Les aspects industriels de la fin de cycle du combustible nucléaire et du recyclage du plutonium en France

Industrial aspects of the end of the nuclear-fuel cycle and of plutonium recycling in France. In France, 58 Pressurised Water Reactors, 60 GWe altogether, are producing 410 TWh per year. A part of UOX spent fuel is reprocessed and 8.5 metric tons of Pu are recycled in MOX fuel by EDF. The performances of the two reprocessing plants, UP3 and UP2, are outstanding. Volumes of radioactive residues are very low. The EPR reactor is ready for the next future. A national debate will take place in 2006 to select the options for radioactive-waste management. New ways will have to be implemented to draw the best of Pu recycling and several concepts of reactors are investigated for the far future. Numerous challenges will have to be met. To cite this article: H. Masson et al., C. R. Chimie 7 (2004).     

Effect of gleysation on remobilization of radionuclides in waterlogged soils

Joint leaching of iron and other macro- and microelements including radionuclides from radioactively contaminated floodplain soil of the Yenisei River is studied on miniecosystems. In developing anaerobic conditions, the redox potential E_h decreased to +140 mV, causing partial reduction of iron and solubilisation of iron hydrous oxide and organomineral films of the soil. Leaching of stable elements and radionuclides with water proceeds parallel to that of iron. To accelerate reductive mobilization of iron, 0.4% aqueous ascorbic acid was used instead of water as the leaching liquid phase. In this case, E_h approached –60 mV, and the leaching of stable and radioactive elements considerably increased, the effect being most pronounced for plutonium. To cite this article: E.K. Legin et al., C. R. Chimie 7 (2004).

Résumé

Effet du passage en milieu anaérobie sur la mobilisation des radionucléides dans les sols humides. La lixiviation simultanée du fer et d’autres éléments de sols contaminés de la vallée de la rivière Yénései a été étudiée. En imposant des conditions anaérobies, le potentiel redox chute à 140 mV, causant une réduction partielle du fer trivalent et la solubilisation de l’hydroxyde de fer et du film organominéral du sol. Les lixiviations du fer, des autres éléments et, par conséquent, des radionucléides contaminant le sol vont de pair. Pour accélérer la mobilisation réductive du fer, une solution d’acide ascorbique à 0,4% a été utilisée à la place de l’eau. Dans ces conditions, la valeur de E_h approche –60 mV et la lixiviation des éléments augmente considérablement, cet effet étant plus prononcé pour le plutonium. Pour citer cet article : E.K. Legin et al., C. R. Chimie 7 (2004).     

Study of Am and Pu behavior in alkaline media

The redox chemical behavior of americium and plutonium species in alkaline media has been studied, mainly two dis/reproportionation reactions: 2 Pu(VI) = Pu(V) + Pu(VII) and Am(III) + Am(V) = 2Am(IV). It was found that Am(VI) is readily reduced by the simple action of water, and that radiolysis has no effect. This is a marked departure from the behavior of Am in other media, where alpha particle self-irradiation generates redox active species that affect the Am oxidation state. An Am(VII) compound has been synthesized. All results are interesting for the treatment of alkaline high-level radioactive wastes. To cite this article: A.P. Nikonov et al., C. R. Chimie 7 (2004).

Résumé

Étude du comportement de Am et Pu en milieu alcalin. Le comportement redox de Am et Pu a été étudié en milieu alcalin, notamment les dismutations 2 Pu(VI) = Pu(V) + Pu(VII) et 2 Am(IV) = Am(III) + Am(V). Am(VI) est rapidement réduit par l’eau et la radiolyse n’a pas d’effet, contrairement à ce qui se produit dans d’autres milieux. Des indications ont été obtenues sur un composé de Am(VII). Les résultats obtenus sont intéressants pour le traitement des déchets nucléaires encore entreposés en milieu basique. Pour citer cet article : A.P. Nikonov et al., C. R. Chimie 7 (2004).     

