Effects of magnesium and zinc additives on corrosion and cobalt contamination of stainless steels in simulated BWR coolant

In water-cooled nuclear reactors, measures to decrease the radiation fields due to the deposition of 60Co on out-of-core components are important to control occupational doses of radiation and to reduce costs. In this respect, dosing of minor amounts of certain metal ions to the primary coolant can be effective. The beneficial effect of adding Zn to boiling water reactors has been known for some time now. As an alternative to Zn, Mg is investigated in the present study. The deposition of 60Co onto typical materials of construction is investigated in neutral, partially oxygenated water at 562 K and 10.2 MPa in the presence and the absence of Mg. For comparison, studies are also conducted in the presence of Zn; rather high concentrations of both additives in the coolant are employed to indicate their relative effectiveness in a reasonably short testing period. The ability of the additives to exclude 60Co pickup by oxides on three types of stainless steel is evaluated. The corrosion rates of stainless steels, the morphology of their oxides, and the pickup of Mg or Zn are determined. Preliminary decontamination studies of the exposed materials are also described. A qualitative mechanism to describe the observed 60Co pickup is discussed