Conceptual Model Uncertainty Research Articles

Performance Assessments of Geologic Repositories for High-Level Nuclear Waste: Are TheyNecessaryorSufficient?

AbstractPerformance assessments of geologic repositories for high-level nuclear waste will be used to determine regulatory compliance. The determination, that with a “reasonable expectation” regulatory limits are met, is based on the presumption that all of the relevant physical, chemical and biological processes have been modeled with enough accuracy to insure that a confident judgment of safety may be made. For the geologic disposal of high-level nuclear waste, this generally means that models must be capable of calculating radiation exposures to a specified population at distances of tens of kilometers for periods of tens to hundreds of thousands of years. A total system performance assessment will consist of a series of cascading models that are meantin tototo capture repository performance. There are numerous sources of uncertainty in these models: scenario uncertainty, conceptual model uncertainty and data uncertainty. These uncertainties will propagate through the analysis, and the uncertainty in the total system analysis must necessarily increase with time. For the highly-coupled, non-linear systems that are characteristic of many of the physical and chemical processes, one may anticipate emergent properties that cannot, in fact, be predicted. For all of these reasons, a performance assessment is not in and of itself a sufficient basis for determining the safety of a repository, but it remains a necessary part of the effort to develop a substantive understanding of a repository site.

Development of the Post-Closure Safety Case for the Low Level Waste Disposal Site at Drigg, United Kingdom

ABSTRACTThis paper provides an overview of the development of the post-closure safety case (PCSC) for the solid low level radioactive waste (LLW) disposal site at Drigg. The paper outlines the background to the site, the implementation of a systematic approach to understanding the site and undertaking a post-closure radiological safety assessment (PCRSA), communication activities, key factors in the PCSC and issues to be investigated further in the forward programme.The Drigg site is the UK's principal near-surface facility for disposal of LLW. Disposals commenced in 1959 to trenches whereas current practice involves disposal to concrete vaults. Disposals are carried out under an authorisation from the UK Environment Agency. Periodically the Environment Agency reviews the authorisation to ensure consistency with current regulatory requirements. British Nuclear Fuels plc (BNFL) will produce an updated PCSC in 2002; in preparation for this, BNFL published in March 2000 a status report on the development of the PCSC.BNFL and the Environment Agency conduct an information exchange process on the development of the PCSC. This process has led to notable improvements, as well as affording a mechanism for providing clarifications and building confidence in the PCSC. BNFL undertakes other communication activities including scientific publication, conference presentation, participation in international programmes (such as those of the IAEA and NEA) and liaison with local communities. Peer review of the PCSC is also being carried out.Central to the safety case is a systematic PCRSA, which promotes transparency and traceability and demonstrates a rigorous treatment of relevant uncertainties. The approach aligns with international best practice for near-surface disposal facilities and includes the assessment of features, events and processes and the use of tools such as conceptual model uncertainty forms. This formalisation helps to identify and understand the key factors for the PCSC. Additional qualitative or simple quantitative information is used to augment the PCRSA so as to provide a wider context to inform decision-making.

Reducing Uncertainty in Site Characterization Using Bayes Monte Carlo Methods

A Bayesian uncertainty analysis approach is developed as a tool for assessing and reducing uncertainty in ground-water flow and chemical transport predictions. The method is illustrated for a site contaminated with chlorinated hydrocarbons. Uncertainty in source characterization, in chemical transport parameters, and in the assumed hydrogeologic structure was evaluated using engineering judgment and updated using observed field data. The updating approach using observed hydraulic head data was able to differentiate between reasonable and unreasonable hydraulic conductivity fields but could not differentiate between alternative conceptual models for the geological structure of the subsurface at the site. Updating using observed chemical concentration data reduced the uncertainty in most parameters and reduced uncertainty in alternative conceptual models describing the geological structure at the site, source locations, and the chemicals released at these sources. Thirty-year transport projections for no-action and source containment scenarios demonstrate a typical application of the methods.

The EVEREST project: sensitivity analysis of geological disposal systems

The main objective of the EVEREST project is the evaluation of the sensitivity of the radiological consequences associated with the geological disposal of radioactive waste to the different elements in the performance assessment. Three types of geological host formations are considered: clay, granite and salt. The sensitivity studies that have been carried out can be partitioned into three categories according to the type of uncertainty taken into account: uncertainty in the model parameters, uncertainty in the conceptual models and uncertainty in the considered scenarios. Deterministic as well as stochastic calculational approaches have been applied for the sensitivity analyses. For the analysis of the sensitivity to parameter values, the reference technique, which has been applied in many evaluations, is stochastic and consists of a Monte Carlo simulation followed by a linear regression. For the analysis of conceptual model uncertainty, deterministic and stochastic approaches have been used. For the analysis of uncertainty in the considered scenarios, mainly deterministic approaches have been applied.

Probability networks for handling uncertainty in the performance assessment of high-level nuclear waste repositories

Abstract This paper proposes the use of probability networks to handle uncertainty in the performance assessment of high-level nuclear waste repositories. Probability networks provide coherent methods for: • describing uncertain expert opinion; • providing both qualitative and quantitative representations of an expert's knowledge; • combining different sources of uncertainty including the uncertainty in scenarios, conceptual models, mathematical models, parameters, and data; • performing sensitivity analyses to determine influential components of the performance model; • making decisions regarding the data that should be collected to reduce uncertainty; • reconciling inconsistencies among experts; and • assessing the regulatory compliance of a particular repository. These features will be illustrated through the use of a simplified example in this paper.