Just wondering what are you preparing for? When was the last time there has been a war between USA and another country on the North American continent?
Agreed, modern missiles and bombs can destroy vast areas. However, knowing that the Chernobyl explosion put 400 times more radioactive material into the Earth's atmosphere than the atomic bomb dropped on Hiroshima, yet the Exclusion Zone is "only" 1.000 sq mi, 20 mi in diameter.
Further agreed, there's been bombs built over 3000 times the power of the Fat Man. How many of those would be needed to blow the entire planet into smithereens, ending all worrying?
And one further question: What kind of ammo would you stock to stop such bombs? Or are you planning to prepare for helping the enemy by killing your neighbours?