  We studied the microstructural and helium bubbling evolutions of Gd2Zr2O7
waste form with immobilized TRPO (50 wt%) under multi-energy He ion
irradiation. Three structurally heterogeneous regions for the Gd2Zr2O7 waste
form were found as a function of the depth from the He-irradiated surface.
Specifically, at a depth less than 40 nm below the He-irradiated surface
(Region I) the Gd2Zr2O7 waste form is completely amorphous with large spherical
He bubbles (5-25 nm). In the intermediate region, Region II, (40-800 nm)
partially amorphized Gd2Zr2O7 waste form accompanied with ribbon-like He
bubbles that may lead to the formation of microcracks is observed. The
crystallinity is not impacted in Region III for a depth of more than 800 nm.
For the first time, we elucidated that the Gd2Zr2O7 waste form, which was
considered to be structurally intact at 100 dpa, is completely amorphized at
6.5 dpa with the synergistic displacement damage, electronic energy loss, and
He concentration enabled. This study leads to new physical insights into
amorphization and He bubbles formation mechanisms of Gd2Zr2O7 waste form under
multi-energy He irradiation, which is essential for the design and optimization
of irradiation-resistant ceramic waste matrices.
