Experiments on flammability limits, ignition energies, and flame speeds were carried out in a 11.25-and a 400-liter combustion vessel at initial pressures and temperatures of 100 kPa and 295 K, respectively. Flammability maps of hydrogen-nitrous oxide-nitrogen, methane-nitrous oxide-nitrogen, ammonia-nitrous oxide-nitrogen, and ammonia-nitrous oxide-air, as well as lean flammability limits of various hydrogenmethane-ammonia-nitrous oxide-oxygen-nitrogen mixtures were determined. Ignition energy bounds of methane-nitrous oxide, ammonia-nitrous oxide, and ammonia-nitrous oxide-nitrogen mixtures have been determined and the influence of small amounts of oxygen on the flammability of methane-nitrous oxide-nitrogen mixtures has been investigated. Flame speeds have been measured and laminar burning velocities have been determined for ammonia-air-nitrous oxide and various hydrogen-methane-ammonianitrous oxide-oxygen-nitrogen mixtures. Lower and upper flammability limits (mixing fan on, turbulent conditions) for ignition energies of 8 J are:Flammability limits of methane-nitrous oxide-nitrogen mixtures show no pronounced dependence on small amounts of oxygen (< 5%). Generally speaking, flammable gases with large initial amounts of nitrous oxide or ammonia show a strong dependence of flammability limits on ignition energy.

The detrimental effects of the fission gas Xe on the performance of oxide nuclear fuels are well known. However, less well known are the mechanisms that govern fission gas evolution. Here, in order to better understand bulk Xe behavior (diffusion mechanisms) in UO2±x we calculate the relevant activation energies using density functional theory (DFT) techniques. By analyzing a combination of Xe solution thermodynamics, migration barriers and the interaction of dissolved Xe atoms with U, we demonstrate that Xe diffusion predominantly occurs via a vacancy-mediated mechanism. Since Xe transport is closely related to diffusion of U vacancies, we have also studied the activation energy for this process. In order to best reproduce experimental data for the Xe and U activation energies, it is critical to consider the active charge-compensation mechanism for intrinsic defects in UO2±x. Due to the high thermodynamic cost of reducing U 4+ ions, any defect formation occurring at a fixed composition, i.e. no change in UO2±x stoichiometry, always avoids such reactions, which, for example, implies that the ground-state configuration of an O Frenkel pair in UO2 does not involve any explicit local reduction (oxidation) of U ions at the O vacancy (interstitial).

The distribution and atomic structure of grain boundaries has been investigated in UO2. Our scanning electron microscopic/electron backscatter diffraction experiments on a depleted UO2 sample showed real nuclear fuels contain a combination of special coincident site lattice (CSL) and general boundaries. The experimental data indicated that ∼16% of the boundaries were CSL boundaries and the CSL distribution was dominated by low Σ boundaries; namely Σ9, Σ3, and Σ5 Based on our experimental observations, the structures of select low Σ (Σ5 tilt, Σ5 twist, Σ3 tilt) and a random boundary were analyzed in greater detail using empirical potential atomic‐scale calculations. Our calculations indicate that the boundaries have very different structures and each CSL boundary had multiple minima on the γ‐surface. The presence of a significant fraction of CSL boundaries and the differences in their structures are expected to have important consequences on fuel properties.

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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...