What Is It?

The atom bomb originated in 1939. Dr. Albert Einstein convinced then-President Franklin Roosevelt to fund development of the bomb. U.S. scientists worked in extreme secrecy to finish the bomb, which they told the Army was ready for use by 1943. On August 6, 1945, the B-29 "Enola Gay" flew from Tinian to strike Hiroshima, Japan, on the world's first atomic bombing mission. Three days later "Bockscar" dropped the plutonium bomb on Nagasaki. The Japanese surrendered in the following days thereby ending World War II.

These are bombs whose huge explosive power is caused by a sudden release of energy resulting from the splitting of nuclei of a heavy chemical element such as plutonium or uranium by neutrons in a very rapid chain reaction.

Uranium Bomb - the nuclear bomb that was dropped on Hiroshima was a gun-type uranium bomb. Uranium is supposedly the material of choice for terrorists since the design is simple, although highly enriched U-235 - the main element - is not easy to make.

Plutonium Bomb - the bomb dropped on Nagasaki was a plutonium bomb, using the element Pu-239. Plutonium is not hard to obtain and is produced in most nuclear reactors. It can then be separated using chemistry. A higher component of the element, Pu-240, is highly radioactive and often tends to pre-detonate bombs before the chain reaction is complete. An implosion design is used to prevent this, but because the process is difficult, it isn't very likely a small terrorist group could do it without some sort of state help.

In this type of bomb, the plutonium is often arranged as a hollow shell, with explosives on the outside. The explosives drive the shell into a blob and compress it enough to set it off.

How Are They Spread?

There are various deployment methods for nuclear weapons.

Intercontinental ballistic and cruise missiles are long-range, rocket-powered ballistic missiles capable of carrying atomic warheads. Specific types include the Peacekeeper, Midgetman, Minutemak, Navaho and Snark. Submarine-launched ballistic missiles are another option. Bombs can also be dropped by various types of bomber and tanker aircraft, and other types of missiles.

Nuclear weapons kill people not only by their conventional impact and explosion, but also with the radioactive material they emit. Here are the various ways the radiation can be spread to humans:

External radiation - occurs when either part of or all of the body is exposed from an external source, such as when a person is standing near the site of where a radiological device is set off and he or she is exposed to radiation, which can be absorbed by the body or can pass completely through it.

Contamination - occurs  when radioactive materials in the form of solids, liquids or gases are released into the air and contaminate people externally, internally or both. This happens when body parts such as the skin become contaminated and/or if the harmful material gets inside the body via the lungs, gut or wounds.

Incorporation of radioactive material - occurs when body cells, tissues and organs such as bone, liver, thyroid or kidney, are contaminated.

Gamma radiation can travel many meters in the air and many centimeters once in human tissue; therefore they represent a major external threat. Dense material is needed as a shield. Beta radiation can travel meters in air and is moderately penetrating of human skin, but clothing and some protection can help. Alpha radiation travels a very short distance through the air and can't penetrate the skin, but can be harmful if inhaled, swallowed or absorbed through open wounds.

Radiation in the first hour after an explosion is about 90 percent, with it going down to about 1 percent of the original level after two days. Radiation only drops to trace levels after 300 hours.

What Are Symptoms of Exposure?

People in the immediate vicinity would likely die from the force of the conventional explosion itself. Some survivors of the blast might die of radiation poisoning in the weeks afterward. Those farther away from the explosion might suffer radiation sickness in the days and weeks afterward, but recover. Over time, risks of cancer in the affected area would rise, but perhaps only slightly.

A mix of physical symptoms must be used to judge the seriousness of exposure. Impact of radiation poisoning also changes if the body has experienced burns or physical trauma. In the case of treatable victims, extensive medical treatment may be needed for more than two months after exposure.

Some symptoms may include vomiting, headache, fatigue, weakness, diarrhea, thermal burn-like skin effects, secondary infections, reoccurring bleeding and hair loss.

How Are They Treated?

If detection and decontamination occurs soon after exposure, about 95 percent of external radioactive material can be removed by taking off the victim's clothing and shoes and washing with water. Further decontamination may require the use of bleaches or other mild abrasives.

Treatment of a victim within the first six weeks to two months after exposure is vital and is determined by what types of radioactive isotopes to which the victim was exposed.

Medical personnel will treat victims for hemorrhage and shock. Open wounds are usually irrigated to cleanse them of any radioactive traces. Amputation of limbs may occur if a wound is highly contaminated and functional recovery isn't likely.

If radioactive material is ingested, treatment is given to reduce absorption and enhance excretion and elimination. It includes stomach pumping or giving the victim laxatives or aluminum antacids, among other things.

If radioactive material has gotten into a victim's internal organs and tissues, treatment includes giving the patient various blocking and diluting agents, such as potassium iodide, to decrease absorption. Mobilizing agents such as ammonium chloride, diuretics, expectorants and inhalants are given to a patient to force the tissues to release the harmful isotopes. Other treatments involve chelating agents. When ingested, these agents bind with some metals more strongly than others to form a stable complex that, when soluble, are more easily excreted through the kidneys.

Who Has Them?

The United States has a stockpile of 12,500 nuclear weapons - all of which aren't in active use - and 103 nuclear power plants. Russia has a similar stockpile involving a combination of fusion and fission bombs. A big international issue now - particularly for the United States and Russia - is to reduce their stockpiles.

The United Nation's International Atomic Energy Agency is responsible for overseeing 900 of the world's nuclear facilities.

The former Soviet republics of Ukraine, Belarus, and Kazakhstan - where the Soviets based many of their nuclear warheads - safely returned their weapons to post-communist Russia in the 1990s, but all three countries continue to have stockpiles of weapons-grade uranium and plutonium. Ukraine and Kazakhstan have nuclear power plants whose byproducts could not be used to make a nuclear bomb but might still tempt terrorists trying to make a "dirty bomb."

Some experts also worry about Pakistan and its untested security systems, dozens of nuclear weapons and many Islamist militants who also are Usama bin Laden sympathizers. Pakistan reportedly began quietly accepting American help to improve its nuclear security measures in early November 2001.

In the 1990s, U.S. authorities discovered several Al Qaeda plots to obtain nuclear materials, and the CIA recently told Congress that bin Laden tried to "acquire or develop a nuclear device."