Air Ingress Research Articles

Study on oxidation kinetics of three kinds of candidate superalloys for VHTR under air ingress accident

Inconel 617 alloy, Incoloy 800H alloy and Hastelloy X alloy are three alternative superalloys for heat transfer tubes used in the steam generator (SG) in the very high temperature gas cooled reactors (VHTR). In this paper, the oxidation kinetics of isothermal oxidation of three superalloys in the circulating air at 950 ℃ during the air ingress accident is investigated. Oxidation kinetic parameters of superalloys, such as the oxidation rate indexes (np and nt), mass parabola rate constant (kp) and oxide layer thickness parabola rate constant (kt) are calculated by the experimental data. In addition, tested samples of superalloys were analyzed by Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (XRD). The results show that these three superalloys exhibit parabolic oxidation behavior in air atmosphere. The oxidation behavior of Incoloy 800H alloy is closer to the cubic oxidation law as time increases. The mass gain of three alloys does not fully match with the growth law of the oxide layer, which should be attributed to the internal oxidation degree of the alloy. The arithmetic square root of kp and kt, L, are proposed to quantify the internal oxidation degree. And the L of Inconel 617 alloy is the largest compared to Inconel 800H alloy and Hastelloy X alloy. This work is helpful to explain the behavior of oxidation of superalloys.

Comparison of unirradiated and irradiated AGR-2 TRISO fuel particle oxidation response

The silicon carbide (SiC) coating in a tristructural isotropic (TRISO) particle acts as a barrier to fission product release during reactor operation and accident scenarios. Oxidation and subsequent failure of the SiC layer during a rare air ingress event is a proposed mechanism for fission product release in a high-temperature gas-cooled reactor (HTGR). Although previous oxidation studies have analyzed unirradiated TRISO particle response, this study compared the oxidation behavior of irradiated and unirradiated TRISO particles from the second Advanced Gas Reactor Fuel Development and Qualification Program irradiation experiment (AGR-2). Particles with exposed SiC were subjected to six varying oxidizing tests in the Furnace for Irradiated TRISO Testing (FITT), examined for failure fraction with the Irradiated Microsphere Gamma Analyzer (IMGA) and characterized with focused ion beam and scanning/transmission electron microscopy techniques to analyze the oxide layer. Uncorrelated unirradiated particle failures throughout the series of exposures suggests that external factors inherent to the experiment increased particle failure sensitivity. However, irradiated particle observations indicated an increased failure response at 400 h 1400 °C in both 2% and 21% O2 atmospheres above failure associated with external factors. Oxide thickness measurements after 400 h at 1400 °C revealed a greater oxidation rate than predicted by parabolic growth, which was attributed to the increased complexity of the oxide structure at longer exposure times. Altering the atmosphere from 21% to 2% O2 reduced the average oxide thickness by approximately 12%–14% in both irradiated and unirradiated particles at 400 h 1400 °C. Overall, the minor variations observed between irradiated and unirradiated particles in this study led to the conclusion that unirradiated TRISO particles can be used to approximate irradiated TRISO oxidation kinetics.

Insight into proactive inertisation strategies for spontaneous combustion management during longwall mining of coal seams with various orientations

ABSTRACT Spontaneous combustion of coal occurs in the longwall (LW) goaf during mining cycles due to coal oxidation at low temperatures and air ingress. Coal seam orientations, dictated by the elevations of maingate (MG) and tailgate (TG) and the height of working face and starting-up line, significantly impact the gas distribution in the goaf. Despite this, there has been limited study on the effects of coal seam orientations on spontaneous heating management. To fill this knowledge gap, extensive computational fluid dynamics (CFD) modeling was conducted based on the actual conditions of an Australian underground coal mine and verified with onsite gas monitoring data, after which extensive parametric studies of how coal seam orientations influenced gas distribution were conducted. Simulation results indicate that coal seam orientations significantly impact spatial gas distribution in the LW goaf and the development of proactive goaf inertisation strategies. Regardless of coal seam orientations, nitrogen performs better than carbon dioxide in reducing the oxidation zone area. In addition, at least 1.5 m3/s of nitrogen is required to effectively prevent spontaneous heating, and the area ratio of oxidation zone to active goaf is approximately 10%, reducing by about 15% compared to scenarios without inertisation. The modeling results shed insight into the goaf gas distribution characteristics under various coal seam orientations and provide guidance on developing corresponding proactive inertisation strategies for managing spontaneous heating in the goaf, thus improving mining safety.

