The vaporization and thermal stability of lithium molybdates

Two lithium molybdates, δ-Li₄MoO₅ and Li₂MoO₄, were evaporated and measured by high temperature mass spectometry. Various lithium and molybdenum oxide ions were observed, and their partial pressures were obtained. From the thermochemical calculation of evaporation, the heats of formation of the molybdates were obtained for the following reactions, $\text{Li}{}_2\text{O(c) +}\text{1}\text{2}\text{MoO}{}_3\text{(c) =}\text{1}\text{2}\text{δ-Li}{}_4\text{Mo0}{}_5\text{(c), ΔH}{}_{\mathrm{r.298}}{}^{\mathrm{o}}\text{= ? 120.4 kj.mol}{}^{\mathrm{?1}}$, and $\text{Li}{}_2\text{O(c) + MoO}{}_3\text{(c) = Li}{}_2\text{MoO}{}_4\text{(c), ΔH}{}_{\mathrm{r.298}}{}^{\mathrm{o}}\text{= ? 154.7 kj.mol}{}^{\mathrm{?1}}$. Thermochemically, $\text{Li}{}_2\text{MoO}{}_4$ is less stable than $\text{δ-Li}{}_4\text{MoO}{}_5$.     

Solubility of nitrogen in liquid lithium and thermal decomposition of solid Li3N

The solubility of nitrogen in liquid lithium was determined from 195 to 441°C by direct sampling and equilibrium nitrogen pressure over solid Li₃N were measured at eight temperatures between 660 and 778° C. The solubility data may be represented by log₁₀S = 3.323 ? 2107 T^?1, where S is in mol% Li₃N and T is in K. From a thermodynamic analysis of the combined solubility and decomposition data, the standard free energy of formation of solid Li₃N was estimated to be ΔG°_f(kcal/mol) = 33.2 × 10^?3T ? 39.1. For dilute solutions of Li₃N in lithium, the Sieverts' law constant, $\text{K}{}_{\mathrm{S}}\text{=}\text{N}{}_{\mathrm{Li3N}}\text{p}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, is given by In $\text{K}{}_{\mathrm{S}}\text{(}\text{atm}{}^{\mathrm{<mtext>?1</mtext><mtext>2</mtext>}}\text{) = ? 13.80 + 14 590 T}{}^{\mathrm{?1}}$, where N_Li3N is the mole fraction of Li₃N and p is the nitrogen pressure. The system Li-Li₃N appears to conform to a simple eutectic diagram in which the eutectic point occurs at ≈ 0.05 mol % Li₃N and 180.3° C. The melting point of Li₃N was found to be (813 ± 1)° C. Implications of the results of this study regarding the compatibility of liquid lithium with the strucutral materials of interest to fusion reactors are discussed.     

Solutions of lithium salts in liquid lithium: The interaction of hydride and of nitride with lead and with tin

Electrical resistivity studies of Li-Pb-H(N) ternary solutions ($\text{x}{}_{\mathrm{Li}}\text{? 0.96}$; $\text{T = 675 K}$) have shown that lead does not interact chemically with either LiH or Li₃N in liquid lithium solutions; similar studies of Li-Sn-H(N) ternary solutions ($\text{x}{}_{\mathrm{Li}}\text{? 0.96}$; $\text{T = 775 K}$) have shown that tin is also inert to both LiH and Li₃N in these solutions. Solubility data for LiH in liquid lithium-lead solutions ($\text{x}{}_{\mathrm{Pb}}\text{= 0.01}$; $\text{675 ? T(K) ? 735}$) have also been determined and compared with those for LiH in pure lithium. Enhanced solubilities are observed for the lithium-lead solutions; they are attributed to a decreased LiH activity in the ternary solutions vis-àvis the binary solutions. The significance of the results to the use of lead as a neutron multiplier in reactor lithium is discussed.     