Sur un nouvel hétéropolytungstate lacunaire à squelette X2W21 dérivé du polytungstate [H6X2W22O76] (X = Sb,Bi)

A novel vacant heteropolytungstate of skeleton X₂W₂₁ derived from the polytungstate [H₆X₂W₂₂O₇₆]^8– (X = Sb,Bi). Five heteropolytungstates Na₈[H₆Sb₂W₂₁O₇₃]·16.5 H₂O, Na₈[H₆Sb₂W₂₁O₇₃]·19 H₂O, Na₈[H₆Bi₂W₂₁O₇₃]·16.5 H₂O, Li₂Na₆[H₆Sb₂W₂₂O₇₆]·14.5 H₂O, and Na₅[H₇Sb₂NiW₂₁O₇₃(H₂O)₃]·20 H₂O were prepared and studied by X-ray diffraction. The values of isotropic temperature factors of the external tungsten atom and of its three terminal oxygen atoms were compared to those of same-type atoms, firstly of the same polytungstate, and secondly inside this five-compound family. This analysis led to the conclusion that the vacant assembly X₂W₂₁O₇₃ (X = Sb,Bi) does exist and that the vacancy may be filled up with a first row transition metal such as nickel. To cite this article: Y. Jeannin, C. R. Chimie 7 (2004).     

Lavoisier and meat stock

Antoine-Laurent de Lavoisier published his results on ‘meat stock’ preparation in 1783. Measuring density, he stated that ‘food principles’ were better extracted using a large quantity of water. This result was checked. To cite this article: H. This et al., C. R. Chimie 9 (2006).     

Synthesis of the Heaviest Chemical Elements—Results and Perspectives

All 17 artificially produced elements heavier than uranium were discovered by nuclear-chemical synthesis. The three heaviest ones–elements 107, 108 and 109–were synthesized at the heavy-ion accelerator UNILAC in Darmstadt by nuclear fusion from the heaviest stable nuclei, lead-208 and bismuth-209, and the most neutron-rich stable isotopes of chromium and iron: element 107 from bismuth-209 (atomic number Z = 83) and chromium-54 (Z = 24), element 108 from lead-208 (Z = 82) and iron-58 (Z = 26), and element 109 from bismuth-209 and iron-58. The first isotopes detected were those with mass numbers 262 (Z = 107), 265 (Z = 108) and 266 (Z = 109); these nuclei are short-lived α-emitters with half-lives of 8.2 ms, 1.8 ms and 3.4 ms, respectively. The yields of these reactions are extremely small; only three atoms of element 109 have ever been observed. Experiments to synthesize element 110 have given ambiguous results. All attempts to detect the “superheavy” elements with proton numbers near Z = 114 and neutron numbers near N = 184 have thus far failed. These elements have been predicted theoretically, and many attempts to synthesize them have been made, e.g. at UNILAC by fusion of calcium-48 (Z = 20) with curium-248 (Z = 96), or by transference of protons in the collision of two very heavy nuclei such as uranium-238 (Z = 92). Surprisingly, the heaviest known nuclei are far more stable toward spontaneous fission into two fragments than was expected, but their synthesis is strongly hindered–much more than initially anticipated. It is this hindrance rather than the decreasing nuclear stability which seems to presently limit the extent of the periodic table: even heavier elements should be able to exist, but no way has yet been found to produce them.     

Radioxenon signatures from activation of environmental xenon

For distinguishing between nuclear explosions and other man made releases, atmospheric monitoring of radioactive noble gas isotopes, xenon isotopes in particular, is of interest to the non-proliferation community. Radioxenon releases can originate from nuclear weapons tests (atmospheric, underground, and underwater), research and commercial reactors, and medical isotope production facilities. Their impacts on atmospheric sample analysis have to be well understood to distinguish between them. This work focuses on the contribution of xenon activation by thermal, epithermal, and 14 MeV neutrons to radioxenon signatures. Calculations to estimate radioxenon production from activation in research reactors are performed and compared to experimental data.     

Solid-state synthesis of boron subnitride, B6N: myth or reality?

Solid-state synthesis of boron subnitride, B₆N, as a result of chemical interaction between boron and boron nitride at 7.5 GPa and 1700 °C has been previously reported by Hubert et al. However, a critical analysis of the results has shown that the evidence for the formation of boron subnitride with B₆O-like structure is inconclusive. We have studied in situ the interaction between boron and BN at the same p–T conditions using X-ray diffraction with synchrotron radiation. At 7.4 GPa and 1700 °C the formation of a new phase has not been observed. At the same time, HP–HT treatment has resulted in strong and unpredictable preferred orientation of boron crystallites. This leads to the rise of some weak boron reflections that might be erroneously attributed to the appearance of a new phase. To cite this article: V.L. Solozhenko et al., C. R. Chimie 9 (2006).     

