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New data on microstructure of 16 years old (La, Pu)PO4 monazite ceramics doped with 8.1 wt% of 238Pu are presented. It is shown that the sample consists from at least two phases differing in La/Pu ratio and small precipitates of Pu-phosphate. Possible mechanisms of the compositional heterogeneity are discussed. Formation of Pu-containing rhabdophane after sample storage in air is observed. This phenomenon together with gradual mechanical destruction of the ceramic pellet formation of submicron particles will likely increase rate of radionuclides loss from the monazite-based waste form. X-ray emission lines produced by recoil uranium ions from Pu decay are analysed. It is suggested that careful examination of their relative intensities may provide important information about behaviour of "hot" recoils in nuclear waste forms.
Mechanical damage of non-metallic nuclear wasteforms can be caused by electrical fields induced by decaying clusters of radionuclides surrounded by an insulating matrix. We assess the electric fields near clusters with decaying radionuclides 244Cm, 241Am, 238,239Pu and 137Cs in a glass matrix determining that matrix destruction can gradually occur via electric breakdown discharges and diffusion-controlled change in form of clusters. The most important parameters that control potential matrix destruction are the radioactive cluster (inhomogeneity) size, radionuclide specific radioactivity and effective electrical conductivity of the matrix.
Results of characterization of 238Pu-doped Eu- and La-monazites using
single crystal XRD, Raman and XAFS spectroscopy and TEM are presented. It is
shown that despite significant accumulated doses (up to 9x1018
α-decays/gram) the Eu-monazite remains a single crystal. Unusual foamy
structures are observed by TEM and are interpreted as recrystallisation of
domains damaged by recoil U-ions. Partial recrystallisation of the surface
material is also supported by Raman and luminescence data.
Samples of 238Pu-doped single-phase ceramics based on cubic zirconia,
monazite, La0.9Pu0.1PO4, have been studied by
static leach test in distilled water. Before leach test accumulated doses were
(in alpha-decays/m3 x 1026): from 1.6 to 1.7 –
for cubic zirconia; and 1.0 – for monazite. Despite high radiation
damage both phases remained crystalline according to XRD analysis. The results
of static leach tests demonstrate the following Pu normalized mass loss (in
g/m2, 90°C, 28 days): from 0.3 to 0.7 – for
cubic zirconia; and 1.6 – for monazite. These data are discussed in
comparison with results of previous leach tests carried out at lower accumulated
Natural metamict mineral found as large (1-3 cm in size) homogeneous grains (as assumed, former single crystals), was investigated by X-ray powder diffraction (pXRD), high-temperature pXRD, scanning electron microscopy (SEM) and electron microprobe analysis (EMPA). The average chemical composition obtained by EMPA is (wt. %): Nb2O5 – 42.6; Ta2O5 – 4.4; TiO2 – 9.2; UO3 – 4.4; ThO2 – 1.0; MnO – 1.3; FeO – 19.4; Y2O3 – 16.6.
The untreated (original) sample is X-ray amorphous. The sample remained amorphous after annealing at 400 °C for 1 hour. The sample became almost fully crystalline after annealing at 700 °C for 1 hour with an X-ray diffraction pattern similar to that of Fe-columbite (ICCD: 01-074-7356). Further annealing at 1000 °C and higher temperatures caused changes in the phase composition of the sample. It was proposed that under self-irradiation a single-phase U-Th-bearing solid solution, based on monocrystalline Y-niobate, became metamict but remained homogeneous without evidence of solid solution destruction. However, this metamict solid solution is unstable under thermal treatment and recrystallization.
Technetium-99 is considered as one of the most dangerous nuclear environmental pollutants due to its long half-life (210,000 y.) and high mobility in aqueous solutions under oxidizing conditions. Development of sorbents, which are capable of irreversible uptake of Tc and further direct conversion into durable ceramic waste forms, is an important field of research. Titanate ceramic doped with up to 10 wt. % Tc was successfully synthesized using Layered Hydrazinium Titanate, LHT-9 (PCT/EP2010/001864) as starting precursor. LHT-9 is a new advanced compound of general formula (N2H5)1/2Ti1.87O4xH2O containing 6-7 wt. % of hydrazine chemically incorporated into a TiO2-based matrix. It was demonstrated that LHT-9 (5g/l) can reductively adsorb up to 90.2 wt. % of Tc from aqueous solutions containing 0.5g Tc/l. The obtained adsorption products can be easily converted into stable titanate ceramic by one-step sintering in argon atmosphere at 1200°C. Phase and chemical composition of synthesized Tc-doped ceramic are discussed.
