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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.
The first cores in this concept are metallic U-Pu-Zr, not high-assay low-enriched uranium. The fissile part is separated civilian plutonium. The fertile part is depleted uranium. Both already exist in quantity. HALEU does not, at the scale a fleet would need, and this concept does not depend on it.
Light-water reactors already use civilian plutonium, as MOX (mixed uranium–plutonium oxide). That industry is real, and fresh MOX is guarded as Category I material in the same way a pure plutonium store is. A fast core does not get a lighter security regime by being smaller. The cost of a guard force is mostly a site cost. It does not shrink in proportion to megawatts. One way to avoid a new perimeter is to stand the plant inside a site that already holds separated plutonium. That is a siting choice. It is not a property of the reactor drawing.
After one pass in a thermal reactor, even-mass plutonium builds up and the fuel is a poor candidate for a second thermal pass. In a fast spectrum those isotopes can fission. Spent MOX and separated plutonium are therefore plausible feeds. Startup still needs a fissile inventory. A conversion ratio near one is a design goal, not a result the screening model calculates. Losses in recycle still have to be made up.
A fee paid by a state for taking separated plutonium is a contract and a policy question. It is not estimated here, and it is not plant revenue in this concept.
Clean U-Pu-Zr is handled in gloveboxes: plutonium is an alpha emitter, and americium that has grown in from Pu-241 adds gamma dose. Once the fuel has been irradiated, or if it still contains fission products, the dose rate itself denies casual access. Fresh, fully decontaminated metal does not have that property. See ReCore-Processing for why the pyro plant is a later step, and why americium is awkward to cast.
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 multi-recycle can convert them into shorter-lived fission products — a fuel-cycle goal, dependent on industrial recycle performance (ReCore-Processing.
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.