Mechanistic insights into the dynamics of radionuclides retention in evolved POFA-OPC and OPC barriers in radioactive waste disposal

[Display omitted] •Pulverized POFA possesses good strength activity index.•Structural evolution of the cementitious materials was traced over 2 years.•POFA-OPC has sorption saturation capacity five times higher than OPC.•Langmuir meta-stable model describes Cs retention during water intrusion progress. Cementitious barriers are widely used in all radioactive waste disposal facilities to ensure the fulfillment of several safety functions. A realistic quantification of radionuclide retention characteristics into these barriers under the dynamic changes that occur in the disposal facility is necessary to optimize the design of the facility and ensure the reliability of these safety functions. Herein, the beneficial use of Palm Oil Fuel Ash (POFA) as supplementary material to tailor the radionuclide retention characteristics of Ordinary Portland Cement (OPC) is investigated. The characterizations of evolved OPC and POFA-OPC samples revealed that the use of POFA slows down the carbonation process; enhances the formation and polymerization of low Ca/Si C-S-H and reduces the degradation state of the barrier. The effects of the progressive release and cooling down of Cs sources and progressive water intrusion on the retardation coefficients, relative contaminant retention, and capacity factors were investigated assuming the validity of the local linear and Langmuir based models. In comparison with reference OPC samples, it was found that POFA improved greatly the containment performance, where the retardation coefficients in the supplemented material are in the range (33–35.42) under the coupled effects of progressive release and cooling down of Cs sources. The retardation coefficients and the barrier capacities increased largely in very diluted systems. The adaptation of the local linear isotherm to describe the dynamic changes in the disposal facility was found to be associated with considerable uncertainties that need to be thoroughly identified.