The near field modeling of a hyphotetical surface disposal at Serpong site has been performed. Considering a realistic downward water flux of 10 -10 m/ s throught the conditioned waste zone and the concrete barriers, transport of radionuclide by advection and dispersion below the bottom of the repository was calculated using PORFLOW computer code. The result shows that the highest fluxes were observed H-3, Cs-135, Nb-94 and Ni-59, all about 10 4 Bq/ a/ m 2 . Intermediate fluxes were obtained for Cs-137, I-129 and Co-60. Lowest were due to Ni-63 and C-14. The effect on radionuclide fluxes by having an unsaturated soil of 1.0-m depth below the bottom of the repository also was investigated. The results indicated that the highest fluxes were due to Cs-135, Nb-94 and Ni-59, approximately 10 4 Bq/ a/ m 2 . The fluxes owing to H-3 had decreased to 10 -9 Bq/ a/ m 2 owing to decay. Radionuclides that were not absorbed onto lateric clay soil, C-14 and I-129 did not decrease in flux. All other radionuclides had significantly smaller fluxes compared to the ones calculated at the bottom of the repository. This indicates that the lateric clay soil at Serpong site plays an important role in retarding and dispersing the radionuclide migration towards the ground water. E. Lubis, D. Mallants, G. Valckaert
