Challenges in the Fabrication of Ceramic Technetium Waste Forms

MRS Advances ◽  
2018 ◽  
Vol 3 (51) ◽  
pp. 3041-3052
Author(s):  
Thomas Hartmann ◽  
Annita Martinelli-Becker
Keyword(s):  
High Temperature ◽  
Hot Pressing ◽  
Strong Impact ◽  
Chemical Coprecipitation ◽  
Waste Forms ◽  
High Temperature Synthesis ◽  
Metallic Inclusions ◽  
Coprecipitation Synthesis ◽  
Wet Chemical ◽  
Tetravalent State

ABSTRACTIn this research we can demonstrate, that fission technetium-99 can be successfully immobilized as tetravalent cation in solid state refractory oxides such as pyrochlores and perovskites. Pyrochlores show excellent performance in ASTM C1220-10 type corrosion testing and have the ability to structurally bond Tc-99 and therefore avoid the formation of highly-mobile, pertechnetate species under conditions of a generic repository. We have fabricated lanthanide technetium oxides using either dry-chemical ceramic processing, or wet-chemical coprecipitation methods. Tc pyrochlores have shown better Tc retention and corrosion resistance compared with Tc-containing LAWE4-type borosilicate glass, combined with 50-times higher waste loading. However, mechanical properties (fracture toughness, compressive strength) of the pyrochlores are lacking and the microstructure shows high open porosity of about 50 %. To improve these properties we tested a variety of measures such as hot-pressing or the combination of hot pressing and high-temperature synthesis, but the improvement was minor and Tc and the surrogate Ru were partly reduced. The presence of metallic inclusions has strong impact on Tc retention and release rates increased more than tenfold. We have further developed a wet-chemical coprecipitation synthesis route followed by calcination and a 4-days high-temperature sintering cycle for the model composition Sm2(Ru0.5Ti0.5)2O7 where titanium oxide was added as sintering agent. The ceramic surrogate waste forms showed improved theoretical densities of about 73 % combined with sufficient mechanical strength, while maintaining ruthenium in the tetravalent state.

Thermodynamic Evaluation for the Synthesis of T2 Phase from Elemental Powders via In Situ Reactively Hot-Pressing

2012 ◽  
Vol 538-541 ◽  
pp. 2082-2085
Author(s):  
Lai Qi Zhang ◽  
Lei Huang ◽  
Yong Ming Hou ◽  
Jun Pin Lin
Keyword(s):  
High Temperature ◽  
Hot Pressing ◽  
Initial Temperature ◽  
Adiabatic Temperature ◽  
Advanced Technology ◽  
Thermodynamic Evaluation ◽  
Temperature Synthesis ◽  
High Temperature Synthesis ◽  
Different Temperatures

T2 phase(Mo5SiB2) is a key component of the two Mo-Si-B tri-phase alloys under hot research. However, there is little research on T2 phase, especially its mechanic characteristics, due to the difficulty of fabrication of pure T2. In this present work, the thermodynamics of an advanced technology to fabricate pure T2 phase i.e. IRHP (In-situ Reactively Hot-Pressing) using elemental powders was analyzed. Formation free enthalpies at different temperatures for the compounds in Mo-Si-B system were calculated. Adiabatic temperatures and molten fractions of T2 phase at different initial temperatures for the reaction of synthesizing T2 phase were evaluated. The results show that it is feasible to in-situ synthesize T2 phase from elemental powders. T2 phase can not be synthesized using SHS(Self-propagating High-temperature Synthesis) mode of combustion. On the contrary, the explosion mode of combustion (IRHP) is receivable. Adiabatic temperature and molten fraction of T2 phase are relevant to initial temperature.

scholarly journals Rapid Immobilization of Simulated Radioactive Soil Waste Using Self-Propagating Synthesized Gd2Ti2O7 Pyrochlore Matrix

Materials ◽  
2019 ◽  
Vol 12 (7) ◽  
pp. 1163 ◽  
Author(s):  
Jiali Xue ◽  
Kuibao Zhang ◽  
Zongsheng He ◽  
Wenwen Zhao ◽  
Weiwei Li ◽  
...  
Keyword(s):  
Phase Composition ◽  
High Temperature ◽  
Contaminated Soil ◽  
Chemical Durability ◽  
Leaching Rate ◽  
Rapid Synthesis ◽  
Temperature Synthesis ◽  
Main Work ◽  
Waste Forms ◽  
High Temperature Synthesis

A rapid and effective method is necessary in the disposal of severely radioactive contaminated soil waste. Simulated Ce-bearing radioactive soil waste was immobilized by self-propagating high-temperature synthesis (SHS) within 5 min in this study. The main work includes the rapid synthesis of soil waste forms, the analysis of phase composition, microstructure and chemical durability. These results show that the simulated nuclide Ce was successfully immobilized into the pyrochlore-rich waste matrice, whose main phases are SiO2, pyrochlore (Gd2Ti2O7) and Cu. The normalized leaching rates of Si and Na on the 42nd day are 1.86 × 10−3 and 1.63 × 10−2 g·m−2·d−1, respectively. And the normalized leaching rate of Ce also remains at low level (10−5–10−6 g·m−2·d−1) within 42 days.

