Here, P i m p a c t is the probability that a particular neutron will strike a fuel nucleus, P f i s s i o n is the probability that the neutron, having struck the fuel, will cause that nucleus to undergo fission, P a b s o r b is the probability that it will be absorbed by something other than fuel, and P e s c a p e is the probability that it will "escape" by leaving the core altogether. n a v g is the number of neutrons produced, on average, by a fission event—it is between 2 and 3 for both U and Pu (e.g., for thermal neutrons in U, n a v g = 2.4355 ± 0.0023 ).