Online Burnup Monitoring of Pebble Bed Reactor Fuel Using Passive
Gamma-Ray Spectrometry

Dr. Jianwei Chen
University of Cincinnati

The Pebble Bed Reactor (PBR) is a helium-cooled, graphite-moderated high temperature nuclear power reactor. The fuel assemblies for this reactor are spherical pebbles that are composed of an outer graphite shell (~0.5 cm thick) surrounding an inner fuel zone (~2.5 cm radius). The fuel zone has a graphite matrix in which silicon carbide (SiC) coated TRISO fuel microspheres are embedded. The SiC coating assures that no fission products are released from the microsphere even at elevated temperatures. Each fuel pebble contains 9 g of Uranium enriched to approximately 8% in U-235. Present designs describe core that is 3.7 m in diameter and 9.0 m in height. When fully loaded, the core would contain 360,000 fuel pebbles. Compared to conventional LWRs, the PBR has a higher thermal efficiency due to its higher coolant outlet temperature; it has a large passive heat removal capability of the reactor design due to the slender core (high surface area to volume ratio) and low power density; it could achieve the steady state of optimal moderation since it is graphite-moderated with online refueling.

Online refueling is a unique feature of PBR. The reactor is continuously refueled with fresh or reusable fuel pebbles from the top of the reactor, while used fuel is removed from the bottom. For each discharged fuel pebble, its burnup is measured by an online burnup monitoring system to determine if it reaches the prescribed End-of-Life burnup limit (~80,000 MWD/MTU). If not, it will be reloaded back to the core. Otherwise, it is discarded to the spent fuel facility.