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Abstract
The risk assessment approach has been applied to support numerous radioactive waste management activities over the last 30 years. A risk assessment methodology provides a solid and readily adaptable framework for evaluating the risks of CO2 sequestration in geologic formations to prioritize research, data collection, and monitoring schemes. This paper reviews the tasks of a risk assessment, and provides a few examples related to each task. This paper then describes an application of sensitivity analysis to identify important parameters to reduce the uncertainty in the performance of a geologic repository for radioactive waste repository, which because of importance of the geologic barrier, is similar to CO2 sequestration. The paper ends with a simple stochastic analysis of idealized CO2 sequestration site with a leaking abandoned well and a set of monitoring wells in an aquifer above the CO2 sequestration unit in order to evaluate the efficacy of monitoring wells to detect adverse leakage.

Introduction
Risk assessment provides a framework for placing information in context. Over the last three decades, the United States has applied risk assessment to key decisions concerning radioactive waste disposal. During this same period, risk concepts have been applied to nuclear reactors, nuclear fuel storage and transportation systems, and critical infrastructure such as national treasures, dams, and water supplies. A repository for radioactive waste disposal is conceptually similar to CO2 sequestration in that the geology is providing an important isolation function. Furthermore, the heterogeneous and uncertain geology must be characterized to reduce uncertainty and, thereby, develop a general understanding of its structure and behavior as a disposal system, unlike an engineered barrier. Similar to radioactive waste disposal, the extent of characterization necessary to adequately understand behavior of the geologic system will be an important unknown in any CO2 sequestration project. Using a risk framework as the hub for decisions related to CO2 sequestration will aid in decisions on research and data collection priorities for characterizing the site. The risk framework can also be used to evaluate monitoring schemes. A qualitative benefit of adopting a risk framework is that it will help the CO2 sequestration community develop a sensible regulatory framework. We elaborate upon these points below.

Risk Assessment Definitions
Risk assessment is a type of policy analysis of what can go wrong in human affairs, in which the current state of scientific and technological knowledge is made accessible to society as input to risk management decisions, with time and resource constraints specified by the policy decision makers. Although risk has several connotations (if not denotations) inside and outside the profession of risk analysis, risk is generally used in this paper to express some measure that combines “the gravity of harm” to something valued by society and “the probability of the event.” Frequently, within the risk profession, the measure of risk is the expected value of the consequence, e.g., probability times consequence based on average values, as used in simple annuity analysis. For financial investments, the measure is often the variance of the return on investment. For situations with large uncertainty, such as disposal of radioactive wastes, the measure of risk is the entire distribution of possible consequences.