Method of the Safety Margins Management of Nuclear Power Plants

Abstract

Background. Nowadays, there is widely used only one methodology that allows numerically measure NPP safety level and ensures safety optimization. This is Probabilistic Safety Assessment. Therefore, NPP safety is still justified using deterministic approach. As a main tool for deterministic approach Design Basis Accident Analysis is used for NPP designing and safety justification. It has widely recognized and sustainable methodology, procedure and experience of application. Thus, there is contradiction between safety measurement and safety justification.

Objective. Article provides concept of the method that on a basis of deterministic approach allows evaluate and manage safety margins for Nuclear Power Plant for safety and expenses optimization purposes.

Methods. As a main tool for deterministic approach and safety margins calculation the Design Basis Accident (DBA) Analysis was applied. 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 and justified in the NPP design. Since, acceptance criteria have different physical background, dimensions and numerical values it is proposed to transform them into dimensionless form by dividing value of corresponded calculated parameter on value of acceptance criterion. 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. Also, it is proposed to establish for each acceptance criterion a 10% zone (its lower boundary corresponds to 0.9 – value of limiting safety deficit) as a deterministic safety criterion and apply it in the nuclear regulations. If calculated value of safety deficit is within this zone than it is proposed to use time limitation and recognize necessity to develop and apply safety measures on decreasing of a safety deficit. Such method allows reveal as safety deficiencies so excessive safety margins.

Results. The proposed method was applied for DBA Analysis of Zaporizhzhya NPP unit 5. Safety deficits were evaluated for each initiating event and corresponding safety profiles were drawn for each acceptance criterion.

Conclusions. Methodol is recommended for usage in regulatory activity, during the NPP designing and operation, and, for justification of the safety systems maintenance and repair activity at reactor power operation.