system containing 10 ppm O 2 is a promising technology to prevent severe corrosion of the SiC fuel compacts while ensuring the material integrity of the graphite core components.This option would be largely helpful for the design of advanced-HTGR to increase the core power density.The findings suggest that the low O 2 concentration used in the study is suitable for long-term service without significant degradation of the components.

Conclusion
This research investigates the corrosion behavior of IG-110, a nuclear grade graphite used in the HTGR core structural material, in the presence of more than 10 ppm of O 2 in the helium coolant system.This high O 2 concentration is required for the sleeveless fuel compact with the SiC-matrix design under the accidental conditions postulated by HTGR.The study reveals that IG-110 does not experience severe corrosion with 10 ppm O 2 , as the oxidation kinetics are not dominant at temperatures below 600°C due to limited O 2 diffusion into the graphite specimen's pore.The material integrity of the graphite core components surrounding the SiC fuel compacts is maintained during the postulated long-time service of HTGR normal operation in the proposed helium coolant system containing 10 ppm O 2 .This finding suggests that this option holds promise as a technology to increase the core power density of advanced-HTGRs.These findings contribute to R&D of an innovative nuclear fuel for future HTGRs, demonstrating higher power density operation under normal service and preferable fuel safety during accidents.

Figure 1 .
Figure 1.Schematic illustration of oxidation at both SiC surface (fuel compact) and graphite surface (core block) which is estimated to occur in a HTGR during the normal operation service due to He/O 2 coolant flow.

Figure 2 .
Figure 2. The appearance of the IG-110 test specimen (nuclear-grade-graphite for HTTR) manufactured by Toyo Tanso Co.

Figure 5 .
Figure 5. Classification of the graphite oxidation mode based on mass changes: graphite burning with gas release at higher O 2 level (left) and possible structures of basic surface sites on a graphite layer (graphene oxide) at lower O 2 level (right).
Figure 7 shows that 1 Pa O 2 partial pressure roughly corresponds to 10 ppm O 2 concentration at atmospheric pressure.Temporary mass gain phenomena were observed up to 10 Pa O 2 at 600°C, with the 10 Pa O 2 case showing a slight decrease in mass earlier than the other cases.These results provide valuable insight into the role of O 2 concentration in the oxidation behavior of IG-110 graphite at high temperatures, which can be useful for optimizing the performance and longevity of graphite components in postulated HTGR operation.

Figure 6 .
Figure 6.The relationship between calculated reaction rates and corresponding temperatures in logarithm scale vs reciprocal temperatures (Arrhenius plots), showing the average values by twice measurements.

Figure 7 .
Figure 7. Mass changes of IG-110 test samples at 600°C and partial pressures of 0.1-10 O 2 for 10 h of oxidation.

Table 1 .
Summary of reported kinetics of IG-110 air oxidation obtained from several experiments.