Risk-informed Regulation: Handling Uncertainty for a Rational Management of Safety

Classically, the control of the risk associated to the operation of a nuclear power plant has been founded on the definition of a group of events representing credible worst-case accident scenarios (the so-called Design Basis Accidents, DBAs) and on the prediction and analysis of their consequences by deterministic calculations. Then, the safety and protection of the system is designed against such events, to prevent them and to protect from, and mitigate their associated consequences. This traditional approach to regulating nuclear safety by the verification that a nuclear plant can withstand a set of prescribed accident scenarios judged as most adverse, conjectures that if a plant can cope with the DBAs, it will also be capable of handling any other accident. In this view to safety, the underlying concept for protecting a nuclear power plant is the so called defensein-depth which has become the design philosophy for attaining acceptable levels of safety. This structuralist defense-in-depth viewpoint and the safety margins derived from it, have been embedded into conservative regulations aimed at enveloping all credible accidents, for what concerns the challenges and stresses posed on the system and its protections. In fact, such view to nuclear safety has been embraced into a number of design and operating regulatory requirements, including [1]: i) the use of redundant active and/or passive engineered safety systems, to avoid the risks from single failures; ii) the use of large design safety margins to cope with the uncertainty in the actual response of the safety systems under accident conditions; iii) the demand of quality assurance practices on materials, manufacturing and construction; iv) the restriction of system operation within predetermined bounds; v) the definition of requirements for the testing, inspection and maintenance of the structures, systems and components to guarantee the desired safety levels. The approach to safety above illustrated has been regarded effective in providing a conservative means for managing the uncertainties in the system behaviour and its modelling within the safety analyses. However, it is widely recognized that the reliance on purely deterministic analyses for the verification of nuclear safety may not be A risk-informed regulatory approach implies that risk insights be used as supplement of deterministic information for safety decision-making purposes. In this view, the use of risk assessment techniques is expected to lead to improved safety and a more rational allocation of the limited resources available. On the other hand, it is recognized that uncertainties affect both the deterministic safety analyses and the risk assessments. In order for the risk-informed decision making process to be effective, the adequate representation and treatment of such uncertainties is mandatory. In this paper, the risk-informed regulatory framework is considered under the focus of the uncertainty issue. Traditionally, probability theory has provided the language and mathematics for the representation and treatment of uncertainty. More recently, other mathematical structures have been introduced. In particular, the Dempster-Shafer theory of evidence is here illustrated as a generalized framework encompassing probability theory and possibility theory. The special case of probability theory is only addressed as term of comparison, given that it is a well known subject. On the other hand, the special case of possibility theory is amply illustrated. An example of the combination of probability and possibility for treating the uncertainty in the parameters of an event tree is illustrated.