Chromium (Cr) is a frequent constituent of the metal alloys proposed for molten salt nuclear reactor (MSR) applications, and is typically the least noble metal ion present. Consequently, chromium is preferentially corroded into molten salt solutions. The redox poise and redox cycling of chromium ions in the salt can greatly influence its corrosivity towards structural alloys, ultimately impacting the longevity of MSR systems. Radiation-induced chemistry is expected to play a significant role in determining the chromium oxidation state distribution during MSR operations. In the present research, electron pulse radiolysis techniques were employed to characterize the reactivity of Cr(II) and Cr(III) ions with primary radiolysis products in molten lithium chloride-potassium chloride (LiCl-KCl) eutectic over a temperature range of 400-600 °C. Both chromium oxidation states were found to rapidly react with the primary products of molten chloride salt radiolysis, i.e., the solvated electron (eS-) and the dichlorine radical anion (Cl2˙-). For reactions with the eS-, second-order rate coefficients (k) of k = (4.1 ± 0.2) and (6.1 ± 0.3) × 1010 M-1 s-1 at 400 °C for Cr(II) and Cr(III), respectively, were determined. Temperature-dependent measurements allowed for the derivation of activation parameters for electron capture by Cr(II) and Cr(III). Both chromium ions also react with Cl2˙-, k = (7.2 ± 0.3) and (1.4 ± 0.1) × 109 M-1 s-1 at 400 °C for Cr(II) and Cr(III), respectively.

Nuclear materials, such as uranium-bearing solids, are exposed to high levels of ionizing radiation throughout the nuclear fuel cycle; thus, it is important to develop a molecular-level understanding of how these materials behave and degrade in the presence of gamma (γ) irradiation. In the current study, three U(VI) tetrachloride complexes, M2[UO2Cl4]·xH2O (where M = K+, Rb+, or Cs+ and x = 0 or 2), and their respective chloride salts were exposed to 1-50 kGy of γ radiation using a 60Co source. Irradiated materials were evaluated by using electron paramagnetic resonance (EPR) and Raman spectroscopy and were further explored by using density functional theory (DFT) methods. EPR spectra of the irradiated materials suggest the formation of a Cl-based radical for both the alkali salts and the uranyl tetrachloride compounds, and DFT calculations provide evidence that the Cl2-• radical is formed within these materials. The presence of water in the K+ and Rb+ compounds leads to additional spectroscopic signatures that could be traced back to water radiolysis and the formation of peroxide and superoxide species. DFT results support the formation of HO2• in the lattice and potentially the formation of a [UO2Cl3(O2)]3- species, highlighting the impact of water within the hydrated material to alter U(VI) speciation by radiolysis.

Metal-organic frameworks (MOFs) are promising candidates for applications in the nuclear fuel cycle due to their high porosity and tunable properties. However, for effective use in this context, these materials must be stable under ionizing radiation conditions. While previous studies have explored variations in metal node identities, topologies, and linker types, this study focuses on maintaining consistent metal and linker components to identify structural features that enhance radiation stability. We investigated the radiation resistance of three thorium-terephthalate hybrid materials─Th(BDC)2(DMF)2 (1,4-benzenedicarboxylic acid, dimethylformamide), Th(BDC)2, and Th-UiO-66─irradiated with He-ions up to a dose of 227 MGy. Structural stability was assessed through powder X-ray diffraction (PXRD), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations. The radiation stability thresholds were identified for Th(BDC)2(DMF)2 and Th-UiO-66, with Th(BDC)2 demonstrating exceptional stability even at the highest radiation dose. The observed stability trend is Th(BDC)2 > Th(BDC)2(DMF)2 > Th-UiO-66. Notably, the inclusion of DMF in Th(BDC)2(DMF)2 enhanced its radiation tolerance, likely due to DMF acting as a sacrificial ligand, preserving linker integrity at higher doses. Additionally, more unique node-linker connections and shorter interligand distances contributed to the improved radiolytic stability of these materials.

