This is a fast reactor. That is, a reactor in which the neutrons, instead of being moderated down to thermal energies, remain at high energy.<p>The fission cross section for such energetic neutrons is much lower than for thermal neutrons. Therefore, there has to be a much greater density of fissionable material in the reactor core.<p>The lack of a moderator also means rearrangement of the core in an accident is potentially much more dangerous. If the fuel itself rearranges to become more compact, say by melting and flowing, the reactivity could increase. This is not possible in (say) a light water reactor, where such a rearrangement would reduce reactivity.<p>The nightmare scenario for any fast reactor, warned about by Edward Teller in 1967, is a rearrangement that causes the core to become supercritical on prompt neutrons alone (that is, on only the neutrons released promptly at the moment of fission, not on those + the delayed neutrons emitted by some fission products as they decay). A fast prompt supercritical configuration could potentially explode with great violence, greater than Chernobyl. An atomic bomb is a prompt fast supercritical system.<p>I will want to see how the NRC does or does not license their design, a process that has just started. I will not be surprised if their approach ends up being unlicensable in the US because safety cannot be assured by analysis under accident conditions.