The use of iron phosphate glass as a wasteform is contingent both on its response to the addition
of waste products and on its evolution in response to radiation emitted by these waste products.
We perform molecular dynamics simulations of high-energy radiation damage in caesium iron
phosphate glasses to study the mobility of caesium, a nuclear decay product, in vitreous
wasteforms and their effect on the glass network. We simulate overlapping 70 keV cascades and
examine the structural and topological effects that caesium has on the iron phosphate glasses
before and after these cascades. We find that the glass network is substantially altered by the
presence of caesium as a potent network modifier and that radiation cascades produce
qualitatively different effects from those in pure iron phosphate glasses. Overlapping cascades
produce minimal effects on the mobility of caesium at low loading. At higher loading, the glass
network accommodates caesium atoms less well, particularly after irradiation. We explain this
in terms of caesium’s role as an excluded network modifier in comparison to iron, which is
tightly incorporated into the glass network.