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Materials selection

Selection of materials for the High Temperature Reactor Intermediate Heat Exchanger.

The indirect-cycle Very High Temperature gas cooled Reactor (VHTR) is one of the leading contenders for the next generation of nuclear reactor. It allows a common heat source to be used for both electricity generation and hydrogen production while minimising the complexity and risk associated with the nuclear part of the cycle. The VHTR produces heat at temperatures in the order of 1000°C. This heat can be used for power generation, via a Brayton or combined cycle, and hydrogen production, via a variety of high temperature processes. A key component of the VHTR is the Intermediate Heat Exchanger (IHX). Regardless of the exchanger type used in this duty, selection of an appropriate material is key. A number of issues need to be taken into account and the material used must: be suitable for use at temperatures up to 1000°C have a design life of up to 60 years be of high strength and be suitable for operation in corrosive environments there may also be design code issues, as at present there are no "nuclear approved" materials available On this basis a number of materials are being qualified by Heatric for IHX manufacture and include: Alloy 617, Alloy 230, Alloy X and XR, Alloy 800 variants and Alloy 602CA. The main factors in the preliminary selection of these materials are: mechanical properties - tensile properties - creep and creep-fatigue properties physical properties - thermal conductivity - thermal expansion corrosion resistance - helium contamination - nitridation resistance fabrication and joining technology  experience Preliminary materials selection Max temp limit: ASME VIII I (2006) The review of the materials would suggest that either Alloy 617 or 230 would be the most suitable material for an IHX. These being the only commercially available alloys that have the appropriate combination of menchanical, physical and corrosion resistance properties. Further more they are alloys that are available in appropriate product forms (ie. sheet and plate fin) and lend themselves to the diffusion-bonding process involved in the manufacture of Heatric heat exchangers.

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