Layered hydrazinium titanate LHT-9, (N2H5)1/2Ti1.87O4 is a new nanohybrid material related to lepidocrocite-type titanates. Unique combination of ion exchange, reductive properties, surface activity due to Brønsted acid sites and occurrence of surface titanyl groups allows exploring LHT-9 for simultaneous uptake of almost all components of liquid nuclear wastes. LHT-9 irreversibly removes technetium, molybdenum, palladium and selenium from their aqueous solutions by specific mechanism of reductive adsorption. For removal of cesium, strontium, transition elements, actinides and lanthanides LHT-9 provides mechanisms of ion exchange and surface complexation. Products of adsorption are nanocrystalline and homogeneous powders loaded with 5 to 15 wt. % of radionuclides and non-radioactive elements. LHT-9 can be applied as ready-to-use precursor for one-step synthesis of durable titanate ceramic waste forms similar to SYNROC. An essential advantage of LHT-9 in comparison with other titanate sorbents (monosodium titanate and peroxo-titanate materials) is the absence of Na in its composition that permits arbitrary tailoring of sorbent properties by simple pre-treatment with the desired elements. Results on sorption of americium, cesium, strontium and lanthanides by LHT-9 are discussed.
Durable crystalline actinide host phases of ceramic waste forms are considered as advanced materials which are prospective for safe use of Pu and minor actinides before their final disposal. Development of self-glowing actinide-doped materials with matrices that are chemically inert and resistant to radiation damage may significantly change the approaches to actinide immobilization. Single crystals of zircon doped with different amount of Tb and 238Pu were synthesized by the flux method. Different non-radioactive crystals of Tb-doped zircon were studied first by cathodoluminescence method in order to identify the optimal content of Tb3+ that provides the highest luminescence emission. Then self-glowing crystals of zircon were grown with the optimal Tb content and small admixture of 238Pu (less than 0.1 wt. %). It was proposed that the valence state of Tb incorporated into zircon crystals can be (3+) and (4+), but only trivalent Tb is responsible for intensive luminescence. It is demonstrated that a small addition of Zr-phosphate to the flux supports Tb incorporation into zircon lattice and stabilizes preferably Tb3+. At the same time the addition of Zr-phosphate caused the crystallization of zirconia as a minor phase. Zircon crystals with very intensive self-glowing were successfully synthesized. The 238Pu content was 0.02 wt.% and the Tb concentration varied between 0.2 and 0.3 wt.%. Zirconia crystals obtained from the same experiment are characterized by weak self-glowing, although the Tb content was only 0.02 wt.%, while the content of 238Pu was comparable to that of zircon, i.e. 0.03 wt. %.
Some crystals doped with radionuclides glow in the dark. Such materials are prospective for certain industrial scale applications. Durable self-glowing crystalline solids, which were initially suggested for development of actinide waste forms, are considered as advanced materials. Well-known durable actinide host phases, such as zircon, xenotime, and monazite are main focus of current research. Single crystal samples of these host phases doped with 239Pu, 238Pu, 241Am and 237Np have been grown by flux methods. It is demonstrated that incorporation of small amounts of non-radioactive elements such as Eu, In and Tb increases the self-glowing intensity. The optimal content of such luminescence ions supporting intensive glowing of 238Pu-doped zircon and xenotime has, at first, been identified by cathodoluminescence study of non-radioactive samples. Subsequently, the results of this study were used to grow intensively glowing crystals of zircon and xenotime doped with 0.01 wt. % and 0.1 wt.% 238Pu, respectively.
Immobilization of long-lived 99Tc requires development of chemically resistant inorganic matrices. Samples of ceramics based on crystalline Fe-Mn- and Zr-Mn-oxide compounds were synthesized at 1150°C in air, reducing or inert atmosphere from precursors doped with 5-12 wt.% Tc. All the samples obtained were studied using optical and scanning electron microscopy (SEM); powder X-ray diffraction (XRD) and microprobe analysis (EMPA). Content of Tc varied from 0.5-0.8 to 3-6 wt.% in oxide host phases and from 54 to 93 wt.% in metallic inclusions. It was demonstrated that synthesis of oxide host-phases under oxidizing or reducing conditions was not optimal due to partial Tc volatilization or metallic phase formation, respectively. The use of inert atmosphere for ceramic synthesis supports Tc incorporation into crystalline structure of stable host-phases. Development of optimal methods of precursor preparation and synthesis conditions of Tc-doped ceramic are being discussed.
