Speaker
Description
Thermal scattering law for ice based on neutron time-of-flight experiments carried out at the SEQUOIA spectrometer at the Oak Ridge National Laboratory
Vaibhav Jaiswal$^{1,*}$, Luiz Leal$^1$, and Alexander I. Kolesnikov$^2$
$^1$Insititut de Radioprotection et de Sûreté Nucléaire (IRSN), Fontenay-aux-Roses, France
$^2$Neutron Scattering Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6473, USA
$*$ Corresponding author: vaibhav.jaiswal@irsn.fr
Neutrons generated in the fission process are in the MeV energy range that are slowed down to thermal energies (eV) by the scattering process with moderator materials. During the scattering process, when the neutron energy is comparable to the bound energies of the scattering atom/molecule, a special treatment must be performed to characterize this effect correctly, which is described by the thermal scattering law (TSL).
The Institut de Radioprotection et de Sûreté Nucléaire (IRSN) is working on the development of improved TSL for light water ice to support research on advanced nuclear systems using ice as a neutron moderator (operating around 80 K) [1]. International recommendation from institutes like IAEA and UKAEA highlights the need to design packages to be transported at low temperature (up to -40°c) [2,3]. This recommendation emphasizes the need for ice neutron scattering data in the temperature range around 253-293 K. Many polymorphic phases of water ice exist depending on the thermodynamic conditions. The most common type of ice at normal pressure and near water freezing point temperature (273.15 K) is ice-Ih [4]. Following the importance and the need for accurate modeling of neutron scattering distributions in energy and angle for light-water ice-Ih, TSLs for ice-Ih were recently developed for the ENDF/B-VIII.0 and JEFF-3.3 nuclear data libraries. Total cross-section and inelastic neutron scattering experimental data were used in the evaluation. However, the detailed structure of the TSL was developed independently of neutron scattering measurements carried out at required temperatures. Also, the integral performance of the ENDF/BVIII.0 ice-Ih TSL has been validated with an experimental pulsed-neutron die-away (PNDA) diffusion benchmark [5], which does not necessarily validate the TSL physics model or guarantee that the TSL will perform acceptably in conditions similar to those of practical applications.
Existing experimental double-differential scattering data for ice-Ih are extremely sparse and of limited quality. New high-quality double-differential measurements for ice-Ih over multiple temperatures and incident energies would directly support the validation and improvement of TSL models for criticality safety applications. Following this, IRSN carried out a series of double-differential neutron scattering and transmission experiments for light-water ice-Ih at low temperatures (starting at 273 K and down to 5 K) at the SEQUOIA spectrometer at the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory (ORNL), the United States.
A commonly used tool by the reactor physics community for generating thermal scattering cross-section data is the LEAPR module of the NJOY code. The process of energy exchange between the neutron and the scattering atom leads to the excitation/deexcitation of vibrational and rotational modes of the moderator material, represented by the phonon spectrum. LEAPR calculates the thermal scattering cross-section data using the phonon spectrum of moderator materials as an input. This paper will present the TSL for ice-Ih based on the phonon spectrum measured at SEQUOIA at different temperatures using the LEAPR module of NJOY. The outcome of this study will lead to a next-generation TSL evaluation for ice-Ih based on significantly improved material and physics models.
[1] IAEA website, “Advanced Moderators for Intense Cold Neutron Beams in Materials Research,” https://www.iaea.org/projects/crp/f12026.
[2] Technical report, “IAEA Safety Standards for protecting people and the environment: Regulations for the Safe Transport of Radioactive Material,” Specific Safety Requirements, No. SSR-6.
[3] SG-3: Criticality Benchmark: A proposal for a benchmark examining the effect of temperature on the neutron multiplication factor for PWR fuel assemblies.
[4] M. K. Gupta, R. Mittal, Baltej Singh, S. K. Mishra, D. T. Adroja, A. D. Fortes, and S. L. Chaplot, “Phonons and anomalous thermal expansion behavior of H2O and D2O ice Ih,” PHYSICAL REVIEW B 98, 104301 (2018).
[5] J. Holmes, M. Zerkle and David Heinrichs, “Benchmarking a first-principles thermal neutron scattering law for water ice with a diffusion experiment,” EPJ Web of Conferences 146, 13004 (2017).