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Multiscale modeling of nitride fuels
KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics. (Reaktorfysik)ORCID iD: 0000-0002-4158-0123
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. , p. 107
Series
TRITA-FYS, ISSN 0280-316X ; 73
Keywords [en]
Uranium Nitride Ab Initio Modelling
National Category
Other Physics Topics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-202538ISBN: 978-91-7729-182-4 (print)OAI: oai:DiVA.org:kth-202538DiVA, id: diva2:1077192
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
List of papers
1. GGA plus U study of uranium mononitride: A comparison of the U-ramping and occupation matrix schemes and incorporation energies of fission products
Open this publication in new window or tab >>GGA plus U study of uranium mononitride: A comparison of the U-ramping and occupation matrix schemes and incorporation energies of fission products
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2016 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 478, p. 119-124Article in journal (Refereed) Published
Abstract [en]

Uranium mononitride is studied in the DFT + U framework. Its ground state is investigated and a study of the incorporation of diverse fission products in the crystal is conducted. The U-ramping and occupation matrix control (OMC) schemes are used to eliminate metastable states. Beyond a certain amount of introduced correlation, the OMC scheme starts to find a lower total energy. The OMC scheme is chosen for the second part of this study. Furthermore, the influence of the magnetic ordering is studied using the U-ramping method, showing that antiferromagnetic order is the most stable one when the U parameter is larger than 1.75 eV. The effect on the density of states is investigated and elastic constants are provided for comparison with other methods and experiments. The incorporation energies of fission products in different defect configurations are calculated and these energies are corrected to take into account the limited size of the supercell.

Place, publisher, year, edition, pages
Elsevier, 2016
Keywords
Antiferromagnetism, Ground state, Uranium, Antiferromagnetic orderings, Defect configurations, Density of state, GGA + U, Meta-stable state, Mononitride, Super cell, Total energy
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-192387 (URN)10.1016/j.jnucmat.2016.06.007 (DOI)000381644500015 ()2-s2.0-84974560146 (Scopus ID)
External cooperation:
Note

QC 20160913

Available from: 2016-09-13 Created: 2016-09-12 Last updated: 2017-11-21Bibliographically approved
2. Investigation of the ground- and metastable states of AnN (An=Th..Pu)
Open this publication in new window or tab >>Investigation of the ground- and metastable states of AnN (An=Th..Pu)
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-202549 (URN)
Note

QC 20170228

Available from: 2017-02-27 Created: 2017-02-27 Last updated: 2017-02-28
3. Ab-initio study of C and O impurities in uranium nitride
Open this publication in new window or tab >>Ab-initio study of C and O impurities in uranium nitride
2016 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 478, p. 112-118Article 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
Keywords
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:nbn:se:kth:diva-192386 (URN)10.1016/j.jnucmat.2016.06.008 (DOI)000381644500014 ()2-s2.0-84974575046 (Scopus ID)
External cooperation:
Note

QC 20160912

Available from: 2016-09-12 Created: 2016-09-12 Last updated: 2017-11-21Bibliographically approved
4. Transport properties in dilute UN(X) solid solutions (X = Xe, Kr)
Open this publication in new window or tab >>Transport properties in dilute UN(X) solid solutions (X = Xe, Kr)
2016 (English)In: Physical Review B, ISSN 2469-9950, E-ISSN 2469-9969, Vol. 94, no 17, article id 174302Article in journal (Refereed) Published
Abstract [en]

Uranium nitride (UN) is a candidate fuel for current GEN III fission reactors, for which it is investigated as an accident-tolerant fuel, as well as for future GEN IV reactors. In this study, we investigate the kinetic properties of gas fission products (Xe and Kr) in UN. Binding and migration energies are obtained using density functional theory, with an added Hubbard correlation to model f electrons, and the occupation matrix control scheme to avoid metastable states. These energies are then used as input for the self-consistent mean field method which enables to determine transport coefficients for vacancy-mediated diffusion of Xe and Kr on the U sublattice. The magnetic ordering of the UN structure is explicitly taken into account, for both energetic and transport properties. Solute diffusivities are compared with experimental measurements and the effect of various parameters on the theoretical model is carefully investigated. We find that kinetic correlations are very strong in this system, and that despite atomic migration anisotropy, macroscopic solute diffusivities show limited anisotropy. Our model indicates that the discrepancy between experimental measurements probably results from different irradiation conditions, and hence different defect concentrations.

National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-196974 (URN)10.1103/PhysRevB.94.174302 (DOI)000386894400002 ()2-s2.0-84994624441 (Scopus ID)
Note

QC 20161213

Available from: 2016-12-13 Created: 2016-11-28 Last updated: 2017-11-29Bibliographically approved
5. Transport properties of C and O in UN fuels
Open this publication in new window or tab >>Transport properties of C and O in UN fuels
(English)Manuscript (preprint) (Other academic)
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-202550 (URN)
Note

QC 20170228

Available from: 2017-02-27 Created: 2017-02-27 Last updated: 2017-02-28
6. Towards the inclusion of open fabrication porosity in a fission gas release model
Open this publication in new window or tab >>Towards the inclusion of open fabrication porosity in a fission gas release model
2015 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 466, p. 351-356Article in journal (Refereed) Published
Abstract [en]

A model is proposed for fission product release in oxide fuels that takes into account the open porosity in a mechanistic manner. Its mathematical framework, assumptions and limitations are presented. It is based on the model for open porosity in the sintering process of crystalline solids. More precisely, a grain is represented by a tetrakaidecahedron and the open porosity is represented by a continuous cylinder along the grain edges. It has been integrated in the TRANSURANUS fuel performance code and applied to the first case of the first FUMEX project as well as to neptunium and americium containing pins irradiated during the SUPERFACT experiment and in the JOYO reactor. The results for LWR and FBR fuels are consistent with the experimental data and the predictions of previous empirical models when the thermal mechanisms are the main drivers of the release, even without using a fitting parameter. They also show a different but somewhat expected behaviour when very high porosity fuels are irradiated at a very low burn-up and at low temperature.

Keywords
Fission products, Fuels, Sintering, Temperature, Crystalline solids, Fission gas release, Fitting parameters, Low temperatures, Mathematical frameworks, Sintering process, Tetrakaidecahedron, Thermal mechanisms, Porosity
National Category
Materials Engineering Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-175606 (URN)10.1016/j.jnucmat.2015.08.022 (DOI)000364883400044 ()2-s2.0-84940194137 (Scopus ID)
Note

QC 20151102

Available from: 2015-11-02 Created: 2015-10-19 Last updated: 2017-12-01Bibliographically approved

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