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Helium Filled Bubbles in Solids: Nucleation, Growth and Swelling
KTH, School of Engineering Sciences (SCI), Physics, Reactor Physics.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Heliumfyllda bubblor i fasta material : Kärnbildning, tillväxt och svällning (Swedish)
Abstract [en]

When nuclear fuel, fabricated for the purpose of transmuting spent fuel is irradiated, significant amounts of He is produced from alpha particles mainly emitted when 242Cm decays into 238Pu. From irradiation experiments it is known that the presence of He in the solids alters the swelling behaviour of the material. The thesis presents the theoretical background from which nucleation models of He bubbles can be formulated. Such models are presented for He in metals, and the case of He in Mo is studied as an example. MgO, which together with Mo is suggested as a matrix material in transmutation fuel is also studied and the stability of He containing bubbles in this material is discussed.

By calculating parameters for a rate theory model derived from atomistic modelling, it is shown that He can stabilise vacancy clusters and cause cluster growth at temperatures and irradiation doses where nucleation and growth would not otherwise occur. At the initial stages of nucleation He can stabilise small bubbles while larger bubbles are unstable. This results in an incubation time of swelling, which implies that He does not always cause increased swelling, but can at certain irradiation conditions slow down the growth of large vacancy clusters and thereby delay swell\-ing beyond the time of the irradiation.

When comparing the behaviour of bubble nucleation in Mo and MgO, it is found that He has a significant impact even at very low concentrations in Mo. In contrast, the concentration of He has to be considerably higher in MgO to affect the swelling behaviour. For an inert matrix fuel, designed for transmutation purposes, this implies that the Mo matrix will have a tendency to swell considerably at rather high temperatures due to He stabilised vacancy clusters. If operated at lower temperatures, the swelling could instead be reduced due to the incubation time. In a MgO matrix, the swelling behaviour will instead depend largely on the production rate of He. For a low production rate, the material will have a swelling behaviour similar to the one seen when He is not present in the material. A high production rate implies that He will remain in vacancy clusters, thereby stabilising the clusters and enhancing the growth and swelling.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , 83 p.
Series
Trita-FYS, ISSN 0280-316X ; 2012:06
Keyword [en]
Helium, Radiation Damage, DFT, molybdenum, MgO
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:kth:diva-90960ISBN: 978-91-7501-263-6 (print)OAI: oai:DiVA.org:kth-90960DiVA: diva2:507565
Public defence
2012-04-04, F3, Lindstedtsvägen 23, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2012-03-12 Created: 2012-03-05 Last updated: 2012-03-12Bibliographically approved
List of papers
1. Helium cluster dissolution in molybdenum
Open this publication in new window or tab >>Helium cluster dissolution in molybdenum
2009 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 21, no 33, 335401- p.Article in journal (Refereed) Published
Abstract [en]

Helium retention and diffusion in molybdenum is studied on an atomistic scale with ab initio methods. The thermal stability of helium–vacancy clusters is quantified within the framework of density functional theory. Calculated helium emission rates are used to derive a desorption spectrum which is compared with experimental results. The agreement between the current calculations and available experiments is satisfactory except in the high temperature end of the spectrum. The current results indicate that above 1100 K He migration is assisted by lattice defects such as vacancies, rather than through interstitial diffusion.

Keyword
Ab initio, Atomistic scale, Desorption spectra, Helium clusters, Helium emissions, Helium retention, Helium-vacancy clusters, High temperature, Interstitial diffusion, Lattice defects, Thermal stability
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-10920 (URN)10.1088/0953-8984/21/33/335401 (DOI)000268413300014 ()2-s2.0-70349097246 (Scopus ID)
Note
QC 20101101Available from: 2009-08-24 Created: 2009-08-19 Last updated: 2017-12-13Bibliographically approved
2. Self-diffusion in MgO-a density functional study
Open this publication in new window or tab >>Self-diffusion in MgO-a density functional study
2011 (English)In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 23, no 34, 345402- p.Article in journal (Refereed) Published
Abstract [en]

Density functional theory calculations have been performed to study self-diffusion in magnesium oxide, a model material for a wide range of ionic compounds. Formation energies and entropies of Schottky defects and divacancies were obtained by means of total energy and phonon calculations in supercell configurations. Transition state theory was used to estimate defect migration rates, with migration energies taken from static calculations, and the corresponding frequency factors estimated from the phonon spectrum. In all static calculations we corrected for image effects using either a multipole expansion or an extrapolation to the low concentration limit. It is shown that both methods give similar results. The results for self-diffusion of Mg and O confirm the previously established picture, namely that in materials of nominal purity, Mg diffuses extrinsically by a single vacancy mechanism, while O diffuses intrinsically by a divacancy mechanism. Quantitatively, the current results are in very good agreement with experiments concerning O diffusion, while for Mg the absolute diffusion rate is generally underestimated by a factor of 5-10. The reason for this discrepancy is discussed.

Keyword
TOTAL-ENERGY CALCULATIONS, WAVE BASIS-SET, MAGNESIUM-OXIDE, LATTICE-DYNAMICS, INITIO, 1ST-PRINCIPLES, PRESSURE, CRYSTALS, FIELD
National Category
Condensed Matter Physics
Identifiers
urn:nbn:se:kth:diva-39014 (URN)10.1088/0953-8984/23/34/345402 (DOI)000294061500007 ()2-s2.0-80051930670 (Scopus ID)
Available from: 2011-09-06 Created: 2011-09-06 Last updated: 2017-12-08Bibliographically approved
3. Helium induced void and bubble formation in MgO
Open this publication in new window or tab >>Helium induced void and bubble formation in MgO
2012 (English)In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 60, 53-58 p.Article in journal (Refereed) Published
Abstract [en]

Trapping of helium in magnesium oxide vacancies and vacancy clusters is studied using empirical potential calculations. Based on the results, a rate-theory model is formulated in order to simulate the kinetics and long time dynamics of helium trapping. The simulations are in good agreement with helium annealing experiments in the literature. In particular, the tendency for helium to stabilise vacancy clusters is found to be strongly dependent on He concentration and temperature.

Keyword
Empirical potentials, Helium, Magnesium oxide, Pair potentials, Rate theory
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-91284 (URN)10.1016/j.commatsci.2012.03.025 (DOI)000303657700007 ()2-s2.0-84860511313 (Scopus ID)
Note
QC 20120529. Updated from submitted to published.Available from: 2012-03-12 Created: 2012-03-12 Last updated: 2017-12-07Bibliographically approved
4. The Impact of Helium on Void Nucleation in Molybdenum
Open this publication in new window or tab >>The Impact of Helium on Void Nucleation in Molybdenum
(English)Manuscript (preprint) (Other academic)
Keyword
PACS
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-91286 (URN)
Note
QS 2012Available from: 2012-03-12 Created: 2012-03-12 Last updated: 2012-03-12Bibliographically approved

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