Infrared spectroscopic analysis of OH− and OD− in crystalline Li2O as a function of chemical treatment

Infrared spectroscopy has been used to study the chemical form and approximate concentration of OH^? and OD^? in Li₂O single crystals as a function of chemical treatment. Infrared absorption maxima at $\text{(3671±0.5)}\text{cm}{}^{\mathrm{?1}}\text{and}\text{(2711±3.3)}\text{cm}{}^{\mathrm{?1}}$ were observed for OH^? and OD^?, respectively. The absorption coefficient for OD^? was determined to be $\text{4.0±0.4}$ absorbance units per mol part per million OD^? per mm of sample thickness. Vacuum baking of Li₂O crystals reduced the OH^? and OD^? concentrations to <50 mppm; baking in a low moisture-level D₂ environment at 600 to 800°C appeared to lead to volatilization of LiD from the Li₂O crystals; and baking in D₂ containing ($\text{350±50}$) mppm D₂O at 600 to 800°C produced a measurable quantity of LiOD. In all cases, the observed spectra indicated the presence of only one distinguishable form of OH^? or OD^? in the Li₂O lattice. Because of the close correspondence of the observed absorption maxima to reported values for pure LiOH and LiOD, the most consistent (although not conclusive) interpretation is that the OH^? and OD^? are present as a separate LiOH or LiOD phase at room temperature. Only limited conclusions can be drawn regarding the chemical state of OH^? and OD^? during the elevated temperatures exposures. An estimate of the approximate value for the solubility of tritium in Li₂O at 800°C was made using data from D₂/Li₂O isothermal exposure experiments — this value was ? 25 wppm.     

Tritium solubility in SUS-316 stainless steel

Tritium solubility in SUS-316 stainless steel was determined with a gas absorption method, in which tritium gas diluted by protium was used. The tritium absorption experiments were carried out at temperatures of 703, 804 and 903 K under pressures of 10, 30, 50 and 100 torr of tritiated hydrogen gas. The radioactivity of tritium dissolved in the specimen was measured by the method of liquid scintillation counting.The tritium solubility was derived from the experimental data by taking into consideration of isotopic equilibrium among H₂, T₂ and HT molecules. The determined tritium solubility can be expressed by the equation:

     

Isotope Exchange Capacity on Li4SiO4 and Comparison of Tritium Inventory in Various Solid Breeder Blankets

It has been pointed out by the present authors that it is essential to understand such mass transfer steps as diffusion of tritium in the grain of a breeder material, absorption of water vapor into bulk of the grain, adsorption of water on surface of the grain, and exchange capacity of tritium to be trapped to surface of the grain together with two types of isotope exchange reactions for evaluation of the tritium inventory in a solid breeder blanket under various conditions. The isotope exchange capacity on the Li₄SiO₄ surface is experimentally obtained in this study. Most of the properties required for evaluation of the tritium inventory for various blanket materials have been already quantified by the present authors. Then it has become possible to compare the tritium inventory in solid breeder blankets packed with either Li₂O, LiAlO₂, Li₂ZrO₃, Li₂TiO₃ or Li₄SiO₄ using the calculation model previously presented by the present authors.     

Synthesis and characterization of lithium silicate powders

Lithium-based ceramics, such as Li₂O, LiAlO₂, Li₄SiO₄, Li₂SiO₃, Li₂TiO₃and Li₂ZrO₃, have long been recognized as promising tritium breeding-materials for D-T fusion reactor blankets. Among these candidate materials, lithium orthosilicate (Li₄SiO₄) and lithium metasilicate (Li₂SiO₃) are recommended by many ITER research teams as the first selection for the solid tritium breeder. Li₄SiO₄ has even been selected as the breeder material for the helium-cooled solid breeder test blanket module (HCSB TBM) in China and EU. In present study, the processes of solid-state reaction between amorphous silica and Li₂CO₃ powders was studied by thermogravimetry analysis-differential scanning calorimetry (TGA/DSC); the lithium silicate powders were synthesized at 700, 800 and 900 ° C with Li:Si molar ratios of 0.5, 1, 2 and 4, respectively, using solid-state reaction method. The as-prepared lithium silicates were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results show that the phase composition and morphology of the as-prepared samples change with the different synthesis conditions. At low temperature of 700 °C, all samples contain the amorphous silica, and the major crystalline phase is Li₂SiO₃ with different microstructure for Li/Si ratio of 0.5, 1 and 2. As for Li/Si=4, 98% purity of Li₄SiO₄ can be obtained at 700 °C. At high temperature of 900 °C, the significant sinterization effect will occur in all samples and Li₄SiO₄ will even decompose. The results also show that pure Li₄SiO₄ can be synthesized by calcining at 800 ° C for 4 h, and its’ solid-state reaction synthesis may be divided into two steps:

