Evaluation of activity build-up experiments

Maintaining radioactivity low at system surfaces in BWR-plants is of importance for minimizing doses to staff working with maintenance work at the plants according to the ALARA, As Low As Reasonable Achievable, principle. The composition and microstructures of metal oxides that are formed on system piping surfaces are of great importance for the activity levels in BWR, which are in turn closely related to water chemistry environments. An experimental system has been built at Studsvik Nuclear AB to study activity build-up as a function of different experimental parameters. Up to now, the influence of different concentrations of iron, nickel and zinc species have been studied under simulated BWR-conditions. The objective of this report is to identify parameters that may be of importance for activity build-up. This has been done through evaluation of the experimental data collected so far within the project frame. It may provide an improved understanding about the nature of various activity changes observed. It is also helpful for designing further experimental studies. The importance of precipitation and dissolution reactions in the oxide/water interface for activity build-up has been pointed out in the report. This has been highlighted through correlation of data from activity release studies to the dissolution rate of oxide (NiFe2O4) in the oxide/water interface which in turn has been correlated to the corrosion rate of stainless steel in BWR-environments.