Hydrothermal conversion of mixed uranium(IV)-cerium(III) oxalates into U1-xCexO2+δ·nH2O solid solutions.

Uranium-cerium oxide solid solutions, U1-xCexO2+δ·nH2O, were prepared through hydrothermal conversion of mixed U(IV)-Ce(III) oxalate precursors, cerium being used as a surrogate for plutonium. Whatever the starting pH, the fluorite-type structure of AnO2 was obtained after heating at 250 °C for 24 h. The initial pH of the reaction media appeared to affect significantly the oxide morphology: for pH ≤ 2, the powder was found to be composed of microspheres, whereas for more alkaline pH values, agglomerates of nanocrystallites were found. Furthermore, a study of the hydrothermal treatment duration (T = 250 °C, pH = 8, t = 1-48 h) showed that fluorite-type mixed dioxides started to form after only 1 h, and then became single phase after 3 h. SEM and TEM/EDS analyses revealed that the cationic distribution narrowed with time to finally form highly homogeneous mixed oxides. Such a preparation route was then applied to various cerium incorporation rates and it was found that the formation of U1-xCexO2+δ·nH2O mixed oxides was possible for 0.1 ≤ x ≤ 0.75. In all the systems investigated, the speciation of uranium and cerium was questioned in both the solid and liquid phases. Thermodynamic calculations and evaluation of the O/M ratio in the final oxides led us to understand the complex redox behaviour of uranium and cerium in solution during hydrothermal processes and to propose a conversion mechanism.

A multiscale in situ high temperature high resolution transmission electron microscopy study of ThO2 sintering.

Two-grain model systems formed by ThO2 nanospheres have been used to experimentally study for the first time the initial stage of sintering from room temperature to 1050 °C using high temperature high resolution transmission electron microscopy. In each grain, oriented attachment drove the reorganization and growth of the crystallites up to 300 °C to form a pseudo single crystal. Crystallite size kept growing up to 950 °C. At this temperature, a fast transformation probably corresponding to the elimination of stacking faults or dislocation walls led to the formation of single-crystals. The contact formed at room temperature between the two grains was stabilized during heat treatment by a slight reorientation of the crystallographic planes (T≈ 400 °C), leading the neck to be formed by numerous boundaries between the crystallites. At higher temperatures, the neck evolved and stabilized in the form of a plane of crystallographic orientation mismatch between the grains, which corresponds to the usual definition of the grain boundary. The growth of the neck by the addition of atomic columns was further observed in real time and quantified. At T = 950 °C, the evolution of the microscopic sintering parameter λ was obtained from HT-HRTEM images and indicated that the neck formation mostly proceeded through volume diffusion.

Monazite, rhabdophane, xenotime & churchite: Vibrational spectroscopy of gadolinium phosphate polymorphs.

Rare-earth phosphates with the general formula REEPO4·nH2O belong to four distinct structural types: monazite, rhabdophane, churchite, and xenotime. We report herein the first direct comparison between vibrational spectra of these compounds for the same metal cation i.e. gadolinium. The four GdPO4·nH2O samples were prepared through wet chemistry methods and first characterized by X-ray diffraction. Three distinct spectral domains, associated to the deformation and stretching modes of phosphate tetrahedra (PO4) and to water molecules vibrations were then analyzed from FTIR and Raman data, and discussed regarding the structural characteristics of each sample. The most obvious differences between the spectra were associated to δ(H2O) and δs(PO4) modes and led to propose a simple method to rapidly and unambiguously discriminate the four polymorphs.

Vibrational spectroscopy of synthetic analogues of ankoleite, chernikovite and intermediate solid solution.

Ankoleite (K(UO2)PO4·nH2O), chernikovite (H3O(UO2)PO4·nH2O) and intermediate solid solutions are frequently encountered in the uranium ores that result from the alteration of uranium primary minerals. This paper reports a thorough FTIR and Raman study related to synthetic analogues for these minerals. First, the vibration bands associated to the UO2(2 +) uranyl ion were used to calculate the U = O bond length which appeared in good agreement with the data coming from PXRD. Then, the examination of the phosphate vibration modes in both sets of spectra confirmed the general formulation of the samples and ruled out the presence of hydrogenphosphate groups. Finally, the presence of H2O as well as protonated H3O(+) and/or H5O2(+) species was also pointed out, and could be used to clearly differentiate the various phases prepared. Vibrational spectroscopy then appeared as an efficient method for the investigation of such analogues of natural samples. It should be particularly relevant when identifying these phases in mineral ores or assemblies.

