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Chapter 3
Nuclear Weapons
The energy generated by fission is around million times bigger than the energy generated by a chemical reaction. This happens because chemical reactions occur at the electron level of the atom, where a typical chemical bond stores energy in the order of 1 eV, while a nuclear bond is in the order of 1 MeV, as seen in Chapter 1.1 This advantage is clear when comparing different options for the types of fuel that could be used for energy generation production for civilian purposes. For example, to generate 1GW in a year using a traditional fossil fuel plant would require eight train cars full of coal per day. While the same 1GW in a nuclear power plant would consume only half a train car of uranium-235 per year. The big difference in energy release per unit mass is also the main strategic advantage of a nuclear bomb. Imagine that one single nuclear bomb can destroy an entire city, while 100,000 bombs with conventional explosives would be needed to achieve the same result.
Proliferation pathways
There are primarily two nuclear materials that can be used in nuclear weapons: uranium-235 and plutonium-239. Creating one or both of these elements in a weapons-grade form, i.e. suitable purity for a successful weapon design, means following one or both of the two paths to nuclear proliferation, i.e. the uranium and the plutonium path. Both paths were developed and pursued simultaneously during the early years of nuclear weapons technology,2 and each path requires specialised nuclear technologies and facilities.
The technology and infrastructure used to produce nuclear material for a weapon is closely related to those necessary to produce nuclear energy. To understand this relation, we will have a look at the civilian fuel cycle steps and identify the main activities required to produce a nuclear weapon.
The nuclear fuel cycle
The nuclear fuel cycle as defined by IAEA3, can be described as the various processing steps necessary to use nuclear fuel in the production of electricity or for producing weapons material.
Nuclear Fuel Cycle
Source: Grübelfabrik, CC BY NS 4.0
The cycle starts with the mining of uranium ore from the ground. In a mill, the uranium ore is crushed and ground to a fine slurry to allow the separation of uranium from the waste rock. The the uranium is recovered from the solution and precipitated as uranium oxide – yellow cake – in the milling step. Next, in the conversion step, the uranium oxide is then converted to a form suitable for enrichment. Since the most common enrichment process is gaseous diffusion*, the solid uranium oxide is converted to gaseous form. Natural uranium is composed mainly of uranium-238 (99.3%) and uranium-235 (0.7%). Therefore, to allow significant generation of energy from the fuel material, an enrichment process that increases the concentration of uranium-235 in the material is performed. Although there are nuclear reactors that run on natural uranium fuel, the majority need fuel with enrichment of around 3–5% of uranium-235. The enriched uranium gas is reconverted into solid uranium dioxide and sent to a fuel fabrication plant. Once inside the nuclear reactor, fissions occur in the uranium. After some time of the fuel being consumed (typically after three years), the number of neutrons generated by fission events are not enough to maintain a chain reaction and the fuel needs to be changed. The consumed fuel is called spent fuel. In a so-called “open fuel cycle”, the spent fuel is treated as waste and deposited in an isolated and secure location for thousands of years. In a “closed fuel cycle”, the spent fuel is sent to a reprocessing facility. In the reprocessing step, the spent fuel is chemically dissolved, uranium and/or plutonium are separated to be re-used as fuel, or sometimes, in the case of plutonium as weapons material.
For the uranium path, the creation of weapons-grade uranium requires creating a highly enriched uranium (HEU) with an enrichment level of at least 90% (i.e. 90% of the mass is uranium-235 and 10% uranium-238). This can be achieved in enrichment plants with large arrays of centrifuges; often several thousand units are needed to increase the fraction of uranium-235 from 0.7% (in natural uranium) to 90%.
For the plutonium path, the starting point is spent nuclear fuel, since plutonium is an element that is not present naturally on Earth in more than trace amounts. Hence, plutonium-239 has to be created by neutron irradiation of uranium inside a nuclear reactor. The process involves capture of a neutron in uranium-238 followed by two beta decays.
