Question

The “rain” of radioisotope dust following a nuclear explosion is called ____________.

Answer 1

Its called the " Nuclear Fallout". Nuclear fallout is the radio residue material propelled into the out atmosphere following a nuclear blast. That is the reason for calling it as 'fall out' after the explosion and after the shock waves have passed.

Even though it is commonly called as dust or ash created due to the explosion. The fallout also contains some products of pyrocumulus clouds and then fall back as black rain. The longest nuclear fallout was by the atomic bomb blasted on Hiroshima and Nagasaki , which lasted for 30 minutes.

You may get a dought that what the dust contains, the dust usually contains fission products which are mixed with bystanding atoms that are neutron activated due to the radioactive exposure.