Heat removal capabilities of boron pebble rods for plasma-facing components in magnetic fusion reactor divertors*

This work demonstrates that boron-based pebble rods used as plasma-facing components can remove reactor-relevant steady-state front-surface heat loads (up to 40 MW/m²) without a cooling system. Hot pebbles are released by thermal shock of the binder, resulting in front surface recession rates up to 0.5 cm/s at 40 MW/m². The surface is promptly recovered as the rods are continuously extruded. This concept allows high heat load handling, sputtering resilience and tritium recovery, making it a promising solution for the first wall in the divertor region of fusion power plants. Boron-based pebble rods were produced by baking a mixture of ~1 mm diameter boron pebbles with a liquid binder precursor, forming solid pebble rods consisting of pebbles held together in a porous hexagonal boron nitride matrix. Boron melting was generally not observed, and front surface temperature remained below 2500 K, demonstrating large steady-state heat removal by surface recession without the use of close-in cooling channels. Front surface outgassing rates below 1000 Torr-L/m²/s were achieved, about 10x lower than carbon-based pebble rods under similar conditions, indicating that boron could be an excellent material choice for a pebble-rod first wall solution.