Corrosion of M5 in PWRs: Quantification of Li, B, H and Nb in the Oxide Layers Formed Under Different Conditions

Until now, most of the detailed characterizations of the M5 corrosion behaviour were performed under standard PWR operating conditions, under moderate Li content and moderate temperature of the primary coolant. In this study, in addition to these standard conditions, two demanding operating conditions were explored: increased Li chemistry and elevated temperature. The objective is to establish whether these more demanding conditions have an impact on the structure of the oxide layers formed, on Nb, Li and B contents in these layers and on Hydrogen pickup of the cladding. The structure of oxide layers was studied by microscopy, the Nb content and distribution by Electron Probe Micro Analysis, the Li and B contents and distributions by Nuclear Reaction Analysis and the hydrogen pickup by gas extraction. It was observed that the stability of the corrosion behaviour of M5 is not affected by increased Li or elevated temperature conditions. The hydrogen pickup fraction of M5 is not modified by increased Li conditions or by irradiation temperature with measured contents (<100 ppm) below the solubility limit at irradiation temperature (586–631 K range). Moreover, no significant release of Nb from the oxide in the primary coolant was observed, whatever the operating conditions can be, with quantification accuracy below 0.05%. The respective Li and B contents reached in oxides are of the same order of magnitude under the all three conditions, with average Li content around 15 ppm and average B content varying from 70 to 140 ppm, corresponding to a range where enhanced corrosion is neither observed, nor expected.