Multiscale Modeling of Plutonium Radiation Chemistry in Nitric Acid Solutions. 1. Cobalt-60 Gamma Irradiation of Pu(IV)

Careful manipulation of the plutonium oxidation states is essential in the study and utilization of its rich redox chemistry. To achieve this level of control, a comprehensive mechanistic understanding of radiation-induced plutonium redox chemistry is critical due to the unavoidable exposure of plutonium to ionizing radiation fields, both inherent and from in-process applications. To this end, we have developed an experimentally evaluated multiscale computer model for the prediction of gamma radiation-induced Pu(IV) redox chemistry in concentrated nitric acid solutions (1.0, 3.0, and 6.0 M). Under these acidic, aqueous solution conditions, cobalt-60 gamma irradiation afforded marginal net conversion of Pu(IV) to Pu(VI), the extent of which was dependent on the concentration of HNO3 and absorbed gamma dose. Multiscale calculations, which are in excellent agreement with experimental data, indicate that this observation is due to a combination of inherent plutonium disproportionation reactions and several radiation-induced processes, including redox cycling between Pu(IV) and Pu(III), as achieved by the reduction of Pu(IV) by nitrous acid and hydrogen peroxide, the oxidation of Pu(III) by nitrate and hydroxyl radicals, and the sequential oxidation of Pu(IV) to Pu(V) and Pu(VI) by the remaining available yield of nitrate radicals.

Gamma Radiation-Induced Degradation of Acetohydroxamic Acid (AHA) in Aqueous Nitrate and Nitric Acid Solutions Evaluated by Multiscale Modelling

Acetohydroxamic acid (AHA) has been proposed for inclusion in advanced, single-cycle, used nuclear fuel reprocessing solvent systems for the reduction and complexation of plutonium and neptunium ions. For this application, a detailed description of the fundamental degradation of AHA in dilute aqueous nitric acid is required. To this end, we present a comprehensive, multiscale computer model for the coupled radiolytic and hydrolytic degradation of AHA in aqueous sodium nitrate and nitric acid solutions. Rate coefficients for the reactions of AHA and hydroxylamine (HA) with the oxidizing nitrate radical were measured for the first time using electron pulse radiolysis and used as inputs for the kinetic model. The computer model results are validated by comparison to experimental data from steady-state gamma ray irradiations, for which the agreement is excellent. The presented model accurately predicts the yields of the major degradation products of AHA: acetic acid, HA, nitrous oxide, and molecular hydrogen.

Multiscale modelling of the radical-induced chemistry of acetohydroxamic acid in aqueous solution

Acetohydroxamic acid (AHA) is a small organic acid with a wide variety of industrial, biological, and pharmacological applications. A deep fundamental molecular level understanding of the mechanisms responsible for the radical-induced reactions of AHA in these environments is necessary to predict and control their behaviour and elucidate their interplay with other attendant chemical species, for example, the oxidative degradation products of AHA. To this end, we present a comprehensive, multiscale computer model for interrogating the radical-induced degradation of AHA in acidic aqueous solutions. Model predictions were critically evaluated by a systematic experimental radiation chemistry investigation, leveraging time-resolved electron pulse irradiation techniques for the measurement of new radical reaction rate coefficients, and steady-state gamma irradiations for the identification and quantification of AHA degradation products: acetic acid, hydroxylamine, nitrous oxide, and molecular hydrogen, with formic acid and methane as minor products. Excellent agreement was achieved between calculation and experiment, indicating that this fundamental model can accurately predict the degradation pathways of AHA under irradiation in acidic aqueous solutions.

A comprehensive multiscale model determines the fundamental reaction mechanisms of the radical-induced degradation of acetohydroxamic acid in acidic aqueous solutions.

