Vaporization study on UO2 and (U1−yNby)O2+x by mass-spectrometric method

The vapor pressures over UO_2.000 and (U_1?yNb_y)O_2+x (y = 0.01, 0.05, x = 0.000–0.022) were measured by the mass-spectrometric method in the temperature range 2025–2343 K. The main gas species over UO_2.000 were observed to be UO₃(g) and UO₂(g) and those over (U_1?yNb_y)O_2+x were NbO₂(g), NbO(g), UO₃(g) and UO₂(g). The partial vapor pressures of almost all gas species over (U_1?yNb_y)O_2+x increased with increasing O/M (M = U + Nb) ratio. With increasing Nb content in (U_1?yNb_y)O_2.000, the partial vapor pressures of UO₂(g) and UO₃(g) decreased and those of NbO(g) and NbO₂(g) increased. The congruently vaporizing composition in the (U_1?yNb_y)O_2+x phase was estimated to be ($\text{U}{}_{\mathrm{0.985\pm 0.005}}\text{Nb}{}_{\mathrm{0.015\pm 0.005}}\text{)O}{}_{2.000}$ from the compositional dependence of the total vapor pressures. The partial molar enthalpy and entropy of oxygen of (U_1?yNb_y)O_2+x calculated from the partial pressures of gaseous species NbO₂(g) and NbO(g) were in fairly good agreement with those previously obtained by the present authors with a thermobalance.     

Etude aux rayons X a haute temperature du systeme U-C-N en presence d'un exces de graphite et sous pression controlee d'azote

Pressed samples of initial compositions $\text{“UC}{}_2\text{” + 1 C}$ and $\text{“UC}{}_2\text{” + 2 C}$ were made to undergo progressive reaction under a gradually increasing pressure of nitrogen () in a high-temperature X-ray diffraction apparatus which operated in the range 800–2000°C. In this way, the various equilibrium domains [ α or β “UC₂”, C, N₂], [α or β “UC₂”, UC_yN_x, C, N₂] and [ UC_yN_x, C, N₂], were successively made manifest; this permitted the lattice parameters and equilibrium pressures of the carbonitrides to be determined, and also their standard free enthalpies of formation and compositions to be evaluated. It was established, in particular, that the “dicaibide” and the “monocarbonitride” in equilibrium on the monovariant “plateau” are hypostoichiometric and hyperstoichiometric, respectively; the stability of these compounds is enhanced by their large nitrogen contents, which increase with the temperature. It was also shown that above 1410°C, a temperature rise leads to a substantial drop in the nitrogen content of the virtually stoichiometric monocarbonitride which is in equilibrium with graphite under Torr. Between 800°C and 1410°C and Torr, an excess of carbon coexists with α “U₂N₃” and β “U₂N₃”, and the lattice parameters of these phases were likewise determined.     

On the decomposition of the UN-CeN solid solution

The decomposition temperature of CeN, U_0.5Ce_0.5N and U_0.75Ce_0.25N has been measured as a function of nitrogen pressure. The results are , , , where p_N2 is in atm and T in K. Thermodynamic analysis based on the reaction, in which the solid solution, U_1?xCe_xN, decomposes into U_1?xCe_x(1) and $\text{1}\text{2}\text{N}{}_2\text{(g)}$, results in a very simple relation: logp_N2[U_0.75Ce_0.25N] = (1 ? x) logp_N2 [UN] +xlogp_N2 [CeN], where the assumption that the interaction parameters for U_1?xCe_xN and U_1?xCe_x(1) have the same value was made. The influence of solubility of nitrogen in U_1?xCe_x (1) has been also taken into consideration. The present experimental data are in good agreement with the equation derived from the present thermodynamic analysis.     

Mass spectrometric study of the evaporation of LiCrO2 as a corrosion product in the compatibility experiment of Li2O pellets with Fe-Ni-Cr alloys

A mass spectrometric Knudsen effusion study of the vaporization of LiCrO₂ in the temperature range of 1673–1873 K has shown the following: (1) The major vapor species over solid LiCrO₂ are Li(g), Cr(g), CrO(g), CrO₂(g) and LiCrO₂(g). (2) The vaporization process involves a sublimation reaction, LiCrO₂(s) = LiCrO₂(g), and a dissociation reaction, $\text{LiCrO}{}_2\text{(s) =}\text{1}\text{2}\text{Cr}{}_2\text{O}{}_3\text{(s) + Li(g) +}\text{1}\text{4}\text{O}{}_2\text{(g)}$. (3) The standard enthalpy of solid LiCrO₂ at 298 K is derived to be (?935 ± 21) and (?966 ± 18) kJ/mol from the 2nd and 3rd law treatments, respectively.     

