The self-assembly of uranyl-peroxide nanocapsules in aqueous solution is unique in uranium chemistry and has potential applications in the fabrication and reprocessing of actinide-based materials. We present the first study of these species in aqueous solution by means of classical molecular dynamics simulations. To this end, we parametrized a uranyl-peroxide force field from interaction energies computed with second order Møller-Plesset perturbation theory and fit to a Born-Huggins-Mayer potential. Bonded parameters were fit from density functional theory calculations. The solvent and counterion structures surrounding four different systems ([(UVIO2)]2+, [(UVIO2)2(μ2-O2)]2+, [(UVIO2)5(μ2-O2)5], and [(UVIO2)20(μ2-O2)30]20-) were studied in aqueous solution. The largest studied system is predicted to encapsulate an ice-like water cluster.