Year
2017
Abstract
From nuclear materials accountability to detection of special nuclear materials, the need for better modeling of fissions has grown over the past decades. Conventional radiation transport codes compute average quantities with great accuracy and performance, but performance and averaging comes at the price of limited interaction-by-interaction modeling. For fission applications, these codes often lack the capability of modeling interactions exactly: energy is not conserved, energies of emitted particles are uncorrelated, prompt fission neutron and gamma multiplicities 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. The computational model FREYA (Fission Yield Event Yield Algorithm) aims to meet this need by developing models that include the treatment of fluctuations and any correlation resulting from conservation of energy and momentum. FREYA is based on modeling and sampling complete fission events. FREYA 2.0 has been integrated into the LLNL fission library 2.0, which has become an integral part of MCNP6.2R, Geant4.10 and TRIPOLI-4.10R. The fluctuations and correlations introduced by FREYA into Monte Carlo simulations lead to significant differences that can be measured experimentally. Measurements of two-neutron angular correlations were carried out using scintillators for the two spontaneous fission sources 252Cf and 240Pu. These experiments were modeled and simulated using Monte Carlo to tune and validate FREYA 2.0. The first part of this paper will focus on recent developments in FREYA 2.0, the second part on the experimental measurements, the third part will concentrate on results of Monte Carlo simulations and comparisons to experimental data.