Vaporization of thoria-urania solid solution

In order to elucidate the vaporization behavior of thoria-urania solid solution, its vapor species identification and partial vapor pressure measurement have been made in a high temperature mass spectrometer. Partial vapor pressure of $\text{UO}{}_3$(g) over $\text{Th}{}_1\text{-}{}_{\mathrm{y}}\text{U}{}_{\mathrm{y}}\text{0}{}_2$ was observed to remarkably change with time and this behavior can be explained from the extended Blackburn's thermochemical model. Activities of $\text{U0}{}_2$(s) and $\text{Th0}{}_2$(s) in $\text{Th0}{}_2\text{-U0}{}_2$ show positive deviation from Raoult's law. Absolute vapor pressures of $\text{U0}{}_2$(g) and $\text{UO}{}_3$(g) over $\text{Th}{}_1\text{-}{}_{\mathrm{y}}\text{U}{}_{\mathrm{y}}\text{0}{}_2$ are evaluated.     

Thermodynamics applied to compatibility of UN with Ni,Cr and Fe

The compatibility of UN with Ni, Cr and Fe has been studied from the viewpoint of phase equilibrium relationships. The free energies of formation of UNi₅ and U₂CrN₃ have been determined from the measurement of equilibrium pressures of nitrogen for the reactions, $\text{U}{}_2\text{N}{}_3\text{+ 10 Ni ? 2UNi}{}_5\text{+}\text{3}\text{2}\text{N}{}_2$ and $\text{U}{}_2\text{CrN}{}_3\text{? 2 UN +}\text{1}\text{2}\text{Cr}{}_2\text{N +}\text{1}\text{4}\text{N}{}_2$, respectively, and the values are expressed by $\text{ΔG(UNi}{}_5\text{) = ?67 650 + 22.9T}$, $\text{ΔG(U}{}_2\text{CrN}{}_3\text{) = ? 165 900 + 54.64 T}$. Combination of these and other thermodynamic data with some additional experimental work has generated a log $\text{P}$ versus $\text{1/T}$ diagram which shows the phase relationships in the U-N-Ni-Cr system. It is concluded that UN is not compatible with Ni and Cr simultaneously but, for practical purposes, is compatible with Fe below 1400°C.     

Transport properties of U0.7Ce0.3O2 − x

We have determined a number of transport properties of $\text{U}{}_{0.7}\text{Ce}{}_{0.3}\text{O}{}_{\mathrm{2-x}}$ at 1273 K for various deviations from stoichiometry and compared them with available results on $\text{(UPu)O}{}_{\mathrm{2\; ?\; x}}$. They are: the electrical conductivity, Seebeck coefficient, effective charge number and chemical diffusion coefficient.A very characteristic behaviour is observed for the electronic properties of $\text{(UCe)O}{}_{\mathrm{2\; ?\; x}}$. A p-type conduction for all the studied deviations from stoichiometry (up to $\text{x = 3 × 10}{}^{\mathrm{?2}}$) is interpreted in terms of a high electronic disorder in the stoichiometric compound. Electronic disorder at stoichiometry is probably less important in $\text{(UPu)O}{}_{\mathrm{2\; ?\; x}}$, which presents a sharp p-n transition at $\text{x = 5 × 10}{}^{\mathrm{?3}}$.Ionic transport properties obtained on $\text{(UCe)O}{}_{\mathrm{2\; ?\; x}}$ indicate an approximate proportionality between the ionic conductivity resulting from oxygen ions transport and the deviation from stoichiometry. Results available on $\text{(UPu)O}{}_{\mathrm{2\; ?\; x}}$ do not appear to be compatible with ours and some arguments are presented which cast doubt on their validity.     

Role of FP iodine in irradiation behaviors of nuclear fuel pins

During the operation of a reactor, the fuel ($\text{(U,Pu)O}{}_{\mathrm{2-x}}$ or $\text{UO}{}_{\mathrm{2\; +\; x}}$) reacts with its cladding (stainless steel or zircaloy). The role of iodine, a fission product, in this reaction has been examined. Out-of-pile experiments have been done to study the chemical vapor transport of components of stainless steel or pure iron due to iodine released by the electron bombardment of CsI vapor, with or without the excess cesium vapor, using X-ray microprobe analysis. Though addition of excess cesium vapor diminishes the transport rate of stainless steel components, there is still a possibility that cladding components are transported due to iodine in a reactor. Thermodynamic marker tests have been done to check the iodine pressure in the electron-irradiated CsI atmosphere. A short review of the previous relevant studies is also given.     