Sonolysis of actinide(IV) β-diketonates in alkanes

Ultrasonic irradiation with the frequency of 20–22 kHz and absorbed acoustic power about 0.4 W ml^–1 causes degradation of An(IV) tetrakis-β-diketonates, AnL₄, where An(IV) is Th(IV), Np(IV), and Pu(IV), and HL is hexafluoroacetylacetone (HFAA) and dibenzoylmethane (HDBM), in hexadecane solutions in the presence of argon. The rate of the sonochemical process corresponds to a first-order kinetic law with respect to metal β-diketonate concentrations. The first-order rate constant of sonolysis increases with the increase in the volatility of the metal complexes. Solid sonication products consisted of a mixture of actinide carbides and partial degradation products, PDP, of initial metal β-diketonates. It is assumed that metal carbides are formed within the cavitating bubbles as a result of high-temperature process with participation of actinide(IV) β-diketonates and solvent vapours. PDP formation is attributed to the thermolysis of the complexes in a liquid reaction zone surrounding the cavitating bubble. To cite this article: S.I. Nikitenko et al., C. R. Chimie 7 (2004).

Résumé

L’irradiation ultrasonore avec une fréquence de 20–22 kHz et une intensité acoustique de 0,4 W ml^–1 conduit à la décomposition de An(IV) tétrakis-β-dicétones, AnL₄, où An(IV) est Th(IV), Np(IV) et Pu(IV), et HL est l’hexafluoroacétylacétone (HFAA) et le dibenzoylméthane (HDBM) en milieu hexadécane sous argon. La cinétique de décomposition sonochimique est d’ordre 1 par rapport à la concentration du complexe et la cinétique augmente avec la volatilité du complexe métallique. Les produits solides de décomposition sont un mélange de carbure d’actinides et de produits de décomposition partielle PDP. Il a été observé que les carbures d’actinide sont formés dans la bulle de cavitation compte tenu des fortes températures et de la présence simultanée du complexe métallique volatil et du solvant (hexadécane). La formation des PDP est attribuée à la thermolyse du complexe à l’interface de la bulle de cavitation. Pour citer cet article : S.I. Nikitenko et al., C. R. Chimie 7 (2004).     

The synergistic effect of cobalt-dicarbollide anions on the extraction of M lanthanide cations by Calix[4]arenes: a molecular dynamics study at the water–‘oil’ interface

We report molecular dynamics studies on the effect of CCD^– (chlorinated cobalt-dicarbollide) anions on the Eu³⁺ lanthanide cation extraction by a calix[4]arene-CMPO ligand L, focusing on the water–‘oil’ interface, where ‘oil’ is modelled by chloroform. The free L ligand and its EuL³⁺ complex are found to adsorb and to concentrate at the interface, but are too hydrophilic to be extracted. Addition of CCD^– anions in diluted conditions (either covalent linked to L or as separated CCD^– H₃O⁺ ions) also leads to adsorption of these species at the interface. However, at high concentrations, CCD^– anions saturate the interface and promote the extraction of EuL³⁺ to the oil phase. Another important feature concerns the uncomplexed Eu(CCD)₃ salt: accumulation of CCD^– anions at the interface creates a negative potential which attracts the hydrated Eu³⁺ ions, therefore facilitating their complexation by interfacial ligands. These features allow us to better understand the synergistic effect of lipophilic anions in the assisted liquid-liquid extraction of trivalent M³⁺ lanthanide or actinide cations. To cite this article: B. Coupez, G. Wipf, C. R. Chimie 7 (2004).

