1. A computer simulator implemented method of modeling a fissile system in a neutronics modeling method comprising: inputting, into the computer simulator, neutronics characteristics of at least one first geometric construct; inputting, into the computer simulator, an indication that the at least one first geometric construct completely fills a second geometric construct; performing, by the computer simulator, a criticality analysis of the fissile system including the first and second geometric constructs, the performing including defining at least one neutronic characteristic of the second geometric construct as if the second geometric construct were completely filled with a plurality of the first geometric constructs without inputting the at least one neutronic characteristic of the second geometric construct; and outputting the criticality analysis of the fissile system including the first and the second geometric constructs.