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Reference : UPR3079-MARBAR1-001
Workplace : ORLEANS
Date of publication : Thursday, June 25, 2020
Scientific Responsible name : Marie-france Barthe
Type of Contract : PhD Student contract / Thesis offer
Contract Period : 36 months
Start date of the thesis : 1 October 2020
Proportion of work : Full time
Remuneration : 2 135,00 € gross monthly
Description of the thesis topic
Uranium dioxide is the most widely used fuel material in fission reactors. It is contained in the form of a stack of sintered pellets in a metal cladding constituting a rod. During its burn-up, 235U fission causes the formation of various defects and introduces fission products that can interact with each other leading to structural and chemical evolution and to a new material generally called spent nuclear fuel (SNF) containing up to about 10 % of plutonium and minor actinides and 10-30 % of fission products depending on the initial composition of the fuel and its burning time. The program seeks to contribute in the understanding of the fundamental out-of-core behaviour of SNF to ultimately ensure safe, reliable and economical use of storage and disposal systems. In a repository, the release of radionuclides from spent nuclear fuel (SNF) rods will strongly depend on the pellet microstructure existing when water comes into contact with the spent fuel surface, i.e. after 10,000 years of disposal. During this period, a large quantity of He atoms is produced by α- disintegrations of actinides in the spent fuel. If He is released into the free volumes in the rod it would contribute to the rising internal pressure and cladding creep strain. Moreover, helium accumulation in the form of bubbles at the grain boundaries can increase either the pressure in the bubbles, and thus the stresses exerted on the grain boundaries, or the size of the bubbles (vacancy migration), diminishing the mechanical strength of the grain boundaries. These phenomena could eventually lead to failure at the grain boundaries, releasing the accumulated radionuclides into the free volumes in the rod.
It is of major importance to know the He behavior in SNF that means its solubility and diffusion properties to predict its location in the rods (in bubbles in grain or grain boundaries, in the free volume between SNF and the cladding).
Various theoretical and experimental studies of He behavior in UO2 have been published. It depends together on its own properties of solubility and diffusion and on its interaction with the damage (point and extended defects) induced in the ceramic. However less is known about He behavior in the SNF. Indeed, the introduced new elements can change the defects properties and have an impact on the He behavior.
Among these FP, lanthanides can be dissolved in the fluorite matrix and their valence states could have an impact on the ratio O/M. They are also used as surrogate to the introduction of minor actinides also produced during burn up. We propose to investigate the effects of lanthanides doping on the He behaviour in uranium oxides. This work relies on separate effects experiments carried out in “model” materials to be carried out in CEMHTI laboratory (CNRS-Orleans, France, ) in the framework of the EURAD European project . They will include three parts
- the characterization of the doped materials with a special investigation of the native defects,
- the measurement of He migration and desorption during annealing
- the evolution of the defects during introduction of He and the annealing.
Helium will be introduced in samples using MeV ions implantation principally and some test experiments will be carried out using low energy He plasma exposure. In such tests we would like to check if it would be possible to avoid the formation of defects during He introduction. Three different He fluences will be introduced in agreement with the experiments already performed in UO2 [3,4] in order to bring in light the effects of Ln doping.
For both types of introduction, the 3He isotope will be used to allow its detection using NRA (Nuclear Reaction Analysis) and the 3He(d,α)1H nuclear reaction as described in . The He release and some He depth profiles will be measured as a function of annealing temperature and duration for various doped materials. The annealing conditions will be chosen to favor release or migration and cover temperature range of the Interim conditions: the temperature will vary from 100 to 600°C and the duration of isochronal heating will change in the range from 1 to several tens of hours.
Besides these experiments, samples will be characterized before and after implantation and also after annealing using positron annihilation spectroscopy (PAS) and Raman spectroscopy. It should help to get information on defects that could be involved in the trapping and migration of He. Two types of PAS techniques will be used positron annihilation lifetime spectroscopy (PALS) for characterisation of the as received materials in the volume [6,7] and Doppler broadening spectrometry coupled to a slow positrons beam to characterise the defects introduced close to the surface  during He implantation and follow their evolution versus the annealing conditions. Raman spectroscopy will also be used for the characterization of the first µms [9,10,11].
