Program Brief

Statement of Problem: Performance assessments of the potential high-level waste repository at Yucca Mountain, Nevada, rely on numerical simulations of radionuclide transport through unsaturated fractured rock. Model predictions of contaminant transport in heterogeneous media are highly uncertain and often impossible to verify. Predictive uncertainty and the challenges of model validation are compounded by the 10,000-year time period of repository performance. Natural analog sites provide the opportunity to collect data on trace element transport during this long time scale. Thus, model prediction uncertainties can be evaluated by comparing transport simulations with natural analog data.

Approach and Accomplishments: A team of geochemists and geohydrologists from the CNWRA^® conducted sampling and characterization testing at an archeological site on the island of Santorini in the Mediterranean. Bronze artifacts, buried by a volcanic eruption around 1600 BC, provided a temporally and spatially constrained source of trace metals. Laboratory analyses of samples detected evidence of a plume of trace metals emanating from the artifacts; however, model simulations using hydrological parameters determined from site characterization tests predicted substantially more trace metal mobilization and transport than could be inferred from the sample data. A more extensive modeling study, using the reactive transport simulator MULTIFLO, developed at CNWRA, demonstrated that uncertainties in model boundary conditions and spatial heterogeneity in model parameters could produce the trace metal distributions inferred from the data. Comparisons of the range of model predictions with the data illustrated uncertainties inherent in performance assessments of the potential repository at Yucca Mountain.

Client Benefits: This field, laboratory, and modeling study is a unique application of natural analog data to evaluation of uncertainties in transport models supporting performance assessments. It has enhanced the U.S. Nuclear Regulatory Commission’s ability to make technically sound decisions regarding the licensing of a high-level waste geological repository.