Numerical Simulation of Airflow and Dust Migration in Vacuum Vessel During Loss of Vacuum Accident for Updated CFETR

A loss of vacuum accident (LOVA) occurs during in-vessel component failure and air ingress. The airflow characteristics of a LOVA are determined by many factors like initial pressure, location of a break, and size of a break and have a great impact on dust migration, which could cause a serious explosion with incoming air and H2. In this paper, a computational fluid dynamics method is adopted, and the k-ε Shear Stress Transport model for airflow and the Discrete Phase Model for dust are used to simulate a LOVA with the updated Chinese Fusion Engineering Test Reactor (CFETR) tokamak device. The effects of initial pressure, break size, and break location on airflow during the LOVA are discussed, and the effects of dust size, break size, and break location on dust migration during the LOVA are investigated as well. The results indicate that the initial pressure and size of a break have a greater impact on airflow of a LOVA than the location of the break and that both the dust size and the characteristics of the airflow have a greater impact on the distribution of the dust. A break located in the upper port has even more dust chaos. This research is the basis for the safety analysis of the CFETR device, and it provides a reference for subsequent studies on dust removal, mitigation of dust explosions, and radioactive substances.

Study on corrosion of T-22 alloy under high temperature condition of VHTR

ABSTRACT T-22 alloy is applied to the steam generator of the high-Temperature Reactor–Pebblebed Modules (HTR-PM). In this study, a corrosion experiment of T-22 alloy was carried out for 50 h under the environment of air, vacuum, and impure helium at ultra-high temperature (950°C). After the experiment, the corrosion behaviors of T-22 alloy were characterized and analyzed by electronic balance, scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), carbon sulfur analyzer, and gas chromatography. The results show that the T-22 alloy was seriously corroded and destroyed under the air ingress accident; in the vacuum environment, the alloy appeared mass loss and slight decarburization; in the impure helium environment, the microclimate reaction would occur on the surface of T-22 alloy, and the oxide on the surface of the alloy would be destroyed and the alloy decarburized obviously; CO2 in the ppm range will accelerate the oxidation and decarburization of the alloy.

Computer vision for non-contact monitoring of catalyst degradation and product formation kinetics.

We report a computer vision strategy for the extraction and colorimetric analysis of catalyst degradation and product-formation kinetics from video footage. The degradation of palladium(ii) pre-catalyst systems to form 'Pd black' is investigated as a widely relevant case study for catalysis and materials chemistries. Beyond the study of catalysts in isolation, investigation of Pd-catalyzed Miyaura borylation reactions revealed informative correlations between colour parameters (most notably ΔE, a colour-agnostic measure of contrast change) and the concentration of product measured by off-line analysis (NMR and LC-MS). The breakdown of such correlations helped inform conditions under which reaction vessels were compromised by air ingress. These findings present opportunities to expand the toolbox of non-invasive analytical techniques, operationally cheaper and simpler to implement than common spectroscopic methods. The approach introduces the capability of analyzing the macroscopic 'bulk' for the study of reaction kinetics in complex mixtures, in complement to the more common study of microscopic and molecular specifics.

Investigation of the oxidation behavior of Zircaloy-4 cladding in a mixture of air and steam

Previous studies had shown that in certain conditions, the rate of oxidation of zirconium (Zr) based alloy fuel cladding is higher in air–steam mixtures than in dry air. In severe accidents in the spent fuel pool and in other air ingress accidents in nuclear power plants, the cladding is likely to be oxidized in an air–steam mixture, which makes it crucial to have an in-depth understanding of the nature of oxidation and its kinetics in that environment. Oxidation tests were conducted at 800 °C on Zircaloy-4 specimens in a mix of (air + steam) with various component ratios. Oxidation kinetics, details of the oxide layer, and hydrogen pick-up in the specimen were studied to investigate the mechanism of oxidation in each of these sets of conditions. Zirconium nitride precipitation in the oxide layer during the initial stages of the pre-breakaway oxidation stage and the widespread porous oxide growth on the cladding surface in the latter post-BA oxidation stage are related to the oxidation mechanism in the air–steam mixture. The differences in the mechanism of oxidation of the cladding in dry air and air–steam mixtures are discussed based on the experimental results.

