b'Integrated Experimental and Modeling Approach to Assessment of Enhanced Grain Size of UO 2Fuel PelletsPrincipal Investigator: Joshua T. WhiteTeam Members/Collaborators: Arjen van Veelen, Brandon S. Battas, Tashiema L Ulrich, Michael Tonks,Michael W.D. Cooper, and Joshua T. WhiteU ranium dioxide (UO 2 ) fuelalso measured the grain size as a pellets power more than 90function of oxidizing atmosphere percent of civilian nuclear(pO 2 ) and concluded that largest and reactors operating today. With thesmallest grain sizes were obtained increasing demands for carbon neutralunder CO 2and Ar atmospheres, energy, the continuous improvementrespectively. We combined this with Methods shown hereof the efficiency and safety nuclearmeso- (MARMOT) and atomic provide a frameworkreactors is a top priority. The additionscale modeling to predict vacancy to integrate modelingof oxide dopants to UO 2fuel providesconcentrations with variations in and experimentation,significant improvement to the micro- O/U ratio. The model sheds light on accelerating thestructural properties, specifically a twothe mechanisms and the energetics validation andto fivefold increase of the grain size1-5.(bulk and grain boundary) that qualification of This is important because it has beengovern sintering, which highlight the model development. demonstrated that grain size affectsstrong thermodynamic driving force fission gas release from fuel pellets, asfor vacancies to diffuse to the grain well as creep and hardness of the fuelboundaries and then diffuse along pellets. In order to develop and licensethe grain boundaries to eliminate enhanced UO 2as Accident Tolerant Fuelinternal porosity. The experimental (ATF), understanding the mechanisticand modeling work correspond well impacts of different dopants on fuelfor near-stoichiometric compositions, performance must be demonstrated.but more work is necessary to account This study aims to develop sinteringfor defect clustering that will impact kinetic models for undoped UO 2withthe applicability of the model for high grain sizes comparable to that of dopedO/U ratios. This work underpins the UO 2with the aim to use this as refer- fundamental mechanisms that will ence material for studying doped-UO 2 further elucidate the understanding fuel performance. of irradiation-enhanced densification Project Description:to better predict material responses This study set out to understand theto irradiation. Research in this area mechanistic impacts of differentsupport the continued interest within sintering conditions on thethe advanced fuel campaign (AFC) to activation energies that control theadvance understanding of near-term microstructural parameters, such asATF fuel through integrated modeling grain size and density. Our resultsand experimental efforts clarifying show that activation energies decreasethe role of fission gas release and with increasing stoichiometry atirradiation induced densification in 0.0003 x 0.2 (Figure 1). Welarge grained UO 2 . 34 2021|AFC ACCOMPLISHMENTS'