Nuclear weapons

How they work

Nuclear weapons are manufactured principally from highly enriched uranium or plutonium. Early nuclear weapons, including the types exploded in warfare in Japan in 1945, employ the principles of nuclear fission to create a chain reaction in a critical mass of either enriched uranium or plutonium.

In the simplest design, the ‘gun assembly’, a mass of fissile uranium is fired at a uranium target in a similar manner to firing a bullet along a gun barrel. The two masses achieve a critical mass when they combine. In more sophisticated implosion devices a fissile mass of uranium plutonium, or a combination of the two is surrounded by high explosives which are triggered to compress the mass and bring about criticality.

Modern thermonuclear weapons derive their destructive power from a two-stage reaction: a ‘primary’ nuclear fission explosion which initiates a much more powerful ‘secondary’ nuclear fusion reaction.


The world’s first nuclear weapons were developed by the US-lead Manhattan Project during World War II. The project resulted in the first nuclear explosion, the ‘Trinity’ test, in July 1945 in New Mexico, followed by the dropping of nuclear bombs on Hiroshima and Nagasaki in Japan in August 1945.

After World War II the USA was joined in 1949 by the Soviet Union as a nuclear weapons power, followed by the UK in 1952 and France and China in the 1960s.

Under the Nuclear Non-Proliferation Treaty (NPT), which was signed in 1970, these five states are recognised nuclear weapon states and have committed to a negotiated disarmament as part of the treaty. The treaty, whose other signatories have promised not to develop or acquire nuclear weapons, has been endorsed by nearly every country in the world.

India and Pakistan have not signed the NPT but have both tested nuclear weapons and declared that they possess nuclear weapons. Israel has not signed the NPT and while it maintains an official policy of neither confirming or denying, it is widely acknowledged as possessing nuclear weapons. North Korea signed the NPT but in 2003 announced that it had withdrawn from the Treaty and in October 2006 conducted its first nuclear weapons test.

The UK’s nuclear weapons

The UK currently deploys only a single nuclear weapons system – the submarine-based Trident system. This has been the case since 1998, when the last WE177 gravity bomb was withdrawn from service.

The UK’s Trident missiles are made in the US and are drawn from a common pool that is shared with the US Trident submarine fleet. They are carried on Vanguard-class submarines and are armed with a warhead that closely resembles the US W76 warhead. The Vanguard submarine fleet is planned to be replaced by Dreadnought-class submarines in the early 2030s. In 2020 the UK government announced its plan to build a new nuclear warhead.

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