A restructuring of the UO2 matrix takes place at the pellet periphery in high burnup LWR fuel. The transformed microstructure is characterised by small (sub-micron) grains, which are depleted of fission gas, and a high density of spherical, inter­granular fission gas bubbles. Since the restructuring occurs at the pellet periphery, the resulting microstructure is often referred to as ‘rim structure’. However, the modified microstructure is a result of the local conditions at the pellet rim — in particular the high local burnup and low fuel temperature — rather than of the radial position per se. Hence, it is better referred to as ‘high burnup structure’, or HBS, as originally used by Lassmann et al. (1995).

The appearance of the HBS is reproduced in Fig. 14.2, which shows a scanning electron microscope (SEM) image of a fractured sample of fuel irradiated to a burnup of 73 MWd/kgU (Noirot et al., 2008). The image constitutes a radial scan, with the pellet surface at the right of the image. The rightmost boxed section is enlarged in Fig. 14.3.

The mechanism for high burnup structure formation is as follows. Resonant neutron capture in 238U causes a build-up of 239Pu near to the pellet surface (239U rapidly decays to 239Pu via 239Np) (Lassmann et al., 1994), which results in a high local fission density and burnup. At the low temperatures prevailing at the pellet surface, the resulting fission damage cannot be fully annealed. Eventually, the accumulated damage results in local recrystallisation, usually starting around the margins of as-fabricated porosity. The recrystallisation causes the xenon and

krypton fission gases to be precipitated into small, isolated bubbles. A very similar microstructural transformation is seen in plutonium-rich agglomerates in LWR (U, Pu)O2 fuel, for those agglomerates located in the cooler outer regions of the pellet.

The local burnup and fuel temperature ranges over which HBS formation occurs have been investigated in the High Burnup Rim Project (HBRP) (Kinoshita

et al. , 2004). The results show that the HBS begins to form at a local burnup of ~45 MWd/kgU, and is fully formed at a local burnup of ~70 MWd/kgU, but only at fuel temperatures below ~1100 °C.

The rim porosity increases the pellet volume and therefore enhances fuel swelling. The rim pores also reduce the thermal conductivity of the rim relative to the non-restructured fuel region. There are several possibilities for enhanced fission gas release associated with formation of the HBS: (a) gas is released from the rim region during the recrystallisation process; (b) athermal release is enhanced by the small size of the restructured grains; (c) gas is released from the rim pores; (d) the reduced thermal conductivity of the rim increases fuel temperatures in the non-restructured regions of the fuel pellets, causing enhanced thermal release. Research on the behaviour of the HBS is ongoing; the currently available results suggest that at most only a small fraction of gas generated in the rim region is released to the pin free volume (Bremier et al., 2000). Thus, only enhanced fission gas release due to (d) is thought to be significant.