Hmm, I think it is readily accepted and established that complete loss of coolant in a LWR will lead to meltdown - although I can't comment on whether that was

Hmm, I think it is readily accepted and established that complete loss of coolant in a LWR will lead to meltdown - although I can't comment on whether that was the scientific opinion immediately after Chernobyl. The problem at Chernobyl is not as you describe. The issue there was that through a process of bad core management, while trying to shut down the reactor and at the same time run an experiment, meant the core ended up in a very unstable state. Mostly the instability was managed automatically but at some point it went wrong; there is much debate over this, though conventionally it is accepted that someone, it is not known who, SCRAMmed the reactor - the poor design of the control rods meant that rather than immediately stop the reaction it actually made things worse as they were inserted, causing the core to overheat causing extreme pressure, cracking the control rods and essentially making the situation unrecoverable. Then a steam explosion broke the pressure seal and the chamber emptied of coolant. After that the exact mechanics of what happened is somewhat unknown (instrument failure) but the lack of coolant lead to a critical "nuclear excursion"; i.e. essentially the neutron emission becomes so large that the fuel becomes supercritical. The excursion heated the fuel so that it expanded and "exploded" out of the pressure vessel; once that had happened the fuel was dispersed and the nuclear reaction ended. At that point the graphite was exposed to the air and caught fire; it was this (and the resulting smoke plume) that spread most of the radiation.