ASME Subcommittee XI Initiatives for the Detection and Correction of Boric Acid Corrosion of Pressure Retaining Low Alloy Steel Components in Pressurized Water Reactor Plants

In pressurized water reactor power plants sodium pentaborate is commonly used as a soluble chemical neutron absorber to control the reactivity level within the reactor core. This chemical solution of boron in the reactor primary cooling water forms a mild boric acid. The main reactor coolant system pressure vessels, i.e., the nuclear reactor and the steam generators, are protected internally from this corrosive solution by a corrosion resistant cladding. The nuclear power industry has recorded instances of leakage from the primary coolant system resulting in corrosion of these pressure vessels on their outer surfaces. In 2002 a primary coolant leak resulted in the nearly complete loss of the pressure retaining capability of the reactor vessel closure head at a domestic nuclear power plant. In response to this event the ASME Subcommittee XI has undertaken to establish inspection rules for pressurized water reactor plants for the detection of primary coolant leakage and the resulting corrosion of low alloy steel components. This paper discusses the history of this issue and the development of inspection rules designed to ensure the early detection of primary coolant system leakage and the possible resulting corrosion prior to loss of the pressure retaining capability of primary coolant system components.