DS508 - Assessment of the Safety Approach for Design Extension Conditions and Application of the Practical Elimination Concept in the Design of Nuclear Power Plants

Status: STEP 8

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Beteiligte IAEO-Komitees: NUSSC, NSGC

Specific Safety Guide

STEP 12        
STEP 11    Kommentare der SSCs und IAEO-Bewertung IAEO-Zusammenfassung nach STEP 11     
STEP 8 Kommentare der Mitgliedsstaaten
und IAEO-Bewertung
IAEO-Zusammenfassung nach STEP 8
STEP 7 Kommentare der SSCs IAEO-Zusammenfassung nach STEP 7
STEP 4 Kommentare der SSCs und IAEO-Bewertung IAEO-Zusammenfassung nach STEP 4
STEP 3a Kommentare der SSCs und IAEO-Bewertung IAEO Zusammenfassung nach STEP 3a
STEP 3 Kommentare der SSCs und IAEO-Bewertung IAEO Zusammenfassung nach STEP 3

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Background Information

As a result of the changes introduced in the new safety requirements for nuclear power plant design and safety assessment, overarching guidance is needed on the application of some requirements. Guidance is needed in particular in relation to the implementation of some important changes in SSR 2/1, such as the inclusion of the so called “design extension conditions” in the plant design envelope as well as the need to demonstrate that event sequences leading to large radioactive releases or early radioactive releases have to be practically eliminated. Other relevant changes made in SSR 2/1 after the Fukushima Daiichi accident include requirements for strengthening the implementation of the defence in depth concept. Since by their nature these changes of the requirements are cross cutting, recommendations for meeting them are better reflected in a single safety guide.

The scope of this safety guide will cover the assessment of the defence in depth implementation and the practical elimination of event sequences leading to early radioactive releases or large radioactive releases. Special attention will be given to the assessment of design extension conditions and the requirement for independence of safety systems from safety features for design extension conditions (especially features for mitigating the consequences of accidents involving the melting of fuel).

The most relevant requirements for this purpose in GSR Part 4 include the requirements:

• 7: Assessment of safety functions and

• 13: Assessment of defence in depth

together with their corresponding requirements for NPP design in SSR 2/1, rev. 1:

• 4: Fundamental safety functions

• 7: Application of defence in depth

The application of these requirements is closely related to other requirements for general plant design in SSR 2/1, rev. 1, such as:

• 13: Categories of plant states

• 16: Postulated initiating events

• 19: Design basis accidents

• 20: Design extension conditions and

• 21: Physical separation and independence of safety systems

which will be taken into account in the safety guide in as much as they are related to the implementation and assessment of defence in depth and safety functions, with specific focus on the aspects of design extension conditions and practical elimination event sequences that would lead an early radioactive release or a large radioactive release.

It is not the purpose of this safety guide to provide recommendations for the full implementation of these requirements. Further guidance on these requirements can be found in the safety guides on the design of nuclear power plant systems and deterministic safety analysis.

The assessment of safety provisions related to the 5th level of defence in depth, i.e. for emergency preparedness and response, will be outside of the scope of this safety guide.