[en] The knowledge of thermodynamic functions is essential to investigate crystal stability and chemical reactivity. These functions are not always experimentally known, as for some crystalline host phases for radioactive waste. Fortunately, it is possible to calculate them. Although possible, the full ab initio calculation is not realistic because the calculation time rapidly becomes too long. These functions are obtained using an analytical model containing physical quantities determined by ab initio calculations. This enabled us to estimate the main thermodynamic functions of zircon ZrSiO₄, fluor-apatite Ca₁₀(PO₄)₆F₂ and stoichiometric iodo-apatite Pb₁₀(VO₄)₆I₂ from the variation of cohesive energy with volume and the harmonic vibration frequencies at the center of the first Brillouin zone for the unit cell corresponding to maximum cohesive energy. These ab initio quantities are calculated with the DMOL³ code that solves the electronic Schroedinger equation using the electronic density functional theory (DFT) in local density approximation, corrected (NLDA) or not (LDA) with its gradient. To limit calculation time, we made additional approximations: - acoustic and optical vibrations beyond the first Brillouin zone center are described by the Debye and Einstein models respectively; - to allow thermal expansion of the crystal we used the quasi-harmonic approximation which assumes that frequency variations are proportional to volume variations. The proportionality coefficients known as Gruneisen coefficients are derived in this study, with specific approximations; - we used the spherical cellular approximation and considered the crystal as isotropic. The thermodynamic functions calculated with this model are the following measurable quantities : enthalpy and free enthalpy, heat capacity at constant pressure, bulk modulus and thermal expansion coefficient. For zircon and fluor-apatite, for which the main thermodynamic functions are experimentally known, the enthalpy of formation are determined with less than 2% uncertainty, and heat capacities at constant pressure are well reproduced: For iodo-apatite, there are as yet no published experimental data to compare with the present theoretical results. This model can be applied to a wide range of crystals. (author)

[en] The fate of old industrial sites is a major problem at the beginning of the new millennium for government organization, agencies owning polluted sites... These brown-field sites often generate considerable pollution of soils and groundwater, the main source of drinking water. The National Center for Research on Polluted Soils and Sites (CNRSSP), in which the CEA is an active partner, is developing a strategy to study and, using several scientific tools, characterize industrial sites polluted by various contaminants: metals, metalloids, hydrocarbons, nitrates. The present study offers an example of the investigation of an old industrial site (coke plant), built in 1919 (in operation for 50 years), located in northern France. The storage of tars and wastewaters on these sites causes soil and groundwater contamination with organic compounds such as Polycyclic Aromatic Hydrocarbons (PAHs), mono-aromatic hydrocarbons (benzene, toluene, ethyl-benzene and xylene BTEX), volatile organic compounds (VOCs), phenolic and other hydrocarbons, and inorganic compounds including metalloids (free and complexed cyanides) and metals (As, Pb, Ni, Cd and Hg). The more soluble constituents tend to migrate to adjacent areas with the groundwater flow and thereby reach receptors such as drinking water resources. Among the above pollutants, the 16 PAHs from the priority list of the Environmental Protection Agency (EPA) are of great environmental concern due to their toxic, mutagenic and carcinogenic properties. The remediation of PAH-polluted sites poses a crucial challenge given the large number of sites contaminated by these pollutants and the difficulty in eliminating the pollution. A better evaluation of the PAH transfer and transport in soils and aquifers will also help draw up remediation strategies. In this study, a numerical model, called SIMUSCOPP and developed by the French Petroleum Institute (IFP), BURGEAP and ENI group for organic pollutants, was used to simulate 3D transfer and transport of PAHs under an old coke plant, taking biodegradation and soil sorption into account. The simulation results discussed in this paper were obtained by assuming tar pollution 7 m deep underground to be the only source. This aquifer pollution in a chalky formation mainly consists of PAHs (naphthalene, fluoranthene, etc). A broad field investigation was conducted on the site by monitoring the water (16 piezometers, piezometric measurements, pollutant concentration analysis) to characterize the location and extent of the PAH plume. The numerical modeling was aimed to characterize PAH migration in the unsaturated and saturated zones and to assess the vulnerability of a drinking water well