High Temperature Gas-Cooled Reactor Applications Research Articles

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.

Failure analysis of nuclear transient-tested UN tristructural isotropic fuel particles in a 3D printed SiC matrix

Fully ceramic microencapsulated fuel elements containing UN tristructural isotropic (TRISO) fuel particles within a 3D printed SiC matrix were subjected to transient testing with varying energy depositions. Detailed post-irradiation examinations were performed, including leaching in hot HNO3 and post-leaching X-ray computed tomography, to quantify the percentage of failed TRISO particles and crack propagation within the particles and surrounding fuel matrix. In parallel, detailed finite element analyses were performed for comparison with experimental findings and to better evaluate transient failure modes. The lowest transient energy deposition—which still exceeded bounding values for high-temperature gas-cooled reactor applications—resulted in no detectable TRISO particle failures or matrix cracking, which was consistent with the simulations. Simulations of the higher-energy transients for which significant TRISO particle failure was expected were generally able to reproduce the transient temperatures and matrix cracking. Thus, the TRISO particle failures were explained based on the effects of local SiC matrix thickness and porosity. Results generally confirmed the high strength of the additively manufactured SiC matrix but also affirmed the need for a modified UN TRISO architecture to prevent SiC matrix cracks from propagating through TRISO layers. This unique failure mode has not historically been considered for TRISO fuels contained in weaker graphite matrices.

High-temperature tribological behavior of structural materials after conditioning in impure-helium environments for high-temperature gas-cooled reactor applications

Incoloy 800HT and Inconel 617 have been selected as candidate structural alloys for the high-temperature gas-cooled reactor (HTGR) concept. Helium, the primary coolant, contains impurities (e.g., H2O and CH4) that can induce corrosion reactions at high temperatures, which in turn can affect the tribological behavior of components in sliding contact such as valves and control-rod drive systems. This paper presents results from the study of the tribological behavior of both alloys before and after conditioning them in either an impure-helium oxidizing environment or in an air environment. Both alloys were conditioned for 22 days at elevated temperatures in a once-through helium loop with 4 ppmv H2O and tested subsequently at elevated temperatures with a pin-on-disk tribometer in an air environment - 650 °C and 750 °C for 800HT; 850 °C and 900 °C for 617 – with various applied loads - 1 N, 2 N and 5 N. SEM-EDS analysis revealed that conditioning the samples in an oxidizing environment leads to the formation of a mixed Fe/Cr-oxide on alloy 800HT and a Cr-oxide on alloy 617, both increasing the wear resistance compared to that of as-received samples. Alloy 617 exhibited lower steady-state friction coefficients compared to those of alloy 800HT. There was a significant decrease in the scatter of the steady-state friction coefficient of the conditioned samples compared to that of the unconditioned samples. The steady-state friction coefficient for alloy 800HT and 617 were found to be 0.53 ± 0.07 and 0.25 ± 0.04, respectively. The wear resistance of alloy 800HT is approximately one order of magnitude lower than that of alloy 617 in all cases that exhibited measurable wear. After sample conditioning, the wear volumes measured at low loads were undistinguishable from the unworn background, a result attributed to the formation of a compacted glaze layer under high temperatures and high contact stresses.

The Tribological Behavior of Tube Supports in Helium Heat Exchangers

A closed-cycle gas turbine plant for high-temperature gas-cooled reactor application contains a number of heat exchangers for low to medium temperatures (up to 500/sup 0/C). The tubes of these heat exchangers are held in multiple spacers. Since thermal expansions lead to relative motions between tubes and spacers, a predictable tribological behavior of this sliding pair is important. The search for a low-cost treatment to achieve good tribological properties has led to manganese phosphate conversion layers on ferritic steel. Laboratory tests in helium with representative tube/spacer configurations have shown very small wear rates, complete absence of scuffing, and moderate and predictable coefficients of friction. Mechanical testing of phosphated tube material did not reveal any detrimental influence from the phosphate treatment. This work was performed under the joint German/Swiss development agreement for the helium high-temperature turbine project, HHT.

