Radionuclides in Irradiated Graphite of Uranium–Graphite Reactors: Decontamination by Thermochemical Methods

Graphite samples from blocks and sleeves are characterized by extremely heterogeneous spatial distribution of radionuclides on the scale of tens and hundreds of micrometers. The graphite decontamination occurs in three steps: Up to 25% of ¹⁴C is removed in the course of low-temperature annealing or treatment of graphite with liquid reagents (total weight loss of graphite ≤10%); up to 70–80% of ¹⁴C is removed in the course of 20-h heat treatment under oxidizing conditions; and in the final step the degree of removal of ¹⁴C exceeds 90%, with a proportional decrease in the graphite weight. Oxidation of graphite in the first steps of thermochemical treatment mainly occurs along (micro)cracks and other defects, which favors the removal of relatively weakly bound ¹⁴C, e.g., from the pore space. The removal of ¹⁴C from the crystal lattice of graphite requires its breakdown, e.g., by high-temperature heat treatment.     

Radionuclides in Irradiated Graphite of Industrial Uranium–Graphite Reactors: Effect of Irradiation and Thermochemical Treatment on the Graphite Structure

The structure of irradiated graphite from decommissioned industrial uranium–graphite reactors was studied. The extent of disturbance of the graphite structure is closely correlated with temperature and integral neutral fluence. The perfection of the structure of graphite samples (data of X-ray diffraction and Raman spectroscopy) does not correlate with their radioactivity, which is due to low absolute concentration of the radionuclides. Mapping of the samples using Raman spectroscopy reveals spatial heterogeneity of the distribution of graphite lattice damages, which casts doubt on the representativeness of the spectra of individual points. The spatial distribution of domains differing in the crystal lattice perfection was studied for the first time and was compared with the radionuclide distribution. Satisfactory correlation between the radiographic and spectroscopic mapping data is observed for some samples. Irradiated graphite is strongly textured and contains amorphous microvolumes, which are probably radionuclide carriers. Thermochemical treatment (oxidation in O₂, thermal shock) leads to degradation of the irradiated graphite structure on the submicron level, accompanied by a drastic decrease in the mechanical strength of the samples.     

Study of the Process of Leaching of Long-Lived Radionuclides 14С and 36Сl from Irradiated Graphite

Radiochemistry - The release of long-lived radionuclides 14C and 36Cl , which are key products for assessment of disposal variants (near-surface, buried, or deep) of irradiated graphite, was...     

Predictions of Phase Distribution in Liquid–Liquid Two-Component Flow

Ground-based liquid–liquid two-component flow can be used to study reduced-gravity gas-liquid two-phase flows provided that the two liquids are immiscible with similar densities. In this paper, we present a numerical study of phase distribution in liquid–liquid two-component flows using the Eulerian two-fluid model in FLUENT, together with a one-group interfacial area transport equation (IATE) that takes into account fluid particle interactions, such as coalescence and disintegration. This modeling approach is expected to dynamically capture changes in the interfacial structure. We apply the FLUENT-IATE model to a water-Therminol 59^® two-component vertical flow in a 25-mm inner diameter pipe, where the two liquids are immiscible with similar densities (3% difference at 20°C). This study covers bubbly (drop) flow and bubbly-to-slug flow transition regimes with area-averaged void (drop) fractions from 3 to 30%. Comparisons of the numerical results with the experimental data indicate that for bubbly flows, the predictions of the lateral phase distributions using the FLUENT-IATE model are generally more accurate than those using the model without the IATE. In addition, we demonstrate that the coalescence of fluid particles is dominated by wake entrainment and enhanced by increasing either the continuous or dispersed phase velocity. However, the predictions show disagreement with experimental data in some flow conditions for larger void fraction conditions, which fall into the bubbly-to-slug flow transition regime. We conjecture that additional fluid particle interaction mechanisms due to the change of flow regimes are possibly involved.     

Thermodynamics of Liquid–Liquid Criticality in Supercooled Water in a Mean-Field Approximation

In a recent publication, it has been shown how a non-analytic scaling theory of critical phenomena can describe the available experimental information for the thermodynamic properties of supercooled water, when it is assumed to exhibit a liquid–liquid critical point. In this article, we present a mean-field equation of state which also represents the experimental data for supercooled water. Compared to the scaling theory, the mean-field equation has the advantage of simplicity for practical calculations of the properties of supercooled water. The insensitivity to a particular form of a critical equation of state is due to a lack of experimental data asymptotically close to the liquid–liquid critical point. Hence, while the assumed existence of a liquid–liquid critical point in water can account for the observed thermodynamic behavior of supercooled water, the actual location of such a liquid–liquid critical point cannot be determined accurately from the available experimental thermodynamic property data.     