(1)

515-565 °C: Li₂CO₃+SiO₂→Li₂SiO₃+CO₂;

(2)

565-754 °C: Li₂CO₃+SiO₂→Li₂SiO₃+CO₂ and then Li₂SiO₃+Li₂CO₃→Li₄SiO₄+CO₂.

While Li/Si=2, 99% purity of and pure Li₂SiO₃ can be obtained at 800 and 900 °C, respectively.     

High-temperature specific heat of UO2 ThO2, and A12O3

The high-temperature specific heat of solid UO₂, ThO₂, and Al₂O₃ can be represented by an equation of the form $\text{C}{}_{\mathrm{p(s)}}\text{= 3}{}_{\mathrm{n}}\text{RF(?}{}_{\mathrm{D}}\text{/T) + dT}{}^3$, (1) where ?_D is the Debye temperature, F(?_D/T) is the Debye function, $\text{d}$ represents contributions of the anharmonic vibrations within the lattice, and $\text{n}$ denotes the number of atoms per molecule. In the liquid the corresponding equation is $\text{C}{}_{\mathrm{p(1)}}\text{= 3}{}_{\mathrm{n}}\text{RF(?}{}_{\mathrm{D}}\text{/T) +}\text{h}\text{T}{}^2$, (2) where h is the anharmonic term. It is shown that for Al₂O₃ and UO₂, where experimental data for the liquid phase are also available, $\text{d}\text{h}$ has the same value, Indicating that both materials behave identically. If we compare the thermodynamic relationship $\text{C}{}_{\mathrm{p}}\text{? C}{}_{\mathrm{v}}\text{= Vα}{}^2\text{KT}$, (3) where V is the volume, $\text{α}$ the volume expansion coefficient, and $\text{K}$ the bulk modulus, with equation (1), It follows that d must be equal to $\text{Vα}{}^2\text{K}\text{T}{}^2$; the value of $\text{Vα}{}^2\text{K}\text{T}{}^2$ is calculated in the temperature region where d was obtained; within experimental error they are equal.     

Crystalline and electronic structures of lithium silicates: A density functional theory study

Lithium silicates, such as Li₂SiO₃ and Li₄SiO₄, are considered as favorable candidates for the tritium breeding materials of a deuterium–tritium type nuclear fusion reactor. Their bulk structural and electronic properties are investigated using pseudopotential plane wave (PPW) method within density functional theory (DFT). The optimized crystal structure parameters are well consistent with the experimental results. The results indicate that Li₂SiO₃ and Li₄SiO₄ are insulators with band gaps of about 5.36 and 5.53 eV, respectively. The valence electrons density of state reveal the covalency properties mainly resulting from the overlapping of O 2p and Si 3p orbital electrons in both lithium silicates. The nonbridging O (NBO) atoms and bridging O (BO) atoms of Li₂SiO₃ exhibit significantly different electron distributions. The Si 3s and 3p hybridization is observed in Li₂SiO₃, but not in Li₄SiO₄. The mechanical elastic constants reflected Li₄SiO₄ might be unstable in some degrees.     

