Patchy nuclear chain reactions

TL;DR

This paper reports the first experimental detection and characterization of neutron clustering phenomena in nuclear reactors, revealing patchy neutron distributions influenced by intrinsic sources, using a combination of experiments and stochastic modeling.

Contribution

It introduces the first experimental evidence of neutron clustering in reactors and develops a stochastic reaction-diffusion model to explain the phenomenon.

Findings

Detection of patchy neutron spatial distributions

Identification of neutron clustering as influenced by intrinsic sources

Development of a stochastic model explaining neutron correlations

Abstract

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.