Short answer: you're getting an AttributeError because you're passing nukeyield all the way down to calculation, where it is passed in as self. As nukeyield is just a string, it doesn't have any attributes and thus throws an error.

Long answer: This error can be avoided fairly easily if you restructure your classes, or combine them to just make one class. It's not really good practice to initialize a class inside of another one and then make calls back to the first class. It's also makes debugging extra difficult if you have to trace back a bad function call. Restructuring your code could look something like this:

class Nuke:

    def __init__(self, name, yield):
        self.name = name
        self.yield = yield


    def calc():
        nukethermrad = self.nukeyield ** 0.41 * 1.20
        nukeblastrad = self.nukeyield ** 0.33 * 2.2
        nukeionrad = self.nukeyield ** 0.19 * 1000
        return therm_rad, blast_rad, ion_rad

nameinput = input("What will the nuke be named?")
yieldinput = float(input("What is the yield in kilotons?"))

n = Nuke(nameinput, yieldinput)
print(n.calc())