Parameterizing a borosilicate waste glass degradation model

William L Ebert,James L Jerden

doi:10.1038/s41529-019-0093-2

Abstract

Borosilicate waste glass degradation models must quantify the effects of the solution composition on the dissolution rate. Here, we present results of modified ASTM C1285 tests conducted at 90 °C with AFCI and LRM glasses to determine whether dependencies of dissolution rates on the pH, Al, and Si concentrations must be included. Solution compositions were modified from those generated by glass dissolution alone by adding small amounts of K4SiO4 glass, Al(OH)3•2H2O, and a concentrated NaOH solution when the tests were initiated. Results show rate laws for the initial and resumption regimes must include pH dependences, but the residual rate can be modeled independent of the pH, Al, and Si concentrations. Triggering the resumption rate probably depends on the pH, Si, and Al concentrations and perhaps other aspects of the glass composition. A waste glass degradation model using is being parameterized using tests with a range of waste glass compositions to quantify these dependencies.

Highlights

The degradation rates of borosilicate glass waste forms when contacted by seepage water in breached waste packages will be used to define radionuclide source terms in contaminant transport calculations performed to assess the combined performance of the engineered and natural systems and ensure regulatory dose limits will be met throughout the service life of a disposal facility.[1,2] Glass degradation includes the dissolution of glass constituents into solution and the transformation of glass to secondary phases through either restructuring or coupled dissolution/precipitation processes
Stage 2 represents the slowing dissolution owing to attenuation of the kinetic rate by solution feedback effects, wherein solubility limits result in the formation of a clay-like surface alteration layer that may act as a transport barrier that further slows glass dissolution
The composition of the water within the breached and dissolution rates or the surface areas of the glass or secondary package is modified by glass dissolution and tracked by the model phases, the kinetics that is observed experimentally as the residual to calculate dissolution rates used in subsequent time steps

Summary

Introduction

The degradation rates of borosilicate glass waste forms when contacted by seepage water in breached waste packages will be used to define radionuclide source terms in contaminant transport calculations performed to assess the combined performance of the engineered and natural systems and ensure regulatory dose limits will be met throughout the service life of a disposal facility.[1,2] Glass degradation includes the dissolution of glass constituents into solution and the transformation of glass to secondary phases through either restructuring or coupled dissolution/precipitation processes. The finite time span and transition can be modeled as occurring instantaneously at Point P, but the dependencies of ratef(secondary phase) values for the nucleating phase that triggers Stage 3 and the stable phase that maintains the Stage 3 rate on the solution composition can be different.

Results

Conclusion