Thermal localization leads to reaction initiation in granular materials. Observations show that it occurs in the vicinity of large pores and, thus, depends on a material's microstructure. Since the spatial variability of the latter cannot be ascertained in all its relevant details, we treat the material’s initial porosity as a random field. The so-called "hotspots" are then modeled as rare events in a complex nonlinear dynamical system. Their probability of occurrence is quantified by the tails of the distributions of the temperature and the corresponding reaction rate. These are computed via Monte Carlo simulations of a two-phase five-equation dynamic compaction model, which are supplemented with a mesoscale model of the thermal localization at the solid-gas interface. Our results demonstrate a strong nonlinear dependence of the probability of hotspot initiation on the variance of the initial porosity.