Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

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Hydrogen and radiation embrittlement of RPV steels

A distinct reduction of ductility at comparable conditions of hydrogen charging has been ascertained for irradiated and unirradiated A302B steel,5unirradiated steels A533B and A542,6and irradiated pure iron and A542 steel.7 The effect of hydrogen on the time to fracture depends on the charging current density and appeared in the CrMoV steel with hydrogen content above 3 ppm. The lower threshold stress Rit of the CrNiMoV steel was found to be twice that of the CrMoV steel. The higher resistivity of the CrNiMoV steel can be explained by a higher micropurity, causing a higher cohesion strength of the grain boundaries and therefore a higher resistivity against intercrystaUine failure. The critical development of initial defects proved to be located on the grain boundaries. It is evident that the quantity of hydrogen trapped on the defects both inside grains and on the grain boundaries as well as the quantity of necessary additional hydrogen on the potential failures increased with increasing current density. Under these conditions the leading r61ecan be considered to be played by the interaction of hydrogenwith the steel structure. At lower charging current densities hydrogen trapping on structural defects does not lead to stable defects: after hydrogen release from the specimens ductility reverted to initial values. Increased current densities of hydrogen charging and radiation damage of the steel result in irreversible hydrogen embrittlement. When the critical hydrogen content is exceeded, trap sides are formed. The given defects remain in the steel structure even after hydrogen release and are capable of initiating and propagating specimen failure. Hydrogen also contributes to crack growth by means of diffusion in the direction of the stress gradient, by which a specific local critical concentration of hydrogen in a crack front region can be reached.8'9 From this point of view, radiation-induceddefectscould play a significant part as sources for hydrogen diffusion to regions of stress concentrators, thus affecting the behaviour of defects in the RPV wall. In PWR operating conditions with the corrosion effect of the coolant, the influenceof hydrogen on the crack propagation process is determined mostly by hydrogen overpotential (in model conditions it is dependent on the current density of hydrogen charging) and by the degree of hydrogen trapping in stainless steel cladding and in the base material of the RPV. For cladding specimens irradiated for a long time in overheated water under PWR operating conditions, a hydrogen content