Fabrication and Comparison of Fuels for Advanced Gas Reactor Irradiation Tests

As part of the program to demonstrate tristructural isotropic (TRISO)- coated fuel for the Next Generation Nuclear Plant (NGNP), a series of irradiation tests of Advanced Gas Reactor (AGR) fuel are is being performed in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL). In the first test, called "AGR-1," graphite compacts containing approximately 300,000 coated particles were irradiated from December 2006 until November 2009. The AGR-1 fuel was designed to closely replicate many of the properties of German TRISO-coated particles thought to be important for good fuel performance. No gaseous fission product release indicative of particle coating failure was detected in the nearly 3- year irradiation to a peak burn up of 19.6% at a time-average temperature of 1038- 1121°C. Following fabrication of AGR-1 fuel, process improvements and changes were made in each of the fabrication processes. Changes in the kernel fabrication process included replacing the carbon black powder feed with a surface-modified carbon slurry and shortening the sintering schedule. AGR-2 TRISO particles were produced in a six-inch diameter coater using a charge size about twenty-one times that of the two-inch diameter coater used to coat AGR-1 particles. Changes in the compacting process to increase matrix density and throughput included increasing the temperature and pressure of pressing and using a different type of press. Irradiation of AGR-2 fuel began in late spring 2010. Properties of AGR-2 fuel compare favorably with AGR-1 and historic German fuel. Kernels are more homogeneous in shape, chemistry, and density. TRISO-particle sphericity, layer thickness standard deviations, and defect fractions are also comparable. In a sample of 317,000 particles from deconsolidated AGR-2 compacts, 3 exposed kernels were found in a nitric acid leach test. No SiC defects in a sample of 250,000 deconsolidated particles and no IPyC defects in a sample of 64,000 particles were found. The primary difference in properties between AGR-1 and AGR-2 compacts is that AGR-2 compacts have a higher matrix density, 1.6 g/cm 3 compared to about 1.3 g/cm 3 for AGR-1 compacts. Based on fuel properties, excellent performance is expected for the AGR-2 UCO fuel.