It's a mock-up of a hydrogen bomb! This is a model of which a few were still deployed until very recently. Unfortunately I didn't get a legible photo of the interpretive sign. It's at the Atomic Testing Museum in Las Vegas(!). If you're ever in Vegas and want Something Completely Different:
nationalatomictestingmuseum.org

Update: tracked down some more information on this model:
en.wikipedia.org/wiki/B53_nuclear_bomb
UPDATE: The old link was broken, so I just linked the Wikipedia article.

If only they were confined to museums!

Per Wikipedia a few have been saved for possible use in planetary defense--i.e., asteroid deflection. That's a legitimate application! ;)