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Objective of this study: Ø Impact of more demanding conditions (Li, T°) on the corrosion behaviour of M5 And in particular, to state on : Ø The Nb content in the oxide layers formed on M5 / possibility of 94 Nb release in the primary coolant Ø The Li and B content in the oxide layers formed on M5 / possible corrosion enhancement due to the Li increase Ø The impact on hydrogen pickup of M5

The very high temperature reactor (VHTR) has been selected as among the Generation IV nuclear power system. The coolant, namely helium, has been shown to contain low level of oxidizing as well as carbon-bearing impurities (such as H 2 , CO (and CO 2 ), CH 4 , N 2 ranging from a few microbars to hundreds of microbars and water vapour in the microbar range) that can interact with metals at high temperature. In this specific gas phase, there is a competition between the formation of protective oxide scales and deleterious reactions towards carbon. A possible oxide layer reduction could be observed in a particular case detailed in this paper. This destruction of the oxide can imply consequences on the behaviour of the alloy as strong carburization or deep internal oxidation could occur. The influence of the level of impurities was analysed through a parametric approach. The experiments were carried out in a purposedesigned device that allows the control of low impurity partial pressures.

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