Fig. 1: Schematic cross-sectional view of the spent nuclear fuel microstructure.

a Magenta and yellow elements are metallic precipitates formed by fission products, whereas green elements are bubbles or pores formed through fission gas release such as Xe, Kr, I. The fission products can segregate in the crack and decorate both the surfaces of the grain boundaries and grain interiors, resulting in further intragranular/intergranular bubble swelling. The grain restructuring (subdivision) occurs at the transition region and fully develops into submicron grains toward the fuel rim. b Selected EBSD scans in a fuel from the Limerick NPP are provided as an example to demonstrate the grain structure at five different radial positions. The transition region has a combination of both large and submicron grains, and the rim indicates the fully developed HBS.