Enrichment of the Uranium Isotopes by Electromigration in a Cation Exchange Membrane

The γ-radiolysis of water subjected to gas bubbling has been studied using a specially desinged gasloop. During the irradiation, N₂ gas was bubbled from the bottom of the irradiation vessel. As the N₂ gas feed rate was raised, the apparent G(H₂) value increased in keeping therewith, from 5 × l0^?3 to 0.26. However in the presence of a sufficient amount of O₂ or H₂O₂, G(H₂) was raised almost to the level of the molecular yield. With reasonable assumptions, it could be concluded that 3～5 × 10^?6 mol/l of H₂O₂ was sufficient to reduce the back reaction of molecular products to less than 10% under the present experimental conditions. It was also found that the G(H₂) value increased with CH₃OH concentration roughly in proportion to log(CH₃OH), and reached 3.1 with 0.1 mol/l CH₃OH.     

Radical Reaction Mechanisms in Infrared Multiphoton Dissociation of UF6 with Scavenger Gases

Abstract

The radical reaction mechanisms in the presence of F-atom scavenger gases were investigated in the p-H₂ Raman laser-induced infrared multiphoton dissociation (IRMPD) of gaseous ²³⁶UF₆/²³⁸UF₆ cooled to —35°C in a static gas cell. When CH₄ was added as a scavenger of F-atoms produced via IRMPD of UFC, the dissociation rate of UF6 became several tens of times larger than when no scavenger gas was added. Gas-chromatographic analysis revealed that as low as 7% of the nascent CH₃ radicals were involved in the radical reaction with UF₆. On the other hand, H₂ and C₂H₆ were found to increase both the dissociation rate of UF₆ and the contribution of this non-selective reaction. These results agreed with those obtained in the UV photolysis of UF₆ with scavengers.     

Relative Reactivities of Radioiodine Released from U3O8 in Oxygen and Helium Atmospheres toward Propane

Chemical form of radioiodine released from U₃O₈ will be UI_x in helium and I₂ in oxygen atmospheres. Relative reactivities of these iodines toward organic compounds have been studied at a temperature range of 250~500°C, by choosing propane as a reactant. The UI_x has a higher reactivity than I₂, particularly at a temperature range of 300~400°C. Above that temperature, both iodine species show a similar reactivity. In the reactions, CH₃I, C₂H₅I, i, and n-C₃H₇I are formed: relative yields depend on both temperature and atmosphere. Two reaction processes participate in the formation of organic iodides. One is an iodine-initiation reaction and is relatively important at a lower temperature. The other is a radical-initiation reaction and become predominant as the temperature rises.     

Radiation Damage of Organic Extractant in Partitioning of High-Level Liquid Waste, (I)

Di(2-ethylhexyl)phosphoric acid (DEHPA), which is a useful extractant for the treatment of high-level liquid waste, was exposed to ⁶⁰Co γ-rays and the radiolysis products and their yields were determined.

The major radiolytic decomposition process of DEHPA was found to be a stepwise splitting of two alkyl groups resulting in MEHPA and H₃PO₄. 1-Methyl-1-ethylpentyl radical, C₄H₉-C(CH₃)-C₂H₅, was found to form with a large G value in the γ-irradiated DEHPA. Reactions involving 1-methyl-1-ethylpentyl radical were also discussed.     

Radiolysis of Dicarboxylic Acid Lithium Salt with Neutron Irradiation

A series of dicarboxylic acids of Li—(CH₂)_n (COOLi)₂, n= 0–5, i.e., Li-oxalate, malonate, succinate, -glutalate, -adipate and -pimelate, was irradiated with mixed radiations in a nuclear reactor. The subject of study was the decarboxylation and the reaction of labelling with T atoms brought about by the exposure of the dicarboxylic acid lithium salts to neutrons.

The T-labelled hydrocarbons were determined by radiogaschromatography, while gaschro- matography was used to measure macroscopic amounts of compounds. The gaseous products of radiolysis produced from the neutron irradiation of dicarboxylic acid lithium salt were CO₂, CO and hydrocarbons.

