In an interview last weekend, Iran’s ambassador to the UN said his country’s nuclear enrichment ‘will never stop’ because it is permitted for ‘peaceful energy’ purposes. It is the latest development in an escalation of tensions over Iran’s nuclear programme, which erupted when Israel targeted the country’s nuclear facilities in June. To understand why enrichment is so important, Madeleine Finlay talks to Robin Grimes, professor of materials physics at Imperial College London. He explains what goes into creating a nuclear weapon, and why getting to the stage of weaponisation is so difficult. Help support our independent journalism at theguardian.com/sciencepod