Nuclear and radiochemistry

Processing and Recycling Nuclear Fuel

Chemical separation of irradiated nuclear fuel recovers uranium and plutonium, isolates long-lived minor actinides, and vitrifies high-level waste for geologic disposal.

IntuitionIntuition: closing the nuclear fuel cycle

When a nuclear fuel assembly is discharged from a commercial light-water reactor after several years of power generation, it is far from empty. Over 95% of the original heavy metal atoms remain unreacted uranium, alongside roughly 1% plutonium bred in situ by neutron capture, 4% intensely radioactive fission products, and minor actinides. In an open once-through cycle, this entire assembly is discarded into cooling ponds and geological repositories, entombing vast energy while requiring radiation containment for over 100,000 years. Chemical reprocessing treats spent fuel as an artificial ore, extracting reusable fissile material while shrinking the long-term toxicity footprint.

SchoolSchool level: spent fuel inventory and the PUREX process

Commercial reprocessing is dominated by the PUREX (Plutonium URanium EXtraction) process. In the head-end stage, bundles are sheared into small lengths ($3\text{-}5\text{ cm}$) and dissolved in boiling $3\text{-}6\ \text{M}\ \ce{HNO3}$. The Zircaloy cladding hulls remain intact and are removed. Uranium dissolves as uranyl ion $[\ce{UO2^2+}]$, plutonium oxidizes to $\ce{Pu^4+}$, while volatile gases ($\ce{^{85}Kr}, \ce{^{129}I}, \ce{^{14}C}, \ce{^3H}$) are trapped in off-gas scrubbing columns. Insoluble residues of noble fission metals (fines of Ru, Rh, Pd, Mo, Tc) are separated by high-speed centrifugal clarifiers. The clarified aqueous solution is fed to a countercurrent extraction contactor where it meets an organic phase composed of $30\%\ \text{v/v}$ tri-$n$-butyl phosphate (TBP, $\ce{(C4H9O)3P=O}$) diluted in an aliphatic hydrocarbon like $n$-dodecane.

UndergraduateUndergraduate: extraction equilibria, U/Pu partition, and solvent degradation

Liquid-liquid extraction in PUREX relies on the selective formation of neutral, lipophilic adducts between metal nitrates and tri-$n$-butyl phosphate. The distribution ratio $D_{\text{M}} = [\text{M}]_{\text{org}} / [\text{M}]_{\text{aq}}$ defines extraction efficiency. In $3\text{-}4\ \text{M}\ \ce{HNO3}$, both hexavalent uranyl $\ce{UO2^2+}$ and tetravalent plutonium $\ce{Pu^4+}$ are readily solvated by TBP, yielding $D > 10$. In sharp contrast, trivalent lanthanides ($\ce{Ln^3+}$), trivalent minor actinides ($\ce{Am^3+}, \ce{Cm^3+}$), and common fission fragments like $\ce{^{137}Cs+}$ and $\ce{^{90}Sr^2+}$ exhibit $D < 0.001$, remaining quantitatively in the aqueous nitric acid raffinate (High-Level Liquid Waste).

Once uranium and plutonium are co-extracted into the TBP phase, they must be partitioned into separate product streams. In the partition contactor, a reducing aqueous stream containing uranous nitrate ($\ce{U^4+}$, stabilized by hydrazine to scavenge nitrous acid) or hydroxylamine nitrate ($\ce{NH3OH+ NO3-}$) selectively reduces plutonium from $\ce{Pu^4+}$ to $\ce{Pu^3+}$: $\ce{2Pu^4+ + U^4+ + 2H2O -> 2Pu^3+ + UO2^2+ + 4H+}$. Because trivalent plutonium has a very low distribution ratio ($D_{\text{Pu(III)}} \approx 0.01$), it strips quantitatively into the aqueous stream, while hexavalent $\ce{UO2^2+}$ remains unreduced in the organic phase. In a subsequent column, uranium is back-extracted into dilute nitric acid ($\sim 0.01\ \text{M}\ \ce{HNO3}$), where $D_{\text{U(VI)}}$ falls below 0.1.

AdvancedAdvanced: minor actinide separation, pyrochemistry, and vitrification

Advanced aqueous reprocessing partitions minor actinides ($\ce{Am^3+}, \ce{Cm^3+}$) from trivalent lanthanides ($\ce{Ln^3+}$) in PUREX raffinate. Because lanthanides possess massive neutron-absorption cross-sections, minor actinides cannot be transmuted without first removing lanthanides. The DIAMEX process uses neutral malonamides (such as DMDOHEMA, adhering to the completely incinerable CHON principle) to co-extract $\text{An(III)} + \text{Ln(III)}$. Subsequently, the SANEX process utilizes soft nitrogen-donor heterocyclic ligands (like bis-triazinylpyridines, BTP, and $\text{CyMe}_4\text{-BTBP}$). The slightly greater spatial extension and covalency of actinide $5f$ orbitals over localized lanthanide $4f$ orbitals allows soft nitrogen donors to selectively coordinate $\ce{Am^3+}$ and $\ce{Cm^3+}$ with separation factors $\beta_{\text{An/Ln}} > 100$.

Pyroprocessing replaces nitric acid and organic solvents with high-temperature molten inorganic salts ($\ce{LiCl-KCl}$ eutectic at $450\text{-}500^\circ\text{C}$). Chopped metallic or oxide fuel (electroreduced to metal) is loaded into an anode basket in an electrorefiner. An electric potential dissolves actinides at the anode: uranium deposits onto a solid steel cathode, while plutonium, americium, and curium co-deposit into a liquid cadmium or bismuth cathode. Because pure plutonium cannot be chemically separated from minor actinides in this regime, pyroprocessing provides intrinsic proliferation resistance and directly cycles fuel into sodium-cooled fast reactors. For aqueous waste, high-level liquid waste is calcined into dry oxides, blended with borosilicate glass frit, and induction-melted at $\sim 1150^\circ\text{C}$ into stable glass canisters for long-term geologic disposal.

ResearchResearch and frontier: closed fuel cycles, partitioning, and nonproliferation