Radiation effects on rare earth chemistry in molten salts for nuclear applications

Across the nuclear energy sector, molten salts are shaping the next generation of nuclear technologies, from advanced power reactors to used nuclear fuel recycling.

In molten salt reactors (MSRs), molten salts uniquely serve as both the primary coolant and fuel medium, a dual role that sets MSRs apart from conventional reactor designs. In parallel, molten salts enable pyrochemical reprocessing of used nuclear fuel, allowing the recovery of valuable fissile material that can be fabricated into new fuel, supporting both energy security and nuclear waste management.

In both applications, nuclear radiation continuously modifies the salt’s chemistry; understanding exactly how it does so is critical to making these technologies work safely and efficiently. Recent work from our group is beginning to map radiation-induced changes in detail, contributing to a mechanistic understanding of how fission products (FPs) behave and how radiation drives nonequilibrium chemistry in high-temperature molten chloride salt systems.

Fission product behavior in molten salts

In pyrochemical reprocessing, used nuclear fuel is dissolved in a molten chloride salt matrix, enabling chemical reduction of uranium dioxide and other FPs to the metallic state. This process is followed by electrochemical reduction to selectively isolate desired materials, such as uranium, onto a solid cathode.¹

However, some FPs are thermodynamically stable in the salt matrix, resulting in their accumulation over time. Accumulation of FPs gradually degrades the salt’s usefulness and limits its recyclability, leading to additional nuclear waste.

In this regard, multivalent rare earth elements pose significant challenges because they are notoriously difficult to separate and tend to undergo comproportionation and disproportionation reactions, complicating recovery efforts. Neodymium, for instance, is typically recovered at efficiencies below 55%.²

The same rare earth element FPs pose challenges for MSRs because some are strong neutron poisons, absorbing neutrons needed to sustain a nuclear chain reaction. Their accumulation must be carefully controlled, making effective management of these FPs essential for long-term reactor performance.

Accurately predicting the long-term behavior of these hard-to-separate rare earth elements requires a fundamental understanding of how ionizing nuclear radiation influences their chemistry in molten salts, an area that remains largely underexplored despite its importance to both reactor operation and salt waste management.

Radiation-induced speciation of neodymium ions

In our recent work,² we explored what happens to neodymium ions when they are exposed to nuclear radiation in molten lithium chloride–potassium chloride (LiCl–KCl) eutectic salt.

When the salt absorbs radiation energy, it generates two key reactive species: 1) strongly reducing solvated electrons (e_S^–) and 2) oxidizing chlorine atoms (Cl^•), which rapidly react with surrounding chloride anions (Cl^–) to form dichlorine radical anions (Cl₂^•–). In a pure salt with no other solutes, e_S^– either reduce trace metal ion impurities or recombine with Cl^•, while Cl₂^•– predominantly undergo disproportionation, ultimately generating chlorine gas (Cl₂). In the presence of neodymium ions (Nd²⁺ and Nd³⁺), predicting the radiolytic redox landscape becomes more complex. Mapping out these interactions required us to combine electrochemistry, spectroscopy, and a time-resolved technique called electron pulse radiolysis.

Electron pulse radiolysis works by firing intense, ultrashort pulses of high energy electrons into a sample, generating a burst of reactive species (e_S^– and Cl^• among them) in concentrations large enough to monitor in real time. Using transient absorption spectroscopy, researchers can watch these species form and decay on timescales ranging from nanoseconds to milliseconds, directly observing radiation-induced reaction pathways and deriving their chemical kinetics.

Our experiments were performed at the Brookhaven National Laboratory Laser Electron Accelerator Facility,³ a specialized facility designed for exactly this kind of ultrafast, time-resolved work. We measured how the molten salt absorbed light at different time intervals after each electron pulse, both with and without neodymium ions present, building up a picture of how the chemistry evolved in real time (Figure 1).

Figure 1. Transient absorption spectra for molten LiCl-KCl eutectic in the a) absence and b) presence of 10 mM NdCl₃ at select times following an electron pulse at 673 K. Reprinted with permission from Reference 2. Copyright 2026 American Chemical Society. Credit: Baldivieso et al., Inorganic Chemistry

These measurements revealed that irradiating molten LiCl–KCl eutectic produces two dominant radiolytic species: short-lived e_S^– (< 5 μs) and longer lived Cl₂^•– (> 5 μs) (Figure 1a). Adding Nd³⁺ to the salt drastically changed the absorption landscape (Figure 1b). The e_S^– signal essentially vanished, indicating that Nd³⁺ rapidly reacts with them to become reduced to Nd²⁺.

Using these spectra, we analyzed the decay kinetics of the e_S^– signal at 700 nm and found that the data are consistent with the reduction of Nd³⁺ at 673 K. Meanwhile, Cl₂^•– (350 nm absorption maximum) remained detectable as a transient species. We also observed a small but consistent increase in absorbance in the region between 450 and 500 nm, suggesting the formation of Nd²⁺, which was confirmed by complementary electrochemical measurements as well.

In molten LiCl–KCl eutectic salt, neodymium ions are predominantly stable in the +3 oxidation state,⁴ but under the right conditions, they can be reduced to the +2 oxidation state. Using spectroelectrochemistry, we demonstrated that the 450–500 nm feature corresponds to the characteristic absorption of Nd²⁺, confirming that ionizing radiation can drive this reduction directly in molten chloride salts.

Additional insights into the behavior of Nd²⁺ were gained with a simpler experiment: immersing a neodymium metal rod into a molten NdCl₃–LiCl–KCl salt mixture. Under these conditions, the metal rod reacted with the salt through comproportionation, steadily forming Nd²⁺ (Figure 2). Leaving the rod submerged for more than 30 minutes noticeably shifted the overall neodymium speciation in the salt, reinforcing the idea that both radiolytic and electrochemical pathways can generate Nd²⁺ under these conditions.

Figure 2. Absorption spectra for molten 36 mM NdCl₃-LiCl-KCl at select times after submerging a neodymium metal rod at 773 K. Reprinted with permission from Reference 2. Copyright 2026 American Chemical Society. Credit: Baldivieso et al., Inorganic Chemistry

Toward a safer and more reliable nuclear future

As pyrochemical reprocessing and MSRs continue to mature, understanding how radiation shapes the chemistry of FPs in molten salts will be essential for improving process efficiency, predicting long-term materials behavior, and designing more robust nuclear fuel cycles. Our findings highlight the importance of considering both radiation driven and electrochemical pathways when evaluating multivalent rare earth element behavior in molten salts. They contribute to the ever-growing institutional knowledge of molten salt radiation chemistry, thereby helping pave the way for safer and more reliable nuclear technologies.