South Korea nuclear summit: factbox Concern had been building following the September 11, 2001 US attacks, the worldwide proliferation of nuclear material, and efforts by North Korea and Iran to develop atomic programmes in defiance of international wishes. Aside from direct casualties from an explosion or radiation, security experts warn an attack could have far-reaching psychological impact on targeted societies, severely disrupt commerce, and provoke security crackdowns that could impinge on civil liberties. Estimates say as much as 1,600 tons of weapons-grade highly enriched uranium (HEU) and 500 tons of plutonium exist in the world, sometimes stored under questionable security in former Soviet states and elsewhere. Obama said in a speech in Seoul Monday that "just the smallest amount of plutonium - about the size of an apple - could kill hundreds of thousands and spark a global crisis." There are millions of other radioactive sources such as nuclear power plants, research facilities and hospitals, which store isotopes used in treating cancer and other conditions. Security experts say this could be used by terrorists to spread contamination through a "dirty bomb." Only about 55 pounds of HEU or 17.6 pounds of plutonium would be required to build a working nuclear bomb, according to experts. Even less material would be needed for a "dirty bomb," a combination of conventional explosives and radioactive substances that would release far less radiation but pack a high terror punch. The International Atomic Energy Agency has confirmed 20 cases of theft or loss of HEU or plutonium in the past two decades and hundreds more cases of other nuclear material going missing.