The yield of CO₂ from the different dicarboxylic acid lithium salts was higher than obtained from γ-ray irradiation, and decreased with increasing number of methylene groups in the original salt. Also, while with γ-radiolysis, CO is only generated in negligibly small amounts, it was found quite significant in the present case of neutron irradiation. The yield of tritiated hydrocarbon was proportional to the number of methylene groups in the salt molecule. Hydrocarbon was mainly produced by the decarboxylation of the original compounds. Several kinds of degradated hydrocarbon were also observed from the mass peaks as also from activity peaks.     

Nuclear Characteristics of Molten-Salt-Cooled D-D Fusion Reactor Blankets

To consecutively decompose ¹⁴CO₂ into carbon (¹⁴C) through its reaction with Hz, an apparatus using microwave discharge and its conditioning were investigated. The reaction produces CO as an intermediate, and proceeds in the two steps of (1) “CO₂+H₂ → CO+H₂O” and (2) “CO+H₂+C_n → C_n+1+H₂O”, where C_n denotes the carbon already deposited on the wall of the discharge tube. Preliminary dispersion of carbon to the wall of the discharge tube by sputtering of a graphite particle was effective to promote the reaction. Two silica discharge tubes (6 mm O.D., 4 mm I.D., and 150 mm length each) were connected in series to proceed the former reaction in the first discharge tube and the latter one in the second one. When a 1:3 mixture of CO₂ and H₂ (total pressure 0.67kPa) was passed through the discharge tubes at a linear gas velocity of approximately 30mm/s and discharged for 60 h under microwave of 30–40 W supplied from two 2,450MHz power generators (200 W each), more than 90% of CO₂ was converted into CO in the 1st tube and about 23% of the CO was then decomposed into carbon in the 2nd tube. However, about 50% of the CO escaped from the tube without being decomposed, and about 0.5% and 1% of the carbon fed were hydrogenated into CH₄ and C₂H₂, respectively. The rest about 25% which was not confirmed was probably evacuated from the 2nd tube as microparticles of carbon. To completely decompose CO₂ into carbon, additional discharge tubes are necessary downstream of the 2nd tube.     

Overall Conductivity and Electromotive Force of SrZr0:9Yb0:1O3–a Cell System Supplied with Moist CH4

A proton-conducting ceramic cell for recovering tritium from process streams was investigated for its application to a fusion reactor system. The ceramic cell tested here was composed of a SrZr_0:9Yb_0.1O_3–a tube, one end of which was closed, and Ni/SiO₂ and NiO/SiO₂ porous electrodes. Its anode was supplied with moist CH₄ or H₂ and its cathode with moist O₂. All of the j-V curves obtained by a direct-current method were correlated to the relation V = E ₀ ? jd/σ at 600–700°C regardless of the two different conditions of the CH₄ + H₂O and H₂ + H₂O supply. The rate-controlling step of charged hydrogen ion transfer was determined from the dependences of the overall conductivity σ and the electromotive force E ₀ on the anode H₂O partial pressure and temperature. The E ₀ value under the condition of the CH₄ + H₂O supply was affected by the diffusion of reaction products of CH₄ + H₂O = CO + 3H₂ through the porous anode. On the other hand, the σ value was limited by the oxygen reduction rate at the cathode interface between the ceramic and the Ni electrode regardless of the different conditions between CH₄ + H₂O and H₂ + H₂O. These results were consistent with our results obtained by an alternating-current method. The activation energy of the overall conductivity was 60 kJ/mol.     

Hydrolysis of Thorium Nitrides and Carbonitrides

Studies were made on the hydrolysis by water and water vapor of thorium nitrides—ThN, Th₃N₄, Th₂N₂O—and carbonitrides. All the thorium nitrides and carbonitrides were found to decompose in water below 100°C, changing thereby into ThO₂. The order among the thorium nitrides in their propensity toward hydrolysis is: ThC_1-xN_x>ThN>Th₃N₄>Th₂N₂O. Upon hydrolysis, ThN produced NH₃ and H₂, but in the case of the higher nitrides no H₂ was found to evolve.

In the reaction between ThN and water vapor, no higher nitride was produced, in contrast to the case of UN. The difference in behavior between ThN and UN was studied from the standpoint of differences in crystallographic conditions for the transformation from mononitride into higher nitrides. Through the hydrolysis of ThC_1-xN_x in water vapor, products containing C-C, C-C-C and C-N bonds, such as ethane and amines, were found in smaller quantities than for the case of UC_1-xN_x. This fact as well as the difficulty of formation of higher nitrides has resulted in a fairly simple hydrolysis behavior of thorium nitrides.     

