Dense sintered samples of Th1−xUxO2 solid solutions were prepared from the initial precipitation of oxalate precursors through two different wet chemical routes, based either on the direct precipitation of the cations or on the use of hydrothermal method. For both low-temperature precursors, the specific surface area was followed versus the heating temperature and the influence of the conversion step on the oxide powder reactivity was evidenced since it allowed to obtain reactive surfaces in the range of 15–45m2g−1 without any additional grinding step. From dilatometric studies, the operating conditions required for the complete densification of the Th1−xUxO2 pellets were set to a heat treatment of 3h at 1500°C. In these conditions, the density of the samples lies between 94% and 99% of the calculated value whatever the preparation method chosen which appeared very promising compared to the results already reported under inert atmosphere. The initial precipitation of low-temperature precursors thus allowed to lower the sintering temperature by about 100°C while the use of hydrothermal conditions significantly improved the cationic distribution in the sintered samples, as shown from EPMA statistical experiments.

Mixed actinide dioxides are currently studied as potential fuels for several concepts associated to the fourth generation of nuclear reactors. These solids are generally obtained through dry chemistry processes from powder mixtures but could present some heterogeneity in the distribution of the cations in the solid. In this context, wet chemistry methods were set up for the preparation of U1−xThxO2 solid solutions as model compounds for advanced dioxide fuels. Two chemical routes of preparation, involving the precipitation of crystallized precursor, were investigated: on the one hand, a mixture of acidic solutions containing cations and oxalic acid was introduced in an open vessel, leading to a poorly-crystallized precipitate. On the other hand, the starting mixture was placed in an acid digestion bomb then set in an oven in order to reach hydrothermal conditions. By this way, small single-crystals were obtained then characterized by several techniques including XRD and SEM. The great differences in terms of morphology and crystallization state of the samples were correlated to an important variation of the specific surface area of the oxides prepared after heating, then the microstructure of the sintered pellets prepared at high temperature. Preliminary leaching tests were finally undertaken in dynamic conditions (i.e. with high renewal of the leachate) in order to evaluate the influence of the sample morphology on the chemical durability of the final cohesive materials.

It is almost a decade since the first tabletop x-ray laser experiments were implemented at the Lawrence Livermore National Laboratory (LLNL). The decision to pursue the picosecond-driven schemes at LLNL was largely based around the early demonstration of the tabletop Ne-like Ti x-ray laser at the Max Born Institute (MBI) as well as the established robustness of collisional excitation schemes. These picosecond x-ray lasers have been a strong growth area for x-ray laser research. Rapid progress in source development and characterization has achieved ultrahigh peak brightness rivaling the previous activities on the larger facilities. Various picosecond soft-x-ray based applications have benefited from the increased repetition rates. We will describe the activities at LLNL in this area.

Extended X-ray absorption fine structure (EXAFS) has been utilized to investigate the local atomic structure around Th, U, and Pu atoms in polycrystalline mixed dioxides Th(1-x)M(x)O2 (with M = U, Pu) for x ranging from 0 to 1. The composition dependence of the two first-coordination-shell distances was measured throughout the entire composition range for both solid solutions. The first-shell distances vary slightly across the solid-solution composition with values close to those of the pure dioxide parents, indicating a bimodal cation-oxygen distribution. In contrast, the second-shell distance varies strongly with composition, with values close to the weighted amount average distances. Nevertheless, in both systems, the lattice cell parameters, deduced from the first- and second-shell bond determined by EXAFS, are very close to those measured from X-ray diffraction (XRD). They vary linearly with composition, accurately following Vegard's law.

XAFS spectroscopy using synchrotron radiation is an extremely suitable technique to study local atomic and electronic structure of mixed Th1-xUxO2 and Th1-xPuxO2 oxides. Despite of XAFS technique overall success, a pico-meter barrier (10(-2) angstrom) exists in XAFS data analysis. Here we present the dependence of the mixed oxide structure on composition, probed by XAFS with picometer accuracy. Complimentary XAFS spectra were measured at the Th, U, and Pu L-edges. We found that opposite to the lattice parameter obtained by XRD, the distances given by XAFS for the first and second shells do not follow completely neither Vegard's law nor the virtual crystal approximation (VCA). The Th-O, U-O and Pu-O distances obtained vary slightly upon dilution. These values are close to the ones expected from Vegard's law but are always smaller than the ones expected in VCA. The U-U(Th) and Th-Th(U) distances vary strongly upon dilution and the values are close to the ones expected from the VCA model but are always smaller than the ones expected by the Vegard law. The average lattice parameter calculated from XAFS data agrees well with a random distribution of metal (Th, U, Pu) and with one calculated from XRD data. With this complementary XAFS data on the local structure around two selected metal ions it is then possible to give a better view on the mixed oxides structure, the distribution of the two metal ions and local distortions in such medium crystal structure studied by XRD.