High-temperature mass-spectrometric studies of Te(s) and FeTe0.9(s)

The nature and composition of the vapour over a two phase mixture of Fe(s) and FeTe_0.9(s) as well as over Te(s) were determined by Knudsen effusion mass spectrometry. The partial pressuretemperature relationship of Te₂(g), $\text{log p(Pa) = ? 10759/T(K) + 11.12}$, and that of Te(g), $\text{log p(Pa) = ? 12227/T(K) + 11.03}$, were obtained over Fe-FeTe_0.9(s) in the temperature range 885–1048 K. The enthalpy change for reactions $\text{FeTe}{}_{0.9}\text{(s) = Fe(s) + 0.45 Te}{}_2\text{(g); FeTe}{}_{0.9}\text{(s) = Fe(s) + 0.9 Te(g)}$ and Te₂(g) = 2 Te(g) were derivedu'yas 98.5 ± 7, 214.4 ± 14and 257.0 ± 13 kJ respectively at 298 K. Tkie enthalpy and free, energy of formation of FeTe_0.9(s) at 298 K were determined as ? 28.5 kJ mol and ? 29.4 kJ mol respectively.     

The mechanism and kinetics of U2C3 formation by the synthetic reaction

In fast breeder reactors it is planned to use the fuel in the form of (U, Pu)C with a slight carbon hyperstoichiometry. It is therefore important to know under what conditions the synthetic reaction $\text{UC + UC}{}_2\text{U}{}_2\text{C}{}_3$ occurs, since the hyperstoichiometric carbon, which exists as a uranium dicarbide phase (UC₂) after manufacture, is to be converted to U₂C₃. The literature concerning the reaction conditions is partly contradictory. In this paper, the influence of thermal or mechanical pretreatments on the formation of U₂C₃ was investigated experimentally and is discussed in connection with other published data. It was found that the relative increase of the U₂C₃ nuclei density caused by grinding corresponds to the increase in surface of the ground material. A quantitative kinetic examination of the U₂C₃ formation was made and the activation energy for the synthetic reaction in powder was found to be $\text{394 ± 30}$ kJ/mol.     

Cross sections for the photoionization of H2(X1Σg+, vi = 0–14) with the formation of H2+(X2Σg+, vf = 0–18), and vibrational overlaps and Rn-centroids for the associated vibrational transitions

Cross sections for the photoionization of $\text{H}{}_2\text{(X}{}^1\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{)}$, initially in vibrational levels v_i = 0–14, with the production of $\text{H}{}_2{}^{\mathrm{+}}\text{(X}{}^2\text{Σ}{}_{\mathrm{g}}{}^{\mathrm{+}}\text{)}$ in vibrational levels v_f = 0–18 are tabulated for the full vibrational array at 24 photon wavelengths ranging from 912 Å to 450 Å. The associated vibrational overlap integrals 〈v_ifv_f〉 and R-centroids, $\text{〈v}{}_{\mathrm{i}}\text{|R}{}^{\mathrm{n}}\text{|v}{}_{\mathrm{f}}\text{〉}\text{〈v}{}_{\mathrm{i}}\text{|v}{}_{\mathrm{f}}\text{〉}\text{, n = 1}\text{and}\text{2}$ are also presented together with accurate curve fits of the bound-free ($\text{H}{}_2\text{-H}{}_2{}^{\mathrm{+}}\text{+ e}$) electronic matrix elements.     

Stopping power and energy straggling for small and large energy losses of MeV protons transmitted through polyester films

The stopping power S(E) and the energy straggling $\text{Ω}{}^2\text{(E}{}_{\mathrm{<mtext>i</mtext>}}\text{,}\text{ΔE}\text{)}$ of 0.5 MeV to 2 MeV proton beams in polyester (C₁₀H₈O₄) have been measured in transmission geometry. S(E) is found to be 2% to 4% lower than the Andersen and Ziegler tabulations, assuming the Bragg's additivity rule. The energy straggling $\text{Ω}{}^2\text{(E}{}_{\mathrm{<mtext>i</mtext>}}\text{,}\text{ΔE}\text{)}$ was measured at various beam energies E_i, mean energy losses$\text{ΔE}$ and target thicknesses (from 5.6 μm to 47 μm). The ratio $\text{Ω}\text{Ω}{}_{\mathrm{<mtext>B</mtext>}}$, (where $\text{Ω}{}_{\mathrm{<mtext>B</mtext>}}$ refers to the free electron model in the small energ approximation) of the order of 1.07 for small energy losses ($\text{ΔE}\text{E}{}_{\mathrm{<mtext>i</mtext>}}\text{, ≈ 0.05}$), increases to about 1.5 to 2 for large energy losses $\text{(}\text{ΔE}\text{/E}{}_{\mathrm{<mtext>i</mtext>}}\text{≈ 0.75)}$. For not too small beam energies (E_i?700 keV) and for our targets, a nearly universal curve $\text{Ω(E}{}_{\mathrm{<mtext>i</mtext>}}\text{,}\text{ΔE}\text{)}\text{Ω}{}_{\mathrm{<mtext>B</mtext>}}$ is obtained as a function of the mean relative energy loss $\text{ΔE}\text{E}{}_{\mathrm{<mtext>i</mtext>}}$. For $\text{ΔE}\text{E}{}_{\mathrm{<mtext>i</mtext>}}\text{?0.7}$, the beam energy sp Gaussian, were compared to theoretical predictions $\text{Ω}{}_{\mathrm{T}}$, valid for nearly symmetrical energy loss spectra. Our experimental results Ω are from 0 to 7% larger than the theoretical values $\text{Ω}{}_{\mathrm{<mtext>T</mtext>}}$ up to $\text{ΔE}\text{E}{}_{\mathrm{<mtext>i</mtext>}}\text{≈ 0.8}$. The surface roughness of the targets was ch influence on the experimental results was generally between 1% and 5%, depending on the targets and on E_i.     