From thorite to coffinite: a spectroscopic study of Th(1-x)U(x)SiO4 solid solutions.

Coffinite (USiO4), along with Th(1-x)U(x)SiO4 uranothorite solid solutions, are frequently present in reduced economically exploitable uranium ores. They could also control the concentration of uranium in the environment in the case of accidental release from underground radwaste repository. This paper reports for the first time a thorough FTIR and Raman study relative to the Th(1-x)U(x)SiO4 system, including synthetic analogues of thorite and coffinite end-members. Both sets of spectra confirmed the formulation of the samples and allowed to rule out the presence of structural water molecules and/or hydroxyl groups in the coffinite. Also, no characteristic signal of UO2(2+) uranyl ion was recorded, ensuring that uranium was fully incorporated under its tetravalent oxidation state. The variation of the positions corresponding to SiO4 internal vibration modes was then followed versus the chemical composition of the samples. If the FTIR spectra did not revealed any significant shift in the bands position, several Raman modes followed a linear trend as a function of the uranium incorporation rate. On this basis, Raman spectroscopy could be considered as a promising tool for the semi-quantitative determination of chemical composition of uranothorite samples, particularly for those coming from mineral ores. Finally, the data collected for the coffinite end-member, as the first to be obtained on pure synthetic samples, allowed a review of the results previously reported in the literature for this compound.

Dissolution of cerium(IV)-lanthanide(III) oxides: comparative effect of chemical composition, temperature, and acidity.

The dissolution of Ce(1-x)Ln(x)O(2-x/2) solid solutions was undertaken in various acid media in order to evaluate the effects of several physicochemical parameters such as chemical composition, temperature, and acidity on the reaction kinetics. The normalized dissolution rates (R(L,0)) were found to be strongly modified by the trivalent lanthanide incorporation rate, due to the presence of oxygen vacancies decreasing the samples cohesion. Conversely, the nature of the trivalent cation considered only weakly impacted the R(L,0) values. The dependence of the normalized dissolution rates on the temperature then appeared to be of the same order of magnitude than that of chemical composition. Moreover, it allowed determining the corresponding activation energy (E(A) ≈ 60-85 kJ·mol(-1)) which accounts for a dissolution driven by surface-controlled reactions. A similar conclusion was made regarding the acidity of the solution: the partial order related to (H(3)O(+)) reaching about 0.7. Finally, the prevailing effect of the incorporation of aliovalent cations in the fluorite-type CeO(2) matrix on the dissolution kinetics precluded the observation of slight effects such as those linked to the complexing agents or to the crystal structure of the samples.

How to explain the difficulties in the coffinite synthesis from the study of uranothorite?

The preparation of Th(1-x)U(x)SiO(4) uranothorite solid solutions was successfully undertaken under hydrothermal conditions (T = 250 °C). From XRD and EDS characterization, the formation of a complete solid solution between x = 0 (thorite) and x = 0.8 was evidenced. Nevertheless, additional (Th,U)O(2) dioxide and amorphous silica were systematically observed for the highest uranium mole loadings. The influence of kinetics parameters was then studied to avoid the formation of such side products. The variation of the synthesis duration allowed us to point out the initial formation of oxide phases then their evolution to a silicate phase through a dissolution/precipitation process close to that already described as coffinitization. Also, the uranium mole loading initially considered was found to significantly influence the kinetics of reaction, as this latter strongly slows down for x > 0.3. Under these conditions, the difficulties frequently reported in the literature for the synthesis of pure USiO(4) coffinite were assigned to a kinetic hindering associated with the coffinitization reaction.

Stability and structural evolution of Ce(IV)(1-x)Ln(III)(x)O(2-x/2) solid solutions: a coupled µ-Raman/XRD approach.

Several CeO(2)-based mixed oxides with general composition Ce(1-x)Ln(x)O(2-x/2) (for 0 ≤ x ≤ 1 and Ln = La, Nd, Sm, Eu, Gd, Dy, Er, or Yb) were prepared using an initial oxalic precipitation leading to a homogeneous distribution of cations in the oxides. After characterization of the Ce/Nd oxalate precursors and then thermal conversion to oxides at T = 1000 °C, investigation of the crystalline structure of these oxides was carried out by XRD and µ-Raman spectroscopy. Typical fluorite Fm ̅3m structure was obtained for relatively low Ln(III) contents, while a cubic Ia ̅3̅ superstructure was evidenced above x ≈ 0.4. Moreover, since Nd(2)O(3) does not crystallize with the Ia ̅3̅-type structure, two-phase systems composed with additional hexagonal Nd(2)O(3) were obtained for x(Nd) ≥ 0.73 in the Ce(1-x)Nd(x)O(2-x/2) series. The effect of heat treatment temperature on these limits was explored through µ-Raman spectroscopy, which allowed determining the presence of small amounts of the different crystal structures observed. In addition, the variation of the Ce(1-x)Ln(x)O(2-x/2) unit cell parameter was found to follow a quadratic relation as a result of the combination between increasing cationic radius, modifications of cation coordination, and decreasing O-O repulsion caused by oxygen vacancies.

