The three-dimensional fracture network within a spent fuel pin is characterised using sequential grinding, and its permeability is numerically estimated. Advanced Gas-cooled Reactors (AGRs) produce spent fuel pins consisting of an outer steel cladding enclosing ceramic uranium dioxide (UO₂) pellets. During irradiation, fuel pellets may undergo fracturing due to thermal, densification and swelling effects. Fracture patterns are usually observed on the surface of the pellet or through a cross section or longitudinal plane along the pellet. In this work, the three-dimensional fracture pattern within the pellet is characterised using an optical microscope. The pellet is progressively ground and polished, providing sequential cross sections, which together yield a three-dimensional discrete fracture pattern. Multiple large fractures grow to connect the cladding to the internal region of the pellet. Multiple surface fractures are observed that do not penetrate into the matrix of the pellet. The porosity of the UO₂ and apertures of the fractures are estimated by sampling microscopic images. Darcy flow is numerically solved using the finite element method, computing flow through the matrix and fractures simultaneously. The equivalent tensorial permeability of the system is estimated for various approximate fracture apertures. The fracture network raises the permeability of the pellet by an order of magnitude.

使用顺序研磨来表征乏燃料销内的三维断裂网络，并对其渗透率进行数值估算。先进的气冷反应堆（AGR）产生乏燃料销，该乏燃料销由外层金属包壳和陶瓷二氧化铀（UO ₂）颗粒。在辐射过程中，燃料颗粒会因热，致密化和溶胀作用而破裂。通常在粒料的表面上或沿着粒料的横截面或纵向平面观察到断裂图案。在这项工作中，使用光学显微镜对丸粒内的三维断裂模式进行了表征。粒料被逐步研磨和抛光，提供了连续的横截面，这些横截面一起产生了三维离散的断裂图案。出现了多个大裂缝，将包层连接到药丸的内部区域。观察到没有渗入颗粒基质的多个表面裂缝。UO ₂的孔隙率并通过对显微图像进行采样来估计裂缝的孔径。使用有限元方法对达西流进行数值求解，同时计算流经基体和裂缝的流量。对于各种近似的裂缝孔径，估计了系统的等效张量渗透率。断裂网络将粒料的渗透性提高了一个数量级。