Late last week, Israel targeted three of Iran’s key nuclear facilities – Natanz, Isfahan and Fordow, killing several Iranian nuclear scientists. The facilities are heavily fortified and largely underground, and there are conflicting reports of how much damage has been done.

Natanz and Fordow are Iran’s uranium enrichment sites, and Isfahan provides the raw materials, so any damage to these sites would limit Iran’s ability to produce nuclear weapons.

But what exactly is uranium enrichment and why does it raise concerns?

To understand what it means to “enrich” uranium, you need to know a little about uranium isotopes and about splitting the atom in a nuclear fission reaction.

All matter is made of atoms, which in turn are made up of protons, neutrons and electrons. The number of protons is what gives atoms their chemical properties, setting apart the various chemical elements.

Atoms have equal numbers of protons and electrons. Uranium has 92 protons, for example, while carbon has six. However, the same element can have different numbers of neutrons, forming versions of the element called isotopes.

This hardly matters for chemical reactions, but their nuclear reactions can be wildly different.

When we dig uranium out of the ground,

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