Iran insists its purposes are purely civilian, asserting it needs enriched uranium to make medical isotopes.

BBC: Obama Nowruz message: Iran must 'reduce nuclear tension'

Deep underneath your feet is a hellish stone soup, kept hot by a torrent of radiation from poisonous isotopes of uranium, thorium and potassium in the earth's superheated mantle.

FORBES: Looking For Energy, Google Goes To Hell

The most obvious attempt was when we gave them an offer that said we are going to provide the conversion of some of the low-enriched uranium that they already have into the isotopes that they need for their medical research and for hospitals that would serve up to a million Iranian citizens.

WHITEHOUSE: President Obama Holds News Conference

Subsequently, France would convert the uranium into fuel rods for use in a reactor in Tehran that produces medical isotopes.

BBC: Iran urged over enrichment plan

Iran says it needs enriched uranium for its Bushehr nuclear power plant to generate electricity, and to produce medical isotopes.

BBC: Iran unveils uranium activities at Saghand and Ardakan

Finding certain isotopes - xenon gases in particular - would help experts determine whether a plutonium or uranium-based device was used.

BBC: North Korea nuclear test: No radiation detected