Step by step: How to build a nuclear bomb - explainer

Israel and the world are waiting for President Trump’s decision. In the coming days or hours he will determine whether to join the effort and help Israel destroy the Fordow reactor, located 90 meters below ground, the stronghold completing the production of nuclear bombs capable of obliterating entire cities.

What steps are involved in creating a nuclear bomb? What does it look like, which facilities are needed, and what does it mean when reports say Iran is enriching uranium to 90 percent?

A nuclear bomb is a weapon that uses the immense energy released from within an atom’s nucleus—the innermost part of all matter. In such a device, a chain reaction is triggered that causes the nuclei of certain isotopes (such as uranium or plutonium) to split, releasing colossal energy, extreme heat, and a devastating blast.

Before explaining bomb types, it’s crucial to understand the basic materials—uranium and plutonium—and why they are so suitable for such destructive weapons.

Uranium is a chemical element found in nature, primarily in rocks and ores worldwide. It appears as a silvery-gray metal, but its nuclei are exceptionally heavy.

Natural uranium contains several isotopes, but only one—uranium-235—readily undergoes fission and releases vast energy. However, uranium-235 makes up less than one percent of natural ore.

To use it, one must “enrich” the uranium—separate and concentrate the rare isotope. Only once its concentration exceeds about 90 percent can it serve as bomb fuel.

When a small sphere of enriched uranium reaches a “critical mass,” it can sustain a runaway chain reaction. Each fission event releases enormous energy—this principle underpinned the Hiroshima bomb in 1945, which killed over 100,000 people within seconds.

Unlike uranium, plutonium does not occur naturally in significant quantities. It is produced in nuclear reactors when uranium-238 absorbs neutrons. The resulting plutonium-239 behaves similarly to uranium-235 but is even more prone to fission, making it highly attractive for bombs.

Because it is easier to isolate plutonium in a reactor than to enrich uranium-235, some states prefer plutonium-based weapons. Plutonium bombs are more powerful but require precise conventional explosive lenses to compress the core. The Nagasaki bomb detonated days after Hiroshima used plutonium.

Natural uranium contains only about 0.7 percent U-235. Enrichment raises this proportion:

Atoms consist of nuclei made of protons (defining the element) and neutrons (affecting mass and stability). Isotopes share the same proton count but differ in neutrons.

Uranium-238 (146 neutrons) is stable and unsuitable for bombs, while uranium-235 (143 neutrons) fissions readily, essential for a chain reaction. The same applies to plutonium isotopes; only plutonium-239 is fissile.

A centrifuge is a cylinder spinning at tens of thousands of revolutions per minute. It exploits tiny mass differences between U-235 and U-238 to gradually concentrate the desired isotope.

Modern plants run thousands of centrifuges in cascades to achieve high enrichment levels.

If a state has enough weapons-grade fissile material, only a few months of technical work and assembly are needed before a bomb is ready. Israel and the US warn Iran is perilously close to that threshold.

The GBU-57A/B Massive Ordnance Penetrator (“MOP”) is a 30,000-lb bunker-buster designed to penetrate deep into reinforced structures before detonation. It can breach over 60 meters of concrete or rock.

Only the B-2 Spirit stealth bomber can carry the MOP, flying undetected over enemy airspace to deliver it precisely.

Eight states publicly acknowledge arsenals—the US, Russia, China, France, the UK, India, Pakistan, and North Korea—and Israel is widely believed to possess them. Iran has not yet joined this group, but it may do so soon.

A Hiroshima-type weapon killed over 100,000 people in seconds. Modern warheads are tens of times more powerful, unleashing blistering heat, blast, and long-lasting radiation.

A reactor’s thick concrete dome serves as a radiation shield and blast barrier. It contains radioactive materials and protects both workers and the public from leaks or internal accidents.

Spent fuel rods sit in deep pools of ordinary (or sometimes “heavy”) water to absorb decay heat and block radiation. The water prevents overheating and shields against dangerous emissions.

The characteristic blue glow is Cherenkov radiation, a phenomenon where charged particles exceed light’s speed in water.

Only specialised bunker-busters like the GBU-57A/B can reach its 90 meter depth. Analysts expect a layered attack—multiple penetrating bombs collapsing successive containment rings—to neutralise the facility.