I would love to see what a sub-critical geometry looks like. I also wonder what kind of math and engineering they did to come up with the shape they ended up with.
This one is just a bunch of separate tanks spaced as far away from each other as possible.
I've also seen a "Bed of nails" design where the liquid salt would flow over a bunch of moderating spikes, seep down between them and freeze.
I assume this is why some Finite Element Analysis packages come with warnings along the lines of "You must not use this to help Kim Jong-Un do you know what"
Feynman wrote of this when he was working at Los Alamos, i.e. the labourers weren't informed as to what they were really working with, so they could come very close to criticality accidents.