- Email: [email protected]
Classification of Plant Components
CHAPTER
CLASSIFICATION OF PLANT COMPONENTS
13
A general agreement exists that classification of systems, structures, and components of a plant from the point of view of safety and from the point of view of resistance to external actions (earthquake, and so on) is necessary to make decisions on the following (IAEA, SS-G2, 2010): • • • •
•
Adequate design, construction, and operation provisions for each class. System characteristics, such as redundancy, emergency power supply, qualification for environmental conditions. Systems to be considered available or not in the deterministic analysis of the postulated initiating events (EUR, Chapter 18: Nuclear Safety Criteria). Gradation of the QA measures, to be proportioned to the importance of the safety component but also to the characteristics of the component such as its complexity and degree of technological innovation. In general, the following classifications should be defined: • Classification on the basis of the safety function, with reference to the requirements above. • Classification for pressure components, on the basis of the mechanical complexity and the pressure level. • Classification for the resistance to earthquakes, with reference to the need that the components continue to be undamaged or functional during and after an earthquake of a certain severity, taking into account the aftershocks and therefore the possible incremental damage. • Classification of the instrumentation and control systems, on the basis of their safety function, which may be different from that of other system types because of the existence of classification schemes specific to their field and commonly used. • Classification for QA requirements.
The various national approaches to the classification systems strongly differ from each other and in every practical case the choice of the classification criteria and the assignment of the various components to the classes identified need a certain degree of reflection and judgment. The subject of classification is made very delicate by the fact that the pertinent choices have a strong economic relevance. Moreover, it is not always possible to correlate the different classes with levels of reliability or unavailability upon demand in the probabilistic safety evaluations, because of the lack of sufficient experimental data. The probabilistic safety analyses should confirm that the structures, systems, and components, which ensure that the risk connected to the plant is low, be classified at the appropriate level. It is necessary to stress the adequacy of the isolation and separation systems adopted for different systems having a possibility of mutual interaction and assigned to different classes. The Nuclear Safety. DOI: https://doi.org/10.1016/B978-0-12-818326-7.00013-5 © 2020 Elsevier Ltd. All rights reserved.
149
150
CHAPTER 13 CLASSIFICATION OF PLANT COMPONENTS
malfunction of a system or component should not cause the malfunction of another system or component assigned to a superior class. If this possibility exists, the affected system or component should also be classified in the superior class. Some examples of adopted classification systems may be found in IAEA (2014), the IAEA Guide on classification (IAEA, 2010) in EUR (Chapter 18: Nuclear Safety Criteria), the EUR criteria, an extract of which, also including the classification system, is included as Appendix 6. Some examples of system classifications are given in the following paragraphs, as an illustration. These examples comply with the above-listed principles and may be found in IAEA (2014) and EUR (Appendix 6). •
• • •
The system of vessels, pipes, and pressure components which form the primary cooling system of a PWR (reactor vessel obviously included) is in Class 1, the highest one, as the failure (break) of the system constitutes a serious LOCA. The core emergency cooling system is placed in Class 2, as its failure does not cause directly and necessarily an accident. The compressed air system which supports the emergency cooling systems is in Class 3 as it is considered a normal, not highly stressed system. The station fire fighting system is not placed in a safety class (or it is in Class 4) as it is considered that the specific industrial standards in force already offer sufficient guarantee by themselves if needed.
These examples make clear the degree of subjectivity in the classification choices and therefore the importance of giving classification adequate attention.
REFERENCES IAEA, 2010. SS-G2 Deterministic Safety Analysis for Nuclear Power Plants. IAEA, 2014, Safety Classification of Structures, Systems and Components in Nuclear Power Plants Specific Safety Guide. IAEA Safety Standards Series No. SSG-30.
CLASSIFICATION OF PLANT COMPONENTS
13
A general agreement exists that classification of systems, structures, and components of a plant from the point of view of safety and from the point of view of resistance to external actions (earthquake, and so on) is necessary to make decisions on the following (IAEA, SS-G2, 2010): • • • •
•
Adequate design, construction, and operation provisions for each class. System characteristics, such as redundancy, emergency power supply, qualification for environmental conditions. Systems to be considered available or not in the deterministic analysis of the postulated initiating events (EUR, Chapter 18: Nuclear Safety Criteria). Gradation of the QA measures, to be proportioned to the importance of the safety component but also to the characteristics of the component such as its complexity and degree of technological innovation. In general, the following classifications should be defined: • Classification on the basis of the safety function, with reference to the requirements above. • Classification for pressure components, on the basis of the mechanical complexity and the pressure level. • Classification for the resistance to earthquakes, with reference to the need that the components continue to be undamaged or functional during and after an earthquake of a certain severity, taking into account the aftershocks and therefore the possible incremental damage. • Classification of the instrumentation and control systems, on the basis of their safety function, which may be different from that of other system types because of the existence of classification schemes specific to their field and commonly used. • Classification for QA requirements.
The various national approaches to the classification systems strongly differ from each other and in every practical case the choice of the classification criteria and the assignment of the various components to the classes identified need a certain degree of reflection and judgment. The subject of classification is made very delicate by the fact that the pertinent choices have a strong economic relevance. Moreover, it is not always possible to correlate the different classes with levels of reliability or unavailability upon demand in the probabilistic safety evaluations, because of the lack of sufficient experimental data. The probabilistic safety analyses should confirm that the structures, systems, and components, which ensure that the risk connected to the plant is low, be classified at the appropriate level. It is necessary to stress the adequacy of the isolation and separation systems adopted for different systems having a possibility of mutual interaction and assigned to different classes. The Nuclear Safety. DOI: https://doi.org/10.1016/B978-0-12-818326-7.00013-5 © 2020 Elsevier Ltd. All rights reserved.
149
150
CHAPTER 13 CLASSIFICATION OF PLANT COMPONENTS
malfunction of a system or component should not cause the malfunction of another system or component assigned to a superior class. If this possibility exists, the affected system or component should also be classified in the superior class. Some examples of adopted classification systems may be found in IAEA (2014), the IAEA Guide on classification (IAEA, 2010) in EUR (Chapter 18: Nuclear Safety Criteria), the EUR criteria, an extract of which, also including the classification system, is included as Appendix 6. Some examples of system classifications are given in the following paragraphs, as an illustration. These examples comply with the above-listed principles and may be found in IAEA (2014) and EUR (Appendix 6). •
• • •
The system of vessels, pipes, and pressure components which form the primary cooling system of a PWR (reactor vessel obviously included) is in Class 1, the highest one, as the failure (break) of the system constitutes a serious LOCA. The core emergency cooling system is placed in Class 2, as its failure does not cause directly and necessarily an accident. The compressed air system which supports the emergency cooling systems is in Class 3 as it is considered a normal, not highly stressed system. The station fire fighting system is not placed in a safety class (or it is in Class 4) as it is considered that the specific industrial standards in force already offer sufficient guarantee by themselves if needed.
These examples make clear the degree of subjectivity in the classification choices and therefore the importance of giving classification adequate attention.
REFERENCES IAEA, 2010. SS-G2 Deterministic Safety Analysis for Nuclear Power Plants. IAEA, 2014, Safety Classification of Structures, Systems and Components in Nuclear Power Plants Specific Safety Guide. IAEA Safety Standards Series No. SSG-30.