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Development of GH3535 Alloy for Thorium Molten Salt Reactor
Abstract
The GH3535 seamless pipe was developed by hot extrusion and cold rolling. The mechanical properties and corrosion resistance of GH3535 seamless pipe under molten salt environment were tested at different temperatures. The results showed that the extrudability of GH3535 alloy was fine at 1150–1250 ℃, and the mechanical properties of hot-extruded GH3535 alloy pipe were excellent and the microstructure was uniform. The average grain size of GH3535 seamless pipe is about 67 μm. The yield strength of GH3535 alloy pipe is above 200 MPa at 650–700 ℃, the tensile strength is above 480 MPa. The corrosion resistance of GH3535 alloy seamless pipe is also good in high temperature (700 ℃) molten salt environment.
... To elucidate the impact of these plasticityimparting operating conditions on the alloy's MSC performance, in the present study we explore the interplay between HTC and MSC. In order to decouple the effect of HTC and MSC, we first subjected the studied NiMoCr (GH3535) alloy [15] to HTC (700 °C/135 MPa) and then MSC in FLiNaK molten salt (750 °C/500 h). Based on the obtained results, we provide direct evidence demonstrating the severe effect of HTC on MSC via a detailed microstructural examination, which connects the dislocation substructure developed during HTC to the accelerated MSC attack. ...
... The studied NiMoCr (GH3535) alloy [15], of the nominal chemical composition shown in Table 1, is an equivalent alloy to the ORNL-developed NiMoCr Hastelloy-N® [16,17], designed for use as a moltensalt-facing material in MSR systems utilizing fluoride molten salts. GH3535 alloy was co-developed by the Institute of Metal Research (IMR) and the Shanghai Institute of Applied Physics (SINAP), Chinese Academy of Sciences, and is being produced by Fushun Special Steel Shares Co., Ltd. ...
Molten-salt-based energy generation and energy-storage systems are expected to function under plasticity-imparting in-service operating conditions. It is thus of utmost importance to understand the effect of these operating conditions on the corrosion performance of molten-salt-facing structural alloys. Here, we investigate the impact of high-temperature creep on the molten salt corrosion of a NiMoCr alloy in FLiNaK molten salt. We show that high-temperature creep leads to an increased susceptibility of the alloy to molten salt corrosion via the development of a dislocation substructure, which promotes mass diffusion of alloying elements towards the salt–alloy interface (exposed surface). In addition, the corrosion-affected near-surface layer undergoes recrystallizations leading to a significant increase in the amount of grain boundaries, which further promote ongoing diffusion processes. This then causes increased molten salt corrosion attack resulting in an accelerated mass loss during the alloy’s exposure to the molten salt. The present results thus unambiguously highlight the importance of the state of the microstructure as well as the evolution of the microstructure in the in-service operating conditions on the alloy’s molten salt corrosion performance.
One Sentence Summary
The dislocation substructure developed during high-temperature creep increases the susceptibility of structural alloys to molten salt corrosion
... HASTELLOY-N®, GH3535® (batch 2014), and MONICR® have been supplied in order to perform static corrosion testing in FLiNaK molten salt. HASTELLOY-N® [24] was developed by Oak Ridge National Laboratory (ORNL, US) and is produced by HAYNES International (US), GH3535 [25] was developed by Institute of Metal Research, Chinese Academy of Science (China) and is produced by Fushun Special Steel Shares Co., Ltd. (China), and finally MONICR® [26] was developed and is produced by COMTES FHT (Czech Republic). ...
In the present study, the corrosion performance of three candidate Ni-Mo-Cr alloys (HASTELLOY-N®, GH3535®, and MONICR®) for Molten Salt Reactor (MSR) systems have been investigated. It is shown that the molten salt corrosion performance of these alloys is not only dependent on their chemical composition, but also on their microstructural characteristic (grain size, grain boundary length, and the presence of second-phase particles). Obtained results suggest that MONICR® alloy is the least affected by the exposure to the FLiNaK molten salt. This is attributed to its chemical composition, high-angel grain boundaries length (grain size) and virtually no presence of large (M6C) carbides.
In order to better understand and predict the behavior of noble metal fission products in molten salt reactor (MSR), the distributions of ⁹⁹Mo,¹⁰³Ru,¹⁰⁵Rh and ¹³²Te between molten salt, graphite and GH3535 alloy were investigated using γ-ray spectroscopy. Experimental results showed that ⁹⁹Mo,¹⁰³Ru,¹⁰⁵Rh and ¹³²Te mainly settle at the bottom of molten salt in the form of metallic particles, and the flow of molten salt can easily make the settled metallic particles suspend in molten salt. In addition, ⁹⁹Mo,¹⁰³Ru,¹⁰⁵Rh and ¹³²Te tend to be deposited more on GH3535 than on graphite, and the deposition behavior of ⁹⁹Mo is particularly sensitive to the oxidation-reduction state of molten salt, which might be used as a redox indicator for the fuel salt of MSR. More importantly, the deposition characteristics provide new ideas for the separation and extraction of medical isotope ⁹⁹Mo from MSR in the future.
