TRISO, or Tri-Structural Iso-Tropic particle fuel, contain uranium, carbon and oxygen fuel kernel. The kernel has three layers of carbon- and ceramic-based materials that prevent the release of radioactive fission products. The DoD recently selected X-energy and BWXT Advanced Technologies, LLC micro-reactor design to continue with their micro-reactor prototypes.
During the Overland Trains development, an MM-1 nuclear micro-reactor was being developed for use to possibly power the Train. I will have more in the book about that development and what eventually came of the nuclear Train concept.