Research and Development on Heat-Resistant Alloys for Nuclear Process Heating in Japan

The developments of the last decade are reviewed on a technical basis for heat-resistant alloys in application to the high-temperature structural components of the process heating high-temperature gas-cooled reactor. The major activities have fallen into two categories: the near-term development for the experimental reactor and the long-term R&D second-generation applications, i.e., for the materials to be used in the second-stage heat exchanger installation in the experimental reactor and those for advanced-stage reactors with very high outlet temperatures. In both categories of programs, significant advances have been made, respectively, in providing and testing a modified commercial alloy with enhanced compatibility with the service environments and in selecting potential high performance alloys from the new developmental candidate alloys. Modification of the existing commercial alloy was achieved through the application of the finding on enhanced oxidation resistance by controlling the common impurities in the material, while the enhanced creep rupture strength recognized in the best performing new alloys has been attributed to the precipitation of a tungsten-rich phase (α₂) during holding at test temperatures. The new alloy development program currently under way is also introduced.