An Experimental and Theoretical Examination of Air Ingress Rates During Small- and Medium-Break Air Ingress Accidents

Air ingress measurements using the 1/20th-scaled Helium Air Ingress gas Reactor Experiment (HAIRE) facility show key geometric variables of interest and their effect on air ingress in small- and medium-sized breaks in High Temperature Gas cooled Reactors. These variables include but are not limited to break diameter, break angle, and break wall thickness. Differing wall thicknesses for the same break diameter can have order-of-magnitude changes to the air ingress rate, which is a key figure of merit in the air ingress accident scenario. Additionally, different break sizes can change the importance of the angle in the break scenario. With smaller breaks, the flow will not transition from intermittent flow, to countercurrent flow, to diffusive flow as the break rotates from vertically upward toward vertically downward. This would lead to less variability with smaller breaks, which in turn would make the accident scenario more predictable for smaller-sized breaks.

Study on corrosion behavior of two superalloys under two extreme atmospheres

Inconel 617 and Hastelloy X alloys are two candidate materials for very high temperature gas-cooled reactor (VHTR) intermediate heat exchanger and steam generator equipment. This work explored the corrosion behavior of two alloys in the atmosphere of extreme air ingress accidents and vacuum environment. After the experiment, the corrosion behaviors of the two alloys were characterized and analyzed by electronic balance, scanning electron microscope(SEM), energy dispersive spectroscopy(EDS) and X-ray diffraction(XRD) and carbon sulfur analyzer. The results showed that the mass change and oxide film thickness of Hastelloy X alloy and Inconel 617 alloy after corrosion in air met the oxidation parabola law, and the kp of Hastelloy is lower. The chromium migration of the two alloys was obvious after corrosion in air. The longer the corrosion time was, the more obvious the phenomenon was. The carbon content of the alloy did not change significantly with the corrosion time. Mass loss and decarburization occurred in both alloys after corrosion in vacuum, and volatile elements volatilized and migrated. The weight loss, decarburization and element volatilization of Hastelloy X were more obvious.

Field performance and numerical simulation study on the toe to heel air injection (THAI) process in a heavy oil reservoir with bottom water

Extra-heavy oil and bitumen (EHOB) comprise 30 percent of the remaining recoverable fossil fuel resources on Earth. This means EHOB could play an important role in a secure transition towards net zero emissions (NZE) by 2050. Technological developments, such as toe to heel air injection (THAI), have been shown to efficiently recover heavy oil with reduced environmental footprint. The Kerrobert project was the first to utilise the THAI technology in presence of bottom water (BW) in the reservoir. The project demonstrated a good performance (with average oil rate of 10 m3/day per well) compared to the conventional ISC operations in a BW situation. Lessons taken from the Kerrobert operational experience can assist the forthcoming THAI operations explicitly in the presence of BW. Dynamic field data for one of the best performing THAI pilot well pairs (K2), were analysed in this work. It was found that the K2 pilot must have experienced interference from K5, which is the closest neighbouring THAI well pair to the K2. Previously developed THAI models have not been validated against actual field data. A new field-scale THAI model in the presence of BW was constructed and, for the first time, validated against the field data from the Kerrobert project in this work. In addition, the quasi-staggered line drive well arrangement, as used for the K2 pilot, was studied. The daily and cumulative oil production rates were predicted well (the final agreement with field data was within 3 percent). The history matched model was then used to investigate the effect of the variation in air injection rates on THAI performance in presence of BW. Major developed zones during the propagation of the combustion front were numerically examined. It was demonstrated that extra air ingress from the neighbouring THAI well pair has caused a reduction in oxygen utilisation throughout the process. As a result, the simulated temperature profile declined with the increasing combustion time. The oxygen profile around the horizontal producer (HP) well was studied via the new history-matched model. An inversely proportional relationship was detected between the coke concentration and the oxygen profile around the HP well. It was found that the size of the steam zone, ahead of the combustion front, differs with variation in air injection rates. It was observed that some of the mobilised oil sank into the BW, leaving a significant amount of oil trapped in the reservoir. To prevent such an event, the location of the HP well was altered as a potential strategy to optimise the THAI efficiency. Consequently, the oxygen utilisation was improved by 13%, resulting in 73% higher cumulative oil production in comparison with the history-matched model.