Zum verhalten von tritium in reaktorgraphiten

An experimental investigation of the behaviour of tritium and hydrogen in HTR graphites is described. The diffusion and adsorption kinetic isotherms were measured using a specially developed vacuum apparatus with a variable temperature/time function. Hydrogen was estimated using a mass spectrometer. Tritium was analysed quantitatively after catalytic oxidation using a liquid scintillator. The diffusion coefficients for tritium in reactor graphite were found to be as follows: $\text{D = 0.024 cm}{}^2\text{/sec}\text{exp}\text{(?2.78 eV/kT)}$ and the desorption energies $\text{E}{}^{\mathrm{I}}{}_{\mathrm{des}}\text{= 1.38 ± 0.07 eV}$, $\text{E}{}^{\mathrm{II}}{}_{\mathrm{des}}\text{= 2.07 ± 0.13 eV}$, $\text{E}{}^{\mathrm{III}}{}_{\mathrm{des}}\text{= 3.27 ± 0.07 eV}$. The results indicated that the actual tritium inventory is largely determined by the buffering action of the graphite core. In the start-up phase of an HTR, the tritium concentration in the coolant gas remains relatively low, despite an increased rate of production because the tritium is being adsorbed by the graphite. In later stages of operation, after equilibrium has been reached, the tritium level will be supplemented by material from the hot core regions, so that the coolant activity is not determined by the production rate alone.     

Electrical conductivity of a sintered pellet of octalithium zirconate

The conductivity of a sintered pellet of Li₈ZrO₆ was measured in dry or wet argon atmosphere by the two-terminal technique with an AC Wheatstone bridge in the temperature range 350–1050 K. The temperature dependence of conductivity multiplied by temperature $\text{(σT)}$ of Li₈ZrO₆ in a dry environment had two transition points at 426 and 833 K. The activation energies of conductivity in high, middle and low temperature regions were 103.2, 52.1 and 37.4 kJ/mol, respectively. The activation energies of conductivity agreed well with those obtained by temperature dependence of spin-lattice relaxation time $\text{T}{}_1$ of ⁷Li with pulsed nuclear magnetic resonance.     

A determination of the solubility of lithium oxide in liquid lithium by fast neutron activation

The solubility of lithium oxide in liquid lithium has been measured in the temperature range of 195 to 734°C. A direct sampling method was employed in which nickel sampling tubes fitted with 2 μm pore size filters were used to take samples of the oxide-saturated lithium. The samples, still contained in their sampling tubes, were analyzed for oxygen content by a fast neutron activation method. A value for the oxygen blank attributable to the sampling tubes was derived from a regression analysis of the individual solubility data points. The resulting solubility values are considerably lower than those reported by previous investigators and can be represented by the equations In S(mol% Li₂O) = 6.054–6669/T, $\text{log S}{}^1\text{(wppm oxygen) = 6.992–2896/T}$. The results indicate that removal of oxygen from lithium to a level of about 7 wppm (the solubility at 200°C) by cold trapping or by filtration should be feasible.     

Effect of Nuclear Data Libraries on Tritium Breeding in a (D–T) Fusion Driven Reactor

In design a Deuterium–Tritium (D–T) fusion driven hybrid reactor, neutronics and nuclear data libraries have an essential role for reliable neutronics calculations. Therefore, nuclear data libraries are very important to calculate of the neutronic parameters and selection of tritium breeder materials to be used in the blanket. In this study tritium breeding performances of candidate tritium breeding materials, namely, Li₂O, LiH, Li₂TiO₃, Li₂ZrO₃ and Li₄SiO₄ in a (D–T) driven fusion–fission (hybrid) reactor is investigated based on three dimensional (3-D) and one dimensional (1-D) neutronic calculations. 3-D and 1-D neutron transport calculations are performed with Monte Carlo transport code (MCNP 4C), SCALE 5 and ANISN nuclear data codes to determine the tritium breeding ratio (TBR) of the blanket. The effects of different nuclear data libraries on TBR are examined and TBR calculation results are comparatively investigated.     

