Your post is total hogwash. Just about everything you have said is completely 100% factually false. By "scientists" I assume you are referring to an actor portraying a scientist in an anti-nuclear power propaganda piece.
a) there is no large pool of water directly below the reactor
b) even if the fuel melted into a large reservoir of water, it could not become critical. It is not physically possible without the precise fuel and moderator arrangement present in an in-tact reactor configuration.
c) even if you somehow made the fuel become critical, it could not explode like a bomb. If a fuel mass became critical, it would simply heat up and disperse. Worse case is the heat would cause additional steam/fire to disperse more fission products. Let me reiterate: it is physically impossible for reactor fuel of any kind to produce a nuclear bomb type explosion. Nuclear bombs require extremely precise arrangement with unimaginably creative engineering to function.
d) if the fuel mass reached the water table, all that would occur would be another path for contamination in the local area which was already heavily contaminated. Fission products would not be magically transported throughout the whole continent, nor could an entire continent be made uninhabitable.
e) a power plant contains a couple orders of magnitude more fuel than a bomb. The contamination from a worse-case-scenario power plant accident is much higher than nuclear bombs would be, unless they were some sort of salted enhanced radiation bomb. For comparison, a typical nuclear power reactor creates as many fissions as a nuclear bomb about every 6 hours.
f) radiation does not spread like a contagion. This is just plain FUD.
The only part of your post which was even remotely accurate was about doomsday enhanced radiation bombs or salted bombs. No one has ever designed, built, or tested such a device and they are only theoretical. But theoretically, with enough salted bombs (meaning about 1000 times more than the entire world's nuclear stockpile at the height of the cold war) you could theoretically make most of the surface uninhabitable.