ASME STP/NU-042

Description / Abstract: When selecting candidate materials, their resistance to environmental degradation caused by exposure to an HTGR helium atmosphere is a key factor. Improving the resistance of commercially available Nickel base super alloys to corrosive oxidation in low oxidizing potential atmospheres such as HTGR-He was discussed in Part I of this report, with reference to, for example, the improved Hastelloy X resulting in Hastelloy XR. With regard to the operating temperature, the required primary helium coolant temperature in the SI process (or IS process) was identified as being 950°C at the reactor outlet.

Review of the available information on Hastelloy XR and Inconel 617 as candidate materials was made in Part I of this report, and several critical issues discussed. Information on Inconel 617 is from a Japanese project. Those issues were identified with help from the author's experiences in developing the HTTR high temperature structural design guide. Some RD needed to obtain approval for Subsection NH construction was then pointed out.

With estimating the strength characteristics, the design creep rupture strength was identified as being 14MPa for Hastelloy XR, even in the HTGR-He atmosphere. The OSDP (Orr-Sherby-Dorn Parameter) method was applied to Hastelloy XR as an extrapolation technique to gain creep rupture strength values, primarily because of scarcity of data on the longer rupture life region.

In Part II of this report, the bounding conditions were briefly summarized for the Next Generation Nuclear Plant (NGNP) that is the leading candidate in the Department of Energy Generation IV reactor program. Metallic materials essential to the successful development and proof of concept for the NGNP were identified. The literature bearing on the materials technology for high-temperature gas-cooled reactors was reviewed with emphasis on the needs identified for the NGNP. Several materials were identified for a more thorough study of their databases and behavioral features relative to the requirements ASME Boiler and Pressure Vessel Code, Section III, Division 1, Subsection NH.

Material properties required for the design and construction of components meeting the rules of ASME Section III Subsection NH (ASME III-NH) were reviewed in Part II. An overview of the data available for candidate "new materials" for the Next Generation Nuclear Plant (NGNP) was undertaken with respect to meeting the needs for incorporation of the materials into ASME III-NH. These materials included alloy 617, alloy 230 and alloy 556 for service to 800°C and above. For service below 800°C, an "enhanced strength" stainless steel typical of a new group of such steels was included. Although not a new material, alloy 800H was included in the review. The data needs identified in the National Laboratories testing plans for the NGNP were considered. In these plans, emphasis was placed on alloy 617 which is the leading candidate for the high-temperature metallic components in the NGNP for components operating above 800°C. It was found that the plans were very comprehensive and identified the data needs for both incorporation of a new alloy into ASME III-NH and the complementary database needed for the application of the Code. A comparison of the strength of several candidate alloys approved for ASME Section I or Section VIII, Division 1 construction was made and this comparison supported the selection of alloy 617 as the leading candidate on the basis of strength. With respect to compact heat exchangers, some concerns about the behavioral features of these alloys as fine-grained strip products were developed, and some comparisons were made between candidate alloys developed for high-temperature recuperators.