The positive void coefficient problem[1] was intractable (without putting the whole reactor vessel inside a pressurized tank), but it was the combination of that and the control rod design that caused the explosion. The control rods were modified trivially. The reactor is basically safe, though you wouldn't repeat the design choices.

[1] An IMHO stupidly obfuscated way of saying that the cooling water around the reactor was an unpressurized pool that would boil if it got hot. Low-density bubbles of steam absorb fewer neutrons than liquid water, so when it boils the core gets hotter, not cooler.