Oxidation and corrosion mechanisms in actinide oxide systems

The principal type of nuclear fuel is uranium dioxide (UO₂). After discharge from a nuclear reactor the spent fuel is highly radioactive and emitting a considerable amount of heat, which necessitates an intermediate storage period of several years before any further handling can take place. Ultimately, safe disposal needs to be applied to manage the spent fuel in a sustainable way. Direct disposal in stable geological repositories is being considered by many countries. However, in most countries, no political decision has yet been made, leading to prolonged intermediate storage periods up to several decades. Also during this intermediate storage period, safety must be ensured, and for any technical solution, incidental or accidental exposure of the spent nuclear fuel to oxidizing atmospheres is part of the safety assessments. Hence, a good understanding on radiological consequences of oxidative reactions of irradiated UO₂ is important.

The response of conventional nuclear fuel to oxidation and corrosion is determined mainly by the physico-chemical properties of uranium. UO₂ is quite susceptible to oxidation, already at room temperature, and  under normal atmospheric conditions it tends to evolve towards the thermodynamically more stable oxide U₃O₈. This transformation is associated with a considerable reorganization of the crystal structure, and results in a volume expansion of about 36% which is detrimental for the integrity of confinement barriers like the fuel cladding or storage containers. Research efforts in this domain have been numerous, because a confinement rupture may expose environmentally critical elements to the biosphere [1-8]. New perspectives on the oxidation behavior of UO₂ were obtained by investigating the kinetic mechanisms of these materials at the nanoscale, using imaging techniques like transmission electron microscopy [9], and atomistic models of crystal structures [10,11]. However, despite sustained efforts for almost a century, the transition and structural relations between the different compounds is not fully described yet.

Research on formation and stability of different binary oxides in heavier actinide systems is equally important, but these oxide systems are not yet characterized to the same extent as that of uranium. This is in part due to their radioactive nature, which makes handling considerably more complicated, and also because of the limited availability of materials in a sufficiently pure form. Transuranium elements are currently under investigation at SCK CEN in the context of optimal spent fuel management, but also for unconventional applications such as “nuclear batteries” for deep space exploration (e.g. the Np-O system), or in niche applications such as actinide-based fission dosimeters. Their handling typically involves liquid-to-solid conversion processes to obtain oxide precursors or final products, and hence, understanding their redox chemistry is of vital importance.

In this PhD project proposal the redox behavior of selected actinides U and Np will be investigated, with the aim to increase our understanding of their binary oxides. The research will focus specifically on the characterization of solid-state transformations and structural relations at the nanoscale between, influenced by external conditions such as temperature, oxygen partial pressure and moisture content. Solid state sample preparation routes as well as more advanced liquid-to-solid conversion routes based on sol-gel chemistry will be used to produce chemically homogeneous materials. In-situ thermogravimetry, calorimetry and X-ray diffraction will be applied to monitor the response to dry and humid gaseous environments (oxidizing, anoxic, reducing, and combinations thereof) at the macroscale. Microscopic observations using scanning and transmission electron microscopy will allow to distinguish features at the micro- and nanoscale, respectively. To unravel the compositional and structural relations occurring between the oxide phases, spectroscopic techniques such as X-ray absorption spectroscopy (XAS), as well as more advanced diffraction techniques like selected-area electron diffraction and neutron diffraction will be considered.

[1] M. Odorowski, C. Jégou, L. De Windt, V. Broudic, S. Peuget, M. Magnin, M. Tribet, C. Martin, Journal of Nuclear Materials, 468 (2016) 17.
[2] K.M. Peruski, K.C. Koehler, B.A. Powell, Environmental Science: Nano, 7 (2020) 2293.
[3] T. Mennecart, K. Lemmens, C. Cachoir, L. Adriaensen, A. Dobney, "Leaching tests for the experimental determination of IRF radionuclides from Belgian high-burnup spent nuclear fuel: Overview of pre-test fuel characterization experimental set-up and first results", 2nd Annual Workshop Proceedings of the Collaborative Project "Fast/Instant Release of Safety Relevant Radionuclides from Spent Nuclear Fuel", KIT Scientific report  KIT- 7676, (2013).
[4] K. Lemmens, et al., Journal of Nuclear Materials, 484 (2017) 307.
[5] T. Mennecart, G. Leinders, C. Cachoir, G. Cornelis, G. Verpoucke, G. Modolo, D. Bosbach, K. Lemmens, M. Verwerft, "First phase of the Spent Fuel Autoclave Leaching Experiments (SF-ALE) at SCK•CEN", Global Top Fuel 2019, Seattle (USA) (2019).
[6] L. Johnson, I. Günther-Leopold, J. Kobler Waldis, H.P. Linder, J. Low, D. Cui, E. Ekeroth, K. Spahiu, L.Z. Evins, Journal of Nuclear Materials, 420 (2012) 54.
[7] E. González-Robles, D. Serrano-Purroy, R. Sureda, I. Casas, J. de Pablo, Journal of Nuclear Materials, 465 (2015) 63.
[8] D. Serrano-Purroy, I. Casas, E. González-Robles, J.P. Glatz, D.H. Wegen, F. Clarens, J. Giménez, J. de Pablo, A. Martínez-Esparza, Journal of Nuclear Materials, 434 (2013) 451.
[9] G. Leinders, J. Pakarinen, R. Delville, T. Cardinaels, K. Binnemans, M. Verwerft, Inorganic Chemistry, 55 (2016) 3915.
[10] G. Leinders, R. Delville, J. Pakarinen, T. Cardinaels, K. Binnemans, M. Verwerft, Inorganic Chemistry, 55 (2016) 9923.
[11] G. Leinders, G. Baldinozzi, C. Ritter, R. Saniz, I. Arts, D. Lamoen, M. Verwerft, Inorganic Chemistry, 60 (2021) 10550.

La durée estimée