Packed-Column Supercritical Fluid Chromatography: Quantitative Determination of Uranium without Liquid Waste Generation

A new procedure for the determination of uranium by packed-column supercritical fluid chromatography is proposed. A nonfluorinated chelating agent selective for copper and uranium, the 2,6-diacetylpyridine bis(benzoylhydrazone), has been chosen. We have studied its chromatographic properties on different stationary phases and the influence of the methanol content in the carbon dioxide mobile phase. The separation of the metal compounds was conducted with and without solvent injection. A calibration curve was obtained for uranium in the range of 52-323 ng injected. The accuracy of the method is 0.5%, the repeatability 4%. The same studies were performed with a new compound, diacetyl-2,6 pyridine bis(4-tert-butyl benzoylhydrazone). An increase in retention and efficiency was then observed.     

Sorption of Radionuclides from the Primary Circuits of Reactors with Pb–Bi Coolant onto the Surface of Structural Materials of the Reactor Core

Sorption and phase distribution of ⁵⁴Mn, ⁵⁹Fe, ⁶⁰Со, ¹⁰⁶Ru, ¹²⁵Sb, ¹³⁷Cs, ¹⁴⁴Се, ^154,155Eu, and ^235,238U radionuclides in contact of the Pb–Bi melt with the steel surface were studied. Radionuclides of the elements with the minimal oxygen affinity (Ru, Te, and Sb) remain mainly in the coolant melt and are weakly sorbed onto the solid phase. The other fission product radionuclides having higher oxygen affinity are sorbed onto the surface of steel structural materials. The sorption increases at 350–400°С.     

Chemical-Spectroscopic Determination of the Isotope Composition and Gravimetric Content of Uranium in Uranium–Aluminum Alloys and Products of Their Processing

Conditions of chemical separation of Al macroamounts and U microamounts, followed by atomic emission spectroscopic determination of the isotope composition and gravimetric content of U, were studied. An algorithm was developed for constructing calibration plots to determine the ²³⁵U/²³⁸U isotope ratio in various samples from nuclear fuel reprocessing. Optimum conditions of the spectral analysis for the U content were found by mathematical design of the experiment: NaCl content 2%, current 18 А, and exposure time 40 s. With the use of these conditions and of a specially developed form of carbon electrode, the uranium detection limit was decreased from 10^–3 to 10^–5%.     

Estimation of the Heat Transfer Coefficient in a Liquid–Solid Fluidized Bed Using an Artificial Neural Network

A non-iterative procedure has been developed using an artificial neural network (ANN) for estimating the fluid–particle heat transfer coefficient, h_fp, in a liquid–solid fluidized system. It is assumed that in a liquid–solid system, the liquid temperature is time dependent, and the input parameters and output parameters for the ANN are considered on a linear scale. The output configuration yields an optimal ANN model with 10 neurons in each of the three hidden layers. This configuration is capable of predicting the value of Bi in the range of 0.1–10 with an error of less than 3%. The heat transfer coefficient estimated using the ANN has been compared with the data reported in the literature and found to match satisfactorily.© Koninklijke Brill NV, Leiden and Society of Powder Technology, Japan, 2008     

Characterization of Dielectric Properties in Liquid–Solid Phase Transition

A method for determining the complex dielectric permeability of liquids based on the temperature changes in geometrical parameters (thickness) of samples and over the phase transition range is proposed. Allowing the measurements to be made in a wide temperature range (–190 to 60°C) at different frequencies (0.1–100 kHz), this technique is used for establishing the complex dielectric permeability of ethanol as a function of temperature. The accuracy of coincidence of the obtained values with the data reported in the literature is 3%.     

Synthesis of Intercalation Compounds in the Graphite–HNO3–H3PO4System

The highly oriented pyrolytic graphite–HNO₃–H₃PO₄system was studied by x-ray diffraction and potentiometry at different acid concentrations. The results demonstrate that chemical and electrochemical intercalation in the graphite–98% HNO₃–85% H₃PO₄system yields graphite nitrate, a binary graphite intercalation compound (GIC). H₃PO₄is shown to have an ambiguous effect on the concentration ranges of different stages of graphite nitrate, shifting them to lower HNO₃concentrations as compared to the graphite–HNO₃–H₂O system. In the graphite–98% HNO₃–100% H₃PO₄system, a stage II ternary GIC is obtained, with an intercalate layer thickness d _i 4.7 Å. Stages II–VI of this GIC were prepared via exchange reaction between graphite nitrate and 100% H₃PO₄. A mechanism for the formation of the ternary GIC is proposed. The synthesis of the cointercalated GIC is likely to involve two steps: in the first step, graphite nitrate is formed; subsequent reaction in the intercalate layer leads to partial replacement of solvated HNO₃by H₃PO₄molecules.     

