Boris E. Burakov

A Raman spectroscopic study of high-uranium zircon from the Chernobyl “lava”

We have studied technogenic, high-uranium zircon, which crystallised from melt formed during the accident at the Chernobyl Nuclear Power Plant in 1986, by confocal Raman spectroscopy, electron microprobe, and backscattered electron imaging. The correlation between Raman and electron microprobe measurements allowed studying mode behaviour as a function of U content of the strongly zoned zircon crystals. The USiO 4 content in the crystals ranges between 0.6 and 11.6 mol. %, corresponding to 0.8 and 15.8 wt. % UO 2 , respectively. The frequency of the v 1 (SiO 4 ) symmetrical and v 3 (SiO 4 ) anti-symmetrical stretching mode decreases by 0.67(3) and 0.75(3) cm -1 per mol. % USiO 4 , respectively, which is a result of an increasing Si-O bond length with increasing U content. The lattice modes show a comparable shift to lower frequencies, whereas the internal v 2 (SiO 4 ) and v 4 (SiO 4 ) bending modes exhibit no or only a small shift to lower frequencies ( -1 per mol. % U). Only the frequency of the lowest energy band (E g ) near 202 cm -1 increases slightly with increasing U content, which is unexpected and indicates that the cation-(SiO 4 ) 4- potentials, the electron orbitals, and/or the cation radius have a strong effect on this mode. The line broadening, reflecting the range of local distortions ( i.e. , microscopic strain), is most pronounced for the lattice modes, in agreement with the large size difference of both cations. We found that the E g lattice modes, involving the movement of the SiO 4 tetrahedron and the cation within the a(b) plane, show significantly larger line broadening with increasing U concentration than the B 1g modes, involving lattice vibrations along the c axis. This suggests that the microscopic strain is significant larger in the a(b) plane than along the c axis, which can be explained by the structural properties of zircon.

Crystalline materials for actinide immobilisation

Physical and Chemical Properties of Actinides Areas of Actinide Use Nuclear Waste Immobilisation of Actinides Synthesis of Chemically Durable Actinides Analytical Methods for Actinide Future Potential of Actinide Containing Materials.

Synthesis and investigation of Pu-doped single crystal zircon, (Zr, Pu)SiO4

Summary Single crystals of zircon doped with 239Pu, up to 2 mm in size, were grown for the first time ever, using a Li-Mo flux. The crystals were transparent and free of inclusions of separated Pu phases. The zircon crystals were studied using optical and scanning electron microscopy, the electron microprobe, and cathodoluminescence imaging and spectroscopy. The incorporation of 239Pu ranged from 0.1 to 1.4 wt.% el. The intensity of the CL emission in the Pu-doped crystals is correlated with the Pu content.

Secondary Uranium Minerals on the Surface of Chernobyl “Lava”

The formation of uranium minerals is still continuing in Chernobyl Unit No. 4. Yellow products of alteration that stain the surface of Chernobyl “lava” have been examined by SEM and X-ray diffraction methods. Secondary minerals of uranium identified are: UO 4 ·4H 2 O studtite; UO 3 ·2H 2 O epiianthinite; UO 2 ·CO 3 rutherfordine; also, Na 4 (UO 2 )(CO 3 ) 3 was identified together with the sodium carbonate phases Na 3 H(CO 3 ) 2 ·2H 2 O and Na 2 CO 3 ·H 2 O. These minerals formed due to the interaction between fuel-containing masses or “lava”, water and air. The matrices of the “lava” do not contain significant amounts of sodium. The source of sodium may be water that has penetrated into the “Sarcophagus”. All identified secondary minerals of uranium are highly unstable, and their continued formation can seriously endanger the radiological situation of the 4 th Unit.

Behavior of 238 Pu-Doped Ceramics Based on Cubic Zirconia and Pyrochlore under Radiation Damage

Crystalline ceramics based on durable actinide host phases such as cubic zirconia and titanate pyrochlore have been suggested for the immobilization of weapons grade plutonium and actinide wastes. Samples of crystalline ceramic based on the gadolinia-stabilized cubic zirconia, (Zr,Gd,Pu)O2, structure doped with 9.9 wt.% 238Pu were synthesized and characterized in comparison with samples of pyrochlore-based ceramic, (Ca,Gd,Hf,U,Pu)2Ti2O7, doped with 8.7 wt.% 238Pu. It was found that a resistance of cubic zirconia to self-irradiation is much higher than that of pyrochlore. At the cumulative dose l.lx1025alpha decays/m3, cubic zirconia retained its crystalline structure. No swelling or cracking were observed in the ceramic matrix. At the same cumulative dose the titanate pyrochlore became nearly amorphous and the density decreased by approximately 10 % in comparison with the initial, unaltered sample. Under self-irradiation, both ceramics demonstrated an increase of normalized Pu mass loss in deionized water depending on cumulative doses, but this increase is significantly greater for the pyrochlore-based ceramic.

