this video I found makes me wonder if Ryuukishi's calculations were correct:
http://www.youtube.com/watch?v=h0Bl2...eature=related
this is the record of a one million pounds of explosives set off to simulate the effect of an atomic bomb shockwave.
one million pounds is roughly the equivalent of 450 tonnes, that is half of the amount that caused the Rokkenjima incident according to the EP7 tea party
the kind of damage generated doesn't seem congruent to what I was expecting. The diameter of the fireball is more or less as big as a destroyer (about 150 meters).
It's hard to imagine that by doubling that amount of explosive you could reach 1 km.
Damage on the ground seems to be negligible, although rokkenjima is probably made of brittle rocks that would crumble more easily.
Judging the shockwave shown in this video I think the Kuwadorian would probably suffer heavy damage, but it wouldn't be razed.
Eva was probably in the basement when the explosion happened and got up only after the shockwave passed.