One of the most unique aspects of material behaviour in a nuclear power plant is the effect of radiation (mainly neutrons) on the dimensional stabil­ity of the reactor components. In fast breeder reactors the Fe and Ni-based alloys creep and swell, that is, they change dimensions in response to a stress and change their volume in response to radiation damage. In LWRs, zirco­nium alloy structural components creep, do not swell, but do change their dimensions through the approximately constant volume process called irradiation growth. Radiation effects are not unexpected since during the lifetime of a typical component every atom is displaced from its normal lattice position at least 20 times (20 dpa). With the possible exception of elastic properties like Young’s Modulus, the properties needed for reliable fuel assembly performance are affected by irradiation. A summary of such effects is given by Adamson (2000).

Practical effects of dimensional instabilities are well known and it is rare that a technical conference in the reactor performance field does not include discussions on the topic. Because of the difference in pressure inside and outside the fuel rod, cladding creeps down on the fuel early in life, and then creeps out again later in life as the fuel begins to swell. A major issue is to have creep strength sufficient to resist outward movement of the cladding if fission gas pressure becomes high at high burnups. PWR guide tubes can creep downward or laterally due to forces imposed by fuel assembly hold down forces or cross flow hydraulic forces — both leading to assembly bow which can interfere with smooth control rod motion. BWR channels can creep out or budge in response to differential water pressures across the channel wall, again leading toward control blade interference. Fuel rods, water rods or boxes, guide tubes and tie rods can lengthen, possibly leading to bowing problems. (For reference, a recrystallized (RX or RXA) Zircaloy water rod or guide tube could lengthen due to irradiation growth more than 2 cm during service; a CWSR component could lengthen more than 6 cm.) Even RX spacer/grids could widen enough due to irradiation growth (if texture or heat treatment was not optimized) to cause uncomfortable interference with the channel. In addition, corrosion leading to hydrogen absorption in Zircaloy can contribute to component dimensional instability due, at least in part, to the fact that the volume of zirconium hydride is about 16% larger than zirconium.

The above discussion leads to the concept that understanding the mecha­nisms of dimensional instability in the aggressive environment of the nuclear core is important for more than just academic reasons. Reliability of materi­als and structure performance can depend on such understanding.

Comprehensive reviews of dimensional stability have been given in the ZIRAT Special Topical Reports (Adamson & Rudling, 2002; Adamson et al, 2009; Cox et al, 2005).The sources of dimensional changes of reactor com­ponents (in addition to changes caused by conventional thermal expansion and contraction) are: irradiation growth, irradiation creep, thermal creep, stress relaxation (which is a combination of thermal and irradiation creep), and hydrogen and hydride formation.

Irradiation effects are primarily related to the flow of irradiation-produced point defects to sinks such as grain boundaries, deformation-produced dis­locations, irradiation-produced dislocation loops, and alloying and impurity element complexes. In zirconium alloys, crystallographic and diffusional anisotropy are key elements in producing dimensional changes.

In the past, hydrogen effects have been considered to be additive to and independent of irradiation. Although this independency has yet to be defin­itively proven, it is certain that corrosion-produced hydrogen does cause significant dimensional changes simply due to the 16-17% difference in density between zirconium hydride and zirconium. A length change in the order of 0.20% can be induced by 1000 ppm hydrogen in an unirradiated material (Fig. 4.60) (King et al., 2002; Seibold et al., 2000).That the presence of hydrides contributes to the mechanisms of irradiation creep and growth is highly suspected but yet to be determined in detail.

Fuel rod diametral changes are caused by stress dependent creep pro­cesses. Fuel rod length changes are caused by several phenomena: [3]

4.60 Dimensional changes in unirradiated ZIRLO and Zircaloy-4 tubing and strip for different sample orientations as a function of hydrogen content. (Source: Reprinted, with permission, from King et al. (2002), copyright ASTM International, 100 Barr Harbor Drive, West Conshohocken, Pa 19428.)

heavily cold worked material, it has been reported that some shrinkage may occur. In a non-textured material such as SS, creep down of the cladding would only result in an increase in cladding thickness, with no change in length.

• Creep due to PCMI after hard contact between the cladding and fuel. This occurs in mid-life, depending on the cladding creep properties and the stability of the fuel.

• Hydriding of the cladding due to corrosion.

Bow of a component such as a BWR channel or PWR control rod assembly can occur if one side of the component changes length more than the other side. Such differential length changes occur due to differential stress and creep, to relaxation of differential residual stresses or to differential growth due to differences in flux-induced fluence, texture, material cold work and hydrogen content (and, although not usually present, differences in temper­ature or alloying content). This is described more in the ZIRAT10 Special Topics Report on Structural Behaviour of Fuel and Fuel Components (Cox et al, 2005).

The next section discusses the effect of irradiation on dimensional stability.

220 Materials’ ageing and degradation in light water reactors