On a Theory of Absolute Reaction Rates
Reaction rates and yield for multichannel molecular collisions, introducing observable yield operator in theory, using motion equations
MOTION EQUATION↗
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Extension of a Kinetic-Theory Approach for Computing Chemical-Reaction Rates to Reactions with Charged Particles
Recently introduced molecular-level chemistry models that predict equilibrium and nonequilibrium reaction rates using only kinetic theory and fundamental molecular properties (i.e., no macroscopic reaction rate information) are extended to include reactions involving charged particles and electronic energy levels. The proposed extensions include ionization reactions, exothermic associative ionization reactions, endothermic and exothermic charge exchange reactions, and other exchange reactions involving ionized species. The extensions are shown to agree favorably with the measured Arrhenius rates for near-equilibrium conditions.
Liechty, Derek S.↗
Computational Workflows for Uncertainty-Quantified Nuclear Reactions: From Nuclear Theory Inputs to Astrophysical Reaction Rates
Reactions on unstable nuclei, particularly those on the neutron-rich side of stability, are important for both fundamental and applied physics. For fundamental science, the most prevalent use case is astrophysi cal nucleosynthesis by rapid neutron capture—the r-process—by which heavy nuclei are formed in extreme astrophysical environments, such as in supernovae and neutron star mergers; see, e.g., Refs. [1–3]. For ap plications, these processes are relevant for the interpretation of radiochemical data from historic nuclear tests, which contribute to our ability to certify the enduring stockpile in the absence of nuclear testing [4]; see Ref. [5] for a broader discussion of applications. However, reaction cross sections involving unsta ble species are generally poorly understood, for the simple reason that useful data become scarce as one moves away from stability. While there are avenues for improving the amount and quality of data for these species [6], one is fundamentally reliant on nuclear theory to make progress on these fields of study.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reaction flows
The use of silane (SiH4) as an effective ignitor and flame stabilizing pilot fuel is well documented. A reliable chemical kinetic mechanism for prediction of its behavior at the conditions encountered in the combustor of a SCRAMJET engine was calculated. The effects of hydrogen addition on hydrocarbon ignition and flame stabilization as a means for reduction of lengthy ignition delays and reaction times were studied. The ranges of applicability of chemical kinetic models of hydrogen-air combustors were also investigated. The CHARNAL computer code was applied to the turbulent reaction rate modeling.
Magnotti, F.↗
Estimation of the rate of volcanism on Venus from reaction rate measurements
Laboratory rate data for the reaction between SO2 and calcite to form anhydrite are presented. If this reaction rate represents the SO2 reaction rate on Venus, then all SO2 in the Venusian atmosphere will disappear in 1.9 Myr unless volcanism replenishes the lost SO2. The required volcanism rate, which depends on the sulfur content of the erupted material, is in the range 0.4-11 cu km of magma erupted per year. The Venus surface composition at the Venera 13, 14, and Vega 2 landing sites implies a volcanism rate of about 1 cu km/yr. This geochemically estimated rate can be used to determine if either (or neither) of two discordant geophysically estimated rates is correct. It also suggests that Venus may be less volcanically active than the earth.
Fegley, Bruce, Jr.↗
A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows
A computationally-viable model describing the interaction between fluid-mechanical turbulence and finite-rate combustion reactions, principally in high-speed flows was developed. Chemical kinetic mechanisms, complete and global, were developed describing the finite rate reaction of fuels of interest to NASA. These fuels included principally hydrogen and silane, although a limited amount of work involved hydrocarbon fuels as well.
Chinitz, W.↗
NCERC-CL TREAT Free Field Characterization Foil Reaction Rate Results
This report summarizes the analysis and preliminary MCNP reaction rate modeling of free field characterization measurements performed in the Transient Reaction Test Facility (TREAT) at Idaho National Laboratory (INL) between 12/18/2023 and 2/22/2024. A series of seven irradiations were performed using the Big-BUSTER (Broad Use Specimen Transient Experiment Rig) core configuration. Foil sets including 19.5% LEU-Zr alloy wire, S, Au, Fe, Ni, Co, Ti, and Zr were deployed in each irradiation. Identical foil sets were supplied to Los Alamos National Laboratory (LANL), Lawrence Livermore National Laboratory (LLNL), and INL. All foils were supplied by INL. Additional details on the experiment can be found in [2]. The foil sets were shipped to the National Criticality Experiments Research Center Counting Laboratory (NCERC-CL) branch at LANL (NCERC-CL NISC) and were received on 3/25/2024. Additional details on receipt and counting are reported in [3]. All reactor dosimetry measurements were performed in adherence to the ASTM standards applicable to reactor dosimetry. Measured reaction rates from Au, Co, Fe, Ni, and Ti are reported. Select reaction rate ratios and simulated reaction rates and ratios are discussed. Reaction rates for all measured reaction products were modeled with MCNP 6.3.1 and a TREAT input deck supplied by Edward Lum. Neutron emission estimates calculated using the measured and simulated reaction rates show excellent agreement between the three fast-threshold n-p reactions with the percent differences being < 9% between the three reactions. The capture reactions had poorer, but still reasonable, agreement with < 20% percent differences between the neutron emission estimates of the three capture reactions. The percent difference between the fast-threshold and capture reactions is ∼ 100% for each reaction. This supports the TREAT model is not accurately modeling the neutron spectrum and additional measurements are required to characterize TREAT and match measurements to simulation.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
A study of the photocatalytic effects of aqueous suspensions of platinized semiconductor materials on the reaction rates of candidate redox reactions
The effectiveness of powdered semiconductor materials in photocatalyzing candidate redox reactions was investigated. The rate of the photocatalyzed oxidation of cyanide at platinized TiO2 was studied. The extent of the cyanide reaction was followed directly using an electroanalytical method (i.e. differential pulse polarography). Experiments were performed in natural or artificial light. A comparison was made of kinetic data obtained for photocatalysis at platinized powders with rate data for nonplatinized powders.