Résumé

Synergie due aux anions dicarbollides lors de l’extraction d’ions lanthanides M³⁺ par des calix[4]arènes : simulations de dynamique moléculaire à l’interface eau–« huile ». Nous étudions par simulations de dynamique moléculaire l’effet de synergie dû aux anions CCD^– (cobalt-dicarbollides) lors de l’extraction de Eu³⁺ par un calix [4]arène L, en se focalisant sur l’interface eau–« huile », l’huile étant modélisée par du chloroforme. On montre que le ligand L et son complexe EuL³⁺ s’adsorbent à l’interface, mais sont trop hydrophiles pour être extraits. L’addition d’anions CCD^– (qu’ils soient sous la forme d’ions CCD^– H₃O⁺ séparés ou greffés de façon covalente au calixarène) conduit aussi à l’adsorption de ces espèces à l’interface. Cependant, aux plus fortes concentrations, les anions CCD^– saturent l’interface et induisent l’extraction du complexe EuL³⁺ vers l’huile. Un autre résultat remarquable concerne les sels Eu(CCD)₃ : l’accumulation des anions CCD^– à l’interface y crée un potentiel négatif, ce qui attire les cations Eu³⁺ et facilite ainsi leur complexation par des ligands à l’interface. Ces résultats permettent de mieux comprendre l’effet de synergie dû aux anions CCD^– lors de l’extraction d’ions lanthanides ou actinides M³⁺ et, d’une manière générale, ce qui se passe à l’interface entre l’eau et des liquides non miscibles. Pour citer cet article : B. Coupez, G. Wipf, C. R. Chimie 7 (2004).     

Titanate-based ceramics for separated long-lived radionuclides

Studies have been conducted in France to minimize the potential long-term impact of nuclear waste by enhanced chemical separation of the minor actinides (Np, Am, Cm) and some long-lived fission products (I, Cs, Tc). Two options may be considered following this work: (i) the initial reference option is transmutation by neutron bombardment in nuclear facilities, (ii) the second option would be to incorporate the separated elements into an inorganic matrix ensuring long-term stability. In the case of specific conditioning, zirconolite and hollandite are the potential host phases for the minor actinides and caesium, respectively. Both of these matrices have shown strong potential: (i) for incorporating the respective radioelement in the crystalline structure, (ii) for fabricating the ceramic by natural sintering in air, (iii) for chemical durability with a very low initial alteration rate (about 10^–2 g m^–2 d^–1 at 100 °C), then very rapidly reach alteration rates more than four orders of magnitude lower. In the case of zirconolite ceramics, the high chemical durability is conserved even after amorphization of the crystalline structure by external irradiation with heavy ions or by self-irradiation in natural zirconolites 550 million years old. To cite this article: C. Fillet et al., C. R. Chimie 7 (2004).

Résumé

Pour réduire l'impact potentiel à long terme des déchets nucléaires, des études sont conduites en France sur la séparation chimique poussée des actinides mineurs (Np, Am, Cm) et de certains produits de fission à vie longue (I, Cs, Tc). À l'issue de cette étape, deux options sont envisageables : (i) la transmutation par bombardement neutronique dans des systèmes nucléaires est la première option de référence, (ii) la seconde option consisterait à incorporer les éléments séparés des autres déchets nucléaires dans une matrice minérale stable à long terme. Dans le cadre d'un conditionnement spécifique, la zirconolite et la hollandite sont des phases hôtes potentielles respectivement pour les actinides mineurs et le césium. Ces deux matrices ont montré une potentialité élevée (i) d'incorporation du radioélément considéré dans la structure cristalline, (ii) d'élaboration de la céramique par des procédés de frittage naturel sous air, (iii) de durabilité chimique, avec une vitesse initiale d'altération très faible (de l'ordre de 10^–2 g m^–2 j^–1 à 100 °C), puis très rapidement des diminutions des vitesses d'altération de plus de quatre ordres de grandeur. Pour la céramique zirconolite, cette propriété de durabilité chimique élevée reste conservée, même après amorphisation de la structure cristalline par irradiation externe aux ions lourds ou par auto-irradiation sur des zirconolite naturelles âgées de 550 millions d'années. Pour citer cet article : C. Fillet et al., C. R. Chimie 7 (2004).     

