Formation of the rim structure in high burnup fuel

The process of grain subdivision or polygonization in UO2 was studied by using ion irradiation techniques and by investigating the rim structure in high burnup UO2 fuel. Ion irradiation with fission product ions caused first the formation of subgrain boundaries, observed in high resolution transmission electron microscopy, HRTEM. At higher doses, polygonization was completed at a well defined threshold. High burnup UO2 fuel showing well developed rim zones was investigated in optical, scanning and transmission electron microscopy. The oxygen potential Delta<(G)over bar(O-2), the hardness H and the fracture toughness K-Ic of the restructured material and the radial dependence of porosity, grain subdivision, H and K-Ic were measured. K-Ic was found to change at smaller radial positions than the other properties, indicating a nucleation process not affecting all properties to the same extent. The emerging picture for the formation of the rim structure in UO2 is discussed. (C) 1997 Elsevier Science B.V.