Assessment of site conditions for disposal of low- and intermediate-level radioactive wastes: a case study in southern China.

Near surface disposal of low- and intermediate-level radioactive wastes (LILW) requires evaluating the field conditions of the candidate site. However, assessment of the site conditions may be challenging due to the limited prior knowledge of some remote sites, and various multi-disciplinary data requirements at any given site. These situations arise in China as in the rest of the industrialized world, particularly since a regional strategy for LILW disposal has been implemented to protect humans and the environment. This paper presents a demonstration of the site assessment process through a case study focusing mainly on the geologic, hydrogeologic and geochemical characteristics of the candidate site. A joint on-site and laboratory investigation, supplemented by numerical modeling, was implemented in this assessment. Results indicate that no fault is present in the site area, although there are some minor joints and fractures, primarily showing a north-south trend. Most of the joints are filled with quartz deposits and would thus function hydraulically as impervious barriers. Investigation of local hydrologic boundaries has shown that the candidate site represents an essentially isolated hydrogeologic unit, and that little or no groundwater flow occurs across its boundaries on the north or east, or across the hilly areas to the south. Groundwater in the site area is recharged by precipitation and discharges primarily by evapo-transpiration and surface flow through a narrow outlet to the west. Groundwater flows slowly from the hilly area to the foot of the hills and discharges mainly into the inner brooks and marshes. Some groundwater circulates in deeper granite in a slower manner. The vadose zone in the site was investigated specially for their significant capability for restraining the transport of radionuclides. Results indicate that the vadose zone is up to 38m in thickness and is made up of alluvial clay soils and very highly weathered granite. The vadose zone has low saturated hydraulic conductivities on the order of 10(-5)cm/s and in this respect is well-suited for the disposal of LILW. The saturated formations are primarily made up of silt and moderately-to-slightly weathered granite, which exhibit even lower hydraulic conductivities, on the order of 10(-6)cm/s, also favorable for restraining the transport of radionuclides. Chemical analyses indicate that the groundwaters at the site are of the HCO(3)-Na · Ca and HCO(3) · SO(4)-Na · Ca types and are weakly corrosive to concrete and steel. Geochemical analyses indicate that the rock and soil materials (particularly weathered granite) at the site contain very small fractions of colloidal particles and exhibit low Cation Exchange Capacities (CEC), and would therefore have limited capacity for sorption of radionuclides. Groundwater flow and solute transport models of the candidate site have been developed using MODFLOW and MT3DMS, incorporating the data obtained during the assessment program. Calibration was based on the available measured groundwater level fluctuations and tracer concentrations from in situ dispersion tests. The longitudinal dispersion coefficient as determined in calibration is equal to 5.0 × 10(-3) m(2)/d. Numerical sensitivity analyses indicate that the hydraulic conductivity and the longitudinal dispersion coefficient are the key parameters controlling the transport of radionuclides, while the numerical model is not sensitive to changes in the effective porosity and the specific yield. Preliminary predictions have been performed with the calibrated model both for the natural setting of the site and the graded site in which the valleys of the site are backfilled with low permeable materials. Results indicate that the proposed site grading increases the safety of the site for disposal of LILW by reducing both the groundwater level and the hydraulic gradient and that radionuclide transport would not likely be a problem or cause groundwater contamination. Although there are some problems remaining to be addressed in future work, the conclusion of the assessment is that the conditions at this site are appropriate for LILW disposal. This study provides an example of the procedures necessary in an assessment of site conditions relevant to the safe disposal of LILW. Such an assessment is crucial to the site selection process and to subsequent environmental impact assessment.