As noted before, radiation exposure generally leads to increased rates of corrosion and oxidation. Three basic effects on materials such as zircaloys are (i) oxida — tion/corrosion, (ii) hydriding, and (iii) stress corrosion cracking and corrosion fatigue.

Oxidation

In LWRs, radiolytic decomposition of water can lead to the creation of various free radicals (such as hydrogen, oxygen, hydroxyl radicals, and hydrogen peroxide), which may accelerate the corrosion effect. Some of the reactions occurring in water during radiolysis are given below:

Figure 6.42 shows the schematic representation of zircaloy corrosion in the tem­perature range of 260-400 °C. In zircaloys, ZrO2 forms as protective layer following a cubic rate law, meaning that oxidation rate decreases as ZrO2 layer thickens. However, after a certain point (breakaway transition), the oxidation rate (as meas­ured in terms of weight gain) becomes constant following a linear rate law, as shown in Figure 6.42. The breakaway transition occurs due to the destruction of the ZrO2 film. It is thought that polymorphic phase transformation of ZrO2 takes place as the stress develops in the ZrO2 film as the thickening continues. This poly­morphic phase transformation leads to sudden volume expansion, thus resulting in

cracking/flaking of the protective film (Figure 6.43). This also means that the lon­ger the breakaway time, the better would be the component sustaining the chemi­cal environment. Recently, research has shown that Nb-containing zirconium alloys (like Zirlo) has greater longer term corrosion resistance in LWR environment.

Generally, materials chosen to serve in nuclear reactors should have the ability to form self-healing, passivating oxide films. But prior experience has shown that it is quite difficult to maintain the passivating oxide film in the long term in a reactor environment. Various flaws can be introduced as a result of stress and irradiation, which may turn into crack initiation sites. Hence, both uniform corrosion and nod­ular corrosion have been observed to be an issue. Evidence of nodular corrosion on zircaloy fuel cladding tube is shown in Figure 6.44.