A nuclear weapon, also known as a nuclear bomb or a nuke, is a weapon that suddenly releases the energy in the nucleus of certain types of atoms. When triggered, the device releases a huge amount of energy in the form of a nuclear explosion.  

Nuclear explosions can destroy a city and kill most of its people. They also make nuclear fallout, which may also make people very sick. Nuclear weapons are the most damaging weapons to have been created. 

The first nuclear weapons were built by the United States during World War II. Two nuclear weapons were used to attack cities in Japan. They were the only times nuclear weapons were used in war. 

There are two ways to make nuclear weapons: fission weapons (also called atomic bombs or A-Bombs) and fusion weapons (also called hydrogen bombs, H-Bombs, or thermonuclear weapons). They make energy for the nuclear explosion in different ways. Fusion weapons make bigger explosions. Fission weapons use a special isotope of uranium or plutonium. Fusion weapons use a special isotope of hydrogen. Nuclear weapons have been used twice in war, both by the United States in the atomic bombings of Hiroshima and Nagasaki.

Since the atomic bombings of Hiroshima and Nagasaki, nuclear weapons have been detonated over 2,000 times for testing and demonstration. Only a few nations possess such weapons or are suspected of seeking them. The only countries known to have detonated nuclear weapons and to acknowledge possessing them are (chronologically by date of first test) the United States, the Soviet Union (succeeded as a nuclear power by Russia), the United Kingdom, France, China, India, Pakistan, and North Korea. Israel is believed to have nuclear weapons, but in a policy of deliberate ambiguity, it does not acknowledge having them. Germany, Italy, Turkey, Belgium and the Netherlands share nuclear weapons with other countries. South Africa is the only country that has independently developed and later renounced and dismantled its nuclear weapons.

The Treaty on the Non-Proliferation of Nuclear Weapons aims to reduce the spread of nuclear weapons, but its effectiveness has been questioned. Modernisation of weapons has continued to this day.

History 

In the years after 1895, people studying physics begin to understand how atoms are made. Around 1915, people began to have the idea that breaking special atoms can release large quantities of energy and can be used to make a bomb.

In 1939, people studying physics began to understand the theory of nuclear fission weapons, but no country knew how to build one. When World War II started, Germany, the United Kingdom, and the United States wanted to build nuclear weapons. Germany did not finish building them, partly because many of the best people studying physics fled Germany after Nazi rule had started. The United Kingdom started working in 1939 but found it so expensive that it gave up in 1942. Later that year, the United States started a very large program to build nuclear weapons. Building upon the work done in the United Kingdom, the program was called the "Manhattan Project". 

By August 1945, the Manhattan Project had built three nuclear fission weapons. Two of the bombs were used by the United States to attack the cities of Hiroshima and Nagasaki in Japan. People from the Manhattan Project believe that around 105,000 people were killed and 94,000 were hurt when the bombs were used. Medical professionals later came to believe that more than 225,000 people died when everyone who was affected after long periods of time has been counted. Japan announced its surrender after the atomic bombings of Hiroshima and Nagasaki.

After World War II, the Soviet Union also began working to create nuclear weapons.

How they work 
One way that nuclear weapons release energy is by breaking atoms apart. Called nuclear fission, that the basis for atomic bombs. Specific isotopes of uranium or plutonium are typically used in the weapons. Those elements can be made to undergo nuclear fission and have a nuclear chain reaction.  

Another process can be used to create nuclear weapons with even bigger explosions and then releasing much more energy by fusing atoms together.  That process is called nuclear fusion, and weapons based on the process are called hydrogen bombs or thermonuclear weapons. Specialized isotopes of hydrogen are typically used in the weapons.

Nuclear weapons produce a very large amount of energy and radiation, which can kill people or animals within several kilometers. Most of the radiation is X-rays, which heats the air to produce a huge nuclear fireball. The rapid expansion of the fireball creates a dangerous shock wave that can destroy houses or buildings several kilometers away. The radiation can cause radiation poisoning and also has the potential to cause mutations in the DNA, which can cause cancer. 

