Creep rupture properties of Ni-Cr-W alloys developed for nuclear steel making, which precipitate α-W, were investigated in the temperature range from 900°C to 1 050°C in He environment (He-2) which simulates the primary coolant of High Temperature Gas Cooled Reactor (HTGR), and at 900°C in reducing gas environment which is to be used when direct steelmaking system is connected with the primary system of the HTGR. The results were discussed in terms of environmental factors such as carburization, decarburization and oxidation. It was found that the two alloys have the possibility to satisfy the target of R & D, that is, "the alloy should have the 50 000 h rupture strength of more than 1 kg/mm2". Further, the degradation of the rupture strength in the alloy which has α-W as a main strengthener, did not appear in spite of severe decarburization of the alloys.