located downstream. The model helped estimate the time for dissolved naphthalene to reach steady state (20 years) and the plume dimensions (1200 m long, 300 m wide and 15 m thick). Simulation were performed of water samplings in piezometers inside and outside the polluted area and the naphthalene concentration in water samples was reproduced, in good agreement with real observations on boreholes of the site. However, for two piezometers located between the two lagoons of coal tars, a large contribution of a second source of PAH, enriched in PAHs with 2 or 3 rings and located at the base of aquifer, was identified in water samples, in the pumped water. This significant share of the pollution is not transported with groundwater flow, but only with water pumping. Thus SIMUSCOPP served to identify the dissolved phase of PAH and to quantify the rest of the pollution, possibly a colloidal phase. As a consequence, a new water sampling strategy could be developed in order to adjust the pump discharge, use specific filters, etc. The major conclusion of the transport simulation is that the PAH plume does not reach the drinking water well located 4 km downstream of the site. However, some remediation measures are necessary and the polluted soils are currently being excavated. (author)

[en] This study addresses the durability of concrete subjected to freezing conditions. The solidification of pore water solution generates different local pressures in the cement matrix. It is possible to distinguish between 'Hydraulic Pressures', 'Osmotic Pressures' and pressures resulting from undercooled fluid circulations according to Litvan's Theory. By overcoming the local tensile strength of the material, these pressures can cause the material to crack. In the pores, ice does not form at 0 deg C but at lower temperatures, as low as -40 deg C. In the literature, this mechanism is explained by the superimposition of different effects, i.e. size restrictions and the presence of solutes. An attempt was made to evaluate numerically the undercooling of pore water solution due to these effects, using the thermodynamic equilibrium between phases for this calculation. An experimental study was carried out using a year old saturated Ordinary Cement Paste with a water to cement mass ratio of 0.5. The cement paste was cured for three months in NaOH and KOH solutions at 3 and 10 g/l, respectively, and then stored in a plastic bag to prevent water release. Solute concentrations were selected from values given by Longuet, who made pore water solution chemistry analyses. The ice formation temperature was measured by Differential Scanning Calorimetry on the BT 2.15 Setaram Calorimeter, and the values are shown. Two different peaks were observed for ice formation, centered near -10 deg C and -40 deg C respectively. According to the classic porous description of the material with capillary and gel pores, peak 1 would correspond to ice formation in capillary pores, and peak 2 in the coarser hydrate pores. Almost all the ice melted near -5 deg C. Undercooling of pore water solution was evaluated using thermodynamic laws. Based on the work done by Radjy, complete equilibrium between phases is described by Equation 1. The effects evaluated were: (i) size restrictions, (ii) presence of solutes, and (iii) confinement pressure of solution in the pores. All these effects tend to lower the freezing point of the solution. Integrated equations corresponding to each of the effects and results shown as graphs are presented respectively in Equations 2, 3 and 4 and Figures 2, 3 and 4. In order to evaluate undercooling of pore water solution, a medium capillary pore and a medium hydrate pore were modeled with the characteristics given in Table 1, and the associated undercooling then calculated, as -2.8 and -6.4 deg C respectively. The calculated undercooling were significantly lower than experimentally observed undercooling. In fact, calculated undercooling correspond to the lowering of the pore water solution freezing points as well the stability temperatures of ice crystals in the medium pores. This explains why the calculated values show good agreement with measured ice melt temperatures. One possible explanation of the lowest ice formation temperatures observed can be found in the mechanisms of ice germination. These mechanisms are described by nucleation theory [8]. Ice forms from clusters or nuclei which are heaps of H₂O molecules tied by hydrogen bonds. At any given temperature below 0 deg C, a nucleus must reach a critical size before nucleation can occur, followed by ice crystal growth. At the stability temperature, the concentration in a critical nucleus is very low, and the probability of nucleation is 0. With decreasing temperature, the critical nucleus concentration, increases exponentially up to a threshold value, where nucleation probability suddenly becomes 1 and ice forms. In a volume of pure H₂O molecules, the spontaneous nucleation point is close to -40 deg C, as in the case of ice formation in clouds. For a volume of ordinary water, impurities and surfaces act as nucleation sites. They modify the surface energy of the clusters and permit nucleation at a temperature closer to the crystal stability temperature, which is generally 0 deg C. Considering all these factors, it appears possible to study the ice formation temperature in a cement paste, not in the conventional way invoking only size restrictions and presence of a solute, but also considering the mechanisms of ice germination described by nucleation theory. (authors)

