Effects of the central graphite column dimension and pebble size on power density distribution in annular core pebble-bed HTR

Pebble-bed HTR utilizes the configuration of randomly distributed graphite and fuel pebbles which contain randomly dispersed TRISO particles, causing the double heterogeneity effect and making simulation get complicated. To establish a high-fidelity whole core model of the annular core pebble-bed HTR, this article proposes a two-step whole core modelling scheme with flexibility. This scheme is verified by comparing the HTR-10 initial critical benchmark results with the HTR-10 experiment results. Based on the geometry modelling method and Monte Carlo simulation, this study investigates the effect of the central graphite column dimension and the pebble size upon the nuclear heating power density distribution in annular core pebble-bed HTR. Results show that the annular core reactor has a more edgy distribution of neutron flux and nuclear heating power density and a higher peak value, compared with the cylindrical configuration core reactor. The annular core reactors with a higher thermal power could realize a higher helium outlet temperature with a precondition that the outlet helium flow is carefully separated and mixed. Accordingly, a higher thermoelectric conversion efficiency could be achieved. Reactors filled with smaller pebbles reach the criticality more quickly. However, the radius of the pebbles in the range from 2.5 to 3.5 cm does less effect than the size of the central graphite column does to the neutron flux and nuclear heating power density distribution. The running-in phase of the annular configuration core reactor is investigated in the last section. The heating power density gradually flattens as the initial core pebbles fall and new fuel pebbles are loaded into the cavity. In this running-in phase, we adopt a one-to-one mapping technique that sets the temperature of pebbles to their real value, varying from their locations. This enables us to do further work of the neutronics/thermal-hydraulics analysis and the dynamic simulation which fit the realistic engineering practice, and to explore the fuel management scheme of the annular core high-temperature gas-cooled pebble-bed reactor.