The use of high nickel alloys in a PWR (stainless steel, inconel 600) is responsible for the Co-58 pro­duction from Ni-58 during early reactor life. In con­trast, Co-59 present in hard facing alloys and as an impurity in other materials determines the Co-60 pro­duction later in life. The relative contribution of each of these sources to the Co-60 production depends upon operational aspects such as valve seat lapping and upon the established metal corrosion and corro­sion product release rates. There is therefore an ad­vantage in minimising the Co impurity level in inconel and stainless steel, and developing Co-free or very low Co hard facing alloys. Typical Co specification for inconel and stainless steel is 0.1% with inconel available at 0.05%Co.

Table 1.21 shows that since approximately 73% of the surface area is inconel compared to 7% for stain­less steel, and 20% for zircaloy, there is a clear in­centive to pursue lower inconel Co levels, provided the cost benefit analysis is favourable.

Similarly, although only 0.04% of the surface area is hard facing alloy, a typical Co level for stellite is 66% and some material is located in the vicinity of
the core (control rod drive mechanisms) and in valves. Direct release of Co-60 species is consequently pos­sible by wear and corrosion mechanisms. There is therefore a clear long term benefit in developing low or Co-free hard facing alloys, and this is being ad­dressed worldwide. However, apart from the need to resolve questions of availability and fabrication it is possible that a Co-free alloy may wear more exten­sively and require more maintenance. This increased maintenance dose will, to some extent offset the bene­fits of a low system Co inventory.