Système ternaire : H2O–Fe(NO3)3–Co(NO3)2 isotherme : 30 °C

Ternary system: H₂O–Fe(NO₃)₃–Co(NO₃)₂ isotherm: 30 °C. The H₂O–Co(NO₃)₂ binary system has been investigated in the –28 to 50 °C temperature range. The solid–liquid equilibria of the ternary system H₂O–Fe(NO₃)₃–Co(NO₃)₂ were studied by using a synthetic method based on conductivity measurements. One isotherm is established at 30 °C, and the stable solid phases that appear are iron nitrate nonahydrate: Fe(NO₃)₃·9 H₂O, iron nitrate hexahydrate: Fe(NO₃)₃·6 H₂O, cobalt nitrate hexahydrate: Co(NO₃)₂·6 H₂O, and cobalt nitrate trihydrate: Co(NO₃)₂·3 H₂O. To cite this article: B. El Goundali et M. Kaddami, C. R. Chimie 9 (2006).     

High solubility and optical transparency of novel polyimides containing 3,3′,5,5′-tetramethyl pendant groups and 4-tert-butyltoluene moiety

A series of novel polyimides (3a–d) were prepared from 3,3′,5,5′-tetramethyl-4,4′-diaminodiphenyl-4”-tert-butyltoluene (1) with four aromatic dianhydrides via a one-step high-temperature polycondensation procedure. The obtained polyimides showed excellent solubility, with the dissolvability at a concentration of 10 wt% in most amide polar solvents and chlorinated solvents. Their films were nearly colorless and exhibited high optical transparency, with the UV cutoff wavelength in the range of 322–350 nm and the wavelength of 80% transparency in the range of 395–414 nm. They also showed low dielectric constant (2.72–2.91 at 1 MHz) and low water absorptions (0.37–0.62%). Moreover, these polyimides possessed high glass transition temperatures (T_g) (above 321 °C) and good thermal stability with 10% weight loss temperatures in the range of 526–547 °C in nitrogen atmosphere. In comparison with the analogous polyimides non-containing 3,3′,5,5′ -tetramethyl pendant groups, the resultant polyimides 3a–d showed better solubility, higher optical transparency and lower dielectric constant.     

Novel M(II)–Hg(II) coordination polymers generated from metal-containing building blocks M(2-pyrazinecarboxylate)2 · (H2O)2 (M=Cu, Ni, Co) and HgCl2

A new class of M(II)–Hg(II) (M=Cu(II), Co(II), Ni(II)) mixed-metal coordination polymers, Cu(2-pyrazinecarboxylate)₂HgCl₂ (4), [Co(2-pyrazinecarboxylate)₂(HgCl₂)₂] · 0.61H₂O (5) and [Ni(2-pyrazinecarboxylate)₂(HgCl₂)₂] · 0.77H₂O (6), have been prepared by self assembly of metal-containing building blocks, M(2-pyrazinecarboxylate)₂ · (H₂O)₂(M=Cu(II), Co(II), Ni(II)), with HgCl₂. Compounds 4–6 were characterized fully by IR, elemental analysis and single crystal X-ray diffraction. Compound 4 crystallized in the monoclinic space group C2/c, with a=17.916(5) Å, b=7.223(2) Å, c=13.335(4) Å, β=128.726(3)°, V=1346.2(6) ų, Z=4. It contains alternating Hg(II) and Cu(II) metal centers that are cross-linked by 2-pyrazinecarboxylate spacers and chlorine co-ligands to generate a unique three-dimensional Hg(II)–Cu(II) mixed metal framework. Compound 5 crystallized in the triclinic space group P , with a=6.3879(7) Å, b=6.6626(8) Å, c=13.2286(15) Å, α=96.339(2)°, β=91.590(2)°, γ=113.462(2)°, V=511.71(10) ų, Z=1. Compound 6 also crystallized in the triclinic space group P , with a=6.3543(8) Å, b=6.6194(8) Å, c=13.2801(16) Å, α=96.449(2)°, β=92.263(2)°, γ=113.541(2)°, V=506.67(11) ų, Z=1. Compounds 5 and 6 are isostructural and in the solid state the Hg(II)M(II)Hg(II) units are connected by Hg₂Cl₂ linkages to produce a novel M(II)–Hg(II) (M=Co(II), Ni(II)) zigzag mixed-metal chain, in which a new type of M–M′–M′–M array was observed. The metal containing building blocks, M(2-pyrazinecarboxylate)₂ · (H₂O)₂ (M=Cu(II), Co(II), Ni(II)), exhibit different connectivities to HgCl₂ depending on the metal cation contained within them.     

