Overview: Imagine the size of a 6-pack of soda able to cause a nuclear explosion. That's about all the HEU (highly enriched uranium) it would take and it's not so difficult to get. Scott Pelley explores how easy it is to get a hold of the materials necessary to make a nuclear bomb. Matthew Bunn, security expert at Harvard's Kennedy School of Government, concurs that access to HEU is not as difficult as it should be.