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A mathematical simulation approach based on the general purpose Monte Carlo N-particle transport code MCNP was developed to predict the response of the XRF branch of the hybrid K-edge/K-XRF densitometer (HKED). The respective MCNP models for two different versions of HKED instruments currently in use were set up and experimentally validated. The setting up of the models involved comprehensive simulations of a bremsstrahlung photon source, the examination of different particle transport models, as well as the examination of different photon attenuation and X-ray fluorescence data libraries. The computation speed was significantly increased through the extensive use of the variance reduction techniques. The models were validated through the series of benchmarking experiments performed with a representative set of uranium, plutonium and mixed U/Pu reference solutions. The models and simulation approach developed are intended for: (i) establishing a consistent mathematical calibration approach for the XRF branch of the HKED instruments, which will require minimum calibration effort and time, (ii) extending the applicability of the HKED method to non-standard samples (e.g. U/Pu mixtures with unusual element ratios) and non-standard sample matrices (e.g. HM matrices from the pyro-processing of irradiated nuclear fuel) without investing a great deal of extra calibration work, and (iii) improving the accuracy of the measurements through the modelling of special measurement effects (e.g. the secondary excitation effect, the interference with X-ray escape peaks, the inconsistent unfolding of the overlapping peaks and peak background delineation in the measured XRF spectrum), which are difficult or sometimes impossible to account for experimentally.

tau-Ag{sub 1/2}Cu{sub 1/2}V{sub 2}O{sub 5} compound crystallises in the monoclinic system space group C2/m with cell parameters a=11.757(4) A, b=3.6942(5) A, c=9.463(2) A, and beta=114.62(2){sup o}. The structure is build up with V{sub 4}O{sub 10} D4 double layer. The silver and copper ions are located in two different oxygenated tunnels. Examination of electronic density maps shows that while the silver ions are located in defined crystallographic sites, the copper ones are fully delocalised over the whole tunnel. Comparison with delta-Ag{sub x}V{sub 2}O{sub 5} and epsilon-Cu{sub x}V{sub 2}O{sub 5} refined structure allows to define crystal chemistry parameters governing the ionic delocalisation and give clues to predict from structural consideration the expected electrical behaviour with the aim to make possible a structural design to enhance guest species reactivity. - Graphical abstract: The role of nature and amount of guest species on their respective localisation Evidence for full delocalisation of copper ions and diffusion pathways visualisation Display Omitted

One can design a critical system with fissile material in the form of a thin layer on the inner surface of a cylindrical neutron moderator such as graphite or beryllium. Recently, we have investigated the properties of critical and near critical systems based on the use of thin actinide layers of uranium, plutonium and americium. The thickness of the required fissile layer depends on the type of fissile material, its concentration in the layer and on the geometrical arrangement, but is typically in the μm–mm range. The resulting total mass of fissile material can be as low as 100 g. Thin fissile layers have a variety of applications in nuclear technology—for example in the design neutron amplifiers for medical applications and “fast” islands in thermal reactors for waste incineration. In the present paper, we investigate the properties of a neutron booster unit for spallation sources and isotope production. In those applications a layer of fissile material surrounds the spallation source. Such a module could be developed for spallation targets foreseen in the MYRRHA (L. Van Den Durpel, H. Ait Abderrahim, P. D’hondt, G. Minsart, J.L. Bellefontaine, S. Bodart, B. Ponsard, F. Vermeersch, W. Wacquier. A prototype accelerator driven system in Belgium: the Myrrha project, Technical Committee Meeting on Feasibility and Motivation for Hybrid concepts for Nuclear Energy generation and Transmutation, Madrid, Spain, September 17–19, 1997 [1]). or MEGAPIE (M. Salvatores, G.S. Bauer, G. Heusener. The MEGAPIE initiative: executive outline and status as per November 1999, MPO-1-GB-6/0_GB, 1999 [2]) projects. With a neutron multiplication factor of the booster unit in the range 10–20 (i.e. with a k eff of 0.9–0.95), considerably less powerful accelerators would be required to obtain the desired neutron flux. Instead of the powerful accelerators with proton energies of 1 GeV and currents of 10 mA foreseen for accelerator driven systems, similar neutron fluxes can be obtained with a medical cyclotron with proton energies of 350 MeV and currents of 2 mA. A basic limitation of such booster units is of course the limited lifetime of the thin fissile layer due to burn up and fission product poisoning. Nevertheless, for initial investigations of full power ADS systems, such units would allow the generation of the full neutron flux without the use of ADS class accelerators. The properties of the neutron booster have been intensively studied through systematic investigations of the different parameters (thickness, Hohlraum, reflector, etc.) of the concept. The calculations were performed with deterministic codes solving the neutron transport equation, and those results have been partially verified with the probabilistic code MCNP (Monte Carlo N-Particle Transport Code System). Layer of pure 235 U and 242m Am fuel have been initially investigated, but non-proliferation concerns led to the use of more conventional fuel layer. A specific geometry of a booster spallation source is presented. The effects of layer-burn up and lifetime are discussed.

(M2) and (M3) VO2 high pressure phases have been obtained at 65kbars by Chamberland. Both phases crystallize in the monoclinic system, (M2) with the space group C2/m and unit cell dimensions a = 9.083A, b = 5.763A, c = 4.532A, β = 90.3 ° whereas (M3) belongs to P2/m with a = 4.506A, b = 2.899A, c = 4.617A and β = 91.79 °. Based on this data ab initio structures have been elaborated and adjusted to fit their experimental powder patterns. (M2) structure exhibits two crystallographically different infinite parallel [VO4]n4n− strings of VO6 edge shared octahedra interconnected by apices alike in the rutile structure but the oxygens are hexagonally close packed. The (M3) variety shows also two different [VO4]n4n− strings but the general network now is rutile like slightly distorted. Vanadium atoms are situated in distorted oxygen octahedra, the V O bond lengths ranging from 1.66A to 2.15A with the V4+-V4+ pairing, V1-V1 = 2.70A and V2-V2 = 2.89A in (M2). In (M3) the V O distances range from 1.75A to 2.10A and V1-V1 = V2-V2 = 2.90A. The homopolar V4+ pairs evidenced in the (M2) form and the general unsymmetrical arrangement of oxygen about V4+ are in excellent agreement with the unusual physical properties of these two high pressure varieties.

The thermal decomposition of sodium oxalate in the presence of vanadium pentoxide in 1:2 molar ratio affords the vanadium bronze α'-NaV 2 O 5 . Complementary techniques such as thermogravimetry (TG), differential thermal analysis (DTA), gas analysis (GC), X-ray powder diffraction (XRD) and infrared spectroscopy (IR) were applied to provide information on the operative mechanism. A promotive effect of vanadium pentoxide on the decomposition of sodium oxalate takes place. Carbonate species and carbon monoxide could not be detected as intermediates during the decomposition which occurs over a wide range of temperatures, typically between 200 and 525°C. The progressive formation of Na x V 2 O 5 bronzes with x increasing up to unity with temperature leads us to propose that the carbon-carbon bond cleavage of the oxalate is promoted by electron transfer between the oxalate anion and the vanadium pentoxide matrix. Such behaviour is dependent on the redox potential of the metal ion initially present in the oxalate and of vanadium pentoxide.

The application of “classical” optimization methods of optimal control to large scale dynamic systems produces various problems because of the amount and complexity of the required calculation. An efficient means of getting round these difficulties is certainly the introduction of multilevel dynamic optimization techniques. Three possible methods are presented from the calculus of variations, for interconnected subsystems. The limitations of these techniques are given and suggestions are made in order to avoid this problem. A study of the optimal control of a synchronous machine is given to illustrate the application of this Kind of method”.

An inexpensive, high capacity counting device has been made for time analysis of light pulses by time-amplitude conversion. In order to allow for particularly long storage times, it has two channels with a capacity of 10 12 pulses. The result is protected from power cuts. The total recording time is measured by a quartz clock.

Ce(OSiPh 3 ) 4 · x DME (0.5 i )4 by exchange reaction with Ph 3 SiOH in dimethoxyethane (DME). Crystallization from a toluene-ether mixture yielded monocrystals of Ce(DME)(OSiPh 3 ) 4 · C 6 H 5 ,CH 3 , the crystal structure of which was determined by X-ray single-crystal techniques. It crystallizes in the monoclinic system, sp.gr. = P 2 1 / c , a = 28.078(6) A, b = 14.628(2) A, c = 18.739(5) A, β = 92.24(2)°, V = 7.691 A 3 , Z = 4, R = 0.074 for 4406 observations with I > 3σ( I ) and 319 variables. The Ce IV ion exhibits a (4 + 2) coordination by oxygen atoms with four short metal-silyloxy bonds (2 x 2.10(1) A; 2 x 2.13(1) A) and two long metal-DME bonds (2.58(1) A; 2.59(1) A).

Low pressure sublimation of tri-2,2′-bipyridinesilicon(0) leads to di-2,2′-bipyridinesilicon crystals. The unit cell is orthorhombic, space group Pbca, a 21.509, b 15.995, c 10.127 », Z = 8. The crystal structure was solved using 490 reflexions recorded at room temperature and refined down to a reliability index R = 0.046. The silicon atom is surrounded tetrahedrally by the bidentate 2,2′-bipyridine ligand with a mean NSiN angle of 100° and a mean SiN bond length of 1.72 ».

In this paper a new approach is suggested which enables one to avoid singularities in the goal coordination method for systems with state variable coupling. This makes the method of practical utility for the control of large dynamical systems. The new approach has the additional advantage of reducing the subproblem computations considerably. The method is illustrated on two classical examples, one which is linear and the other, non-linear.

The thermal decomposition of sodium and copper oxalates in the presence of vanadium pentoxide in a 0.5:1 molar ratio affords the vanadium bronzes α′-NaV 2 O 5 and β-Cu 0.5 V 2 O 5 . The use of complementary techniques such as TG, DTA, gaz analysis and X-ray powder diffraction permit to propose a reaction scheme for the formation of the above-mentioned compounds. Copper oxalate leads to CO 2 and metallic copper which then incorporates to V 2 O 5 . The reaction mechanism for sodium oxalate is less straightforward. A new phase, β′Na x V 2 O 5 has been characterized during the course of the formation of NaV 2 O 5 .

The structure of CuSe 2 O 5 is characterized by chains with Cu(II) ions aligned along the c -axis of the monoclinic lattice and bridged by Se 2 O 2− 5 ligands. The intrachain Cu-Cu separation is 3.98 A. The magnetic susceptibility of this compound, investigated in the 3.8–300 K temperature range, reveals a strong intrachain antiferromagnetic coupling. The J exchange parameter of the isotropic Heisenberg Hamiltonian — J Σ N i =1 S i S i + 1 ( N →∞) is found equal to − 95.1 cm −1 . From a previously described orbital model, an interpretation of this behavior is proposed: the interaction of the magnetic orbitals along the chain leads to a MO band, the width of which is related to the magnitude of the antiferromagnetic coupling. The top of the band corresponds to an antibonding interaction of the CuO 4 chromophores through the selenium lone pairs whereas the bottom of the band corresponds to a non-bonding interaction of these chromophores.