The standard cladding material in a LWR is a dilute zirconium-base alloy containing some other elements such as tin, niobium, iron, nickel, chro­mium and oxygen (Zircaloy-2 for BWR and Zircaloy-4 for PWR). Being a hexagonally close packed (HCP) crystal structure and hence inherently anisotropic, zirconium acquires further anisotropic properties after fabri­cation due to induced texture (Fig. 1.25b) with the <c> axis of the HCP crystal oriented at ~30° from the radial direction of the tube. Minor modi­fications in the chemistry of the alloys are made to reduce the water side corrosion in the clad tubes. Formations of inter-metallic precipitates (which increase the corrosion rate) are avoided by giving the clad material a beta quench (fast cooling from the beta phase). The fuel and the pressure bound­ary (clad tube) experience time-related ageing and degradation; the former may affect linear power rating while the latter can lead to catastrophic clad failure. The burning of the fuel leads to release of fission gases and to fuel swelling. The fuel makes contact with clad which has picked up hydrogen from the coolant and leads to degradation of the clad tube.7 0 In order to improve the structural rigidity, spacer grids made of Inconel/Zircaloy are placed at definite intervals along the length of the assembly which holds the fuel elements in the assembly with spring forces. When these forces relax (due to creep) a gap is created between the grid and rod, and the rod can vibrate. This fretting may result in a breach in the clad integrity.71