I’m not quite sure as the tiny details get more specific, but I believe the salt need not be heated very high before it liquifies, I think about 150 C, so a reasonable external heater is used before the reactor starts up and keeps it hot...MUCH hotter, more like 650 C in operation.
Yes, the neutron source is another piece of non-trivial engineering and indeed, my readings seem to point both towards high-enriched uranium and/or some sort of accelerator device that generates protons, then accelerates them at a metallic target, generating neutrons by a process known as spallation...literally having them split off the backside of the target. I can’t claim a deep understanding of the details.
Ah well, there’s a reason I didn’t major in nuclear physics, but it’s an interesting topic nonetheless.