No chance of surviving the direct hit of the bomb, but cockroaches can survive the surrounding fallout
First and foremost, nothing can survive intense heat which prevails in an injured area. For example, the power of the bomb [15 pounds - a ton] thrown on the Hiroshima firestorm ignited at a temperature of a thousand degrees Celsius, which destroyed everything within a radius of two kilometers. However, the cockroaches can survive the fallout that comes later.
The average cockroach is able to withstand radiation of 6400 rad [official unit of radiation exposure]. Lethal dose for humans, however, is only 500 Rad – equivalent, more or less, to 42 CT scans.
One theory attributes to the resilience of cockroaches dropping larval weekly, during which the rate of cell division is about half of normal. Adult rate sharing of information is even lower still. Since most of the radiation causes mutations in DNA replica – a phenomenon that occurs mainly in cells division – the slowdown in the rate of replication protects against cockroach radiation.
However, even cockroaches can not match the bacterium Dinokokos Rdiodiorens. In 1956, scientists discovered a thriving organism in a can of spoiled meat, which suffered a dose of gamma rays. Subsequent studies revealed that the bacteria can survive even in a dose of 1.5 million Rudd. Reasonable therefore to assume that this is enough to survive a 1.2 mega bomb – tons – which experts estimate that is stockpiled in the U.S.A.
Scientists are still not clear why DNA repair mechanism allows bacteria Dinokokos Rdiodiorens to reach such high levels of radiation. One possibility is that the moment the chains degrade the bacterial DNA proteins protect the ends of the chain long enough until it connects again (like the edges of a torn rope fuse immediately, even before they can separate itself apart). If it turns out that this is indeed the case, scientists could offer a similar protein in human cells – they can survive better then cockroach cells.