[en] One of the main concerns related to the disposal ire deep geological repository of the conditioned nuclear wastes generated after reprocessing of spent fuels is related to the presence within these wastes of long lived radionuclides (LLRNs), and particularly those of the so-called minor actinides, neptunium, americium (Am) and curium (Cm). Within its 4. Framework programme (FWP), covering the period 1996-1999, the European Union supported several research contracts in order to define processes for the Partitioning of the LLRNs from the wastes in view of their subsequent Transmutation (PT scenario) or Conditioning (P-C scenario). NEWPART was one of these contract aiming to define solvent extraction processes for Am and Cm partitioning. The main achievements obtained in the course of this research are highlighted in this article. The main choices of the selected strategy are the following: (i) among the LLRNs, those of Am and Cm were selected for partitioning, (ii) the target waste to be treated consists in the high active waste (HAW) issuing the reprocessing of the spent fuels by the PUREX process, (iii) no modification of the oxidation state of the target elements to be extracted will be done; consequently the chemistry will concern the trivalent Am and Cm (An(III)), (iv) most of the reagents to be used should fulfill the CHON principle. Among the difficulties to solve one can mention: (i) how to extract the An(III) from the acidic HAW? (ii) how to separate the An(III) from the most abundant lanthanides(III) (Ln(Ill)) from acidic feeds? To overcome these problems one process including two steps was studied: (i) the first one consists in the co-extraction of An(III)+Ln(III) from the acidic HAW by a diamide (malonamide) extractant. This corresponds to the so-called DIAMEX process, (ii) the second process, named SANEX, performs the An(III)/Ln(III) group separation by the selective actinide extraction from rather acidic feeds. Two ways were developed for the definition of SANEX processes, both based on the use of extractants bearing soft donor atoms either nitrogen of sulphur. For both DIAMEX and SANEX research areas, fundamental studies and process development were carried out. Basic understanding of the extractive properties of the malonamide ligands was improved. This was done in particular by the determination of the structure of the solvates which are formed between the extractant E and the M(III) nitrate. In all cases, malonamides were found acting as bidendate ligand towards M(III) ions, as shown in Figure 1. Process developments were carried out principally using the dimethyl-dibutyl-tetra-decyl-malonamide (DMDBTDMA) extractant. Several flow-sheets were developed and tested. In particular, a DIAMEX process flow-sheet was successfully tested at the ITU (Karlsruhe) in a centrifugal contactor test-loop using a genuine HAW. Moreover, a new diamide, the DMDOHEMA, was chosen by CEA to design an improved DIAMEX process. Several families of N-terdendate ligands were studied. But the most exciting results obtained so-far are certainly those related to a new family of ligands developed by Kolarik at FZK, the bis-1,2,4-tri-azinyl-pyridines, the so-called BTPs, able to selectively extract the An(III) from the Ln(III) from 1 to 2 mol/L nitric acid feeds. Figure 3 shows the crystal structure of a solvate formed between a Gd(III) ion and a BTP ligand. In this crystal the Gd(III) ion is bound to nine N atoms belonging to three BTP molecules. A SANER process based on the use of a BTP extractant was successfully designed and tested at Marcoule first, then at the ITU. New bis-substituted di-thio-phosphinic acids were developed at FZJ. It was found that these new acidic S-bearing ligands are able, in synergistic mixtures with neutral 0-bearing ligands, to extract selectively the An(III) over the Ln(III) from highly acidic solutions. A process using a mixture of bis-chloro-phenyl-di-thio-phosphinic acid+tri-n-octyl-phosphine oxide (TOPO) was designed at FZJ and successfully tested with synthetic spiked solutions in a centrifugal contactor test loop. The research carried out within the NEWPART programme was highly successful: several DIAMEX and SANER processes were developed and successfully tested in hot conditions for the partitioning of Am and Cm. About fifty papers were published in International Journals and Proceedings of International Conferences. The European collaboration in this field will be pursued within the 5. FWP of the European Union during the period 1999-2002. (authors)