Diffusion of carbon in zirconium and some of its alloys

Diffusion of carbon in zirconium, zircaloy-2 and Zr- 2.5% Nb has been studied in the temperature range 873–1523K for zirconium and zircaloy-2 and 753–1523K for Zr-2.5% Nb alloy, using the residual activity technique. The diffusivities (in m²/s) in the α and β phases could be represented by $\text{D}{}_{\mathrm{C/\alpha -Zr}}\text{(873–1123K) = (2.00 ± 0.37) × 10}{}^{\mathrm{?7}}\text{exp [}\text{?(151.59 ± 2.51)}\text{RT}\text{]}$$\text{D}{}_{\mathrm{C/\alpha -Zircaloy-2}}\text{(873–1043K) = (1.41 ± 0.32) × 10}{}^{\mathrm{?7}}\text{exp [}\text{?(158.99 ± 3.14)}\text{RT}\text{]}$$\text{D}{}_{\mathrm{C/\alpha -Zr-Nb-alloy}}\text{(753–873K) = (4.68 ± 0.88) × 10}{}^{\mathrm{?7}}\text{exp [}\text{?(159.98 ± 2.91)}\text{RT}\text{]}$$\text{D}{}_{\mathrm{C/\beta \; Zr}}\text{((1143–1523K) = (8.90 ± 1.60) × 10}{}^{\mathrm{?6}}\text{exp [}\text{?(133.05 ± 1.46)}\text{RT}\text{]}$$\text{D}{}_{\mathrm{C/\beta \; Zircaloy-2}}\text{(1263–1523K) = (2.45 ± 0.61) × 10}{}^{\mathrm{?5}}\text{exp [}\text{?(150.29 ± 1.72)}\text{RT}\text{]}$$\text{D}{}_{\mathrm{C/\beta \; Zr-Nb\; alloy}}\text{(1143–1523) = (1.70 ± 0.42) × 10}{}^{\mathrm{?5}}\text{exp [}\text{?(158.20 ± 2.09)}\text{RT}\text{]}$The activation energies are given in kJ/mole. In the phase transition region, the diffusivities could be represented by the empirical relation: D = D_α^Cα · D_β^Cβ, where C_α, C_β are the concentrations of the two phases in the alloy and D_α, D_β are the extrapolated values of diffusion co-efficients in the α and β phases respectively.The results have been explained in terms of the interstitial mechanism of diffusion.     

Heterodiffusion de l'or,de l'argent et du cuivre dans les phases cubiques δ et ε du plutonium

The solute diffusion at infinite dilution of ¹⁹⁸Au and ^110mAg in cubic phases of Pu has been studied using the serial sectroning method. The solute diffusion coefficients in the b.c.c. ? phase can be expressed by: $\text{D}{}^{\mathrm{Au}}{}_{\mathrm{?Pu}}\text{= 5,7 × 10}{}^{\mathrm{?5}}$ exp(?10300/RT) cm²/s and $\text{D}{}^{\mathrm{Ag}}{}_{\mathrm{?Pu}}\text{= 4,9 × 10}{}^{\mathrm{?5}}\text{exp(?9600/RT) cm}{}^2\text{/s}$. The solute diffusion mechanism is interstitial of the dissociative type in both cases. These experiments confirm the activated interstitial model which has been proposed for self diffusion of ?Pu. Indeed the solute diffusion coefficients of Au and Ag are near of the self diffusion coefficients of Pu. The mechanisms are therefore interstitial in both cases. In the f.c.c. δ phase of Pu where self diffusion takes place by a vacancy mechanism, the solute diffusion coefficients of Au and Ag are near of the self diffusion coefficients of δ Pu. Solute diffusion takes place also by a vacancy mechanism. On the other hand, the extrapolation at infinite dilution of experiments of solute diffusion of Cu in ?Pu (Matano-Wagner coupling) gives the following results: $\text{D}{}^{\mathrm{Cu}}{}_{\mathrm{?Pu}}\text{= 1 × 10}{}^{\mathrm{?3}}\text{exp(?12300/RT)}$ cm²/s. The solute diffusion mechanism is interstitial of the dissociative type. In the ? phase the smaller the atomic radius the faster the migration: r_Co < r_Cu < r_?Pu < r_Ag = r_Au, and D^Co_?Pu > D^Cu_?Pu >D^Pu_?PU > D^Ag_?Pu ≈ D^Au_?Pu.     