Application of phosphorus- and nitrogen-containing organic compounds for the isolation of actinides and technetium from acid or basic aqueous solutions

The derivatives of ethers of phosphoric acid, namely O-(2-alkyl) (diethylcarbamoylmethyl) phenylphosphinates (O2ADECMPP), were synthesized and tested for liquid–liquid extraction of transuranium elements, lanthanides and technetium in meta-nitrobenzotrifluoride or 1,2-dichlorethane from acidic solutions. The O-(ethylhexyl) (diethylcarbamoylmethyl) phenylphosphinate (O2EHDECMPP) was used to prepare chelating granulated sorbents. The family of chelating fibrous ‘filled’ sorbents POLYORGS-type shows, according to preliminary experiments, that Pu and Tc at concentrations around 10^–5 M can be recovered almost completely. POLYORGS show fast kinetics for sorption processes. To cite this article: G.V. Myasoedova et al., C. R. Chimie 7 (2004).

Résumé

Utilisation de réactifs phosphorés et azotés pour la séparation des actinides et du technétium de milieux acides et basiques. Des dérivés de l’éther de l’acide phosphorique du type O-2-alkyl diéthylcarbamoylméthyl phénylphosphonates ont été préparés et testés pour extraire des éléments transuraniens, des lanthanides et du technétium dans le meta-nitrobenzène ou le 1,2-dichloroéthane, à partir de solutions aqueuses acides. Le dérivé O-éthylhexyl diethylcarbamoylméthyl phénylphosphonate a été utilisé pour préparer des échangeurs solides par effet de chélation. La famille des chélatants fibreux Polyorgs montre, dans des expériences préliminaires, que Pu et Tc, à des concentration de l’ordre de 10^–5 M, peuvent être complètement récupérés à partir de milieux acides contenant d’autres éléments. Ces composés présentent des cinétiques de sorption très rapides. Pour citer cet article : G.V. Myasoedova et al., C. R. Chimie 7 (2004).     

Agglomération contrôlée de latex par formation de complexes tensioactifs anioniques : copolymères à blocs à base de poly(oxyde d’éthylène)

For polystyrene–poly(ethylene oxide) (PS–PEO) diblock copolymers, as micellar dispersions in aqueous medium, the formation of complexes with anionic surfactants, such as sodium dodecylsulfate (SDS) could be confirmed. The number of SDS molecules fixed per EO unit is close to the values reported for the SDS–PEO homopolymer interaction. Advantage of this type of complexation was taken to develop a controlled agglomeration process for SDS stabilized PS and PVC latexes by using as agglomerants ‘hairy’ latexes of PS and PVC that have been synthesized in the presence of PS–PEO block copolymers and that carry therefore a fringe of PEO sequences on their surface. The complexation of SDS by these surface-anchored PEO chains leads to the destabilization of the anionic latex, which has a tendency to precipitate onto the surface of the agglomerant latex. The average particle size and the size distribution of the agglomerated particles were studied as a function of the weight and number ratio of the two types of latexes involved in the agglomeration process, as well as in function of the surface coverage by SDS and PEO respectively. By adjusting these parameters, it was possible to obtain, with an efficiency of almost 100%, latex agglomerates with a monomodal distribution in the size range of 1 to 40 μm. An agglomeration mechanism could be outlined taking into account the complexation capacity and the specific surface of the agglomerating ‘hairy’ latex. To cite this article: P. Peter et al., C. R. Chimie 6 (2003).     

Curium isotopes in Chernobyl fallout

In nuclear reactors plutonium and transplutonium isotopes are produced by multiple neutron capture of uranium and plutonium and are important for the energy production and their composition reflects the core burnout. Under normal operation these elements are not released to the environment in significant amounts. There are accordingly very few areas or source terms where exotic transplutonium elements, such as curium isotopes, can be studied in the environment. The Chernobyl accident provided a complex spectrum of fission and activation products in fallout while the relative amounts, compared to the core inventory, of refractory elements such as transuranium and transplutonium elements were small. The major alpha-activity consisted of ²⁴²Cm (T _1/2 = 163 d) that would have decayed after a few years. In this study we have demonstrated the presence of so called supported ²⁴²Cm from the long-lived ²⁴²Am^m (T _1/2 = 141 a) in environmental samples, following fallout from the Chernobyl accident. It has also been possible to assess the core burn up by using the data obtained for the Cm isotopes. The curium isotopes ²⁴³Cm (T _1/2 = 29.1 a) and ²⁴⁴Cm (T _1/2 = 18.1 a) cannot be resolved by conventional alpha-spectrometry. The assessment of these isotopes in environmental samples contaminated from the Chernobyl accident has been made by studying the effective half-life of the mixture of the isotopes. The data are compared with those previously obtained by high-resolution alpha-spectrometry and spectral deconvolution.     

