Charge localization, frustration relief, and spin-orbit coupling in U3_3O8_8

Research efforts on the low temperature magnetic order and electronic properties of U$_3$O$_8$ have been inconclusive so far. Reinterpreting neutron scattering results, we use group representation theory to show that the ground state presents collinear out-of-plane magnetic moments, with antiferromagnetic coupling both in-layer and between layers. Charge localization relieves the initial geometric frustration, generating a slightly distorted honeycomb sublattice with N\'eel order. We show, furthermore, that spin-orbit coupling has a giant effect on the conduction band states and band gap value. Our results allow a reinterpretation of recent optical absorption measurements.Comment: 12 pages, including supplemental materia

Solvent Optimization Studies for a New EURO-GANEX Process with 2,2’-Oxybis( N,N -di- n -decylpropanamide) (mTDDGA) and Its Radiolysis Products

The diglycolamide 2,2’-oxybis(N,N-di-n-decylpropanamide) (mTDDGA) is being studied as an extractant for actinides and lanthanides in the European Grouped Actinide Extraction (EURO-GANEX) process. The aim is the development of a more simplified process using a single extractant instead of a mixture of extractants used in the current EURO-GANEX process. This work presents solvent optimization studies of mTDDGA, with regards to the extraction characteristics of the different diastereomers of mTDGA and of mixed diastereomer solutions. Also radiolysis behavior has been studied by irradiation of solvent extraction systems in a gamma irradiation facility using $^{60}$Co. The availability of irradiated organic solutions made it possible to gain valuable insights into the plutonium loading capacity after gamma-irradiation of the solvent up to 445 kGy and to quantify degradation compounds. Solvent extraction characteristic of the major degradation compounds themselves were determined. Like other methylated diglycolamides, we found a remarkable difference in extraction of up to two orders of magnitude between the two diastereomers. High plutonium loading (36 g L$^{−1}$) is feasible using this single extractant, even after absorbing a dose of 445 kGy. This remarkable observation is possibly promoted by the presence of the main degradation compound which extracts plutonium verywell

Blind Benchmark Exercise for Spent Nuclear Fuel Decay Heat

The decay heat rate of five spent nuclear fuel assemblies of the pressurized water reactor type were measured by calorimetry at the interim storage for spent nuclear fuel in Sweden. Calculations of the decay heat rate of the five assemblies were performed by 20 organizations using different codes and nuclear data libraries resulting in 31 results for each assembly, spanning most of the current state-of-the-art practice. The calculations were based on a selected subset of information, such as reactor operating history and fuel assembly properties. The relative difference between the measured and average calculated decay heat rate ranged from 0.6% to 3.3% for the five assemblies. The standard deviation of these relative differences ranged from 1.9% to 2.4%

Comparison of interatomic potentials for UO2. Part I: Static calculations

An improved knowledge of nuclear fuel can be gained from a better description of atomic-scale processes such as point defects behaviour under irradiation. In this perspective the different techniques involving interatomic potentials can play a major role as they permit to simulate such mechanisms at the atomic scale. In this article we will assess the range of applicability of the available interatomic potentials for UO2 by static calculations. Lattice properties have been envisaged, together with defect properties: the formation and activation energies of vacancies and interstitials, the binding energy of small clusters of these defects and the volume change associated with them. © 2007 Elsevier B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Comparison of interatomic potentials for UO2. Part II: Molecular dynamics simulations

An improved knowledge of nuclear fuel can be gained from a better description of atomic-scale processes such as point defects behaviour under irradiation. In these perspectives, computer simulation techniques involving semi-empirical potentials can play a major role as they allow studying some of these processes separately. The range of applicability in static calculations of the available interatomic potentials for UO2 has been previously assessed by the authors. This study complements the static calculations by including dynamical simulations of the temperature evolution of different elastic properties (lattice parameter, specific heat, bulk modulus and Gruneisen parameter) and by calculations of bulk melting temperature. © 2008 Elsevier B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

In-pile Xe diffusion coefficient in UO2 determined from the modeling of intragranular bubble growth and destruction under irradiation

Intragranular bubbles grow in the nuclear fuel by diffusion and precipitation of fission gases, mainly xenon; and are ultimately destroyed, under irradiation, by fission fragments. This article will attempt to determine the in-pile bubble distributions taking into account the evolution of the concentration profile around a bubble during its growth and the destruction process by fission fragments. From these distributions a relation between the bubble mean radius and the diffusion coefficient of xenon can be established, allowing the determination, from experimental measurements of intragranular bubble sizes, of the in-pile Xe diffusion coefficient in UO2. The estimated activation energy (0.9 eV) is about one order of magnitude lower than the widely used value of 3.9 eV determined from out-of-pile experiments. This effect can be attributed to the presence of point defects created by the irradiation. © 2007 Elsevier B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Comparison of interatomic potentials for UO2: Static calculations

info:eu-repo/semantics/publishe

Molecular dynamics simulation of helium and oxygen diffusion in UO2 ± x

Atomic scale simulation techniques based on empirical potentials have been considered in the present work to get insight on helium diffusion in uranium dioxide. By varying the stoichiometry, together with the system temperature, the performed molecular dynamics simulations indicate two diffusion regimes for He. The first one presents a low activation energy (0.5 eV) and suggests oxygen vacancies assisted migration. This regime seems to provide the major contribution to diffusion when structural defects are present (extrinsic defects, imposed, e.g. by the stoichiometry). The second regime presents a higher activation energy, around 2 eV, and dominates in the higher temperature range or at perfect stoichiometry, suggesting an intrinsic migration process. Considering the dependance of He behaviour with oxygen defects, oxygen diffusion has been considered as well in the different stoichiometry domains. Finally, further investigations were made with nudged elastic bands calculations for a better interpretation of the operating migration mechanisms, both for He and O. © 2009 Elsevier B.V. All rights reserved.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Thorium and thorium-plutonium fuel research in Belgium

status: publishe

Two remarkable defect related electron diffraction effects

In this note, two singular diffraction effects are reported and interpreted. For the two cases presented here, the interpretation of the diffraction pattern is only possible when combined with real space information about the defects in the material. A first observation is that of a peculiar case of so called "double" diffraction involving two crystal parts with a different structure. The second observation is that of an interference effect occuring at intergrowth structures.Dans cette note, nous présentons et interprétons deux effets remarquables de diffraction electronique. Dans les deux cas, leur interprétation n'est possible qu'en l'associant à une information obtenue dans 1'expace réel sur les défauts présents localement dans l'échantillon. Une première observation est un cas particulier de phénomène de "double" diffraction mettant en jeu deux portions cristallines de stuctures différentes. La seconde réside dans un effet interférentiel apparaissant dans des structures d'intercroissance