Microstructures of Melt-Processed and Spark Plasma Sintered Ceramic Waste Forms

Abstract

Hollandite-rich ceramic waste forms have been demonstrated to exhibit high durability while simultaneously accommodating a wide range of radionuclides in their matrices. This paper presents preliminary results on the preparation and characterization of ceramic waste forms prepared by two different methods-melt processing and spark plasma sintering (SPS). Both processes resulted in similar phase assemblages but exhibited different microstructures depending on processing method. The SPS samples exhibited fine-grained (<1 lm) and dispersed phases, whereas the melt-processed sample contained larger grains (10-20 lm) of specific phases. Additional data will need to be collected on the aqueous leaching durability and radiation resistance to evaluate each processing method for waste form performance.

Polymorphic Transitions in Cerium-Substituted Zirconolite (CaZrTi2O7)

“…Simulated waste forms have been fabricated in minutes (~30 minutes for the entire process) to high theoretical densities 21, 22 . The powder samples are contained within a graphite die during processing; this, along with the shorter processing times and lower processing temperatures, will limit volatile loss of radionuclides.…”

Heat capacities, standard entropies and Gibbs energies of Sr-, Rb- and Cs-substituted barium aluminotitanate hollandites

“…In particular, barium hollandite is used to immobilize fission products such as Rb, Sr, and Cs, which tend to be water soluble and volatile on heating. Studies have been done for Ba-hollandite waste formed during melt processing and their structure [17][18][19]. The Ba-hollandite family has the general formula A x (Ti 4+ ,B) 8 O 16 in which Ba occupies the A site and can be substituted by Cs, Rb, Sr, etc., while the ratio of other B cations (such as Al 3+ , Fe 3+ , and Mg 2+ ) to Ti 4+ can be varied to maintain charge neutrality [20][21][22].…”

Crystallization behavior during melt-processing of ceramic waste forms

“…2xþy Ti 8À2x-y ]O 16 [7], is the primary host for Cs. The structure consists of a continuous network of edge and corner shared oxygen octahedra forming large tunnels running parallel to the c-axis [8,9].…”

“…Zirconolite (CaZrTi 2 O 7 ) or pyrochlore, with the general formula A 2 B 2 O 7 (A 3þ /B 4þ or A 2þ /B 5þ ), is the preferred host for Zr and minor actinides [12]. Various processes including conventional sintering [13], hot isostatic pressing (HIP) [14], cold crucible induction melting (CCIM) [15], and spark plasma sintering [16] have been used to prepare these ceramic waste forms.d…”