High-burn-up behaviour of the Rapsodie Fortissimo standard fuel

The paper presents the results of the destructive testing of an entire fuel bundle consisting of 61 pins from the Rapsodie Fortissimo reactor which were irradiated to 10 at.%. A series of micrographic sections were cut across the bundle at linear ratings ranging from 290 to 400 W/cm. The temperature distributions evaluated at a given distance from the extremes of the column-grain zones are remarkably similar in the different pins (mean deviation approximately 250C) whereas the diameters of the central hole show a marked spread. These dispersions are clearly greater than those expected from fabrication data range for the fuel. They are shown to be due to several factors, namely random radial distribution of clearance in the cracks, swelling of the cladding steel and variable oxide sintering in the reactor. Pellet sintering in the reactor and cladding swelling result in surface superheating of the oxide and in continuous development of structures. However, the central temperatures are kept low - lower than their value at the beginning of their life span. (author)