Parameter study of the running-in process for the Generic Pebble Bed Reactor (GPBR-200)

The run-in period of a pebble-bed reactor is complex and difficult to model given significant heterogeneity in core composition, power, and temperature. While the initial core composition should eventually result in the same equilibrium core composition, the approach to equilibrium can vary significantly depending on factors such as startup fuel enrichment and power ramp rate. To explore this, we used a high-fidelity model of the General Pebble Bed Reactor to vary power ramp schemes and startup core compositions. Both the initial core composition and power ramp rate had a significant impact on the flow rate of pebbles during early time steps, with higher ramp rates and low enrichment resulting in non-physical flow rates. Power ramp rate alone was found to dictate the maximum pebble power peaking observed during the run-in process, with higher ramp rates resulting in greater peak pebble powers. Startup fuel enrichment and power ramp were found to both impact total fuel consumption, although the impact of startup fuel enrichment was generally secondary to ramp rate.