A method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride

Abstract

This invention relates to the fields of computational chemistry and reaction mechanism technology, and particularly to a method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride. The method includes the following steps: 1) thermodynamic assessment; 2) reaction pathway design; 3) pathway calculation and screening; 4) rate-determining step location; and 5) kinetic analysis and process guidance. Through this complete analytical chain, the microscopic reaction mechanism of sodium-thermal reduction of UF6 is revealed, its optimal pathway and rate-determining step are identified, and precise energy data and process parameters for the UF6 reduction process are provided, offering reliable theoretical support for targeted design in experimental research and efficient upgrading of industrial processes.

Description

Technical Field

[0001] This invention relates to the fields of computational chemistry and reaction mechanism technology, and in particular to a method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride. Background Technology

[0002] Sodium thermal reduction of depleted uranium hexafluoride (DUF6) is a promising method for resource utilization, converting it into a stable, easily stored, or further usable element. However, insufficient understanding of its reaction mechanism severely restricts process development and optimization. Traditional experimental methods struggle to capture transient intermediates and transition states at the atomic scale, failing to accurately depict reaction pathways and energy changes. While first-principles calculations can be used to study elementary reactions, a systematic and efficient analytical method is lacking for designing complex reaction systems with multiple valence changes and various possible intermediates (such as mononuclear and polynuclear cluster models), designing possible pathways, screening dominant pathways, and identifying steps that determine the overall rate. Current research on sodium thermal reduction of DUF6 still lacks a clear understanding of the reaction mechanism and the defluorination pathway of uranium fluoride intermediates. Furthermore, insufficient understanding of the fundamental thermodynamics and kinetics of this reaction system hinders precise guidance for reducing harsh reaction conditions and optimizing process parameters. Summary of the Invention

[0003] The technical problem to be solved by this invention is to provide a method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride. Through a complete analytical chain of "thermodynamic evaluation - multi-path comparison - rate-determining step location - kinetic analysis", the microscopic reaction mechanism of sodium thermal reduction of UF6 is revealed. The reaction mechanism is revealed at the atomic scale, providing reliable theoretical support for targeted design of experimental research and efficient upgrading of industrial processes. To achieve the aforementioned objective, the technical solution of the present invention is as follows: a novel method and approach for the direct reduction and conversion of depleted uranium hexafluoride, comprising: Step 1: Thermodynamic Assessment Calculate the Gibbs free energy change of sodium thermal reduction of uranium hexafluoride in different reactant and product phase ranges. This clarifies the spontaneous trend of the reaction and the feasible temperature boundary.

[0004] Step 2: Reaction Pathway Design Based on the valence state evolution of uranium, reaction model pathways including mononuclear cluster intermediates (e.g., UF6→UF5→UF4→UF3→U) and multinuclear cluster intermediates (e.g., UF6→UF4→UF5→U ...4→UF4→UF4→U 4.5 There are multiple possible reaction pathways, including →UF4→UF3→U).

[0005] Step 3: Path Calculation and Filtering First-principles calculations based on density functional theory (DFT) were used to construct and optimize adsorption models of all reactants, intermediates, and products involved in each reaction pathway on the surface of the reducing agent support. The CI-NEB (climbing image nudged elastic band) method was used to locate the quasi-steady transition state configuration and calculate the activation energies of key elementary steps in each pathway. The DFT+U correction is preferred to treat the strong correlation effect of 5f electrons of U. The activation energy and energy barrier of the key steps of each path are calculated and compared, and the path with the lowest overall activation energy barrier is determined as the dominant path.

[0006] Step 4: Speed ​​Step Positioning In the dominant reaction pathway, the reaction energy barrier ΔEa of each elementary reaction is calculated at different temperatures. The step with the highest ΔEa value at the same temperature is the reaction rate controlling step at that temperature.

[0007] Step 5: Kinetic Analysis and Process Guidance For a given rate-determining step, the reaction rate constant k is calculated under different temperature conditions using the transition state theory (TST) combined with the Arrhenius equation.

[0008] By analyzing the variation of k value with temperature, a temperature-dependent curve of the reaction rate is obtained. Then, the low-temperature optimization range where the reaction rate increases exponentially with temperature and the high-temperature range where the rate growth slows down are divided, providing quantitative temperature guidance for balancing reaction efficiency and energy consumption.

