Kerry joins nuclear talks as Iran, powers push for breakthrough

* Nascent compromise on Iran's "right to enrich" uranium * Fate of Iran's Arak heavy-water reactor remains dispute * France's Fabius says in Geneva to reach "solid deal" * China, UK, French, German, Russia foreign ministers arrive * Iranian negotiator sees deal close, differences remain * Powers aim to ease risk of Iran developing nuclear weapon (Adds British comments, detail) By Parisa Hafezi ...