Evolution of porosity in the high-burnup fuel structure

The evolution of the high-burnup structure (HBS) porosity is investigated. Electron probe microanalysis (EPMA) and scanning electron microscope (SEM) measurements of UO2 fuel with ≈105 GWd/tHM rod average burnup show the formation of an ultra-high burnup structure with a local burnup of 300 GWd/tHM in the proximity of the fuel–cladding interface. Such structure is characterized by gas pores of sizes up to 15 μm. A large population of pores with 3–5 μm pores is also observed in more inner regions of the HBS. An analysis of the pore size distributions indicates predominance of 3.5 μm and 7.5 μm pores. A simple model accounting for vacancy diffusion kinetics and coalescence is used to interpret the observations: the 3.5 μm pores are obtained by growth of 1 μm pores with an initial overpressurization of 50–70 MPa. The extra-large pores with diameters ≈7–8 μm result by coalescence of the intermediate size pores, assuming that: (1) such pores are only slightly overpressurized and that (2) the coalescence process occurs at constant porosity, as observed experimentally.