Metrology of XUV beams (X-ray lasers, high-harmonic generation and VUV free-electron lasers) is of crucial importance for the development of applications. We have thus developed several new optical systems enabling us to measure the optical properties of XUV beams. By use of a Michelson interferometer working as a Fourier-transform spectrometer, the line shapes of different X-ray lasers have been measured with a very high accuracy (Δλ/λ∼10 -6 ). Achievement of the first XUV wavefront sensor has enabled us to measure the beam quality of laser-pumped as well as discharge-pumped X-ray lasers. A capillary discharge X-ray laser has demonstrated a very good wavefront allowing us to achieve an intensity as high as 3×10 14 W cm -2 by focusing with a f=5 cm mirror. The sensor accuracy has been measured using a calibrated spherical wave generated by diffraction. The accuracy has been estimated to be as good as λ/120 at 13 nm. Commercial developments are underway. At Laboratoire d’Optique Appliquée, we are setting up a new beamline based on high-harmonic generation in order to start the femtosecond, coherent XUV optic .

The dissolution of Th1−xUxO2 was investigated through leaching experiments combined with X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) analyses. These experiments were performed in acidic and in oxidizing conditions (nitric solutions), for several compositions of solid solutions ranging from x=0.24 to 0.81. Static sequential experiments in acidic media performed at room temperature confirmed that higher concentration of uranium in the solid solution leads to higher release of uranium in the leachate whatever the pH. The normalized dissolution rate in oxidizing media is increasing all the more the content of uranium is increases in the mixed oxide. While for Th enriched solids, kinetic parameters remain similar to that of ThO2, in the case of uranium enriched solids, a drastic change is observed, and kinetic parameters are similar to that of UO2 ones. For x>0.50, the saturation is reached in the leachate after 100 days. XPS and EXAFS analysis on leached samples pointed out an oxidation of U(IV) at the surface for x<0.5, and in the bulk for x>0.5. Enrichment in Th is also observed at the surface of the solid, indicating the formation of a protective layer of hydrated thorium oxide, or hydroxide. Finally, the solubility product of secondary phase was determined. The values obtained are in good agreement with that of ThO2, Th(OH)4 and ThO2, xH2O reported in the literature.

Nous presentons dans une premiere partie une etude de miroirs interferentiels multicouches composes de divers materiaux pour des longueurs d'onde de l'ordre de 30 nm. Les quatre systemes etudies, Mo/Si recouvert de Si ou de B 4 C, B 4 C/Si et Mo/B 4 C/Si/B 4 C, presentent des pouvoirs reflecteurs de l'ordre de 24% autour de 30 nm. Les multicouches B 4 C/Si et Mo/B 4 C/Si/B 4 C presentent une meilleure selectivite que les multicouches Mo/Si. Sur ces dernieres, la couche de protection en B 4 C ne permet pas d'ameliorer la stabilite en temperature, par contre l'ajout de B 4 C aux interfaces ameliore considerablement cette stabilite. Dans une seconde partie, nous presentons la realisation et la calibration de lames separatrices a 13,9 nm pour un interferometre de Michelson. Les lames separatrices que nous avons developpees presentent des proprietes symetriques : les reflectivites de chaque face ne different que de 6%. Le produit reflectivite - transmission de ces lames peut etre optimise au-dessus de 2% pour une longueur d'onde fixee dans la gamme 12,6-15,5 nm.

