Stochastic Analog Neutron Transport with TRIPOLI-4 and FREYA: Bayesian Uncertainty Quantification for Neutron Multiplicity Counting

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From nuclear safeguards to homeland security applications, the need for the better modeling of nuclear interactions has grown over the past decades. Current Monte Carlo radiation transport codes compute average quantities with great accuracy and performance; however, performance and averaging come at the price of limited interaction-by-interaction modeling. These codes often lack the capability of modeling interactions exactly: for a given collision, energy is not conserved, energies of emitted particles are uncorrelated, and multiplicities of prompt fission neutrons and photons are uncorrelated. Many modern applications require more exclusive quantities than averages, such as the fluctuations in certain observables (e.g., the neutron multiplicity) and correlations between neutrons and photons. In an effort to meet this need, the radiation transport Monte Carlo code TRIPOLI-4® was modified to provide a specific mode that models nuclear interactions in a full analog way, replicating as much as possible the underlying physical process. Furthermore, the computational model FREYA (Fission Reaction Event Yield Algorithm) was coupled with TRIPOLI-4 to model complete fission events. FREYA automatically includes fluctuations as well as correlations resulting from conservation of energy and momentum.Neutron multiplicity counting (NMC) exploits the correlated nature of fission chains and thus requires analog neutron transport. With the latest analog neutron transport developments in TRIPOLI-4, we show that NMC can now be properly simulated by reconstructing the mass and multiplication of two objects by analyzing the measured signal from 3He tubes in a well counter.