Radiation-induced reaction kinetics of Zn2+ with eS- and Cl2˙- in Molten LiCl-KCl eutectic at 400-600 °C

Molten chloride salts are currently under consideration as combined coolant and liquid fuel for next-generation molten salt nuclear reactors. Unlike complementary light-water reactor technologies, the radiation science underpinning molten salts is in its infancy, and thus requires a fundamental mechanistic investigation to elucidate the radiation-driven chemistry within molten salt reactors. Here we present an electron pulse radiolysis kinetics study into the behaviour of the primary radiolytic species generated in molten chloride systems, i.e., the solvated electron (e_S^-) and di-chlorine radical anion (Cl₂˙^-). We examine the reaction of e_S^- with Zn²⁺ from 400-600 °C (E_a = 30.31 ± 0.09 kJ mol^-1), and the kinetics and decay mechanisms of Cl₂˙^- in molten lithium chloride-potassium chloride (LiCl-KCl) eutectic. In the absence of Zn²⁺, the lifetime of e_S^- was found to be dictated by residual impurities in ostensibly "pure" salts, and thus the observed decay is dependent on sample history rather than being an intrinsic property of the salt. The decay of Cl₂˙^- is complex, owing to the competition of Cl₂˙^- disproportionation with several other chemical pathways, one of which involves reduction by radiolytically-produced Zn⁺ species. Overall, the reported findings demonstrate the richness and complexity of chemistry involving the interactions of ionizing radiation with molten salts.

Gamma radiation-induced defects in KCl, MgCl2, and ZnCl2 salts at room temperature

Room temperature post-irradiation measurements of diffuse reflectance and electron paramagnetic resonance spectroscopies were made to characterize the long-lived radiation-induced species formed from the gamma irradiation of solid KCl, MgCl2, and ZnCl2 salts up to 100 kGy. The method used showed results consistent with those reported for electron and gamma irradiation of KCl in single crystals. Thermal bleaching of irradiated KCl demonstrated accelerated disaggregation of defect clusters above 400 K, due to decomposition of Cl3-. The defects formed in irradiated MgCl2 comprised a mixture of Cl3-, F-centers, and Mg+ associated as M-centers. Further, Mg metal cluster formation was also observed at 100 kGy, in addition to accelerated destruction of F-centers above 20 kGy. Irradiated ZnCl2 afforded the formation of Cl2- due to its high ionization potential and crystalline structure, which decreases recombination. The presence of aggregates in all cases indicates the high diffusion of radicals and the predominance of secondary processes at 295 K. Thermal bleaching studies showed that chloride aggregates' stability increases with the ionization potential of the cation present. The characterization of long-lived radiolytic transients of pure salts provides important information for the understanding of complex salt mixtures under the action of gamma radiation.

Radiation-Assisted Formation of Metal Nanoparticles in Molten Salts

Knowledge of structural and thermal properties of molten salts is crucial for understanding and predicting their stability in many applications such as thermal energy storage and nuclear energy systems. Probing the behavior of metal contaminants in molten salts is presently limited to either foreign ionic species or metal nanocrystals added to the melt. To bridge the gap between these two end states and follow the nucleation and growth of metal species in molten salt environment in situ, we use synchrotron X-rays as both a source of solvated electrons for reducing Ni²⁺ ions added to ZnCl₂ melt and as an atomic-level probe for detecting formation of zerovalent Ni nanoparticles. By combining extended X-ray absorption fine structure analysis with X-ray absorption near edge structure modeling, we obtained the average size and structure of the nanoparticles and proposed a radiation-induced reduction mechanism of metal ions in molten salts.

Design and performance of high-temperature furnace and cell holder for in situ spectroscopic, electrochemical, and radiolytic investigations of molten salts

To facilitate the development of molten salt reactor technologies, a fundamental understanding of the physical and chemical properties of molten salts under the combined conditions of high temperature and intense radiation fields is necessary. Optical spectroscopic (UV-Vis-near IR) and electrochemical techniques are powerful analytical tools to probe molecular structure, speciation, thermodynamics, and kinetics of solution dynamics. Here, we report the design and fabrication of three custom-made apparatus: (i) a multi-port spectroelectrochemical furnace equipped with optical spectroscopic and electrochemical instrumentation, (ii) a high-temperature cell holder for time-resolved optical detection of radiolytic transients in molten salts, and (iii) a miniaturized spectroscopy furnace for the investigation of steady-state electron beam effects on molten salt speciation and composition by optical spectroscopy. Initial results obtained with the spectroelectrochemical furnace (i) and high-temperature cell holder (ii) are reported.

