The Energy Policy Act of 2005 authorized the Department of Energy to develop a research and development program that could deliver a high-temperature gas-cooled reactor prototype to increase domestic energy supplies, reduce greenhouse gas emissions, and move more quickly towards a national hydrogen economy. Westinghouse and its partners plan to build a pebble bed modular reactor that uses fuel balls surrounded by a hollow sphere of graphite moderator. These are stacked in a close-packed lattice and are cooled by helium, not water. The term "pebble bed" derives from the use of spheres. Rods are used to control fission in conventional nuclear reactors.

I didn't know the US even had a PBMR project. That we at least have one is something though they're now saying 2021 :/