TRISO Fuel
Enriched TRISO fuel is used in pebblebed reactors to produce energy. This is produced by mines. When fuel becomes spent in a pebblebed reactor, it can be ejected. This depleted fuel can be reprocessed over some time with energy to produce higher tier fuels.

Mines
Mines produce metal ore based on the amount of extractible ore. The metal ore is used for the construction of even more mines and pebblebed reactors. Once a mine is depleted, it is no longer active and cam be salvaged for metal ore to be used for other mines. Mines also produce enriched fuel which is used for pebblebed reactors.

Pebblebed Reactors
Pebblebed reactors are simple fission reactors that produce energy with enriched fuel. You can load the fuel using the "Load with Enriched Fuel" buttons. Each reactor has a maximum capacity that both enriched and spent fuel contribute towards. The amount of enriched fuel in a reactor increases the efficiency of reactor logarithmically. Energy can be spent on them to increase the amount of reactors, increasing capacity, and the efficiency of the reactors.