The dissolution of thorium–uranium (IV) dioxide solid solutions has been investigated in nitric media as a function of several parameters (leaching time, acidity of the leachate, temperature and uranium substitution rate in the solid solution) by using batch experiments. The normalized dissolution rates were evaluated for Th1−xUxO2 with x<0.5, leading to the determination of the partial order related to the proton concentration, n, and to the corresponding normalized dissolution rate constant at pH=0, k′T,298K. The normalized dissolution rate of Th1−xUxO2 increases with the acidity of the leachate, and with the amount of uranium in the solid for a given pH. As the thermodynamic equilibrium is not yet reached in acidic media after two years for the solid solutions with x<0.5, only kinetics of the dissolution is described. The stoichiometry of the release of both actinides was verified until the precipitation of thorium occurred in the leachate for pH>2, while uranium was released in the solution under uranyl form. The partial order related to the proton concentration was determined for ThO2 and three compositions of solid solutions. The variation of the normalized dissolution rate with temperature allowed to determine the activation energy following the Arrhenius law (20, 33 and 16 kJmol−1 for ThO2 Th0.63U0.37O2 and Th0.47U0.53O2 respectively) at pH 2. The dissolution mechanism of Th1−xUxO2 solid solutions was explained by three steps: initial oxidation of the uranium at the surface, protonation at the U(VI), Th(IV), U(IV) sites, then finally detachment of the surface complexes.

The first longitudinal coherence measurement of the transient inversion collisional x‐ray laser is presented. The scheme under study is the picosecond output of the Ni‐like Pd x‐ray laser at 14.68 nm generated by the COMET laser facility at LLNL. Interference fringes were generated using a Michelson interferometer setup in which a thin multilayer membrane was used as a beam splitter. Longitudinal coherence measurements were made for this transition by changing the length of one interferometer arm and measuring the resultant variation in fringe visibility. The nature of this dependence also allows for an estimation of the linewidth of the lasing transition to be made. Analysis indicates a linewidth of ∼0.3 pm which is a factor of four less than previous measurements on quasi‐steady state x‐ray laser schemes.

We present the longitudinal coherence measurement of the transient inversion collisional x-ray laser for the first time. The Ni-like Pd x-ray laser at 14.68 nm is generated by the LLNL COMET laser facility and is operating in the gain-saturated regime. Interference fringes are produced using a Michelson interferometer setup in which a thin multilayer-coated membrane is used as a beam splitter. The longitudinal coherence length for the picosecond duration 4d(1)S(0) --> 4p(1)P(1) lasing transition is determined to be similar to400 mum (1/e HW) by adjusting the length of one interferometer arm and measuring the resultant variation in fringe visibility. This is four times improved coherence than previous measurements on quasi-steady state schemes largely as a result of the narrower line profile in the lower temperature plasma. The inferred gain-narrowed linewidth of similar to0.29 pm is also substantially narrower than previous measurements on quasi-steady state x-ray laser schemes. This study shows that the coherence of the x-ray laser beam can be improved by changing the laser pumping conditions. The x-ray laser is operating at 4-5 times the transform-limited pulse.

Metrology of XUV beams and more specifically X-ray laser (XRL) beam is of crucial importance for development of applications. We have then developed several new optical systems enabling to measure the x-ray laser optical properties. By use of a Michelson interferometer working as a Fourier-Transform spectrometer, the line shapes of different x-ray lasers have been measured with an unprecedented accuracy (deltalambda/lambdasimilar to10(-6)). Achievement of the first XUV wavefront sensor has enable to measure the beam quality of laser-pumped as well as discharge pumped x-ray lasers. Capillary discharge XRL has demonstrated a very good wavefront allowing to achieve intensity as high 3*10(14) Wcm(-2) by focusing with a f = 5 cm mirror. The measured sensor accuracy is as good as lambda/120 at 13 nm. Commercial developments are under way.

With the aim of realizing a Michelson interferometer working at 13.9 nm, we have developed a symmetrical beam splitter with multilayers deposited on the front and back sides of a silicon nitride membrane. On the basis of the experimental optical properties of the membrane, simulations have been performed to define the multilayer structure that provides the highest reflectivity-transmission product. Optimized Mo-Si multilayers have been successfully deposited on both sides of t he membrane by use of the ion-beam sputtering technique, with a thickness-period reproducibility of 0.1 nm. Measurements by means of synchrotron radiation at 13.9 nm and at an angle of 45 degrees provide a reflectivity of 14.2% and a transmission of 15.2% for a 60% s-polarized light, close to the simulated values. Such a beam splitter has been used for x-ray laser Michelson interferometry at 13.9 nm. The first interferogram is discussed.

