Abstract <p>Calculated and experimental periods of thermal autoignition delay τ are compared for samples of NEPE-type propellant with characteristic dimensions of 20–100 mm. Experimental data obtained in the isoperibol mode are taken from literary sources. Calculations are made using a model chain reaction of the thermal decomposition of nitroester plasticizers in the composition of NEPE. Calculated values τ depend weakly on the size of the samples, but they are considerably lower (by several times for small samples) than those obtained in experiments. The reasons for the discrepancy between the calculated and experimental periods of autoignition delay are shown to be the loss of active particles (products of the decomposition nitroesters, primarily NO₂) due both to interaction with stabilizers and their leaking into the environment because of poor packaging. The role of such leaking is considered by solving an equation of diffusion that allows for the generation, reproduction and death of active particles.</p>