Project title: Cyclic loading and delayed hydride cracking in Zr-alloys
Supervisor: Professor Adrian Sutton, Dr Daniel Balint, Mark Wenman & Rolls Royce
The Zr-alloy fuel cladding in fission reactors has to safely withstand thermomechanical cycles in a corrosive environment. On one side the hydrogen from the corrosion reaction accumulates ahead of stress raisers and forms hydrides, which break and form cracks at whose tip new hydrides form because of the associated tensile strain field. This is known as delayed hydride cracking (DHC). On the other side though, the cyclic plastic strain leads to the development of slip bands and to a complex network of dislocations, to which completely different failure mechanisms are associated.
The interplay between fatigue and DHC is manifold. Among other things, the dislocations resulting from fatigue are sinks of hydrogen and are likely to interfere with conventional DHC, while at the same time their interactions are altered by the hydrogen Cottrell atmospheres which are not considered in conventional fatigue models. Moreover, the surface roughening arising from cyclic slip may be the site for DHC nucleation, and the network of hydrides interacts with the fatigue slip bands as well. The model should finally take also radiation damage into consideration, bringing its characteristic population of defects into the picture.
On top of this, the hydrides are hierarchically organised. Macrohydrides spanning the micron to millimetre size are made of stacked platelets hundreds of nanometres in thickness. The effect of atomic diffusion of hydrogen and dislocation slip mechanisms is propagated up to crack nucleating at macroscopic notches. Considering also that the fatigue models have to guarantee a life expectation of several years shows that this problem is inherently multiscale both in space and in time.
This project is being carried out under the industrial supervision of Rolls-Royce. Its ultimate goal is to understand to what extent the cyclic loading might influence their current modelling of DHC, and develop a mechanistic understanding of how all the different characters at play may threaten the structural integrity of the cladding.