Lattice parameter changes associated with the rim-structure formation in high burn-up UO2 fuels by micro X-ray diffraction

Radial variations of the lattice parameter and peak width of two high burn- up UO2-fuels (67 and 80 GWd/tM) were measured by a specially developed micr o-X-ray diffraction technique, allowing spectra acquisition with 30 mu m sp atial resolution. The results showed a significant but constant peak broade ning, and a lattice parameter that increased towards the pellet edge and de creased again within the rim-zone. This lattice contraction coincided with other property changes in the rim region, i.e., porosity increase, hardness decrease and Xe depletion. In terms of local burn-ups, the lattice contrac tion followed the rate of the matrix Xe depletion measured by EMPA, exceedi ng greatly the contraction rate due to dissolved fission products. The obse rved behaviour can be equally explained by a saturation of single interstit ials with subsequent recombination with excess vacancies, as by the saturat ion and enlargement of dislocation loops. The concentration and sizes of de fects involved and their possible relation to the rim structure formation a re discussed. (C) 2000 Elsevier Science B.V. All rights reserved.