I too am grateful for this link.

The words about a "catch basin" design at the bottom are reassuring that a hot molten uranium slag going to the bottom has been designed for to spread it out and keep it from going critical.

BTW- the word critical means that the nuclear fission reaction is self-sustaining. Sub-critical means it's dying off, supercritical means it's building up.
To startup a reactor you have to take it from sub-critical to critical then just a little tiny bit supercritical in order to get the fission rate up to where it's making power.

So don't fear "supercriticality" unless it's uncontrolled.