THE PEÑA BLANCA NATURAL ANALOGUE MODEL

The Nopal I uranium deposit at Peña Blanca is a natural analogue to the Yucca Mountain repository. The Peña Blanca Natural Analogue Model (PBNAM) simulates mobilization and transport of radionuclides from the deposit. The results were compared to the performance the Yucca Mountain repository Total System Performance Assessment Model for the License Application (TSPA-LA Model). The PBNAM provides probabilistic simulations of the outcomes of hydrogeologic processes operating at the mine.

The Nopal I uranium deposit lies in fractured, welded, and altered rhyolitic ash flow tuffs and is geologically, climatically, geochemically, and hydrologically analogous to a repository at the Yucca Mountain site, and has been exposed to oxidizing conditions for 3.2 to 3.4 my. Uranium oxide and uranium silicates in the Nopal I deposit are modeled as being directly analogous to uranium-oxide spent nuclear fuel.

The PBNAM source term was two waste packages of uranium oxide with a defined steady-state dissolution rate in a recharge condition with lateral to vertically downward groundwater flow. Radionuclide transport assumes an estimated 99Tc production generated primarily by the spontaneous fission of 238U. The PBNAM predicts saturated-zone radionuclide concentrations downgradient from the ore body as compared to observed concentrations in groundwater.

The PBNAM results indicate radionuclide concentrations are dominated by 238U, with lesser contributions from other uranium isotopes. The range of results obtained from the simulations bracket the observed concentrations beneath the ore deposit. Sensitivity analyses show that the results are sensitive to the sorption coefficients of the uranium isotopes. The PBNAM assists in the validation of TSPA LA Model forecasts of the mobilization of radionuclides and their unsaturated and saturated-zone transport from Yucca Mountain.