Hidden order and multipolar exchange striction in a correlated f-electron system

The nature of order in low-temperature phases of some materials is not directly seen by experiment. Such “hidden orders” (HOs) may inspire decades of research to identify the mechanism underlying those exotic states of matter. In insulators, HO phases originate in degenerate many-electron states on localized f or d shells that may harbor high-rank multipole moments. Coupled by intersite exchange, those moments form a vast space of competing order parameters. Here, we show how the ground-state order and magnetic excitations of a prototypical HO system, neptunium dioxide NpO2, can be fully described by a low-energy Hamiltonian derived by a many-body ab initio force theorem method. Superexchange interactions between the lowest crystal-field quadruplet of Np4+ ions induce a primary noncollinear order of time-odd rank 5 (triakontadipolar) moments with a secondary quadrupole order preserving the cubic symmetry of NpO2. Our study also reveals an unconventional multipolar exchange striction mechanism behind the anomalous volume contraction of the NpO2 HO phase.

PRODUCTION AND RESEARCH OF TABLETS OF MIXED URANIUM, PLUTONIUM, AMERICIUM AND NEPTUNIUM NITRIDES

The method of carbothermic synthesis produced 260 g tablets of mixed nitrides of uranium, plutonium, americium and neptunium with a mass fraction of americium 0.61%. For the production of tablets were used uranium oxide manufactured by the water method, as well as plutonium dioxide containing impurities of uranium and americium oxides with a mass of 0.9 g americium, obtained by volumetric crystallization method in a NaCl — 2CsCl melt. Neptunium dioxide and americium oxide were added to the mixture of uranium and plutonium oxides before carbothermic synthesis. The resulting tablets had a density of 11.6-11.9 g / cm3. By methods of gamma-spectrometry, scanning electron microscopy and x-ray microanalysis is showed that the heat treatment of the mixture of the source chemicals - oxides of uranium, plutonium, americium, neptunium with carbon black at a temperature 1600-1500 °C for 72 h (24 h in nitrogen and 48 h in nitrogen-hydrogen atmospheres), and also the subsequent sintering synthesized by the press powders for 48 h at a temperature of 1800°C in nitrogen-hydrogen atmosphere doesn't leads in average to the significant losses of americium.

Effect of calcination temperature on neptunium dioxide microstructure and dissolution

Solubility of neptunium dioxide decreases as microstructure grain size increases, likely due to decreasing surface free energy and surface area.

Thermomechanical Safety Analyses for a 238Pu Production Target at the HFIR

Safety analyses at the high flux isotope reactor (HFIR) are required to qualify irradiation of production targets containing neptunium dioxide/aluminum cermet (NpO2/Al) pellets for the production of plutonium-238 (238Pu). High heat generation rates (HGRs) due to a fertile starting material (237Np), low melting temperatures, and previously unstudied material irradiation behavior (i.e., swelling/densification, fission gas release) require a sophisticated set of steady-state thermal simulations in order to ensure sufficient safety margins. Experience gained from previous models for preliminary target designs is incorporated into a more comprehensive production target model designed to qualify a target for three cycles of irradiation and illuminate potential in-reactor behavior of the target.

Thermo-Mechanical Safety Analyses of Preliminary Design Experiments for 238Pu Production

Safety analyses at the high flux isotope reactor (HFIR) are required to qualify experiment targets for the production of plutonium-238 (238Pu) from neptunium dioxide/aluminum cermet (NpO2/Al) pellets. High heat generation rates (HGRs) due to fissile material and low melting temperatures require a sophisticated set of steady-state thermal simulations in order to ensure sufficient safety margins. These simulations are achieved in a fully coupled thermo-mechanical analysis using comsolmultiphysics for four different preliminary target designs using an evolving set of pre- and postirradiation data inputs, and subsequently evolving solution scopes, from the unique pellet and target designs. A new comprehensive presentation of these preliminary analyses is given and revisited analyses of the first prototypical target designs are presented to reveal the effectiveness of evolving methods and input data.

Intrinsic formation of nanocrystalline neptunium dioxide under neutral aqueous conditions relevant to deep geological repositories

Simple dilution of an aqueous Np(iv) bicarbonate solution triggers the intrinsic formation of nanocrystalline neptunium dioxide (NpO2). This new formation route could be a likely scenario in the repository and disposal of radioactive waste.

In situX-ray diffraction study of point defects in neptunium dioxide at elevated temperature

High-temperature X-ray diffraction measurements have been performed on neptunium dioxide up to 2000 K for the first time under He, He/5%H2and air atmospheres. Up to 1643 K, NpO2remains stoichiometric under all the considered atmospheres, and the coefficients of thermal expansion have been evaluated. Above 1643 K, the lattice parameter departs from linearity towards higher values. The atomic displacement parameters of the O and Np atoms were determined from Rietveld refinement and the Debye temperature subsequently obtained. This was used to study the contribution of point defects to the evolution of the lattice parameter at elevated temperature by estimating the energy of formation of vacancies. It is shown that only the chemical reduction of NpO2to NpO2−xis responsible for the departure from linearity below 1750 K. Above this temperature, the evolution is due to the simultaneous effect of reduction and the formation of oxygen Frenkel pairs.