Modeling Carbonation of High-Level Waste Tank Integrity and Closure

Abstract

The Cementitious Barriers Partnership (CBP) is focused on reducinguncertainties in current methodologies for assessing cementitious barrier performanceand increasing the consistency and transparency in the assessment process. Oneimportant set of US Department of Energy challenges is assessing the integrity andclosure of the high-level waste (HLW) tanks that currently store millions of gallons ofhighly radioactive wastes. Many of these tanks are decades past their design lives, haveleaked or been overfilled, and must be emptied and closed to satisfy regulatoryagreements. Carbonation-induced corrosion has been identified as a primary degradationand possible failure mechanism for the HLW tanks prior to closure. After closure theimpact of carbonation (and concurrent oxidation) may be to increase the release andshort-range transport of contaminants of concern. HLW tanks may be significantlyempty for many years (and possibly decades) prior to closure; the performance of theclosed tank over centuries, if not millennia, must be assessed to evaluate the potentialrelease of residual radionuclides to the environment. CBP is developing models to evaluate a representative HLW tank closure scenarioincluding the potential impacts of carbonation on waste tanks prior to and post closure.CBP modeling tools, including LeachXS™/ORCHESTRA, are being used to simulatewaste tank carbonation, major constituent leaching, and contaminant releases to evaluatethe source term and near-field conditions. Simulations presented here include sensitivityanalysis for uncracked concrete to varying input parameters including composition,effective diffusivities, and thermodynamic parameters.