Think of the atoms in the crystal lattice of the glass as marbles in the holes of a Chinese checkers board. Radiation has caused one of the atoms (marbles) to be knocked out of its hole. A UV photon can push the atom (marble) back into its hole.
Even in a throium glass the majority of atoms in the glass lattice are either silicon or oxygen atoms. For simplicity I say atoms but in actuality they are present as ions in the lattice.