[en] Inconel 617 has been chosen as the main reference alloy for the Prototype Nuclear Process Heat system (PNP). Although this alloy is deemed to be adequate for current needs, there is nonetheless a longer term requirement for a tailor-made alloy suited specifically to the rigorous constraints imposed by the PNP system. Accordingly, within the PNP Project, there is a considerable effort being expended on alloy development, particularly by MFI, Duisburg, and TEW, Krefeld (1). The work described herein is a complementary effort undertaken by the High Temperature Materials Programme (HTMP), also under the auspices of the PNP Project. Outside the Project, but within the HTR field, there is also considerable effort on alloy development being expended by research and industrial groups in Japan (2-4). In particular the Japanese report significant improvements in rupture strength for Ni-based alloys with high tungsten content (2).