What Is It?

First developed by the United States in the early 1950s, the hydrogen or thermonuclear bomb is perhaps a thousand times more powerful than a uranium- or plutonium-based fission bomb, making it effectively the nuclear weapon's nuclear weapon.

Only the five permanent United Nations Security Council members — the United States, Russia, Britain, France and China — are known to possess hydrogen bombs.

The difference between a hydrogen bomb and a regular uranium or plutonium bomb is that a hydrogen bomb uses fusion instead of fission to generate the main explosion.

In a fission reaction, unstable isotopes of the heavy elements uranium and plutonium are split into smaller atoms, releasing a large amount of energy proportional to the amount of material used.

In a fusion reaction, two atoms of the lightest element, hydrogen, fuse to create one atom of helium, the next-lightest element — and release much more energy.

Since a nuclear explosion is one of the few proven methods for creating temperatures high enough for hydrogen nuclei to overcome their natural repulsion and fuse — about 50 million degrees — the detonation of regular plutonium- or uranium-based fission bombs is essential to ignite the process. Hence, a hydrogen bomb is really a composite weapon.

In a typical hydrogen bomb, the detonation of a fission bomb compresses and heats a core of lithium deuteride, lithium combined with the hydrogen isotope deuterium. The neutrons emitted by the fission reaction strike the lithium atoms, creating atoms of another hydrogen isotope, tritium. The heat of the fission reaction begins to fuse the tritium and deuterium atoms, creating helium along with a large amount of neutrons.

The neutrons created by both the fission and fusion reactions strike a specially shaped uranium casing, which begins to undergo a fission reaction of its own, creating still more neutrons, accelerating the fusion reaction until it all culminates in an enormous explosion.

After the Soviet Union and Britain followed the U.S. in developing thermonuclear bombs, there was worry that conflict involving them would mean the end of the world.

"You can't have this kind of war," President Dwight D. Eisenhower told a committee that urged preparedness for nuclear conflict during the . "There just aren't enough bulldozers to scrape the bodies off the streets."

How Are They Spread?

Nuclear and thermonuclear weapons kill people not only by their conventional impact and explosion, but also with the radiation they emit.

External radiation exposure occurs when either part of or all of the human body is exposed to an external source of 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. This happens when body parts such as the skin become contaminated and/or if the harmful material gets inside the body via the lungs, digestive tract or wounds.

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

The most destructive form of radiation, gamma radiation, can travel many meters in the air, and many centimeters in human tissue, after being emitted from a radioactive source. Dense material, such as lead, is needed to shield against gamma radiation.

Beta radiation can travel meters in air and will moderately penetrate human skin, but clothing will provide some protection.

Alpha radiation travels a very short distance through the air and will not penetrate the skin, but can be harmful if inhaled, swallowed, or absorbed through open wounds.

Radiation within one hour after a nuclear explosion is still about 90 percent of the original level, but goes down to about 1 percent after two days. Radiation only drops to trace levels after 300 hours.

What Are the Symptoms of Radiation Exposure?

People in the immediate vicinity of a nuclear bomb's detonation 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.

The distances for all the effects of a nuclear explosion would vary proportionally with the amount of fissile material used in a fission bomb, and be exponentially greater in the case of a hydrogen bomb.

A mix of physical symptoms must be used to judge the seriousness of radiation 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 Is Radiation Exposure 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 the victim was exposed to.

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 in to 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 Thermonuclear Weapons?

The first thermonuclear bomb was exploded in 1952 at Enewetak atomic proving station in the Marshall Islands by the United States, the second in 1953 by Russia. Great Britain, France and China have also exploded thermonuclear bombs. These five nations comprise the so-called "nuclear club," nations that have the capability to produce nuclear weapons, both fission and fusion, and admit to maintaining an inventory of them.

Several other nations either have tested fission nuclear devices or claim to have the capability to produce them, but officially state that they do not maintain a stockpile of such weapons; among these are India, Israel, and Pakistan. South Africa's apartheid regime built six nuclear bombs but dismantled them later.

The United States has a stockpile of 12,500 nuclear weapons — not all of which are 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 Nations International Atomic Energy Agency, based in Vienna, Austria, 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 still have stockpiles of weapons-grade uranium and plutonium.

Ukraine and Kazakhstan also have nuclear power plants whose by-products could not be used to make a nuclear bomb but might still tempt terrorists trying to make a "dirty," or radioactive, conventional bomb.

Experts also worry about Pakistan and its untested security systems, dozens of nuclear weapons, and many Islamic militants. Pakistan reportedly began quietly accepting American help to improve its nuclear security measures in early November 2001.

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