[en] The search for ligands which specifically separate actinides(III) from lanthanides(III) by liquid-liquid extraction has prompted considerable research in the Process Design and Modeling Department ('Service d'Etude et de Modelisation des Procedes'- SEMP). Ligands with soft donor atoms AS) that are able to perform this separation have already been investigated and research is currently under way to improve their performance for high acidic feeds. Theoretical chemistry research is conducted in the Theoretical and Structural Chemistry Laboratory ('Laboratoire de Chimie Theorique et Structurale') to improve our understanding of the complexation and extraction of these cations with such ligands. Theoretical studies were first carried out for the ter-pyridine (TPY) and bis-triazinyl-pyridine (BTP) ligands that display fairly good ability to separate and extract actinide(III) from lanthanide(III) ions. Molecular dynamics simulations were performed on ter-pyridine and bis-triazinyl-pyridine complexes with three lanthanide cations (La³⁺, Eu³⁺ and Lu³⁺) for vacuum and for water solutions. These calculations were carried out without counter-ions, with three nitrate (NO₃^-) ions, and, in the case of ter-pyridine, with three α-bromo-caprate anions that are likely to be used experimentally as synergistic agents for the separation and extraction of An(III) from Ln(III). Molecular dynamics simulations were first performed for vacuum to evaluate the distances between nitrogen and lanthanide atoms (Ln³⁺,N) and intrinsic interaction energies to poly-nitrogenous ligands with or without NO₃ ions, and for both ligands. The (Ln³⁺,N) distances decrease and the cation/ligand interaction energies increase along the La³⁺, Eu³⁺, Lu³⁺ series, with decreasing Ln(III) ion radii. The introduction of nitrate counter-ions makes the (Ln³⁺,N) distances slightly higher, and the TPY/Ln³⁺ and BTP/In³⁺ interaction energies lower, compared with complexes without NO₃^-. By contrast, with a-bromo-caprate anions, the TPY/Ln³⁺ interaction energy is the highest for Eu³⁺ owing to the strong interaction of the counter-ions with Ln³⁺ cations and to the small size of Lu³⁺. Gibbs free energy differences (ΔΔG) can be calculated in molecular dynamics simulations using the free energy perturbation theory. These calculations serve to account for entropy and were made for the vacuum phase first to assess the selectivity of TPY and BTP with respect to the lanthanide(III) cations, and then to assess the selectivity of each cation for the two ligands. With or without nitrate counter-ions, both ligands are selective for the smaller Lu³⁺ cation. Without NO₃^- anions, every Ln³⁺ cation is selective towards BTP versus TPY whereas with nitrate ions, the ΔG differences approach zero. For the water phase, Ln³⁺ complexes with TPY and with BTP, including NO₃- ions or without counter-ions, dissociate after a few picoseconds of molecular dynamics simulations. The only complexes that do not dissociate are those with La³⁺ or Eu³⁺, ter-pyridine, and three α-bromo-caprate anions. For these two complexes, one water molecule is bound to the cation according to recent Time-Resolved Laser-Induced Fluorescence results. The Gibbs free energy difference between these two complexes in water solution reveals a slight preference for Eu³⁺, highlighting the future difficulty of calculating separation of lanthanide(III) from actinide(III) cations. The calculations reported here call for further investigations. First, new simulations can be performed with different soft donor atom ligands, and from the theoretical standpoint, the description of 'non-bonded' interactions and the introduction of an explicit polarization term in the potential energy expression need to be examined in greater detail. (author)

[en] To understand the properties of some systems able to extract actinides (III) from lanthanides(III) selectively, the solution chemistry of lanthanide(III) and actinide(III) cations with poly-hetero-aromatic nitrogen-containing ligands was studied by Time-Resolved Laser Induced Fluorimetry (TRLIF) and UV-visible spectrophotometry, combined with chemo-metric methods. Three soft donor ligands (L) were selected for the study: 2,2':6;2^-ter-pyridine (Tpy),4,6-tri-(pyridine-2-yl)-1,3,5-triazine (Tptz) and 2,6-bis-(5,6-dimethyl-1,2,4-triazine-3-yl)-pyridine (MeBtp). Tpy and Tptz exhibit moderate affinity (distribution ratio) and selectivity when used in the synergistic liquid-liquid extraction of americium(III) (with a lipophilic carboxylic acid). MeBtp is also very efficient, and extracts Am(III) with high selectivity; The TRLIF study analyzed the Eu(III) fluorescence emission spectrum. By analyzing the respective changes in the band intensities, and the lifetimes of the Eu(III) excited states, when the ligands were added in homogeneous phase, the following conclusions