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Experimental and theoretical studies of nitride fuels
KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.ORCID iD: 0000-0003-4136-6458
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With respect to nitrides being considered as potential fast reactor fuels, research is conducted on the out-of-pile thermophysical properties, sintering and fabrication processes, gas migration mechanisms, self-diffusion and point defect behaviour of actinide nitrides, their surrogate materials, and the inert matrix material ZrN . The experimental research, carried out in the framework of qualifying fuel for the European Lead Cooled Training Reactor (ELECTRA), shows that sintered ZrN and (Dy,Zr)N pellet densities are influenced by the oxygen concentration in the material. The effect is confirmed in sintered (Pu,Zr)N pellets. Oxygen concentration also plays a role in the thermophysical properties of inert matrix nitride fuels, but does not have an impact on the electrical properties of these materials. With the fuel fabrication methods applied here, clean nitride powders can be synthesized. However, the subsequent fabrication phases, including milling and solid solution formation, increases the impurity levels significantly. Research of equal importance is performed on materials free of fabrication-induced impurities, whose properties are studied by employing first-principles methods. ZrN, UN and (U,Zr)N are studied, whereas the results from ZrN are expected to be applicable for actinide nitrides as a first approximation. The migration of noble gases in ZrN, on the atomic scale, confirms the experimentally observed tendency for noble gases with higher atomic number to be retained in the fuel matrix, while the majority of He is released to the fuel pin. Materials modelling implies that self-diffusion of nitrogen and metal atoms in inert matrix nitride fuels is accelerated under irradiation, since noble gas retention reduces migration barriers which govern self-diffusion. Unlike Kr and Xe, He has the capacity to be released into the fuel matrix, after having been trapped in a vacancy. The results are expected to aid in providing an explanation to the macroscopic diffusion phenomena in nitride fuels, as the diffusion behaviour of noble gases is sparsely studied. In addition, a study on the miscibility of ZrN and UN in a narrow composition range suggests solubility, based on the negative mixing energies. The results obtained from research on inert matrix nitride fuel underline several beneficial properties which are desirable in a fast reactor fuel. The relevance of these results is analyzed and contextualized in the thesis, from the perspective of current research and development in the field.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , x, 74 p.
Series
Trita-FYS, ISSN 0280-316X ; 12013:18
Keyword [en]
fast reactor fuel, ab-initio modelling, fuel fabrication, inert matrix nitride fuel
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-123521ISBN: 978-91-7501-794-5 (print)OAI: oai:DiVA.org:kth-123521DiVA: diva2:626972
Public defence
2013-06-14, FD5, AlbaNova University Center, Roslagstullsbacken 21, Stockholm, 14:00 (English)
Opponent
Supervisors
Note

QC 20130611

Available from: 2013-06-11 Created: 2013-06-10 Last updated: 2013-06-11Bibliographically approved
List of papers
1. Vacancy formation and solid solubility in the U-Zr-N system
Open this publication in new window or tab >>Vacancy formation and solid solubility in the U-Zr-N system
2010 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 406, no 3, 351-355 p.Article in journal (Refereed) Published
Abstract [en]

For the purpose of developing a nuclear fuel with enhanced thermophysical properties and better irradiation performance density functional theory calculations are used to explore UN, ZrN and (U, Zr)N. Negative deviation of ground state energy from the ideal solution model as well as energetically favourable maximal distance between substitutional metal atoms in respective nitrides indicate mutual solubility of UN and ZrN at all temperatures. Nitrogen vacancy formation energies in UN (1.81 eV) and ZrN (1.40 eV) are considerably lower than metal vacancy formation energies. A substitutional Zr atom in UN has little effect on nitrogen vacancy formation energies (similar to 1.79 eV), while U in ZrN decreases the value by similar to 0.1 eV (similar to 1.30 eV) due to elastic stress and charge density redistribution in the material. The relative distance between a substitutional metal atom and a vacancy in UN has little influence over the radially declining displacement pattern induced by the substitutional atom, while in ZrN the relaxation of atoms is governed by the position of the vacancy. The calculated vacancy formation energies indicate a lower surface energy of ZrN in comparison with UN.

