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Alloys 617 and 230 are currently identified as two leading candidate metallic materials in the down selection for applications at temperatures above 760°C in the Gen IV nuclear reactor systems. Qualifying the materials requires significant information related to codification, mechanical behavior modeling, metallurgical stability, environmental resistance, and many other aspects. In the present paper, material requirements for the Gen IV nuclear reactor systems are discussed; available information regarding the two alloys for the intended applications are reviewed and analyzed; and further R&D activities are suggested. In the United States the major requirement for qualifying the materials is to satisfy the ASME Subsection NH, with adequate considerations for NRC, ASME NQA-1, and Section XI. In comparison, Alloy 617 is more studied with larger existing databases in air and helium, while Alloy 230 may have highly desired potentials but needs more exploration. To provide a sound technical basis for the material selection decision, more data should be generated to characterize behaviors of both alloys in creep, loading rate sensitivity, fatigue, creep-fatigue, crack resistance, toughness, product form dependency, and metallurgical stability.

Fluoride Salt-Cooled High-Temperature Reactors (FHRs) are a promising new class of thermal-spectrum nuclear reactors. The reactor structural materials must possess high-temperature strength and chemical compatibility with the liquid fluoride salt as well as with a power cycle fluid such as supercritical water while remaining resistant to residual air within the containment. Alloy N was developed for use with liquid fluoride salts and it possesses adequate strength and chemical compatibility up to about 700°C. A distinctive property of FHRs is that their maximum allowable coolant temperature is restricted by their structural alloy maximum service temperature. As the reactor thermal efficiency directly increases with the maximum coolant temperature, higher temperature resistant alloys are strongly desired. This paper reviews the current status of Alloy N and its relevance to FHRs including its design principles, development history, high temperature strength, environmental resistance, metallurgical stability, component manufacturability, ASME codification status, and reactor service requirements. The review will identify issues and provide guidance for improving the alloy properties or implementing engineering solutions.

2.3 GT-MHR reactor system cutaway showing the metallic internals structures, core, control rod guide tubes, and cross vessel (labeled as hot duct) .

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