- •Iaea safety standards
- •Objective
- •Structure
- •Relationship to other standards
- •2. Management systems for I&c design
- •2.8. The management systems for development of I&c systems should comply with the recommendations of Safety Guides gs-g-3.1, Ref. [5] and gs-g-3.5, Ref. [6].
- •Generic management system processes
- •Configuration management
- •2.15. All I&c configuration items and their associated configuration documents should be designated, given a unique identification, and placed under configuration control.
- •2.31. Insights gained from probabilistic safety assessments (psAs) should be considered in the design of I&c systems.
- •Documentation
- •2.33. Before I&c systems are declared operable their documentation should be complete and should reflect the as-built configuration.
- •2.34. I&c documentation should:
- •2.36. I&c documents should be grouped according to their primary or secondary role in the design process.
- •2.38. Documentation for I&c systems and components should, as a minimum, cover the following topics:
- •3. Design bases
- •Inputs to I&c design bases
- •Identification of I&c functions
- •3.4. The required functions of the I&c systems should be determined as part of the nuclear power plant design process.
- •Content of I&c design bases
- •3.7. The overall I&c system architecture and each I&c system should have a design basis.
- •3.9. The I&c systems required for the safety of the plant should be identified systematically.
- •3.10. I&c system design bases should specify the following:
- •3.12. In addition to the above the design basis for the reactor protection system should specify the following:
- •Variables and states that must be displayed so that the operators can confirm the operation of protective system functions;
- •4. Guidance for overall I&c system architecture architectural design
- •4.3. The overall I&c architecture should:
- •4.4. The inputs to the overall I&c architecture design process should refer to the plant safety design basis documents, which should provide the following information:
- •Defence in depth
- •4.28. When diverse I&c systems are provided to meet requirements for defence-in-depth, the diverse systems should not both be subject to the same errors in design or fabrication.
- •5. Safety classification of I&c functions, systems, and equipment
- •6. Life cycle activities
- •Process implementation
- •Verification that the effects of automatic control system failures will not exceed the acceptance criteria established for anticipated operational occurrences.
- •6.58. The I&c architecture should be designed to fully satisfy the system requirements, including system interfaces and non-functional requirements (e.G., performance and reliability).
- •6.109. The benefits of changes should be weighed against potential negative safety consequences and this assessment documented as part of the justification for the changes.
- •Design for reliability
- •Single failure criterion
- •7.10. Each safety group should perform all actions required to respond to a pie in the presence of:
- •7.15. Non-compliance with the single failure criterion should be exceptional and clearly justified in the safety analysis.
- •7.19. I&c systems should be redundant to the degree needed to meet design basis reliability requirements.
- •Independence
- •7.27. When isolation devices are used between systems of different safety importance, they should be a part of the system of higher importance.
- •7.29. The adequacy of design features provided to meet the independence recommendations should be justified. Physical separation
- •7.31. Items that are part of safety systems should be physically separated from items of lower safety classification.
- •7.32. Redundant items of safety systems should be physically separated from each other.
- •Electrical isolation
- •Diversity
- •7.49. The decision to use diversity or not use diversity should be justified.
- •7.50. Where diversity is provided to cope with ccf several types of diversity should be used.
- •7.51. Where diversity is provided the choice of the types of diversity used should be justified.
- •Failure modes
- •7.57. The failure modes of I&c components should be known and documented.
- •7.60. Failures of I&c components should be detectable by periodic testing or self-revealed by alarm or anomalous indication.
- •7.73. Analysis that is part of the evidence of equipment qualification should include a justification of the methods, theories and assumptions used.
- •7.75. Traceability should be established between each installed system and component important to safety and the applicable evidence of qualification.
- •Suitability and correctness
- •7.81. The equipment qualification program should demonstrate that the as-built I&c systems and installed components correctly implement the qualified design.
- •7.90. Environmental qualification of safety components that must operate in harsh environments should include type testing.
- •7.102. Detailed emc requirements should be determined for safety systems and components and their compliance with the requirements demonstrated.
- •7.105. Equipment and systems, including associated cables, should be designed and installed to withstand the electromagnetic environment in which they are located.