Nuclear bombs also release fallout, which is nuclear material and dust that has been irradiated and become radioactive. Over time, the radioactive fallout can potentially kill people farther away, depending on how much was released. Fallout from a nuclear explosion can be blown by the wind over large distances from the explosion and can remain dangerous for long periods of time.

A hydrogen bomb, also known as a fusion bomb, uses hydrogen isotopes (deuterium and tritium) in addition to uranium or plutonium. Hydrogen bombs have the potential to be much more powerful than fission bombs. Despite the name, a typical hydrogen bomb has only enough hydrogen to produce additional neutrons to detonate a casing made of natural uranium. The fuel in hydrogen bombs is thus mostly unrefined uranium.

Building nuclear weapons 
Nuclear weapons are difficult to build because they need special isotopes of uranium or plutonium, as well as specialized technology.  Tht is why so few countries have them.  When countries without nuclear weapons create their own, that is commonly referred to as nuclear proliferation.

Getting nuclear weapons to target 
Getting a nuclear weapon to its target can be as hard as making one. The explosive material can be placed in a bomb or artillery shell or into a missile. When a nuclear device is placed on a missile it is commonly called a nuclear missile and can be carried by airplanes, submarines, or trucks or placed into underground missile silos. Some kinds of airplanes like the B-29 Superfortress, B-36 Peacemaker, B-52 Stratofortress, and B-2 Spirit have carried nuclear weapons. 

They are also carried by missiles, such as intercontinental ballistic missiles (ICBMs) or submarine-launched ballistic missiles (SLBMs). Some missiles travel to the border of space and then launch a number of separate nuclear weapons back toward the ground, with each weapon travelling to a different target. That is called a MIRV Warhead, or Multiple Independent Reentry Vehicles. Very large nuclear bombs have been produced, but in practice a weapon with multiple warheads can produce much more damage by attacking more targets.

Nuclear weapons take many resources to make because the materials they are made of are very rare, and it takes many scientists to make them. However, several countries have managed to create nuclear weapons and many have them today. The countries that have nuclear weapons are listed here in the order that they were invented: United States (1945), Russia (1949), United Kingdom (1952), France (1960), China (1964), India (1974), and Pakistan (1998). Other countries are believed to secretly have nuclear weapons or develop them. Some countries used to have nuclear weapons but have since then said that they have gotten rid of them. 

Some countries have lost nuclear weapons while transporting them. There are 92 known instances of atom bombs being lost at sea by all of the countries known to posses them. Bombs have been lost in 15 different cases. However, there could be more lost bombs.

Nuclear explosions to date 
This is a list is of the main nuclear explosions which have happened. As well as the atomic bombings of Hiroshima and Nagasaki, the first nuclear test of a given weapon type for a country is included, and tests which were otherwise notable (such as the largest test ever). All yields (explosive power) are given in their estimated energy equivalents in kilotons of TNT.

Compensation for victims
Over 500 atmospheric nuclear weapons tests were done at various sites around the world from 1945 to 1980. As public awareness and concern grew over the possible health hazards associated with exposure to nuclear fallout, various studies were done. A Centers for Disease Control and Prevention study says that nuclear fallout might have led to 11,000 excess deaths, most caused by thyroid cancer linked to exposure to iodine-131.

People associated with nuclear weapons

Notable individuals who have been associated with nuclear weapons and related issues include:

 Albert Einstein
 David Lange
 Hermann Joseph Muller
 J. Robert Oppenheimer
 Manuel Pino
 Jonathan Schell
 Peter Shumlin
 Edward Teller

References

Other websites 

 WW2DB: Operation Trinity and the Manhattan Project
 WW2DB: Destruction of Hiroshima and Nagasaki
 The natural voice of A-bomb victims. VOSHN.com
 Nuclear weapon Citizendium

Nuclear weapons