Hydrologic considerations for placement and design of disposal facilities

Below ground or earth mounded concrete vaults are a design option being considered for disposal of low-and intermediate-level radioactive wastes at a variety of locations around the world. The overall goal of these facilities is to isolate the waste from the environment for time periods ranging from hundreds to thousands of years depending upon the type of waste and applicable regulatory environment. A number of factors influence the longevity and performance of subsurface concrete vaults including concrete composition and quality, design of the vault, design and maintenance of the cover, geochemical environment, climate, and site hydrology. As an aid to predicting long term performance of concrete barriers, mathematical models of concrete degradation and fluid flow and mass transport through concrete have been developed. The ultimate goal of the modeling exercises is to predict future performance of these systems. In this paper two aspects of concrete vault performance are considered. First the flow of water through a vault located in the unsaturated zone is contrasted with flow through an identical vault located below the water table. The exercise suggests that frequently, the vault located below the water table will have lower flow rates and perhaps represent a superior location. Just as importantly the calculations give several suggestions concerning the design of vaults located in the unsaturated zone. In the second portion of the paper mass transport through cracks in a concrete vault is considered. From a structural perspective, some controlled or limited cracking of concrete is allowable. Frequently structural design and reinforcement placement are based on controlling crack width and spacing. From the perspective of fluid flow and radionuclide transport through the vault, cracking is a much more serious problem