In LWRs (in particular PWRs), the relatively high differential pressure across the cladding wall (due to the high coolant pressure and the low pin fill pressure) in the first year or two of irradiation induces a compressive stress on the cladding, which causes the cladding to creep inwards. The resulting ‘clad creepdown’ leads to reduction of the pellet-cladding gap and, consequently, a reduction in fuel centreline temperatures. The creep is primarily irradiation induced, since the clad temperatures are not high enough for thermal creep to be significant.

14.2.4 Oxygen migration

In thermal reactor oxide fuel, the fuel temperatures are relatively low and the as-manufactured oxygen/metal molar ratio (O/M) is close to 2.00, i. e. the material is stoichiometric. Oxygen migration is then negligible. In contrast, in fast reactor oxide fuel, the fuel temperatures are relatively high and the as-manufactured O/M is usually less than two (typically in the range 1.97 to 1.99), i. e. the material is hypostoichiometric. Oxygen migration then rapidly occurs, i. e. as soon as the fuel is heated up at the start of irradiation. The oxygen migrates down the large fuel pellet radial temperature gradient, such that the local O/M increases in transitioning from the pellet centreline to the pellet rim, with the as-manufactured O/M maintained on a pellet average basis (Bailly et al., 1999). This is important because local fuel properties, including thermal conductivity, creep and fission product diffusion coefficients, are strongly dependent on local O/M — in particular, regions that have become more hypostoichiometric have a degraded conductivity, while regions that have become more stoichiometric (with an O/M closer to two) have an improved conductivity. There is some uncertainty over the dominant mechanism, or mechanisms, for the oxygen migration, which may be complex (Olander, 1976) — thermal diffusion (also known as the Soret effect) in either the solid or gaseous phase appears most likely.