Phase stability and oxidation behavior of high Pu content fast reactor MOX fuel


Vleugels Jef, (Katholieke Universiteit Leuven (KULeuven))

SCK•CEN Mentor

Delville Rémi,, +32 (0)14 33 31 65

Expert group

Fuel Materials

SCK•CEN Co-mentor

Verwerft Marc , , +32 (0)14 33 30 48

Short project description

For future fast reactors, like the lead-bismuth-cooled reactor MYRRHA, hypo-stochiometric uranium-plutonium oxides (mixed oxides=MOX) (O/(U/Pu)=O/M<2.0) with Pu content up to 30% are envisaged. For Pu level above 20%, it is known [1-3] that phase separation into two f.c.c. phases with different oxygen content is likely to occur during cooling after sintering. Recent studies [4] revealed that depending on cooling rate and final O/M ratio, phase separation can induce severe macroscopic cracks that can potentially impact fuel properties and cause in-pile safety issues (e.g. decrease in thermal conductivity). This calls for a careful re-appraisal of conditions used for high plutonium MOX fabrication, in particular on the determination and control of oxygen stoichiometry and cooling rate. A correlated issue is the spontaneous oxidation of MOX pellets at room temperature, even if subjected to very low oxygen and moisture contents as it is the case during their stay in glove boxes. Using XRD measurements, it has been recently shown [5] that spontaneous oxidation was mostly dictated by the variation in stoichiometry of the low-oxygen phase, coupled to the decrease in its fraction in bi-phasic MOX. The knowledge of the extent and kinetics of the oxidation process is therefore crucial when fabricating and/or handling hypo-stoichiometric uranium–plutonium mixed oxides with high plutonium content. Low temperature oxidation is influenced by temperature, initial stoichiometry, presence of one or several phases, oxygen and humidity content of atmosphere. Some basis knowledge on the respective influence of these parameters exists, also from MOX manufacturers such as Belgonucléaire, but a finer study of the mechanisms at play is still needed. From a broader perspective it is also of relevance for spent fuel storage studies and can be connected to a more fundamental study on phase equilibria in the complex U-Pu-O system.


SCK•CEN is currently setting up a brand new line of glove boxes for MOX production and characterization, building up on the experience acquired in the UO2 lab and from Belgonucléaire. The new line will allow the fabrication UO2/PuO2 powder via wet routes, the mixing and co-milling of powders, pressing of pellets and sintering under perfectly controlled conditions (T, p(O2)) in a high temperature furnace in order to control fuel stoichiometry. A low temperature furnace with controlled atmosphere may be used for oxidation studies. High-end characterization technique (BET surface area analysis, thermogravimetry, XRD) will be available for characterizing powder characteristics, stoichiometry and phase in powder/pellets.  This will allow performing a set of parametric studies to study the phase stability and oxidation behavior of high Pu content MOX in the framework of a PhD. 

[1]          R. Vauchy, A.-C. Robisson, F. Audubert, F. Hodaj, Ceram. Int., 40 (2014) 10991-10999.

[2]          R. Böhler, et al., J. Nucl. Mater., 448 (2014) 330-339.

[3]          C. Sari, U. Benedict, H. Blank, J. Nucl. Mater., 35 (1970) 267-277.

[4]          R. Vauchy, R.C. Belin, A.-C. Robisson, F. Hodaj, J. Eur. Ceram. Soc., 34 (2014) 2543-2551.

[5]          R. Vauchy, et al., J. Nucl. Mater., 465 (2015) 349-357.

The minimum diploma level of the candidate needs to be

Master of sciences , Master of sciences in engineering

The candidate needs to have a background in

Physics , Chemistry
Before applying, please consult the guidelines for application for PhD.