Multiphase Titanate-Based Ceramic Waste Forms: Cerium Incorporation in Zirconolite and Pyrochlore and Radiation Stability

Abstract

This thesis aims at developing ceramic waste forms as a potential replacement for the conventional glass waste forms for safe immobilization and disposal of nuclear wastes from legacy weapons programs as well as commercial power production. The body of work consists of two parts. The first part focused on fabrication and characterization of multiphase waste forms containing hollandite as the major phase with perovskite, pyrochlore, and zirconolite as secondary phases. The second part focuses on single phase pyrochlores and zirconolites for cerium (Ce) incorporation.

Part I: Multiphase waste forms: Compositions of simulated multiphase waste forms were developed at Savannah River National Laboratory (SRNL) and processed at Alfred University using melt-processing and spark plasma sintering (SPS). The resulting multiphase waste forms contained a majority of hollandite along with perovskite, pyrochlore, and zirconolite phases as determined by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). Cesium (Cs) incorporation into the hollandite phase is more prevalent in melt-processed sample, requiring further optimization of the SPS process.

Radiation damage resistance is a vital performance requirement for waste forms. This was simulated in these multiphase ceramics by bombarding the samples with Au3+ and He+ ions. Heavy ions (Au3+) initiated ballistic processes that caused all phases in the affected volumes to amorphize. Regardless of the processing method, the amorphization behavior remained the same in all samples bombarded with Au3+ ions. The ion penetration depth was reduced in SPS-processed samples, attributed to the smaller grain sizes of these materials. Light ion (He+) irradiation caused the breakdown of the hollandite phase, while the remaining phases appeared to be unaffected. This behavior was confirmed in single phase hollandite samples.

Part II: Single phase waste forms: High amounts of Ce can be incorporated into zirconolite and pyrochlore structures, up to 50 mol% for Zr in the case of zirconolite and 25 mol% for Nd in Nd2Ti2O7 pyrochlore, via solid state reaction. Perovskite is observed in small amounts in zirconolite materials (up to 7 wt%). With increasing Ce content, a transition from the 2M to 4M-zirconolite polymorph has been reported. The valence state of Ce will affect which atomic site that the Ce substitutes on. 2M-zirconolites contain trivalent Ce, which supports Ce substitution on both Ca and Zr sites. Both trivalent and tetravalent Ce are present in 4M-zirconolites, indicating greater partitioning onto the Zr site. All Ce substituted into pyrochlore materials is converted from tetravalent to trivalent during the reaction process to maintain charge neutrality of the structure.

Ce-substituted zirconolite and pyrochlore materials were consolidated using SPS. The reducing environment of the SPS causes 4M-zirconolite to convert to perovskite and 2M-zirconolite due to the reduction of Ce4+ to Ce3+ causing a redistribution of Ce onto the Ca and Zr sites of zirconolite and stabilizing perovskite. The original phase assemblage of the materials can be restored by a heat treatment in air post-sintering. CaCeTi2O7 forms as an intermediate phase up until 1300°C, and 4M-zirconolite begins at 1350°C. The transformation to 4M-zirconolite is slow, but complete conversion to the original phase assemblage is achieved with a 24h heat treatment in air. The sintering behavior of Ce-substituted pyrochlore is unaffected by Ce content in the material.

The integral waste form performance properties of radiation damage stability and chemical durability of Ce-substituted zirconolite and pyrochlore were simulated by implanting samples with He+ or Kr3+ with ion fluences equating to 0.5 dpa and Product Consistency Testing (PCT) and Materials Characterization Center (MCC-1) leach tests, respectively. Gracing-incidence x-ray diffraction (GIXRD) of the samples revealed that light ion (He+) irradiation had very little effect on the materials, whereas heavy ion (Kr3+) irradiation caused near complete amorphization of all materials tested, which corroborated well with the results from multiphase samples. No effect of Ce content on radiation damage behavior was seen. For zirconolite materials, Ce was only released during the PCT of CaZn0.9Ce0.1Ti2O7 indicating that the 4M-zirconolite polymorph is more chemically durable than 2M-zirconolite. Ce was released from Nd1.5Ce0.5Ti2O7 during both the PCT and MCC-1. During MCC-1 tests longer than 7 days, no Ce above the detectable limit of the ICP-AES was released. The Ce release values in these studies were comparable to those in similar tests performed previously in the literature.