Fig. 9-4 Test pit with a depth 30 m below the ground surface Soil cores doped with radionuclides were buried in the wall of the test pit to investigate radionuclide migration in situ.

Fig. 9-5 Distribution profiles of the radionuclides in the loess soil after the test period of almost three years 90Sr moved slightly from the tracer source position (0 cm). 237Np and 238Pu did not move due to strong adsorption onto the soil.

Fig. 9-6 Comparison of the measured distribution of 90Sr and two-dimensional simulation The calculation simulated the measured distribution of 90Sr. This agreement indicates the validity of the sorption-equilibrated, advection-dispersion model to predict mass transfer in the loess.

Low-level radioactive waste generated at nuclear power plants is solidified, mainly with cement, and buried in a subsurface concrete pit (called a repository), under institutional control for a certain period. During this period the radioactivity decays to a safe level. The long-term safety assessment for disposal is evaluated based on a scenario whereby radionuclides are assumed to leak from a repository and through the ground to inhabitants. To demonstratively investigate the behavior of radionuclides leaked from a repository into the underground soil, we conducted a field migration test for 90Sr, 237Np, and 238Pu in a loess layer in China in a cooperative research project between JAERI and the China Institute for Radiation Protection. Migration of the radionuclides in the loess was observed for three years (Fig. 9-4). Based on the results of the field test, shown in Fig. 9-5, the migration rates of 90Sr, 237Np, and 238Pu were calculated to be less than 1/20, 1/500 and 1/5000 of the groundwater flow velocity, respectively. These rates demonstrate quantitatively the retardation performance of natural soil against underground mass transfer. The hydrological, hydrogeological, and geochemical data obtained in this series of tests were used for validation of a part of our safety assessment code, GSA-GCL, developed by JAERI (the GSA-GCL code has been widely used to establish the safety criteria of shallow land disposal). The simulation (Fig. 9-6) using the GSA-GCL code indicates that the migration of 90Sr in the soil could be predicted based on the sorption-equilibrated, advective-dispersive chromatographic theory.

Reference M. Mukai et al., Plan and Progress of a Cooperative Research Program on Field Migration Test between JAERI and CIRP (Phase-2), J. Nucl. Fuel Cycl. Environ., 7, 31 (2001).

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