A performance assessment methodology for high-level radioactive waste disposal in unsaturated, fractured tuff Page: 4 of 46
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ABSTRACT
Sandia National Laboratories, under contract to the U.S. Nuclear Regulatory
Commission, has developed a methodology for performance assessment of deep geologic
disposal of high-level nuclear waste. The applicability of this performance assessment
methodology has been demonstrated for disposal in bedded salt and basalt; it has since
been modified for assessment of repositories in unsaturated, fractured tuff. Changes to
the methodology are primarily in the form of new or modified ground water flow and
radionuclide transport codes. A new computer code, DCM3D, has been developed to
model three-dimensional ground-water flow in unsaturated, fractured rock using a dual-
continuum approach. The NEFTRAN II code has been developed to efficiently model
radionuclide transport in time-dependent velocity fields, has the ability to use externally
calculated pore velocities and saturations, and includes the effect of saturation-
dependent retardation factors. In order to use these codes together in performance-
assessment-type analyses, code-coupler programs were developed to translate DCM3D
output into NEFTRAN II input.
In addition to flow and transport codes, other portions of the performance assessment
methodology were evaluated as part of modifying the methodology for tuff. The scenario
methodology developed under the bedded salt program, considered adequate, was not
altered, but has been applied to tuff. An investigation of the applicability of uncertainty
and sensitivity analysis techniques to non-linear models indicates that Monte Carlo
simulation remains the most robust technique for these analyses. No changes have been
recommended for the dose and health effects models, nor the biosphere transport
models. Additionally, a number of outstanding, but unresolved, technical issues have
been identified.iii
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Gallegos, D. P. A performance assessment methodology for high-level radioactive waste disposal in unsaturated, fractured tuff, report, July 1991; United States. (https://digital.library.unt.edu/ark:/67531/metadc628299/m1/4/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.