Elastic collisions of fast neutrons with metal atoms in the cladding knock the metal atoms from their lattice sites. The result of this irradiation damage is concentrations of vacancies and interstitials (‘point defects’) in the cladding crystal structure, which are well above those due to thermal effects. The irradiation damage causes hardening (a higher resistance to plastic deformation) and embrittlement (loss of ductility) of the cladding. Additional hardening and embrittlement can be caused by: precipitation, and subsequent growth due to diffusion of helium bubbles; hydrogen pick-up from the coolant (in water-cooled reactors only); diffusion of oxygen from the cladding oxide layer into the cladding metal (in water-cooled reactors only, and only during accident conditions where high clad temperatures are achieved); and selective dissolution of cladding constituents (see 14.2.13). The helium is formed by neutron capture of the cladding alloy constituents and impurities. The hydrogen picked up is primarily created by the chemical reaction of the cladding and the coolant. Diffusion of the point defects can lead to recombination, absorption at sinks (dislocations, grain boundaries and surfaces), formation of two-dimensional dislocation loops, or, in the case of vacancies, formation of three-dimensional clusters known as voids. Nucleated voids can themselves subsequently act as vacancy sinks — the result is void swelling of the cladding.

In reactors with zirconium alloy clad fuel, the combination of the manufacturing process and the hexagonal close-packed structure of zirconium leads to an anisotropic crystal structure (the cladding is said to have ‘texture’), with the basal planes of the unit cells tending to orient in the axial direction. Since dislocation loops composed of interstitials are more favourably formed along the basal planes, while dislocation loops composed of vacancies are more favourably formed normal to this direction, the lattice expansion due to the interstitial loops and the lattice contraction due to the vacancy loops tend to occur in different directions. The net result is an elongation of the cladding such that the cladding volume is maintained constant. In the case of PWR fuel assemblies, this axial growth is also exhibited by the guide tubes, which can lead to deformation of the fuel assembly. (One consequence of this is the operational fault condition known as incomplete rod insertion, whereby bowing of the guide tubes prevents full insertion of control rods.)

Hydrogen pick-up is significant in zirconium alloy clad fuel (~16% for Zircaloy-4 in an LWR (Kaczorowski et al.. 2008)). The hydrogen is primarily generated by the chemical reaction of zirconium and water, i. e. Zr + 2H2O ^ ZrO2 + 2H2 . In the case of fresh cladding and cladding irradiated to moderate burnups, the hydrogen levels in the cladding are low and the hydrogen is generally in the form of a solid solution. However, at higher burnups clad hydrogen contents are greater, and significant quantities of hydrogen can precipitate out as zirconium hydride platelets, which are brittle. Both void swelling and helium generation are negligible in zirconium alloy cladding.

I n reactors with stainless steel clad fuel, the cladding crystal structure is isotropic, so dislocation loops are randomly oriented and there is no axial growth (neither is there radial or circumferential growth). However, in fast reactors the high flux of fast neutrons leads to extensive nucleation of voids, and the clad temperatures are high enough that significant diffusion of vacancies can occur. Thus, void swelling is generally significant (there is a strong dependence on steel type — the austenitic steels tending to swell more than the ferritic or ferritic/martensitic varieties — and the manufacturing process, in particular the amount of cold work). In fact, void swelling is such that it is often the limiting phenomenon with respect to the fast neutron dose that can be accumulated in a fast reactor. Generation of helium via neutron capture is also significant in stainless steel cladding. This is mainly due to thermal neutron capture of nickel and boron, and to fast neutron capture of iron, chromium, nickel, boron and nitrogen. The former dominates in thermal reactors (principally AGRs), while the latter dominates in fast reactors. The result is hardening and embrittlement of the cladding. Hydrogen pick-up is negligible in stainless steel cladding.