Modeling Carbonation of High-Level Waste Tank Integrity and Closure

Abstract

Abstract. The Cementitious Barriers Partnership (CBP) is focused on reducing uncertainties in current methodologies for assessing cementitious barrier performance and increasing the consistency and transparency in the assessment process. One important set of US Department of Energy challenges is assessing the integrity and closure of the high-level waste (HLW) tanks that currently store millions of gallons of highly radioactive wastes. Many of these tanks are decades past their design lives, have leaked or been overfilled, and must be emptied and closed to satisfy regulatory agreements. Carbonation-induced corrosion has been identified as a primary degradation and possible failure mechanism for the HLW tanks prior to closure. After closure the impact of carbonation (and concurrent oxidation) may be to increase the release and short-range transport of contaminants of concern. HLW tanks may be significantly empty for many years (and possibly decades) prior to closure; the performance of the closed tank over centuries, if not millennia, must be assessed to evaluate the potential release of residual radionuclides to the environment.CBP is developing models to evaluate a representative HLW tank closure scenario including the potential impacts of carbonation on waste tanks prior to and post closure. CBP modeling tools, including LeachXS™/ORCHESTRA, are being used to simulate waste tank carbonation, major constituent leaching, and contaminant releases to evaluate the source term and near-field conditions. Simulations presented here include sensitivity analysis for uncracked concrete to varying input parameters including composition, effective diffusivities, and thermodynamic parameters.

Reactive transport codes for subsurface environmental simulation

“…Recent applications of the model include coupling of ORCHESTRA with LeachXS database to improve understanding and prediction of the long-term performance of cementitious barriers used in nuclear applications. This thermo-hydro-mechanical (THM) coupling has also been used to assess long-term release behavior of sulfidic mining wastes [134], carbonation-induced corrosion in highlevel waste tanks [135], and uncertainty quantification of thermodynamic parameters [136]. More information on code development and example applications is available at http://www.macaulay.ac.uk/ORCHESTRA/.…”

Solid Secondary Waste Data Package Supporting Hanford Integrated Disposal Facility Performance Assessment

“…SSW encapsulation and solidification grout is not expected to contain any steel near environmental exposure surfaces, assuming a minimum thickness of encapsulation grout on the order of 10 cm (4 inches). Furthermore, reactive transport simulations performed by Brown et al (2013) indicate that a carbonation front effectively stops advancing after initial penetration due to dissolved calcium migrating to the reaction front from unreacted zones. Also, SSW grouts are expected to fully saturated once equilibrated with surrounding backfill.…”

Degradation of Saltstone Disposal Unit Cementitious Materials

“…The conservative-estimate assumes 95% liquid saturation / 5% gas saturation due to combined effects, or conditions otherwise more pessimistic than the CV case. As a point of reference, Brown et al (2012) performed detailed simulations of carbonation in "VCT" concrete, which is essentially Saltstone SDU 2/6/7 concrete, using LeachXS™/ORCHESTRA in a developmental CBP Software Toolbox carbonation module (Brown et al 2013a, b). The carbonation depth was defined as the location where pH < 9.…”