In PUREX process for spent fuel reprocessing plants, nitric acid solutions are treated in many components. Particularly, heating portions in the components are severely corroded in the boiling solution under heat transfer (HT) conditions. Although corrosion behavior under the conditions has been investigated by many researchers, corrosion mechanism has not been sufficiently understood. Consequently, because of its safety, we need to study dominant factors of corrosion mechanism and to develop a method to predict corrosion progress.
Surface temperature, heat flux, concentration and compositions of the solution have been previously considered as important factors for corrosion rate (CR). In this paper, authors have focused on the effects of surface temperature and heat flux on CRs of stainless steels in boiling nitric acid under HT conditions. We performed experimental study in consideration of dissolvers and concentrators. Two types of cells for HT and immersion conditions were applied for corrosion tests and the effects were compared. Test solution used was 33 mol·m−3 (0.033 M) vanadium added to 3 kmol·m−3 (3 M) nitric acid solution. The solution was heated at boiling temperature under atmospheric pressure. Additionally, a boiling curve which was indicated the relation between heat flux and degree of superheating was investigated experimentally. Surface temperatures during corrosion tests were estimated from a boiling curve. The relationship among measured CRs, surface temperature and heat flux were studied.
The results showed that CR did not depend on heat flux, but depended on surface temperature. Arrhenius plots on CRs indicated that CR was accelerated by solution boiling against non-boiling. The cause of the acceleration of CR under boiling nitric acid solution was discussed.