The 2nd design flaw is the positive void coefficient due to it using light water as a coolant and graphite as a moderator. In its scheme, the light water is a neutron absorber, and if steam voids form in it, there's less water absorbing neutrons. More neutrons -> more split atoms -> more heat -> more/bigger voids, this repeats until the reactor sufficiently disassembles itself: https://en.wikipedia.org/wiki/Void_coefficient

Such designs are illegal in the US, one of the reasons all but the latest version of the CANDU reactor can't be used here.