Nuclear weapons: a beginner’s guide to the threats

In the run-up to new UN negotiations on a treaty to ban nuclear weapons, Dr Philip Webber, SGR, outlines the key scientific and technological information on the threat from these weapons of mass destruction.

Latest update: 24 March 2017

Concern about nuclear weapons is again high. Whether you’re worried about Donald Trump now having his finger on the button, North Korea’s latest missile test or the misfire by the UK’s Trident system, the risk that these weapons may once again be used is increasingly in the public mind.

Yet, despite this, there has been very little media attention devoted to upcoming UN negotiations – beginning on 27 March in New York – on a treaty to ban these weapons of mass destruction. These negotiations are supported by over 100 governments, the International Red Cross and many civil society organisations.

In the run-up to the negotiations, SGR is publishing a series of short online articles (with a new one each day or so) to help public understanding of what is at stake. We will summarise key scientific and technological background information – for example, about the destructive capabilities of the various weapons held by the nuclear-armed states. The articles will be fully referenced so that readers can dig deeper into the issues if they wish.


1. What is a nuclear weapon?

2. Nuclear weapons: the basic science

3. How many nuclear weapons are there?

4. How much destructive power do the nuclear-armed nations have?


1. What is a nuclear weapon?

A nuclear weapon has two key parts: an extremely powerful warhead which explodes over its target; and a 'delivery system', usually a missile. Nuclear-armed missiles can be launched from an underground 'silo', a ground-based mobile launcher (basically a large truck), a submarine or by a bomber flying at high altitude. Nowadays, nuclear bombs which simply drop on to a target are very rare because of the high likelihood of destroying the bomber as well as the target. [1] Some nuclear warheads can also be fired from heavy artillery or multiple rocket launchers or placed in the ground as a nuclear landmine.  

A typical modern warhead can be quite small. For example, the USA’s W-80 cruise missile warhead is less than a metre long, about 30cm in diameter – about the size of a domestic waste bin – and weighs 130kg. [2]

All nuclear weapons deliver a huge destructive power by releasing the very powerful forces which hold atoms together. Most nuclear weapons deployed today have explosive powers ranging from 100,000 tonnes of TNT equivalent (100 kilotonnes or kT) up to several million tonnes (MT). [3]

Many nuclear-armed missiles have an intercontinental range, in which case the warheads are launched briefly into space. These missiles carry several warheads (typically between 3 and 12). [4] Such warheads re-enter the atmosphere individually high above their target moving at several times the speed of sound, thus one missile launch can strike several targets hundreds of kilometres apart. Nuclear weapons are very robust – during re-entry they glow white hot. They can reach their targets less than 30 minutes after launch.

While comparisons are made with conventional high explosives, a nuclear warhead has many more damaging effects. It produces an intense electro-magnetic pulse (EMP) of energy which can knock out electronic equipment, a blinding flash of light, intense nuclear radiation, an intensely hot fireball capable of starting fires and causing burns at great distances, an extremely powerful blast wave and radioactive particles that can be carried for many kilometres downwind. [5] [6] [7]


[1] Some recent general sources (with various levels of technical detail) are:
Wikipedia (2017). Nuclear weapon.
Encyclopaedia Britannica (2016). Nuclear weapon.
BBC News online (2009). Trident missile factfile.

[2] For example, see: Nuclear Weapon Archive (2006). Complete List of All U.S. Nuclear Weapons.

[3] International Institute for Strategic Studies (2016). The Military Balance 2016.

[4] Webber P, Wilkinson G, Rubin B (1983). Crisis Over Cruise. Penguin Books.
Rotblat J (1981). Nuclear radiation in warfare. Stockholm International Peace Research Institute (SIPRI). p.21.
National Security Archive (1997/ 1976). Multiple Independently Targetable Re-entry Vehicles (MIRVs).

[5] Webber P, Parkinson S (2015). UK nuclear weapons: a catastrophe in the making? Scientists for Global Responsibility.

[6] Greene O, Rubin B, Turok N, Webber P, Wilkinson G (1982). London after the bomb: what a nuclear attack really means. Oxford University Press.

