Evaluation and computer modeling of long-term durability of concrete engineered barriers in the tumulus technology for the disposal of solid low-level radioactive waste

The above-grade tumulus disposal technology can provide a major means for the disposal of low-level radioactive waste provided that the concrete pad and disposal vaults (which are structural components as well as engineered barriers against migration of radionuclides) are designed and fabricated for long-term durability.;The objective of this study was to examine the longevity of the concrete used for the engineered barriers. Alkali-aggregate reactivity, permeability (measured as rapid chloride permeability), freeze-thaw resistance, and strength were identified as key properties of hardened concrete that are important to ensure the long-term durability of tumulus concrete, and the testing and evaluation of the properties were conducted on laboratory concrete samples prepared with the concrete component materials and/or fly ash and silica fume field concrete samples. The second part of the research program was to develop computer model that can predict the long-term performance of the concrete engineered barriers utilized in the tumulus disposal technology; the overall strategy consisted of three parts: hydration modeling of portland cement paste, pore solution modeling of portland cement paste, and mass transport modeling in the concrete engineered barrier.;Although there were minor variations in properties, the mix design of both the fly ash concrete and silica fume concrete appeared to be adequate for the intended applications, assuming the concretes were produced properly. Based on the information generated for the both concretes, the merits of the silica fume concrete over the fly ash concrete in the light of long-term durability, particularly in the application for the tumulus disposal technology, are not conclusive yet.;It was predicted that, during the entire expected service life of 300 years, the pore solution of the tumulus pad concrete will maintain highly alkaline conditions remaining at pH 12.5 or above depending on whether or not the equilibrium release of the adsorbed alkali back into the pore solution is taken into account. In addition to the function as a physical barrier offered by the tumulus concrete pad, the highly alkaline environments being maintained inside the concrete pad during the entire expected service life will also function adequately as a chemical barrier against migration of radionuclides and other hazardous species in the case of an accidental breach of the steel waste containers and concrete waste vaults.