A stainless-steel loop containing recirculating, pressurized water at 300°C and neutral pH has been used to investigate the processes by which surfaces in nuclear reactor coolant circuits can become contaminated by radioactive corrosion products. By measuring deposition rates of activity onto metals (usually stainless steel) under different conditions and by examining exposed surfaces, it was deduced that the corrosion of the test surface controls the activation. Hence, the traditional phenomenological method of describing activation by first-order deposition and release coefficients is an oversimplification.