The perturbations of the crystal lattice due to the displacement of atoms, that is point defects (intersti­tials, vacancies) and extended defects (defects clusters, dislocations, voids), contribute to the degradation of the thermal conductivity by scattering or limiting the mean free path of the phonons. This perturbation at the atomic scale is of the same nature as for soluble fission products and is also interpreted in terms of phonon scattering centers. Two aspects introduce uncertainties into the prediction of the effect of radia­tion damage: the concentration is difficult to calculate as a function of the irradiation conditions and the effect on the thermal conductivity is nonlinear (satu­ration occurs). Because of the large number of displa­cements produced by fission, the point defect concentration is expected to saturate early, probably before a burnup of 1 MWd kg HM~ , the saturation level depending on temperature. Supplementary radi­ation damage of a dynamical nature is present only in pile because of the fission spikes and its concentration depends on the fission rate; it disappears immediately when irradiation stops.

2.17.2.2.2 Stoichiometry

Stoichiometry has a major effect on the thermal conductivity of fresh fuels.19 UO2 is generally stoi­chiometric when introduced in pile and the O/U ratio of UO2 irradiated under LWR conditions remains close to 2.00 up to a burnup of 100 MWd kg HM~120-22 This parameter is, however, difficult to define for an irradiated fuel because of the complex chemical composition. Also, the effect of the oxygen defects due to the nonstoichiometry can be expected to be reduced because of the large number of oxygen defects created by irradiation.

2.17.2.2.3 Additives in UO2 (Pu, Gd, Cr)

The addition of Pu, Gd, or Cr to UO2 reduces the thermal conductivity of the fresh fuel. These addi­tives may form solid solutions with UO2 or be present as precipitates. The perturbation decreases with burnup because of the other burnup effects, as observed for (U, Gd)O2 by Sonoda et a/.,23 for (U, Pu) O2 by Fujii et a/.24 and Staicu et a/.,25 and for Cr by Caillot et a/.26