Segregation of xenon to dislocations and grain boundaries in uranium dioxide

Abstract

It is well known that Xe, being insoluble in UO 2 , segregates to dislocations and grain boundaries, where bubbles may form resulting in fuel swelling. Less well known is how sensitive this segregation is to the structure of the dislocation or grain boundary. In this work, we employ pair potential calculations to examine Xe segregation to dislocations (edge and screw) and several representative grain boundaries (Σ5 tilt, Σ5 twist and random). Our calculations predict that the segregation trend depends significantly on the type of dislocation or grain boundary. In particular, we find that Xe prefers to segregate strongly to the random boundary as compared to the other two boundaries and to the screw dislocation rather than the edge. Furthermore, we observe that neither the volumetric strain nor the electrostatic potential of a site can be used to predict its segregation characteristics. These differences in segregation characteristics are expected to have important consequences for the retention and release of Xe in nuclear fuels. Finally, our results offer general insights into how atomic structure of extended defects influence species segregation.I. INTRODUCTION Segregation can be understood as the interaction between an isolated zero dimensional defect (in this case, an impurity) and multidimensional defects such as dislocations (1D), grain boundaries (GB) and free surfaces (3D). 11 The driving force for this process is the energy difference (segregation energy) between the isolated impurity in the bulk and that associated with an extended structural defect. If the energy at the structural defect site is lower than in the bulk, the local impurity concentration is higher at the structural defect than in bulk and conversely, if the energy at the defect site is higher than in the bulk, the impurity will remain in the bulk. Segregation phenomena influence many material properties such as ion transport (which has a strong effect, for example on sintering rates), electrical and chemical reactivity and grain growth.2 At higher concentrations, segregating impurities are known to form glassy or ordered phases and can cause structural transformations of the GB. 2,3 In the case of UO 2 nuclear fuel, fission gases such as xenon (Xe) are insoluble in the fuel matrix. 45 Therefore, Xe tends to segregate to dislocations and GBs forming fission gas bubbles. Xe may diffuse along the short circuit paths provided by these two defects and be released into the plenum region between the fuel rod and the cladding. 5 If the gases are released from the fuel, they contribute to the gaseous atmosphere within the fuel pin and the fuel pin internal pressure correspondingly increases; this can contribute to failure of the fuel pin. If these gases are retained inside the fuel, they form bubbles, which lead to swelling of the fuel matrix. Swelling is detrimental to fuel performance as it contributes to fuel-cladding mechanical interaction (FCMI); the resulting stresses can shorten the lifetime of the pin. 7 Swelling and release are compleme...

Edge dislocation slows down oxide ion diffusion in doped CeO2 by segregation of charged defects

“…The dislocation core structure (Fig. 1b) we obtained is in good agreement with the previous geometric description of the dislocation core 41 and computational models for other fluoritestructured oxide materials 42,43 . This edge dislocation has a non-uniform strain-stress field as mapped in Fig.…”

Impact of homogeneous strain on uranium vacancy diffusion in uranium dioxide

“…These anisotropies mean that, in the complex strain fields associated with dislocations, the flux of defects to the dislocations will be quite convoluted, in contrast to the assumptions made by simple cylinder models of defect fluxes to dislocations. 83 For example, while the strain fields of edge dislocations indicate that defects and fission products will be attracted to the core 84,85 , the coupling with the dipole tensors suggests that greater enhancements in diffusion might occur along the edge dislocation line rather than toward the core (perpendicular to the line direction). These insights have important implications in understanding fuel creep and point to the need of comprehensive models that account for these anisotropies.…”

Uranium vacancy mobility at the Σ5 symmetric tilt and Σ5 twist grain boundaries in UO2

“…Thus, to understand how the distribution of fission gases evolves in the fuel, we must understand the underlying transport mechanisms, tied to the concentrations and mobilities of defects within the material, and how these gases interact with microstructural features that might act as sinks. The mobility of cation defects and fission products in bulk urania (UO 2 ) has received significant attention in the literature [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] as has the segregation of various fission products to microstructural features such as bubbles, dislocations and grain boundaries [20][21][22][23]. However, once a fission product has reached a sink, such as a grain boundary, its mobility may be different there than in the grain interior and predicting how, for example, bubbles nucleate within grain boundaries necessitates an understanding of how fission gases diffuse within boundaries.…”