The influence of Fe and Zn addition upon activity build-up in BWR system piping

Maintaining a low radioactivity on system surfaces in BWR-plants is important for minimizing doses to staff during maintenance work. The composition and microstructures of metal oxides formed on system piping surfaces, which in turn is closely related to water chemistry environments, are of great importance for the activity levels in BWRs. An experimental system has been built at Studsvik Nuclear AB to study activity build-up as a function of i.a. water chemistry conditions. The influence of different concentrations of iron and nickel species under simulated BWR conditions have been studied and reported in earlier stages within this project. In this work the effect of combined Zn and Fe addition upon activity build-up has been studied. The overall objective was to study the effect of Zn addition in connection to the nature of various activity changes observed in BWRs. The results show that a low concentration of Zn (0.5 ppb) together with Fe (2 ppb) is enough to significantly decrease activity build-up compared to the addition of only Fe separately. For higher concentrations of Zn (5 and 10 ppb), the activity build-up is in principle suspended. The effect of Zn addition to the water is instantaneous, which means that as soon as the Zn addition to the water stops, the activity build-up increases significantly. (authors)