*How advanced reactor designs are transforming used nuclear fuel into clean energy*
One of the most persistent criticisms of nuclear power is its waste: spent fuel rods that have remained dangerously radioactive for tens of thousands of years. But a growing class of advanced reactor designs — particularly sodium-cooled fast reactors — can use that ‘waste’ as fuel, dramatically reducing both the volume and the hazard lifetime of nuclear byproducts. The technology is not theoretical; it’s being built right now.
The Problem with Conventional Nuclear Waste
When a uranium fuel rod is ‘spent’ in a conventional light-water reactor, it still contains roughly 95% of its original uranium — mostly U-238 — plus about 1% plutonium and a small fraction of highly radioactive fission products and minor actinides. These minor actinides (neptunium, americium, curium, and others) are the primary source of nuclear waste’s extraordinary longevity. Eliminating them would reduce the hazardous lifetime from hundreds of thousands of years to a few centuries.
Fast Reactors: Key Technology
Conventional reactors use ‘thermal’ neutrons — slowed-down neutrons that are very efficient at splitting U-235 but poor at ‘burning’ U-238 and the minor actinides. Fast reactors use high-energy (‘fast’) neutrons that are effective at fissioning these heavier atoms. In a fast reactor, spent fuel from conventional plants becomes a valuable feedstock rather than a disposal problem. TerraPower’s Natrium reactor, currently under construction in Kemmerer, Wyoming, is a sodium-cooled fast reactor that converts some U-238 in its core into plutonium-239, which then burns with high efficiency. TerraPower estimates that the ultimate 1,000 MWe Natrium reactor will generate approximately 33 times more electrical energy per ton of mined uranium than present-day light-water reactors. The United States holds 700,000 metric tons of depleted uranium — a byproduct of the enrichment process that fast reactors could theoretically use as fuel for centuries.
The Natrium Demonstration Project
TerraPower’s Natrium demonstration project is the most advanced fast reactor program in the Western Hemisphere. The first non-nuclear construction began in June 2024 at the site of a retiring coal plant in Kemmerer, Wyoming, with nuclear island construction planned for 2026 and commercial operation targeted for 2031. The project is supported by up to $2 billion from the U.S. Department of Energy’s Advanced Reactor Demonstration Program, matched dollar-for-dollar by TerraPower. The NRC completed its final safety evaluation in December 2025, finding no issues that would prevent issuing a construction permit. The project has also secured a 10-year HALEU fuel supply agreement, addressing one of the sector’s key vulnerabilities.
Breed and Burn
The most radical application of fast reactor technology is the ‘breed-and-burn’ approach, in which the reactor breeds new fissile material from U-238 faster than it consumes the fuel it started with. TerraPower’s original traveling wave reactor concept was designed around this principle. A single fuel loading could power the reactor for 100 years without refueling, using only depleted uranium stockpiles. The mature 1,000 MWe Natrium design incorporates breed-and-burn capabilities, and TerraPower notes that natural or unenriched uranium could serve as the only input fuel for these larger reactors, eliminating the need for enrichment entirely and dramatically reducing proliferation risk.
Global Fast Reactor Programs
Russia has operated the BN-800 sodium fast reactor since 2016 and is constructing the larger BN-1200. China is advancing its China Experimental Fast Reactor program. India’s Prototype Fast Breeder Reactor (PFBR) was scheduled to enter service in late 2024. France operated the Superphénix fast reactor from 1985 to 1997, accumulating valuable operational experience. The European Union’s advanced reactor programs include lead-cooled fast reactor designs specifically optimized for minor actinide burning.
The Path Forward
Nuclear waste recycling via fast reactors represents one of the most compelling long-term solutions to both the waste disposal problem and the supply constraints on uranium fuel. It does not eliminate nuclear waste — fission products still require disposal — but it dramatically reduces the volume and the geological timeframe of concern. As TerraPower and its peers demonstrate these technologies over the coming decade, the case for closing the nuclear fuel cycle will only strengthen.