Synthesis and Study of 239Pu-Doped Gadolinium-Aluminum Garnet

Garnet solid solutions, Y 3 A1 5 O 12 -Gd 3 A1 5 O 12 -Gd 3 Ga 5 O 12 (YAG-GAG-GGG), are being considered as prospective durable host phases for the immobilization of actinide-containing waste with complex chemical compositions. Garnet samples with the suggested simplified formula: (Gd,Ce…) 3 (Al,Ga,Pu,…) 5 O 12 containing from 3.4 to 5.3 wt.% 239 Pu and 3.6-5.5 wt.% Ce have been synthesized through melting of oxide starting materials in air using a hydrogen torch. Calcium and Sn were added to increase the Pu incorporation into the garnet lattice through ion charge and size compensation for Pu 4+ . Polycrystalline materials obtained in the experiments consist of garnet, perovskite and other phases and were studied by scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). Our results confirmed that the use of compensating elements such as Ca and Sn allow for significant incorporation of Pu and Ce (not less than a few wt.%) into the garnet structure. The preliminary conclusions thus so far indicate that garnet solid solution compositions may incorporate simultaneously trivalent and tetravalent actinides in significant quantities because they occupy different positions in the garnet structure

The Behavior of Nuclear Fuel in First Days of the Chernobyl Accident

Various types of Chernobyl fuel containing masses named black “lava”, brown “lava”, porous “ceramic” and “hot” particles that formed during first days of the accident at the Chernobyl Nuclear Power Plant 4th Unit were studied by methods of optical and electron microscopy, microprobe and x-ray diffraction. Data about their chemical, phase and radionuclide composition are summarized. The products of interaction between fuel, zircaloy and concrete, produced under experiments in laboratory were examined for comparison with samples of Chernobyl “lava” and “hot” particles. The behavior of nuclear fuel in first days of the Chernobyl accident was a three-stage process. The first stage occurred before the moment of the Chernobyl explosion and was exceptionally short-lasting, perhaps, less than a few seconds. It was characterized by reaching a high temperature, ≥2600 °C, in the epicenter of accident and formation of a Zr-U-O melt in a local part of the core, which is estimated to be not more than 30% of whole core volume. The second stage lasted for about 6 days since the explosion, during which there was interaction between uranium products of the destroyed reactor: UOx, UOx with Zr, Zr-U-O, with the environment and silicate structural materials of the 4th Unit. The third stage, after 6 days involved the process of final formation of the radioactive silicate melt or Chernobyl “lava” at one of the sections of the destroyed 4th Unit. During this stage the melts lamination occurred, followed by a break-through of the “lava” reservoir on the 11 th day of the accident and penetration of the “lava” into space under the reactor.

Synthesis of zircon for immobilization of actinides

A new method of synthesis for actinide-doped zircon is presented based on studies of zircons formed by crystallization from the reactor core melt generated in the course of the accident at the Chernobyl Nuclear Power Plant. These zircons have compositions in the range (Zr{sub 0.94}, U{sub 0.06})SiO{sub 4} to (Zr{sub 0.9}, U{sub 0.1})SiO{sub 4}. Hot-pressing of oxides was studied to make Zr-based waste forms. The results demonstrate the efficacy of using metallic zirconium in synthesizing high-actinide zircons. In the event of deviations from zircon`s ideal stoichiometry, ZrO{sub 2} forms, which is also an effective host phase for actinide elements. Waste streams high in zirconium and actinides could be converted into Zr-based waste forms. The adaptation and modification of the mixed-oxide reactor fuel (MOX) production process is proposed as a process for the production of (Zr,Pu)SiO{sub 4}, a durable waste form for excess weapons plutonium.

Multiscale Speciation of U and Pu at Chernobyl, Hanford, Los Alamos, McGuire AFB, Mayak, and Rocky Flats.

The speciation of U and Pu in soil and concrete from Rocky Flats and in particles from soils from Chernobyl, Hanford, Los Alamos, and McGuire Air Force Base and bottom sediments from Mayak was determined by a combination of X-ray absorption fine structure (XAFS) spectroscopy and X-ray fluorescence (XRF) element maps. These experiments identify four types of speciation that sometimes may and other times do not exhibit an association with the source terms and histories of these samples: relatively well ordered PuO2+x and UO2+x that had equilibrated with O2 and H2O under both ambient conditions and in fires or explosions; instances of small, isolated particles of U as UO2+x, U3O8, and U(VI) species coexisting in close proximity after decades in the environment; alteration phases of uranyl with other elements including ones that would not have come from soils; and mononuclear Pu-O species and novel PuO2+x-type compounds incorporating additional elements that may have occurred because the Pu was exposed to extreme chemical conditions such as acidic solutions released directly into soil or concrete. Our results therefore directly demonstrate instances of novel complexity in the Å and μm-scale chemical speciation and reactivity of U and Pu in their initial formation and after environmental exposure as well as occasions of unexpected behavior in the reaction pathways over short geological but significant sociological times. They also show that incorporating the actual disposal and site conditions and resultant novel materials such as those reported here may be necessary to develop the most accurate predictive models for Pu and U in the environment.

Behavior of 238 Pu-Doped Cubic Zirconia under Self-Irradiation

To investigate the resistance of cubic zirconia to accelerated radiation damage, which simulates effects of long term storage, 238 Pu-doped polycrystalline samples of cubic zirconia, (Zr,Gd,Pu)O 2 , were obtained and studied using X-ray diffraction analysis (XRD), electron probe microanalysis (EPMA), optical and scanning electron microscopy (SEM), and modified MCC-1 static leach test. The ceramic material was characterized by the following chemical composition (from EPMA in wt.% element): Zr = 50.2, Gd = 15.4, Pu = 12.2. This corresponds to the estimated formula, Zr 0.79 Gd 0.14 Pu 0.07 O 1.99 . The content of 238 Pu estimated was approximately 9.9 wt.%. The XRD measurements were carried out after the following cumulative doses (in alpha decay/m 3 × 10 23 ): 3, 27, 62, 110, 134, 188, 234, and 277. Even after extremely high self-irradiation, cubic zirconia retained its crystalline structure. All XRD analyses showed no phases other than a cubic fluorite-type structure. The following results of normalized Pu mass loss ( NL , in g/m 2 , without correction for ceramic porosity) were obtained from static leach tests (in deionized water at 90°C for 28 days) for 4 cumulative doses (in alpha decay/m 3 × 10 23 ): The results obtained confirm the high resistance of cubic zirconia to self-irradiation. This allows us to consider zirconia-based ceramic as the universal material that is suitable for actinide transmutation and geological disposal.