High magnification SEM observations for two types of granularity in a high burnup PWR fuel rim

Discussion About HBS Transformation in High Burn-Up Fuels

“…The generation of free surfaces allows partial relaxation of the local energy through a surface undulation induced by surface diffusion of point defects. This surface diffusion leads to the formation on the pore surface of 50 to 100 nm "round grains" as reported by N.Lozanno [12] et al Looking closer a minimum of three subdivision levels can be observed, as shown on figure 1. Bubbles tend then to become quasi-spherical due to the internal gas pressurization (figure 15, step C), achieving at the end micrometric sizes.…”

“…It is considered that the round sub-grains (50 to 150 nm) observed on the bubble inner surface is related to a surface relaxation mechanism as suggested by N.Lozanno [12]. Polygonisation is a bulk mechanism leading to polyhedral grains 200 to 600 nm.…”

Detailed characterisations of high burn-up structures in oxide fuels

“…He suggested that it could be U 4 O 9 . The nature of these defects was discussed in [3] and linked to the planar defects observed with TEM [14,15]. It was then suggested that these defects could form the boundaries of the polyhedral sub-grains.…”

“…The first of these evaluations consists in using the total HBS porosity measured by SEM with the xenon EPMA measurements. This method was first applied to UO 2 -Zr4 (2) -as exposed in [3] -before being applied to other fuels including MOX fuel. The aim of this evaluation is to obtain an underestimation of the amount of gas inside the HBS bubbles by systematically over-estimating all parameters resulting in a low pressure:…”

“…Demonstrating the presence of two types of sub-grains in the HBS; round around the bubbles, and polyhedral elsewhere [3]. It also gave porosity values, discussed the nature of planar defects observed near HBS areas to conclude that such defects might play a role in the formation of polyhedral sub-grains or bubbles.…”