Objective.TOPAS-nBio enables users to simulate dose rate-dependent radiation chemical yields in water radiolysis accounting for inter-track and long-term chemistry for pulsed irradiation. This study aims to extend the TOPAS-nBio chemistry for the special case of continuous high-dose rate scenario, where both intertrack and longer time reactions need to be considered, and to quantitatively validate the extended framework by comparing the results with experimental data.Approach.The inter-track chemistry and escapeG-values were first evaluated by the independent reaction time method. The escaping molecules were assumed to have a temporally continuous distribution based on theG-values using the Gillespie algorithm. The simulation results were comprehensively validated by comparing with the experimental data at different dose rates, temporal pulse shapes, and solutions. In addition, the influence of various factors, such as the chemistry model, simulation volume, temperature, pH concentration, and organic carbon contamination, was evaluated.Main results.The validation results showed that the H2O2concentration and O2consumption increased with dose rate, and agreed within 3% with experimental data. Computational factors related to the chemistry model and volume size were negligible. pH and temperature had an impact of less than 10% in the experimental range. The presence of organic carbon and resulting reactions doubled H2O2yields and significantly increased O2consumption by about an order of magnitude at lower dose rates, while the results are almost unchanged at higher dose rates. Consequently, the dose rate dependence of H2O2yields and O2consumption were reversed at a certain organic carbon concentration compared to the pure water results.Significance.The extended TOPAS-nBio chemistry framework enables the reproduction of the dose-rate dependent radiation chemical yields of several experimental studies at different dose rates, temporal pulse shapes, and solutions. This new functionality is necessary to investigate recent high dose rate (FLASH) experimental results.

The generation and stabilization of gamma radiation-induced hydrogen atoms in gibbsite (Al(OH)3) nanoplates is directly related to the nature of residual ions from synthetic precursors used, whether nitrates or chlorides.

Extraction of uranium from water is an essential step in in situ leach (ISL) mining and environmental decontamination. This is often done by precipitating uranium in solution as the uranyl peroxide studtite, [(UO2)(O2)(H2O)2](H2O)2, by adding hydrogen peroxide, which is energy-intensive to produce and hazardous to transport. Here, we present a method for synthesizing studtite, by generating reactive oxygen species in solution using a nonthermal plasma. Precipitation of studtite is observed within 5 min of the onset of plasma treatment as confirmed by X-ray diffraction and Raman spectral analysis. The faradaic efficiency of studtite formation is analyzed to estimate the values of hydrogen peroxide yield, 1.23 molecules per incident ion, and the rate constant of the studtite-forming reaction, 4.44 x 107 M-1 s-1. This work is a proof of concept and identifies significant parameters for the future development of a larger scale, higher throughput system.

The solid form of the uranyl peroxide cage (UPC) cluster LiU28 (Li28[(UO2)28(O2)42]) was irradiated by 5 MeV He2+ ions to achieve doses up to 42 MGy. An intermediate compound formed that reacts with atmospheric CO2 to form uranyl carbonates. The role of water in the UPC to uranyl carbonate transformation was studied by flowing either dry or hydrated Ar over samples during He2+ irradiation, and by storing samples in dry and humid environments before and after irradiation. Raman, infrared, and X-ray photoelectron spectroscopies and electrospray ionization mass spectrometry were used to characterize solid Li-U28 salts before and after He2+ irradiation. The highest yield of uranyl carbonates occurred when hydrated Ar gas was flowed across the sample during He2+ irradiation. Electron paramagnetic resonance spectroscopy provided evidence of hydroxyl and superoxide radicals in both unirradiated and gamma-irradiated Li-U28. He2+-beam irradiation of the uranyl peroxide cage cluster Li-U28 to high doses leads to breakdown of the cage cluster and ultimately to the formation of uranyl carbonate species.

