Post-irradiation investigations of corrosion and deuterium pickup by Zr-2.5 wt% Nb alloy pressure tubes: 14C, 13C and 11B tracers in outside surface oxides

During oxide growth, on the outside surfaces of pressure tubes in CANDU reactors, there is an attendant production of carbon 13C and 14C and the boron isotope 11B from nuclear transmutation reactions; their depth distribution in the oxide provides a fingerprint of the growth characteristics of the oxide. SIMS depth profiles of 14C and 11B, products of the nuclear transmutation of 14N, can discriminate between oxide grown in a nitrogen annulus and oxide grown in a carbon dioxide annulus. The higher concentrations of these isotopes and their depth distributions in oxides grown in a nitrogen environment indicate nitrogen incursion from the annulus into the oxides. By comparison, oxides grown in carbon dioxide annuli show higher concentrations of 12C which indicate carbon dioxide incursion from the annulus. The relative concentrations of 14C and 11B in the oxide indicate that a loss of carbon has also occurred, suggesting that the same porosity, responsible for the incursion of annulus gas, has provided a means for the release of oxidised carbon compounds. The carbon tracer 13C is produced by the transmutation of 16O. Depth profiles of this isotope, corrected for contributions from indigenous carbon impurity, essentially provides a retrace of history of the oxide growth kinetics. In both the nitrogen and carbon dioxide annuli, oxide growth is found to be initially linear followed by a transition to an increased linear rate at about 2 μm. (orig.)