How Nuclear Weapons Work

The document describes how nuclear weapons generate energy through fission (splitting heavy nuclei) and fusion (combining light nuclei). It details the transition from early 'gun-type' fission weapons to modern thermonuclear weapons that use a primary fission stage to trigger a secondary fusion stage. It also outlines the technical challenges of uranium enrichment and plutonium reprocessing, and provides estimated warhead counts for nuclear-armed states as of 2025.

• Nuclear weapons derive their explosive power from either fission, where heavy nuclei split into smaller nuclei, or fusion, where lightweight nuclei combine under extreme temperature and pressure. Modern thermonuclear weapons combine both processes, using a primary fission stage to drive a secondary fusion stage, often resulting in energy releases exceeding all weapons used in World War II combined.
• The primary isotopes used in nuclear weapons are uranium-235 and plutonium-239. Uranium-235 requires a costly and difficult 'enrichment' process because it makes up less than one percent of naturally occurring uranium. Plutonium-239 is produced as a byproduct in nuclear reactors and is isolated through 'reprocessing,' a highly radioactive process requiring remote-handling equipment.
• Modern fission weapons achieve critical mass—the minimum material needed to maintain a reaction—by using chemical explosives to compress a sub-critical sphere of nuclear fuel, rather than colliding two pieces of fuel. Thermonuclear weapons further enhance yield by using 'boosted' fission (adding deuterium and tritium) and a fission 'spark plug' to facilitate fusion, with a final uranium 'blanket' often contributing more than half of the total explosive yield.
• As of 2025, the largest nuclear arsenals are held by Russia (estimated 5,459 warheads) and the United States (estimated 5,177 warheads). Other nations with nuclear weapons include China (~600), France (~370), the United Kingdom (~225), India (~172), Pakistan (~170), and North Korea (suspected ~50). Israel has not officially acknowledged its capabilities, though estimates are typically around 80 warheads.
• Global nuclear stockpiles have decreased by more than 75 percent from a mid-1980s peak of over 60,000 warheads to approximately 15,000 today. Despite this, risks remain due to the potential for severe climate effects from nuclear exchange and the continued 'hair-trigger alert' status of hundreds of U.S. missiles.

APA

Union of Concerned Scientists (2016). How Nuclear Weapons Work. https://www.ucs.org/resources/how-nuclear-weapons-work

Chicago

Union of Concerned Scientists. How Nuclear Weapons Work. 2016. https://www.ucs.org/resources/how-nuclear-weapons-work.

Wikipedia

{{cite report |author=Union of Concerned Scientists |title=How Nuclear Weapons Work |date=29 September 2016 |url=https://www.ucs.org/resources/how-nuclear-weapons-work |access-date=23 October 2026 |via=Climate Insights Directory}}

BibTeX

@techreport{unionofconcernedscientists2016how, author = {{Union of Concerned Scientists}}, title = {{How Nuclear Weapons Work}}, institution = {Union of Concerned Scientists}, year = {2016}, month = sep, url = {https://www.ucs.org/resources/how-nuclear-weapons-work}, urldate = {2026-10-23}, note = {Indexed by Climate Insights Directory} }