were drawn: - for Tpy and Tptz, only one EuL³⁺ complex species was detected, with a low symmetry in the first coordination sphere, and the Eu(III) hydration number (number of water molecules in the Eu(III) first sphere of coordination) in these complexes was found to be around 5-6; - for MeBtp, two species were detected, one with a low symmetry and a hydration number close to 5-6, the other with a high symmetry and almost completely dehydrated. This is indicative of the formation of the complexes: EuL³⁺ for L =Tpy and Tptz, and Eu(MeBtp)³⁺ and Eu(MeBtp)₃³⁺ in the case of MeBtp. The formation of these complexes, as well as the protonated ligands, was quantitatively studied using UV-visible spectrophotometry. In each case, the variation in the absorption spectrum of one species was monitored, while the concentration of the other was varied. The complex formation constants were then evaluated by mathematical processing of the whole spectrum. The following conclusions were drawn: - the protonation constants for MeBtp are far lower than for Tptz or Toy. This can be correlated to the fact that MeBtp can extract M(III) nitrates from solutions up to 2 molar nitric acid, whereas Tpy and Tptz are limited to acidities below 0,1 mol/l and require the use of a lipophilic anion; - all three ligands behave differently towards lanthanides(III) and americium(III): they all are quite stable AmL₂³⁺ complexes, and MeBtp forms a very stable Am(MeBtp)₃³⁺ complex. This is consistent with the fact that both types of ligand selectively extract Am(III) cation, extractants of the Btp family demonstrating singular behavior. The number, stoichiometries and stability constants of the complexes formed in aqueous homogeneous conditions between lanthanide(III) and americium(III) cations and poly-hetero-aromatic nitrogen-containing ligands were determined through the combined use of Time-Resolved Laser Induced Fluorimetry and UV-visible spectrophotometry. Data obtained in homogeneous conditions confirmed the ability of nitrogen-containing ligands to discriminate between 4f and 5f cations, an ability already observed in solvent extraction studies. Evidence for the very high selectivity of 2,6-bis-(1,2,4-triazine-3-yl)-pyridine derivatives (Btp's) for americium(III) vs lanthanide(III) cations could also be obtained. This constitutes the first clue for a correlation, at least qualitative, between homogeneous and solvent extraction results. (authors)

[en] The CEA is currently developing the DIAMEX process, the first step in the strategy for the separation of minor actinides from high-level radioactive waste. The extractant belongs to the diamide family of molecules and is able to co-extract trivalent actinides and lanthanides. This study focuses on the thermodynamic properties (ΔH, ΔG, ΔS) of lanthanide extraction by malonamide in order to better understand the mechanisms involved and to account for differences in the behavior of various diamide extractants. The main technique used is microcalorimetric titration. The Thermal Activity Monitor (TAM) microcalorimeter is a modular system with a highly stable (± 0.1 mK) temperature-controlled bath containing up to four calorimetry vessel units. The sensor bulbs inserted in the reaction vessel can measure heat flows in static or dynamic conditions. Micro-calorimetry, and calorimetric titration in particular, is a fast growing field due to technical improvements in both hardware and software. In the case of an equilibrium reaction, titration allows both Δ_rG and Δ_rH (and thus Δ_rS) to be determined simultaneously. It was decided to initiate this thermochemical investigation with a homogeneous phase reaction, and the first study concerned the aqueous phase complexation of a trivalent lanthanide ion by a water-soluble diamide, tetraethyl-malonamide (TEMA: (C₂H₅)₂NCO-CH₂CON(C₂H₅)₂). In the test system, the heat of dilution of the diamide in water is preponderant over the heat arising from the complexation reaction; the result is a positive value corresponding to an endothermic reaction. However, the equilibration constant K and Δ H are both very small, and cannot be calculated from the resulting Q_vf(n_TEMA) curves. Moreover, in aqueous phase, the reactions involved are different from those observed when neodymium(III) is extracted into an organic phase, and the medium must be further characterized before the calorimetry data can be fully interpreted. Additional studies were performed using specific methods (time-resolved laser-induced fluorescence spectrometry) to confirm the existence of one or more complexes and to more accurately determine the relevant complexation constant(s). The overall heat values measured were compared using a McMillan-Mayer approach; the thermodynamic excess functions (reflecting the deviation from ideality) were expressed as a sum of pair and triplet interactions among the solutes in solution: H^E = h_iic_i² + h_jjc_j² + 2h_ijc_ic_j + h_iiic_i³ +... where hii and hiii are the