The Phase 3 Total System Performance Assessment (TSPA) sponsored by the
Electric Power Research Institute (EPRI) has led to new insights into
critical models and parameters affecting estimated doses to humans from a
potential repository of high-level radioactive wastes at Yucca Mountain,
Nevada. The Phase 3 model has been extended to encompass time-varying
climate and infiltration, detailed modeling of the source term and
hydrology, and detailed specification of possible interaction between
percolating ground water and waste containers. The model estimates doses to
a time of one million years.
The three key radionuclides contributing to estimated total doses are
Tc-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 as
follows. Infiltration and percolation assumptions, including the amount of
lateral diversion of infiltration water, are important and need verification
with site data and/or more detailed modeling. Parameters of the unsaturated
zone (UZ) and saturated zone (SZ) determine dilution and delay of
concentrations and peak doses downstream. The fraction of containers that
become wet are not critical in our model, but this lack of sensitivity
reflects our coupling of the fraction with a model of focused flow past the
containers; an different model might indicate higher sensitivity. Also, the
degree of coupling between fracture and matrix flow is important in
affecting the times of peak doses but not their magnitudes.
Other critical design assumptions that could lead to reduced and/or delayed
doses are a more robust container design, a capillary barrier around each
container, the dilution during hydrologie transport from the repository to a
potential agricultural community downstream, and the characteristics of an
“average” individual in that community who might receive a dose.