Radiation buildup control in BWR recirculation piping

Boiling water nuclear reactors (BWRs) employ stainless steel (Type 304 or 316 NG) pipes in which high-purity water at temperatures in the vicinity of 2750C are circulated. Various components of the system, such as valves and bearings, often contain hard facing metal alloys such as Stellite-6. These components, along with the stainless steel tubing and feedwater, serve as sources of 59Co. The presence of even small amounts of 60Co on the walls of the cooling water system is the dominant contributor to inplant radiation levels. Typical long-term surface activities due to this isotope on the walls of the cooling pipes range from 2.7 to 24 μCi/cm2. The resulting radiation field limits the allowable exposure time of maintenance and inspection personnel. The object of the research was twofold: (a) to form a thin film diffusion barrier against the outward diffusion of cobalt from a cobalt-containing surface and (b) to prevent the growth of a 60Co-containing corrosion film. The latter goal was most important since most of the radioactive cobalt will originate from sources other than the stainless steel piping itself. To meet these objectives, an experimental program was carried out using stainless steel rods to simulate the BWR piping and then exposing the rods to simulated BWR conditions in the General Electric Vallecitos 60Co test loop. It has been shown that the use of protective thin film coatings will reduce 60Co pickup. Many coatings were tried, but the three best were palladium, nickel, and gold prepared in various ways