PNNL

Abstract

A can-in-canister waste package design has been proposed for disposal of excess weapons plutonium at the proposed mined geologic repository at Yucca Mountain, NV. by the U. S. Department of Energy Office of Fissile Material Disposition. 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 ceramic (~15% of the total canister volume). The larger canister is then filled with HLW glass, SRL-202, (~85% of the total canister volume). A 6-year 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. Volumetric water content was observed to increase steadily during PUF testing from accumulation of water mass as waters of hydration associated with alteration phases formed on the glass surface. 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. Thermodynamic modeling of select effluent solution samples suggests the dominant secondary reaction product for the SRL-202 glass is a smectite di-octahedral clay phase(s), possibly nontronite [Na0.33Fe2(AlSi)4O10(OH)2•n(H2O)] or beidellite [Na0.33Al2.33Si3.67O10(OH)2]. This phase was identified in SEM images as discrete spherical particles found growing out of a gel-layer on reacted SRL-202 glass. Plutonium analyses of filtered and unfiltered solutions indicate that >80% of the Pu exiting the PUF system is as filterable particulates. In this advection-dominated system, Pu is migrating principally as colloids after being released from the LaBS glass. Analysis of reacted LaBS glass using SEM-EDS illustrates that Pu has segregated into a discrete disk-like phase, possibly PuO2. Alteration products that contain the neutron absorber Gd have not been positively identified. Separation of the Pu and its neutron absorbers during glass dissolution and transport is a potential criticality concern in the proposed repository. However, the translation and interpretation of these long-term PUF test results to actual disposed waste packages requires further analysis.