ORNL-TM-2010-199

If a coated or layered structure is used, a number of issues must be considered. First is the question of the adequacy of the high-nickel layer to provide protection for the microstructure of the underlying superalloy.

Evaluation of diffusion through the surface layer of various alloying elements and environmental impurities (e.g., oxygen and carbon) to and from the free surface of the layered component must demonstrate stability of both the high-temperature properties of the superalloy and the integrity of the layer (see Fig. 11.1). Fabricability is clearly an issue. Simple geometries can be fairly easily obtained via co-extrusion, roll bonding, weld overlay cladding, etc., but complex geometries such as tube-totube sheet, manifolds and headers, compact heat Fig. 11.1. Schematic representation exchangers, etc., will be a much more significant of diffusion through the protective challenge. Long-term integrity of the surface layer bond surface layer in a layered metallic and inspectability of the layered component must structure. be ensured.

For external components, such as the RPV or piping, it may be possible and desirable to internally insulate the component, allowing the use of a lower service temperature alloy, such as A508 or A533B steel, for the pressure boundary. This approach would require an engineering solution to corrosion protection on the inside surface of the component and an insulation layer sufficient to limit the operating temperature of the alloy used to an acceptable range, such as that schematically depicted in Fig. 11.2.

With such an approach, it would be possible to increase the operating temperature of the coolant to 850°C or significantly beyond and still enable the use of existing structural alloys codified for nuclear service, but only for components with a coolable surface. Reactor internals, such as the core barrel or control rods, or heat exchangers that would have to operate at temperatures at or near that of the coolant would still require materials of construction with higher usage temperatures.

The long-term solution for higher temperature Fig. 11.2. Use of insulated structures operation of SmAHTR up to 850°C or slightly higher will for pressure boundaries. likely require the development of a new superalloy with both adequate mechanical properties at the operating temperature and sufficient compatibility with the liquid salt and other required environments. Given the major overall advances in materials science and especially in predictive thermodynamic-based modeling tools for alloy development and evaluation, this is a very realistic possibility but would still likely require several years of development and qualification before such an alloy could be codified for nuclear service.

Alternately, the use of carbon–carbon or SiC-SiC composites or monolithic SiC would allow for uncooled components to temperatures up to and beyond 1,000°C, but would include very significant qualification, codification, and regulatory activities for the design and use of such materials, as well as ways to ensure the limited permeability needed for any pressure boundary applications. It is also expected that the radiation resistance of this class of materials would be adequate for use in the reactor internals for virtually all of the conditions envisioned for SmAHTR. SmAHTR’s graphite structures are expected to be of the fixed prismatic type with fairly fine webs, so the use of a high-strength, fine-grained graphite will be needed. Two grades of Toyo Tanso 11-3