interaction parameters for the i-i pairs or i-i-i triplets, and c_i and c_j are the concentrations of solutes i and j. Unlike the measured heat values, the interaction parameters are intensive, i.e. independent of the experimental conditions. After evaluating the heat of dilution of TEMA in water, the theory was applied to the Nd(Ill)-TEMA complexation reaction. The linearity of the Q_r/V[Nd]_tot versus [TEMA] curve is consistent with the expression of the excess enthalpy H^E h_Nd-TEMA [TEMA][Nd], confirming the existence of an interaction between the TEMA ligand and neodymium in the aqueous phase. The results obtained were analyzed in this way, and the h_Nd-TEMA interaction parameters compared. A study of various media revealed the following variations h_Nd-TEMA(neutral NO₃^-) > h_Nd-TEMA(HNO₃) > h_Nd-TEMA(H₂O) indicating a weaker interaction between Nd(111) and TEMA in the presence of nitrate ions. The variation over the lanthanide series was also investigated in water: the h_Nd-TEMA parameter was characterized by a monotonic increase for the light lanthanides, followed by stabilization. Tests were also conducted to measure the enthalpy of Nd(III) extraction by DMDBTDMA, a lipophilic diamide (N,N'-dimethyl N,N'-dibutyl tetra-decyl-malonamide, the first reference molecule proposed for the DIAMEX process [5]): Α_extH was determined to be near -5 to -15 kJ-mol^-1. The extraction reaction is exothermic, and the numerical values are consistent with published values on comparable systems. The diamide concentration has a major influence on the reaction enthalpy. The preliminary test results have been highly encouraging. The technique has now been mastered both for homogeneous phase reactions and for extraction reactions in two-phase media. The first tests with a trivalent lanthanide/diamide system were very productive, and highlighted the influence of the medium on the thermodynamic properties: the reactions are endothermic in a homogeneous aqueous phase-resembling interaction more than true complexation-while extraction is exothermic and highly sensitive to the ligand concentration. Forthcoming studies will address extraction reactions to obtain further data on the medium. (authors)

[en] One alternative selected by the CEA for partitioning minor actinides from aqueous solutions containing fission products is the selective extraction of oxidized americium. This is the SESAME process (Selective Extraction and Separation of Americium by Means of Electrolysis) aimed to convert americium to oxidation state (VI) and then extract it with a specific extractant of high valences. This paper presents the study of the electrochemical oxidation of americium in nitric medium which represents an important stage of the process. The reaction can be divided into two main steps: oxidation of americium (III) to americium (IV), and then of americium (IV) to americium (VI). For the first oxidation step, a ligand L is needed to stabilize the intermediate species americium (IV) which disproportionates in its free form into americium (III) and (V). Phospho-tungstate or silico-tungstate are appropriate ligands because they are stable in concentrated nitric acid and show a great affinity for metallic cations at oxidation state (IV) (Table 1 lists the stability constants of americium (IV) complexes). The presence of the lacunary poly-anion lowers the potential of the americium (IV) / americium (Ill) redox pair (see Figure 5 for the diagram of the apparent formal potential of americium versus ligand concentration). This makes it thermodynamically possible to oxidize americium (III) into americium (IV) at the anode of an electrolyzer in nitric acid. For the second oxidation step, a strong oxidant redox mediator, like silver (II), is needed to convert complexed americium at oxidation state (IV) to oxidation state (V). The AmVL complex is then hydrolyzed to yield americyle (V) aqua ion. A spectroscopic Raman study with ¹⁸O labeled species showed that the oxygen atoms of the americyle moiety came from water. This indicates that water hydrolyzes the americium (V) complex to produce americyle (V) aqua ion, AmO₂⁺. This cation reacts with silver (Il) to give americyle (VI) ion. Figure 8 shows the kinetics of the oxidation of americium in the presence of poly-anion and silver nitrate. Americium (III) is completely converted into americium(VI) within 1 h 30. These results led us to propose a reaction path for the electrochemical oxidation of americium in nitric medium and to model kinetics of americium oxidation. This work was validated by electrolysis experiments and the model is extremely useful for optimizing the operating parameters or for calculating the ideal dimensions of future electrolyzers. (authors)