[7] Glasstone S, Dolan PJ (1977). The Effects of Nuclear Weapons, US Dept of Defense and Energy.


2. Nuclear weapons: the basic science

To understand just how destructive nuclear weapons are, it is useful to understand the basic physics behind them.

The first nuclear weapons released enormous amounts of energy through splitting the nucleus of atoms of uranium or plutonium. This process is called ‘nuclear fission’.  The enormous explosion arises as a small amount of matter is converted into energy as defined by the famous equation e = m x c2 where e is the energy of the explosion (in joules), m is the mass of the material converted to energy (in kg) and c is the speed of light (which is 300 million metres per second). So the energy released in the nuclear explosion is 90,000,000,000,000,000 times the mass! A nuclear explosion occurs when a large enough ‘critical mass’ of uranium or plutonium [8] is brought together. The usual way of measuring the size of the explosion created by a nuclear weapon – called the ‘yield’ – is to compare it with an amount of the common explosive TNT. For example, the nuclear bomb dropped on the Japanese city of Hiroshima towards the end of World War II had a yield of about 15,000 tonnes of TNT or 15 kilotonnes (kT).

A nuclear weapon which just relies on the process of nuclear fission is commonly called an atomic bomb or A-bomb. The simplest form of this type of weapon uses purified (‘enriched’) uranium-235 extracted from naturally occurring uranium. An explosive nuclear reaction is created by suddenly bringing two smaller pieces of this enriched uranium together to form a critical mass. In a critical mass, a nuclear chain reaction takes place, releasing enormous energy in less than a millionth of a second. Countries which have weapons of this type include India, Pakistan and North Korea. Uranium-235 atomic bombs made with this simple design can have yields up to around 20kT, beyond which it becomes very difficult to hold a larger than critical mass together long enough to create a bigger explosion. [9]

With a bomb made using plutonium, simply creating a critical mass causes a fairly ‘weak’ nuclear explosion as the weapon blows itself apart before the chain reaction has gone very far. In a plutonium (or specialist uranium-235) bomb, specialist high explosives called ‘shaped charges’ crush a hollow sphere of plutonium into a critical mass and hold it there for a millionth of a second. Other special materials such the metal beryllium are also used to reflect neutrons making the explosion stronger. [10] Additional neutrons may also be fired in from another source to set off a powerful fission explosion. This type of weapon can only be made by countries with advanced knowledge of materials, high explosives, detonation circuitry and neutron generators as well as access to plutonium. 

Most industrialised countries with nuclear power plants – for example, Japan, South Korea, Germany and Brazil – could build nuclear weapons within a few months, but they choose not to do so. Brazil, Sweden and South Africa once had nuclear weapons programmes, but decided to abandon them. [11]

As the USA, Soviet Russia, the UK and France developed nuclear weapons in the 1950’s, scientists tested and built a new type of nuclear weapon using a combination of fission and another process called ‘nuclear fusion’. This newer weapon is often called the hydrogen bomb, thermonuclear weapon or H-bomb. In this type of weapon, the lightest element, hydrogen, is fused together to form helium. This is the same process that fuels the Sun – thus every thermonuclear bomb creates a short-lived but fierce release of energy like a miniature Sun. Again, a small amount of matter is converted into enormous amounts of energy – and it is possible to build weapons with very large yields. In this weapon type, a compact plutonium-based atomic bomb is detonated, generating intense gamma, neutron and x-ray radiation. Before the radiation and explosive forces can destroy the weapon itself, the nuclear radiation is used to bombard a lightweight foam containing hydrogen causing a nuclear fusion reaction, forming helium. But crucially, in this type of weapon, very high yields can be achieved simply by increasing the amount of the fusion ‘fuel’. Weapons with yields of up to 50 million tonnes (MT) have been tested. Finally, this enormous fusion reaction and explosion creates a further large fission reaction in the uranium or other heavy metal warhead casing. 

