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Many astrochemical models today explicitly consider the species that comprisethe bulk of interstellar dust grain ice-mantles separately from those in thetop few monolayers. Bombardment of these ices by ionizing radiation - whetherin the form of cosmic rays, stellar winds, or radionuclide emission -represents an astrochemically viable means of driving a rich chemistry even inthe bulk of the ice-mantle, now supported by a large body of work in laboratoryastrophysics. In this study, using an existing rate equation-basedastrochemical code modified to include a method of considering radiationchemistry recently developed by us, we attempted to simulate two such studiesin which (a) pure O$_2$ ice at 5 K and, (b) pure H$_2$O ice at 16 K and 77 K,were bombarded by keV H$^+$ ions.  Our aims are twofold: (1) to test the capability of our newly developedmethod to replicate the results of ice-irradiation experiments, and (2) todetermine in such a well-constrained system how bulk chemistry is best handledusing the same gas-grain codes that are used to model the interstellar medium(ISM). We find that our modified astrochemical model is able to reproduce boththe abundance of O$_3$ in the 5 K pure O$_2$ ice, as well as both the abundanceof H$_2$O$_2$ in the 16 K water ice and the previously noted decrease ofhydrogen peroxide at higher temperatures. However, these results require theassumption that radicals and other reactive species produced via radiolysisreact quickly and non-diffusively with neighbors in the ice.

On Simulating the Proton-Irradiation of O$_2$ and H$_2$O Ices Using Astrochemical-type Models, with Implications for Bulk Reactivity