Molten salts are proposed as liquid fuels or coolants in a new fleet of molten salt nuclear reactors that would have operational and safety advantages over present reactor systems. Under those conditions, the salt will be exposed to high radiation levels, and understanding the chemical effects of radiolysis on the molten salt fuel or coolant is essential to reliable, efficient and sustainable reactor operation. Building this understanding begins with identifying primary salt radiolysis products (solvated electrons (esolv –) and Cl2 •–) and characterizing their reactivities, for which we conduct high-temperature pulse radiolysis transient absorption spectroscopy at the BNL Laser-Electron Accelerator Facility. Here we report on the reaction kinetics of reactions of metal ions with esolv – and Cl2 •– in different molten salt compositions. Salt mixtures containing mono- and divalent cations are particularly interesting because of their tunable Lewis acidity-basicity that can be used to control the solubility and redox poise of dissolved metal ions in the reactor. Previously we showed how varying the MgCl2:KCl mixing ratio alters the absorption spectra of radiolytically-produced excess electrons, with increasing blue shifts related to the likely number of Mg2+ ions adjacent to the cavity electron. The strong blue shifts indicate significant changes in the electron’s energetics and reactivity that we now have probed by measuring reaction kinetics with metal ion electron acceptors. We observed that reaction rates of the solvated electron depend strongly on the composition of the salt. Reactivity trends among first-row transition metals in eutectic LiCl-KCl will be discussed. This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.

Actinides are inherently radioactive; thus, ionizing radiation is emitted by these elements can have profound effects on its surrounding chemical environment through the formation of free radical species. While previous work has noted that the presence of free radicals in the system impacts the redox state of the actinides, there is little atomistic understanding of how these metal cations interact with free radicals. Herein, we explore the effects of radiation (UV and gamma) on three U(VI) trinitrate complexes, M[UO2(NO3)3] (where M=K+, Rb+, Cs+), and their respective nitrate salts in the solid state via electron paramagnetic resonance (EPR) and Raman spectroscopy paired with Density Functional Theory (DFT) methods. We find that the alkali salts form nitrate radicals under UV and gamma irradiation, but also note the presence of additional degradation products. M[UO2(NO3)3] solids also form nitrate radicals and additional DFT calculations indicate the species corresponds to a change from the bidentate bound nitrate anion into a monodentate NO3 center dot radical. Computational studies also highlight the need to include the second sphere coordination environment around the [UO2(NO3)3]0,1 species to gain agreement between the experimental and predicted EPR signatures. Uranyl trinitrate solid-state materials are exposed to gamma radiation and characterized by electron paramagnetic resonance (EPR) and Raman spectroscopy. Density functional theory calculations confirm the spectroscopic signatures and the presence of a nitrate radical bound in a monodentate fashion to the uranyl cation. Including alkali cations in the theoretic models is crucial to accurate calculation of the g-values. image

Here, we examine how radiation impacts the dissolution behavior of boehmite by subjecting dry nanoparticles of different sizes to Co-60 gamma radiation and subsequently analyzing their dissolution behavior in caustic solutions as a function of temperature. The measured kinetics show that irradiation with an amount 228.24 Mrad significantly slows the dissolution rate, particularly for smaller sizes at lower temperatures. Specifically, the temperature-dependent dissolution rates of irradiated 20 nm boehmite versus pristine material in 3 M NaOH solutions were several times lower (e.g., rate constant of 0.026 vs 0.075 h(-1) at 60 degrees C), with an apparent activation energy 40 kJ mol(-1) higher. Although various imaging techniques and X-ray diffraction measurements consistently revealed no obvious differences between pristine and irradiated samples, after irradiation significant binding energy shifts were detected in the X-ray photoelectron Spectroscopy peaks of Al 2p and O 1s, and a change in their relative intensities indicated a lower O/Al ratio. This suggests that gamma-irradiation may stabilize boehmite particle surfaces by driving their chemistry and structure toward more stable aluminum oxide forms. This finding may help explain slower dissolution rates of boehmite in nuclear waste and may be useful for the development of more robust predictive models and effective strategies for waste processing.