The Ln doped uranium oxides will be prepared at CNRS/ICSM using wet chemistry routes for precursors, and their conversion into the final actinide-based oxides by heating. It allows to get homogeneous dissolution of Ln in the UO2 matrix. We plan to investigate 3 different Lanthanides: La, Ce principally and some experiments will be carried out in (U,Nd)O2. We will start with La doped materials to check on different microstructures and La concentration the first results obtained in MYRTE project. Then the Ce doped materials will be investigated, and some tests will be performed in (U,Nd)Ox to check no special differences in the He behaviour compared to La doped. The Lanthanides content is provisionally fixed in the range between 3 and 14%. The highest concentration is essentially chosen to amplify effects and observe them in a first step. It has to be noticed that it is also on the same order than the total concentration of Pu, minor actinides and Ln in MOX spent fuel . It could be considered as a “model” oxide of such fuel. The lowest content of 3 % is close to the Ln concentration in spent UO2 fuel . The Nd concentration in Nd doped UO2 will be chosen in regard with the results obtained in (U,La)O2 samples.
3 S. Guilbert, T. Sauvage, P. Garcia, G. Carlot, M.F. Barthe, P. Desgardin, G. Blondiaux, C. Corbel, J.P. Piron, J.M. Gras, He migration in implanted UO2 sintered disks, Journal of Nuclear Materials. 327 (2004) 88–96. https://doi.org/10.1016/j.jnucmat.2004.01.024.
4 G. Martin, T. Sauvage, P. Desgardin, P. Garcia, G. Carlot, M. Barthe, Accurate automated non-resonant NRA depth profiling: Application to the low 3He concentration detection in UO2 and SiC, Nuclear Inst. and Methods in Physics Research, B. 258 (2007) 471–478.
5 Chamssedine, F., T. Sauvage, and S. Peuget, DIADDHEM set-up: New IBA facility for studying the helium behavior in nuclear glasses. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms, 2010. 268(11): p. 1862-1866.
6 Krause-Rehberg, R. and H.S. Leipner, Positron Annihilation in Semiconductors. solid-state Sciences. Vol. 127. 1999, Berlin: Springer Verlag. 378.
7 Corbel, C., et al., Positron-annihilation spectroscopy of native vacancies in as-grown GaAs. Physical Review B, 1988. 38(12): p. 8192-8208.
8 Desgardin, P., et al. Slow positron beam facility in Orleans. in Materials Science Forum. 2001. Trans Tech Publications Ltd., Zurich-Uetikon, Switzerland.
9 Lebreton, F., et al., Peculiar Behavior of (U,Am)O2−δ Compounds for High Americium Contents Evidenced by XRD, XAS, and Raman Spectroscopy. Inorganic Chemistry, 2015. 54(20): p. 9749-9760.
10 Talip, Z., et al., Raman and X‐ray Studies of Uranium–Lanthanum‐Mixed Oxides Before and After Air Oxidation. Journal of the American Ceramic Society, 2015. 98(7): p. 2278-2285.
11 Desgranges, L., et al., Raman Spectrum of U4O9: A New Interpretation of Damage Lines in UO2. Journal of Raman Spectroscopy, 2012. 43: p. 455-458.
12 RAPPORT CEA-R-6084 – 2005, Cécile FERRY, Christophe POINSSOT, Véronique BROUDIC, Chantal CAPPELAERE, Lionel DESGRANGES, Phillipe GARCIA, Christophe JEGOU, Patrick LOVERA, Pierre MARIMBEAU, Jean-Paul PIRON, Arnaud POULESQUEN, Danièle ROUDIL (CEA) with the contribution of Jean-Marie GRAS & Pol BOUFFIOUX (EDF), «Référentiel scientifique sur l'évolution à long terme des combustibles usés.» from PRECCI project.
13 RAPPORT CEA-R-6084 – 2005, Cécile FERRY, Christophe POINSSOT, Véronique BROUDIC, Chantal CAPPELAERE, Lionel DESGRANGES, Phillipe GARCIA, Christophe JEGOU, Patrick LOVERA, Pierre MARIMBEAU, Jean-Paul PIRON, Arnaud POULESQUEN, Danièle ROUDIL (CEA) with the contribution of Jean-Marie GRAS & Pol BOUFFIOUX (EDF), «Référentiel scientifique sur l'évolution à long terme des combustibles usés.» from PRECCI project.
CEMHTI is a CNRS own research unit spread over two sites (High temperature and Cyclotron), bringing together around a hundred people. The laboratory develops original expertise and tools at the national and international level to study in particular the physicochemical properties of materials in extreme conditions.
The researcher will join the team “Impurity Defects, Radiotracers: Properties, Materials, Imaging (DEFIR)” based on the cyclotron site and will be attached to the theme “Defect properties: Nuclear materials and ICT”.
Constraints and risks
The work will include travels during the duration of the thesis.
Work under ionizing radiation: Training on radiation protection will be provided internally.
Deadline for application : 15/07/2020
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