Nuclear reactors are engineered to reduce <math>P_{escape}</math> and <math>P_{absorb}</math>. Small, compact structures reduce the probability of direct escape by minimizing the [[surface area]] of the core, and some materials (such as [[graphite]]) can [[Reflection (physics)|reflect]] some neutrons back into the core, further reducing <math>P_{escape}</math>.
The probability of fission, <math>P_{fission}</math>, depends on the nuclear physics of the fuel, and is often expressed as a [[cross section (physics)|cross section]].