1. Nuclear Bombs

2. The First Nuclear Bombs

3. There are two main types of bombs which release energy from the nuclei of atoms. The simplest kind is an atomic bomb. Like a nuclear power plant, it releases great quantities of energy through a process called nuclear fission, or 'splitting', of a large unstable (radioactive) element like uranium or plutonium.

4. The energy source is a mass of radioactive material such as uranium or plutonium. This material is very unstable; every atom's nucleus is ready to fall apart ('decay') at the slightest nudge, releasing unneeded energy and extra neutrons. In the diagram, the plutonium (B) is given that nudge by the outer casing of TNT (A), which explodes all around it.

5. The plutonium is unstable, or radioactive
The plutonium is unstable, or radioactive. Its atoms are constantly 'falling apart', breaking up into smaller elements that are more stable. Every time one nucleus does this, it releases the extra energy it no longer needs to hold it together, as well as a few left-over neutrons. This energy, and the escaping neutrons, is what we describe as the radiation being emitted from the radioactive plutonium. This energy and flow of escaping neutrons can damage human cells, so radioactivity is dangerous. Enough atoms in the chunk of plutonium are breaking down at any one time to make the chunk of plutonium warm up, but not enough to be considered an explosion.

6. What happens in the bomb, however, changes that
What happens in the bomb, however, changes that! The force of the TNT explosion causes the plutonium to be squashed, or compressed in size, and become very dense. This is called its 'critical mass'; the plutonium is now so densely packed together that the neutrons escaping from the decaying nuclei of plutonium cannot escape from the plutonium without bumping into another plutonium atom!

8. When they hit another atom, they cause that nucleus to break down too, whether it was ready to or not. That second nucleus releases more energy, and more neutrons, which in turn go on to hit and break up further nuclei. The decaying nuclei cause more decaying nuclei, and so on, in a rapidly escalating chain reaction ... and all because the plutonium has been squeezed into such a dense state (by the TNT) that the escaping neutrons that normally would fly out of the material now can't, without hitting other nuclei!

9. Within a very tiny fraction of a second, all the nuclei in the chunk of plutonium have been hit by escaping neutrons, and have broken down. The extra energy in trillions of atomic nuclei is all released at once! This energy is considerable; the atomic bomb dropped on Hiroshima in WWII was an example of this process.

13. Initial Explosive Conditions
Maximum temperature at burst point: several million degrees centigrade. A fireball of 15-meters radius formed in 0.1 millisecond, with a temperature of 300,000 degrees centigrade, and expanded to its huge maximum size in one second. The top of the atomic cloud reached an altitude of 17,000 meters.

14. Black Rain
Radioactive debris was deposited by "black rain" that fell heavily for over an hour over a wide area.

15. Damaging Effects of the Atomic Bomb
Intense thermal heat emitted by the fireball caused severe burns and loss of eyesight. Thermal burns of bare skin occurred as far as 3.5 kilometers from ground zero (directly below the burst point). Most people exposed to thermal rays within 1-kilometer radius of ground zero died. Tile and glass melted; all combustible materials were consumed.

16. Blast
An atomic explosion causes an enormous shock wave followed instantaneously by a rapid expansion of air called the blast; these represent roughly half the explosion's released energy. Maximum wind velocity: 440 meters per second. Wooden houses within 2.3 kilometers of ground zero collapsed. Concrete buildings near ground zero (thus hit by the blast from above) had ceilings crushed and windows and doors blown off.

17. Bodily Injuries
Acute symptoms. Symptoms appearing in the first four months were called acute. Besides burns and wounds, they included: general malaise, fatigue, headaches, loss of appetite, nausea, vomiting, diarrhea, fever, abnormally low white blood cell count, bloody discharge, anemia, loss of hair.
Aftereffects. Prolonged injuries were associated with aftereffects. The most serious in this category were: keloids (massive scar tissue on burned areas), cataracts, leukemia and other cancers.

18. Radiation
People exposure within 500 meters of ground zero was fatal. People exposed at distances of 3 to 5 kilometers later showed symptoms of aftereffects, including radiation-induced cancers.