Plutonium-239
Source: Authors’ own illustration/Grübelfabrik, CC BY NS 4.0
Both neutrons and uranium-238 are present in large amounts in a nuclear reactor, but in a reactor used for civilian purposes, plutonium is partly used as reactor fuel and the fraction of the relevant isotope, plutonium-239, decreases with reactor operation time. Nevertheless, civilian reactor are strictly monitored to prevent proliferation of plutonium (see LU05).
Reactors that are operated with the purpose of generating plutonium for a nuclear weapon are operated somewhat differently from civilian reactors, for instance by exchanging the fuel at shorter time intervals. Once plutonium intended for weapons production has been created in the reactor, it must be separated from the remaining uranium and fission products in the spent nuclear fuel. This is done by reprocessing that fuel.
There are several definitions of weapons-grade plutonium, but in general, it is considered that it should contain at least 93% plutonium-239, or less than 7% plutonium-240. It is important to note, however, that with suitable technology, even plutonium that does not fulfil the weapons-grade criteria can be used as weapons material so it can be difficult to assess exactly what plutonium product aspiring nuclear weapons builders need and want.
Nevertheless, the civilian fuel cycle activities, especially the enrichment, the nuclear reactor operation and reprocessing, are strictly monitored by international nuclear safeguards (for states that are part of the non-proliferation treaty) and security systems to guarantee that nuclear material and facilities are being used for peaceful purposes (see LU05).
Uranium is not uncommon on Earth, but the ore deposits are unevenly distributed and their suitability for mining varies.
Countries with available uranium resources
Countries with available uranium resources
Australia 28%28
Kazakhstan 13%
Canada 10%
Russia 8%
Namibia 8%
Brazil 5%
South Africa 5%
Niger 5%
China 4%
Ukraine 2%
Usbekistan 2%
Mongolia 2%
USA 1%
Tanzania 1%
Botswana 1%
Global distribution of identified recoverable uranium resources
Data source: Piro, M,/Lipkina, K. (2020): 8 - Mining and milling. In: Piro, M. (ed.): Advances in Nuclear Fuel Chemistry, Woodhead Publishing
A country that pursues an indigenous and independent nuclear weapons programme might face uranium constraints and have to make choices on how to use the available uranium reserves to feed both or one of the proliferation pathways (and perhaps feed electricity-producing nuclear reactors as well). It might, for example, not be possible to feed both an enrichment plant for making HEU and a reactor for making plutonium. Some reactors (heavy water or graphite moderated) can run on natural uranium, removing the demand for enriched fuel, but they still consume uranium.
Different nuclear weapons designs
Over the years different models of nuclear warheads were invented to overcome the challenge of putting together, in one gadget, the necessary nuclear material mass that would lead to a supercritical stage at the correct time of detonation (but not beforehand!).
The size or yields of nuclear weapons are often measured in kilotons. This unit relates to conventional explosives, with 1 kiloton corresponding to the yield of 1,000 tons of TNT (trinitrotoluene).
Gun-type weapons
This type of warhead is the easiest to produce and contains two subcritical pieces of uranium-235 and a detonator with a conventional explosive charge bringing the two pieces together. The design also contains an initiator which produces the neutrons to induce the fission chain reaction.
Gun-Type Nuclear Weapon
Source: Grübelfabrik, CC BY NS 4.0
The Hiroshima bomb, “Little Boy” (15 kilotons), which contained 64 kg of HEU was also a gun-assembly type warhead
Little Boy
Source: U.S. National Archives, ID 61-55, unrestricted use
Implosion-type weapons
For plutonium bombs, a slightly more complicated design is needed to assemble the critical mass at the time of detonation and it takes quite some craftsmanship to master it. The reason for this is that one of the plutonium isotopes inevitably present is plutonium-240, in which there is a fairly large chance of spontaneous fission occurring. This is a process that cannot be controlled, and risks setting off parts of the bomb material prematurely, due to the neutrons created by the spontaneous fission events. A much faster assembly time than can be achieved with gun-type warheads is necessary to be certain that the whole plutonium bomb will fission at the time of detonation. The solution is the implosion-type bomb.
