2018 | AFC ACCOMPLISHMENTS 107 Figure 2. Radiograph of welded targets prior to irradiation. Ultimately this capability will help deliver advanced fuel forms that offer enhanced performance and improve the safety and economics of nuclear power production. The first mini fuel experiments have been inserted into the HFIR starting in cycle 480 (June 2018).This first set of experiments is investigating irradiation effects on uranium mononitride (UN) kernels with a nominal diameter of 800 µm.A wide variety of kernels have been included in the testing including variations in kernel density (87-95% of theoretical density), carbon impurities, and burnable absorbers (Gd).The fuel kernels were fabricated at ORNL using a sol-gel process.Tristrictural isotropic (TRISO) coated particle fuel utilizing the same UN kernels was also included in the test matrix.The design temperature for these irradiations is 550°C, which is relevant for light water reactor applications. Higher temperatures can be achieved using a more insulating fill gas. One set of fueled capsules is targeting a lower burnup of 1% fission of initial metal atoms (FIMA).A second set of capsules will be irradiated to a target burnup of 6% FIMA. All fuel kernels use either depleted or natural uranium. Despite the lower enrichment, very high particle powers (on the order of 500 mW , or 150 W/g UN) can be achieved due to the extremely high neutron flux in the HFIR. Neutron absorption in 238U results in rapid breeding of 239Pu. Eventually, an equilibrium between 239Pu production and fission is achieved.At this point, the fuel fission rate remains nearly constant for the remainder of the irradiation.This greatly reduces the effects of fuel burnup on the fuel temperature over the course of the irradiation. In addition, the small size of the fuel naturally minimizes the sensitivity of the fuel temperature to fluctuations in fission rate (i.e., the total experiment heating is dominated by gamma heating in the structure). After irradiation, the experiments will be shipped to the Irradiated Fuels Examination Laboratory where the experiment targets will be cut open and the individual capsules will be extracted. Fission gas release will be measured for the fuel contained inside each capsule via puncturing and cold trapping of xenon and krypton isotopes. Swelling measurements will be performed using either gas pycnometry or X-ray computed tomography. Finally, microstructural characterization will be performed to evaluate irradiation effects on the kernels and (for theTRISO particles) the coating layers.