Simulation of the inhibition of water α-radiolysis via H2 addition

The continuous formation of H₂, O₂, and H₂O₂ observed in water during α-radiolysis may be suppressed by the addition of H₂ above the threshold hydrogen concentration (THC). Using the FACSIMILE simulation code, water radiolysis was reproduced in order to determine the THC and clarify the mechanism at room temperature. Using the reaction set and rate constants reported by Ershov and Gordeev together with the primary yields for water decomposition products generated using 12 MeV α-particles, the THC was found to be 165 μM. Further simulation results clearly showed that the value of THC is strongly dependent on the reaction set and rate constants. In addition, a possible mechanism involving a chain reaction governed by the two reactions OH + H₂ → H + H₂O and H + H₂O₂ → OH + H₂O was proposed. Furthermore, the same inhibition effect was found when a high-temperature simulation (300 °C) was performed, but the concentration range and THC were much smaller than the values obtained at room temperature. The importance of the reverse reaction OH + H₂ → H + H₂O was also investigated.     

Mixed Desorption of He,H2, and CH4 Adsorbed on Charcoal Maximally Cooled at 10K

Desorption rates of mixtures of He, H₂, and CH₄ adsorbed on activated carbon placed in an enclosed vessel are determined under the condition where the temperature is elevated from 10K to room temperature. Activated carbon is selected because of its good adsorption performance at the cryogenic temperature. The carbon used in the present study is covered with mesopores or micropores from 1 to 10 nm in diameter. The adsorption and desorption of their respective gases proceed independently, and the desorption behavior of each component is well expressed in terms of a Langmuir-Freundlich isotherm. Since desorption curves under different heating conditions can be correlated to the same one on a (T-pt) plot, it is considered that gas desorption proceeds under equilibrium condition between the adsorbent and gaseous phases. Two- or three-component desorption of He, H₂, and CH₄ from activated carbon can be described using an extended multicomponent Langmuir-Freundlich isotherm without any correction. Desorption curves calculated for their respective gases are in comparatively good agreement with experimental data.     

Volatilization of Technetium from Simulated Reprocessing Solutions

Volatilization behavior of technetium was investigated in order to evaluate the decontamination efficiency of an evaporator for reprocessing waste solutions. The gaseous species of technetium volatilized over HTcO₄ solutions had been considered as the mixture of Tc₂O₇ and HTcO₄. Estimation of the decontamination efficiency needs to determine whether the dominant gaseous species is Tc₂O₇ or HTcO₄. The continuous distillation of dilute HTcO₄ solution was carried out and we obtained the purification factor (PF) defined as the concentration ratio of technetium in the concentrate and distillate. The PF was normalized by total pressure, HTcO₄ molarity and mole number in 1l solution. The normalized PF was reciprocal to the second power of HTcO₄ molarity and was proportional to the activity of water. These findings suggested the dominant gaseous species were Tc₂O₇ and volatilization was the dehydration of HTcO₄ forming gaseous Tc₂O₇. The vapor pressure of Tc₂O₇ was obtained from PF and the equilibrium constant of volatilization reaction was calculated considering that the gaseous species was dominantly Tc₂O₇. The thermodynamic functions controlling the volatilization reaction were obtained from the dependence of equilibrium constant on temperature as, ΔH=82.3 kJ·mol^?1 and ΔS=205.4 J·mol^?1·K^?1, respectively.     

Influence of hydrocarbons on vibrational excitation of H2 molecules

The influence of light hydrocarbons on vibrational excitation of H₂ generated in a special source of excited molecules has been studied. Molecule dissociation on the hot tungsten filament and atom recombination on cooled walls is used for the production of molecular excitation in the source. Specific influence of CH₄, C₂H₄ and C₂H₆ on vibrational distribution of hydrogen molecules from the source is presented here. Production of vibrationally excited H₂ molecules due to the thermal dissociation on the hot filament is observed when C₂H₄ (ethene) and C₂H₆ (ethane) are introduced alone in the source. These molecules appear to be produced by direct process on the hot filament in the case of ethene while in the case of ethane H₂(v) is produced by the similar process as for H₂. Production of H₂(v) is not observed when only CH₄ is introduced into the source. In this case vibrational relaxation of H₂(v) is observed when CH₄ is introduced into the source in addition to H₂. Studied processes are relevant for the modelling of edge plasma in tokamaks.     

