Performance modeling of high-temperature gas-cooled reactor fuel during accident simulations

This research attempts to provide a technologically sound phenomenological basis for estimating the performance of Pebble Bed Reactor (PBR) TRISO-coated particle fuels. An extensive database on the performance of reference-quality coated fuels has been utilized, and a broad range of parameters that could influence particle integrity have been systematically investigated.

The PANAMA-I code for predicting fuel performance of PBR fuels was determined to have several shortcomings. In a significant number of cases, the code didn't provide fuel performance results that agreed with experimental measurements obtained during fuel irradiation and subsequent exposure to simulated accident conditions. As a result, a revised code, PANAMA-R, based on the diffusion of fission products through particle layers and matrix grains, has been developed at the University of Tennessee to predict fission product release. PANAMA-R also considers random "pressure-vessel" failure of fuel coatings as a method of fission product release; however, such failures are considered rare. Overall, the new PANAMA-R code was found to give good agreement with nearly all experimental measurements.

High fuel burnup and high fast neutron fluence are considered the most important parameters affecting fuel performance. High fuel burnup can result in a significant increase in fission gas build-up within the coated fuel particle and can eventually lead to pressure- induced failure of coatings. Similarly, a high fluence can decrease the strength of all coatings increasing the likelihood of particle failure(s). The effect, however, is actually less than predicted in the PANAMA-I code. Also, very long irradiation times with relatively high fuel burnup appears to increase fission product corrosion of the SiC layer, resulting in SiC "thinning" and increased "pressure-vessel" failures.