Conceptual design of a passively safe thorium breeder Pebble Bed Reactor

Results show that high conversion ratios (CR  > 0.96) and passive safety can be combined in a thorium PBR within a practical operating regime, which means a thermal power of 100 MW or higher, 1000 days total residence time of the breeder pebbles and fuel pebble handling times longer than 14.5 s, like in the HTR-PM. With an increased U-233 content of the fresh driver pebbles (18 w%), breeding (CR = 1.0135) can already be achieved for a 220 cm core and 80 cm driver zone radius. While the decay heat removal is sufficient in this design, the temperature feedback of the undermoderated driver pebbles is too weak to compensate the reactivity insertion due to the xenon decay during a DLOFC without scram. With a lower U-233 content per driver pebble (10 w%) it was found possible to combine breeding (CR = 1.0036) and passive safety for a 300 cm core and 100 cm driver zone radius, but this does require more than a doubling of the pebble handling speed and a high reprocessing rate of the fuel pebbles. The maximum fuel temperature during a DLOFC without scram was simulated to be 1481 °C for this design, still quite a bit below the TRISO failure temperature. The maximum reactivity insertion due to an ingress of water vapour is also limited with a value of +1497 pcm.