My understanding was that modern nuclear weapons technology had developed enough to permit the destruction of 1 regular city block, and with minimal fall out.
That said, the only reason I can see that Israel might want to use nuclear weapons in this pre-emptive strike, would be to render the site mildly radioactive to prevent them from ever recovering anything, or rebuilding the site.
That said, I sure hope Israel is on the same page as Rob because the moment they use a nuclear weapon, all the nations around them will feel threatened, and possibly into action.