Workshop on the Application of the Concept of Defence in Depth and Practical Elimination

Defence in depth is a fundamental concept in nuclear safety that involves creating multiple, independent layers of protection to prevent accidents and mitigate their consequences if they occur. This aims to ensure that if one layer fails, others will still be in place to achieve the fundamental safety objective of protecting people and society from harmful effects of ionising radiation, and the fundamental safety principle (number 8) of making all practical efforts to prevent and mitigate nuclear or radiation accidents (IAEA Safety Standards, Safety Fundamentals No. SF-1). One of the key aspects of defence in depth is the independence among the layers, in particular between the third layer, focused on control of accidents within the severity level of design basis accidents, and the fourth, focused on prevention and mitigation of the progression of accidents with severe consequences. With the aim to fulfil the independence between these two layers, the IAEA Specific Safety Requirements for Nuclear Power Plants: Design (SSR-2/1) introduced the concept of ‘design extension conditions’ and the associated additional safety features to be required in the design, together with the acknowledgment of the concept of ’practical elimination’ (of plant event sequences that could lead to an early radioactive release or a large radioactive release).

The concept of practical elimination in nuclear safety has evolved since it was first introduced. The idea of ‘practical elimination’ was suggested in the 1996 report INSAG-10, concerning the elimination of possible weaknesses in defence in depth. In this context, INSAG-10 discussed strengthening the containment function for advanced reactors; it indicates that by using probabilistic and deterministic means certain severe accident scenarios that could lead to large radioactive releases due to early containment failure would be demonstrated to be essentially eliminated with a high degree of confidence. The term later became ’practical elimination’.

Following INSAG-10, the concepts of practical elimination and design extension conditions were further developed and refined through various international safety standards and guidelines. The International Atomic Energy Agency (IAEA) played a crucial role in this process by publishing SSR-2/1 (Rev.1) in 2016 that provided detailed requirements on achieving practical elimination and implementing additional safety features for design extension conditions in nuclear power plant designs. This publications aims to enhance the understanding of the interconnections between the defence-in-depth concept, design extension conditions, and practical elimination, which are key components of the overall safety approach for nuclear power plants. IAEA's Specific Safety Guide No. SSG-88, published in 2024, marked a significant milestone in achieving consensus on the guidance related to practical elimination and design extension conditions. Indeed, SSG-88 outlines the necessary steps to demonstrate that certain accident scenarios have been practically eliminated, including the use of advanced safety features, thorough safety assessments, and continuous improvement of safety measures, thereby enhancing global nuclear safety.

The purpose of the event is to disseminate Design Extension Conditions and the Concept of Practical Elimination in the Design of Nuclear Power Plants (IAEA Safety Standards Series No. SSG-88) on the implementation of the concepts of defence in depth and the practical elimination of plant event sequences that could lead to an early or large radioactive release from nuclear power plant.

The event will provide a forum for NPP designers, operators, technical support organizations, and regulators to familiarize first, with the concepts of defence in depth, practical elimination and design extension conditions and their interrelations, and second, with the methodology to demonstrate the practical elimination implementation at the design of nuclear power plants for those plant event sequences that could lead to large or early radioactive releases.