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This page explains what spent nuclear fuel from light-water reactors actually is, why long-lived waste is driven by transuranics, and how pyroprocessing chemistry relates to a closed metal-fuel cycle. How that chemistry supports the overall concept is summarised under Why?.
Spent nuclear fuel (SNF) from a light-water reactor (LWR) is the irradiated fuel assembly after it leaves the core — typically ceramic uranium dioxide (UO₂) pellets in zirconium-alloy (Zircaloy) cladding, held in a metal assembly structure. It is not “empty ash”: most of the mass is still uranium, and a large fraction of the original energy potential remains unused in a once-through cycle.
Commercial LWR fuel usually starts as low-enriched uranium (LEU): mostly U-238, with a few percent U-235 (often roughly 3–5% for power reactors, exact values depend on design and cycle). The ceramic form is UO₂; the cladding is zirconium-based metal chosen for neutron economy and corrosion behaviour in water.
In the core, neutrons fission U-235 (and later some plutonium) and also transform U-238 by capture. After several years in the reactor, a discharged assembly is a mixture of:
| Rough class | What it is | Why it matters |
|---|---|---|
| Unused uranium | Mostly U-238 plus residual U-235 | Still fertile / partly fissile; most of the heavy-metal mass |
| Plutonium | Built mainly from U-238 capture + β decay | Fissile and fertile isotopes; long-term radiotoxicity and safeguards significance |
| Minor actinides | Neptunium, americium, curium, … | Small mass share, large share of very long-lived radiotoxicity |
| Fission products | Lighter fragments of fission (Cs, Sr, Tc, I, lanthanides, …) | Dominate heat and short-to-medium-term radiotoxicity |
| Activation products | In cladding and structural metals | Secondary waste streams after reprocessing or disposal |
| Cladding & hardware | Zircaloy tubes, grids, end pieces | Physical waste form; chopped or disposed with fuel in once-through paths |
Exact percentages depend on burnup, enrichment, power history, and cooling time. Order-of-magnitude pictures often quoted for typical PWR discharged fuel (after some cooling) look roughly like: about 95%+ uranium by heavy-metal mass, about 1% plutonium, under 0.1% minor actinides, and about 3–5% fission products
After discharge, SNF is intensely radioactive and generates decay heat. It is usually cooled for years in wet storage, then often moved to dry casks. Early on, fission products dominate heat and dose; over centuries to millennia, actinides (especially plutonium and americium) increasingly dominate the long radiotoxicity curve that drives multi-hundred-thousand-year isolation arguments for once-through waste.
SNF is therefore simultaneously:
In a once-through cycle, the whole assembly is treated as waste for deep geologic disposal (or long interim storage). In a closed cycle, the goal is to separate and reuse actinides as fuel and immobilize fission products in compact waste forms. See ReCore-Processing for the pyro path used in this concept.
Transuranic elements (TRU) are elements with atomic number greater than uranium (92): principally plutonium, neptunium, americium, and curium.
In LWR SNF, TRUs are a small mass fraction but a large driver of long-term radiotoxicity. They form mainly by successive neutron captures and decays starting from U-238 (and chains involving Pu isotopes). In thermal spectra, many higher actinides are hard to fission efficiently, so they accumulate rather than burn.
In a fast spectrum, many TRU isotopes fission more readily. Recovered TRU (with uranium) can be fuel, not only waste. The ReCore concept aims to return TRU to a metallic U-TRU-Zr core via pyroprocessing so that multi-recycle can convert them into shorter-lived fission products.
The liquid cadmium cathode (LCC) is a process option in electrorefining used in metal-fuel pyroprocessing programmes.
In the electrorefiner, electricity moves metal ions through a molten salt bath (see How spent fuel is pyroprocessed:
Because of the electrochemistry of the cadmium pool, the process is designed so that pure plutonium is not isolated as a separate product stream of the kind associated with classical PUREX-style pure Pu oxide. The product remains a highly radioactive actinide mixture that requires remote handling.
That is safeguards-by-process design, not a claim that no diversion analysis or international safeguards are needed. Operational detail and material accountancy remain essential for any real facility. Broader recycle context: ReCore-Processing.