Fast Rossi-alpha Measurements of Plutonium using Organic Scintillators

In this work, Rossi-alpha measurements were simultaneously performed with a $^3$He-based detection system and an organic scintillator-based detection system. The assembly is 15 kg of plutonium (93 wt$\%$ $^{239}$Pu) reflected by copper and moderated by lead. The goal of Rossi-alpha measurements is to estimate the prompt neutron decay constant, alpha. Simulations estimate $k_\text{eff}$ = 0.624 and $\alpha$ = 52.3 $\pm$ 2.5 ns for the measured assembly. The organic scintillator system estimated $\alpha$ = 47.4 $\pm$ 2.0 ns, having a 9.37$\%$ error (though the 1.09 standard deviation confidence intervals overlapped). The $^3$He system estimated $\alpha$ = 37 $\mu$s. The known slowing down time of the $^3$He system is 35-40 $\mu$s, which means the slowing down time dominates and obscures the prompt neutron decay constant. Subsequently, the organic scintillator system should be used for assemblies with alpha much less than 35 $\mu$s.Comment: PHYSOR 2020: Transition to a Scalable Nuclear Future Cambridge, United Kingdom, March 29th-April 2nd, 202

Patchy nuclear chain reactions

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