Comparative behavior of britholites and monazite/brabantite solid solutions during leaching tests: a combined experimental and DFT approach.

In the field of the specific immobilization of actinides, several phosphate-based ceramics have already been proposed as suitable candidates. Among them, britholite and monazite/brabantite (now called monazite/cheralite) solid solutions have been considered as serious candidates on the basis of several properties of interest. Although both matrices appear almost similar from a chemical point of view, their chemical behavior during leaching tests appear to be strongly different with normalized dissolution rates of typically (2.1 +/- 0.2) g.m(-2).day(-1) for Th-britholites (10(-1)M HNO(3), theta = 25 degrees C, dynamic conditions) and (2.2 +/- 0.2) 10(-5) g.m(-2).day(-1) for Th-brabantites (10(-1)M HNO(3), theta = 90 degrees C, dynamic conditions). To understand such difference from a crystallographic point of view, comparative leaching tests have been performed using either high or low renewal of the leachate. The results obtained clearly revealed a lower chemical durability of An-britholites compared to that of (Ln, Ca, An)-monazite/brabantite solid solutions. As a confirmation of this point, density functional theory calculations clearly showed some great differences in the cohesive energy of calcium in both crystal structures, which can explain this strong difference in the chemical durability of both materials.

Hydrothermal method of preparation of actinide(IV) phosphate hydrogenphosphate hydrates and study of their conversion into actinide(IV) phosphate diphosphate solid solutions.

Several compositions of Th2-x/2AnIVx/2(PO4)2(HPO4).H2O (An=U, Np, Pu) were prepared through hydrothermal precipitation from a mixture of nitric solutions containing cations and concentrated phosphoric acid. All the samples were fully characterized by X-ray diffraction, UV-vis, and infrared spectroscopies to check for the existence of thorium-actinide(IV) phosphate hydrogenphosphate hydrates solid solutions. Such compounds were obtained as single phases, up to x=4 for uranium, x=2 for neptunium, and x<4 for plutonium, the cations being fully maintained in the tetravalent oxidation state. In a second step, the samples obtained after heating crystallized precursors at high temperature (1100 degrees C) were characterized. Single-phase thorium-actinide(IV) phosphate-diphosphate solid solutions were obtained up to x=0.8 for Np(IV) and x=1.6 for Pu(IV). For higher substitution rates, polyphase systems composed by beta-TAnPD, An2O(PO4)2, and/or alpha-AnP2O7 were formed. Finally, this hydrothermal route of preparation was applied successfully to the synthesis of an original phosphate-based compound incorporating simultaneously tetravalent uranium, neptunium and plutonium.

Investigation in hydrated thorium phosphates by NMR I-relation proton phosphorus.

The relations between (1)H and (31)P nuclei are investigated by multinuclear and multidimensional nuclear magnetic resonance (NMR) spectroscopy in order to obtain structural information along the transformation of the thorium phosphate hydrogen phosphate hydrate (TPHPH) into the thorium phosphate diphosphate (TPD) when heating. The raw sample obtained at 120 degrees C was heated at 300, 400, 600, 800 or 1100 degrees C and then studied at room temperature. Single acquisition on (1)H and (31)P nuclei, cross-polarization (CP) at the magical angle spinning, Lee-Golburg homonuclear decoupling in two-dimensional experiments, rotational echo double resonance (REDOR and CP-REDOR) and heteronuclear correlation (HETCOR) were performed. These experiments contribute to evidence the differences between the raw sample and that heated. Indeed, above 300 degrees C, hydrogen phosphate groups (HPO(4)) are completely condensed as diphosphate entities (P(2)O(7)). These results confirm that the TPHPH is successively transformed into a low-temperature form of the TPD (called alpha-TPD), then into its well-known beta-form above 950 degrees C.

Synthesis, characterization, sintering, and leaching of beta-TUPD/monazite radwaste matrices.