Binary and ternary Ni-Mo based alloys under the service conditions of high temperature show precipitation and chemical ordering which influence their microstructure, mechanical and corrosion properties. In the present work, the phase transformation behaviour of Ni-Mo and Ni-Mo-Cr alloys is studied to identify the role of Cr in altering the stability of brittle ordered intermetallic phases. TEM characterization demonstrates that the addition of Cr destabilizes not only the long-range ordered D1a phase but also the short-range order represented by <1 ½ 0> ordering wave vectors. EXAFS studies on Ni-Mo and Ni-Mo-Cr alloys establish preferential bonding of Cr with Ni as the first nearest neighbour in the fcc lattice replacing Mo. The cluster-expansion based calculations on Ni-Mo, Ni-Cr and Cr-Mo systems show that the nearest neighbour pair and multisite interaction energies are changed in magnitude as well as in sign for Ni-Mo and Ni-Cr systems signifying that the D1a type ordering tendency is reduced with Cr addition and the disordered fcc phase is stabilised. A combination of experimental and theoretical studies unequivocally establishes the stability of the disordered fcc structure, which is crucial for the long-term use of the selected Ni-Mo-Cr alloy at elevated temperatures.
Corrosion behaviors of GH3535 immersed in NaOH, HF and H2O2 solutions were investigated at RT for 600 h. The micro-morphology of surface and cross section was systematically characterized by SEM, EDS, EPMA, XRD and Nano Indenter. GH3535 showed excellent corrosion resistance in NaOH solution, but suffered serious pitting corrosion and slight grain boundary corrosion in HF solution, whereas only distinct hole corrosion in H2O2 solution due to the depletion of carbides. Element depletion, fall-off of carbides, decrease of hardness and Young’s modulus of GH3535 were detected in HF and H2O2 solution. However, NaOH solution is chemically inert towards GH3535. Corrosion mechanism was also explored based on the experimental measurement and thermodynamic calculation. Results not only provide the theoretical guidance for understanding the anticorrosion mechanism of GH3535 and purifying the molten salt, also expand the application of GH3535.
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The effect of solution heat treatment on the microstructure and properties of hot-rolled GH3535 superalloy has been investigated by scanning electron microscope (SEM), electron back scattered diffraction (EBSD) and tensile tests. Results show that significant grain growth and carbide dissolution occur after solution treatment at 1177 °C for 20 min. With the rise of the solution temperature up to 1220 °C and 1260 °C, the grain size grows abnormally and the amounts of M6C carbides decrease significantly. Tensile tests indicate that higher solution temperature and longer solution time decrease the ultimate tensile strength and improve the elongation. The grain growth and carbide dissolution are responsible for the evolution of tensile properties in GH3535 superalloy after different solution heat treatments.
Though Hastelloy N is basically a solid-solution alloy, various thermomechanical treatments do change its mechanical properties and micro-. structure. Since it is intended for-use up to 850°C and is now used in molten fluoride salt reactor systems, we must understand the nature of the precipitation processes and the effect on mechanical behavior. Identifying and characterizing the precipitates involved several complementary techniques: optical metallography, transmission electron microscopy, extraction replication, x-ray.diffraction, and electron probe microanalysis. In addition, chemical analysis with a microprobe attach-ment for the electron microscope and electron diffraction were employed to identify individual particles, agglomerates, and grain-boundary films on extraction replicas without interference from the matrix. These techniques help resolve differences in precipitates and relate the microstructure to mechanical properties. The microstructure is characterized by stringers of massive primary precipitates of the Ni3Mo3Ctype. Exposure between 500 and 1000°C results in precipitation of particles of the NisI%AC type in the grain boundaries. In air-melted heats that contain approximately 0.6s Si, the carbide-type precipitates are enriched in silicon and are not dissolved at high annealing temperatures but melt and transform to-a noncarbide phase. In vacuum-melted heats with low silicon contents, carbides go into solid solution. The only precipitates that form in air-melted alloys at temperatures as high as 1180°C are complex pseudocarbides of the Ni3(Mo,Cr)3(C,Si) and Niz(Mo,Cr)h(C,Si) types. The amount, and behavior of precipitates are highly silicon dependent; this impurity stabilizes the particles, pre-venting their being taken into solid solution at high annealing tempera-tures and causing them to transform to the high-temperature phase. This latter phase is possibly the 6-NiMo intermetallic and is probab,ly responsible for the increased embrittlement at high annealing temperatures.
