Secondary ion mass spectrometry measurements of volcanic tuffs containing radionuclides from underground nuclear tests

The in situ distribution of anthropogenic radionuclides in volcanic tuffs w as measured using secondary ion mass spectrometry (SIMS). Samples were obta ined from boreholes drilled in close proximity to expended underground nucl ear test cavities at the Nevada Test Site, USA. SIMS measurements revealed the presence of Na-22, Cs-137, and enriched U-235/U-238 ratios. The radionu clides are distributed heterogeneously and occur at concentrations ranging from 1 to <0.1ppm. Two processes govern the distribution of radionuclides o utside the nuclear test cavity region: the prompt injection of radionuclide s along fractures at the time of the nuclear detonation, and the post-test sorption of radionuclides from groundwater. A zone of prompt injection 37 m above the static water table is characterized by the correlated presence o f all three radionuclides - in one case, all within a single (60 <mu>m) poi nt of analysis. Cs-137 is the only radionuclide detected in rocks from the saturated zone. Since these volcanic tuffs are highly zeolitized, containin g up to 60 wt % clinoptilolite, it is likely that zeolite minerals control Cs-137 sorption from groundwater. However, the heterogeneous distribution o f radionuclides within the zeolitized samples indicates that micro-fracture s in the rock are also important in determining where the radionuclides are deposited.