Protective Coatings for Ontario Power Generation Inc. Used Nuclear Fuel Dry Storage Container

Abstract

Ontario Power Generation (OPG), formerly Ontario Hydro, has been storing used CANDU fuel in the irradiated fuel bays (IFBs) at its nuclear generating stations since they began operation. As the IFBs began reaching capacity, the dry storage of previously cooled used fuel became an economically viable alternative to the construction of additional wet fuel bays and the OPG Dry Storage Container (DSC) was developed. The OPG DSC is a free standing reinforced concrete container, with an inner and outer carbon steel shell, for the storage and transportation of used CANDU fuel. The outer steel shell of the DSC is protected by an applied coating system to facilitate decontamination of the outer shell and to provide protective corrosion resistance. In 1990 a study was performed to determine the optimal commercial coating system to be considered as a protective coating on the DSC outer shell. An experimental program was undertaken to identify the optimal commercial coating system which had the best decontamination characteristics as well as maximum resistance to abrasion, weathering and durability. A total of nine coating systems were selected for study, five epoxy coatings, three epoxy/polyurethane coatings, and one polyurethane coating. Sand blasted carbon steel coupons, similar to the DSC outer shell, were coated by the manufacturers and submitted for testing of the decontamination characteristics such as activity uptake, decontamination of the coating, and the activity ‘sweating' phenomenon. Tests identified four commercially available pure epoxy and epoxy/polyurethane protective coating systems as being the most promising for the exterior of the DSC surface. Of these four, the coating system chosen for use on the DSC was an epoxy/polyurethane system. After a decade of use, however, several safety and environmental concerns centering on the isocyanate content present in the polyurethane and the Volatile Organic Component (VOC) content of the coating system have arisen. These concerns prompted a second study to select an alternative DSC coating system which has a low VOC content and is isocyanate free. The selection procedure followed a similar pattern as previously, and consisted of a survey of coating system manufacturers followed by decontamination characteristics tests of the more promising coating systems. More than 40 coating systems were considered for use but the four most promising systems were selected for testing. The four systems tested consisted of a polysiloxane coating, two high-solids epoxy coatings, and a modification to the present coating system with the polyurethane topcoat replaced by a high solids epoxy. All the coating systems were isocyanate free and had VOC levels near or below 300 g/l. Activity uptake, decontamination, and sweating tests similar to those performed previously were performed on coated carbon steel coupons with the above coatings as well as coupons with the present coating system. The alternative coating system with the best performance in decontamination tests was the polysiloxane coating which was recommended as an alternative coating system. Based on further discussion with the coating manufacturers it was recommended that the polysiloxane coating be used in combination with a high solid epoxy primer. This combination would give a superior coating for abrasion resistance and weathering.