Grain Subdivision Fission Gas Swelling Model for UO 2
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Grain Subdivision Fission Gas Swelling Model for UO2 Thomas Winter1, Richard Hoffman1 and Chaitanya S. Deo1 Nuclear & Radiological Engineering Program, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, United States
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ABSTRACT Under high burnup UO2 fuel pellets can experience high burnup structure (HBS) at the rim also known as rim effect. The HBS is exceptionally porous with fine grain sizes. HBS increases the swelling further than it would have achieved at a larger grain size. A theoretical swelling model is used in conjunction with a grain subdivision simulation to calculate the swelling of UO2. In UO2 the nucleation sites are at vacancies and the bubbles are concentrated at grain boundaries. Vacancies are created due to irradiation and gas diffusion is dependent on vacancy migration. In addition to intragranular bubbles, there are intergranular bubbles at the grain boundaries. Over time as intragranular bubbles and gas atoms accumulate on the grain boundaries, the intergranular bubbles grow and cover the grain faces. Eventually they grow into voids and interconnect along the grain boundaries, which can lead to fission gas release when the interconnection reaches the surface. This is known as the saturation point. While the swelling model used does not originally incorporate a changing grain size, the simulation allows for more accurate swelling calculations by introducing a fractional HBS based on the temperature and burnup of the pellet. The fractional HBS is introduced with a varying grain size. Our simulations determine the level of swelling and saturation as a function of burnup by combining an independent model and simulation to obtain a more comprehensive model. INTRODUCTION Under high burnup UO2 fuel pellets can experience high burnup structure (HBS) at the outer edge also known as rim effect. The HBS is exceptionally porous with fine grain sizes. HBS increases the swelling further than it would have achieved at a larger grain size. A theoretical swelling model [2] is used in conjunction with a grain subdivision simulation to calculate the swelling of UO2. While the swelling model used does not originally incorporate a changing grain size, the simulation allows for more accurate swelling calculations by introducing a fractional HBS based on the temperature and burnup of the pellet. The fractional HBS is introduced from two sources. One has an instantaneous grain division at a critical fission density [3] whereas the other gives us additional information for a more continuous approach. THEORY UO2 Swelling Model In UO2 the nucleation sites are at vacancies and the bubbles are concentrated at grain boundaries. Vacancies are created due to irradiation and gas diffusion is dependent on vacancy migration. The gas diffusion is dependent on three parts; intrinsic diffusion at high temperatures, vacancy assisted diffusion dependent on both temperature & fission rate, and irradiation enhanced athermal diffusion [1,2]. The bubbles born within the grains at vacancies
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