Excess plutonium not destined for burning as MOX or in Generation IV reactors is both a long-term waste management problem and a security threat. Immobilisation in mineral and ceramic-based waste forms for interim safe storage and eventual disposal is a widely proposed first step. The safest and most secure form of geological disposal for Pu yet suggested is in very deep boreholes and we propose here that the key to successful combination of these immobilisation and disposal concepts is the encapsulation of the waste form in small cylinders of recrystallized granite. The underlying science is discussed and the results of high pressure and temperature experiments on zircon, depleted UO2 and Ce-doped cubic zirconia enclosed in granitic melts are presented. The outcomes of these experiments demonstrate the viability of the proposed solution and that Pu could be successfully isolated from its environment for many millions of years.
To investigate the resistance of actinide host phases to accelerated radiation damage, which simulates radiation induced effects of long term storage, the following samples doped with plutonium-238 (from 2 to 10 wt. %) have been repeatedly studied using XRD and other methods: cubic zirconia, Zr0.79Gd0.14Pu0.07O1.99; monazite, (La,Pu)PO4; ceramic based on Pu-phosphate of monazite structure, PuPO4; ceramic based on zircon, (Zr,Pu)SiO4, and minor phase tetragonal zirconia, (Zr,Pu)O2; single crystal zircon, (Zr,Pu)SiO4; single crystal monazite, (Eu,Pu)PO4; ceramic based on Ti-pyrochlore, (Ca,Gd,Hf,Pu,U)2Ti2O7. No change of phase composition, matrix swelling, or cracking in cubic zirconia were observed after cumulative dose 2.77×1025 alpha decay/m3. The La-monazite remained crystalline at cumulative dose 1.19×1025 alpha decay/m3, although Pu-phosphate of monazite structure became nearly amorphous at relatively low dose 4.2×1024 alpha decay/m3. Zircon has lost crystalline structures under self-irradiation at dose (1.3-1.5)×1025 alpha decay/m3, however, amorphous zircon characterized with high chemical durability. The Ti-pyrochlore after cumulative dose (1.1-1.3)×1025 alpha decay/m3 became amorphous and lost chemical durability. Radiation damage caused crack formation in zircon single crystals but not in the matrix of polycrystalline zircon. Essential swelling and crack formation as a result of radiation damage were observed in ceramics based on Ti-pyrochlore and Pu-phosphate of monazite structure, but not so far in La-monazite doped with 238Pu.
Cubic zirconia is a well known, highly durable material with potential uses as an actinide host phase in ceramic waste forms and inert matrix fuels and in containers for very deep borehole disposal of some highly radioactive wastes. To investigate the behaviour of this material under the conditions of possible use, a cube of ∼ 2.5 mm edge was made from a single crystal of yttriastabilized cubic zirconia doped with 0.3 wt.% CeO2. The cube was enclosed in powdered granite within a gold capsule and a small amount of H2O added before sealing. The sealed capsule was held for 4 months in a cold-seal pressure vessel at a temperature of 780°C and a pressure 150 MPa, simulating both the conditions of a deep borehole disposal involving partial melting of the host rock and the conditions under which the actinide waste form might be encapsulated in granite prior to disposal. At the end of the experiment the quenched, largely glassy, sample was cut into thin slices and studied by optical microscopy, EMPA, SEM and cathodoluminescence methods. The results show that no corrosion of the zirconia crystal or reaction with the granite melt occurred and that no detectable diffusion of elements, including Ce, in or out of the zirconia took place on the timescale of the experiment. Consequently, it appears that cubic zirconia could perform most satisfactorily as both an actinide host waste form for encapsulation in solid granite for very deep disposal and as a container material for deep borehole disposal of highly radioactive wastes (HLW), including spent fuel.
In order to study americium incorporation into calcite, CaCO3, under conditions of crystal growth, two samples of single crystal Am-doped calcite were synthesized and studied by cathodoluminescence (CL) spectroscopy in comparison with undoped and Eu-doped artificial calcite. Americium contents in calcite crystals were (in kBq/g): 1) 6.9; 2) 1.9(E+4). The CL emission of undoped and Am-Eu-doped calcitesamples was characterized by three broad bands at 2.03; 2.47 and 2.96 eV. Weak CL lines related to typical transitions 5D07F1,2,4 of Eu3+ and Am3+ions were observed at 1.68; 1.99, 2.06 eV and 1.60; 1.98 eV, respectively. Degrading of calcite structure under irradiation has been studied using CL emission of high power electron beam.
