Roughly 80K were killed INSTANTLY from the blast. Blown to bits, vaporized, incinerated, or rather more prosaically by killed shrapnel. Others were killed when the city burned - lots of wooden construction, especially far from the city center. Many more were fatally wounded but did NOT die in the blast - burn or shrapnel victims who would die of infection or lack of treatment days, or even weeks later. The fatality list would have been smaller had the bombing not eliminated most medical staff and facilities within the city, leading to treatable wounds becoming septic.
You can't take the total fatalities, subtract the immediate deaths and arrive at the figure for "death by radiation poisoning". In point of fact, there were a great many people killed from radiation, but anyone close enough to the blast to receive a lethal dose ALSO received other injuries, meaning that, rather than having discrete categories for radiation poisoning, burns, shrapnel/blast and infection casualties, you have people who were wounded in multiple ways, and died of more than one cause.