The performance of Canadian deuterium uranium (CANDU) nuclear steam generators has been outstanding. In the 88 reactor years of operation that ended in August 1980, only 61 of ∼300 000 tubes have been plugged and only 12 of these were in large commercial units. This excellent performance is ascribed to the high recirculation ratio and very open tube support design of the steam generators and the relatively noncorrosive cooling water at most sites. In May 1979, 37 tubes were plugged at the Ontario Hydro Nuclear Power Demonstration (NPD) Nuclear Generating Station. The location of the defects suggests they may be the first corrosion-induced failures in a CANDU steam generator. Chemical cleaning of the NPD steam generator in November 1979 resulted in the removal of 900 kg of deposits, cleaning of the tubes, and a return to full-power capability. Examination of a steam generator removed from the Douglas Point Generating Station (GS) in 1977 has revealed corrosion of carbon steel tube support plates and Monel 400 tubes but no denting. No tubes have been plugged at Pickering since the manufacturing defect discovered in 1974. Two of the 11 Inconel 600 tubes plugged at Bruce GS in 1977 and 1979 had leaked at tube-to-tubesheet seal welds, and others showed primary side tube wall defects, the cause of which has not been determined. Specification of low cobalt Inconel 600 has contributed to a tenfold reduction in 60Co radiation fields at Bruce compared with Pickering. To ensure good performance from steam generators at seawater-cooled stations, corrosion-resistant tube supports, high integrity condensers, and automated steam generator chemistry control have been installed