There are three possible mechanisms for clad wall thinning due to the action of the coolant: oxidation (i. e. the chemical reaction of the cladding material with an oxygen-bearing coolant, or with oxygen dissolved in the coolant), erosion (i. e. the wearing away of the metal due to the forces induced by the flowing coolant) and dissolution (i. e. leaching of the cladding material). Oxidation and dissolution are also commonly known as corrosion. Excessive wall thinning can lead to failure of the cladding.

Which of the three wall thinning mechanisms dominates depends upon the coolant type, the cladding material, the coolant dissolved oxygen concentration, the clad surface temperature and the coolant velocity. Only oxidation is pertinent to light or heavy water coolants; dissolution is of primary importance in sodium; dissolution and erosion are both significant in lead or a lead-bismuth eutectic. Oxidation, erosion and dissolution can all potentially be reduced or eliminated by applying a suitable coating to the cladding surface, or by surface treatment. Erosion can also be prevented by limiting the coolant velocity. If selective dissolution of cladding constituents occurs the cladding may become embrittled.

The extent of corrosion is very much dependent upon the cladding material and the coolant type. The two most common combinations are austenitic steel cladding in sodium, and zirconium alloy cladding in water. The corrosion is generally limited in the case of the former, with 20 to 30 ^m of the cladding wall dissolved away (Bailly et al., 1999). In the case of the latter, the extent is more variable and is dependent upon the exact cladding alloy. With respect to Zircaloy-2 and Zircaloy-4, the corrosion can be considerable, with an oxide thickness of ~100 ^m (above this thickness oxide spalling tends to occur), although on average the oxide thickness tends to be significantly less than this — approximately 50 ^m for a PWR Zircaloy-4 clad pin with a typical pin average burnup of 45 MWd/kgU (Chapin et al., 2009). The oxide thicknesses are generally significantly less with more modern cladding alloys. Corrosion tends to be uniform in PWRs and CANDU reactors, but may be nodular in BWRs.

The clad corrosion considered above pertains to corrosion of the outside of the cladding by the coolant. However, corrosion of the inside of the cladding may also occur due to chemical reactions between the cladding, the free oxygen in the fuel (liberated by fission in oxide fuel), and the fission products. The potential for corrosion by free oxygen in oxide fuel is enhanced by oxygen migration (see 14.2.10). In the case of LWR fuels, where discharge burnups are moderate, clad temperatures are relatively low and a zirconium alloy cladding is used, oxidation of the clad inner wall is limited. However, for fast reactor fuels, where burnups and clad temperatures are considerably higher and a steel cladding is used, corrosion of the clad inner wall is more extensive.

There are three distinct types of clad internal corrosion that occur in fast reactors: early-in-life, ROG and RIFF (Bailly et al., 1999). Early-in-life corrosion is localised and occurs only at high powers in the first ten days of irradiation, but can cause clad penetration. It is caused by free iodine and tellurium (which after ten days have mostly reacted with rubidium and caesium to form non-corrosive compounds). This type of corrosion can be prevented by limiting the pin powers during the first ten days of irradiation. ROG corrosion (named after the French term — reaction oxyde gaine) is a general fuel-cladding reaction, and is the main corrosion type of interest. RIFF corrosion (named after the French term — reaction a I’interface fissile-fertile) can occur at the interface between the fissile and fertile fuel in a pin with axial blankets. Both are due to complex chemical reactions between the free oxygen in the fuel, the fission products (principally tellurium) and the metallic elements in the cladding (principally iron, chromium and nickel). The ROG corrosion rate is mainly dependent upon burnup and cladding strain. RIFF corrosion is less well understood.