The Phase 3 Total System Performance Assessment (TSPA) sponsored by theElectric Power Research Institute (EPRI) has led to new insights intocritical models and parameters affecting estimated doses to humans from apotential repository of high-level radioactive wastes at Yucca Mountain,Nevada. The Phase 3 model has been extended to encompass time-varyingclimate and infiltration, detailed modeling of the source term andhydrology, and detailed specification of possible interaction betweenpercolating ground water and waste containers. The model estimates doses toa time of one million years.

The three key radionuclides contributing to estimated total doses areTc-99,1–129, and Np-237. Five other nuclides contributing to dose in lesser(but significant) amounts are U-233, Th-229, Pa-231, Ac-227, and Se-79.These results are consistent with other TSPAs.

From sensitivity studies, the most critical models and parameters are asfollows. Infiltration and percolation assumptions, including the amount oflateral diversion of infiltration water, are important and need verificationwith site data and/or more detailed modeling. Parameters of the unsaturatedzone (UZ) and saturated zone (SZ) determine dilution and delay ofconcentrations and peak doses downstream. The fraction of containers thatbecome wet are not critical in our model, but this lack of sensitivityreflects our coupling of the fraction with a model of focused flow past thecontainers; an different model might indicate higher sensitivity. Also, thedegree of coupling between fracture and matrix flow is important inaffecting the times of peak doses but not their magnitudes.

Other critical design assumptions that could lead to reduced and/or delayeddoses are a more robust container design, a capillary barrier around eachcontainer, the dilution during hydrologie transport from the repository to apotential agricultural community downstream, and the characteristics of an“average” individual in that community who might receive a dose.