Concrete Barrier Performance in Radioactive Waste Disposal in the Unsaturated Zone
- PDF / 482,474 Bytes
- 7 Pages / 420.48 x 639 pts Page_size
- 44 Downloads / 182 Views
CONCRETE BARRIER PERFORMANCE IN RADIOACTIVE WASTE DISPOSAL IN THE UNSATURATED ZONE
JOHN C. WALTON* AND MARK D. OTIS** *Idaho National Engineering Laboratory, P.O. Box 1625, Idaho Falls, Idaho 83415 "**Science Applications International Corporation, 101 S. Park Ave., Idaho Falls, Idaho 83415 ABSTRACT Concrete barriers are an important component of many designs for disposal of radioactive waste in the unsaturated zone. In order to evaluate the effectiveness of the concrete barriers performance assessment models representing material degradation rates and transport properties must be developed. Models for evaluation of fluid flow and mass transport through partially failed concrete barriers located in the unsaturated zone are presented. Implications for the use of impermeable barriers in vault design are discussed. Concrete of highest quality may not always be desirable for use in all components of waste disposal vaults. INTRODUCTION In the US and other countries low level radioactive waste disposal facilities will take the form of earth covered concrete bunkers (Figure 1). In order to evaluate the performance of these disposal systems an evaluation must be made of the role of the concrete barriers. The concrete can serve several functions in the disposal system including structural support, limiting of water percolation, geochemical conditioning of water contacting the waste and waste containers, and slowing of radionuclide release rates. Because high quality concrete is relatively impermeable, most of the flow through a concrete system is likely to be through cracks. The permeability of intact concrete with low water/cement ratio is very low [1]. For this reason, cracks, particularly microcracks, are thought to actually control concrete permeability in service environments. Microcracks are caused by a variety of phenomena including response of concrete and reinforcement bars to physical loading, drying shrinkage, and expansion/contraction from temperature changes. Methodologies for estimation of crack width and spacing in concrete slabs have been developed by Bazant and others [2]. Crack like openings can also be present at joints in the concrete. Rather than attempt to determine the extent of cracking, we instead look at the performance implications of cracks. FLUID FLOW THROUGH CRACKS The first analysis assumes saturated flow through the concrete layer. Although the concrete vaults will typically be located in the unsaturated zone, areas of saturation may be expected in and above portions of the concrete as illustrated in Figure 2. A phenomenon called capillary perching also leads to near saturation in very fine grained barrier materials which are underlain by more coarse grained material. Because of the very small size of the pores in the barrier material and the large pore sizes in the coarse grained material, the barrier material will not drain into the large underlying pores until saturation is reached. If an infinitely long, parallel-sided crack through the concrete is assumed, the flow though the crack is:
Data Loading...
