Degradation of Saltstone Disposal Unit Cementitious Materials

Abstract

The Saltstone facilities at the DOE Savannah River Site (SRS) are used to stabilize and dispose of lowlevel radioactive salt solution originating from liquid waste storage tanks at the site. The Saltstone Production Facility (SPF) receives treated salt solution and mixes the aqueous waste with dry cement, blast furnace slag, and fly ash to form a grout slurry which is mechanically pumped into concrete disposal cells that compose the Saltstone Disposal Facility (SDF). The solidified grout is termed "saltstone".Cementitious materials play a prominent role in the design and long-term performance of the SDF. The saltstone grout exhibits low permeability and diffusivity, and thus represents a physical barrier to waste release. The waste form is also reducing, which creates a chemical barrier to waste release for certain key radionuclides, notably Tc-99. Similarly, the concrete shell of a saltstone disposal unit (SDU) represents an additional physical and chemical barrier to radionuclide release to the environment. Together the waste form and the SDU compose a robust containment structure at the time of facility closure. However, the physical and chemical state of cementitious materials will evolve over time through a variety of phenomena, leading to degraded barrier performance over Performance Assessment timescales of thousands to tens of thousands of years. Previous studies of cementitious material degradation in the context of low-level waste disposal have identified sulfate attack, carbonation-influenced steel corrosion, and decalcification (primary constituent leaching) as the chemical degradation phenomena of most significance under SRS exposure conditions.

Geochemical Model of E_h and pH Transitions in Pore Fluids during Saltstone and SDU Concrete Aging: Saltstone Disposal Facility PA Revision

“…The Saltstone Production Facility, located in Z-Area at SRS, receives treated aqueous salt solution and mixes the waste with dry cement, blast furnace slag (BFS), and fly ash to form a grout slurry that is mechanically pumped into concrete disposal cells that compose the Saltstone Disposal Facility (SDF). The solidified grout formed in this process is called "saltstone" (Flach, 2018).…”

“…The waste form also acts as a chemical barrier to contaminant release for radionuclides such as Tc-99, whose solubility and mobility are a function of the reducing capacity of the grout (Flach, 2018).…”

“…Similarly, the concrete shell of the saltstone disposal unit (SDU) serves as an added physical and chemical barrier to radionuclide release (Flach, 2018). Considered together, the saltstone waste form and the SDU represent a robust containment structure at facility closure.…”

“…However, the physical and chemical properties of the cementitious materials will change over time via sulfate attack, carbonation-influenced steel corrosion, and decalcification. During the thousands of years of interest in a Performance Assessment (PA), barrier performance will degrade (Flach, 2018).…”

PORFLOW Simulations Supporting the Saltstone Performance Assessment Revision

“…Also, PORFLOW simulations of aquifer transport have been revised to reflect a recent update to the General Separations Area (GSA) groundwater flow model (Flach 2018c) and improved treatment of plume dispersion based on recommendations of Flach (2018a). Other updates to the PA inputs include Hommel (2018a), Flach (2018b), andWatkins (2018).…”