They will use encapsulated uranium fuel aka a pebble bed nuclear reactor. It will have heat pipes to move the heat from the core.

The full sized reactor model and design will be completed at the end of 2023. The reactor will be about the size of a family car. The current model is about the size of a microwave but the actual system will be larger.

Each TRISO particle is made up of a uranium, carbon and oxygen fuel kernel. The kernel is encapsulated by three layers of carbon- and ceramic-based materials that prevent the release of radioactive fission products.

The particles are incredibly small (about the size of a poppy seed) and very robust.

They can be fabricated into cylindrical pellets or billiard ball-sized spheres called "pebbles" for use in either high temperature gas or molten salt-cooled reactors.

TRISO fuels are structurally more resistant to neutron irradiation, corrosion, oxidation and high temperatures (the factors that most impact fuel performance) than traditional reactor fuels.

Each particle acts as its own containment system thanks to its triple-coated layers. This allows them to retain fission products under all reactor conditions.

Robust TRISO Fuel Research
TRISO fuel was first developed in the United States and United Kingdom in the 1960s with uranium dioxide fuel. In 2002, the Department of Energy (DOE) focused on improving TRISO fuel using uranium oxycarbide fuel kernels and enhancing its irradiation performance and manufacturing methods in order to further develop advanced high-temperature gas reactors.