Effect of Ionizing Radiation on the Redox Chemistry of Penta- and Hexavalent Americium

The recent development of facile methods to oxidize trivalent americium to its higher valence states holds promise for the discovery of new chemistries and critical insight into the behavior of the 5f electrons. However, progress in understanding high-valent americium chemistry has been hampered by americium's inherent ionizing radiation field and its concomitant effects on americium redox chemistry. Any attempt to understand high-valent americium reduction and/or disproportionation must account for the effects of these radiolytic processes. Therefore, we present a complete, quantitative, mechanistic description of the radiation-induced redox chemistry of the americyl oxidation states in aerated, aqueous nitric acid, as a function of radiation quality (type and energy) and solution composition using multiscale modeling calculations supported by experiment. The reduction of Am(VI) to Am(V) was found to be most sensitive to the effects of ionizing radiation, undergoing rapid reductions with the steady-state products of aqueous HNO₃ radiolysis, i.e., HNO₂, H₂O₂, and HO₂^•, which dictated its practical lifetime under acidic conditions. In contrast, Am(V) is only susceptible to radiolytic oxidation, mainly through its reactions with NO₃^•, and is notably radiation-resistant with respect to direct one-electron reduction to produce Am(IV). Our multiscale modeling calculations predict that the lifetime of Am(V) is dictated by its rate of disproportionation, 2AmO₂⁺ + 4H_aq⁺ → AmO₂²⁺ + Am⁴⁺ + 2H₂O, with a fourth-order dependence on [H_aq⁺] in agreement with previous experimental findings, giving an optimized rate coefficient of k = 2.27 × 10^-6 M^-5 s^-1. This disproportionation initially produces Am(IV) and Am(VI) species, but the lack of any spectroscopic evidence in our study for Am(IV) suggests that solvent reduction of this cation occurs rapidly. The ultimate product of all the Am(VI)/Am(V) irradiations is Am(III), which shows great stability in an irradiation field.

Methods for the Simulation of the Slowing of Low-Energy Electrons in Water

A computational Monte Carlo simulation approach for modeling the thermalization of low-energy electrons is presented. The simulation methods rely on, and use, experimentally based cross sections for elastic and inelastic collisions. To demonstrate the different simulation options, average numbers of interactions and the range of low-energy electrons with initial energies ranging from 1 to 20 eV are calculated for density normalized gaseous water. Experimental gas-phase cross sections for (subexcitation) electrons of energies in the range of 1-20 eV were taken from the compilation of Hayashi. The ballistic collision-by-collision simulations provide information on the intricacies of the thermalization processes not available experimentally. © 2018 Wiley Periodicals, Inc.

Molecular Hydrogen Yields from the α-Self-Radiolysis of Nitric Acid Solutions Containing Plutonium or Americium

The yield of molecular hydrogen, as a function of nitric acid concentration, from the α-radiolysis of aerated nitric acid and its mixtures with sulfuric acid containing plutonium or americium has been investigated. Comparison of experimental measurements with predictions of a Monte Carlo radiation track chemistry model shows that, in addition to scavenging of the hydrated electron, its precursor, and the hydrogen atom, the quenching of excited state water is important in controlling the yield of molecular hydrogen. In addition, increases in solution acidity cause a significant change in the track reactions, which can be explained as resulting from scavenging of e_aq^- by H_aq⁺ to form H^•. Although plutonium has been shown to be an effective scavenger of precursors of molecular hydrogen below 0.1 mol dm^-3 nitrate, previously reported effects of plutonium on G(H₂)_α between 1 and 10 mol dm^-3 nitric acid were not reproduced. Modeling results suggest that plutonium is unlikely to effectively compete with nitrate ions in scavenging the precursors of molecular hydrogen at higher nitric acid concentrations, and this was confirmed by comparing molecular hydrogen yields from plutonium solutions with those from americium solutions. Finally, comparison between radionuclide, ion accelerator experiments, and model predictions leads to the conclusion that the high dose rate of accelerator studies does not significantly affect the measured molecular hydrogen yield. These reactions provide insight into the important processes for liquors common in the reprocessing of spent nuclear fuel and the storage of highly radioactive liquid waste prior to vitrification.

