Evaluation of cobalt sources in Westinghouse-designed plants

Light water reactor radiation fields after shutdown are generally increasing with plant operation. After the first few years, Co-60 becomes the dominant isotope. The source of Co-60 is neutron activation of Co-59 present as an impurity in some of the materials of the reactor coolant system components. In addition, those component surfaces subject to wear are typically hardfaced with cobalt-based alloys which are about 60 percent Co-59. Deposition on and erosion from fuel surfaces results in the release of Co-60 to the coolant and subsequent deposition on coolant system surfaces. It was found that about 70 percent of the cobalt input arises from the reactor coolant system. Almost all of this is from the corrosion release of the cobalt impurity in Inconel steam generator tubing. Corrosion of high cobalt alloys accounts for about 16 percent of the cobalt input, and wear of these alloys accounts for 10 percent