nuclear

Very unlikely.

Also neither were nuclear explosions:Chernobyl was a massive steam explosion in the cooling system, and Japan was a combination of steam explosions and maybe hydrogen/oxygen explosions. The nuclear releases in both cases were due to breaches of the containment by these explosions.

If they had been nuclear explosions many miles from the plants would have been leveled and that did not happen.

Prohibits nuclear weapon test explosions and any other nuclear explosions in three environments: in the atmosphere, in outer space and underwater, but does not prohibit underground nuclear explosions

Yes.

Nuclear explosions. Thousands of them.