A nuclear reaction is at its most basic nothing more than a reaction process that occurs in an atomic nucleus. They typically take place when a nucleus of an atom gets smacked by either a subatomic particle (usually a "free neutron," a short-lived neutron not bound to an existing nucleus) or another nucleus
but alot of things happen to be honest to much to put down but the basic is a blast of energy that is super heated by friction that is followd by nuclear after fall and this can be very deadly even miles away
hope this helps