LImplosion-Type Nuclear Weapon
Source: Grübelfabrik, CC BY NS 4.0
This bomb design contains a hollow sphere of plutonium that is rapidly compressed by a shockwave created by the ignition of the surrounding explosives. The resulting increase in density of the plutonium causes the sample to become critical and at the same time, neutrons from the initiator start the fission chain reaction. The second bomb the US released over Nagasaki in 1945 was called the “Fat Man” (21 kilotons) and contained 6.4 kg of plutonium.
'Fat Man' (mockup)
Source: U.S. Department of Defense
Note that plutonium has a much smaller critical mass than uranium-235, making it suitable for lighter bomb designs.
Thermonuclear weapons
A thermonuclear weapon is an even more advanced type of warhead than the gun-assembly and implosion-type described before.
Another term for thermonuclear bomb is hydrogen bomb, indicating that this type of device relies not only on energy released from fission, but from fusion as well. Fusion is the nuclear reaction when two light atomic nuclei fuse together while releasing large amounts of energy. It is this process that takes place inside stars. Isotopes of the lightest element hydrogen – such as deuterium (hydrogen-2) or tritium (hydrogen-3) – can be made to fuse, if the temperature is high enough.
Fusion inside the sun
Source: NASA/Wikimedia, public domain
A thermonuclear bomb works in a two-step process. The first step is the detonation a fission device with either HEU or plutonium (primary), which causes the temperature to rise to millions of degrees Celsius.
Thermonuclear Weapon
Source: Grübelfabrik, CC BY NS 4.0
At this high temperature, much of the energy is released in the form of X-rays, which starts the second step, the compression and ignition of the secondary, setting off the fusion device.
Effects of nuclear explosions
It has been estimated that 140,000 people died in Hiroshima within months of the detonation of the first nuclear bomb and that another 74,000 people died in Nagasaki in 1945 after the second bomb was dropped.4
Physical effects
A nuclear explosion releases large amounts of energy in a very short time. The energy is divided into the blast, heat and radiation.
Effects of a Nuclear Explosion
Source: EU Non-Proliferation and Disarmament Consortium eLearning, CC BY NS 4.0
The largest part of the energy is in the blast, which is a shock wave that can reach velocities of hundreds of kilometres per hour and cause severe injuries to humans and animals as well as raze structures and buildings to the ground.
U.S. 23 Kiloton Nuclear Test, 1953
Source: Photo courtesy of National Nuclear Security Administration / Nevada Site Office
The heat wave also travels fast. In fact, the heat comes first since it consists of thermal radiation that travels at the speed of light. This heat can cause severe burns and even vaporise material. Radiation is released immediately at the time of the explosion (direct initial radiation), but also over a long period of time (residual radiation). The half-lives of radioactive isotopes produced in the nuclear fission or as a result of irradiation will determine how long the residual radiation lasts, but many commonly produced radioisotopes have half-lives of several years or decades.5
Fallout is a term used to describe radioactive material that “falls from the sky” after a nuclear detonation and it contains both fission products from the explosion itself and material that has become radioactive through the irradiation of neutrons from the fission processes. Here, the term radiation refers to either the particles that are released by radioactive decays (alpha, beta, gamma rays), or neutrons from the fission itself.
In this context, it should be noted that the highly penetrating particles will travel far, but also have a higher chance of travelling right through a biological body than the short-range particles (see Chapter 1). For example, ingesting or inhaling an alpha-emitting substance will probably mean that the particles will not exit the body but rather deposit all energy inside it, accompanied by any potential harm. Different particles also pose different dangers to biological systems; alpha particles are the most destructive, while beta and gamma are least destructive.
Biological effects
One of the biological effects of ionising radiation is that atoms in body tissues are ionised, which can cause chemical imbalance of the cells through the creation of chemically reactive ions, such as free radicals, which can be harmful. Another biological effect is that energetic particles (especially alpha particles) can physically break one or both strings of DNA which can lead to mutations such as cancer. When subjected to a high dose of radiation at one time, the risk of severe damage is greater than if the same dose is received over a long time period. The reason is that the cell repair systems do not have time to respond if multiple cell injuries occur at the same time.
