The simulation of nuclear systems requires a variety of data, which depend on the typical energies of the active particles. For many well-known systems, such as light water, heavy water or graphite-moderated nuclear reactors, cold neutron source facilities (e.g. spallation neutron sources) and others, neutrons are slowed down to low energies where the chemical structure of materials plays a central role in the neutron physics. The simulation of thermalised neutronic systems is highly sensitive to neutron scattering. The evaluation and validation of these data are therefore of great importance and represent a specialised discipline within the nuclear data community. Over the past two decades, great advances have been made in the use of atomistic simulation techniques, including density functional theory (DFT) and molecular dynamics (MD) that, alongside the increase in computational power, have provided a wealth of theoretical information for thermal scattering law (TSL) evaluations. Combined with new experimental data obtained in the past decade, these advances have resulted in an active area of research.
A subgroup was launched in 2015 under the Nuclear Science Committee (NSC) Working Party on International Nuclear Data Evaluation Co-operation (WPEC) to co‑ordinate international activities in this area. The work of this subgroup has stimulated numerous activities, directly or indirectly resulting in a suite of new evaluated TSL data evaluations that have been adopted in the most recent nuclear data libraries of the United States (Evaluated Nuclear Data File [ENDF]/B-VIII.0, February 2018) and the NEA Data Bank (Joint Evaluated Fission and Fusion File [JEFF]-3.3, November 2017). Ainclude new evaluations for novel materials such as uranium nitride (UN), silicon carbide (SiC), silicon oxide (SiO2) and aluminium oxide (Al2O3), as well as the re‑evaluation of critical materials, including water (H2O) and heavy water (D2O), and enhanced evaluations, including graphite at multiple levels of porosity and phase Ih ice. Using the new evaluation techniques, opportunities to provide additional data that were previously unavailable – including correlated uncertainties – have been explored. New frontiers have been established through Subgroup 42, including the development and utilisation of new data, the application of novel techniques to modern neutronic systems and the use of the most recent experimental data, which will be addressed in future WPEC activities.
The NEA's nuclear data evaluation co-operation activities involve the following evaluation projects: ENDF (United States), JENDL (Japan), ROSFOND/BROND (Russia), JEFF (other Data Bank member countries) and CENDL (China) in close co-operation with the Nuclear Data Section of the International Atomic Energy Agency (IAEA).