An Estimation of Gas Pressure in a UN TRISO of a 12Χ12 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 in a TRISO of a 16Χ16 FCM fuel assembly (FA) whose kernel diameter was 700 μm was estimated to be 12 MPa at 1000 effective full power days (EFPD). A core design of a 12Χ12 FA is also an option for the FCM PWR, in which the kernel size increases to 800 μm. This study treats the quantitative analysis of gas buildup in a TRISO of 12Χ12 FCM PWR FA. Table 1 shows the layers of the CFP used in a FCM fuel and their thicknesses and densities. The enrichment of the UN kernel is 19.7 atom %. It was assumed that the fuel temperature was 1000 .deg. C. The maximum fuel burnup is 139 GWd/tHM at 1000 EFPD. The nuclide inventory according to the fuel burnup was calculated using the McCARD code. The HSC software was used to calculate the thermo-chemical equilibrium. It is very difficult to calculate the thermochemical equilibrium for all nuclides. The major gas species is xenon, and its pressure is about 38 MPa at 1000 EFPD. The minor species are krypton and helium, and their gas pressures are negligible