Abstract
Convenient access to accurate nuclear data, particularly data describing low-energy neutrons, is crucial for trustworthy simulations of thermal nuclear systems. Obtaining the scattering kernel for thermal neutrons (i.e., neutrons with energy ~1 eV or less) can be a difficult problem, since the neutron energy is not sufficient to break molecular bonds, and thus the neutrons must often interact with a much larger structure. The “scattering law” S(α; β), which is a function of unitless momentum α and energy β transfer, is used to relate the material’s phonon frequency distribution to the scattering kernel. LEAPR (a module of NJOY) and GASKET are two nuclear data processing codes that can be used to prepare the scattering law and use different approaches to approximate the same equations. LEAPR uses the “phonon expansion method” which involves iterative convolution. Iteratively solving convolution integrals is an expensive calculation to perform (to ease this calculation, LEAPR uses trapezoidal integration for the convolution). GASKET uses a more direct approach that, while avoiding the iterative convolutions, can become numerically unstable for some α; β combinations. When both methods are properly converged, they tend to agree quite well. The agreement and departure from agreement is presented here.
Highlights
The accuracy of a nuclear system simulation is highly dependent on the nuclear data available
The “scattering law” S(α, β), which is a function of unitless momentum α and energy β transfer, is used to relate the material’s phonon frequency distribution to the scattering kernel
The quality of thermal neutron scattering data greatly impacts the reliability of thermal reactor analysis and safety margin calculations, due to the important role that thermal neutrons play in many reactor designs
Summary
The accuracy of a nuclear system simulation is highly dependent on the nuclear data available. The quality of thermal neutron scattering data greatly impacts the reliability of thermal reactor analysis and safety margin calculations, due to the important role that thermal neutrons play in many reactor designs. Thermal neutrons have energy on the order of ∼1 eV, which is not sufficient to break molecular bonds. A neutron that hits an atom effectively scatters from a much larger target. Thermal neutron wavelengths are on the order of interatomic spacing, which means neutrons can exist atop multiple nuclei at once, further complicating these interactions [1]. Thermal scattering behavior is represented by use of the scattering law ∗ S(α, β), which is a function of unitless momentum exchange α and unitless energy exchange β. It can be used to calculate the double-differential scattering cross section,
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