- •7.109. Limits on radiated and conducted electromagnetic emissions should be established for all plant equipment.
- •7.112. The equipment qualification program should show that electromagnetic emissions of plant equipment are within the defined limits.
- •7.114. Instrumentation cables should have twisting and shielding sufficient to minimize interference from electromagnetic and electrostatic interference.
- •Design to cope with ageing
- •7.119. Ageing mechanisms that could significantly affect I&c components and means for following the effects of these mechanisms should be identified during design.
- •7.122. Maintenance programs should include activities to identify any trend towards degradation (ageing) that could result in the loss of operability of equipment.
- •Control of access to systems important to safety
- •7.130. Access to equipment in I&c systems should be limited to prevent unauthorized access and to reduce the possibility of error.
- •Testing and testability during operation
- •Test provisions
- •7.150. Arrangements for testing should neither compromise the independence of safety systems nor introduce the potential for common cause failures.
- •Test interfaces
- •7.153. Provisions for testing I&c systems and components should:
- •7.154. Where equipment to be tested is located in hazardous areas, facilities should be provided to allow testing from outside the hazardous area.
- •7.164. The test program should define processes for periodic tests and calibration of systems that:
- •Individually test each sensor, to the extent practicable.
- •7.165. In addition to the recommendations of paragraph 7.164, the processes defined for periodic tests and calibration of safety systems should:
- •Independently confirm the functional and performance requirements of each channel of sense, command, execute, and support functions;
- •Include as much of the function under test as practical (including sensors and actuators) without jeopardizing continued normal plant operation;
- •Maintainability
- •7.169. The design of I&c systems should include maintenance plans for all systems and components.
- •Setpoints
- •7.185. Trip setpoints used to initiate safety actions should be selected to ensure that required mitigating actions occur before the monitored variable reaches the analytical limit.
- •Operational identification of items important to safety
- •7.186. A consistent and coherent method of naming and identifying all I&c components should be determined and followed throughout the design, installation and, operation phases of the plant.
- •7.190. I&c components in the plant should be marked with their identifying information.
- •8.4. To the extent practicable, the plant conditions of concern should be monitored by direct measurement rather than being inferred from indirect measurements.
- •8.17. Means should also be provided to manually initiate the mechanical safety systems and the individual components necessary to initiate and control performance of their safety functions.
- •Digital computer systems and digital equipment
- •8.68. Specific skilled staff should be available during operation to allow controlled software and configuration data changes to be made when necessary to computer based systems.
- •8.91. Data received and data transmitted should be stored in separate, pre-determined memory locations.
- •8.154. Tools should be used to support all aspects of the I&c life cycle where benefits result through their use and where tools are available.
- •8.173. Confirmation of the suitability and correctness of industrial digital devices for their intended functions should produce evidence:
- •V&V at each stage of development for the final product;
- •9.4. The I&c system should allow the operator in the control room to initiate or take manual control of each function necessary to control the plant and maintain safety.
- •9.21. Instrumentation performing the functions given in 9.20 items a, b, and c should be classified as safety systems.
- •9.32. The main control room, the supplementary control room, and the Emergency Control Centre should have at least two diverse communications links with:
- •9.42. The Human System Interface (hmi) design should retain positive features and avoid hfe issues and problems of previous designs.
- •9.57. Where hmi stations are distributed, plant staff should have means to access these different locations in a safe and timely manner.
- •10.4. Development of software for systems should follow a previously defined life cycle, be duly documented and include thorough verification and validation. (See Chapter 6.)
- •10.49. Coding rules should be prescribed and adherence verified.
- •10.72. Verification should include the following techniques:
- •Software tools
- •Glossary
- •Annex I defense in depth in I&c systems
- •Annex II traceability to previouse I&c safety guides
- •Annex III bibliography of supporting international standards
4.4. The inputs to the overall I&c architecture design process should refer to the plant safety design basis documents, which should provide the following information:
The defence in-depth concepts of the plant,
The groups of functions to be provided to address Postulated Initiating Event (PIE) sequences,
The safety categorization, and the functional and performance requirements of the plant functions important to safety,
The role of automation and prescribed operator actions in the management of anticipated operational occurrences and accident conditions,
The assignment of functions to operators and to automatic means,
The information to be provided to the operators,
The priority principles between automatically and manually initiated actions;
Member State requirements for I&C licensing, e.g. security, software qualification; and
Member State requirements with respect to operational requirements (i.e., The I&C design as it affects the interface with plant operators) for systems important to safety.
