Exposure of vitreous silica in a nuclear reactor is shown to introduce ionization expandability which is superimposed upon the ionization contractibility of ordinary vitreous silica and upon the ionization expandability associated with certain impurities. The ionization expandability developed in pile exposure consists of two parts. The principal part requires ∼104eV of ionization for a volume increase of an oxygen ionic volume (O.V.) and can exceed 5×1019O.V./cm3. Capacity for an earlier phase of ionization expansion, pre‐evident expansion, grows on pile exposure to a maximum ∼5×1018O.V./cm3, requires ∼32 eV/O.V., and declines when electron irradiation is intermitted. Accompanying the reversion is a recuperation phenomenon resulting in an increase in principle expansion on resumption of irradiation. The recuperation phenomenon is explained as caused by a redistribution of bond strain in the reversion of the pre‐evident expansion. The supervenient expansion observed after maximum compaction during pile exposure is explained as predominantly associated with the slowing down of atoms scattered by fast neutrons (also the cause of the original compaction) and indicates the existence of structure in the microregion being compacted.