Through major programmes of development work – which included hundreds of actual nuclear test explosions – the nuclear weapons possessed by the USA, Russia and several other nations are now much smaller and lighter than the first designs of the 1950’s. For example, the US cruise missile warheads with yields of up to 150kT are around 30cm in diameter, 80cm in length and weigh around 130kg. [12]


[8] Specific types or ‘isotopes’ of uranium or plutonium are needed to construct a nuclear weapon. For a uranium weapon, the most important isotope is uranium-235 or U-235 for short. For plutonium, it is plutonium-239 or Pu-239 for short. The figure refers to the number of neutrons in the nucleus of the atom of a particular material. Complex industrial processes – collectively known as ‘enrichment’ – are needed to produce the necessary amounts of the U-235 isotope from naturally occurring uranium. Plutonium does not occur naturally in nature and is made in the core of a nuclear reactor through the irradiation of uranium nuclear fuel. Thus nuclear power is an intrinsic part of developing a military nuclear weapons capability. What is unusual about these isotopes is that the atomic nucleus can split in two if struck by a sub-atomic particle called a neutron. This is what is called nuclear fission. Fission occurs naturally in these materials at a very low level due to neutrons that are part of cosmic rays which pass through the earth all the time. Every time a nucleus splits it emits two or three more neutrons. If there is sufficient mass – known as a critical mass (typically just a few kilograms) – each fission generates several more fissions and more neutrons in a very fast and run-away explosive reaction called a nuclear chain reaction. For more details, see:
Glasstone S, Dolan PJ (1977). The Effects of Nuclear Weapons. US Dept of Defense and Energy.

[9] SIPRI (1981). Nuclear Radiation in Warfare. (Written by J Rotblat.) p.7.

[10] As note 9 – p.5

[11] Cirincione J (2004a). A Brief History of the Brazilian Nuclear Program. Carnegie Endowment for International Peace.
Cirincione J (2004b). South Africa's Nuclear Free Decade. Carnegie Endowment for International Peace.

[12] Nuclear Weapons Archive (2006). Complete List of All U.S. Nuclear Weapons.


3. How many nuclear weapons are there?

Nine nations have nuclear weapons: the USA, Russia, China, France, the UK, India, Pakistan, Israel and North Korea. 

Altogether there are over 4,000 ready-to-fire (‘deployed’) warheads, with over 10,000 more held in various stockpiles. The Federation of American Scientists estimates that the current overall total number of warheads held by all nine nations is about 14,900. [13] 93% of these are held by the USA and Russia. [14]

And only Russia and the USA have nuclear missiles ready to fire at very short notice.

Russia and the USA each have around 900 warheads – on roughly 300 missiles each – ready to fire within minutes. [15] The intention is to be able to launch these missiles against a potential attacker before any incoming warheads strike their targets. This status is known as ‘launch on warning’. In such a situation, there is very little time – typically less than 20 minutes – to decide if a warning is real and a President would typically have less than ten minutes – possibly as little as five minutes – to decide whether or not to fire. [16] Maintaining this launch on warning status thus creates a high chance that some sort of error could cause an accidental nuclear launch.

The UK’s nuclear warheads are carried on Trident missiles – leased from the USA – in nuclear-powered submarines. Currently, eight missiles can be fired, carrying 40 x 100kT warheads, with a few hours’ notice from a submerged submarine. [17] The UK’s total nuclear weapons arsenal consists of 215 warheads. [18]

The French nuclear posture is similar to that of the UK. They have 16 missiles carried on a submarine on patrol, armed with up to 96 warheads of approximately 100kT. [19] Their total arsenal consists of 300 warheads. [20]

The four other larger nuclear-armed nations – China, India, Pakistan and Israel – do not keep nuclear weapons ready to fire as they judge that the risks of accidental launch are too risky. They store launchers and warheads separately and launchers are not ready to fire. [21]

Of these nations, only China has missiles with the ability to hit targets in Russia and the USA. They possess submarines which can be armed with nuclear weapons but their navy lacks the skills to navigate them reliably within range of the US and they do not normally carry nuclear weapons. [22] In total, China has 62 long range missiles and about 260 warheads. [23]

India and Pakistan have around 120 atomic weapons of a similar size to those dropped on Hiroshima and Nagasaki at the end of World War II. [24] Their missiles are of limited range and are capable of hitting targets within the sub-continent.