Inhibition of Radiolytic Molecular Hydrogen Formation by Quenching of Excited State Water

Comparison of experimental measurements of the yield of molecular hydrogen produced in the gamma radiolysis of water and aqueous nitrate solutions with predictions of a Monte Carlo track chemistry model shows that the nitrate anion scavenging of the hydrated electron, its precursor, and hydrogen atom cannot account for the observed decrease in the yield at high nitrate anion concentrations. Inclusion of the quenching of excited states of water (formed by either direct excitation or reaction of the water radical cation with the precursor to the hydrated electron) by the nitrate anion into the reaction scheme provides excellent agreement between the stochastic calculations and experiment demonstrating the existence of this short-lived species and its importance in water radiolysis. Energy transfer from the excited states of water to the nitrate anion producing an excited state provides an additional pathway for the production of nitrogen containing products not accounted for in traditional radiation chemistry scenarios. Such reactions are of central importance in predicting the behavior of liquors common in the reprocessing of spent nuclear fuel and the storage of highly radioactive liquid waste prior to vitrification.

Multi-Scale Modeling of the Gamma Radiolysis of Nitrate Solutions

A multiscale modeling approach has been developed for the extended time scale long-term radiolysis of aqueous systems. The approach uses a combination of stochastic track structure and track chemistry as well as deterministic homogeneous chemistry techniques and involves four key stages: radiation track structure simulation, the subsequent physicochemical processes, nonhomogeneous diffusion-reaction kinetic evolution, and homogeneous bulk chemistry modeling. The first three components model the physical and chemical evolution of an isolated radiation chemical track and provide radiolysis yields, within the extremely low dose isolated track paradigm, as the input parameters for a bulk deterministic chemistry model. This approach to radiation chemical modeling has been tested by comparison with the experimentally observed yield of nitrite from the gamma radiolysis of sodium nitrate solutions. This is a complex radiation chemical system which is strongly dependent on secondary reaction processes. The concentration of nitrite is not just dependent upon the evolution of radiation track chemistry and the scavenging of the hydrated electron and its precursors but also on the subsequent reactions of the products of these scavenging reactions with other water radiolysis products. Without the inclusion of intratrack chemistry, the deterministic component of the multiscale model is unable to correctly predict experimental data, highlighting the importance of intratrack radiation chemistry in the chemical evolution of the irradiated system.

Decay Mechanism of NO3(•) Radical in Highly Concentrated Nitrate and Nitric Acidic Solutions in the Absence and Presence of Hydrazine

The decay mechanism of NO3(•) has been determined through a combination of experiment and calculation for 7 mol dm(-3) solutions of deaerated aqueous LiNO3 and HNO3, in the absence and presence of hydrazine (N2H4, N2H5(+), and N2H6(2+)). In the absence of hydrazine, the predominant NO3(•) decay pathways are strongly dependent upon the pH of the solution. For neat, neutral pH LiNO3 solutions (7 mol dm(-3)), NO3(•) produced by the pulse is fully consumed within 160 μs by OH(•) (37%), H2O (29%), NO2(-) (17%), and NO2 (17%). For acidic HNO3 solutions (7 mol dm(-3)), radiolytically produced NO3(•) is predominantly consumed within 1 ms by HNO2 (15%) and NO2 (80%). Intervening formulations exhibit the mechanistic transition from neat LiNO3 to neat HNO3. In highly acidic nitric acid solution, hydrazine exists mainly as N2H5(+) and N2H6(2+), both of which rapidly consume NO3(•) in addition to other decay mechanisms, with rate constants of 2.9 (±0.9) × 10(7) and 1.3 (±0.3) × 10(6) dm(3) mol(-1) s(-1), respectively.

Note: Establishing α-particle radiation damage experiments using the Dalton Cumbrian Facility's 5 MV tandem pelletron

Evaluating the radiation stability of mineral phases is a vital research challenge when assessing the performance of the materials employed in a Geological Disposal Facility for radioactive waste. This report outlines the setup and methodology for efficiently allowing the determination of the dose dependence of damage to a mineral from a single ion irradiated sample. The technique has been deployed using the Dalton Cumbrian Facility's 5 MV tandem pelletron to irradiate a suite of minerals with a controlled α-particle ((4)He(2+)) beam. Such minerals are proxies for near-field clay based buffer material surrounding radioactive canisters, as well as the sorbent components of the host rock.

