COLLOID FACILITATED TRANSPORT OF PLUTONIUM THROUGH FRACTURED CARBONATE AND VOLCANIC TUFF

During the U.S. nuclear test program, over 800 underground nuclear tests were detonated at the Nevada Test Site (NTS), NV. As a result, a radiologic source term consisting of tritium, fission products, activation products and actinides remains underground following the nuclear test. The U.S. Department of Energy is tasked with investigating the distribution and /or potential migration of the radionuclides in groundwater at the NTS. An accurate assessment of risk for human health down-gradient from the testing centers requires understanding of how the radionuclides migrate. Our lack of knowledge regarding the extent to which colloids facilitate transport of low-solubility radionuclides, such as plutonium (Pu), currently limits our efforts to develop reliable transport models.

Two sets of experiments were carried out to understand better the mechanisms controlling colloid-facilitated transport of Pu in fractured rock. The first set of experiments was conducted using rhyolitic tuff and the second using carbonate fractured rock. One set of fractures were injected with a radionuclide cocktail of dissolved species, and the second injected with pseudocolloids consisting of Pu(IV) sorbed to either zeolite or clay. Tritium and Re (analog to Tc) tracers were also added to a NTS-type synthetic groundwater (4.5mM NaHCO3-). The cocktail was injected into a smooth fracture surfaces and the effluent analyzed. Efforts to characterize the fractured surface to investigate the location of the sorbed Pu by autoradiography and secondary ion mass spectrometry will be presented. In the rhyolitic tuff, less than 5% of the dissolved Pu was eluted through the column. In contrast, approximately 30% of the Pu sorbed onto the zeolite colloids was eluted through the tuff. Pu was transported more effectively through the carbonate cores than the tuff. In both cases, Pu appears to be mineralogically controlled within the cores.

This work was performed under the auspices of the U.S. Department of Energy by University of California, Lawrence Livermore National Laboratory under Contract W-7405-Eng-48.