Molten fluoride salts, because of their radiation stability and ability ; to contain both Th and U, offer important advantages as high-temperature fuel ; solutions for nuclear reactors and as media suitable for nuclear fuel processing. ; Both applications have stimulated experimental and theoretical studies of the ; corrosion processes by which molten salt mixtures attack potential reactor ; materials. Corrosion experiments with fluoride salts which were conducted in ; support of the Molten-Salt Reactor E xperiment and analytical methods employed to ; interpret corrosion and masstransfer behavior in this reactor system are ; discussed. The products of corrosion of metals by fluoride melts are soluble in ; the molten salt; accordingly passivation is precluded and corrosion depends ; directly on the thermodynamic driving force of the corrosion reactions. ; Compatibility of the container metal and molten salt, therefore, demands the ; selection of salt constituents which are not appreciably reduced by useful ; structural alloys and the development of container materials whose components are ; in near thermodynamic equilibrium with the salt medium. Utilizing information ; gained in corrosion testing of commercial alloys and in fundamental ; interpretations of the corrosion process, an alloy development program was ; conducted to provide a high temperature container material that combined ; corrosion resistance with useful mechanical properties. The program culminated ; in the selection of a high-strength Nibase alloy containing 17% Mo, 7% Cr, and 5% ; Fe. The results of several long-term corrosion loops and in-pile capsule tests ; completed with this alloy are reviewed to demonstrate the excellent corrosion ; resistance of this alloy composition to fluoride salt mixtures at high ; temperatures. Methods based on thermodynamic properties of the alloy container ; and fused salt are presented for predicting corrosion rates in these systems. ; The results of radiotracer studies conducted to demonstrate the proposed ; corrosion model also are discussed. (auth)
Molten fluorides of uranium, thorium, plutonium, and other elements potentially have wide applicability as fuels for power reactors. Because of their low vapor pressure they can be used in very high-temperature but low-pressure liquid-fuel reactors. In addition, they possess great chemical flexibility—the molten-salt principle can be applied to burners, thorium-uranium thermal breeders, plutonium-uranium converters, and possibly even to fast plutonium breeders. Because of the very high thermal efficiency obtainable in reactors using molten salt fuel, the fuel cost in a simple burner using enriched U²³⁵ is of the order of 2–3 mills/kwhr.
A high-temperature reactor using molten uranium salts (Aircraft Reactor Experiment) was operated for a short time at the Oak Ridge National Laboratory. The reactor was of the circulating-fuel type, with a BeO moderator. The maximum outlet temperature achieved was greater than 1500°F. It is believed that with further development the ARE could be a prototype for an economical uranium burner.
Experience to date has shown that in a molten-salt reactor environment the alloy Hastelloy N is embrittled by irradiation and suffers shallow intergranular cracking due to the fission product tellurium. From January 1974 through September 1976 these problems were actively researched. Hastelloy N modified with 1 to 2 percent NB was found to have good resistance to irradiation embrittlement and to intergranular cracking by tellurium. The severity of cracking by tellurium was noted to be influenced by the oxidation state of the salt so that cracking could be prevented even in standard Hastelloy N. This observation opened up other possibilities for materials selection.
The Generation-IV consortium seeks to develop a new generation of nuclear energy systems for commercial deployment by 2020–2030. These systems include both the reactors and their fuel-cycle facilities. The aim is to provide significant improvements in economics, safety, sustainability, and proliferation resistance. The systems selected for development are the very high-temperature gas-cooled reactor (VHTR), the sodium-cooled fast reactor (SFR), the gas-cooled fast reactor (GFR), the lead-cooled fast reactor (LFR), the molten salt reactor (MSR) and the super-critical water-cooled reactor (SCWR). UK organisations plan to contribute to the first three of these systems because of its existing capabilities and experience with gas-cooled systems, graphite cores, and SFRs. The science base for the VHTR and SFR systems is reasonably established, although there are gaps. For the VHTR, these include the performance of graphite at high neutron doses, and the performance of the fuel. For the SFR, the behaviour of fuels containing minor actinides, and processes for their recycling and refabrication into new fuel, must be established. The GFR presents many technical challenges, because it would need fuel and structural materials capable of withstanding extremes of fast neutron flux and high temperatures. Adequate heat removal from the core under fault conditions is likely to determine its feasibility.
MSRE Design and Operation Report, Part I, Description of Reactor Design
- R C Robertson
Development of Structural Material and Equipment for Molten Salt Technology
- P Hosnedl
The Study of Microstructure and Properties of a Corrosion Resistant Nickel-base GH3535 Superalloy
- T Liu