Fracture properties of SiC layer in triso-coated fuel particles

The fracture properties of coated SiC layers and their relationship with microstructure and processing condition have been investigated. A crush testing method for hemispherical shell specimens was developed based on the results of finite element (FE) analysis on stress distribution and applied to the evaluation of fracture strength for the chemical vapor deposited (CVD) SiC layers of tri-isotropic (TRISO) fuel particles. A blanket material was used in the crush tests as the FE analysis confirmed that a relatively soft metal (brass foil) inserted between the specimen's convex surface and plunger tip produced a highly uniform stress under the contact area. Nine sets of hemispherical shell specimens were prepared from different versions of coated fuel particles, which include several relevant lots of coated fuel particles: AGR (advanced gas-cooled reactor) fuels, a German reference fuel, and other development fuels available at Oak Ridge National Laboratory. The coatings display variations in grain size and density, which originated from different conditions in coating processes. At least 30 specimens were tested at room temperature for each material and the fracture stress data were analyzed using Weibull statistics. The local fracture stress calculated was converted to the fracture stress for the whole inner surface area of each particle. Mean fracture stress varied with test material in the range of 330-650 MPa, however, could not be clearly connected to the microstructural characteristics.