Israel refuses to confirm that it possesses nuclear weapons but is believed to keep 80 nuclear warheads and missiles stored separately with sufficient range to target countries in the Middle East such as Iran, Syria or Saudi Arabia. They also have nuclear-capable submarines and aircraft. [25]

In the final case of North Korea, they have detonated several small nuclear weapons in tests. North Korea is also testing missiles which have sufficient range to hit neighbouring countries such as China or Japan, although whether these missiles could carry nuclear warheads remains unverified. [26]

There are three countries – Brazil, Argentina and South Africa – who originally had covert nuclear weapons programmes but then decided to halt them, regarding nuclear weapons as more of a risk than a benefit. [27] Most countries with advanced manufacturing facilities and access to nuclear materials via nuclear power programmes could build a simple nuclear bomb maybe within a few months. Examples are Japan and Finland. This is a key point. Opponents of nuclear disarmament often state that you cannot get rid of nuclear weapons because they cannot be dis-invented. Yet there are international treaties banning chemical and biological weapons, landmines and some other weapons technologies. These treaties have been essential in helping to delegitimise these weapons and moving towards their total elimination. This is the process which the UN now seeks to follow for nuclear weapons during negotiations in 2017.


[13] Federation of American Scientists (2017). Status of World Nuclear Forces. 17 February update.

[14] Kristensen HM, Norris RS (2016a). United States nuclear forces 2016. Bulletin of the Atomic Scientists, vol.72:2, pp.63-73.
Kristensen HM, Norris RS (2016b). Russian nuclear forces 2016. Bulletin of the Atomic Scientists, vol.72:3, pp.125-134.
Federation of American Scientists (2017) – as note 13.

[15] Union of Concerned Scientists (2015). Taking Nuclear Missiles Off Hair-Trigger Alert.

[16] Bloomberg (2017). To Launch a Nuclear Strike, Donald Trump Would Follow These Steps.

[17] A ‘few hours’ refers to technical requirements rather than political decision-making. Section 40, Chapter 2 of: House of Commons Defence Committee (2006). The Future of the UK's Strategic Nuclear Deterrent. HC 986.

[18] As note 13.

[19] International Institute for Strategic Studies (IISS) (2016). The Military Balance 2016.

[20] As note 13.

[21] Kristensen HM, Norris RS (2016c). Chinese nuclear forces 2016. Bulletin of the Atomic Scientists, vol.72:4, pp.205-211.

[22] As note 19.

[23] As note 13.

[24] As note 13.

[25] As note 19 – Chap.7.

[26] Nuclear Threat Initiative (2017). North Korea.

[27] Cirincione J (2004a). A Brief History of the Brazilian Nuclear Program. Carnegie Endowment for International Peace.
Cirincione J (2004b). South Africa's Nuclear Free Decade. Carnegie Endowment for International Peace.
These countries are now very active proponents of nuclear disarmament at the UN.


4. How much destructive power do the nuclear-armed nations have?

Over cities, the consequences of exploding just one nuclear warhead are so extreme that leading medics, aid agencies including the Red Cross and Red Crescent, and several studies have concluded that an effective medical and humanitarian response would be impossible. [28] If a few tens or hundreds of warheads are detonated they would ignite huge fires in cities, oil refineries and other highly flammable targets. The resultant, persistent, high altitude smoke particles would disrupt the global climate, causing widespread agricultural collapse and famine. [29]

Modern nuclear weapons are up to 50 times more powerful than the single atomic bombs which devastated the Japanese cities of Hiroshima (bomb yield of 15,000 tonnes TNT or 15kT) and Nagasaki (21kT) in 1945. [30]

During the six years of World War II it is estimated that all the bombs dropped, including the two nuclear bombs, had a total explosive power equivalent to 3 million tonnes of TNT (3MT). [31]

The largest Russian warhead is the RS-20 with an explosive power of 800kT. This is equivalent to 40 times the size of the bomb dropped on Nagasaki. The Russian SS-18 missile (given the ‘Satan’ designation by NATO) can carry ten such warheads, giving this one missile a total destructive power of 8MT. In other words, this one nuclear tipped missile has a destructive power more than twice that of all the bombs dropped during WWII and of 400 times the Nagasaki atomic bomb! The Russian have 46 of these missiles ready for use and a further 507 launchers (missiles, and bombers) carrying smaller warheads. [32]

The total destructive power of Russian nuclear weapons ready to fire is approximately 510MT, equivalent to 170 times that of all the bombs dropped during WWII. [33] This is obviously an incredible level of destructive capability – and does not include weapons kept in stockpiles.