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 $\text{δ-Li}{}_4\text{MoO}{}_5$.     

Sublimation thermodynamics of UO2 − x

The uranium activity has been measured between 1667 and 2175 K, and between $\text{U}\text{(l) +}\text{UO}{}_{\mathrm{2\; ?\; x}}$ and UO_2.0 using a mass spectrometer to study the behavior of UO(g). Equations are presented which allow the pressure of the vapor species to be calculated as a function of temperature and composition. The composition variation of activity agrees well with the average of the results from previous activity measurements, but the variation in the measured enthalpy of vaporization is in direct conflict.     

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.     

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.     

Electrical conductivity measurement and thermogravimetric study of pure and niobium-doped uranium dioxide

The electrical conductivity and nonstoichiometric composition of UO_2+x and (U_1?yNb_y)O_2+x (y = 0.01, 0.05 and 0.10) were measured in the range $\text{1282 ≦ T ≦ 1373}$ K and Pa by tie four inserted wires method and thermogravimetry, respectively. The electrical conductivity of (U_1?yNb_y)O_2+x plotted against the oxygen partial pressure indicated a minimum corresponding to the transition between $\text{n}$- and $\text{p-}\text{type}$ cone uction. The band-gap energy of (U_1?yNb_y)O_2+x was calculated to be $\text{(248 ± 12)}$ kJmol.^?1, independent of niobium content, which is nearly the same as that of UO_2+x. From the oxygen partial pressure dependences of both the electrical conductivity and the deviation $\text{x}$ of UO_2+x and (U_1?yNb_y)O_2+x, the defect structures in these oxides were discussed with the complex defect model consisting of oxygen vacancies and two kinds of interstitial oxygens.     

Electrical conductivity of UO2-ThO2 solid solutions

The electrical conductivities of UO_2+x. ThO₂ and their solid solutions, in thermodynamic equilibrium with the gas phase, were measured as a function of temperature, and of oxygen partial pressure in the temperatnre range 800 to 1200°C. The slope of the plot log α versus 1/$\text{T}$ for UO_2+x and UO₂-rich solid solutions exhibits a single region, whereas in the ThO₂-rich solid solutions it exhibits two regions. The pressure dependence of the conductivity (σ) in the UO₂-rich solid solutions can be represented by σ ∝ [O_i] ∝ in the range of $\text{0.01 < x < 0.1}$. Here, O_i is an interstitial oxygen and the partial pressure of oxygen, and it varies with the ThO₂ content. At greater deviation from stoichiometry ($\text{x ? 0.1}$) the presence of U₄O₉ or (Th U)₄O₉ phases influences the conductivity data. In ThO$\text{2}$ or ThO₂-rich solid solutions. P-type conduction at high oxygen pressures is interpreted as arising from the incorporation of excess oxygen into oxygen vacancies.     

Oxygen potential studies on hypostoichiometric uranium-cerium mixed oxide

Oxygen potential measurements on $\text{(U,Ce)0}{}_{\mathrm{2?x}}$ with $\text{Ce/U + Ce}$ ratios of 0.2 to 0.5 were carried out at 1073 and 1173 K by a gas equilibration method employing $\text{H}{}_2\text{0/H}{}_2$ carrier gas. A new wet chemical method based on potentiometric titrations was used to determine and cross check the O/M ratios of the equilibrated samples. Lattice parameters of stoichiometric oxides were determined by x-ray diffraction. The oxygen potentials and their dependence on O/M and composition are discussed and compared with available literature data.