There was at no stage the risk of any thermonuclear explosion or any explosion even approaching the megaton range during or after the Chernobyl disaster. There was no risk of a nuclear explosion of any kind, at any stage, not even in the low kiloton range. And every actual expert involved in the recovery at the time of the accident had to be fully aware of this because it is physically impossible for a nuclear explosion to occur by accident within the molten remains of a former reactor core. You need highly enriched fission fuel to achieve that, which was not present in the core even before the accident. If anything, the fuel was being diluted by molten concrete and non-fissionable reactor metals in the accident. But even if it had all been 100% weapons-grade material, you would also need a complex mechanism functioning with extreme precision to not only create a supercritical mass but keep it assembled and highly compressed for long enough to release the explosion energy before it blows itself apart. This simply cannot occur by accident and every person with a modicum of scientific literacy knew that even at the time.
At one point it is claimed that a nuclear explosion ion the three-to-five megaton range would "raze" a city even at a distance of 320 km. In reality, at such a range, people might notice the flash dimly in the distance if they happened to look in the right direction, and some time later notice a muffled rumble. There would not even be superficial blast damage past about 25 km and no burn victims past about 48 km. There might be fallout, hours or days later, but that would not raze a city and not even make it uninhabitable in this case.