To study the simultaneous incorporation of both tri- and tetravalent actinides in phosphate ceramics, we prepared several beta-TUPD/monazite-based radwaste matrices through two different chemical routes (called dry and wet routes) involving the initial precipitation of crystallized precursors of each phase, i.e., TUPHPH solid solutions on the one hand, and rhabdophane (LnPO(4).xH(2)O) on the other. The final material was obtained after heating above 1000 degrees C, and no additional phase was detected from elementary analyses and XRD. Moreover, the complete segregation of tri- and tetravalent cations was evidenced when using dry chemical processes. This method also allows for the preparation of dense pellets (90% < d(exp)/d(calc) < 95%) after only 10-20 h of heat treatment at 1250 degrees C. Finally, the chemical durability of the pellets was examined through several leaching experiments in acidic media. The normalized dissolution rate determined from the uranium release in the leachate varies from (8.2 +/- 0.7) x 10(-6) to (2.7 +/- 0.4) x 10(-2) g m(-2) day(-1) between 25 and 120 degrees C in 10(-1) M HNO(3). Near equilibrium, thorium and lanthanide ions were found to quickly precipitate as phosphate-based neoformed phases in the back end of the initial dissolution process. These phases were identified as uranium-depleted T(U)PHPH and as rhabdophane or monazite.

[The impact of a French guideline on the clinical management of severe head injury (SHI) published in 1999]. / Enquête sur l'impact des recommandations pour la pratique clinique (RPC) << Prise en charge des traumatisés crâniens graves à la phase précoce >>

OBJECTIVE: To evaluate the impact of a French guideline on the clinical management of severe head injury (SHI) published in 1999. STUDY DESIGN: Nationwide survey. METHODS: 182 medical centres answered the inquiry. RESULTS: 111 centres declared to manage such patients suffering from SHI. 68% of doctors from these clinical departments did report to have read these guidelines. Fifty percent of them found in these guidelines useful data for their clinical practice. Forty four percent of centres could not monitor ICP lacking neurosurgical facility in their hospital. Conversely, all hospitals with neurosurgery available did monitor ICP. Seventy six percent of centres reported difficulties to find a facility, which would take these patients in charge when discharged from ICU. We analysed the main items included in the guidelines and report their impact. Seventy four percent of responders found that clinical management of SHI patients had improved during the last years. CONCLUSION: The French guidelines had a significant impact and seem to have provided a useful aid to clinical management of SHI patients. ICP monitoring and the shortage in post-ICU facilities are remaining issues.

[Analysis of indications fo interhospital transfer]. / Analyse des indications de transfert interhospitalier.

In France there are no clear guidelines for the transfer of severely head-injured patients, due to the difficulties of combining the requirements of a safe transfer with the necessity of a rapid, adapted and specialized management. These various aspects are discussed in order to facilitate the definition of local strategies for the initial orientation of severely head-injured patients to a centre adapted for severe head trauma management.

[Protective effect of hypothermia in cerebral ischemia]. / Rôle protecteur de l'hypothermie dans l'ischémie cérébrale.

The use of hypothermia to protect the brain from ischaemic insults is an old concept. During the last decades, studies have mainly shown a modulating effect of hypothermia on biochemical cerebral responses to ischaemia, in addition to the basic effect of energy savings. Thus, the beneficial effects of decreased brain temperature, even when limited during transient ischaemic insults, whether global or focal, complete or incomplete, have been established. The deeper the hypothermia, the longer the ischaemia can be prolonged with acceptable neurological outcome. Conversely, the effects of postischaemic hypothermia remain unclear, while extracerebral deleterious effects cannot be overlooked, and many parameters remain to be evaluated before undertaking a beneficial clinical trial. The only indication for therapeutic postischaemic hypothermia in the near future could be in the control of impending intracranial hypertension occurring after cerebral ischaemia.

No evidence for cerebral hypoperfusion during cerebral malaria.

