Klevtsov S. Methodology of the Evaluation of Safety Margins for Nuclear Power Plants

Thesis provides methodology that on a basis of deterministic approach allows evaluate safety margins and safety level as for Nuclear Power Plant, so for each physical safety barrier as well as is capable to satisfy provisions of the fundamental safety principle – “Optimization of Protection”. These could be performed by independent implementation of the methodology or in the framework of Integrated Risk-Informed Decision Making approach. As a tool for deterministic approach and safety margins calculation the Design Basis Accident Analysis was applied since, it is world widely used during NPP designing and safety justification. It has widely recognized and sustainable methodology, procedure and experience of application. Verified and validated computer codes for thermohydraulic, neutron physics and strength analysis are developed and applied for a long time. The main objective of DBA analysis is to demonstrate based on conservative approach exceeding or non-exceeding of so called acceptance criteria that are established in the NPP design or in the norms, rules and standards on nuclear and radiation safety. In the theses it is proposed do not limit the research by the fact of exceeding or non-exceeding of acceptance criteria. It is proposed to develop the research further – to calculate differences between the acceptance criteria and calculated value that is to evaluate the margin or other words – safety margin. Since, acceptance criteria have different physical background, units and numerical values it is proposed to transform them into dimensionless form by dividing value of corresponded calculated parameter on value of acceptance criterion in order to establish the basis for the possibility of comparative analysis. The result of ratio is the dimensionless safety deficit while difference between the dimensionless acceptance criterion (that is always equal to 1) and safety deficit is the dimensionless safety margin. Such an approach allows evaluate for each initiating event (transient or anticipated accident) the safety deficits for each acceptance criterion. Introduction of the average and integrated safety deficit makes possible to perform comparative analysis between different initiating events, acceptance criteria, physical protection barriers and types of nuclear reactors. Also, it is proposed to establish for each acceptance criterion a 10% zone (corresponds to 0.9 value limiting safety deficit) as a deterministic safety criterion and apply it in the norms, rules and standards on nuclear and radiation safety. If calculated value of safety deficit is within this zone than it is propose to use time limitation and recognize necessity to develop and apply safety measures of decrease of a safety deficit. Graphical representation of safety deficits evaluated for the initiating events, acceptance criteria, physical protection barriers reflects the safety profile and demonstrate violation of the deterministic safety criterion and deviation from the average value of safety deficit. This allows reveal as safety deficiencies so excessive safety margins. If the first one requires development of the corresponded safety measures then the second one – establishes the basis on implementation of “Optimization of protection” fundamental safety principle and develop measures aimed on decrease of expenses on reliable and safe NPP operation along with keeping established high level of safety. Process of the achievement of the both purposes establishes basis of the safety margin management. The proposed methodology was applied for Design Basis Accident Analysis of Zaporizhzhya NPP unit 5. Safety deficits were evaluated for each initiating event and corresponded safety profiles were draw for each acceptance criterion. As a result, the violation of acceptance criteria on primary, secondary and containment pressure were revealed. Limitations on term of plant operation were evaluated for each of the safety deficit. Safety measure on decrease of safety deficit for primary and secondary side pressure in case of the transients and postulated accidents was proposed. RELAP5 Mod3.2 was used to perform calculation analysis to evaluate safety measure effectiveness and to demonstrate decrease of safety deficit. Also, excessive safety margins were revealed and ways on optimization of operational expenses were proposed. Methods for independent application and application as part of the IRIDM approach were developed. Methodology is recommended for usage in regulatory activity, during the NPP designing and operation, and, for optimization of the safety systems maintenance and repair activity and its fulfillment on operating at power reactor.