[0009] In step one, the overall reaction for the sodium-thermal reduction of uranium hexafluoride is as follows: Among these factors, the contact area of ​​the reactants and the possible reaction conditions are taken into account. Feed in gaseous state; In step three, low-valent fluorides of uranium are used as intermediates, mainly uranium pentafluoride (UF5) and uranium tetrafluoride (UF4, UF5). 4.5 UF 4.25 ), uranium trifluoride (UF3); In step two, considering atomic coordination, for UF 4.5 and UF 4.25 The intermediates are essentially binuclear cluster non-integer fluorination intermediates U2F9 and tetrauclear cluster non-integer fluorination intermediates U4F. 17 ; In step three, the U value is set to 4.5 eV; In step three, the reducing agent carrier is the crystal surface of metallic sodium, preferably the Na(101) crystal plane; In step four, vibrational frequencies of the stable and transition states of all uranium fluoride intermediates are calculated. Frequency analysis verifies the rationality of the stable structure (no imaginary frequency) and transition state (only one imaginary frequency) of the uranium fluoride intermediates, and the zero-point vibrational energy (ZPE) of the system is obtained. Based on the uncorrected electronic energy Eelec (0 K), ZPE, and the Gibbs free energy G(T) corrected for the temperature term, the free energy change of the reaction step is calculated. G) and the correction value of the reaction barrier. By comparing the magnitudes of the reaction barriers, the step with the highest reaction barrier value at the same temperature is the rate-controlling step at that temperature.

[0010] The beneficial effects of this invention are reflected in: (1) The method provided by the present invention directly compares the energy of different reaction paths by first-principles calculation, objectively selects the most advantageous path in terms of thermodynamics and kinetics, and avoids the arbitrariness of path selection.

[0011] (2) The method provided by this invention is the first to clearly demonstrate that the rate-determining step in the sodium thermal reduction of UF6 reaction is temperature-dependent. This discovery is of great significance for guiding the optimization of actual high-temperature processes.

[0012] (3) The final kinetic analysis of the method provided by the present invention gives a quantitative relationship between the reaction rate and temperature, and clearly points out the law that "efficiency is significantly improved in the low temperature range and energy consumption benefit ratio decreases in the high temperature range", which provides a direct and specific theoretical basis for selecting the optimal reaction temperature window in experiments and engineering.

[0013] (4) The method provided by this invention is not only applicable to the sodium thermal reduction UF6 system, but its core idea of ​​“path design-calculation screening-rate-step location-kinetic analysis” can also be extended to the study of other similar complex multiphase or metal thermal reduction reaction mechanisms. Attached Figure Description

[0014] Figure 1 A flowchart illustrating the method of this invention; Figure 2 This is a schematic diagram of various possible reaction paths designed in the embodiments of the present invention; Figure 3 This invention provides a reactant model diagram constructed by DFT calculation. Figure 4 The reaction energy barrier diagrams of the elementary reactions at different temperatures provide the method advantages of this invention. Figure 5 The reaction rate diagrams of the elementary reactions at different temperatures provide the method advantages of this invention. Detailed Implementation

[0015] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0016] Example like Figure 1-5 As shown, this invention provides a method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride, providing a precise theoretical tool for process optimization. The specific principle is as follows: First, starting from the reaction thermodynamics, the key thermodynamic parameters of the sodium reduction DUF6 reaction are systematically calculated. The spontaneous tendency and thermodynamic feasibility boundary of the reaction at different temperatures are theoretically demonstrated, and its thermodynamic feasibility is explored. Secondly, within a thermodynamically feasible framework, multiple possible stepwise reduction reaction paths are designed. Through energy calculation and analysis based on first principles, the reaction energy barriers and energy changes of the elementary steps in each path are compared, thereby identifying the most likely dominant reaction path and accurately locating the "rate-determining step" that restricts the overall reaction rate. Finally, a kinetic analysis was performed on the rate-limiting step to investigate the effect of temperature on its reaction rate constant, in order to optimize the reaction conditions.

[0017] Taking the sodium-thermal reduction of depleted uranium hexafluoride reaction system as an example, the specific implementation steps are as follows: Step 1: First, conduct a thermodynamic feasibility assessment.

[0018] Based on the changes in the physical states of UF6, Na, U, and NaF with temperature, multiple temperature ranges were divided. Calculations were performed using both classical calculation methods and the first approximation method. It was found that ΔG < 0 in a wide range of temperatures below ~4800 K, indicating that the reaction is thermodynamically spontaneously feasible. This established the temperature boundary for subsequent mechanism studies.

[0019] Step 2: Design the reaction pathway.

[0020] Based on the valence state changes of uranium, seven possible stepwise reduction pathways are designed, as shown in the appendix. Figure 2 As shown.

[0021] There are two main types: one is the path via single-core cluster intermediates such as UF5, UF4, and UF3 (Path I); the other is the path via U2F9 and U4F... 17 The paths for non-integer fluorinated intermediates in dual-core or quad-core clusters (paths II, III, etc.). All paths have the same initial and final states, UF6 and U.

[0022] Step 3: Screening for dominant response pathways.

[0023] First-principles calculations were performed using the VASP software package to optimize all possible intermediates (such as UF5, UF4, UF3, UF2, UF, U2F). 11 U2F 10 Adsorption configurations of (e.g., U2F9, etc.) on Na surfaces.

[0024] The calculation results show that the activation barrier of the initial step UF6→UF5 in the mononuclear cluster path (path I) is 0.33 eV, while the activation barrier of the initial step U2F in the binuclear cluster path (path II) is... 12 →U2F 11 The activation barrier is 0.70 eV. Comparison shows that the mononuclear cluster pathway is more energy-efficient and is therefore identified as the dominant reaction pathway: UF6 → UF5 → UF4 → UF3 → UF2 → UF → U.

