1 Copyright © 2009 by ASME Proceedings of the 12th International Conference on Environmental Remediation and Radioactive Waste Management ICEM2009 October 11-15, 2009, Liverpool, UK ICEM2009-16220 COUPLING TIME-DEPENDENT SORPTION VALUES OF DEGRADING CONCRETE WITH A RADIONUCLIDE MIGRATION MODEL Janez Perko Belgian Nuclear Research Centre SCKCEN Mol, Belgium Dirk Mallants Belgian Nuclear Research Centre SCKCEN Mol, Belgium Diederik Jacques Belgian Nuclear Research Centre SCKCEN Mol, Belgium Lian Wang Belgian Nuclear Research Centre SCKCEN Mol, Belgium ABSTRACT Safety assessment of radioactive waste disposal facilities is usually carried out by means of simplified models. Abstraction of the numerical model from the real physical environment is done in several steps. One of the most challenging issues in safety assessment concerns the long time scales involved and the evolution of engineered barriers over thousands of years. For some processes occurring in specific engineered barriers the uncertainties related to long time scales are addressed by implementing conservative assumptions in the radionuclide migration models. Other processes such as chemical concrete degradation, however, can be estimated for long time periods by the use of coupled geochemical transport models. For many near-surface disposal facilities, concrete is a very important engineered barrier because it is used in the construction of disposal modules or vaults, in production of high-integrity monoliths and their backfilling and for waste conditioning. Knowledge on the durability of such concrete components and its relation to radionuclide sorption is important for a defensible safety assessment. Chemical degradation typically occurs as the result of decalcification, dissolution and leaching of cement components and carbonation. These reactions induce a gradual change in the solid phase composition and the concrete pore-water composition, from “fresh” concrete porewater with a pH above 13 to a pH lower than 10 for “evolved” porewater associated with fully degraded concrete. In this study the time-dependency of the concrete mineralogy and porewater was coupled with sorption values that are characteristic for the four concrete degradation states: (i) State I with a pH larger than 12.5, controlled by the dissolution of alkali-oxides, (ii) State II with a pH at 12.5 controlled by the dissolution of portlandite, (iii) State III with a pH between 12.5 and 10 when all portlandite has been dissolved and the pore water composition is determined by different cement phases including calcium-silicate hydrates (C-S-H phases), and (iv) State IV with a pH lower than 10 with calcite and aggregate minerals present. Above mentioned pH values are valid for a system with a temperature of 25 o C. Sorption values were obtained from a literature review. The time-dependency of the sorption values R d was implemented in a one-dimensional radionuclide migration model used for release calculations from the planned near-surface disposal facility at Dessel, Belgium. Calculated releases will be discussed for radionuclides typical of low- and intermediate level short-lived (LILW-SL) waste. INTRODUCTION Engineered barriers in radwaste disposal facilities are used to retard the migration of radionuclides and consequently prevent hazardous species from getting into contact with humans and other biota. The engineered barriers represent an important component of the safety of a disposal system. Crucial for evaluating the long-term safety is to know the timeframe over which the barrier is assumed to function. The degradation of concrete is principally understood to involve chemical and thermo-mechanical processes that disrupt the integrity of concrete and degrade its hydraulic, mechanical and chemical properties. In safety assessment of LILW-SL disposal facilities it is often assumed that the chemical buffering function provided by the concrete is preserved over several thousands of years [1]. Therefore the past practice on evaluation of degradation of concrete barriers is based primarily on hydraulic