Current status of studies on high temperature metallic materials for HTGR applications in JAERI

Progress made in the studies on high-temperature metallic materials for HTGR applications in JAERI conducted during the last several years is reviewed, and future direction of the studies is also mentioned. The items of research activities carried out are classified into three different categories, i.e. those for the near-term target structural alloys, for the long-term target heavy duty structural alloys and for the cladding material for the neutron absorber rods. As for near-term target alloys for applications in the High-Temperature Engineering Test Reactor (HTTR), comprehensive qualification tests such as creep, fatigue, corrosion resistance and other fracture-relevant properties on Hastelloy XR which is a modified version of Hastelloy X have been carried out to accumulate the data for structural design and safety evaluation. For further improvement of the performance and optimization of the specification of Hastelloy XR, a creep resistant version with control of boron content with 40 to 70 ppm has also been developed. Significance of the impurity content in the helium environment on creep behaviour is highlighted. The development of new heavy duty Ni-Cr-W superalloys as the next generation candidates has been promoted for extended use at service temperatures up to 1,000 deg. C, the basal composition of the experimental alloys had been already specified. The investigation on the effects of minor elements such as C, Nb, Fe, Mn, Si, Ti, B and Y is underway to determine the optimized chemical composition of selected candidate alloys. For the selection of cladding materials for neutron absorber rods of HTTR, the post-irradiation tensile and creep tests have been performed, and Alloy 800H has been tested as one of the currently most realistic candidate materials. (author). 18 refs, 18 figs, 4 tabs