The impact of γ radiation on the bioavailability of Fe(III) minerals for microbial respiration

Conservation of energy by Fe(III)-reducing species such as Shewanella oneidensis could potentially control the redox potential of environments relevant to the geological disposal of radioactive waste and radionuclide contaminated land. Such environments will be exposed to ionizing radiation so characterization of radiation alteration to the mineralogy and the resultant impact upon microbial respiration of iron is essential. Radiation induced changes to the iron mineralogy may impact upon microbial respiration and, subsequently, influence the oxidation state of redox-sensitive radionuclides. In the present work, Mössbauer spectroscopy and electron microscopy indicate that irradiation (1 MGy gamma) of 2-line ferrihydrite can lead to conversion to a more crystalline phase, one similar to akaganeite. The room temperature Mössbauer spectrum of irradiated hematite shows the emergence of a paramagnetic Fe(III) phase. Spectrophotometric determination of Fe(II) reveals a radiation-induced increase in the rate and extent of ferrihydrite and hematite reduction by S. oneidensis in the presence of an electron shuttle (riboflavin). Characterization of bioreduced solids via XRD indicate that this additional Fe(II) is incorporated into siderite and ferrous hydroxy carbonate, along with magnetite, in ferrihydrite systems, and siderite in hematite systems. This study suggests that mineralogical changes to ferrihydrite and hematite induced by radiation may lead to an increase in bioavailability of Fe(III) for respiration by Fe(III)-reducing bacteria.

Aqueous solution of UCl6(2-) in O2 saturated acidic medium: an efficient system to scavenge all primary radicals in spurs produced by irradiation

Absorbance measurements find the yield of the oxidation of U(IV) to be (8.75 +/- 0.05) x 10(-7) mol J(-1) in the (60)Co gamma radiolysis of aqueous solutions containing 4.4 x 10(-3) mol L(-1) U(IV) in the presence of O(2) saturated 2 mol L(-1) Cl(-) at pH = 0. This high value of oxidation yield suggests that all primary radicals formed by water decomposition are scavenged in these solutions. Simulations using a nonhomogeneous stochastic kinetic track model agree with the experimental results and are used to explain the mechanism for scavenging radicals and oxidation of U(IV).

An apparatus for the study of high temperature water radiolysis in a nuclear reactor: calibration of dose in a mixed neutron/gamma radiation field

The cooling water of nuclear reactors undergoes radiolytic decomposition induced by gamma, fast electron, and neutron radiation in the core. To model the process, recombination reaction rates and radiolytic yields for the water radical fragments need to be measured at high temperature and pressure. Yields for the action of neutron radiation are particularly hard to determine independently because of the beta/gamma field also present in any reactor. In this paper we report the design of an apparatus intended to measure neutron radiolysis yields as a function of temperature and pressure. A new methodology for separation of neutron and beta/gamma radiolysis yields in a mixed radiation field is proposed and demonstrated.

Products of the Triplet Excited State Produced in the Radiolysis of Liquid Benzene

The radiation chemical yields of the products derived from the triplet excited state produced in the radiolysis of liquid benzene with gamma-rays, 10 MeV 4He ions, and 10 MeV 12C ions have been determined. Iodine scavenging techniques have been used to examine the formation and role of radicals, especially the H atom and phenyl radical. For all irradiation types examined here, the increase in hydrogen iodide yields with increasing iodine concentration matches the increase in iodobenzene yields. This agreement suggests that the benzene triplet excited state is the common precursor for the H atom and the phenyl radical. Pulse radiolysis studies in liquid benzene have determined the rate coefficients for the reactions of phenyl radicals with iodine and with the solvent benzene to be 9.3 x 10(9) M(-1) s(-1) and 3.1 x 10(5) M(-1) s(-1), respectively. Direct measurements of polymer formation, which refers to trimers (C18) and higher order compounds (>C18), in liquid benzene radiolysis using gamma-rays, 4He ions, and 12C ions at relatively high doses have been performed using gel permeation chromatography. The yields of trimers increase from gamma-rays to 12C ions due to the increased importance of intratrack radical-radical reactions that can be scavenged by the radical scavenging reactions of iodine. On the other hand, the >C18 product yields decrease from gamma-rays to 12C ions. The structure of the polymer consists of a partly saturated ring as determined by infrared and gas chromatography/mass spectrometry studies. A schematic representation for the radiolytic decomposition of the benzene triplet excited state is presented.