[en] Satisfactory completion of decommissioning operations requires precise knowledge of the radiological condition of the facilities. Radiological balances are currently based on dose rate measurements. Laboratory analysis on samples are sometimes carried out. Experience has shown that the gamma activity is often localized in discrete zones, called ''hot spots''. Their identification allows them to be quickly eliminated to remove a great part of the activity and to obtain lower irradiation levels. Over the last years, Nuclear Facilities Decommissioning Unit has been developing and implementing radiation imaging prototype devices ''gamma camera'', suitable for site work. Once the source has been localized, spectrometric data on the source may now be obtained by the use of collimated. compact high resolution probes, associated to the camera body. Use of computer codes, based on geometric assumptions, enables the evaluation of the gamma activity from spectrometry results. This association has been validated on a highly irradiating reprocessing cell. The gamma camera uses a double cone collimator to form a gamma image in the same way as in a pinhole camera. The image is formed on a scintillator plate, Cesium Iodine or organic material, depending on the operating conditions. Gamma photons interact in the scintillator to generate luminous photons, forming an image. The image is then amplified by an intensifier tube, collected on a CCD array and transmitted to a video acquisition card in a PC. The article shows examples of images, obtained before and after the first decontamination phase in a reprocessing cell. The elimination of the main hot spot is observed. In its present configuration, the device is capable of localizing a ¹³⁷Cs point source irradiating a 250 nGyh^-1 within 10 minutes, with a signal to noise ratio reaching 11. After a precise calibration, it is possible to roughly evaluate the dose rate generated by a point source of known energy. However, due to the thermal drift of the amplifier tube gain, this can not be considered as a precise measurement yet. A deeper characterization of the hot spots, previously identified by gamma imaging, has been field tested, using a compact high resolution spectrometric probe. The small size of CdTe detectors, their ability to operate at room temperature and their high energy resolution (2 % at 662 keV) made them fit to the need. A 50 mm³ planar detector was selected. Due to its small volume, the energy range is limited to 60 keV - 1000 keV. Moreover, the efficiency sharply drops when the incident energy increases. Another drawback is the incomplete charge collection of the holes, due to significant trapping effects. The resulting asymmetrical photo-peaks lead to a loss of resolution. However, this effect can be significantly corrected by an electronic circuit ''charge loss corrector''. Improvement of the gamma camera signal to noise ratio and of the image resolution by image convolution is still in progress. In the same time, other possibilities of compact and room temperature spectrometric detectors operations are investigated. (authors)

[en] Fission chambers are basically gas ionization detectors where one electrode is coated with fissile material. For many years, the Fuel Cycle Division has been contributing to the manufacture of fission chambers (traditional domain of the Nuclear Research Division) by providing the fissile material deposits. In 9999, a transfer of knowledge about the manufacture of deposits on cylindrical electrodes (i.e. imposed current electrolysis) took place between the two Division. This transfer resulted in the installation of a new generation of experimental rigs for electrolysis, the development of procedures designed to obtain very high accuracy on the fissile mass deposited, and the performance of several electro-depositions of actinide radionuclides. Thanks to the use of machined parts and the study of the shape of certain parts, the new electrolysis rig offers easy installation and disassembly of the various parts, maintenance of a constant distance between the electrodes, and minimized trapping of hydrogen bubbles formed at the cathode during electrolysis. The latter two qualities are necessary to obtain a homogeneous deposit. The knowledge of the mass deposited and its uncertainty are essential data for the users of the fission ionization chambers. In the procedure selected, isotopic dilution is combined with ICP-MS analysis to measure the radioelement content of the electrolyte before and after electrodeposition, and to weigh the electrolyte. These measurement techniques help determine the fissile mass deposited with a low uncertainty (approximately 2 %) (see Tables I and II). To validate the procedures, ten electrolytic deposits of plutonium and uranium with deposition mass ranging from 100 μg to 5 mg were carried out on cylindrical metallic electrodes 0.7 mm, 2.5 mm and 6.5 mm in diameter (Table I). The deposits were uniform and displayed good adherence. However, the electrolysis yields varied (20 to 99 %) with electrodeposition yields ranging from 80 % to 100 %. A parametric study and further expertise will serve to improve the reproducibility of the electro-depositions. (authors)