The quest for boundless energy | Science

The quest for boundless energy | Science: Although designs abound, reactors that can breed fuel remain mostly a dream.

For all their innovations, NuScale Power's small modular reactors remain conventional in one way: They would use ordinary commercial reactor fuel that's meant to be used once and safely disposed of. But for decades, nuclear engineers envisioned a world powered by “fast reactors” that can breed an essentially boundless supply of fuel as they operate while producing less long-lived radioactive waste to boot. The dream lives on today in dozens of designs for advanced fast reactors meant to be cheaper and safer than their predecessors.

The uranium fuel for a typical nuclear reactor contains less than 5% of the isotope uranium-235. Its nuclei can split, or fission, to release energy and neutrons. The dilute fuel sustains a chain reaction only if the neutrons are slowed by a moderator, typically the reactor's cooling water, to increase the probability that they'll split other nuclei. In contrast, a fast reactor runs without a moderator by using a fuel richer in uranium-235, or one containing plutonium. Both fuels produce copious neutrons. They enable a fast reactor to breed more fuel as neutrons shower nuclei