A two-dimensional bimetallic oxide constructed from ζ-octamolybdate clusters and Ag(I)-tpyprz cationic polymer components (tpyprz = tetra-2-pyridylpyrazine)

The hydrothermal reaction of Na₂MoO₄, Ag(CH₃CO₂), tetra-2-pyridylpyrazine (tpyprz), HF and water at 170 °C for 48 h yields [ Ag₄(tpyprz)₂(H₂O) Mo₈O₂₆] (1) as colorless plates. The structure of 1 is a two-dimensional network, constructed from Ag₄(tpyprz)₂(H₂O) ⁴ⁿ⁺_n chains linked through ζ- clusters. The chains exhibit three distinct Ag(I) environments: a four coordinate AgN₄ site, a three coordinate AgN₂O_aqua site, and four coordinate AgN₃O_oxo site which links to the molybdate cluster. Each cluster is connected by bridging oxo-groups to two silver/tpyprz chains to form the network architecture. Crystal data: C₂₄H₁₇Ag₂Mo₄N₆O_13.5, fw 1204.94, orthorhombic Pccn, a=20.8234(9) Å, b=14.5076(6) Å, c=20.3537(9) Å, V=6148.9(5) Å³, Z=8, D_calc=2.603 g cm⁻³.     

Synthèse et stabilité sous irradiation électronique d'une céramique Ba1,16Al2,32Ti5,68O16 de structure hollandite envisagée pour le confinement de césium radioactif

Synthesis and stability under electron irradiation of a hollandite structure-type Ba_1.16Al_2.32Ti_5.68O₁₆ ceramic envisaged for radioactive cesium immobilization. Hollandite structure-type Ba_xCs_y(M,Ti)₈O₁₆ (x + y < 2, M trivalent cation) ceramics are currently envisaged as a specific waste form for radioactive cesium immobilization. In order to simulate the effect of cesium β decay on this kind of matrix, the structural modifications and the paramagnetic point defects induced by external electron irradiations near room temperature in a simplified Ba_1.16Al_2.32Ti_5.68O₁₆ hollandite composition were studied mainly by EPR and NMR. Modifications of Al³⁺ and Ti⁴⁺ ions' environment were observed and are due to both the formation of oxygen vacancies and to barium ions displacement. Electron (Ti³⁺) and hole (O₂⁻) centres were observed. The stability of these centres was good at room temperature but thermal treatments performed between 50 and 850 °C generated new paramagnetic defects originating from previous defects. These new defects correspond to titanyl-type Ti³⁺ ions located on grain surface and to oxygen aggregates in their bulk.     

Synthesis of polydimethylsiloxane microemulsions by self-catalyzed hydrolysis/condensation of dichlorodimethylsilane

The preparation of PDMS microemulsions was carried out by adding at controlled rate dichlorodimethylsilane (DCMS) in a solution of sodium dodecylpolyoxyethylene(8) sulphate. The instantaneous hydrolysis of DCMS and subsequent condensation of the corresponding dihydroxysilane generate dispersions of cyclosiloxanes of small lengths (4 to 6 D units). The high load of chloride ions released during the hydrolysis step requires the presence of the above-mentioned electrosteric surfactant to avoid rapid coagulation of the dispersion. In addition, its sulphate end-group captures a proton that catalyses the ring-opening polymerization of cyclosiloxane as well as the polycondensation of disilanol PDMS chains. Final particles exhibit a diameter of about 50 nm for a polydispersity index of less than 1.1. They are constituted of PDMS chains exclusively linear ( ; ) and of small cycles in low contents (less than 5 wt% in the best conditions). To cite this article: G. Palaprat, F. Ganachaud, C. R. Chimie 6 (2003).