Explainer-Iran's strongest card in nuclear talks: its highly enriched uranium

By Francois MurphyVIENNA, May 29 (Reuters) - Iran and the United States are in discussions to extend their ceasefire so as to start negotiations on issues including Tehran's nuclear program, where Washington insists Iran must not be able to make a nuclear weapon.While much of Iran's uranium enrichment infrastructure was destroyed or badly damaged when Israel and the U.S. bombed it in June, a large part of the highly enriched uranium it amassed is thought to have survived. That is the biggest U.S. concern ahead of nuclear talks.