There are many amazing advanced nuclear technologies, but Oklo wanted to focus on the technology with the most demonstration history, with inherent safety, while having the capability to use waste as fuel.
By far, liquid-metal-cooled, metal-fueled fast reactors have the most demonstration history of the advanced fission technologies at over 400 reactor-years of operating experience worldwide.
In fact, the very first power plant to produce useful electrical power from fission was a liquid-metal-cooled, metal-fueled fast reactor – the Experimental Breeder Reactor-I (EBR-I). It's successor, the Experimental Breeder Reactor-II (EBR-II), operated for decades and showed that it could easily remain safe during challenges as severe as those that led to the Fukushima accident. The tests done with EBR-II showed that the coolant could be shut off and all shutdown systems removed, and the reactor would naturally stabilise and shut itself down without damage.
Only fast reactors can use waste as fuel, and in fact, this capability too was demonstrated already with EBR-II. Oklo is currently doing work with the Idaho National Laboratory to take the waste fuel from EBR-II and use it for our very first Aurora Powerhouse.