Waste Form Release Calculations for Performance Assessment of the Hanford Immobilized Low-Activity Waste Disposal Facili
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Waste Form Release Calculations for Performance Assessment of the Hanford Immobilized Low-Activity Waste Disposal Facility Using a Parallel, Coupled Unsaturated Flow and Reactive Transport Simulator Diana H. Bacon and B. Peter McGrail Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, U.S.A. ABSTRACT A set of reactive chemical transport calculations was conducted with the Subsurface Transport Over Reactive Multi-phases (STORM) code to evaluate the long-term performance of a representative lowactivity waste glass in a shallow subsurface disposal system located on the Hanford Site. Technetium, the main contributor to a drinking water dose, is assumed to be released congruently with the dissolution of the glass. Sodium is released at a higher rate via a kinetic ion-exchange reaction. Aqueous equilibrium reactions involving sodium and other dissolved glass constituents increase the pH, and hence the rate of glass dissolution. The precipitation of secondary minerals can also lower the amount of aqueous dissolved silica, which can increase the rate of glass dissolution. Predicted technetium release rates, however, remain several orders of magnitude lower than required by drinking water regulations.
INTRODUCTION A large inventory of radioactive and mixed waste is currently stored in 177 buried single- and doubleshell tanks at the U.S. Department of Energy’s (DOE) Hanford Site in southeastern Washington State. Liquid waste recovered from the tanks will be pretreated to separate the low activity fraction, which will be immobilized in glass and placed in a near-surface disposal system [1]. Before the Immobilized LowActivity Waste (ILAW) may be disposed, the DOE must approve a performance assessment (PA).
METHODS
Vertical Distance (m)
A computer model, STORM (Subsurface Transport Over Reactive Multi-phases) that couples unsaturated flow through the subsurface disposal facility with geochemical reactions controlling the waste glass dissolution has been developed at Pacific Northwest National Laboratory (PNNL) to perform the waste form release calculations for the ILAW PA [2]. In order to simulate the weathering of the waste glass over 10,000 years with the detail necessary to represent the complex, multidimensional flow through the subsurface disposal system, STORM has been modified to run in parallel on the high-performance computing cluster at the Molecular Science Computing Facility (MSCF) at PNNL.
15
Waste Glass
Individual Waste Package (1.22 m x 2.3 m)
10
Backfill
Hanford Sand
5
0 50
55
60
65
70
75
80
Horizontal Distance (m)
85
90
Figure 1. Cross-section of ILAW subsurface disposal facility trench
95
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Simulations were based on the latest conceptual design [3] of the ILAW subsurface disposal facility trench (Figure 1). The modeled domain was 100 m wide by 15 m tall, with 410 nodes in the horizontal direction and 65 nodes in the vertical direction. Three material zones are included in the simulation: waste glass, backfill in the trench surrounding the waste glas
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