> Nuclear plants should never fail by way of nuclear explosion. If this is a possibility it is a bad reactor design.

Fast reactors have always had this potential problem, since they require a much higher density of high enrichment material. A larger fast reactor might have a ton of plutonium in it; in an accident where fuel is melting and moving around unpredictably it's hard to prove that a prompt supercritical mass of Pu won't assemble somewhere. Edward Teller famously warned about this in 1967.

This is one advantage of MSRs as fast reactors: the fuel is already dispersed in salt, so such rearrangement is less plausible.