  The neutron population in a nuclear reactor is subject to fluctuations in
time and in space due to the competition of diffusion by scattering, births by
fission events, and deaths by absorptions. As such, fission chains provide a
prototype model for the study of spatial clustering phenomena. In order for the
reactor to be operated in stationary conditions at the critical point, the
population of prompt neutrons instantaneously emitted at fission must be in
equilibrium with the much smaller population of delayed neutrons, emitted after
a Poissonian time by nuclear decay of the fissioned nuclei. In this work, we
will show that the delayed neutrons, although representing a tiny fraction of
the total number of neutrons in the reactor, have actually a key impact on the
fluctuations, and their contribution is very effective in quenching the spatial
clustering.