Keyword
Material Property Correlations, Uranium-Nitrogen System, Advanced Nuclear-Fuels, Thermodynamic Properties, Point-Defects, Mononitride, Nitride, 1st-Principles
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-27681 (URN)10.1016/j.jnucmat.2010.09.006 (DOI)000284750400009 ()2-s2.0-78049241493 (Scopus ID)
Funder
Swedish Research Council, 90399101
Note

QC 20101221

Available from: 2010-12-21 Created: 2010-12-20 Last updated: 2017-12-11Bibliographically approved
2. He, Kr and Xe diffusion in ZrN: An atomic scale study
Open this publication in new window or tab >>He, Kr and Xe diffusion in ZrN: An atomic scale study
2013 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 438, no 1/3, 7-14 p.Article in journal (Refereed) Published
Abstract [en]

The atomic scale diffusion mechanisms for He, Kr and Xe in the nitride fuel component ZrN are developed from first principles. The vacancy formation energies reveal a prevalent N vacancy concentration in the material. However, a high N self-diffusion barrier hinders vacancy-aided Kr and Xe diffusion. High, attractive binding energies of interstitial Xe and Kr to a N vacancy effectively eliminate interstitial diffusion mechanism for these gases. In comparison, He exhibits considerable degrees of freedom, as it is weekly bound to a N vacancy, enhances N-vacancy aided diffusion, has the lowest interstitial migration barrier, and has the capacity to be reintroduced into the ZrN lattice as an interstitial. N self-diffusion barriers are lowered if the diffusing N is in close proximity to a substitutional atom. The obtained results suggest a high release of He, while the majority of Kr and Xe is retained, in agreement with experiments.

National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-123529 (URN)10.1016/j.jnucmat.2013.02.077 (DOI)000319481700002 ()2-s2.0-84875751907 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20130611

Available from: 2013-06-11 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved
3. Sintering and characterization of ZrN and (Dy,Zr)N as surrogate materials for fast reactor nitride fuel
Open this publication in new window or tab >>Sintering and characterization of ZrN and (Dy,Zr)N as surrogate materials for fast reactor nitride fuel
2014 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 444, no 1-3, 7-13 p.Article in journal (Refereed) Published
Abstract [en]

Pellets of inert matrix material ZrN, and surrogate nitride fuel material (Dy0.4Zr0.6)N, are fabricated for the purpose of investigating the origin and the effect of carbon and oxygen impurity concentrations. Oxygen concentrations of up to 1.2 wt% are deliberately introduced into the materials with two separate methods. The achievable pellet densities of these materials, as a function of O content, sintering temperature and dimensional powder properties are determined. O dissolved into (Dy,Zr)N increases the achievable densities to a larger extent than if dissolved into ZrN. The segregation of O-rich phases in ZrN indicates a low O solubility in the material. Oxygen pick-up during the fabrication of the product as well as its exposure to air is demonstrated. The quality of the materials is monitored by the systematic analysis of O, N and C contents throughout the fabrication and sintering processes, supported by XRD and SEM analyses.

Keyword
Effect of carbons, Oxygen concentrations, Oxygen impurity, Powder properties, Sintering process, Sintering temperatures, Surrogate materials, Systematic analysis
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-123530 (URN)10.1016/j.jnucmat.2013.09.001 (DOI)000329888100002 ()2-s2.0-84885090542 (Scopus ID)
Note

QC 20140214. Updated from accepted to published.

Available from: 2013-06-11 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved
4. Sintering and characterization of (Pu,Zr)N
Open this publication in new window or tab >>Sintering and characterization of (Pu,Zr)N
2014 (English)In: Journal of Nuclear Materials, ISSN 0022-3115, E-ISSN 1873-4820, Vol. 444, no 1-3, 421-427 p.Article in journal (Refereed) Published
Abstract [en]

Nitride fuel, with the composition of (Pu0.4Zr0.6)N, is fabricated for studying the sinterability of nitride fuel as a function of oxygen concentration in the material. Oxygen concentration of up to 0.6 wt% evidently enhances the densification of the material. Increasing the sintering temperature from 1923 to 1973 K improves the sintered pellet densities by up to 3.8%TD. In addition, the measured thermophysical and electrical properties of (Pu0.4Zr0.6)N reveal that the values are close to those of PuN. Elevated oxygen concentration in the material decreases its thermal conductivity. Oxygen concentration of 0.34 wt% in (Pu,Zr)N is a consequence of the fabrication process, considering the relatively pure ZrN (0.03 wt% O) and PuN (0.08 wt% O) powders initially fabricated.

Keyword
Fabrication process, Nitride fuel, Oxygen concentrations, Sinterability, Sintered pellets, Sintering temperatures, Thermophysical
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-123531 (URN)10.1016/j.jnucmat.2013.09.060 (DOI)000329888100053 ()2-s2.0-84887849778 (Scopus ID)
Note

QC 20140214. Submitted to published.

Available from: 2013-06-11 Created: 2013-06-11 Last updated: 2017-12-06Bibliographically approved

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