J. Wayne Jones

Professor Emeritus

jonesjwa@umich.edu

2022 Gerstacker

T: (734) 764-7503

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Alloys for 1000C Service in the Next Generation Nuclear Plant

Collaborators: James Crum and Gaylord Smith, Special Metals Inc.,
Sponsor: Department of Energy
The very high temperature reactor (VHTR) has been selected by the Department of Energy for the Next Generation Nuclear Plant (NGNP) Project. This design calls for outlet gas temperatures of 1000¡C and a lifetime of 60 years. These are extremely challenging conditions for the operation of metallic components that will be required in the intermediate heat exchanger and primary-to-secondary piping. Nickel-base alloy 617 has been identified as the leading candidate for such applications. However, the microstructure stability, surface stability (oxidation/carburization), creep strength, and interaction between these processes in a He environment containing impurities at temperatures up to 1000¡C is still poorly understood and insufficient to qualify the alloy for service. The objective of this new research program is to investigate the oxidation/carburization behavior, microstructural stability and creep behavior of alloy 617 in an impure He environment in the temperature range 900-1000¡C, with the aim of understanding the synergisms between these three critical processes, and of providing data for long-term prediction of properties. Because the upper end of the temperature range will challenge this alloy, alloy and microstructure modifications to enhance properties of the alloy will also be investigated. Surface oxidation and carburization reactions in alloy 617 will be studied over a range of conditions in a system designed to control H2/H2O, CO2/CO and CH4/H2 levels in the He environment. Degradation of surface microstructure for varying impurities, exposure times and temperatures will be studied in detail. The influence of impurity-induced surface degradation along with evolution of carbide and grain structure during creep of alloy 617 will be investigated. The synergistic effects of these degradation mechanisms will be captured in a continuum-damage mechanics-based model for creep. Since it is likely that alloy improvements will be required to reach the goal of 1000˚C operating temperature, alloying additions that are likely to improve carbide stability and/or influence diffusion, and the modification of the grain boundary structure through grain boundary engineering will be investigated.