In light water reactor (LWR) UO₂ fuels, the evolution of volatile fission products is one of the critical areas of fuel behavior that is yet to be fully understood. In UO₂ irradiated to high burnups, it is well known that most released fission gases come from the central region of the fuel as opposed to the highly porous high burnup structure (HBS) on the periphery of the pellets. However, fuels with and without interconnected bubble networks at the fuel center showed high to moderate release fractions, which conceals the mechanisms responsible for the gas release in the latter scenario. In this work, focused ion beam tomography was used to investigate the three-dimensional bubble structure in an irradiated LWR UO₂ fuel pellet with high degree of fission gas retention so that the degree of bubble interconnection could be assessed. Six radial locations with different burnups and temperatures were serially sectioned and imaged to reconstruct the three-dimensional bubble structure. As expected, the highest porosity was observed at the periphery of the fuel (HBS). The porosity then decreased towards the pellet center, except for the centermost location. This location had a slightly higher porosity than its adjacent mid-radial location, which was attributed to the temperature difference between the two locations. This study provides a first-time volumetric evaluation of the porosity at different radial locations on a UO₂ fuel pellet. During this investigation, no significant bubble interconnection was noted at any of the six radial locations.

在轻水反应堆（LWR）UO ₂燃料中，挥发性裂变产物的演变是燃料行为的关键领域之一，尚待充分了解。众所周知，在UO ₂辐照到高燃尽状态中，大多数释放的裂变气体来自燃料的中心区域，与颗粒周围的高孔隙率高燃耗结构（HBS）相反。但是，在燃料中心有或没有相互连接的气泡网络的燃料显示出较高至中等的释放比例，这掩盖了在后一种情况下导致气体释放的机制。在这项工作中，使用聚焦离子束层析成像技术研究了照射后的LWR UO ₂中的三维气泡结构具有高度裂变气体保留能力的燃料芯块，以便可以评估气泡相互连接的程度。对具有不同燃耗和温度的六个径向位置进行了连续切片和成像，以重建三维气泡结构。如所预期的，在燃料（HBS）的外围观察到最高的孔隙率。然后，除最中心位置外，孔隙率朝着丸粒中心减小。该位置的孔隙度比其相邻的中径向位置略高，这是由于两个位置之间的温差所致。这项研究首次对UO ₂上不同径向位置的孔隙率进行了体积评估燃料芯块。在此研究过程中，在六个径向位置中的任何一个位置都未发现明显的气泡互连。