Disposition of High-Level Waste and Spent Nuclear Fuel The Continuing Societal and Technical Challenges (2001) / Chapter Skim
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6. Scientific and Technical Issues in Radioactive Waste Management
6. Scientific and Technical Issues in Radioactive Waste Management
Pages 85-113
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From page 85... ...
The first part of the chapter describes the scientific basis on which repository behavior can be analyzed. The second part of the chapter describes the quantitative methodology, known as performance assessment, which has been developed for such analyses by the waste management community, and the measures taken to enhance confidence in its reliability.
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From page 86... ...
It is also beyond established practice to predict accurately whether or not some of the radionuclides disposed in the repository may move through the geological formations and eventually come in contact with human beings and the environment in the future and cause them harm. As emphasized above, however, the challenge is not to accomplish these impossible tasks, but rather to assess the range of potential future behaviors with sufficient confidence to allow the appropriate societal decisions to be made.
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From page 87... ...
. Assessment of future repository performance must be based upon modeling of the physical and chemical processes involved.
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From page 88... ...
The behavior of the host rock will be affected by the temperature of the waste package, the chemical composition (as a function of time) of the water contacting the waste package, the mechanical properties of the rock, and the water content and water velocity close to the waste package.
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From page 89... ...
Methodologies for hydrogeochemical modeling of the near-field environment have also been developed and tested. They are generally based on the measurement of the present composition of the water; on its potential evolution due to temperature increases; and on the nature of the host rock, the waste package, and possibly a surrounding buffer material.
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From page 90... ...
Knowing the flux of water in contact with the waste package is necessary to model the chemical degradation of the waste. This flux will vary throughout the repository due to small-scale spatial variability of the rock.
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From page 91... ...
Another difficulty encountered in modeling radionuclide transport in geological media is that of properly accounting for the numerous coupled hydrogeochemical interactions that take place among the solutes, host rock, and particulate or colloidal matter that may be present. Much has been learned about these complex interactions, mostly in laboratory experiments and also in some of the transport experiments done in URLs, as mentioned above.
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From page 92... ...
If the model does not properly account for the physical, hydrogeochemical, and when appropriate, biological processes and system properties that actually control radionuclide migration in both the near- and far-fields of the repository system, then modelderived estimates of radionuclide transport are very likely to have very large even orders-of-magnitude systematic errors (see Sidebar 6.1~. The present state of the art for transport modeling involves the use of many different kinds of numerical models to represent radionuclide migration in the environment.
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From page 93... ...
In many disposal concepts, one approach to this challenge of understanding complex geology is to use other safety barriers to reduce the importance of the geological medium. A durable waste form and container, along with an effective, low-permeability buffer material around the waste packages, will reduce the performance requirements on the geology.
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From page 94... ...
94 D~OS1~N ~ FEW USE ~ Saw N~E~R F~[ 1 .
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From page 96... ...
96 DISPOSITION OF HIGH-LEVEL WASTE AND SPENT NUCLEAR FUEL Steel S= for Ground I Control DmjfPIIaCement Drip Shield Commercial Spent Nuclear Fuel Waste Package Codisposal Five Waste Package Containing High-Level Waste Canisters with One DOE Spent Nuclear Fuel Assembly Commercial Spent Nuclear Fuel Waste Package Emplacement Pallet Gantry Crane Rail
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From page 97... ...
Accordingly, the acceptability of project decisions based, at least in part, upon the results of safety assessments will depend on the level of confidence placed in the methodology by the technical experts within the implementer and regulator organizations, political decision makers, and the public. The following sections expand on the principles, technical approaches, and confidence-building measures that have been developed for analyzing the safety of geological .
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From page 98... ...
Proper site selection is therefore an important aspect of repository design, but it can only reduce the probability of release, not eliminate it. In all cases, the occurrence of a release, even in the distant future, must be evaluated in terms of the potential harm it can cause.
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From page 100... ...
This uncertainty, which cannot be easily quantified, can be reduced by using alternative conceptual models and should always be kept in mind. There is a danger that a legalistic, prescriptive regulatory environment or a project forced to meet deadlines can induce scientists charged with developing a performance assessment to assume that they have no uncertainty in their conceptual models (see Sidebars 6.1, 6.2~.
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From page 101... ...
The combination of short-term, exact experiments and observations with long-term analogue studies can also contribute to elimination of unsuitable models and to increase confidence in the models retained for the final safety assessments. ~ The use of risk numbers for comparison among alternatives ("relative risk")
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From page 102... ...
Efforts to raise public confidence have also been widely undertaken by the scientific and technical community. In their 1991 collective opinion on safety assessment methodology (NEA, l991b)
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From page 103... ...
For instance, the examples concerning transport processes in Sidebars 6.1 and 6.2 should motivate the examination of processes and mechanisms that were not anticipated when many national programs started one or two decades ago. It is necessary that new ideas be examined, which implies that scientists inside and outside the project can submit proposals and receive funding for performing work designed to disprove the prevailing conceptual model or to collect data that may support an alternative conceptual model.
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From page 104... ...
assuring that all potentially negative processes are analyzed; and (3) being relatively insensitive to parameter and conceptual model changes.
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From page 105... ...
The special challenges of involving the public in nuclear and radioactive waste issues are dealt with in detail in Chapters 5 and 8. Although no deep geological repository for long-lived wastes has yet been licensed through a standard procedure,2 regulators and implementers are relatively confident that workable procedures have been, or can be, 2 The WIPP is a special case, certified by the EPA for limited types of DOE waste (see Sidebar 4.1)
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From page 106... ...
· What is the role of the regulator in site selection and choice of disposal method? · Legal and regulatory issues what are the roles of government agencies and regulators?
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From page 108... ...
The most conspicuous result of these developments has been an intensive debate on monitoring repositories, on phased or stepwise implementation programs, and on measures needed to guarantee reversibility of steps including retrieval of emplaced wastes. The technical community accepted the validity of these points rather grudgingly.
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From page 109... ...
It is nearly impossible to quantify risk when we are also uncertain of the appropriate conceptual model underlying the phenomena of interest. The issues involved in gaining public acceptance under conceptual uncertainty include acknowledging the importance of ignorance and 3 The precautionary principle as a concept is a matter of intense debate in the risk community as well as in the international legal community.
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From page 110... ...
Alternative assumptions and value judgments should be explored fully. Such an incremental approach, which makes sense from the technical standpoint, also offers a way to provide for the open-ended societal learning that is an essential quality of the successful waste management program.
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From page 111... ...
Safety assessments alone are not the only considerations governing the acceptability of any disposal facility. · The feasibility of performing assessments of sufficient quality is accepted by technical experts within the waste management community.
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From page 112... ...
This is a fact that is true also for every other comparable technical undertaking, and it is important to assure that unique, unattainable requirements to the contrary are not placed on radioactive waste disposal. · The extensive technical efforts that are being put into specific, technical validation programs, centered around comparisons of calculations, experiments, and observations of analogue objects, should be complemented increasingly by further confidence-building measures.
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From page 113... ...
SCIENTIFIC AND TECHNICAL ISSUES IN RADIOACTIVE WASTE MANAGEMENT 113 have a long period of monitored controls and enhanced retrievability before being converted into their final closed state. · Adequately safe geological repositories can be implemented with various combinations of carefully selected host rocks, sites, and engineered barriers as long as decisions are taken commensurate with understanding.
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