The largest comparable US warhead is the W-88 Trident-II Mk-5 with a yield of 475kT. One US Trident missile carries four of these warheads making a total destructive power of 1.9MT. This one missile is thus equivalent to 95 times the Nagasaki weapon or more than half of all the bombs dropped during WWII! The US have 96 such missiles ready to fire and a further 692 launchers (missiles and bombers) carrying smaller warheads. [34]

The total destructive power of the USA’s ready to fire arsenal is approximately 431MT, equivalent to 144 times that of all the bombs dropped during WWII. [35] Again, this is a staggering level.

One UK Trident warhead has a yield of 100kT, and the total destructive power of all the warheads carried by a single British submarine is currently 4MT. [36] Although modest by Russian and US standards, this is still more destructive power than all the bombs dropped in WWII. Hence even the UK with its supposedly “minimum” nuclear arsenal (which is comparable to that of most of the other smaller nuclear-armed states) deploys the capability for devastation with global consequences.

Nuclear weapons are so incredibly destructive that it is very hard to hold in your head a clear picture of the levels of destruction of which they are capable. Even what are now considered to be small nuclear bombs caused incredible levels of devastation, deaths and injuries in Hiroshima and Nagasaki in 1945. [37] Nuclear weapons tests provide some insight with images and pictures of huge fireballs and shockwaves overwhelming whole fleets of (retired) battleships moored in test areas such as Mururoa Atoll in the Pacific, and the complete destruction of target housing, bridges and other civilian infrastructure. As a result, there are fairly reliable ways of estimating the casualties that would result from the use of nuclear weapons over a modern city and various military targets. This will be the subject of a later article.


[28] See for example:
International Committee of the Red Cross and Red Crescent (2016). A price too high: Rethinking nuclear weapons in light of their human cost.
Moyes R, Webber P, Crowther G (2013). Humanitarian Consequences: short case study of the direct humanitarian impacts from a single nuclear weapon detonation on Manchester, UK. Article 36.

[29] Helfand I (2013). Nuclear Famine: Two Billion People at Risk? International Physicians for the Prevention of War.
Mills MJ, Toon OB, Lee-Taylor J, Robock A (2014). Multidecadal global cooling and unprecedented ozone loss following a regional nuclear conflict. Earth’s Future, vol. 2, pp. 161–176.

[30] Malik JS (1985). The yields of the Hiroshima and Nagasaki nuclear explosions. Los Alamos National Laboratory, report number LA-8819.

[31] The total explosive power of all the bombs dropped in World War II has been estimated by US and Russian physicists to be about 3,000,000 tonnes of TNT. p19 of: Schlosser E (2013), Command and Control. Penguin.

[32] Kristensen HM, Norris RS (2016a). Russian nuclear forces 2016. Bulletin of the Atomic Scientists, vol.72:3, pp.125-134.

[33] My calculation based on data from note 32.

[34] Kristensen HM, Norris RS (2016b). United States nuclear forces 2016. Bulletin of the Atomic Scientists, vol.72:2, pp.63-73.

[35] My calculation based on data from note 34.

[36] Webber P, Parkinson S (2015). UK nuclear weapons: a catastrophe in the making? Scientists for Global Responsibility.

[37] The total casualties (deaths and injuries within a few months of the explosion) due the atomic bomb dropped on Hiroshima were about 200,000. Detailed figures available in: Ishikawa E, Swain DL (translators) (1981). Hiroshima and Nagasaki: The physical, medical, and social effects of the atomic bombings. The committee for the compilation of materials on damage caused by the atomic bombs in Hiroshima and Nagasaki. Hutchinson. (First published in Japanese in 1979 by Iwanami Shoten, Tokyo. © 1981 Hiroshima and Nagasaki Cities.)