Technical Bases for Yucca Mountain Standards (1995) / Chapter Skim
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CHAPTER 3 - ASSESSING COMPLIANCE
CHAPTER 3 - ASSESSING COMPLIANCE
Pages 67-104
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From page 67... ...
Our judgment is that limiting individual risk in this way is also likely to provide adequate radiological protection for all relevant populations that might be exposed to radiation from radionuclides released from the proposed repository at Yucca Mountain (see Chapter 2~. The period over which this level of protection should be assessed should extend over the period of duration of hazard potential of the repository, that is, until the time at which the highest critical group risk is calculated to occur, within the limits imposed by the long-term stability of the geologic environment at Yucca Mountain, which is on the order of 1 o6 years.
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From page 68... ...
While these factors cannot be calculated precisely, we believe that there is a substantial scientific basis for making such calculations, taking uncertainties and natural variabilities into account, to estimate, for example, the concentration of wastes in ground water at different locations and the times of gaseous releases. One critical gap in our understanding is with respect to future human behavior.
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From page 69... ...
Nevertheless, the currently available tools plus additional tools that we believe can be developed as part of the stanciarci-setting and compliance assessment efforts, or through other research, should be adequate for the analyses requires] to evaluate repository performance.
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From page 70... ...
It involves computer calculations using quantitative models of physical, chemical, geologic, anti biological processes, taking uncertainties into account. Modeling repository performance is a challenging task because the rates of geochemical transformation and transport of the radionuclides are generally very slow and the times at which points distant from the repository become significantly affected by ra(iionuclide releases will be in the far future.
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From page 71... ...
In this perspective even the longest times considered for repository performance models are not excessive. Furthermore, even changes in climate at the surface would probably have little effect on repository performance deep below the ground.
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From page 72... ...
For example, uncertainties in waste canister lifetimes might have a more significant effect on assessing performance in the initial 10,000 years than in performance in the range of 100,000 years. Probabilistic Analysis of Risk To judge compliance against a risk-base(i standard of the type proposed, a risk analysis including treatment of all scenarios that might leac!
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From page 73... ...
further in Chapter 4. We believe that performance assessment using numerical models of physical anti chemical processes and quantitative estimates of probabilities is the key approach to assessing compliance.
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From page 74... ...
Figure 3.l The Basic Steps in Performance Assessment Natural Observations Laboratory and Field Experiments develop system understanding (conceptual model) develop quantitative models (mathematical model)
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From page 75... ...
conceptual models of release ant] transport processes specifying, among other things, how and where the ground water flows and exposure scenario mociels specifying where farmers live, what technologies they use ant!
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From page 76... ...
76 Math etnatical mode' YUCCA MOUNTAIN STANDARDS By mathematical mocie} we mean the mathematical relationships that are used to describe the physical system quantitatively. The system of equations that is incorporated in the mathematical mocle!
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From page 77... ...
Although the typical nature of past climate changes is well known, it is obviously impossible to predict in detail either the nature or the timing of future climate change. This fact adds to the uncertainty of the model predictions.
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From page 78... ...
A method used to help implement statistical distributions of a parameter in performance assessment is the Monte CarIo method. In this method, data on the frequency distributions of parameter values are sampled to provide input to the equations.
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From page 79... ...
A further drawback of complex probabilistic modeling is that the results are not very transparent or easily unclerstood. Bounding estimates Analyses using pessimistic scenarios and parameter values are more easily understood than Monte CarIo analysis.
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From page 80... ...
Following a major conference on safety assessment in 1990, a Collective Opinion was prepared by the Radioactive Waste Management Committee of the Nuclear Energy Agency of the Organization for Economic Cooperation and Development. The general conclusions drawn were, first, that appropriate performance assessment tools are currently available for producing results of the quality required for a decision on compliance and, second, that the final quality of the results is restricted primarily by the
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From page 81... ...
the backfill, disturbed rock and other materials of the near-field zone in the vicinity of the waste; (3) the rock, air and water of the unsaturated zone (rock anti pores above the water table)
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From page 82... ...
82 YUCCA MOUNTAIN STANDARDS Figure3.2 Schematic illustration of the major pathways from a repository at Yucca Mountain to humans ':''''''''' Waste Forms ~ (bannisters, ~ etc.) ~ corrosion, etc.
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From page 83... ...
FDr example, release of gaseous radionuclides from waste packages anti their transport through the unsaturated zone to the atmosphere above Yucca Mountain is thought to be a relatively rapid process compareci to dissolution of radionuclides and their transport in solution through the unsaturated zone to the water table. Exposure pathways from the atmosphere or from ground water to humans are represented by jagged arrows.
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From page 84... ...
time-dependencies of uncertainties associated with ~ ~ ~ transport from a Yucca Mountain repository through the various reservoirs shown in Figure 3.2; (2) the effects of potential natural and human modifiers to repository performance; and (3)
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From page 85... ...
Time-dependent temperatures, porosity and humidity in the vicinity ofthe canisters, required for estimates of waste package failure, are currently being calculated using models of two-phase convective flow in the near-field unsaturated zone induced by thermal loading of the repository. Detailed estimates of time for canister failure are less important for much longer-term estimates of individual dose or risk.
