N. Dacheux

Preparation and characterization of lanthanum–gadolinium monazites as ceramics for radioactive waste storage

Several La1u2006−u2006xGdxPO4 solid solutions were prepared in the monazite- or rhabdophane-type structures for various x values using three methods of preparation (direct evaporation, synthesis in closed PTFE containers on a sand bath or in autoclaves). Samples of rhabdophane-type La1u2006−u2006xGdxPO4·nH2O (nu2006≈u20060.5) were prepared at 150u2006°C only for xu2006≥u20060.4. For xu2006≤u20060.3, the solids were precipitated as the monazite-type structure. These results were confirmed by the study of pure rare earth phosphates synthesized under the same conditions. By these means, well-crystallized and monophase samples of MPO4·nH2O (nu2006≈u20060.5–1) in the monazite (La, Ce), rhabdophane (Nd, Sm, Eu, Gd, Tb, Dy) or churchite (Ho, Er, Tm, Yb, Lu) forms were prepared. n On the basis of the variation of the specific area versus the holding temperature and of the dilatometric studies, the optimal temperature of sintering for these solids was found to be between 1250 and 1400u2006°C. The effective relative densities of the pellets of GdPO4 prepared using a two-step procedure (pressing between 200 and 700 MPa, then heat treatment at 1300u2006°C) reached 96% of the value calculated from the XRD data. The chemical durability of sintered samples of GdPO4 was evaluated in several acidic media between room temperature and 90u2006°C. The very low normalized dissolution rates RL n(between 10−6 and 10−3 gxa0m−2xa0day−1) measured even in very acidic media confirmed the very good retention properties of this kind of phosphate-based matrix for the immobilization of radionuclides and especially of trivalent actinides.

Stability and Structural Evolution of CeIV1–xLnIIIxO2–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.

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.

Understanding the peak asymmetry in alpha liquid scintillation with β/γ discrimination

The peak evaluation in alpha liquid scintillation is known to be easy, mostly due to the gaussian shape of the peaks. However, we often observed a high-energy tail in addition to a pure gaussian function. This effect is only detectable with a high resolution α liquid scintillation spectrometer such as the PERALS® system. Indeed, its intrinsic resolution (180 keV for a 4 MeV α particle) is better than that obtained for conventional LSC spectrometers. The peak asymmetry was quantified using the Fisher´s coefficient γ1 (symmetry factor). We show that the main effect responsible for the asymmetry is internal conversion. Indeed, most of the even-even nuclides have low α intensity transitions leading to excited levels of their daughter nuclides. The internal conversion is almost equal to 100% and consequently produces a sum peak at higher energy. No generalization is possible for odd-even nuclides, but the knowledge of their disintegration scheme allows to explain the experimental values obtained and the differences observed (e.g. between 241Am and 243Am). The experimental determinations of γ1 are given for polonium, radon, radium, thorium, uranium, plutonium, americium, and curium isotopes. We show the necessity to take into account the L and M shell contributions for few nuclides like thorium isotopes to get a maximum accuracy in the activity measurements.

Some rules to improve the energy resolution in alpha liquid scintillation with beta rejection

Abstract Two common scintillating mixtures dedicated to alpha measurements by means of alpha liquid scintillation with pulse shape discrimination were tested: the di-isopropylnaphthalene — based and the toluene-based solvents containing the commercial cocktails Ultima Gold AB® and Alphaex®. We show the possibility to enhance the resolution up to 200% by using no-water miscible cocktails and by reducing the optical path. Under these conditions, the resolution of about 200 keV can be obtained either by the Tri CarbTM or by the PeralsTM spectrometers. The time responses, e.g., the time required for a complete energy transfer between the initial interaction alpha particle-solvent and the final fluorescence of the organic scintillator, have been compared. Both cocktails present similar behavior. According to the Förster theory, about 6-10 ns are required to complete the energy transfer. For both apparatus, the detection limits were determined for α emitters. The sensitivity of the Tri Carb was improved by 300% but remains 40% lower than that of Perals, due to a higher α/β discrimination and the use of vials with lower optical path.

Comparison and improvement of the resolution of several alpha liquid scintillation spectrometers for actinide measurements

In order to enhance the resolution in a liquid scintillation with pulse shape discrimination for the measurement of actinide at low level, a study of two scintillating cocktails (Alphaex™ and Ultima Gold LLT™) and two alpha liquid scintillation spectrometers, (PERALS™ and TRI-CARB™) was undertaken. The criteria which can be applied to improve the resolution are the use of no-water miscible cocktail, the optimisation of energy transfer through the knowledge of the spectroscopic data, the employ of P.M. with higher quantum efficiency, scintillators with low Stokes shift and high fluorescence quantum yield and the use of vial with low optical path.

Separation of uranium(VI) from tri- and tetravalent elements in phosphoric acid solutions

Summary A new method of separation of uranyl ions from di-, tri- and tetravalent cations such as thorium, zirconium, cerium(IV) and/or lanthanides(III) involving phosphoric acid medium is described in this paper. The process is based on the difference of solubility between various phosphates: M2(PO4)2(HPO4)·H2O, M(HPO4)2·H2O, M(OH)PO4 (where M is a tetravalent cation) or APO4·n H2O (where A is a trivalent cation) on the one hand, and the uranyl phosphate pentahydrate (UO2)3(PO4)2·5H2O, on the other hand. Taking into account the associated solubility constants, the conditions of separation were calculated then experimented for a mixture of Th4+, Ce4+ (as a plutonium surrogate), Zr4+, La3+, Ba2+, Sr2+ and UO22+. Tetravalent and trivalent cations phosphates were precipitated in 2 mol L−1 nitric acid medium while uranyl ion and divalent cations remained in the solution. In a second step, (UO2)3(PO4)2·5H2O was precipitated at pH ≈ 1.5 and thereby separated from divalent cations present in the solution.

Study of Actinides Incorporation in Thorium Phosphate-Diphosphate/Monazite Based Ceramics

Phosphate materials are usually considered as potential candidates to perform the immobilization of actinides coming from an advanced reprocessing of spent fuel in the field of an underground repository. Among them, Thorium Phosphate-Diphosphate (TPD) and monazites have been already extensively studied. The elaboration of TPD/monazite based materials was thus envisaged in order to immobilize simultaneously tri- and tetravalent actinides and a neutron absorber. Two chemical ways of synthesis were considered and the compounds were easily prepared in the powder and in the pellet form. A good chemical compatibility was found between TPD and monazite since the properties of both phosphates were kept in these composites. Moreover, the relative density of the pellets reached 90 – 95 % of the value calculated from XRD data. The normalized dissolution rates determined in acidic media did not exceed 5.10 −4 g.m −2 .day −1 which confirmed the very good durability of such materials during leaching tests.