Effect of water adsorption on tritium release from Li4SiO4

As a ceramic material proposed for tritium breeding in a fusion reactor blanket, lithium orthosilicate (Li₄SiO₄) is being examined in view of the influence of water uptake on tritium release behavior. In this work, out-of-pile tritium release experiments were performed on Li₄SiO₄ samples that were transferred and stored under different moisture environments. The water content was measured on the samples that were treated in similar conditions. Effects of water adsorption on the chemical form and temperature of released tritium were investigated. It is found that with the water content increases, the gaseous tritium fraction decreases and the proportion of low-temperature desorption of HTO increases. The results of this study can be used later for engineering and design activities for fusion reactor blankets.     

Heat capacity and thermal decomposition of lithium peroxide

The heat capacity of Li₂O₂ was measured by adiabatic scanning calorimetry from 301 to 566 K resulting in $\text{C}{}_{\mathrm{p}}\text{= 59.665 + 52.123 × 10}{}^{\mathrm{?3}}\text{T + 5.0848 × 10}{}^5\text{T}{}^{\mathrm{?2}}\text{(J/K · mol)}$. The thermal decomposition of Li₂O₂ was studied by continuous calorimetric measurements. An endothermic decomposition was observed above 570 K. The enthalpy of the thermal decomposition was determined to be 25.8 kJ/mol. The effects of atmosphere and heating rate of the decomposition were also studied for the powder and the compact specimens.     

Retention and release of water by sintered uranium dioxide

The release of water and hydrogen upon heating sintered $\text{UO}{}_2$ pellets was measured by a direct mass spectrometric method of vacuum outgassing. The technique avoids water loss by sample transfer and measures, rather than the cumulative release, the rate of release with a sensitivity of 1 μg of water (as D₂O) per hour. Exposure of high-density $\text{UO}{}_2$ pellets to water (liquid D₂O) results in negligible water adsorption. Water in pellets fabricated with especially high open porosity (5%) was driven off by a linear temperature ramp below 200°C. A drying model for this process was developed and applied to the data. Strongly bound water was introduced into high-density UO₂by sintering in an atmosphere of D₂O and D₂. Release of the water or hydrogen began at ~500°C and was complete only at the melting point of UO₂(2800°C). The release kinetics are not diffusion-controlled; rather the process is governed by the rates of desorption of bound hydrogen-bearing species from at least three binding sites in the solid characterized by interaction energies between 20 and 50 kcal/mol. The D₂O/D₂ ratio of the desorbed gas was > 1 and did not correspond to thermodynamic equilibrium with stoichiometric urania. Hydrogen and water release kinetics are comparable below 2&#x0303;000°C, suggesting a common bound precursor. The total hydrogen (as D₂O or D₂) absorbed in the specimens was between 2 and 4 μg/g $\text{UO}{}_2$.     

Lattice diffusion coefficient of oxygen in lithium oxide

Lattice diffusion coefficients of the oxygen ion in antifluorite-cubic Li₂O were determined employing polycrystalline samples, in the temperature range of 920 ~ 1130°C, by means of the gas-solid isotope exchange and solid-phase analysis technique; use was made of the relationship between the particle-size and grain-size dependences of the grain-boundary enhanced diffusion. The results were described by D = 1.52 × 10³ exp$\text{(?83.3 × 10}{}^3\text{/RT)}\text{cm}{}^2\text{s}$ and showed a good agreement with the previously reported results determined by the gas-phase analysis technique. Diffusion characteristics of the constituent ions in the antifluorite-cubic structure were discussed in comparison with those in fluorite-cubic crystals.