> All honor to Kairos if they can deliver, but history is against them.
History is not really against them. Our current reactors (mainly pressurized water reactors) are the way they are because Admiral Rickover determined that PWRs are the best option for submarines. He was not wrong, but civilian power reactors are not the same as the reactors powering submarines.
PWRs are expensive mainly because of the huge pressure inside the reactor core, about 150 times higher than the atmospheric pressure. For comparison, a pressure cooker has an internal pressure about 5 times higher than the atmospheric pressure, and such a cooker can explode with a pretty loud bang.
The Kairos Hermes reactor design is based on a design that was tested in the '60s, the Molten-Salt Reactor Experiment [1]. While such a reactor can be used to burn thorium, Kairos decided to go with the far more conventional approach of burning U-235. The reactor operates at approximately regular atmospheric pressure. This should reduce considerably the construction costs.
Of course, there are unknowns. While the world has built thousands of pressurized water reactors, it has built maybe 10 molten salt reactors. For example one quite unexpected effect in the MSRE was the enbrittlement of the reactor vessel caused by tellurium, which shows up as a fission product when U-235 burns.
The Nuclear Regulatory Commission is a very conservative organization, and they don't have much experience with molten salt reactors because nobody has. It took them 6 years to give NuScale an approval for a pressurized water reactor, design that they knew in and out. My guess is that they will not give Kairos an approval without at least 15 years of testing. But Google's agreement with Kairos is quite crucial to keep this testing going.
[1] https://en.wikipedia.org/wiki/Molten-Salt_Reactor_Experiment