[0025] Step Four: Accelerated Positioning For all elementary steps of the dominant path (monocluster path), calculate the activation free energy barrier at different temperatures.

[0026] At 0 K, the ΔEa of the U2→UF step is the highest (0.90 eV); when the temperature rises to 500 K and above, the ΔEa of the UF3→UF2 step (1.06 eV at 500 K) exceeds that of all other steps and continues to increase with increasing temperature (reaching 2.23 eV at 2000 K).

[0027] Therefore, under actual high-temperature reaction conditions (≥500K), the rate-determining step is UF3 → UF2.

[0028] Step 5: Kinetic Analysis and Process Guidance For the rate-determining step UF3 → UF2 at ≥500K, the reaction rate constant k is calculated using transition state theory formulas between 500K and 2000K. The kT relationship curve is then plotted (e.g., ...). Figure 5 (As shown).

[0029] Analysis shows that in the low-temperature range of 500K to 800K, the k value increases by about 890 times, indicating that heating can greatly accelerate the reaction; in the medium-high temperature range of 1100K to 1400K, the k value increases by only about 5 times; and in the high-temperature range above 1700K, the increase further decreases to about 2 times.

[0030] Based on this kinetic principle, it is recommended that in actual process development, the reaction temperature be controlled within the medium-high temperature window of 800K to 1100K. This window can achieve a high reaction rate while avoiding a sharp increase in energy consumption due to limited rate improvement at higher temperatures, thus achieving an optimal balance between efficiency and energy consumption.

[0031] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride, characterized in that, Includes the following steps: Step 1: Thermodynamic Assessment Calculate the Gibbs free energy change of sodium thermal reduction of uranium hexafluoride in different reactant and product phase ranges. ), clearly define the spontaneous tendency of the reaction and the feasible temperature boundary; Step 2: Reaction Pathway Design Based on the valence state evolution of uranium, reaction pathways including mononuclear cluster intermediate reaction model pathways and multinuclear cluster intermediate reaction model pathways are designed. Step 3: Path Calculation and Filtering First-principles calculations based on density functional theory were used to construct and optimize adsorption models of all reactants, intermediates, and products involved in each reaction pathway on the surface of the reducing agent support. The CI-NEB method was used to locate the quasi-steady transition state configuration and calculate the activation energies of key elementary steps in each reaction pathway. The strong correlation effect of 5f electrons of U was corrected by DFT+U, and the activation energies and energy barriers of key steps in each reaction pathway were calculated and compared. The reaction pathway with the lowest overall activation energy barrier was determined as the dominant pathway. Step 4: Speed ​​Step Positioning In the dominant path, the reaction energy barrier ΔEa of each elementary reaction is calculated at different temperatures. The step with the highest ΔEa value at the same temperature is the reaction rate controlling step at that temperature. Step 5: Kinetic Analysis and Process Guidance For a given rate-determining step, the reaction rate constant k is calculated under different temperature conditions using transition state theory combined with the Arrhenius equation. By analyzing the variation of k value with temperature, a temperature-dependent curve of the reaction rate is obtained. Then, the low-temperature optimization range where the reaction rate increases exponentially with temperature and the high-temperature range where the rate growth slows down are divided, providing quantitative temperature guidance for balancing reaction efficiency and energy consumption.

2. The method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride according to claim 1, characterized in that, In step one, the overall reaction for the sodium-thermal reduction of uranium hexafluoride is as follows: ,in, Feed in gaseous state.

3. The method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride according to claim 2, characterized in that, In step three, the intermediate includes uranium pentafluoride, uranium tetrafluoride, and uranium trifluoride; the uranium tetrafluoride includes UF4 and UF... 4.5 and UF 4.25 .

4. The method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride according to claim 3, characterized in that, The UF 4.5 U2F9 is a binuclear cluster non-integer fluorination intermediate; the UF 4.25 U4F is a non-integer fluorinated intermediate for tetranuclear clusters. 17 .

5. The method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride according to claim 2, characterized in that, In step three, the U value in the 5f electron strong correlation effect of the DFT+U correction process is set to 4.5 eV.

6. The method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride according to claim 2, characterized in that, In step three, the reducing agent carrier is the crystal surface of metallic sodium.

7. The method for determining the direct reduction and conversion reaction mechanism of depleted uranium hexafluoride according to claim 2, characterized in that, In step four, the vibrational frequencies of the stable and transition states of all uranium fluoride intermediates are calculated. Frequency analysis is used to verify the rationality of the stable structure and transition state of the uranium fluoride intermediates and to obtain the zero-point vibrational energy of the system. Based on the uncorrected electronic energy Eelec (0 K), ZPE and the Gibbs free energy corrected for the temperature term, the free energy change of the reaction steps and the correction value of the reaction energy barrier are calculated. By comparing the magnitude of the reaction energy barrier, the step with the highest reaction energy barrier value at the same temperature is the rate-controlling step at that temperature.

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