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Why could deuterium and lithium provide energy for billions of years?

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3D illustration of the interior reaction chamber of a tokamak fusion reactor, showing a large, cylindrical, metallic structure with curved walls and bright light panels. — fusion reactor A tokamak is a device used in nuclear-fusion research for magnetic confinement of plasma.

Deuterium and lithium can provide energy for billions of years because they serve as the primary fuel components for deuterium-tritium (D-T) fusion, a process that is effectively inexhaustible due to the abundance of these materials.1

In a D-T fusion reactor, the reaction between a deuteron and a triton produces an alpha particle and a high-energy neutron. Because tritium does not occur naturally in significant quantities, the reactor is designed with a surrounding lithium "blanket". When the neutrons produced by the fusion reaction are captured by this lithium blanket, they generate more tritons, which are then fed back into the plasma as fuel.1

This unique "closed-loop" system allows the reactor to use its own waste products to generate more fuel. Since deuterium is readily available in seawater and lithium is abundant and inexpensive, these resources are sufficient to meet global energy needs for billions of years.1