Nukes need electricity to be launched. You probably mean nuclear meltdown at nuclear power plants, because then need electricity to keep control of the core. That is bigger problem than bitcoin. Bitcoin will not be affected much, because EMP only affects side facing the sun. Night time side has no EMP, so bitcoin will continue to work there. And electricity will be restored within a few days. Wile electricity is down, bitcoin will not work, but neither will banks, or any other financial systems.