As many of you suspected, the actual energy available in the potential steam explosion at the Chernobyl power plant was not in the megatons-of-TNT range. It would have been dramatic and it would have been large enough to disrupt the other reactors. It would have ejected a large quantity of the remaining core into the atmosphere and resulted a larger and more radioactive fallout zone. But it would not have been, as reported, in the 3 to 5 megaton range.
The best discussion on the topic I have found so far was at physics.stackexchange.com.
It isn't the show that came up with the reported numbers, though, so I am still open to further explanation. Here's an older interview reporting the high numbers.