Looking at Hiroshima and Nagasaki, I would guess you could expect a 50% fatality rate within the blast radius.
I don't think nearly that high, Dead. The buildings of Manhattan are far, FAR stronger than both of thos cities. They would serve to contain alot of the blast and heat. If I remember correctly, H and N went up like matchsticks due to everything being built of bamboo/wood. That kind of flash fire wouldn't happen as rapidly in a modern city.