I have now had a look at the video's of the explosion that occurred.
It looks like I have been talking out of my arse. The problem lies not with the cooling system as they stated (used rods, cooling ponds etc). But with the main steam production process.
PWR's work by pumping water, they describe this as coolant, round a sealed system, at high pressure. The fuel rods, giving off their radiation under control, heat this medium to around 600ยบ, thus heating the water in the heat exchanger, which is bolted to but separate from the pressure vessel,to superheated steam, to drive the turbines.
If flow of the heating medium is lost, of course the rods will overheat, but only if they are not withdrawn soon enough and they state they have an automatic shutdown system in place for this eventuality.
So. That leaves me with a few questions. Did the system malfunction, or were the technicians too slow to react and remove the rods manually, I mean we are talking hours here not minutes.????? or worse, the system lost water and pressure rapidly, exposing the rods and hydrogenising the system which has then blown.
The rods would have to be enriched to over 80% for a nuclear explosion to occur. As they melt, they then form a critical mass, below this enrichment that mass will not go supercritical, (chain reaction) That's why a bomb needs super enriched plutonium and a trigger. The resulting mass hits temperatures far higher than any of the surrounding material though and thats why it's fire needs to be extinguished as soon as poss and when radioactivity would be released, which is why they are using seawater and Boron now.
The explosion I saw looks nothing like a nuclear bang Doc, the only damage (worryingly) appeared to be the containment structure of the end reactor building.
A nuclear bang would flatten the whole site and a 5 mile radius round it, with severe damage as far again.
If you have a look at pics of the Jap bombs dropped in WW2, You will get a better idea and they were small by comparison.