Title: Testing and Evaluation of Durability of Concrete Barriers for Tumulus Disposal Facility for Low-level Radioactive Waste
Author(s): J. H. Lee; and D. M. Roy
Publication: Symposium Paper
Appears on pages(s): 67-90
Keywords: alkali-aggregate reactions; carbonate aggregates; concretes; durability; expansion; fly ash; portland cement type 1; portland cement type 2; radioactivity; silica fume; General
The tumulus (an earthen mound) disposal concept can provide a major means for the disposal of low-level radioactive waste (LLRW) provided the concrete structure of the tumulus disposal units is designed and fabricated for the long- term durability. The concrete used in an experimental disposal facility, Tumulus II, was designed to have an excellent resistance to frost attack and a very low permeability to chloride ions. The present study reports numerous research results, including those from an accelerated alkali-aggregate reactivity test (Accelerated Concrete Core Method), which showed that a local coarse aggregate was potentially reactive to alkali. The reactivity to alkali was substantially reduced by incorporating 30 percent fly ash (Class F) by weight of cement. Additional studies were performed on field concrete samples incorporating nine percent silica fume by weight of cement which showed effective reduction in alkali-aggregate reactivity. Expansion mechanisms of the local coarse aggregate and reference alkali- carbonate reactive Pittsburgh aggregate in concrete were studied by digesting the powdered aggregates under the accelerated test condition (1.0 N NaOH solution and 80 C) and monitoring clay mineral phases in the aggregates with x-ray diffraction (XRD) analysis at various digestion ages. The results showed that transformation of non-expansible clay phases (vermiculites/smectites) could occur in a highly alkaline environment which is typical of many concrete pore solutions. The expansible clays thus formed are, at least in part, responsible for the expansion of concrete cores containing the local coarse aggregate and Pittsburgh aggregate, as observed by the accelerated alkali-aggregate reactivity tests.