Once the fuel-clad gap is closed, or nearly so, in oxide fuel pins with zirconium alloy cladding, chemical reactions can occur between the oxygen that has migrated towards the pellet outer radius (see 14.2.10) and the cladding material. This leads to fuel-cladding bonding. The bond is primarily composed of zirconium oxide, but there are also some fission product compounds and (given a tin-bearing zirconium alloy) tin oxide present. A fuel-cladding bond also forms in fast reactor oxide fuel, but in this case the bond (often known as JOG from the French term joint oxyde-gaine) is primarily composed of fission products.

Stress-corrosion cracking (SCC) is an important cladding failure mechanism for zirconium alloy clad fuel during power transients in which strong pellet­cladding mechanical interaction occurs. The necessary requirements for failure are a sufficiently high clad stress, a sufficiently high concentration of corrosive fission product species at the clad inner wall, and sufficient time for crack initiation and crack propagation. Experimental results point to free iodine being the primary corrosive species. Thermodynamic equilibrium considerations suggest that all released iodine would be in the form of caesium iodide, and therefore that there would be no free iodine available for corrosion, and no SCC. The fact that this is not observed shows that radiolysis is crucial for formation of free iodine, and hence for SCC. Stress-corrosion cracking of Zircaloy cladding materials was the subject of an IAEA coordinated research project. More details of this phenomenon can therefore be found in the project report (IAEA, 2000a).