Analysis of air ingress scenario in GEMINI+ plant

The work presented in this paper was performed within the Euratom Horizon 2020 GEMINI+ project. The GEMINI+ reactor is a prismatic block-type High-Temperature Gas-Cooled Reactor (HTGR). Within tasks devoted to safety, an analysis of air ingress scenario was performed by NRG with the thermal–hydraulic system code SPECTRA, while UJV and NCBJ worked with the integral system code MELCOR. Two air ingress scenarios were analyzed:Design Basis Accident (DBA) scenario. The air ingress scenario selected as DBA is a 65 mm break of a Helium purification line on top of the Steam Generator,Large air ingress scenario. The air ingress scenario selected is guillotine break of the coaxial gas duct.The DBA scenario is characterized by a very long period with practically stagnant gas in the primary system after the initial depressurization. However, very small gas flows through the break due to counter flow and diffusion in the break region (gas velocities of 10–2 m/s) are difficult to model with system codes. Therefore the break models in the system code were calibrated by performing CFD simulations. Furthermore, a model of gas mixing by diffusion was introduced in the NRG analysis for the practically stationary gas in the primary system (gas velocities of 10–4 m/s). The large air ingress scenario, with relatively large flow through the break, is easier to model from this standpoint.The NRG SPECTRA results showed that the amount of air that can reach the core is extremely small in the DBA accident (0.5 kg of graphite consumed after 100 h). The maximum depth of oxidation was 2 µm. In the large air ingress scenario the air ingress is significantly larger. At 100 h, approximately 105 kg of graphite was consumed. Even then, the maximum depth of oxidation was very small, approximately 0.2 mm. MELCOR results, performed at NCBJ and UJV, basically confirmed that the amount of air ingress is very small. The numbers were somewhat higher, mainly due to heavy flow oscillations in the break that could not be mitigated in MELCOR calculations.

High-temperature oxidation of sprayed alumina and zirconium‑yttrium-based coatings on Zircaloy-4 in air and oxygen: Investigation on microstructure evolution and phase changes

Zr-Y-C coating was prepared from APS, and its enhancement on the oxidation resistance of Zircaloy-4 in air and oxygen was compared with Al2O3 coating. Detailed characterization techniques including SEM, EDS, AFM, Raman spectroscopy and XRD were employed to analyze microstructure evolutions and phase components. Zr-Y-C coating is less susceptible to oxygen and Al2O3 coating is more resistant to air attack. Oxides and nitrides were both formed on these two coatings. Air ingress made Zr-Y-C coating exhibit a significant weight gain, while it maintained relatively stable in oxygen. As for Al2O3 coating, cracks developed and the porosity increased in air. When exposed to oxygen, metal ion diffusion accelerated, and numerous irregular Zr and Al oxide particles were produced.

