Jc. Helton et al., UNCERTAINTY AND SENSITIVITY ANALYSIS RESULTS OBTAINED IN A PRELIMINARY PERFORMANCE ASSESSMENT FOR THE WASTE ISOLATION PILOT-PLANT, Nuclear science and engineering, 114(4), 1993, pp. 286-331
Uncertainty and sensitivity analysis results obtained in a preliminary
performance assessment for the Waste Isolation Pilot Plant (WIPP) in
southeastern New Mexico are presented. The most appropriate conceptual
model for performance assessment at the WIPP is believed to include g
as generation due to corrosion and microbial action in the repository
and a dual-porosity (matrix and fracture porosity) representation for
solute transport in the Culebra Dolomite Member of the Rustler Formati
on. Under these assumptions, complementary cumulative distribution fun
ctions (CCDFS) summarizing radionuclide releases to the accessible env
ironment due to both cuttings removal and ground-water transport fall
substantially below the release limits promulgated by the U.S. Environ
mental Protection Agency (EPA). This is the case even when the current
estimates of the uncertainty in analysis inputs are included. Perform
ance assessment results are dominated by cuttings removal. The release
s to the accessible environment due to groundwater transport make very
small contributions to the total release. The variability in the dist
ribution of CCDFs that must be considered in comparisons with the EPA
release limits is dominated by the rate constant in the Poisson model
for drilling intrusions. The variability in releases to the accessible
environment due to cuttings removal is dominated by drill bit diamete
r. For a single borehole, whether or not a release due to groundwater
transport from the repository to the Culebra occurs is controlled by S
alado halite permeability, with no releases for small values (i.e., <5
x 10(-21) m2). Further, releases that do reach the Culebra for larger
values of halite permeability are small and usually do not reach the
accessible environment. A potentially important scenario for the WIPP
involves two or more boreholes through the same waste panel, of which
at least one penetrates a pressurized brine pocket and at least one do
es not. For these scenarios, the uncertainty in release to the Culebra
due to groundwater transport is dominated by borehole permeability, b
rine pocket pressure, and the solubilities for individual elements (i.
e., americium, neptunium, plutonium, thorium, uranium). Once a releas
e reaches the Culebra, matrix distribution coefficients for the indivi
dual elements are important, with releases to the Culebra often failin
g to reach the accessible environment over the 10 000-yr period specif
ied in the EPA regulations.