Progress in a four‐wave sum difference frequency generation scheme to convert quasi‐coherent soft X‐ray radiation to shorter wavelengths, using a plasma medium is discussed. Experimental results and calculations involving the Ne‐like Ni XRL (λXRL = 0.023 μm) mixed in phase‐matched mode with a Nd:glass laser at the second harmonic (λOPT = 0.52 μm) in a Na‐like Argon plasma are presented. A 25 mm single slab target of Ni pumped with ∼ 200 J of energy from 5 optical beams has routinely produced ∼ 5 mJ of XRL energy at 23.1 nm. The XRL was split and recombined with the Nd:glass laser in the Na‐like Ar plasma. Considerations for phase matching the XRL and optical beams in the ionized gas jet are also discussed.

A measurement of the time‐resolved emission of transient X‐ray laser pulses is described. An ultra‐fast X‐UV streak camera set at the focal plane of a flat field Spectrometer was used to obtain the temporal evolution of X‐UV spectra of Ni‐like Ag and Ne‐like Ni plasmas in a small wavelength range covering the lasing lines. The total time resolution of the device was of 1,1 ps. The FWHM duration of the X‐ray laser pulse was measured to be 3.5 ps for Ni‐like Ag (3d94d(3/2,3/2)J=0→3d94p(5/2,3/2)J=1, λ=13.9 nm) and 13±2 ps for Ne‐like Ni (2p53p(1/2,1/2)J=0→2p53s(1/2,1/2)J=1, λ=23.1 nm). Lasing signal was also observed on the 4f‐4d laser line (λ=16nm) in Ni‐like Ag,, in both time‐integrated and time‐resolved spectra.

Obtaining an X-ray laser emission from plasmas, created and driven by an intense IR laser, has been pursued at the Laboratoire de Spectroscopie Atomique et Ionique (LSAI) for several years. At present, we operate various types of X-ray lasers driven by IR laser pulses of different durations (600 ps, 100 ps, and 600 ps/1 ps). A review of different techniques used at the LSAI to produce a strongly amplified emission using the collisional excitation pumping is presented. In the second part of this paper, to illustrate the potential of the X-ray lasers for applications, we present the main results obtained with an X-ray laser emitting at 21.2 nm in a study of surface defects of a niobium cathode, induced by strong electrical fields. We also describe a novel imaging interferometry device using an X-ray laser as a source and designed as a tool for high-resolution diagnostic of dense plasmas.

Collision-pumped soft x-ray laser devices operating in quasi-steady-state mode in the 5-25 nm wavelength region now routinely produce saturated outputs, corresponding to about 10(13) - 10(14) photons in pulses of similar to50 psec duration. These partially coherent beams are extremely bright, single-shot sources of radiation with a range of demonstrated and potential applications. Here, we review some recent developments in production and characterisation of Ne-like and Ni-like soft x-ray lasers pumped by the Vulcan laser at the Central Laser Facility, UK. Stemming from this, we examine some application options including radiography, interferometry, Thomson scattering and non-linear optics.

Abstract Thorium dioxide is an important material for the nuclear industry. In the last decade, there has been a renewal of interest in studying the feasibility of thorium based fuel reactor to decrease the minor actinides production during the burn-up. Furthermore the resistance of the thorium dioxide to aqueous corrosion can make this material attractive for immobilizing tetravalent actinides. Leaching tests of powdered samples of thorium dioxide calcinated at 1300°C showed that the normalized dissolution rate is very low (between 10−6 and 10−7 g/(m2.d) in acidic media, and 10−9–10−10 g/(m2.d) after pH>3 when the formation of colloïdes occurs. Thorium dioxide which is isomorphic with the actinide dioxides such as UO2, PuO2 allows the formation of solid solutions whatever the concentration of the actinide. Several solid solutions Th1−xUxO2 were synthesized with mole-ratios Th/(U+Th) ranging from x = 0 to 1. X-ray powder diffraction data allowed to check that the Vegard’s law is respected in all the range, and specific surface area was also measured. The resistance of the solid-solution to aqueous corrosion was measured as a function of several parameters (leaching time, leachate acidity, uranium concentration) and the kinetics of solid solutions dissolution was determined as a function of the uranium concentration. The stoechiometry of the release of both actinides was verified, however due to the oxidization of U (IV) in U (VI) in contact with the leachate, the dissolution rate of both thorium and uranium increases with the thorium substitution in the solid by uranium (TV).