Miles, A. M.↗
A simple reaction-rate model for turbulent diffusion flames
A simple reaction rate model is proposed for turbulent diffusion flames in which the reaction rate is proportional to the turbulence mixing rate. The reaction rate is also dependent on the mean mass fraction and the mean square fluctuation of mass fraction of each reactant. Calculations are compared with experimental data and are generally successful in predicting the measured quantities.
Bangert, L. H.↗
Reaction Rate Ratios for Recent Fast Metal Experiments with Large Plutonium Masses
Reaction rate ratios are integral responses that are used within the criticality experiments field because they contain spectral information. While these types of measurements have been utilized for nuclear data validation with historic experiments, few experiments of this type have been utilized for recent experiments, as few exist. This work focuses on measured reaction rate ratios for two nearly bare plutonium critical assemblies with different geometries: one that is cube like (with a Pu mass of 40 kg) and one that is slab like (with a Pu mass of 109 kg). Irradiations were performed with both configurations in which foils were placed near the center of the assembly. Plutonium, highly enriched uranium, depleted uranium, and Au foils were included in the irradiation and counted via high-purity germanium detectors. From these measurements, reaction rate ratios were calculated. Measured and simulated values and uncertainties are presented for the reaction rate ratios. Ratios utilizing the following reactions are given in this work: 197 Au(n, γ), 197 Au(n,2n), 235 U(n,fission), 238 U(n, fission), 238 U(n,2n), 238 U(n,γ), and 239 Pu(n,fission). Uncertainties for the measured reaction rate ratios ranged from 4% to 7%, and the contribution of various parameters to this uncertainty was investigated. The results are compared to historical experiments and should be used for nuclear data validation for future nuclear data library releases. These measurements are part of the EUCLID (Experiments Underpinned by Computational Learning for Improvements in Nuclear Data) project, which utilizes measurement responses in addition to k eff (such as these reaction rate ratios) to help reduce uncertainties in 239 Pu nuclear data.
73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
The 2025 Evaluation of Experimental Thermonuclear Reaction Rates (ETR25)
This work describes the formalism for estimating thermonuclear reaction rates for astrophysical applications, emphasizing modern statistical approaches such as Monte Carlo sampling and Bayesian models. We discuss related topics including the calculation of resonance energies from nuclear Q values, indirect estimates of particle partial widths, and matching of reaction rates at elevated temperatures to statistical model results. We have evaluated available experimental data on cross sections, resonance energies and strengths, partial widths, lifetimes, spin-parities, and spectroscopic factors. Based on these results, we have estimated numerical values of 78 experimental charged-particle thermonuclear reaction rates for target nuclei in the A = 2–40 mass region, for temperatures ranging from 1 MK to 10 GK. For each reaction, three rate values are provided: low, median, and high, corresponding to the 16th, 50th, and 84th percentiles, respectively, of the cumulative reaction rate probability density distribution. Additionally, we present the factor uncertainty of each rate at each temperature grid point. These results enable users to sample the reaction rate probability density in nucleosynthesis calculations, facilitating uncertainty estimates of nuclidic abundances. The rates presented here refer to their laboratory values. For use in stellar model simulations, these values need to be corrected for the effects of thermal excitations of the interacting nuclei. For each reaction, we include graphs that illustrate the fractional contributions to the overall reaction rate along with the associated uncertainty. These visuals are designed to assist both stellar modelers and nuclear experimentalists by identifying the primary sources of rate uncertain=^texttx);ty at specific stellar temperatures. A graphical comparison with earlier Monte Carlo rates is also provided.
Nuclear astrophysics↗
A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows
Reaction rates in turbulent, reacting flows are reviewed. Assumed probability density functions (pdf) modeling of reaction rates is being investigated in relation to a three variable pdf employing a 'most likely pdf' model. Chemical kinetic mechanisms treating hydrogen air combustion is studied. Perfectly stirred reactor modeling of flame stabilizing recirculation regions was used to investigate the stable flame regions for silane, hydrogen, methane, and propane, and for certain mixtures thereof. It is concluded that in general, silane can be counted upon to stabilize flames only when the overall fuel air ratio is close to or greater than unity. For lean flames, silane may tend to destabilize the flame. Other factors favoring stable flames are high initial reactant temperatures and system pressure.
