> Which of the Gen IV reactors would you prefer?

TBH, probably the SCWR. They seem like the easiest to build without a lot of new surprises.

> Which Gen IV reactor can not melt down, even if explicitly sabotaged?

One like the BREST. https://en.wikipedia.org/wiki/BREST_(reactor) . Funnily my preferred reactor, the SCWR, would probably not be immune to some sabotage, specifically explosives around the reactor. But a reactor which uses a metal coolant would be. It just so happens that the nature of a SCWR cooled with water means that the reactor core has to be much beefier anyways, so it's a lot harder to really damage even if that was an explicit goal.