Kinetics of 403m repassivation after decontamination

The films formed on 403m steel during operation of the primary circuit of a NPS catch some radioactive corrosion products and fission products transported by the cooling agent from the primary circuit components. In this way an increasing of the radioactivity of the 403m components results, which must be removed using a proper chemical decontamination procedure. The surfaces resulted after applying the decontamination treatment must be passivated to increase their corrosion resistance. Taking into account the above mentioned technical data, we used a number of samples which were been first filmed in autoclaves in conditions specific to the primary circuit. These filmed samples were decontaminated using the CAN-DECON procedure. Afterwards, the decontaminated samples were passivated chemically in four solution types. The films formed after the prepassivation being very thins (<1 mm), the prepassivated samples were exposed for about 100 days in autoclaves in the same conditions specific to primary circuit. To establish the most efficient repassivation solution, the kinetics of the 403m oxidation, decontamination and repassivation were studied. Besides with these kinetics evaluations, spectrophotometric analyses of Fe ions delivered in solution from autoclave after several periods of time were made. The superficial films formed after the autoclavization of the prepassivated samples were studied using the metallography and the electrochemical impedance spectroscopy (EIS) method. In conclusions: - 1. The surface of decontaminated 403m components is more susceptible to corrosion in several aqueous environments then any filmed surfaces, hence the necessity of passivation surfaces results; - 2. To achieve passive superficial films on the decontaminated 403m, we proposed four solutions; - 3. The samples chemically prepassivated in those ways were submitted afterwards to an autoclavization in the conditions of primary circuit; - 4. The films formed after the second autoclavization were been characterized by electrochemical methods, metallographic microscopy and XPS; - 5. The best solution for the prepassivation of the decontaminated samples proved to be the solution no.1 (N2H4 + NH3). (authors)