Advances in the Development and Description of Tritium Permeation Barriers in High-Temperature Alloys

Abstract

Recent experimental findings on tritium permeation barriers are described with special emphasis on the interpretation in terms of mechanical behavior. Kinetic measurements of the water vapor corrosion reaction with Incoloy-800 have been performed first by determining hydrogen production and permeation rates on line. Growth laws of the oxide scales have been determined indicating that a visually parabolic phase can be attributed to a scale of enhanced impeding effect against permeation. A certain amount of the hydrogen created by the corrosion reaction permeates spontaneously through the metal at a fraction varying between 1 and 10%.A new quality of oxide layer has been identified that can be characterized by enhanced activation energies for hydrogen permeation of ∼150 kJ/mol as well as a modified pressure dependence proportional p1 in a limited range. Such scales show improved impeding factors ≫ 100. Moreover, the effect of an additional layer on the opposite side of the tube specimen has been studied that shows a different impeding behavior dependent on the direction of the hydrogen/tritium flow. A model has been discussed describing the impeding effect of oxide scales in terms of surface controlled reaction steps rather than bulk diffusion, as has been the usual procedure hitherto. The model proposed offers a qualitative understanding of experimental findings characterizing high-quality layers.Acoustic emission and hydrogen permeation measurements as indicators for cracking have been combined in an attempt to define oxide layer qualities with respect to behavior during temperature cycling. Three alloys with different damage rates have been investigated. The results of such experiments seem to offer the possibility of a quantitative correlation between the intensity of the acoustic emission signal and the loss of impeding effect due to cycling.