Crystalline Ceramic Waste Forms: Comparison Of Reference Process For Ceramic Waste Form Fabrication

Abstract

v SUMMARYThe research conducted in this work package is aimed at taking advantage of the long term thermodynamic stability of crystalline ceramics to create more durable waste forms (as compared to high level waste glass) in order to reduce the reliance on engineered and natural barrier systems. Durable ceramic waste forms that incorporate a wide range of radionuclides have the potential to broaden the available disposal options and to lower the storage and disposal costs associated with advanced fuel cycles. Assemblages of several titanate phases have been successfully demonstrated to incorporate radioactive waste elements, and the multiphase nature of these materials allows them to accommodate variation in the waste composition. Recent work has shown that they can be produced from a melting and crystallization process. The objective of this report is to explore the phase formation and microstructural differences between lab scale melt processing in varying gas environments with alternative densification processes such as Hot Pressing (HP) and Spark Plasma Sintering (SPS).The waste stream used as the basis for the development and testing is a simulant derived from a combination of the projected Cs/Sr separated stream, the Trivalent Actinide -Lanthanide Separation by Phosphorous reagent Extraction from Aqueous Komplexes (TALSPEAK) waste stream consisting of lanthanide fission products, the transition metal fission product waste stream resulting from the transuranic extraction (TRUEX) process, and a high molybdenum concentration with relatively low noble metal concentrations.Melt processing as well as solid state sintering routes SPS and HP demonstrated the formation of the targeted phases; however differences in microstructure and elemental partitioning were observed. In SPS and HP samples, hollandite, pervoskite/pyrochlore, zirconolite, metallic alloy and TiO 2 and Al 2 O 3 were observed distributed in a network of fine grains with small residual pores. The titanate phases that incorporate M +3 rare earth elements were observed to be distinct phases (ex. Nd 2 Ti 2 O 7 ) with less degree of substitution as compared to the more homogeneous melt processed samples where a high degree of substitution and variation of composition within grains was observed. Liquid phase sintering was enhanced in reducing gas environments and resulted in large (10-200 microns) irregular shaped grains along with large voids associated with the melt process; SPS and HP samples exhibited finer grain size with smaller voids. Metallic alloys were observed in the bulk of the sample for SPS and HP samples, but were found at the bottom of the crucible in melt processed trials. These results indicate that for a first melter trial, the targeted phases can be formed in air by utilizing Ti/TiO 2 additives which aid phase formation and improve the electrical conductivity. Ultimately, a melter run in reducing gas environments would be beneficial to study differences in phase formation and elemental partitioning.

Compositional control of radionuclide retention in hollandite‐based ceramic waste forms for Cs‐immobilization

“…Although many compositions and potential B‐site dopants exist, current strategies for waste form processing based on melting and crystallization require temperatures in excess of 1400°C, which poses potential processing limitations and increased volatilization of species such as Ba and Cs . Two prominent advantages of using gallium in the hollandite B‐site are that Ga has potentials to lower the melting point of hollandite compared with traditional Al‐ and Cr‐doped hollandites and that Ga has demonstrated redox stability in a range of oxygen activity conditions encountered in high‐temperature melt processing . Therefore, this study is focused on titanate‐based hollandite of the form Ba x Cs 1.33− x Ga 1.33+ x Ti 6.67−x O 16 ( x = 0, 0.667, 1.33) with Ga 3+ partially substituted on Ti sites.…”

A-site compositional effects in Ga-doped hollandite materials of the form BaxCsyGa2x+yTi8−2x−yO16: implications for Cs immobilization in crystalline ceramic waste forms

“…A number of studies have been conducted on how the M-site dopant affects microstructure and Cs incorporation14. Among those cations studied (Al 3+ , Cr 3+ , Ga 3+ , Fe 3+ ), Ga 3+ demonstrates the ability to lower the melting point of synthetic hollandite and provide redox stability, which is of interest to current US DOE efforts in waste forms aimed at melt processing15161718.…”

Thermodynamic assessment of the hollandite high‐level radioactive waste form

“…Ceramic waste forms offer better durability and higher waste loadings for some species for which existing HLW glass formulations are inappropriate or inefficient . Specifically, titanate ceramics, eg, SYNROC, have been extensively studied for use in immobilizing nuclear wastes due to their inherent leach resistance . Cs is one challenging radionuclide due to its thermal heat load, volatility at high temperatures, and tendency to form water‐soluble compounds .…”

“…Ti‐substituted hollandite, one of the SYNROC phases, is an alternative candidate for Cs immobilization. In these waste forms, 137 Cs (and other constituent radionuclides, ie 137 Ba, 87 Rb) is incorporated into the crystalline structure . Notably, natural analogs of hollandite including ankagite are present in dolomitic marble in the Apuan Alps in Tuscany, Italy, which demonstrates the stability of the hollandite phase over geologic timescales of interest for nuclear waste immobilization.…”

“…Titanate hollandite ceramics can be generally expressed as A x (Ti +4 ,M) 8 O 16 where A represents alkali and alkaline earth metal cations such as Cs +1 , Ba +2 , Rb +1 , K +1 , and Sr +2 and M represents +2/+3 cations such as Al +3 , Fe +3 , Fe +2 , Ga +3 , Cr +3 , Zn +2 , and Mg +2 . 4,11 The structure is composed of edge and corner sharing TiO 6 and MO 6 octahedra that form a framework consisting of tunnels parallel to the c-axis or baxis for tetragonal or monoclinic hollandites, respectively.…”