Types of Nuclear Weapons

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Fission Weapons (also called atomic bombs)

Nuclear fission is a process in which a neutron collides with an atom’s nucleus, splitting the atom into two smaller atoms and releasing a significant amount of energy. Every collision also releases more neutrons, which in a critical mass of fissile material will sustain a chain reaction of fission. By manipulating the size and speed of the chain reaction, nuclear fission can be exploited for power generation or alternatively, for weapons of mass destruction.

Gun-type design (HEU)

This is the “simplest” type of nuclear explosive and was detonated over the city of Hiroshima by the United States in World War II. The design uses highly enriched uranium (HEU) as fissile material, which is obtained by concentrating atoms of the rare U-235 isotope. When uranium is extracted from the ground, less than 1 percent of the ore is U-235. 99.3 percent is the heavier U-238 isotope, which cannot sustain a chain reaction of nuclear fission.

“As the bomb fell over Hiroshima and exploded, we saw an entire
city disappear. I wrote in my log the words: "My God, what have
we done?"Robert Lewis, pilot of the plane that dropped the 'Little Boy' bomb, April 1947

HEU is defined as uranium with a concentration of at least 20 percent U-235; however, at 20 percent enrichment, 400kg of material are needed for a bare critical mass, more than is practical for a weapon. “Weapons-grade” uranium (also used in submarine and icebreaker propulsion) is defined as uranium enriched to at least 90 percent U-235. By contrast, low enriched uranium (LEU), which is used as fuel in the majority of nuclear power plants, generally contains only 3 to 5 percent U-235. The same technology used to make LEU for peaceful purposes can be configured to make HEU for weapons.

The gun-type weapon is produced through a rather simple process in which one mass of U-235 is ‘shot’ into another by conventional explosives, creating a critical mass. The impact generates more neutrons, ensuring a fission chain reaction.The gun-type nuclear explosion is the most inefficient in terms of burning up the fissile material; only about 1.4 percent of the HEU in the Hiroshima bomb actually fissioned.

Yet a large amount of fissile material is required to ensure that a nuclear chain reaction will take place. Therefore, gun-type weapons will necessarily be heavier and bulkier than other types of nuclear weapons. While this suggests that States seeking strategic nuclear weapons would look to more advanced designs, the simplicity of a gun-type device may be attractive to terrorists. A weapon of this type is too large to be mounted on a long-range missile, but it could be dropped from a plane or delivered in a truck or a shipping container.

“There was a blinding electric blue light… I pressed my hands
hard to my eyes, then, realized my hands were covering my
eyes. This terrific light power, or rays, were actually passing
through the tarpaulin, through the towel, and through my head
and body.”Observer of British nuclear test, 16 May 1956

In terms of testing, no nuclear components need to be verified with the gun-type design; only the conventional components must be tested. Although the United States was able to prove that the chain reaction would work, it did not conduct an actual nuclear test of this design before it was used to bomb Hiroshima. South Africa built seven nuclear weapons using this design, but verifiably dismantled them in the 1990s.

As explained above, creating a sufficient amount of HEU is the most challenging step of building this type of nuclear weapon, requiring raw materials, expertise, infrastructure, and massive amounts of energy. Because producing fissile material is so difficult and costly, it is highly unlikely that terrorist organizations would be capable of taking this route to acquiring HEU. It is more likely that they would steal it. Thus, it is crucial that countries around the world secure all existing stockpiles to reduce the risk of theft.