Comment by dontwearitout

They certainly can, see https://en.wikipedia.org/wiki/Resonance_escape_probability. The all-important number in nuclear reactions is "k", the average number of child neutrons a single neutron will produce. The neutron population follows an equation something like N = exp((k - 1) * t). For k<1, you get exponential decay, and for k>1, you get exponential growth (until everything becomes a plasma and k changes). Check out https://en.wikipedia.org/wiki/Six_factor_formula.