Coupling Time-Dependent Sorption Values of Degrading Concrete With a Radionuclide Migration Model

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. The focus of this work is to analyse the behaviour of the disposal facility in terms of radionuclides sorption values depending on the geochemical evolution of engineered barriers. The time-dependency of the concrete mineralogy and porewater is 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 is 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°C. Sorption values are obtained from a literature review. The time-dependency of the sorption values R_d is 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.