Improvement of the pre-filming technique for the reduction of the metal release from TP304L

Water chemistry of LWR is controlled from various points which are corrosion resistance of structural material, reliability of fuel cladding, reduction of radioactivity, minimization of radiation waste and so on. The pre-filming technique has been studied to focus on the Cr release from the inner surface of TP304L seamless tube for the feeder water heater. This technique is a practical application of a selective oxidation of Cr. Cr had lower oxygen potential than that of the other major constituent elements i.e. Fe and Ni. The Cr oxide pre-film grows in the controlled atmosphere during the final heat treatment. The Cr oxide is known to be stable in BWR environment. Basically not only Cr but also Fe and Ni dissolve into the coolant from the stainless steel tubes. Varying the thickness of the pre-film on TP304L, the metal release (Fe, Cr, Ni), the stability of the pre-film, the SCC susceptibility and the electrochemical property in high temperature water are discussed in this study. The metal release decreased with increasing the thickness of the pre-film in water at 250degC. After the metal release test, there were not the corrosion products on the more than 160 nm pre-film. This result also suggests that the pre-film reduced the metal release. The pre-film had been continuous and uniform even in the high temperature water which contained saturated dissolved oxygen. The pre-filming didn't assist the inter granular SCC in high temperature water. (author)