Investigations Into the Irradiation Behavior of High-Temperature Alloys for High-Temperature Gas-Cooled Reactor Applications

Control rods in a pebble-bed-type high-temperature reactor operate at temperatures below 650°C, but in upset conditions short-term excursions up to 850°C can occur. Here, austenitic steels or nicke...

Tensile and Impact Properties of Candidate Alloys for High-Temperature Gas-Cooled Reactor Applications

The tensile properties of solution-treated Incoloy alloy 800H, Hastelloy-X, Nimonic-86, and Inconel-617 have been determined in the temperature range 20 to 1000/sup 0/C. The strength parameters at temperatures above 700/sup 0/C showed a strong dependence on the strain rate; at low strain rates the deformation was dominated by creep effects, the strain rate and maximum stress being related by the Norton creep equation. The tensile and impact properties of the alloys were also determined after exposure at 700 to 1000/sup 0/C for up to 30 000 h. For Incoloy-800H, the results showed good retention of ductility and impact strength. The nickel-base alloys, in contrast, were found to have low room-temperature impact resistance after long time exposure at 700 to 900/sup 0/C, typical values being 10 to 20 J x cm/sup -2/. In impact tests at the exposure temperature, impact strengths were generally above 50 J x cm/sup -2/. At room temperature, allowances in design must be made for the low impact strength of the nickel-base alloys to ensure against brittle fracture. For example, excessive stresses during cooling of components following shutdown should be avoided.

Hastelloy-X for High Temperature Gas-Cooled Reactor Applications

Hastelloy-X is a potential structural material for use in gas-cooled reactor systems. In this application, data are necessary on the mechanical properties of base metal and weldments under realistic service conditions. The test environment studied was helium that contained small amounts of H/sub 2/, CH/sub 4/, and CO. This environment was found to be carburizing, with the kinetics of this process becoming rapid above 800/sup 0/C. Suitable weldments of Hastelloy-X were prepared by several processes; those weldments generally had the same properties as base metal except for lower fracture strains under some conditions. Some samples were aged for up to 20 000 h in the test gas and tested, and some creep tests on as-received material exceeded 40 000 h. The predominant effects of aging were the significant reduction in the fracture strains at ambient temperature and the lower strains for samples aged in high-temperature gas-cooled reactor (HTGR) helium than for those aged in inert gas. Under some conditions, aging also resulted in increased yield and ultimate tensile strength. Creep tests failed to show the effects of environment, aging, or welding on the creep strength of Hastelloy-X; however, the fracture strains for weldments were generally lower than they were formore » base metal. Prior aging in inert gas for 20 000 h at 538 and 871/sup 0/C reduced the fatigue life slightly, but no difference was observed in the fatigue properties of samples aged in air and HTGR helium environments.« less

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Design of Wrought Nickel-Base Alloys for Advanced High-Temperature Gas-Cooled Reactor Applications

The utilization of the high-temperature gas-cooled reactor (HTGR) for advanced or process heat applications will require the use of wrought components operating at temperatures up to 1000°C (1832°F...

Characterization of thermally sprayed coatings for high temperature wear protection applications

Under normal high temperature gas-cooled reactor (HTGR) operating conditions, faying surfaces of metallic components under high contact pressure are prone to friction, wear and self-welding damage. Component design calls for coatings for the protection of the mating surfaces. Anticipated operating temperatures of up to 850–950 °C (1562–1742 °F) and a design life of 40 years require coatings with excellent thermal stability and adequate wear and spallation resistance. In addition, they must be compatible with the HTGR coolant helium environment. Plasma- and detonation-gun-deposited chromium-carbide-base and stabilized zirconia coatings are under consideration for wear protection of reactor components such as the thermal barrier, heat exchangers, control rods and turbomachinery. Programs are under way to investigate the structural integrity, helium compatibility and tribological behavior of relevant sprayed coatings. In this paper the need for the protection of critical metallic components and the criteria for the selection of coatings are discussed. The technical background to coating development and the experience with the steam cycle HTGR are commented on. Coating characterization techniques employed at the General Atomic Company are presented, and the progress of the experimental programs is briefly reviewed. In characterizing the coatings for HTGR applications, it is concluded that a systems approach to establish correlation between coating process parameters and coating microstructural and tribological properties for design consideration is required.