Glass Degradation in Performance Assessment Models
- PDF / 188,233 Bytes
- 10 Pages / 432 x 648 pts Page_size
- 15 Downloads / 195 Views
Glass Degradation in Performance Assessment Models1 William L. Ebert Argonne National Laboratory, Argonne, IL, U.S.A ABSTRACT The interface with reactive transport models used in performance assessment calculations is described to identify aspects of the glass waste form degradation model important to long-term predictions. These are primarily the conditions that trigger the change from the residual rate to the Stage 3 rate and the values of those rates. Although the processes triggering the change and controlling the Stage 3 rate are not yet understood mechanistically, neither appears related to an intrinsic property of the glass. The sudden and usually significant increase in the glass dissolution rate suggests the processes that trigger the increase are different than the processes controlling glass dissolution prior to that change. Application of a simple expression that was derived for mineral transformation to represent the kinetics of coupled glass dissolution and secondary phase precipitation reactions is shown to be consistent with experimental observations of Stage 3 and useful for modeling long-term glass dissolution in a complex disposal environment. INTRODUCTION Performance assessments of geological disposal systems engineered for high-level radioactive waste are conducted to provide confidence that regulations addressing groundwater contamination will be met throughout the regulated service life. The releases of radionuclides and other hazardous constituents to the surrounding biosphere are calculated using reactive-transport models that track contaminant migration through multiple engineered and natural barriers that comprise the disposal system. Waste form degradation models are developed to provide source terms for key contaminants that can be used in transport models throughout the long disposal times. The degradation models quantify the effects of changes in both the waste form surface as it corrodes and the environmental conditions driving the corrosion on the releases of those contaminants. Confidence in the source term values calculated with these models is derived from an understanding of both the processes that control waste form degradation and the influence of environmental conditions on the kinetics of those processes. The modular approach to performance assessment modeling currently being followed in the US has the potential to significantly reduce the degrees of conservatism and empiricism that were necessary in previous waste form models, but adds the need to track solution feed-back and surface alteration effects as waste form corrosion progresses. An approach is being developed by the US Department of Energy for integrating waste form source term models with contaminant transport models for performance assessment. The initial interfaces are being established using the source term model for used oxide fuel, and the insights gained will be applied to source term models 1
This work was under the auspices of the US DOE Fuel Cycle R&D program Materials Recovery and Waste Form Development
Data Loading...
