Two employees walk past a reactor at the Marcoule nuclear site in southeastern France.

The materials, technology, and expertise associated with the nuclear fuel cycle can be used to produce both nuclear power and nuclear weapons. Once a country is capable of enriching uranium so it can be used for nuclear purposes, even peaceful ones, it usually needs just a few months to produce enough material for a nuclear weapon. This makes nonproliferation exceptionally difficult.

It all starts in the Earth’s crust, home to uranium ore. Uranium is found all over the world, though mostly in trace quantities. Five countries – Australia, Canada, Kazakhstan, Niger, and Russia – possess two-thirds of the world’s known supply. Once located, uranium is extracted through mining. It is then converted to gas so it can be enriched and used for nuclear purposes.


The scientific process at the base of all nuclear energy is called fission.

Nuclear Fission

In nuclear energy, fission occurs when neutrons bombard atoms, specifically targeting their nuclei, and split them, releasing massive amounts of energy. Fission occurs easily in only a few isotopes, or types, of elements, typically uranium and plutonium. (Unlike uranium, plutonium isn’t found in nature--we’ll return to it later.)

In nature, uranium is a mix of mostly two isotopes, uranium-235 and uranium-238, and only the former easily undergoes fission. The trouble with U-235 is that it is extremely rare, accounting for less than 1 percent of the world’s natural uranium. So countries with nuclear ambitions, peaceful or not, need to first increase the proportion of U-235 in their uranium samples. That’s where enrichment comes in.


In most cases, nuclear energy production begins with uranium enrichment.

Uranium enrichment most commonly occurs in gas centrifuges; this increases the proportion of U-235 in a uranium sample to facilitate fission. Uranium can be enriched to various levels, which fall into two categories: low-enriched uranium (LEU) and highly enriched uranium (HEU). LEU has up to 20 percent U-235 and is often used for nuclear power or in “nonpower reactors,” which produce materials for medical use, scientific research, and other purposes. HEU has more than 20 percent U-235, and is mainly used for military purposes; this includes naval reactors (such as those used in nuclear-powered submarines) but generally means nuclear weapons.

Any level of HEU works for a weapon, but it’s weapons-grade uranium, enriched to at least 90 percent, that’s most common. This is because the higher the enrichment level, the less material that’s needed--clearly desirable for weapons, because smaller warheads fit more easily on missiles that can travel long distances.


Why is this important? Because of “breakout time.”

Breakout time refers to the theoretical amount of time required for a country with an existing enrichment plant to produce enough HEU for a nuclear weapon. (Other technologies are needed to build the weapon itself.) It takes significantly fewer resources and expertise to enrich LEU to the 90 percent needed for weapons-grade uranium than it does to enrich uranium in its natural state to between 3 and 5 percent U-235 (the enrichment level of LEU used in most nuclear power plants).

Because of this, once a country can enrich uranium at all, its breakout time is often just a matter of months.

And that’s a precarious prospect.

Paths to the Bomb


95 percent of nuclear bombs are actually plutonium-based.

Producing HEU is only one path to a nuclear weapon. But uranium enrichment is actually a necessary intermediate step in the process of creating plutonium, which isn’t found in nature. Plutonium-fueled bombs can be twice as powerful as uranium-fueled bombs.

There are two types of plutonium, created via different means but both derived from spent, or used, uranium fuel found in reactors. Reactor-grade plutonium (55-70% Pu-239) is extracted from spent fuel that’s irradiated, or exposed to radiation, for years in a nuclear reactor. Weapons-grade plutonium (at least 90% Pu-239) is extracted from spent fuel that’s been transferred to a special plutonium production reactor and only irradiated for weeks or months.

Because weapons-grade plutonium is made specifically for military purposes, and this reactor has a different design from a typical nuclear reactor, it is easy to identify as a sign that a country may be trying to build nuclear weapons.

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