Detailed characterisation of the rim microstructure in PWR fuels in the burn-up range 40-67 CWd/tM.

Fuel porosity, volume pore density and pore size distributions were determined as a function of the radial position for specimens with average burn-ups between 40 and 67 GWd/tM. in the rim region, the porosity and pore density increase exponentially towards the pellet edge, whereas the mean pore size remains constant. Accompanying the porosity increase, a band of recrystallised material (submicron-grains) is formed at the pellet edge, followed by a wide transition zone where porous grain-refined areas are mixed with the original matrix grains. Individual pores were found associated with recrystallised surroundings, extending up to a distance of 1 to 3 submicron grains from the pore surface. Accordingly, a representation of the rim material with localised recrystallisation around pores is proposed. As determined by microindentation, the mechanical strength of the rim material appeared to be increased with respect to unirradiated UO2. This improvement is supposed to be caused by grain refinement.