Associate
The, likely worst case, scenario for the designs at ZNPP is as you said, not the worst case ever possible when you include RBMK reactors of the unmodified design or if Russia went to significant lengths to engineer a nuclear disaster beyond just using a moderate amount of explosives to breach the core.
RBMK reactors were dual-use (for civilian/military purposes) as they were originally designed to produce Plutonium-239 for atomic bomb cores and then modified to generate electricity for the civilian population. Because of that they used a very large mass of slightly enriched Uranium in order to bombard the maximum amount of Uranium-238 with moderated thermal neutrons and produce Neptunium-239 which rapidly decays into Plutonium-239. Therefore, RBMK reactors held a huge amount of highly radioactive material, so if the core was breached there was far more radioactive material to release into the environment compared to a PWR.
The PWR was originally designed to generate electricity onboard nuclear submarines and naval vessels. Hence it is more compact and uses a much smaller volume of more enriched Uranium than RBMK reactors. Thus, there is far less radioactive material to release if the core is exposed. The Three Mile Island nuclear accident in 1979 involved a PWR reactor and its containment structure was not breached by an explosion so its partially melted down core was never exposed. A crudely engineered accident with explosives at ZNPP would probably be far worse than that accident.
The fact that Putin's government could even contemplate deliberately causing a nuclear accident at ZNPP proves that they have absolutely no regard for the lives/welfare of Ukrainian civilians or indeed that of their own loyalists in DPR, LPR and Crimea. They truly are selfish vermin.