Field-Scale Migration of 99 Tc and 129 I at the Nevada Test Site
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Field-Scale Migration of 99Tc and 129I at the Nevada Test Site Qinhong Hu and David K. Smith Chemical Biology and Nuclear Science Division, Lawrence Livermore National Laboratory 7000 East Avenue, MS L-231, Livermore, CA 94550, U.S.A. ABSTRACT The groundwater at the Nevada Test Site (NTS) contains many long-lived radionuclides, including 99Tc (technetium) and 129I (iodine), as a result of 828 underground nuclear weapons tests conducted between 1951 and 1992. We synthesized a body of data collected on the distribution of 99 Tc and 129I in groundwater to assess their migration at NTS, at field scales over distances of hundreds of meters and for durations up to forty years and under hydrogeologic conditions very similar to the proposed geological repository at Yucca Mountain. The results of our study show that Tc does not necessarily exist as a mobile and conservative species TcO4–, as has been commonly assumed. This conclusion is corroborated by recent in situ redox potential measurements, which show that groundwaters at multiple locations of the NTS are not oxidizing, and mobility of reduced Tc species (TcO2 · nH2O) is greatly decreased. Speciation of iodine and its associated reactivity is also complex in the groundwater at the NTS, and its effect on the mobility of iodine should be the subject of future studies. INTRODUCTION The current performance assessment model for the proposed Yucca Mountain high-level nuclear waste repository assumes that 99Tc and 129I move at the same rate as water in the subsurface. However, it is probably over-conservative to apply a retardation factor of one for these redox-sensitive radionuclides. At the NTS, a large number of radionuclides (tritium, fission products, activation products, and actinides) are present at various concentrations as a result of underground nuclear weapons tests. Using the body of information that has been gathered since 1973 regarding the distribution of these radionuclides in groundwater, we are continuing to gain insight into radionuclide migration at the NTS. Because Yucca Mountain is located on the western edge of NTS, this insight can be used to assess the performance of a nuclear waste repository in the absence of engineered barriers [1] (Figure 1). DATA ANALYSES A database was set up to compile dissolved radionuclide concentration data, available at Lawrence Livermore National Laboratory, from 18 monitoring wells for 14 nuclear tests at the NTS, with a total of 206 sampling events. In this study, we focused on the measured activity for 3 H (tritium), 99Tc, and 129I, as available. These activities were decay-corrected to September 23, 1992, the date of the last underground nuclear test at NTS, permitting us to directly compare the measured activity with residual radionuclide inventory from all nuclear tests at NTS [2]. This radionuclide inventory (available to the general public) sums the radionuclide totals for individual tests into five principal geographic test areas.
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