Formation of the rim structure in high burnup fuel

Abstract

The process of grain subdivision or polygonization in UO 2 was studied by using ion irradiation techniques and by investigating the rim structure in high burnup UO 2 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 UO 2 fuel showing well developed rim zones was investigated in optical, scanning and transmission electron microscopy. The oxygen potential Δ ¯G(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 UO 2 is discussed.