Titanate Ceramics for the Immobilization of Sodium‐Bearing High‐Level Nuclear Waste

Abstract

The phase chemistries and microstructures of titanate-based ceramics containing simulated high-level nuclear waste with varying sodium contents were compared. Incorporation of relatively low sodium levels (=0.9 wt% NazO) resulted in more complex phase assemblages. The principal hosts for sodium were hibonite and freudenbergite, and, when iron and sodium were present in combination, loveringite was also stabilized. During fabrication, oxygen potential was controlled by Ti-TiOz or TiHz-TiJOS solid-state buffers. These metal and hydride oxygen getters behaved similarly, neither disturbing the phase assemblages nor significantly altering the partitioning of waste elements between radiophases. It is believed that the hydrothermal stability of the sodiumbearing ceramics (containing up to 2.7 wt% NazO) will be comparable to sodium-free material, since less durable sodium-rich phases are encapsulated in a resistant matrix. Extensive formation of glassy phases may cause embrittlement at higher sodium loadings.

Immobilization of sodium and potassium in Synroc

“…Sodium is normally introduced into the reprocessing cycle via (a) NaOH for neutralization and (b) Na2CO 3 for reconditioning tributyl phosphate damaged by irradiation. The leachabilities and mineralogies of Synroc containing 10 wt % oxide equivalent of simulated Na-bearing Japanese waste (JW-A) have been reported by Buykx et al [3], who also studied Synroc containing simulated Na-free Purex waste plus up to 2.7% Na20. It was concluded that the chemical durability of Synroc should be maintained as long as the Na20 content does not exceed ~ 3 wt %, because at this Na20 level, Na-rich phases are encapsulated in a resistant matrix.…”

“…The precursor has the composition (wt %) A120 3 (5.4); BaO (5.6); CaO (11.0); TiO2 (71.4); ZrO z (6.6). The pH was maintained at ~ 9 with NH4OH solution [3] during waste addition and the mixture was stirred continuously during drying to minimize elemental segregation. After calcination for 1 h at 750 ~ in flowing 3.5% H2/N 2, 2 wt % fine ( ~ 325 mesh) Ti powder was thoroughly mixed in to serve as a redox buffer (see e.g.…”

“…After calcination for 1 h at 750 ~ in flowing 3.5% H2/N 2, 2 wt % fine ( ~ 325 mesh) Ti powder was thoroughly mixed in to serve as a redox buffer (see e.g. [3]). The mixtures were then hot-pressed in 40mm diameter graphite dies at 1200 ~ and 20 MPa for 2 h. Selected calcines were also hot-pressed for 2 h at 20 MPa in 10 mm diameter graphite dies at temperatures between 850 and 1100 ~ Densities and open porosities were measured by Archimedes' method, in which samples were boiled in water for 3 h or were evacuated in n-octanol.…”

“…(i) the standard Synroc B composition at higher waste loadings than those previously utilized [3], i.e. up to 28 wt %;…”

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

“…Dopants which have the atomic radius appropriate for the M +3 site in the hollandite are not found in the anticipated waste streams. However, taking into consideration the corrosion products and other additives from the separations processes results in appreciable concentrations of Cr 2 O 3 and/or Fe 2 O 3 (~1-10 wt%) arising from Purex or JW-A processing contamination [18,19]. The presence of Fe +3 (coexisting with Al +3 ) in the B site of the structure facilitated Cs incorporation via calcining at 1000 o C and air sintering at 1200 o C. In addition, iron oxide and chromium oxide additions may improve the densification and melting properties due to the lower melting point as compared to Al 2 O 3 rich compositions.…”