Status of benchmark exercises in the frame of the NUGENIA QUESA project

Severe accidents in nuclear power plants with loss of cooling or loss of coolant and core uncovery can lead to air ingress into fuel assemblies. If air or a steam–air-mixture comes into contact with cladding material, the exothermal character of zirconium oxidation and nitride formation leads to enhanced cladding oxidation, heat-up and degradation up to core melt and fission product release. Two experiments, QUENCH-18 and CODEX-AIT-3 were performed in the frame of EC projects ALISA and SAFEST to fill knowledge gaps on relevant phenomena. Both tests are accompanied by pre- and post-test benchmarks within the NUGENIA project QUESA. The pre-test benchmarks were completed, while the post-test benchmarks are currently running. Results of the pre-test benchmarks indicate that participating codes can qualitatively predict the experimental observations. Generally, the range of the predictions are wider for CODEX-AIT-3 while the trend is close except one for QUENCH-18. The specification of both test procedures was additionally supported by parameter studies. First post-test benchmark results for QUENCH-18 show that code predictions are in good agreement during the pre-oxidation and mixed steam/air phase. In the subsequent reflooding phase with intensive melt release, relocation and oxidation, notable differences between codes and simultaneously deviations from experimental results are observed. In the paper the main findings of the QUENCH-18 test are discussed, but the focus will be on the pre-test analyses of CODEX-AIT-3.

Air Ingression in Ladle Shrouds, Tundish Open Eye Formation, Their Inter-dependance and Correlations: A Physical Model Investigation

Air Ingression in Ladle Shrouds, Tundish Open Eye Formation, Their Inter-dependance and Correlations: A Physical Model Investigation

Oxidation of typical Zr-1 Nb alloy in high-temperature air under 700–900 °C

In order to promote the understanding of Zirconium alloy cladding failure under loss of coolant accidents (LOCA), air oxidation behavior of Zr-1 Nb-0.12 O (M5) alloy at 700–900 °C for 1–2 h was investigated. At different temperatures and times, m-ZrO2 was always the dominant phase of surface oxides, whereas oxide shapes, the integrity and roughness of oxide films differed noticeably. At 900 °C, M5 alloy exhibited accelerated kinetics and breakaway oxidation after approximately 80 min air exposure. The presence of ZrO and ZrO0.35 sub-oxides accounts for the high reaction activation energy (149.6 kJ/mol). During the kinetics acceleration, some intergranular cracks turned transgranular. The oxide/metal (O/M) interface became wavy, massive long transverse cracks and small radial cracks appeared in the oxide film, and pores distributed on the oxide layer and O/M interface. Air ingress through the oxide layer to initiate the matrix oxidation is jointly affected by air transport and ion diffusion.

Infiltration of molten fluoride salts in graphite: Phenomenology and engineering considerations for reactor operations and waste disposal

The possibility and consequences of salt-infiltration in graphite must be evaluated for graphite used in molten salt reactors (MSRs) and fluoride-salt-cooled high-temperature reactors (FHRs), which can be subjected to salt pressures as high as 500 kPa. The volume of graphite porosity infiltrated by salt can be measured by direct infiltration and it can be predicted from the graphite pore size distribution, the surface tension of the salt, and the contact angle between the graphite and the salt. While these three properties are believed to be insensitive to irradiation, the former can be impacted by chronic or acute oxidation, and the latter two are highly sensitive to the chemistry of the salt and to events such as air ingress. For MSRs, predictions based on nominal properties of salt and graphite reveal that few graphite grades would satisfy the 4 vol% limit set in the Molten Salt Reactor Experiment, and even fewer would satisfy the 0.5 vol% design target. For FHRs, infiltration limits have not been defined and depend on the effect of infiltration on graphite properties, which are discussed. A hypothesis is presented for properties that may be impacted by infiltration and for which future studies are needed.

Study of oxidation behavior and tensile properties of candidate superalloys in the air ingress simulation scenario

Air ingress incidents are major safety accidents in very-high-temperature reactors (VHTRs). Air containing a high volume fraction of oxygen may cause severe oxidation of core components at the VHTR, especially for the significantly thin alloy tube wall in the intermediate heat exchanger (IHE). The research objects of this study are Inconel 617 and Incoloy 800H, two candidate alloys for IHE in VHTR. The air ingress accident scenario is simulated with high-temperature air flow at 950 °C. A continuous oxide scale was formed on the surfaces of both the alloys after the experiment. Because the oxide scale of Inconel 617 has a loose structure, whereas that of Incoloy 800H is denser, Inconel 617 exhibited significantly more severe internal oxidation than Incoloy 800H. Further, Inconel 617 showed a significant decrease in ultimate tensile strength and plasticity after aging for 200 h, whereas Incoloy 800H maintained its tensile properties satisfactorily. Through control experiment under vacuum, we preliminarily concluded that serious internal oxidation is the primary reason for the decline in the tensile properties of Inconel 617.

