29 research outputs found
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CATASTROPHIC OXIDATION OF HIGH-TEMPERATURE ALLOYS
The growth of nonprotective, crust-like oxide films was encountered in high-temperature alloy systems that contain molybdenum, vanadium, or tungsten as strengthening additions. The cause of accelerated oxidation in such alloys appears to be associated with the characteristically low melting temperatures of oxides of these refractory elements. The factors that contribute to a breakdown of oxidation protection in these systems are outlined and remedial methods which may be used to avoid catastrophic oxidation are discussed. Commonly encountered service failures that have resulted from catastrophic oxidation are also described. (auth
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Investigation of breached depleted UF sub 6 cylinders
In June 1990, during a three-site inspection of cylinders being used for long-term storage of solid depleted UF{sub 6}, two 14-ton cylinders at Portsmouth, Ohio, were discovered with holes in the barrel section of the cylinders. An investigation team was immediately formed to determine the cause of the failures and their impact on future storage procedures and to recommend corrective actions. Subsequent investigation showed that the failures most probably resulted from mechanical damage that occurred at the time that the cylinders had been placed in the storage yard. In both cylinders evidence pointed to the impact of a lifting lug of an adjacent cylinder near the front stiffening ring, where deflection of cylinder could occur only by tearing the cylinder. The impacts appear to have punctured the cylinders and thereby set up corrosion processes that greatly extended the openings in the wall and obliterated the original crack. Fortunately, the reaction products formed by this process were relatively protective and prevented any large-scale loss of uranium. The main factors that precipitated the failures were inadequate spacing between cylinders and deviations in the orientations of lifting lugs from their intended horizontal position. After reviewing the causes and effects of the failures, the team's principal recommendation for remedial action concerned improved cylinder handling and inspection procedures. Design modifications and supplementary mechanical tests were also recommended to improve the cylinder containment integrity during the stacking operation. 4 refs., 2 figs
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GRAPHITE-STAINLESS STEEL COMPATIBILITY STUDIES
S>The compatibility of type 304L stainless steel in intimate contact with graphite is being studied as a function of temperature and contact pressure. This study is an outgrowth of materials compatibility problems in present and advanced gas-cooled reactors, where structural members in direct contact with graphite provide the possibility of both carburization and self-welding. Initial studies were concerned with surface reactions in the absence of gaseous contaminants under a vacuum of 10/sup -6/ mm Hg at 540 to 705 deg C. Stainless steel specimens are pretreated to provide three surface conditions: H/sub 2/- fired, preoxidized, and Cu-plated. Surface contact pressures ranged from 0 to 10,000 psi. Test results are presented which establish the lower temperature limit for significant diffusion between graphite and stainless steel at approximately 60O deg C. Above this temperature, diffusion between untreated or H2-fired stainless steel surfaces was found to effect complete bonding of the two materials at contact pressures as low as 500 psi. Bonding was effectively prevented by the presence of either an oxide film or a Cu plate at temperatures up to 700 deg C. Where bonding occurred, diffusion rates measured for C in stainless steel were comparable with those reported for stainless steel ln C- saturated Na. However, phases produced in surface reactions between graphite and stainless steel were of higher order than those reported for the Na carrier. Lack of carburization in control specimens not in contact with graphite indicated the role of the gas phase to be unimportant at impurity pressures of 10/sup -6/ mm Hg. (auth
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Liquid metal corrosion considerations in alloy development
Liquid metal corrosion can be an important consideration in developing alloys for fusion and fast breeder reactors and other applications. Because of the many different forms of liquid metal corrosion (dissolution, alloying, carbon transfer, etc.), alloy optimization based on corrosion resistance depends on a number of factors such as the application temperatures, the particular liquid metal, and the level and nature of impurities in the liquid and solid metals. The present paper reviews the various forms of corrosion by lithium, lead, and sodium and indicates how such corrosion reactions can influence the alloy development process
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Mass transfer deposits in lithium-type 316 stainless steel thermal-convection loops
In spatially nonisothermal flowing liquid metal systems, selected constitutents of the containment materials characteristically dissolve into the liquid metal in the hotter zones and are deposited in the colder areas. The accumulation of deposits is often a more serious problem than dissolution because of attendant flow restrictions and, in reactor applications, the aggregation of radioactive species in the coolant circuits. Accordingly, the deposition processes in lithium-type 316 stainless steel thermal convection loops is studied. The morphology and composition of deposits varied with loop operating time. Initially, chromium-rich dendritic crystals formed in the colder region of a loop, but later the deposits changed in structure and contained significant amounts of nickel and iron. Deposition rates were also measured as a function of time and temperature and were correlated with the above observations. A plug extracted from one loop consisted of an aggregate of chromium-rich crystals
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CORROSION BEHAVIOR OF REACTOR MATERIALS IN FLUORIDE SALT MIXTURES
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
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Corrosion in lithium-stainless steel thermal-convection systems
The corrosion of types 304L and 316 austenitic stainless steel by flowing lithium was studied in thermal-convection loops operated at 500 to 650/sup 0/C. Both weight and compositional changes were measured on specimens distributed throughout each loop and were combined with metallographic examinations to evaluate the corrosion processes. The corrosion rate and mass transfer characteristics did not significantly differ between the two austenitic stainless steels. Addition of 500 or 1700 wt ppM N to purified lithium did not increase the dissolution rate or change the attack mode of type 316 stainless steel. Adding 5 wt % Al to the lithium reduced the weight loss of this steel by a factor of 5 relative to a pure lithium-thermal-convection loop
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Examination of bimetallic Inconel 600/316 stainless steel. [LMFBR]
This report describes the results of a detailed examination of a bimetallic sodium-heated steam generator that developed a massive sodium-to-water leak during performance testing in 1970