I've been talking with folks who are certified nuke engs. This event has been rated a 4 on the 7 point scale (TMI was a 3 (correction - 5), Chernobyl a 7)

Backup gens are on site, and are running for reactors 2,3,4

No1 they are pumping seawater and boron in, which will "posion" the reactor.

The explosion was from them venting the gasses - because there IS at least some melting of the rods in No1, there was a mix of hydrogen and oxygen vented - it went "bang" - outside the containment

In the machine spaces (aka where the generators and the like are) the rad levels WERE up to .67 Rem/hr - not great (fairly normal is .007)

Remember, "western' design reactors all have a negative 'radioactivitiy defecit' - basically, they way they are designed and built, the hotter they get, the less well they react - aka they tend to be self slowing (in fact, it is possible to build a reactor that is fail SAFE - turn off the cooling, when the heat gets high enough, it turns off - basically it stabilizes at a very HIGH temp, but a temp that is safe for the design - see the idea of PBRs). Anyway, chernobyl (and that whole series of graphite moderated reactors) had a POSITIVE defecit in certain operating ranges - aka, when they got hot, they got more radioactive, so they got hotter, so they got more radioactive, etc - so you can guess what happened

Graphite reactors have one HUGE advantage (well, two) - fairly cheap, and if you're looking to make plutonium for weapons, it's a great design.....