Evaluation of Subcritical Organic Rankine Cycle by Pure and Zeotropic of Binary and Ternary Refrigerants

The simulation configuration and process analysis of the Subcritical Organic Rankine Cycle (SORC) system are carried out for the potential comparison between pure, binary, and ternary zeotropic mixtures of R1234ze(E), R1234yf, and R134a as refrigerant working fluids based on applying the flue gas as a heat source with medium temperature. The compression pressure was selected as an optimized variable input parameter of SORC with the lower limit of boundary condition (1.4 MPa); to mitigate air ingress and sub-atmospheric pressure that led to approach optimum net power output generated. Increasing the compression pressure has a positive relationship with the superheated temperature and the mass enthalpy change in the evaporation and, therefore, in the expansion process. In parallel, the enthalpy and entropy changes in the flue gas and cold water positively correlate with exergy efficiency. So, R1234ze(E)/ R1234yf/R134a with 68.35% and R1234yf/ R134a with 69.29% as the lowest and highest exergy efficiency in the highest compression pressure; furthermore, the SIC consequences of increasing the cost of each component of the SORC system that has a direct relationship with the PPC and the required exchanger area of evaporation and condensation process and generating a net power output of the turbine. As a result, the maximum to the minimum value of specific investment cost (SIC) achieves R134a with 5807402.18-22455670.61 $.kW-1 and R1234yf with 16.82-17.38% reduction, respectively. To sum up, the lowest payback period (PBP) was R1234yf with 302 days.

Effect of steam and oxygen starvation on severe accident progression with air ingress

The CODEX-AIT-3 experiment simulated an in-vessel air ingress scenario after failure of bottom head of a nuclear reactor. 1600 °C maximum temperature was reached with the electrically heated bundle and slow cool-down was applied at the end of the test to demonstrate the bundle state before quench. Limited air and steam flow rates were intentionally selected in order to slow down the high temperature chemical interactions and provided unique data on nuclear fuel behaviour under oxygen and steam starvation conditions. The individual oxidation and nitriding phenomena were well separated from each other and it was indicated by the complex microstructures in the zirconium cladding with nitrides, oxides and transition zones. The competition between different chemical reactions could be excluded in some locations and some time periods. The post-test numerical analyses of the experiment could be used for the evaluation of the role of numerical models developed for individual chemical reactions in severe accident codes.

A New Inert Gas Delivery Design for Improved Shielding of Ladle Shroud–Collector Nozzle (LS–CN) Assembly: Modeling, Design, and Industrial‐Scale Validations

Argon injection in ladle shrouds is customarily practiced during the teeming of liquid steel from ladle to tundish to minimize air ingression and thereby restrict atmospheric oxidation of steel melt. Despite such practices, nitrogen pickup results in the tundish, implying inadequate shielding of ladle shroud–collector nozzle (LS–CN) joint. Accordingly, the efficacy of different argon injection designs in shrouds is evaluated in the present study, and thereby, a novel argon injection arrangement, embodying a twin, tangential gas discharge system, developed as a curative step to ensure maximum inertization of the LS–CN assembly. Numerical prediction of argon flow and distribution around the LS–CN assembly is accompanied by a homogeneous, non‐isothermal, two‐component turbulent flow model embodied in ANSYS Fluent. These, supported adequately by water modeling results, indicated that inertization of LS–CN assembly is considerably improved with the proposed design at argon flowrates typically practiced in industries (10 Nlit min−1). To substantiate laboratory findings, a plant‐scale investigation is carried out, wherein shrouds with a new gas delivery design are fabricated and deployed in continuous bloom casting operation in a special steel plant. Industrial trials confirm the superiority of the new design over existing ones and demonstrate a 50% reduction in nitrogen pickup, during teeming, under identical operating conditions.