Room-Temperature Reactor Packed with Hydrophobic Catalysts for the Oxidation of Hydrogen Isotopes Released in a Nuclear Facility

A room-temperature reactor packed with hydrophobic catalysts for the oxidation of hydrogen isotopes released in a nuclear facility will contribute to nuclear safety. The inorganic-based hydrophobic Pt catalyst named H1P has been developed especially for efficient oxidation over a wide concentration range of hydrogen isotopes at room temperature, even in the presence of saturated water vapor. The overall reaction rate constant for hydrogen oxidation with the H1P catalyst in a flow-through system using a tritium tracer was determined as a function of space velocity, hydrogen concentration in carriers, temperature of the catalyst, and water vapor concentration in carriers. The overall reaction rate constant for the H1P catalyst in the range near room temperature was considerably larger than that for the traditionally applied Pt/Al₂O₃ catalyst. Moreover, the decrease in reaction rate for H1P in the presence of saturated water vapor was slight compared with the reaction rate in the absence of water vapor due to the excellent hydrophobic performance of H1P. Oxidation reaction on the catalyst surface is the rate-controlling step in the range near room temperature and the rate-controlling step is shifted to diffusion in a catalyst substratum above 313K due to its fine porosity. The overall reaction rate constant in the range near room temperature was dependent on the space velocity and hydrogen concentration in carriers. The overall reaction rate constants in the range of 1;000=T greater than 3.2 correlated to k _overall[s^?1] = 5.59 × 10⁷ × SV[h^?1] × exp (?67.7 [kJ/mol]/R_gT), where the space velocity range was from 600 to 7,200 h^?1.     

Fragmentation by β-Decay in Tritium-Labelled Compounds, (II)

The behaviors of CH₃He⁺ and C₂H₅He⁺ formed by the decay of CH₃T and C₂H₅T were studied theoretically using the STO-3G molecular orbital method and was compared with that of HHe⁺ in the decay of HT. It was clearly shown that the ground state daughter ions CH₃He⁺ and C₂H₅He⁺ dissociate instantly to give CH₃ ⁺ and C₂H₅ ⁺ because their potential energy curves are repulsive, whereas the daughter ion HHe⁺ in the ground state does not dissociate. The transition probability to the ground state ions of CH₃He⁺ and of C₂H₅He⁺ are computed to be 66.5 and 64.8%, respectively. These values are in fairly good accordance with those obtained experimentally.     

Molecular effects in carbon K-shell Auger-electron production by 0.6–2.0 MeV protons and extraction of an atomic cross section

Carbon K-shell Auger-electron production cross sections are reported for 0.6–2.0 MeV protons incident on CH₄ (methane), C₂H₂ (acetylene), C₂H₄ (ethylene), C₂H₆ (ethane), nC₄H₁₀ (normal butane), i-C₄H₁₀ (isobutane), C₆H₆ (b monoxide), and CO₂ (carbon dioxide). A constant-energy mode 45° parallel-plate electrostatic analyzer was used for detection of Auger electrons. The carbon KLL Auger-electron cross sections for all molecules were found to be lower than that found for CH₄ by 9–23%. All carbon KLL Auger-electron data could be brought into agreement when corrected for the chemical shift of the carbon K-shell binding energy in molecules and for intramolecular scattering. KLL Auger-electron production cross sections are compared to first Born and ECPSSR theories and show good agreement with both after the chemical shift of the carbon K-shell binding energy in molecules and the effects of intramolecular scattering are considered.     

Water Chemistry in a Crack Tip under Irradiation, (II)

Under neutron and gamma-ray irradiations, radiolytic species are generated directly in the crack tip, which causes higher oxidant concentrations and subsequently influences crack propagation rate.

A crevice radiolysis model was proposed to estimate the oxidant concentrations in the crack tip water under gamma-ray irradiation. Direct generation of radiolytic species in the crevice water, and their secondary generation and disappearance caused by their interaction with the crevice surface as well as species in the crevice water were included in the model. The diffusion of the radiolytic species through the narrow gap from the bulk water to the crack tip and vice versa were also considered.