PECULIARITIES OF THE SYNTHESIS OF HIGH-TEMPERATURE CERAMICS TASI2–SIC, REINFORCED IN SITU BY DISCRETE SILICON CARBIDE NANOFIBERS

2018 ◽  
pp. 72-76
Author(s):  
S. Vorotilo ◽  
E. D. Polozova ◽  
E. A. Levashov
Keyword(s):  
Fracture Toughness ◽  
Silicon Carbide ◽  
High Temperature ◽  
Relative Density ◽  
Hot Pressing ◽  
Temperature Synthesis ◽  
Similar Composition ◽  
High Temperature Synthesis ◽  
Sic Ceramic

The possibility of the increase of the properties of ceramics in the TaSi2–SiC system via the reinforcement by the SiC nanofibers formed in situ in the combustion wave has been studied. For the formation of nanofibers as well as for increase of the exothermicity of the reaction mixtures, energetic additive PTFE (C2F4) was used. Using the method of self-propagating high-temperature synthesis of the mechanically activated mixtures, 70%TaSi2+30%SiC ceramic was produced, with SiC present as the round-shaped grains and as nanofibers. Ceramic specimens sintered by hot pressing were characterized by relative density up to 98 %, hardness 19,0–19,2 GPa and fracture toughness 7,5–7,8 MPa·m1/2, which is noticeably above the fracture toughness of the ceramic with similar composition produced without the PTFE additions.

Self-Propagating High-Temperature Synthesis of Simulated 90SrO-Contained Radioactive Graphite in N2 Atmosphere

2014 ◽  
Vol 525 ◽  
pp. 45-52 ◽  
Author(s):  
Xi Rui Lu ◽  
Si Jin Su ◽  
Meng Jun Chen
Keyword(s):  
Titanium Dioxide ◽  
High Temperature ◽  
Titanium Carbide ◽  
Exothermic Reaction ◽  
The Self ◽  
Cubic Phase ◽  
Temperature Synthesis ◽  
Radioactive Graphite ◽  
Waste Forms ◽  
High Temperature Synthesis

In order to evaluate the performance of the self-propagating high-temperature synthesis (SHS) to treat 90SrO-contained radioactive graphite in N2 atmosphere, waste forms were prepared with the self-developed SHS reactor according to the waste forms formulation designed with a solid-soluted content of 010 wt% (calibrated in mass, hereinafter the same). The waste forms were made with the exothermic reaction (3C + 4Al + 3TiO2 = 2Al2O3 + 3TiC + Q), where, 88SrO (a stable Sr isotope)-containing 88SrO was used to simulate 90SrO. And the raw materials for the waste forms were the powdery materials of graphite (C), aluminum (Al) and titanium dioxide (TiO2). Then, X-ray diffractometer (XRD) and scanning electron microscope (SEM) were used to test and analyze the phase composition and morphology of the prepared waste forms. According to the results in dealing with the treatment with the given exothermic reaction of the 90SrOcontaining radioactive graphite in N2 atmosphere, the SrO solid solubility could be up to 8 wt%. Besides, with a SrO content of 0~2 wt%, the major composition of the waste forms was including: alumina (Al2O3) in diamond scheme, titanium carbide (TiC) in cubic phase, graphite (C), anatase titanium dioxide (TiO2) and aluminum nitride (AlN) in cubic phase. Comparatively, with a SrO content of 3~8 wt%%, the major composition of the waste forms was including: alumina (Al2O3) in diamond scheme, titanium carbide (TiC) in cubic phase, graphite (C), anatase titanium dioxide (TiO2), aluminum nitride (AlN) in cubic phase and rhomboid aluminum titanate (Al2TiO5). Furthermore, diffractive peaks of unidentified phase occurred at 2θ = 7.7°, 15.6°, 19.8° and 24.1° position, whose intensities were increased with the increasing additional SrO content. The grain sizes of the prepared waste forms are mainly within 515μm, majorly exist in pieces.

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