Sample of natural Zr-silicate gel containing up to 13 wt.% U was characterized using scanning electron microscopy (SEM), X-ray powder diffraction (XRD), high resolution transmission electron microscopy (HRTEM) and electron microprobe analysis (EPMA) method. It was found that gel matrix is amorphous in general; however, it contains non-identified nanocrystallites. No separated oxide phases of U, Zr or Si were observed in a gel matrix. After sintering in air at 1400°C for 1 hour gel transformed largely into crystalline zircon, (Zr,U)SiO4. Uranium was not found in any other phases besides zircon. It was assumed that high chemical durability of natural Zr-P-U-Ti-silicate gel is caused by two competing processes which exist under self-irradiation conditions: 1) crystallization of the gel and 2) metamictization of the crystallized zircon and other phases.
Synthetic samples of Zr-silicate gel doped with Ce, U, Pu and Am were obtained and studied in comparison with natural samples. It was suggested to use artificial solid Zr-silicate gels for durable fixation of actinides for the goal of long-term or intermediate storage.
polycrystalline sample of Ti-pyrochlore, (Ca,Gd,Hf,U,Pu)2Ti2O7, doped with approximately 8.7 wt.% 238Pu and 20.0 wt.% 238U, has been studied at different cumulative doses of alpha induced radiation damage using scanning electron microscopy (SEM) and quantitative electron probe microanalysis (EMPA). All analyses of 238Pu-doped Ti-pyrochlore were done for comparison with 239Pu-doped pyrochlore synthesized under similar conditions. Our results suggest that accelerated alpha-induced radiation damage affects not only the crystalline structure of 238Pu-doped pyrochlore, but also causes changes in the chemical composition of the pyrochlore, and as proposed, destruction of pyrochlore-U-Pu solid solution. Numerous inclusions of a separate U-oxide phase were found in the 238Pu-doped pyrochlore crystalline matrix. The inclusions were observed visually using SEM and increased as a function of the cumulative dose. Also, after a cumulative dose of 1.1 × 1025 alpha decays/m3 the pyrochlore became nearly amorphous and inclusions of Hf-Ti-Ca-O phase were found in the crystalline matrix. In all cases separate phases of U-oxide and Hf-Ti-Ca-O were localized in chemically inhomogeneous areas. The results obtained allow us to assume that under self-irradiation the amorphization of Ti-pyrochlore might be accompanied with destruction of single solid solution, (Ca,Gd,Hf,U,Pu)2Ti2O7, into several pyrochlore phases of different chemical composition and precipitation of some amount of Pu and U into separate oxide phases.
To investigate the behavior of monazite during accelerated radiation damage, which simulates effects of long term storage, 238Pu-doped polycrystalline samples of (La,Pu)PO4 and PuPO4 were synthesized for the first time ever and studied using powder X-ray diffraction (XRD) analysis and optical microscopy. The starting precursor materials were obtained by precipitation of La and (or) Pu from their aqueous nitrate solutions followed by calcination in air at 700°C for 1 hour, cold pressing, and sintering in air at 1200-1250°C for 2 hours. The 238Pu contents in ceramic samples measured using gamma spectrometry were (in wt.% el.): 8.1 for (La,Pu)PO4 and 7.2 for PuPO4. The (La,Pu)PO4 monazite remained crystalline at ambient temperature up to a cumulative dose of 1.19 × 1025 alpha decays/m3. In contrast, the PuPO4 monazite became nearly completely amorphous at a relatively low dose of 4.2 × 1024 alpha decays/m3. Swelling and crack formation due to the alpha decay damage was observed in the PuPO4 ceramic. Also, under self-irradiation this sample completely changed color from initial deep blue to black. The (La,Pu)PO4 monazite was characterized by a similar change in color from initial light blue to gray, however, no swelling or crack formation have so far been observed. The results of this study allow us to conclude that the radiation damage behavior of monazite strictly depends on the chemical composition. The justification of monazite-based ceramics as actinide waste forms requires additional investigation.