Calculation results confirmed that the concentrations of H₂O₂, one of the most important oxidants in BWR environments, in both bulk water and crack tip water under irradiation (energy deposition rate: 0.1 W/cm) were high enough to show high local ECP in both regions under NWC, but were high in the bulk water and low in the crack tip water under HWC. A high H₂ diffusion rate from the bulk to the crack tip enhanced the recombination reaction of H₂O₂ and H₂.     

Kinetics of Reduction of Np(V)by Hydroxylammonium Chloride in Perchloric Acid Solution

The kinetics of the reaction between Np (V) and hydroxylammonium chloride in perchloric acid solution was studied at 70°c. In perchloric acid solutions from 1.0 to 1.5 mol·dm^?3, the reaction rate is described by

?d[Np(V)]/dt = k{Np(V)][NH₃OHCl]^1.0[H⁺]^2.1

where the rate constant k = (6.7 ± 0.3) × 10^?2 mol^?3.7dm^11.1min^?1. Detailed studies of the dependence of the rate on the concentrations of hydroxylammonium ion and chloride ion showed that the reaction is much accelerated by chloride ion.     

Thermodynamic investigation of the possibility of using uranium dioxide to obtain hydrogen and oxygen from water

The equilibrium composition in the system U–O–H at temperatures 300–3000 K is calculated using thermodynamic functions. It is shown that the decomposition of water in H₂ and O₂ can be achieved in steps: first with only hydrogen being released at 1200–1500 K and then only oxygen at 2100–2500 K. When uranium dioxide interacts with water, mainly two uranium oxides are formed – U₄O₉ and UO₃; nine gaseous components can have appreciable partial pressure: UO, UO₂, UO₃, UO₂(OH)₂, H₂O, H₂, H, OH, HO₂. Translated from Atomnaya énergiya,Vol. 106, No. 2, pp. 76–79, February, 2009.     

Simulation Procedure for Multistage Water/Hydrogen Exchange Column for Heavy Water Enrichment Accounting for Catalytic and Scrubbing Efficiencies

The present paper gives a simulation procedure for multistage water/hydrogen exchange columns for heavy water enrichment using hydrophobic catalysts. The Murphree-type efficiencies for H₂O, HDO and D₂O are used as the scrubbing efficiencies for sieve trays. The reaction rate of H₂+D₂?HD is assumed to be very rapid and equilibrated at the outlet of the catalyst bed. The catalytic efficiencies defined for H₂O(g) and D₂O(g) are considered for the nonequilibrated exchange reactions: H₂+HDO(g)?HD+H₂O(g) and H₂+D₂O(g)?D₂+H₂O(g). Those efficiencies are treated as input variables for the simulation. The main calculational loop is based on the Newton-Raphson iteration, but the order of the Jacobian matrix is just equal to the number of sieve trays for vapor/hydrogen scrubbing. The procedure is applicable to solution of operating problems in a wide range of input and output specifications.     

Reactivity enhancement of chemical materials used in packed bed reactor of chemical heat pump

In this study, a mixture of expanded graphite (EG) and magnesium hydroxide (Mg(OH)₂) was used to enhance the thermal conductivity and reactivity of a magnesium oxide/water (MgO/H₂O) chemical heat pump, because EG is chemically stable and has high thermal conductivity and high moldability to form the heat exchange structure. Calcium chloride (CaCl₂) was also introduced into the mixture of EG and Mg(OH)₂ to ensure smooth diffusion of vapor in materials and enhance the fittability between EG and Mg(OH)₂. The reaction kinetics of pure Mg(OH)₂, a mixed material containing Mg(OH)₂ and CaCl₂ (termed MC), and a mixed material containing EG, Mg(OH)₂, and CaCl₂ (termed EMC) were examined under the same reaction conditions by performing thermobalance measurements. EMC exhibited a higher dehydration rate than the other materials. It also exhibited hydration reactivity at temperatures of up to 200 °C; at this temperature, pure Mg(OH)₂ exhibited low reactivity. The addition of CaCl₂ also enhanced the hydration reactivity of MgO because of the high water adsorption ability of CaCl₂ in EMC. A reaction rate equation for the hydration of EMC was proposed on the basis of an assumed reaction model. The thermal performance of a MgO/H₂O chemical heat pump manufactured using EMC was evaluated from this equation. EMC was concluded to have good potential for use as a packed bed material in the MgO/water chemical heat pump owing to its low cost, high hydration reactivity, high thermal conductivity, and high moldability to form the heat exchange structure.