Overview

Simulations of nuclear reactors require that neutron scattering distributions be provided. These distributions describe how the energy and trajectory of neutrons change due to a collision with material in the reactor.

If a neutron has sufficiently high energy prior to a scattering event then we may neglect the molecular bonds of the target atom, and thus assume that the target atom was free and unbound. Thermal neutrons are defined as those for which this assumption is no longer valid, and have energy of a few eV or less.

The LEAPR module of NJOY is to prepare thermal neutron scattering data for use in reactor system simulations, whereas the THERMR module of NJOY prepares and writes the pointwise scattering cross sections onto a PENDF file for further processing or for use by simulation codes.

Note that while THERMR is often used in conjunction with LEAPR, it can also be used to simply add a free-gas scattering component to an existing PENDF.

For more information on the physics of thermal neutron scattering and how LEAPR prepares the scattering data, please visit the LEAPR documentation.

Thermal neutron scattering can be categorized as either elastic or inelastic, both of which can have coherent and incoherent contributions. Modern thermal scattering data processing codes typically combine coherent inelastic with incoherent inelastic in what is called the incoherent approximation.

With this approximation, NJOY differentiates the following thermal scattering types:

Incoherent elastic

This is important for hydrogeneous solids like solid methane, polyethylene, and zirconium hydride. For this, LEAPR calculates the Debye-Waller coefficients for desired temperatures, which can be used to later calculate the incoherent elastic cross sections.

Coherent elastic

This is important for crystalline solids such as graphite and Beryllium. LEAPR prepares the locations and weights of the coherent elastic Bragg edges.

Inelastic scattering

This is important for all materials. To describe this data, LEAPR prepares the scattering law \(S(\alpha,\beta)\). LEAPR works in the incoherent approximation, which allows the scattering law to be calculated using the phonon density of states (also known as the phonon distribution).

While calculating the scattering law from the phonon distribution, LEAPR employs the “phonon decomposition approach” and separates the phonon distribution into three components: a solid-type continuous contribution, a discrete-oscillator contribution, and a translational contribution. These three components of the phonon distribution will be further described in the theory section.

How NJOY handles these three types of scattering will be explained in subsequent sections.