In order to obtain Pu-doped zircon, (Zr,Pu)SiO4, with a maximum Pu content in the form of solid solution, zircon single crystals have been grown using the flux method from starting materials overloaded with Pu. The crystals obtained ranged from 0.2-0.5 to 3.5-4.5 mm in size, are transparent, and characterized by deep pink-brown color. No inclusions of separate Pu phases were observed in the crystals. The distribution of Pu in crystals is zoned and the Pu content varying from approximately 5 to 14 wt.% el. The zircon unit cell parameters calculated from XRD data of bulk powdered sample were: a=6.620(1); c=5.989(2). The results obtained allow us to conclude that the capacity of the zircon structure to incorporate Pu exceeds 10 wt.% el.; however, additional research is required to study the extent of solid solution, (Zr,Pu)SiO4, at higher Pu contents.
Since 1990, the Laboratory of Applied Mineralogy and Radiogeochemistry of the V.G. Khlopin Radium Institute (KRI) has been developing several different types of crystalline host-phases acceptable for the economically feasible and environmentally safe immobilization of actinide wastes. We proposed that ceramics that are based on host phases similar to naturally occurring accessory minerals including zircon, (Zr,Hf,…)SiO4; hafnon, (Hf,Zr,…)SiO4; baddeleyite (monoclinic zirconia), (Zr,Hf,…)O2; tazheranite (cubic zirconia), (Zr,Hf,Ca,Ti,…)O2; garnet, (Ca,Fe,Gd,…)3(Al,Fe,Si,…)5O12; perovskite, (Ca,Gd,…)(Al,Fe,Ti,…)O3, and monazite, (La,Ce,…)PO4, are the most efficient materials for actinide immobilization in deep geological formations. Solid solution of Pu in zirconia, (Zr,Pu)O2, could be used as a ceramic nuclear fuel that is competitive with mixed oxide fuel (MOX). To date, the following crystalline materials doped with 239Pu, 238Pu and 243Am have been successfully synthesized and studied at KRI: zircon; hafnon; cubic and tetragonal zirconia; monazite; aluminate garnet and perovskite. The maximum actinide loading was (in wt.% el.): 239Pu -37; 238Pu-10; 243Am-23. All Pu-Am-doped samples were made in air atmosphere under glove boxes conditions. Polycrystalline (ceramic) materials were made by sintering or melting of sol-gel, co-precipitated hydroxides, oxalates and phosphates or ground oxide precursors; single crystals were grown by a flux method. It was demonstrated that all ceramic samples obtained are characterized by high chemical durability and typical normalized actinide losses in deionized water at 90°C do not exceed 10−2–10−3 g/m2 (without correction for ceramic porosity). However, investigation of long-term behavior of ceramic waste forms requires taking into account the results of accelerated radiation damage study and modeling of ceramic alteration by underground solutions. The principal features of Pu-Am-doped samples obtained so far at KRI and their synthesis conditions are discussed.
The experience of the Laboratory of Applied Mineralogy and Radiogeochemistry of the V.G.Khlopin Radium Institute on synthesis of Pu-Am-doped ceramics is summarized. During the last 5 years, dozens of actinide doped polycrystalline samples and single crystals have been successfully synthesized such as zircon, hafnon, cubic zirconia, monazite, Ti-pyrochlore, perovskite and garnet. Actinide loading has been varied as follows:
-239Pu - from 5–6 wt.% in zircon (polycrystalline and single crystals), hafnon, garnet and perovskite to 10 wt.% in Ti-pyrochlore and up to 37 wt.% in zirconia;
- 238Pu - from 2.5 wt.% in zircon single crystals to 5 wt. % in polycrystalline zircon and 10 wt.% in monazite and cubic zirconia;
- 243Am - 20–23 wt.% in cubic zirconia and monazite.
The weight of each single ceramic pellet varied from 0.2 to 2.0 grams. Special furnaces developed in KRI for ceramic synthesis allowed obtaining up to 7 ceramic pellets simultaneously during the same experiment. The highest amounts of actinides used under glove-box conditions in the same experiment were: 1.5–2.0 g for 239Pu, 0.6 g for 238Pu and 0.3 g for 243Am. Most experiments on synthesis of ceramics and single crystals doped with 239Pu, 238Pu and 243Am carried out at the KRI did not lead to contamination of internal walls of glove boxes. No release of Pu-Am-aerosols was observed as a result of sintering or melting at 1300–1600°C. These results allowed us to conclude that at the present the KRI has developed the experimental basis for transferring laboratory innovations to the industry of actinide immobilization. It is important that adopting ceramic synthesis methods at industrial scale does not require development of new special equipment.