Their names were Alexei Ananenko, Valeri Bezpalov, and Boris Baranov. Here is the relevant section of the Chernobyl wiki article.
10x Hiroshima is a larger estimate than most I’ve seen – we don’t actually know how powerful the steam explosion that they’ve prevented would have been. Most sources I’ve read have estimated it at 5-10x the explosion that already happened at Chernobyl. The general consensus is “really fucking big explosion”.

In the original Chernobyl explosion, only 30% of the core was sent into the atmosphere. The rest melted down. The steam explosion prevented by Ananenko, Bezpalov, and Baranov would have sent the remaining 70% into the air as well. Much of surrounding Ukraine, including Kiev, would likely have had to evacuate.

[SOURCE](http://www.reddit.com/r/undelete/comments/2dq3pj/2751270_til_during_the_chernobyl_meltdown_3/)