Modelling the Impact of Engineered Barrier Degradation on Radionuclide Release from a Lithuanian Near-Surface Repository

Cementitious materials are widely used as engineered barriers in radioactive waste repositories due to their low permeability and ability to sorb radionuclides. However, the degradation of concrete under detrimental environmental impacts alters its ability to sorb radionuclides and may increase radionuclide release from the repository. The aim of this work was to investigate the effect of concrete barrier degradation on radionuclide migration in the near field of the Lithuanian near-surface repository. At first, changes in geochemical conditions in the repository were evaluated, and concrete degradation stages were defined using a reactive transport model. Then, sorption values (Kd values) corresponding to concrete degradation stages at different locations in the repository were selected, and radionuclide migration from the repository was modelled. Temporal as well as spatial changes in radionuclide sorption were taken into account. Long-lived weakly, moderately and strongly sorbed radionuclides (129I, 59Ni and 239Pu, respectively) were considered. It was found that, under the assumed conditions, changes in sorption values had no impact on 129I flux from the repository. Considering concrete degradation, 59Ni release was twice as high as in the case of non-degrading concrete, while 239Pu flux was similar to that obtained assuming constant sorption, as in degraded concrete.