Electron microscope study of Synroc before and after exposure to aqueous solutions
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A well-characterized Synroc was subjected to durability tests in de-ionized water at 70 °C and 150 °C, and silicate and bicarbonate solutions at 70 °C, to study the effect of temperature and solution composition on the mechanisms of aqueous alteration. SEM and TEM were used before and after durability experiments to characterize the primary and secondary phases in and on the Synroc samples, and to describe changes in morphology and chemistry. Leachant compositions after durability testing were analyzed using ICP/OES and ICP/MS and have been reported elsewhere.8 After durability testing, titaniferous surface layers were observed primarily on perovskite, the most soluble Synroc phase. Electron microscopy demonstrates that the surface layer formed at 70 °C is an amorphous T i - 0 film and is fine grained anatase at 150 °C. Congruent dissolution is the major mechanism of perovskite alteration. However, at 150 °C, selective loss of Ca and Sr may occur locally. A number of additional secondary phases were identified, including Al—oxide/hydroxide, Fe-oxide, REE-bearing Ti-oxides, and several silicate phases. The abundance and composition of the secondary phases are related to solution composition and temperature.
I. INTRODUCTION Synroc is a polyphase titanate ceramic designed for the incorporation of high-level radioactive waste (HLW).1 The primary phase assemblage of Synroc consists of zirconolite (CaZrTi2O7), hollandite [Ba^Cs,, (Al,Ti 3+ ) 2 , +> ,(Ti 4+ ) g _ 2 ^O 16 ], perovskite (CaTiO3), titanium oxides (TinO2n-_i), and several minor oxide and intermetallic phases. The first three phases provide the main lattice sites for incorporation of actinides, lanthanides, cesium, and strontium from the waste stream.1'2 Long-term retention of waste elements is the ultimate requirement of any nuclear waste form, therefore waste form integrity must be maintained in aqueous media under various conditions of temperature, fluid composition, and pH. The majority of durability tests conducted on inactive Synroc have been short-term (generally 28 days) quality assurance tests, designed to assess the effect of variations in the temperature and pressure of fabrication, processing impurities, and oxygen fugacity.3 Some longterm data are available for Synroc containing radioactive elements. For example, Reeve et al.4 conducted durability tests for up to 1000 days on Synroc samples that included 90Sr, 134Cs, 140Ba, and several other radionuclides in addition to nonradioactive, simulated waste elements. Their results showed that the amounts of Cs, Mo, Sr, and Ba released decrease exponentially with time. In most cases, solution analysis was employed as the sole post-testing characterization technique. Recent work by Solomah and Matzke5 illustrates the advantage 2218
J. Mater. Res., Vol. 6, No. 10, Oct 1991
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of using a combination of techniques to characterize Synroc after durability testing. These authors used Rutherford backscattering, secondary ion mass spectrometry, elastic recoi
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