Goldstein, D.↗
Reaction rate and products for the reaction O/3P/ + H2CO
A study of reaction kinetics of O + H2CO in a discharge-flow system using mass spectrometric detection of reactants and products is presented. It was performed under both oxygen-atom-rich and formaldehyde-rich conditions over the 296 to 437 K range, showing that the global bimolecular rate constant is in agreement with other studies. This study differs from others in that the reaction products can be observed, and a substantial yield of a primary reaction product was measured with a mass spectral peak at m/e=44. This suggests that the global reaction rate probably consists of combination, as well as of simple abstraction. For the combination, one hypothesis is that triplet dioxymethylene is formed which polymerizes to triplet formic acid; the vibrationally excited triplet formic acid may decompose to form several sets of products, including HCO + OH and HCO2 + H.
Chang, J. S.↗
Energy dependence of methanol proton transfer reaction rate
Methanol proton transfer reaction rate energy dependence
Dunbar, R. C.↗
Effect of temperature oscillation on chemical reaction rates in the atmosphere
The effect of temperature fluctuations on atmospheric ozone chemistry is examined by considering the Chapman photochemical theory of ozone transport to calculate globally averaged ozone production rates from mean reaction rates, activation energies, and recombination processes.
Eberstein, I. J.↗
Thermonuclear 28 P(p, γ ) 29 S reaction rate and astrophysical implication in ONe nova explosion
An accurate 28 P(p, γ) 29 S reaction rate is crucial to defining the nucleosynthesis products of explosive hydrogen burning in ONe novae. Using the recently released nuclear mass of 29 S, together with a shell model and a direct capture calculation, we reanalyzed the 28 P(p, γ) 29 S thermonuclear reaction rate and its astrophysical implication. We focus on improving the astrophysical rate for 28 P(p, γ) 29 S based on the newest nuclear mass data. Our goal is to explore the impact of the new rate and associated uncertainties on the nova nucleosynthesis. We evaluated this reaction rate via the sum of the isolated resonance contribution instead of the previously used Hauser-Feshbach statistical model. The corresponding rate uncertainty at different energies was derived using a Monte Carlo method. Nova nucleosynthesis is computed with the 1D hydrodynamic code SHIVA. The contribution from the capture on the first excited state at 105.64 keV in 28 P is taken into account for the first time. We find that the capture rate on the first excited state in 28 P is up to more than 12 times larger than the ground-state capture rate in the temperature region of 2.5 × 10 7 K to 4 × 10 8 K, resulting in the total 28 P(p, γ) 29 S reaction rate being enhanced by a factor of up to 1.4 at ~1 × 10 9 K. In addition, the rate uncertainty has been quantified for the first time. It is found that the new rate is smaller than the previous statistical model rates, but it still agrees with them within uncertainties for nova temperatures. The statistical model appears to be roughly valid for the rate estimation of this reaction in the nova nucleosynthesis scenario. Using the 1D hydrodynamic code SHIVA, we performed the nucleosynthesis calculations in a nova explosion to investigate the impact of the new rates of 28 P(p, γ) 29 S. Our calculations show that the nova abundance pattern is only marginally affected if we use our new rates with respect to the same simulations but statistical model rates. Finally, the isotopes whose abundance is most influenced by the present 28 P(p, γ) 29 S uncertainty are 28 Si, 33,34 S, 35,37 Cl, and 36 Ar, with relative abundance changes at the level of only 3% to 4%.
Astronomy & Astrophysics↗
A comprehensive model to determine the effects of temperature and species fluctuations on reaction rates in turbulent reacting flows
A principal element to be derived from modeling turbulent reacting flows is an expression for the reaction rates of the various species involved in any particular combustion process under consideration. A temperature-derived most-likely probability density function (pdf) was used to describe the effects of temperature fluctuations on the Arrhenius reaction rate constant. A most-likely bivariate pdf described the effects of temperature and species concentrations fluctuations on the reaction rate. A criterion is developed for the use of an "appropriate" temperature pdf. The formulation of models to calculate the mean turbulent Arrhenius reaction rate constant and the mean turbulent reaction rate is considered and the results of calculations using these models are presented.
Foy, E.↗
The effect of temperature fluctuations of reaction rate constants in turbulent reacting flows
Current models of turbulent reacting flows frequently use Arrhenius reaction rate constants obtained from static or laminar flow theory and/or experiments, or from best fits of static, laminar, and turbulent data. By treating the reaction rate constant as a continuous random variable which is temperature-dependent, the present study assesses the effect of turbulent temperature fluctuations on the reaction rate constant. This model requires that a probability density function (PDF) describing the nature of the fluctuations be specified. Three PDFs are examined: the clipped Gaussian, the beta PDF, and the ramp model. All the models indicate that the reaction rate constant is greater in a turbulent flow field than in an equivalent laminar flow. In addition, an amplification ratio, which is the ratio of the turbulent rate constant to the laminar rate constant, is defined and its behavior as a function of the mean temperature fluctuations is described
Chinitz, W.↗