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Ab-initio study of C and O impurities in uranium nitride
KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.ORCID iD: 0000-0002-4158-0123
KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.ORCID iD: 0000-0002-2381-3309
2016 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 478, 112-118 p.Article in journal (Refereed) Published
Abstract [en]

Uranium nitride (UN) has been considered a potential fuel for Generation IV (GEN-IV) nuclear reactors as well as a possible new fuel for Light Water Reactors (LWR), which would permit an extension of the fuel residence time in the reactor. Carbon and oxygen impurities play a key role in the UN microstructure, influencing important parameters such as creep, swelling, gas release under irradiation, compatibility with structural steel and coolants, and thermal stability. In this work, a systematic study of the electronic structure of UN containing C and O impurities using first-principles calculations by the Density Functional Theory (DFT) method is presented. In order to describe accurately the localized U 5f electrons, the DFT + U formalism was adopted. Moreover, to avoid convergence toward metastable states, the Occupation Matrix Control (OMC) methodology was applied. The incorporation of C and O in the N-vacancy is found to be energetically favorable. In addition, only for O, the incorporation in the interstitial position is energetically possible, showing some degree of solubility for this element in this site. The binding energies show that the pairs (C-N-vac) and (O-N-vac) interact much further than the other defects, which indicate the possible occurrence of vacancy drag phenomena and clustering of these impurities in grain boundaries, dislocations and free surfaces. The migration energy of an impurity by single N-vacancy show that C and O employ different paths during diffusion. Oxygen migration requires significantly lower energy than carbon. This fact is due to flexibility in the U-O chemical bonds, which bend during the diffusion forming a pseudo UO2 coordination. On the other hand, C and N have a directional and inflexible chemical bond with uranium; always requiring the octahedral coordination. These findings provide detailed insight into how these impurities behave in the UN matrix, and can be of great interest for assisting the development of this new nuclear fuel for next-generation reactors.

Place, publisher, year, edition, pages
Elsevier, 2016. Vol. 478, 112-118 p.
Keyword [en]
Binding energy, Bond strength (chemical), Building materials, Calculations, Chemical bonds, Crystallography, Density functional theory, Electronic structure, Fuels, Grain boundaries, Impurities, Light water reactors, Nitrides, Nuclear reactors, Uranium compounds, Density functional theory methods, First-principles calculation, Interstitial positions, Light water reactor (LWR), Meta-stable state, Octahedral coordination, Oxygen migration, Structural steels
National Category
Other Chemistry Topics
Identifiers
URN: urn:nbn:se:kth:diva-192386DOI: 10.1016/j.jnucmat.2016.06.008ISI: 000381644500014Scopus ID: 2-s2.0-84974575046OAI: oai:DiVA.org:kth-192386DiVA: diva2:968772
Note

QC 20160912

Available from: 2016-09-12 Created: 2016-09-12 Last updated: 2017-02-26Bibliographically approved
In thesis
1. Multiscale modeling of nitride fuels
Open this publication in new window or tab >>Multiscale modeling of nitride fuels
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nitride fuels have always been considered a good candidate for GENIV reactors, as well as space reactors, due to their high fissile density, highthermal conductivity and high melting point. In these concepts, not beingcompatible with water is not a significant problem. However, in recent years,nitride fuels started to raise an interest for application in thermal reactors,as accident tolerant or high performance fuels. However, oxide fuels havebenefited from decades of intensive research, and thousands of reactor-years.As such, a large effort has to be made on qualifying the fuel and developingtools to help assess their performances.In this thesis, the modeling side of this task is chosen. The effort istwo-fold: determining fundamental properties using atomistic models andputting together all the properties to predict the performances under irradi-ation using a fuel performance code. The first part is done combining manyframeworks. The density functional theory is the basis to compute the elec-tronic structure of the materials, to which a Hubbard correction is added tohandle the strong correlation effects. Negative side effects of the Hubbardcorrection are tackled using the so-called occupation matrix control method.This combined framework is first tested, and then used to find electronic andmechanic properties of the bulk material as well as the thermomechanicalbehavior of foreign atoms. Then, another method, the self-consistent meanfield (SCMF) one, is used to reach the dynamics properties of these foreignatoms. In the SCMF theory, the data that were obtained performing the abinitio simulations are treated to provide diffusion and kinetic flux couplingproperties.In the second step of the work, the fuel performance code TRANSURA-NUS is used to model complete fuel pins. An athermal fission gas releasemodel based on the open porosity is developed and tested on oxide fuels.A model for nitride fuels is introduced, and some correlations are bench-marked. Major issues remaining are pointed out and recommendations asto how to solve them are made.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 107 p.
Series
TRITA-FYS, ISSN 0280-316X ; 73
Keyword
Uranium Nitride Ab Initio Modelling
National Category
Other Physics Topics
Research subject
Physics
Identifiers
urn:nbn:se:kth:diva-202538 (URN)978-91-7729-182-4 (ISBN)
Public defence
2016-12-16, F3, Valhallavägen 79, Stockholm, 09:30 (English)
Opponent
Supervisors
Note

QC 20170227

Available from: 2017-02-27 Created: 2017-02-26 Last updated: 2017-02-27Bibliographically approved

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