4.5. The independent effectiveness of each of the different I&C levels is achieved by incorporating design features such as redundancy, independence and diversity to avoid the failure of one level causing the failure of subsequent levels. Chapter 7 provides recommendations for implementing such design features.
Defence in depth
4.6. SSR 2/1 Requirement 7 states:
The design shall incorporate defence in depth. The levels of defence shall be independent as far as is practicable.
4.7. INSAG-10 [28] and INSAG-12 [29] further amplify the explanation of SSR 2/1.
4.8. The overall I&C architecture should implement a defence in depth concept.
4.9. The overall I&C architecture should not compromise the Defence in Depth strategy of the plant design.
4.10. For I&C, Defence in depth consists of implementing successive I&C functions designed to limit the consequences of a design basis event to an acceptable level despite the failure of I&C functions designed to respond first.
INDEPENDENCE
4.11. Independence is intended to prevent the propagation of failures from the item affected by the failure to other redundancies, or from system to system.
4.12. The overall I&C architecture should neither compromise the independence of the safety divisions, nor the independence implemented at the different levels of defence in depth.
4.13. Safety systems should be independent from systems of lower safety classification as necessary to ensure that the safety systems can perform their safety functions during and following any design basis event that requires these functions.
4.14. Safety items should be independent of the effects of the design basis accidents to which they must respond.
4.15. The failure of the support features of safety systems should not compromise the independence between redundant portions of safety systems or between safety systems and systems of lower safety classification.
CONSIDERATION OF COMMON CAUSE FAILURE
4.16. SSR 2/1 Requirement 24 states:
The design of equipment shall take due account of the potential for common cause failures of items important to safety to determine how the concepts of diversity, redundancy, physical separation and functional independence have to be applied to achieve the necessary reliability.
4.17. A common cause failure (CCF) is defined as the concurrent failure of two or more structures, systems or components due to a single event or cause.
4.18. Common cause failure might happen, for example, because of human errors, errors in the manufacturing process, inadequate specification, qualification for, or protection against internal or external hazards, high voltages, data errors, data communication errors, or failure propagation between systems or components.
4.19. The consequences of a PIE in combination with a CCF that prevents necessary reactor protection system response to the PIE should be no greater than those accepted for design extension conditions.1
4.20. The accident sequences and consequences resulting from the combination of a PIE and CCF of the reactor protection system may be analysed using best estimate methods.
4.21. Often it is necessary to provide a Diverse Actuation System (DAS) to limit the consequences of the PIE in conjunction with CCF in one or more protection system functions.
4.22. Latent failures and common failure modes which potentially might result in a common failure of the redundancies should be identified, and justification should be provided for any that need not be considered as credible sources of CCF between systems or individual components.
4.23. Justification that a CCF need not be considered may, for example, be based on the component dependability, technology, or feedback gained over its wide usage.
4.24. A complete elimination all vulnerabilities of I&C systems and architecture to CCF is not required, but justification should be provided for accepting identified vulnerabilities that have are not addressed.
Diversity
4.25. Diversity is a way to reduce CCF vulnerability resulting from design, manufacture or maintenance error, and to include conservatism to compensate for the difficulty of demonstrating the necessary level of reliability, or for errors in requirement specifications.
4.26. In any application, it is important to ensure that diversity is in fact achieved in the implemented design and preserved throughout the life of the plant.
4.27. It is necessary to consider both the scope and the type of the diversity provided. The necessary level of conservatism may be achieved by providing diversity to protect against the more frequent PIE, without extending full diversity to cover very unlikely PIEs or low consequence PIEs, since the risk of such events might be acceptable despite the possibility of common cause failure.