We examined the effect of gamma radiation on the stabilities of aluminum hydroxide (gibbsite) and aluminum oxyhydroxide (boehmite) nanoparticles in relation to their thermal decomposition. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images revealed no significant differences in mineral components or morphologies before and after radiation. However, thermogravimetric and differential scanning calorimetry (TGA/DSC) analyses showed that both boehmite and gibbsite nanoparticles experienced a decreased mass loss following irradiation. Raman and attenuated total reflection-Fourier transfer infrared (ATR-FTIR) spectra indicated that a fraction of the hydroxyl content in both cases was selectively cleaved by radiation, primarily at the particle surfaces. Quantitative analyses of thermal mass loss behavior demonstrated that irradiated boehmite and gibbsite nanoparticles had higher activation energies than their pristine counterparts, with the extent of the increase being dependent on the total dose. Taken together, these findings suggest that exposure to a sufficient dose of ionizing radiation alters these materials such that they are less prone to decomposition by dehydration. This increased stability may be due to the decreased hydrous nature of the samples after radiation exposure, which was supported by further high-temperature drop calorimetry. Additionally, a radiation-induced amorphous phase on the nanoparticle surfaces appears to have a permanent and positive influence on the thermodynamic stabilities.

Understanding the dissolution of boehmite in highly alkaline solutions is important to processing complex nuclear waste stored at the Hanford (WA) and Savannah River (SC) sites in the United States. Here, we report the adsorption of model carboxylates on boehmite nanoplates in alkaline solutions and their effects on boehmite dissolution in 3 M NaOH at 80 °C. Although expectedly lower than at circumneutral pH, adsorption of oxalate occurred at pH 13, with adsorption decreasing linearly to 3 M NaOH. Classical molecular dynamics simulations suggest that the adsorption of oxalate dianions onto the boehmite surface under high pH can occur through either inner- or outer-sphere complexation mechanisms depending on adsorption sites. However, both adsorption models indicate relatively weak binding, with an energy preference of 1.26 to 2.10 kcal/mol. By preloading boehmite nanoplates with oxalate or acetate, we observed suppression of dissolution rates by 23 or 10%, respectively, compared to pure solids. Scanning electron microscopy and transmission electron microscopy characterizations revealed no detectable difference in the morphologic evolution of the dissolving boehmite materials. We conclude that preadsorbed carboxylates can persist on boehmite surfaces, decreasing the density of dissolution-active sites and thereby adding extrinsic controls on dissolution rates.

Objective.To present and validate a method to simulate from first principles the effect of oxygen on radiation-induced double-strand breaks (DSBs) using the Monte Carlo Track-structure code TOPAS-nBio.Approach.Two chemical models based on the oxygen fixation hypothesis (OFH) were developed in TOPAS-nBio by considering an oxygen adduct state of DNA and creating a competition kinetic mechanism between oxygen and the radioprotective molecule WR-1065. We named these models 'simple' and 'detailed' due to the way they handle the hydrogen abstraction pathways. We used the simple model to obtain additional information for the •OH-DNA hydrogen abstraction pathway probability for the detailed model. These models were calibrated and compared with published experimental data of linear and supercoiling fractions obtained with R6K plasmids, suspended in dioxane as a hydroxyl scavenger, and irradiated with137Cs gamma-rays. The reaction rates for WR-1065 and O2with DNA were taken from experimental works. Single-Strand Breaks (SSBs) and DSBs as a function of the dose for a range of oxygen concentrations [O2] (0.021%-21%) were obtained. Finally, the hypoxia reduction factor (HRF) was obtained from DSBs.Main Results.Validation results followed the trend of the experimental within 12% for the supercoiled and linear plasmid fractions for both models. The HRF agreed with measurements obtained with137Cs and 200-280 kVp x-ray within experimental uncertainties. However, the HRF at an oxygen concentration of 2.1% overestimated experimental results by a factor of 1.7 ± 0.1. Increasing the concentration of WR-1065 from 1 mM to 10-100 mM resulted in a HRF difference of 0.01, within the 8% statistical uncertainty between TOPAS-nBio and experimental data. This highlights the possibility of using these chemical models to recreate experimental HRF results.Significance.Results support the OFH as a leading cause of oxygen radio-sensitization effects given a competition between oxygen and chemical DNA repair molecules like WR-1065.

