Point defects do not tend to change the thermal expansion coefficient, which have been confirmed experimentally.

6.4

Radiation Effects on Corrosion Properties

The chemical environment found in the nuclear reactors is quite harsh as many of the electrochemically active metals/alloys constitute the materials used in nuclear reactors. Furthermore, existence of crevices allowing chemicals to collect can lead to crevice corrosion. Sometimes “crud” formation also causes problems; the word “Crud” stands for Chalk River Unidentified Deposit. “Crud” is the corrosion prod­uct that is created in the steam generator, piping, and reactor pressure vessel walls, get transported via the core thus acquiring induced radioactivity, and get deposited in various locations of the reactor primary system. These result in problem with heat transfer and exacerbation of corrosion issues. Stobbs and Swallow [36] explained the effect of radiation in terms of metal, protective layer, and environ­ment (corrodent).

6.4.1

Metal/Alloy

Radiation damage by generation of Frenkel defects, spikes, or transmutation can affect corrosion to some degree. Possible mechanisms are as follows:

a) Increased chemical activity: This effect becomes less important at higher temperatures.

b) Irradiation-induced dimensional changes: These may lead to enhanced corro­sion by cracking the surface film so that the surface underneath becomes exposed to the corrodent. Thermal cycling is more important than density changes.

c) Radiation-induced losses: Such losses in ductility accompanied by stressing may result in stress corrosion. This has been observed in control rod alloys of boron in steel, titanium, or zirconium.

d) Radiation-induced phase changes: Such changes (e. g., precipitation) may affect the corrosion behavior.

6.4.2

Protective Layer

Lattice defects introduced into the protective oxide layer may affect the corrosion rate in the metallic substrate.

a) Diffusion of anions or cations through the oxide layer — irradiation would increase the number of defects.

b) Activity changes in the oxide layers are a potential source of enhanced corrosion.

c) Phase changes are a possible source of accelerated corrosion. If irradiation influ­ences the transformation in a system such as ZrO2 (monoclinic to tetragonal), an increase in rate may occur.

6.4.3