- •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
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.
FIG. 5. Setpoint terminology and errors to be considered in setpoint determination
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.
7.187. Such identification ought not require frequent reference to drawings, manuals, or other reference material.
7.188. Coherent and easily understood naming and identification of systems and components is important for engineering, maintenance, and construction staff as well as for use to label the controls, displays and indications.
7.189. Components or modules mounted in equipment or assemblies that are clearly identified do not themselves need identification. Configuration management is generally sufficient for maintaining the identification of such components, modules and embedded computer software. Chapter 2 discusses configuration management.
7.190. I&c components in the plant should be marked with their identifying information.
7.191. Safety systems and their components should be uniquely identified and marked to differentiate them from non-safety systems and to differentiate the different redundancy groups from each other.
7.192. Clear identification of components is necessary to reduce the likelihood of inadvertently performing maintenance, tests, repair or calibration on an incorrect channel.
7.193. Identification may, for example, take the form of tagging or colour coding.
8. SYSTEM AND EQUIPMENT SPECIFIC DESIGN GUIDELINES
SENSING DEVICES
8.1. Measurements of plant variables should be consistent with the requirements of the design basis.
8.2. Measurement of plant variables includes both detection of the present value of a variable within a range, and detection of a discrete state such as are detected by limit switches, auxiliary relay contacts, and temperature, pressure, flow or level switches.
8.3. Measurement of plant variables may be made by direct measurement of a plant variable, or indirect measurements such as calculation of the value of a variable based upon multiple measurements, or determination of the value of a variable based upon measurement of other data with a known relationship to the desired variable.
8.4. To the extent practicable, the plant conditions of concern should be monitored by direct measurement rather than being inferred from indirect measurements.
8.5. The sensor for each monitored variable and its range should be selected on the basis of the accuracy, speed of response, and range needed to monitor the variable in normal, accident, and design extension conditions.
8.6. CCF affecting sensing devices should not result in unacceptable consequences for people and environment.
8.7. No identified CCF vulnerability of sensing devices should have the potential of denying operators the information and parameters that they need to control and mitigate accident conditions.
8.8. If more than one sensor is necessary to cover the entire range of the monitored variable, a reasonable amount of overlap from one sensor to another should be provided at each transition point to ensure that saturation or foldover effects do not prevent the required function from being performed.
8.9. If the monitored variables have a spatial dependence (i.e., the measured value of variable depends upon sensor location), the minimum number and locations of sensors should be identified.
THE PROTECTION SYSTEM
8.10. SSR 2/1 Requirement 61 states:
A protection system shall be provided at the nuclear power plant that has the capability to detect unsafe conditions and automatically to initiate safety actions to actuate the safety systems necessary for achieving and maintaining safe plant conditions.
Automatic and manual safety actions
8.11. The protection system should monitor plant variables and detect deviations from their specified limits so that protection system functions can be initiated and performed.
8.12. SSR 2/1 Paragraph 6.33(2) states:
The design (of the protection system) shall … automate various safety actions to actuate safety systems so that operator action is not necessary within a justified period of time from the onset of anticipated operational occurrences or accident conditions.
8.13. Means should be provided to automatically initiate and control all safety actions except those for which manual action alone has been justified.
8.14. Typically automatic initiation will be necessary for most protection system functions.
8.15. Examples of situations in which manual action alone might be justified include:
Initiation of certain safety tasks after completion of automatic sequences;
Control actions to bring the plant to a safe state in the long term after an accident; and
Initiation of safety actions that are not required until a considerable time after the PIE.
8.16. In order to justify that manual action alone is acceptable it should be shown that:
The operator has sufficient and clearly presented information from sensors and equipment of the safety system to make reasoned judgments on the need to initiate the required safety actions;
The operator is provided with written procedures and training for the safety tasks;
The operator is allowed sufficient time to evaluate the status of the plant and to complete the required actions;
For new designs many Member States consider that it is inadvisable to require that operators take any safety action in less than 30 minutes.
The operator is provided with sufficient means of plant control to perform the required actions; and
The communication links between operators carrying out the actions are adequate to ensure the correct performance of these actions.