Stress Behaviors of a UN TRISO of a 12*12 FCM Fuel Assembly

A coated fuel particle (CFP) with a uranium mononitride (UN) kernel has been recently considered as an advanced fuel option, such as in fully ceramic micro-encapsulated (FCM) replacement fuel for pressurized water reactors (PWRs). In FCM fuel, a large number of tri-isotropic coated fuel particles (TRISOs) are embedded in a silicon carbide (SiC) matrix. The gas pressure produced in a TRISO of the 12*12 FCM FA was estimated throughout 1300 effective full power days (EFPD). This study treats the quantitative mechanical analysis of a TRISO of 12*12 FCM PWR FA using the gas pressure data and related thermo-mechanical properties of TRISO components. The stresses of the coating layers in a TRISO with an 800-micrometer diameter UN kernel has been estimated under the normal operation conditions of an FCM PWR. The following conclusions were drawn through this analysis. - At a high fluence, low temperature accelerates the expansion of IPyC, which gives rise to a severely high tensile hoop stress on the inner surface of the SiC layer. - At a temperature blow 800 .deg. C, the contribution of the gas pressure to the stress buildup is negligible. - It is likely that the SiC layer will survive until a fluence of 13*10²¹ n/cm² (En > 0.18 MeV), which is equivalent to 1100 EFPD. - The PyC IIDC should be modified into a more realistic form over the extended fluence