Accelerated weathering of high-level and plutonium-bearing lanthanide borosilicate waste glasses under hydraulically unsaturated conditions

Abstract

The US Department of Energy (DOE) has proposed that a can-in-canister waste package design be used for disposal of excess weapons-grade Pu at the proposed mined geologic repository at Yucca Mountain, Nevada. This configuration consists of a high-level waste (HLW) canister fitted with a rack that holds mini-canisters containing a Pu-bearing lanthanide borosilicate (LaBS) waste glass and/or titanate-based ceramic (∼15% of the total canister volume). The remaining volume of the HLW canister is then filled with HLW glass (∼85% of the total canister volume). A 6-a pressurized unsaturated flow (PUF) test was conducted to investigate waste form–waste form interactions that may occur when water penetrates the canisters and contacts the waste forms. The PUF column volumetric water content was observed to increase steadily during the test because of water accumulation associated with alteration phases formed on the surfaces of the glasses. Periodic excursions in effluent pH, electrical conductivity, and solution chemistry were monitored and correlated with the formation of a clay phase(s) during the test. Geochemical modeling, with the EQ3NR code, of select effluent solution samples suggests the dominant secondary reaction product for the surrogate HLW glass, SRL-202, is a smectite di-octahedral clay phase(s), possibly nontronite [Na 0.33 Fe 2(AlSi) 4O 10(OH) 2 · n(H 2O)] or beidellite [Na 0.33Al 2.33Si 3.67O 10(OH) 2]. This clay phase was identified in scanning electron microscope (SEM) images as discrete spherical particles growing out of a hydrated gel layer on reacted SRL-202 glass. Alpha energy analysis (AEA) of aliquots of select effluent samples that were filtered through a 1.8 nm filter suggest that approximately 80% of the total measurable Pu was in the form of a filterable particulate, in comparison to unfiltered aliquots of the same sample. These results suggest the filterable particles are >1.8 nm but smaller than the 0.2 μm average diameter openings of the Ti porous plate situated at the base of the column. In this advection-dominated system, Pu appeared to be migrating principally as or in association with colloids after being released from the LaBS glass. Analyses of reacted LaBS glass particles with SEM with energy dispersive X-ray spectroscopy suggest that Pu may have segregated into a discrete disk-like phase, possibly PuO 2. Alteration products that contain the neutron absorber Gd have not been positively identified. Separation of the Pu and the neutron absorber Gd during glass dissolution and transport could be a criticality issue for the proposed repository. However, the translation and interpretation of these long-term PUF test results to actual disposed waste packages requires further analysis.