Letter to the Editor An experimental study of grain growth in mixed oxide samples with various microstructures and plutonium concentrations P. Van Uffelen a,⇑ , P. Botazzoli b,1 , L. Luzzi b , S. Bremier a , A. Schubert a , P. Raison a , R. Eloirdi a , M.A. Barker c a European Commission, Joint Research Centre, Institute for Transuranium Elements, Hermann-von-Helmholtz-Platz 1, D-76344 Eggenstein-Leopoldshafen, Germany b Politecnico di Milano, Department of Energy, Nuclear Engineering Division (CeSNEF), via Ponzio 34/3, I-20133 Milano, Italy c The UK’s National Nuclear Laboratory Ltd., Central Laboratory, Sellafield, Seascale, Cumbria CA20 1PG, UK article info Article history: Received 28 August 2012 Accepted 27 November 2012 Available online 7 December 2012 abstract Samples of (U, Pu)O 2 Mixed Oxide (MOX) with various microstructure and plutonium contents ranging between 4% and 25% have been submitted to a series of heat treatments in order to assess grain growth between 1350 and 1750 °C. XRD measurements on the samples indicated that they were not affected by modifications in the oxygen-to-metal ratio during annealing. The grain size distributions inferred by means of image analysis of metallographic pictures reveal that, when taking into account the experimen- tal uncertainties, the grain growth kinetics are similar to those observed in conventional UO 2 fuel that was also tested under the same conditions. An analysis of experimental data available in the open liter- ature for both UO 2 and MOX fuel leads to the same conclusion. It is therefore suggested that grain growth models for UO 2 fuel can be applied to MOX fuel for fuel performance simulations, when taking into con- sideration the uncertainties pertaining to grain growth measurements. Ó 2012 Elsevier B.V. All rights reserved. The release of fission gas and helium plays an important role in the fuel performance in nuclear reactors, and in mixed uranium– plutonium oxide (MOX) fuel in particular. Indeed, comparative studies with respect to conventional UO 2 fuel reveal that gas re- lease is higher in commercial MOX fuel at discharge [1]. This was considered as an important issue in order to achieve equivalent discharge burnups in both fuel types. This dissimilarity has been mainly attributed to the relatively higher power of the MOX fuel rods. Nevertheless, there is also a discussion about the role of the microstructure resulting from the different commercial fabrication processes for MOX fuel like MIMAS (MIcronised MASter blend) or SBR (Short Binderless Route). The average size of the grains deter- mines the distance that fission gas atoms must migrate before they can be released via the tunnel network of interconnected bubbles along the grain boundaries [2]. In addition, grain boundaries will collect insoluble fission gas atoms while they move across the fuel. The accumulation rate of fission gas along grain boundaries, which determines the onset of thermal fission gas release, is therefore strongly affected by the kinetics of grain growth. Sari [3] published data on grain growth in unirradiated sub- stoichiometric (U 0.8 Pu 0.2 )O 2x fuel for fast breeder reactors. He concluded that grain growth was reduced when decreasing the oxygen-to-metal (O/M) ratio in the range 1.97 < O/M < 2. Bainbridge et al. [4] also studied grain growth of fast breeder reac- tor (FBR) MOX fuel irradiated in a material test reactor to a low burn-up. More recently, Duriez et al. [5] reported grain growth data for stoichiometric MOX fuel with plutonium concentrations between 3 and 15 wt% with an initial grain size of 5–6 lm. Pellets were prepared by co-grinding UO 2 and PuO 2 powders. They annealed their samples for 240 h at 1750 °C, obtaining grain sizes between 15 and 23 lm. Unfortunately, there are insufficient data for a systematic analysis of the effect of the microstructure and plutonium content on grain growth in current commercial MOX fuels. The present study aims at providing experimental data to fill that gap. Five types of stoichiometric MOX samples have been studied, the main features of which are reported in Table 1. The SBR sample is a commercial fuel produced by the British Nuclear Fuels Limited (BNFL), while the MIMAS  and the SOLGEL samples have been manufactured at the laboratory scale at ITU. In particular, the MIMAS  9 and SOLGEL9 samples have been manufactured in the framework of the European OMICO project [6]. Although manufac- tured in the laboratory, the MIMAS  sample is representative of commercial LWR MOX fuels. However, some features of the MIMAS used in the present analysis differ from the fuel manufactured in commercial facilities (e.g., AUC/ADU MIMAS). More precisely, the blend obtained in this study (MIMAS  ) is more homogeneous in comparison with standard MIMAS. The plutonium distribution is compared with that in the SBR sample considered in this study in Fig. 2. Our sample is characterized by a single grain size 0022-3115/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnucmat.2012.11.053 ⇑ Corresponding author. Tel.: +49 7247 951384; fax: +49 7247 95199384. E-mail address: Paul.Van-Uffelen@ec.europa.eu (P. Van Uffelen). 1 Present address: Siemens AG, Freyeslebenstrasse 1, D-91058 Erlangen, Gemany. Journal of Nuclear Materials 434 (2013) 287–290 Contents lists available at SciVerse ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat