Daniel Baron

Ultrasonic study of UO2: effects of porosity and grain size on ultrasonic attenuation and velocities

Abstract Ultrasonic techniques applied to nuclear fuel characterisation are developed in our group since 1996. Before applying our methods to irradiated fuel, we are searching sensitive parameters which could give interesting information. That is the reason why only results concerning non-irradiated UO2 are presented. This paper mainly deals with the investigation of a relevant acoustic parameter: the attenuation. Indeed, the ultrasonic attenuation in UO2 as a function of the operating ultrasonic frequency has been measured on samples with various microstructures: variable fraction volume porosity (1–6%) and grain size (10– 90 μm ). Using a 15 MHz operating frequency, no attenuation has been observed. With frequencies around 40 MHz, we show that the measured ultrasonic attenuation is only sensitive to grain size (no effect of porosity has been observed). On the contrary, the ultrasonic velocities (which are very sensitive to porosity) are not affected by the sizes of the grains. These reversed and non-correlated effects constitute an interesting tool for UO2 study because two aspects of the microstructure can be studied separately with ultrasonic waves.

The δ-selective Opioid Peptide Dermenkephalin and theμ-selective Hybrid Peptide Dermenkephalin-[1-4]-Dermorphin-[5-7] Display Strikingly Different Conformations Despite Identical Tetrapeptide N-Termini. A Quantitative 2-D NMR and Molecular Modeling Analysis

The selective recognition of the aminoterminal binding pharmacophore Tyr-D-Xaa-Phe of the opioid heptapeptide dermorphin, Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2 (DRM)1, and of dermenkephalin, Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2 (DREK), by the mu-opioid receptor and delta-opioid receptor, respectively, depends upon the constitution / conformation of the C-terminal tripeptide. The hybrid peptide DREK-[1-4]-DRM-[5-7] is very potent at, and exquisitely selective for the mu-opioid receptor, and differs only from dermenkephalin by its C-terminal tripeptide. Comparison of the structural features of DREK-[1-4]-DRM-[5-7] and dermenkephalin by nmr analysis and molecular modeling revealed striking differences, as well in the trans (Tyr5 - Pro6) isomer (population 75%) than in the cis isomer.. Whereas the folded C-terminal tail of dermenkephalin influenced the tertiary structure of the N-terminal tetrapeptide and placed the Tyr1 and Phe3 aromatic rings in definite orientations that are best suited for the delta-receptor, there were only weak contacts, as shown by NOE data, between the aminoterminal and carboxyterminal parts of the hybrid peptide. This promoted increased flexibility of the whole backbone and relaxed orientations for the side-chains of Tyr1 and Phe3 that are compatible with the mu-receptor but unsuitable for the delta-receptor. The steric hindrance introduced by Pro6 in DREK-[1-4]-DRM-[5-7], plus the absence of large hydrophobic side-chains in positions 5 and 6 may prevent close contacts between the N-terminal and C-terminal domains and reorientation of the main pharmacophoric elements Tyr1 and Phe3.

Two-Dimensional 1H NMR Study of a Tetradecapeptide with the Consensus Sequence Arg5-Asp-Val-Arg-Gly9: Structural Effects of the Outside Substitution Ser12 by Ala12

Conformation of a tetradecapeptide with a RXVRG consensus sequence, Arg5-Asp-Val-Arg-Gly9, found in several precursors of antibacterian peptides, was investigated in dimethylsulfoxide solution by proton NMR spectroscopy. Complete resonance assignments and conformational parameters were obtained through correlated (COSY) and nuclear Overhauser (NOESY) techniques. The 3J(alpha H, beta H) coupling constants and the intramolecular NOE, NH...beta H, were used to analyse the conformers around the C alpha-C beta bond and, in four cases, to obtain stereospecific assignments. Use of restraints derived from NOE connectivities and 3J(NH, alpha H) coupling constants allows the determination of a range of phi and psi dihedral angles for all the residues in the sequence. The present NMR results provide favourable evidence for the formation of two bends in the consensus sequence of the tetradecapeptide. The first one has most of the features of a Glu4-Val7 beta-turn (low temperature coefficient of the Val7NH chemical shift, Arg5 alpha H...Val7NH and Asp6NH...Val7NH NOE correlations). The second one exhibits only the Asp6 alpha H...Arg7NH and Val7NH...Arg8NH NOE interactions. These consensus sequence organizations proposed were confirmed by molecular modeling based on low potential energy structure on the [4-9] fragment with high agreement of NOE data. Overall, the substitution of Ser12 by Ala12 shifts the conformation of the hydrophobic moiety [10-14] towards a quite random coil structure in this fragment and strongly destabilizes the folded structures of the consensus domain where only one NH (Val7) is solvent-shielded opposed to three (Asp6 to Arg8) in the [Ser12] tetradecapeptide. These conformational changes could be related to the processing enzyme activities on these model oligopeptides.

Ultrasonic method for nuclear fuel rods pressure and gas composition released measurement

The objective of this work was to develop a non-destructive acoustic method giving an easy access to two important pieces of information in the irradiated fuel rods: the pressure and the composition of the internal gas mixture in the upper plenum of a standard LWR fuel rod. A first attempt started in 1993 made possible the development of a focused sensor, able to inject acoustic power from a piezoelectric transducer, through the fuel rod cladding and excite the internal gas mixture. This step was achieved in 1999 and covered by a first patent [1]. However, the initial problem was not totally solved because of the presence of the upper spring in the LWR fuel rod. This spring mainly induced a large decrease in the acoustic amplitude response and this solution was not applicable. The method has been revised recently for several aspects: the design of the sensor to optimize the acousmtic power injection through the cladding rod, and the entire experimental protocol, including the signal processing in the time and frequency spaces. A single λ/2 layer (instead of the standard solution in λ/4) of low impedance material (compared to the transducer and tube wall impedance) as water, was found to be a better matching layer.

Full-scale hot cell test of an acoustic sensor dedicated to measurement of the internal gas pressure and composition of a LWR nuclear fuel rod

A full-scale hot cell test of the internal gas pressure and composition measurement by an acoustic sensor was carried out successfully between 2008 and 2010 on irradiated fuel rods in the LECA-STAR facility at CADARACHE Centre. The acoustic sensor has been specially designed in order to provide a non-destructive technique to easily carry out the measurement of the internal gas pressure and gas composition (mainly Helium-Xenon mixture, with a small amount of Krypton) of a LWR nuclear fuel rod. This sensor has been achieved in 2007 and is now covered by an international patent. We performed the gas characterisation contained in irradiated fuel rods. The acoustic method accuracy is now ±5 bars on the pressure measurement result and {±}0.3% on the evaluated gas composition. The results of the acoustic method were compared to puncture results (destructive sampling). Another significant conclusion is that the efficiency of the acoustic method is not altered by the irradiation time, and possible modification of the cladding properties. The sensor-operating characteristics have not been altered by a two-year exposure in the hot cell conditions. These results make it possible to demonstrate the feasibility of the technique on irradiated fuel rods. The transducer and the associated methodology are now operational.

Structure determination of a tetradecapeptide mimicking the RXVRG consensus sequence recognized by a Xenopus laevis skin endoprotease: an approach based on simulated annealing and 1H NMR.

SummaryThe tetradecapeptide of sequence H-Asp-Val-Asp-Glu-Arg5-Asp-Val-Arg-Gly9-Phe-Ala-Ser-Phe-Leu-NH2 is recognized by a putative maturation endoprotease of the Xenopus laevis skin, which cleaves between Arg8 and Gly9. A conformational search has been performed on this peptide by simulated annealing calculations. Two different models in agreement with the NMR data were found. The conformational difference between the two types of model is located in the consensus sequence, i.e., from Arg5 to Gly9.

Ultrasonic critical angle reflectometry applied to porous nuclear fuel mechanical characterization

An ultrasonic reflectometer working from 2 to 10 MHz on zones around 4 mmhas been built in order to assess in a non destructive way the reflection coefficient R(θ, f) as a function of incident angle and frequency. Many experiments were performed on various materials (glass, aluminium,…) showing that with this goniometer and the related signal processing it is possible to assess very accurately R(θ, f) for bulk or thin samples (Lamb modes). Longitudinal and shear velocities (VL, Vs) deduced from R(θ, f) have been used to determine the elastic moduli E and G which values match very favourably with literature. Hence the Ultrasonic Critical angle Reflectometry is a powerful tool for materials characterization. In a second step, this device has been used on civil power plants fuel (UO2) with various volume fraction of porosity. The elastic moduli obtained are in good agreement with the literature data. The relations between VL, VS and the porosity (p) on the one hand, and E and G with p on the other hand will ...

Nuclear magnetic resonance study of Ln3+ complexes with aspartate and glutamate residues. Thermodynamic and structural analysis

Binding of aspartate and glutamate residues with the lanthanide cation Yb3+ has been studied in D2O, involving two kinds of peptide backbones: N-acetylamino acid methyl esters and cyclic dipeptides. A method is described for the quantitative analysis of the lanthanide-induced shifts (LIS) from which the best model of solution, the binding constants, the induced shifts, and the geometry of the complexes have been determined. Aspartate and glutamate derivatives are found to form both 1 : 1 and 2 : 1 complexes (peptide:cation). Backbone flexibility and, to a lesser extent, side-chain length seem to govern the magnitude of the binding constants. Only one oxygen of the carboxylate group is involved in the complex. The Yb3+⋯ O– distances are found to be different according to the Asp (2.70–2.75 A) and Glu (2.50–2.55 A) residues. The predominant conformer has an extended side-chain so no chelation occurs with the peptide backbone.

Conformational study based on simulated annealing and 1H NMR of peptides comprising the consensus sequence Arg5-Asp-Val-Arg-Gly9: effects of the substitution Ser 12 by Ala 12 or of 3 residues deletion at the N-terminus.

A conformational search by simulated annealing has been performed on two peptides derivated from the tetradecapeptide used to isolate the Xenopus laevis skin maturation RXVRG-endoprotease. The Ala 12 derivative, obtained by substitution in the hydrophobic C terminal fragment and the undecapeptide 4-14, obtained by deletion of an acidic rich tripeptide, were studied. No unique structure has been found for the tetradecapeptide Ala 12. This structural disorganization could explain the loss of activity of the endoprotease towards the substituted peptide. For the undecapeptide, two different models in accordance with the NMR data were found. The conformational differences between these two models are located in the consensus sequence and in each case an hairpin-like conformation is observed. These results could be related to the enhanced cleavage activity of the maturation enzyme. The obtained structures are also compared with those of the original tetradecapeptide.