Experimental Study of Gas–Liquid Two-Phase Flow in Glass Micromodels

To estimate the most important flow variables in reservoir engineering, such as the relative permeability, it is required to know with high precision, other variables such as saturation, pressure drop of each phase, and porous media data such as porosity and absolute permeability. In this study, experimental tests were performed inside a glass micromodel using gas–liquid two-phase flow in steady-state conditions. The liquid-phase flow and the pressure drop of the porous media were determined. Additionally, the flow development inside the porous media was visualized using a high-speed video camera system. These pictures were recorded at 500 fps, and they were used to compute the phase saturation and the gas velocity in the glass micromodel. The visualization was performed in three regions of the glass micromodel demonstrating that saturation gradients were not present. The effect of the capillary number was studied over the gas–liquid relative permeability curves and on the flow mechanisms. It was concluded that high flow rates minimize edge effects, that the capillary number modifies the relative permeability values and the flow patterns inside the micromodel, and that the high-speed visualization is an efficient and accurate technique to determine saturation values and to study the flow patterns in transparent porous media such as glass micromodels.     

Content of Long-Lived Radionuclides of Carbon-14 and Chlorine-36 in Reactor Graphite and in the Biosphere (Is there a Problem with Carbon-14 and Chlorine-36 when It Comes to the Processing of Reactor Graphite?)

Radiochemistry - The amounts of technogenic carbon and chlorine contained in reactor graphite and in the biosphere are compared. The total amount of 14C on Earth and in the atmosphere, according to...     

Evaluation of Erosion Corrosion in Liquid–Solid and Liquid–Gas via Experimental Analysis Inside 90° Copper Elbow

Erosion–corrosion experiments of copper elbow were performed by acidified dichromate. Mass transfer coefficient inside 90° copper elbow has been investigated. The results showed that the mass transfer coefficient increases as solution velocity increases in both cases of one- and two-phase flow. The mass transfer coefficient can be related to the solution velocity in case one-phase flow by the following equations:

$$k \, \alpha \, v^{ 0. 4 4}\quad{\text{for\,Sc\,from\,678 to 845}}$$

$$k \, \alpha \, v^{ 0. 3}\quad{\text{for\,Sc\,from\,1040\,to\,1445}}$$

In case of liquid–solid flow

$$k \, \alpha \, v^{ 0. 3 3}$$

In case of liquid–gas flow

$$k \, \alpha \, vg^{ 0. 2 4}$$

The importance of these equations is to understand and predict erosion corrosion inside 90^o copper elbow.

     

Measuring Turbidity in a Near-Critical, Liquid–Liquid System: A Precise, Automated Experiment

A ground based (1g) experiment is in progress that measures the turbidity of the density-matched, binary fluid mixture methanol–cyclohexane extremely close to its liquid–liquid critical point. By covering the range of reduced temperatures t (T–T _c)/T _c from 10^–8 to 10^–2, the turbidity measurements should allow the Green–Fisher critical exponent to be determined. This paper reports measurements showing ±0.1 % precision of the transmitted and reference intensities, and ±4K temperature control near the critical temperature of 320 K. Preliminary turbidity data show a nonzero consistent with theoretical predictions. No experiment has precisely determined a value of the critical exponent , yet its value is significant to theorists in critical phenomena. Relatively simple critical phenomena, as in the liquid–liquid system studied here, serve as model systems for more complex behavior near a critical point.     

Advances in the Experimental Determination of the Uranium–Oxygen Phase Diagram at High Temperature

Due to its complex phase diagram and the employment of UO₂ as a nuclear fuel, the binary system U–O is of great interest both scientific and technological. Numerous experimental and theoretical studies have been carried out in the last 45 years in order to determine the properties of this system, leading to a precise definition of a considerable part of the state diagram in the region ranging from pure uranium to stoichiometric UO₂, and at temperatures lower than 1500 K, up to the oxide U₄O₉. However, due to the poor chemical stability of O–U compounds with high oxygen content at high temperature (O/U > 2, T > 2000 K), an important part of the phase diagram still lacks experimental data. In this work measurements are presented on the melting transition of the stoichiometric and hyperstoichiometric dioxide UO_2+x up to x=0.21, and on the melting point of the higher oxide U₃O₈. These measurements were performed under buffer gas pressures varying between 10 and 250 MPa, using a method based on subsecond laser heating developed to overcome experimental difficulties encountered by previous researchers. Paper presented at the Seventh International Workshop on Subsecond Thermophysics October 6–8, 2004, Orléans, France.     

Deterministic Model of Homophase and Heterophase Fluctuations in a Liquid–Vapor System

Within the framework of the assembly-type catastrophe model, a nonlinear dynamic equation (DE) homogeneous in the parameter t with an aftereffect is constructed, in which t characterizes the deviation of the reduced density of a thin surface layer on the liquid-vapor interface from the mean density of the vapor-liquid system. This equation is used to treat a second-order nonlinear DE with a variable damping coefficient for a vapor-liquid system excited by periodic impacts (acts of evaporation and condensation of molecules). This DE is integrated over a finite time interval to find a two-dimensional mapping whose numerical solution describes the chaotic dynamics of the density in time, including homophase and heterophase fluctuations. For this system, the bifurcation diagrams are constructed and the Lyapunov exponents are found.