Little Boy: The first atomic bomb used during a war was a gun-type device. In the bomb that destroyed Hiroshima, two pieces of uranium were literally blown together in a device similar to an artillery barrel -- creating the chain reaction that led to the explosion. The destructive force of Little Boy was seven times greater than all the bombs the Allies dropped on Nazi Germany during 1942.
Little Boy:  146 K QuickTime VR
(Picture copyright AJ Software & Multimedia)

Fat Man: The bomb dropped on Nagasaki was an implosion device -- a subcritical mass of plutonium-239, surrounded by a chemical explosive. The weapon's fuse made sure all the conventional, chemical explosive was detonated at the same time. That explosion compressed the plutonium, which created a supercritical mass and then the chain reaction
Fat Man:  389K QuickTime VR
(Picture copyright AJ Software & Multimedia)

Nuclear fission and the chain reaction: An atomic bomb gets its destructive power from a sudden release of energy, created by nuclear fission. In fission weapons, matter is changed into energy when the nucleus, or core, of an atom is split apart. As the nucleus breaks apart, it releases neutrons -- subatomic particles -- which in turn collide and break up other atoms. This causes the chain reaction that gives nuclear weapons a power far greater than conventional weapons.
Fission reaction:  124 K / 4 sec. / 240x180  QuickTime
Chain reaction:  116 K / 5 sec. / 240x180  QuickTime
(Picture copyright AJ Software & Multimedia)

Nuclear fusion: Most modern nuclear devices are hydrogen bombs, also known as thermonuclear weapons. A fission explosion creates intense heat, which fuses lightweight atomic nuclei into a nucleus of heavier mass. The resultant loss in the combined mass is converted into energy, which fuels the explosion.
Fusion Reaction:  203 K / 5 sec. / 240x180  QuickTime
(Picture copyright AJ Software & Multimedia)

The Trinity Test: The birth of the Nuclear Age was carefully documented, beginning with the first nuclear test. Trinity was the culmination of the secret Manhattan Project, the U.S. government's mission to create an atomic bomb before Nazi Germany did. The blast, near Alamogordo, New Mexico, took place on July 16, 1945. Three weeks later, a U.S. warplane dropped an atomic bomb on Hiroshima.
Trinity test:  774 K / 12 sec. / 240x180  QuickTime

Nuclear Winter: Some scientists say a nuclear war would touch off a series of firestorms. The smoke from these would blanket the Earth's atmosphere, causing a "nuclear winter," which would eventually destroy most of the planet's higher life forms.
Nuclear winter simulation:  141 K / 9 sec. / 240x180  QuickTime
(Picture copyright AJ Software & Multimedia)