Low-level radioactive waste (LLRW) must be disposed of in a manner that safeguards the environment and future generations. To this end, engineers should provide reasonable assurance that the proposed methods of disposal and materials of construction will function as intended throughout the design life. This paper addresses design and construction issues related to concrete for the nation's first commercial, above-grade, engineered LLRW disposal facility.

Several engineered disposal technologies involving concrete structures have been proposed for low-level radioactive waste disposal. The long-term performance and behavior of reinforced concrete structures in disposal units have been examined. Under most conditions, the reinforcing steel and concrete work well together to withstand the natural forces. Under certain conditions, however, the reinforcement and concrete may be subject to environmental attack which may cause degradation of the reinforced concrete. Water infiltration through the structure may increase as a result of cracking and increasing permeability and thereby increase the potential for contaminant release. The model for estimating time to onset of reinforcing steel corrosion due to presence of chloride is presented. Requirements for design of reinforced concrete structures for low-level radioactive waste disposal facilities are suggested.

Water is generally involved in every form of concrete deterioration; the permeability of concrete usually determines the rate of deterioration. Since the permeation of water into concrete takes place through the capillary pores, a reduction in the volume of large (greater than about 0.01 micron) capillary voids in the paste matrix will reduce permeability. This can be assisted by the partial substitution of cement with fly ash in the paste. As a pozzolanic material, fly ash reacts chemically with the calcium hydroxide resulting from cement hydration to form compounds (mainly calcium silicate hydrate) with cementitious properties. The effects of fly ash type and content on the permeability characteristics of concrete materials subjected to two different curing conditions were investigated. Four different fly ash contents and three different fly ash types were considered to provide sufficient data for powerful statistical analysis of results. Fly ash was observed to be capable of reducing the permeability of concrete, even at early ages. Selection of fly ash type (Class F vs. Class C), the level of cement substitution with fly ash, and curing conditions had important effects on the permeability characteristics of fly ash concrete. The interactions between these factors were also generally important.

Interest in the use of reinforced concrete structures in LLW disposal facilities has preceded the development of a comprehensive understanding of the long-term performance of these disposal technologies. With this in mind, Rogers and Associates Engineering Corporation has developed a new assessment computer model, restrict, that adopts a more complete, mechanistic approach to modeling concrete degradation, groundwater infiltration, leaching, and radiological risks.

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.