Understanding the speciation and local structure of metal solutes in molten salts is essential for predicting thermal properties and the redox and corrosion potentials of molten salts for next-generation nuclear reactors and concentrated solar power plant applications. We employ X-ray absorption spectroscopy combined with electron paramagnetic resonance (EPR) measurements to investigate the effects of electron irradiation on 0.1 wt % NiCl2 dissolved in molten eutectic KCl-ZnCl(2 )salt. Radiation-driven reduction of Ni2+ leading to nucleation and growth of Ni nanoparticles is studied as a function of electron dose (3-15 MGy) and temperature. Quantitative X-ray absorption near edge structure and linear combination fitting are utilized to investigate the extent of reduction as a function of the electron dose and temperature. A multiple-scattering (MS) approach is used for extended X-ray absorption fine structure analysis to deduce the size of the Ni nanoparticles formed and to understand the effect of the dose on the local structure of the nanoparticles. EPR studies on solidified NiCl2 dissolved in molten eutectic KCl-ZnCl2 salt show the formation of magnetic Ni nanoparticles.

Objective . The TOPAS-nBio Monte Carlo track structure simulation code, a wrapper of Geant4-DNA, was extended for its use in pulsed and longtime homogeneous chemistry simulations using the Gillespie algorithm. Approach . Three different tests were used to assess the reliability of the implementation and its ability to accurately reproduce published experimental results: (1) a simple model with a known analytical solution, (2) the temporal evolution of chemical yields during the homogeneous chemistry stage, and (3) radiolysis simulations conducted in pure water with dissolved oxygen at concentrations ranging from 10 μ M to 1 mM with [H 2 O 2 ] yields calculated for 100 MeV protons at conventional and FLASH dose rates of 0.286 Gy s −1 and 500 Gy s −1 , respectively. Simulated chemical yield results were compared closely with data calculated using the Kinetiscope software which also employs the Gillespie algorithm. Main results . Validation results in the third test agreed with experimental data of similar dose rates and oxygen concentrations within one standard deviation, with a maximum of 1% difference for both conventional and FLASH dose rates. In conclusion, the new implementation of TOPAS-nBio for the homogeneous long time chemistry simulation was capable of recreating the chemical evolution of the reactive intermediates that follow water radiolysis. Significance . Thus, TOPAS-nBio provides a reliable all-in-one chemistry simulation of the physical, physico-chemical, non-homogeneous, and homogeneous chemistry and could be of use for the study of FLASH dose rate effects on radiation chemistry.

Developing a predictive understanding of mineral-phase stability in extreme chemical conditions such as those found in nuclear waste is challenging given the unknown influence of ionizing radiation. The long-term impacts of cumulative radiation damage in the solid state and exposure to radiolysis products in solution can impact mineral precipitation, dissolution, and aggregation behavior. Here, we sought to disentangle some of these effects by examining the dissolution of gibbsite platelets in NaOH solutions using atomic force microscopy with an integrated X-ray source that was used to compare dissolution rates when particles were initially irradiated in a dry state versus irradiated in solution, both of which were compared to unirradiated controls. By tracking particle morphology changes and quantifying material lost over time, dissolution rates and particle roughness were found to be enhanced most when irradiation was carried out during dissolution in NaOH, and to a lesser but significant extent when irradiated dry prior to dissolution. The maximum observed dissolution enhancement in the former case suggests the importance of both a direct effect of absorbed dose on gibbsite stability and an indirect effect arising from surface interaction with solution radiolysis products.

The solid-state transformation of sodium uranyl triperoxide (Na4(UO2)(O2)3·9H2O, NaUT) to sodium uranyl tricarbonate (Na4(UO2)(CO3)3) by radiolysis was observed for the first time. The exposure of NaUT to 3 MGy gamma irradiation resulted in partial breakdown of the peroxides forming a mixed peroxide and carbonate species. The effects of He-ion irradiation on NaUT were also investigated up to 225 MGy using both hydrated argon and dry argon. The complete conversion to the uranyl tricarbonate phase by 56 MGy was done using hydrated argon, while dry argon did not fully convert showing the importance of water in the system. He-ion irradiated NaUT samples all convert to the tricarbonate phase with time in air post radiation exposure. This transition was monitored via Raman spectroscopy, infrared spectroscopy (IR), and powder X-ray diffraction (PXRD) to further confirm the identity of the final product as the sodium uranyl tricarbonate, čejkaite. This transformation outlines a mechanism for the mobility of uranyl in natural environments and in the Hanford tanks.