Supercritical fluid pseudo‐boiling (PB), recently brought to the attention of the scientific community, is the phenomenon occurring when fluid changes its structure from liquid‐like (LL) to gas‐like (GL) states across the Widom line. This work provides the first quantitative analysis on the thermodynamics and the dynamics of water's PB, since the understanding of this phase transition is mandatory for the successful implementation of technologies using supercritical water (scH₂O) for environmental, energy, and nanomaterial applications. The study combines computational techniques with in situ neutron imaging measurements. The results demonstrate that, during isobaric heating close to the critical point, while water density drops by a factor of three in the PB transitional region, the system needs >16 times less energy to increase its temperature by 1 K than to change its structure from LL to GL phase. Above the PB‐Widom line, the structure of LL water consists mainly of tetramers and trimers, while below the line mostly dimers and monomers form in the GL phase. At atomic level, the PB dynamics are similar to those of the subcritical water vaporization. This fundamental knowledge has great impact on water science, as it helps to establish the structure–properties relationship of scH₂O.

Nanodiamonds have attracted attention in recent years for their exceptional albedo of slow neutrons. Several theoretical models have been proposed to compute the total elastic cross section. However, these models neglect the relatively complex internal structure and chemical composition of the diamond nanoparticles, relying often on the monodisperse structureless spheres approximation. In this work, we explore the possibility of adding the small-angle neutron scattering (SANS) process for nanodiamonds to NCrystal, a library that enables calculations for Monte Carlo simulations of neutrons in polycrystalline materials and powders. This approach aims to describe the scattering process also at neutron wavelengths below the diamond Bragg cutoff where simple models usually struggle. The extension relies on modeling the SANS process through the fitting of experimental data as well as theoretical inputs. The code can then be coupled with an existing simulation framework, such as McStas, and benchmarked in different setups. We tested the plugin for two properties of diamond powder nanoparticles: the backward reflection of very cold neutrons and the quasispecular reflections of cold neutrons. The validation of this simulation tool is intended to pave the way for the design of the beam extraction system for a future high-intensity cold neutron moderator at the European Spallation Source.

Transport calculations for neutronic design require accurate nuclear data and validated computational tools. In the Spallation Physics Group, at the European Spallation Source, we perform shielding and neutron beam calculations to help the deployment of the instrument suite for the current high brilliance (top) moderator, as well for the design of the high intensity bottom moderator, currently under study for the facility. This work includes providing the best available nuclear data in addition to improving models and tools when necessary. In this paper we present the status of these activities, which include a set of thermal scattering kernels for moderator, reflector, and structural materials, the development of new kernels for beryllium considering crystallite size effects, nanodiamonds, liquid hydrogen and deuterium based on path integral molecular dynamics, and the use of the software package NCrystal to assist the development of nuclear data in the framework of the new HighNESS project.