Seems dangerous to put nuclear material in a live artillery shell, I'll bet thats why it was never comissioned. Good find (although I vaguely recall hearing about this before)
Tsar Bombas/Davy Crocket are on the complete opposite ends of the scale. Besides, with todays technology we could probably make things that make the Tsar Bomba look like a tic tac or a really good tactical nuke-recoiless rifle much better than the Crocket.