Nothing against Moxie, but this stated fact is clearly nonsense. It certainly would have been terrible for a large volume of water to reach the burning core, but you're not going to get a 5MT blast from a reactor accident. It takes a LOT of things working perfectly to make a nuclear bomb explode correctly. Lots of bad things happen when reactors go critical, a nuclear blast is generally not one of them. It's the spread of contaminated matter that makes reactor accidents bad. Also, sure, explosions can happen with rapid temperature changes like one encounters in these situations, but nothing like an actual nuclear bomb.