Technical Bases for Yucca Mountain Standards (1995) / Chapter Skim
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APPENDIX C - A PROBABILISTIC CRITICAL GROUP
APPENDIX C - A PROBABILISTIC CRITICAL GROUP
Pages 145-152
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From page 145... ...
The main purposes of this example are to show that the approach is feasible and to illustrate the steps necessary to perform such a calculation. The example uses a Monte Cario method for mocleling exposure consistent with that employed in the hydrologic modeling of radionuclide transport.
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From page 146... ...
For example, we were particularly concerned with avoiding strategies that would reward uncertainty in the temporal or spatial distribution of radionuclides in ground water. A procedure in which larger uncertainty in transport parameters leads to a reduction in calculated risk, relative to the risk that would be calculated were transport parameters less uncertain, would provide a strong disincentive to reduce uncertainty through sitecharacterization activities.
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From page 147... ...
The fact that gaseous releases are not included in this example should not be interpreted as a judgment that such releases can be excluded from performance assessment and compliance evaluation. A separate exposure scenario, with a different critical group, would be required for evaluation of the gaseous exposure pathway.
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From page 148... ...
provides a technical basis for limiting the simulation area for exposure analysis, significant reduction in the computational effort required for the calculations would result. In a Monte Cario simulation, each of the pertinent parameters is represented by a distribution of values, from which one value for each is randomly selected for each of many calculations.
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From page 149... ...
Once the plume reaches the aquifer beneath an exposure area, the risk to inhabitants will vary with time as the areal extent of the plume ant} radionuclicle concentrations in the contaminated ground water change cluring plume migration. If the critical group comprises a set of individuals who have the greatest average risk, then the temporal as well as spatial distribution of risk must be considered in identifying the group.
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From page 150... ...
Step 5: Generate exposure realizations Having identified the time period of maximum potential exposure for each plume realization, it is also necessary to determine the spatial distribution of potential doses and health effects to identify the critical group and to calculate the risk to an average individual in that group. The next step, then, is to use the plume snapshots in the Monte CarIo series of exposure simulations.
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From page 151... ...
in the exposure realization. Step 7: Interpretation of exposure simulation results to identify critical subgroups For each of the plume realizations, the results of the exposure simulations can be combined to yield a spatial distribution of expected close, which can then be used to identify tile geographic area inhabited by the critical subgroup for a given plume realization.
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From page 152... ...
The variability in risks between critical subgroups is related primarily to the variability in potential plume concentrations and locations resulting from the probabiliistic simulations of release and transport mechanisms. Using the average critical subgroup risk provides an estimate of the risk to the critical group exposed to the average plume.
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