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From page 86... ...
of Zircaloy claiming on spent fuel can be estimated. Transportfrom canisters to the r~ear-f eld unsaturated zone Inflow of air through failed canisters and oxidation of waste prior to infiltration of water can affect the time-dependent release rates of gaseous radionuclides as well as the later release of dissolved radionuclides.
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From page 87... ...
diffusion of radionuclicles away from the waste canister. Analysis of diffusion for unsaturated zone sites such as Yucca Mountain, with or without sorptive retardation, is currently limited by knowlecige of the effective diffusion coefficients in unsaturated backfill anti tuff.
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From page 88... ...
If those risks were found to be significant, they would be included against the risk limit we propose. Aqueous phase transportirom the unsaturated zone to the water table Mechanisms of aqueous phase transport of dissolved radionuclicles in the unsaturated zone at Yucca Mountain are less well unclerstood than
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From page 89... ...
Although considerable uncertainty currently exists regarding the mechanism and rates of aqueous phase transport in the unsaturated zone, these uncertainties do not preclude incorporation of this transport in a quantitative performance assessment. Site characterization activities currently underway are designee!
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From page 90... ...
Saturate~zone transportirom t/`e aquifer beneath the repository to other locations from which water may be extracted by humans or ultimately reach the surface in a regional discharge area The time at which inhabitants downgradient from a Yucca Mountain repository coup! be exposed to ractionuclides depends on the rates of advective transport in the saturated zone and on modifications to that rate resulting from geochemical processes such as sorption.
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From page 91... ...
the large changes in surface conditions, there are three main potential effects of climate change on repository performance. The first of these is that increases in erosion might significantly decrease the burial depth of the repository.
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From page 92... ...
For this reason, climate changes on the time scale of hundreds of years would probably have little if any effect on repository performance, and the effects of climate changes on the deep hydrogeology can be assessed over much longer time scales. The third type of change that might result from climate change is a shift in the distribution and activities of human populations.
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From page 93... ...
Seismicity is an episodic process, appearing to be essentially a fractal activity involving frequent releases of small amounts of strain energy and progressively less frequent releases of larger amounts of energy. It is possible through careful examination ofthe geologic record to establish a chronological history of the activity over millions of years.
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From page 94... ...
In summary, although the timing of seismic events is unpredictable, the consequences of these events are bounciable for the purpose of assessing repository performance. Volcanism A volcanic intrusion into the proposer!
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From page 95... ...
for a repository at Yucca Mountain. PART III: EXPOSURE SCENARIOS IN PERFORMANCE ASSESSMENT As noted above, we believe that it is feasible to calculate, to within reasonable limits of certainty, potential, defined as possible but not necessarily probable concentrations of radionuclides in ground water and air at different locations and times in the future.
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From page 96... ...
As far as we are able to determine, there is no sound basis for quantifying the likelihood of future scenarios in which exposures do or do not occur; about all that can be said is that both are possible. It is our view, however, that once exposure scenarios have been adopted, performance assessment calculations can be carried out for the specified scenarios with a degree of uncertainty comparable to the uncertainty associated with geologic processes and engineered]
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From page 97... ...
Unfortunately, many human behavior factors important to assessing repository performance vary over periods that are short in comparison with those that should be consiclered for a repository. The past several centuries (or even decades)
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From page 98... ...
The purpose of making exposure scenario assumptions is not to iclentiiTy possible fixtures, but to provide a framework for the analysis and evaluation of repository performance for the protection of public health.2 2 Another argument for using a large number of scenarios is that iterative analysis of repository performance will lead to the most cost-effective repository design. This might be true, but we believe that the regulator must in the end assess compliance with a single level of protection as defined in the standard.
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From page 99... ...
This process will provide a more complete analysis of the advantages and iisadvantages of alternative scenarios than we have been able to perform, and do so with the benefit of full public participation.3 As with other aspects of defining the standards and demonstrating compliance that involve scientific knowlecige but must ultimately rest on policy judgments, we considered what to suggest to EPA as a useful starting point for rulemaking on exposure scenarios. Reflecting the disagreement inherent in the literature, we have not reached complete consensus on this question.
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From page 100... ...
be based on the source of food for today's population near the repository site. The Appendix C approach bases the exposure scenario on a population distribution rierived from observer!
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From page 101... ...
All food eaten over the lifetime of the subsistence farmer would be grown with water drawn from an underground aquifer contaminated with radioactivity from the repository. The water would be withdrawn at a location outside the footprint of the repository and near that maximum potential concentration of the most critical radioactive contaminant in the ground water so that the scenario describes the maximum dose and risk.
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From page 102... ...
Fourth, it makes the most conservative assumption that wherever en c} whenever the maximum concentration of raclionuclides occurs in a ground water plume accessible from the surface, a farmer will be there to access it. These approaches have many elements in common.
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From page 103... ...
Most of the committee regard these as desirable features of exposure scenarios that are intended to be consistent with the critical-group concept. We emphasize, however, that specification of exposure-scenario assumptions is a matter for policy decision.
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From page 104... ...
separately from the performance of an undisturbeci repository, it is reasonable in our view to clefine a region in which human activities are to be regarded as intrusion and to exclude that region from calculation of the undisturbed repository performance. For example, if we assume that all drilling for water wells is vertical, the area directly above the repository plan (or footprint)
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