Radiolysis of Aqueous Solutions of 1,1- and 1,2-Dichloroethane

The yields of chloride ion and molecular hydrogen were determined in the gamma, the fast electron, and the 5 MeV helium ion radiolysis of deaerated and aerated aqueous solutions of 1,1- and 1,2-dichloroethane. In deaerated solutions irradiated with gamma-rays or fast electrons, the yield of chloride ion increases while the yield of molecular hydrogen decreases with increasing dichloroethane concentration. These results are due to the quantitative reaction of both the hydrated electron and the hydrogen atom with the dichloroethane to produce chloride ions. The yield of chloride ions is significantly larger in aerobic than in anaerobic conditions and is dependent upon the dose rate. Formation of peroxyl radicals by the reaction of molecular oxygen with chlorinated hydrocarbon radicals and their subsequent chemistry are responsible for the observed increase in chloride ions. The yield of chloride ion with 5 MeV helium ions is smaller than with gamma irradiation, while the yield of molecular hydrogen is larger reflecting the higher density of reactive species and consequent increase in intratrack reactions in a helium ion track compared to a gamma-ray track.

Hydrated Electron Yields in the Heavy Ion Radiolysis of Water

Experimental measurements coupled with Monte Carlo track simulations have been used to examine the yields of hydrated electrons in the radiolysis of water with protons, helium ions, and carbon ions. Glycylglycine, in concentrations ranging from 10(-4) to 1 M, was employed as a scavenger and the production of the ammonium cation used as a probe of hydrated electron yields from about 2 ns to 20 mus. Monte Carlo track simulations employing diffusion-kinetic calculations of product yields are found to reproduce experimental observations satisfactorily. Model details are used to elucidate the heavy ion track physics and chemistry. Comparison of the heavy ion results with those found in gamma radiolysis shows intratrack reactions are significant on the nanosecond to microsecond time scale as the ion track relaxes, and that a constant (escape) yield is never attained on this time scale. Numerical interpolation techniques are used to obtain both track average and track segment yields for use in practical applications or comparison with other models. The model results give the first hints that initial ( approximately 5 ps) hydrated electron yields, and possibly other water decomposition products, are dependent on the type and energy of the incident radiation.

Rate Coefficient Measurements of Hydrated Electrons and Hydroxyl Radicals with Chlorinated Ethanes in Aqueous Solutions

Rate coefficients for the reactions of hydrated electrons and hydroxyl radicals with various chloroethanes were determined in aqueous solutions using pulse radiolysis techniques. The rate coefficients for the hydrated electron increase from 0.17 x 10(9) to 16.3 x 10(9) M(-1) s(-1) with increasing number of chlorine atoms from monochloroethane to hexachloroethane. Very little difference in rates is found between the isomers. Rate coefficients for the OH radicals range from 1 to 5 x 10(8) M(-1)s(-1) and have very little variation with the number of chlorine atoms except when no H atom is available on a carbon atom. The use of competition kinetics with low concentrations of SCN(-) as a reference is reviewed and suitable model simulations proposed. Possible explanations for the discrepancies between the previously published rate coefficients and the present values are offered.

On (3)H beta-particle and (60)Co gamma irradiation of aqueous systems

The chemistry of water and aqueous solutions is very different after irradiation with (3)H beta particles and high-energy electrons or (60)Co gamma rays. The greater the linear energy transfer (LET) of the medium for (3)H beta particles compared to high-energy electrons or (60)Co gamma rays leads to an increased local concentration of reactants. There is an increased amount of intratrack chemistry, which reduces the escape yield of and OH by about 50%, but increases the yield of H(2) by about 50% and of H(2)O(2) by about 35%. Analysis of stochastic-diffusion kinetic calculations employing simulated track structures reveals that the yield of H(2) produced by diffusion-kinetic processes increases significantly for (3)H beta particles compared to (60)Co gamma radiation, while production of H(2) by sub-picosecond processes is essentially the same. In both (3)H beta-particle and (60)Co gamma radiolysis, the reactions + and are equally important in the production of H(2). In the former case, each reaction has a yield of approximately 0.18, and in the latter a yield of approximately 0.08. In neutral water, the reaction (H + H) is negligible. The yield of Fe(III) in (3)H beta-particle radiolysis of the Fricke dosimeter is much smaller than in radiolysis with more energetic electrons. Simulations show that this change is primarily due to the reduced escape yield of H, formed from the scavenging of by the bulk H(3)O(+) of the acid. The chemical differences observed in experiments, and in calculations, reflect the underlying structure of the electron tracks: Examination of the track structure simulations demonstrates that primary events are considerably more well-separated in high-energy electron tracks compared to (3)H beta-particle tracks.