  Stochastic fluctuations of the neutron population within a nuclear reactor
are typically prevented by operating the core at a sufficient power, since a
deterministic behavior of the neutron population is required by automatic
safety systems to detect unwanted power excursions. Recent works however
pointed out that, under specific circumstances, non-Poissonian patterns could
affect neutron spatial distributions. This motivated an international program
to experimentally detect and characterize such fluctuations and correlations,
which took place in 2017 at the Rensselaer Polytechnic Institute Reactor
Critical Facility. The main findings of this program will indeed unveil
patchiness in snapshots of neutron spatial distributions -- obtained with a
dedicated numerical twin of the reactor -- that support this first experimental
characterization of the 'neutron clustering' phenomenon, while a stochastic
model based on reaction-diffusion processes and branching random walks will
reveal the key role played by the reactor intrinsic sources in understanding
neutron spatial correlations.
