[orthecreedence]

I may be completely ignorant as I'm not an authority on the subject, but I think it's the difference between the radioactive materials being exploded/vaporized vs shot into the atmosphere (without being vaporized/broken down) and distributed in the general area.

With the the A-bombs, the radiation that people suffered wasn't from the radioactive materials in the weapon itself, but the radiation from such a huge explosion.

With Chernobyl, the radioactive materials were sent into the air and settled around the vicinity, making many parts of it still very radioactive to this day.

[mpyne]

Pretty much, but the largest factor was simply the sheer difference in scale between the A-bombs and the reactors at Chernobyl. With the A-bombs there was simply simply less fission fragments and other radioactive products to worry about due to there being far less nuclear fuel and surrounding material that could be irradiated and itself made radioactive.