Numerical study on influences of bed resettling, breeding zone orientation, and purge gas on temperatures in solid breeders

Abstract

We apply coupled computational fluid dynamics and discrete element method (CFD-DEM) modeling tools with new numerical implementations of pebble fragmentation to study the combined effects of granular crushing and ensemble restructuring, granular fragment size, and initial packing for different breeder volume configurations. In typical solid breeder modules, heat removal from beds relies on maintaining pebblepebble and pebble-wall contact integrity. However, contact is disrupted when an ensemble responds to individually-crushed pebbles. Furthermore, restructuring of metastable packings after crushing events are, in part, dependent on gravity forces acting upon the pebbles. We investigate two representative pebble bed configurations under constant volumetric heat sources; modeling heat removed from beds via inter-particle conduction, purge gas convection, and contact between pebble beds and containers. In one configuration, heat is removed from at walls oriented parallel to the gravity vector (no gap formation possible); in the second, heat is removed at walls perpendicular to gravity, allowing for the possibility of gap formation between bed and wall. Judging beds on increase in maximum temperatures as a function of crushed pebble amount, we find that both pebble bed configurations to have advantageous features that manifest at different stages of pebble crushing. However, all configurations benefit from achieving high initial packing fractions.

Blanket/first wall challenges and required R&D on the pathway to DEMO

“…Coupled discrete element with CFD simulation of a ceramic breeder bed showing resultant temperature profiles after 3% of pebbles crushed and simulated uniform nuclear heating (temperature in • C)[148].…”