OBJECTIVE: Among the mechanisms suggested for altered consciousness during cerebral malaria is the hypothesis of cerebral ischemia, which remains controversial, with little supportive O2 conflicting hemodynamic data. The purpose of this study was to test the hypothesis that cerebral ischemia is a main mechanism for altered consciousness during cerebral malaria. SETTING: University hospital pediatric ward in a region with endemic cerebral malaria. DESIGN: Prospective evaluation of cerebral hemodynamics and cerebral oxygenation during cerebral malaria compared with severe malaria anemia without altered consciousness. PATIENTS: During a 2-wk period, we evaluated all patients who were admitted for cerebral malaria (n = 5). Age-matched patients admitted for severe malaria anemia without altered consciousness (n = 3) and outpatients (n = 3) were investigated for comparison. INTERVENTIONS: All patients received the usual treatment according to their needs, which was determined by the physician in charge. Repeated neurologic evaluations were performed during the early management period in patients with cerebral malaria. METHODS AND MAIN RESULTS: We repeatedly measured cerebral blood flow velocity (transcranial Doppler) and simultaneous systemic determinants of cerebral blood flow (arterial pressure, arterial oxygen saturation, PaCO2, rectal temperature, and hemoglobin concentration). The adequacy of cerebral blood flow to oxygen demands during cerebral malaria was assessed by continuous recording of jugular bulb venous oxygen saturation (using a fiberoptic device). Marked cerebral vasodilation was observed during cerebral malaria (systolic velocity, 1.45 +/- 0.09 m/s; diastolic velocity, 0.75 +/- 0.08 m/s; n = 4) and during severe malaria anemia (systolic velocity, 1.18 +/- 0.14 m/s; diastolic velocity, 0.55 +/- 0.05 m/s; n = 3) compared with control children (systolic velocity, 0.84 +/- 0.13 m/s; diastolic velocity, 0.35 +/- 0.06 m/s; n = 3; p < .05). During cerebral malaria, jugular bulb venous oxygen saturation remained stable, including during neurologic recovery, with initial values of 67.5 +/- 4.3%. CONCLUSIONS: Because jugular bulb venous oxygen saturation remained within the normal range, cerebral hyperemia seems to be an adaptive response to altered systemic determinants, which argues against a hemodynamic mechanism for altered consciousness during cerebral malaria.

[Monitoring and maintenance of blood volume in intracranial surgery]. / Monitorage et maintien de la volémie au cours de la chirurgie intracrânienne.

Intraoperative monitoring usually does not assess intravascular blood volume of the patient. The variations of the systolic blood pressure and the right auricular pressure allow to appreciate the intravascular blood volume changes and their consequences on the cerebral perfusion pressure. However, when the autoregulation of the cerebral blood flow is strongly depressed, the evaluation of the cerebral perfusion pressure alone is inadequate. In this case, the optimal monitoring should be metabolic (jugular bulb venous oxygen saturation) to finally assess the cerebral flow-metabolic coupling.

Continuous jugular bulb venous oxygen saturation validation and variations during intracranial aneurysm surgery.

PURPOSE: During intracranial aneurysm surgery, numerous factors may alter cerebral blood flow and oxygen supply-demand balance. Continuous monitoring of jugular bulb venous oxygen saturation (SvjO2) may help in the anesthetic management of such procedures. MATERIALS AND METHODS: Fiberoptic SvjO2 was continuously monitored in seven patients during intracranial aneurysm surgery. Fiberoptic SvjO2 measurement was compared with IL3 CO-OXIMETER determination from 85 paired samples. The occurrence of large SvjO2 variations (SvjO2 variation reaching 10% or more of stable preceding value) during aneurysm surgery was recorded and classified according to the association or not with systemic clinical or therapeutic changes. RESULTS: Fiberoptic SvjO2 showed a limited accuracy, with limits of agreement with IL3 CO-OXIMETER at -16.8% and +10.7% and a small bias (-3.1%). SvjO2 variations were frequent during aneurysm surgery, ranging from 3 to 22 per patient during procedures lasting 6 hours (range 4.5 to 7). Half of these variations occurred in the absence of any systemic clinical or therapeutic change, most often leading to an increased SvjO2. CONCLUSIONS: Although the accuracy of fiberoptic SvjO2 determination is limited, it allows the detection of cerebral blood flow and oxygen supply-demand imbalance during aneurysm surgery. The frequent occurrence of SvjO2 elevations is suggestive of reactive hyperemia mechanisms.

[Monitoring head trauma]. / Monitorage du traumatisme crânien.

Cerebral monitoring after head trauma aims at reducing secondary lesions resulting mainly from additional cerebral ischemia-hypoxia. Such monitoring must include repetitive (preferably continuous) estimation of cerebral blood flow, cerebral oxygenation and their variations. Based on this information the evaluation and modulation of the main determinants of cerebral blood flow (cerebral metabolism, cerebral perfusion pressure, arterial oxygen content and PaCO2 variations) often allows optimization of cerebral blood flow and metabolism balance. This therapeutic strategy may lead to improvement of severe head trauma outcome through reduction of additional cerebral ischemia-hypoxia.