The atomic bomb that incinerated the Japanese city of Hiroshima at the close of World War II contained about 60 kilograms of chain-reacting uranium. When the American "Little Boy" device detonated over the doomed port, one part of the bomb's charge--a subcritical mass--was fired into the other by a relatively simple gunlike mechanism, causing the uranium 235 in the combined mass to go supercritical and explode with the force of 15 kilotons of TNT. The weapon that devastated Nagasaki a few days later used plutonium rather than uranium in its explosive charge and required much more complex technology to set it off.
Despite the production of more than 100,000 nuclear weapons by a few nations and some close calls during the succeeding 60 years, no similar nuclear destruction has occurred so far. Today, however, an additional fearful threat has arisen: that a subnational terrorist organization such as al Qaeda might acquire highly enriched uranium (HEU), build a crude gun-type detonating device and use the resulting nuclear weapon against a city. HEU is uranium in which uranium 235, the isotope capable of sustaining a nuclear chain reaction, has been concentrated to levels of 20 percent or more by weight.