- •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
Digital computer systems and digital equipment
8.59. SSR 2/1 requirement 63 states:
If a system important to safety at the nuclear power plant is dependent upon computer based equipment, appropriate standards and practices for the development and testing of computer hardware and software shall be established and implemented throughout the lifetime of the system, and in particular the software development cycle. The entire development shall be subject to an appropriate quality management system.
Digital computer system functions
8.60. The use of computer-based systems for NPP I&C functions provides advantages that include functional flexibility to provide complex functions, improved plant monitoring and operator interfaces, low physical size and low cabling needs. They can have test and self-check functions that improve reliability.
8.61. Chapters 2 and 6 give guidance on quality management systems for all I&C systems including computer based systems. Chapter 10 gives guidance on software development for I&C systems.
8.62. Computer-based systems should behave deterministically with regard to functions and timing.
8.63. The design of computer systems should ensure that the system will meet the system’s design basis response time and accuracy requirements in all specified operating conditions.
8.64. Response time and accuracy of computer-based systems are heavily influence by sample rate and processor speed.
8.65. The full scope of individual component failures (e.g., computer processors) should be addressed in the design and analysis of computer-based systems.
8.66. Digital computer systems may use one set of equipment to perform numerous system functions. A disadvantage of this is that if one component goes out of service, several functions might fail simultaneously.
8.67. Failure of data communication channels or computers could prevent all data communication or all computer control. It is therefore necessary for the operators to be able to shut down the plant, and to be certain it has shut down, if this is a credible failure in the design basis.
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.
8.69. Loss of power or restart of a computer-based system should not result in modification of configuration data.
Fault detection and self-supervision
8.70. Computer based systems should detect and report their faults and failures in a timely manner.
8.71. Computer based systems should be provided with an alarm covering major redundancy failures.
8.72. When a fault in a system or equipment is detected by self-supervision, appropriate action should be taken.
8.73. Actions taken upon detection of faults by self-supervision should be identified in the system or equipment functional specification, and should be subject to the design requirements and verification appropriate to the importance to safety.
8.74. Fault detection and self-supervision features should not adversely affect the ability of a computer system to perform its safety function, and should not cause spurious actuations of the safety function.
Data communication
8.75. Data communications systems should behave deterministically with regard to functions and timing.
8.76. Data communication channels should satisfy the recommendations for independence given in paragraphs 7.21-7.47.
8.77. The design of the data communications should provide for detection and rejection or correction of invalid, inauthentic, and corrupted data.
8.78. If communications systems encrypt data or use proprietary protocols, these features must not prevent detection of errors.
8.79. The design should ensure that errors and failures of transmission and of the data communication equipment are detected and that suitable alarms are provided to the operators and records made for analysis of performance.
8.80. It is necessary to consider both operational and security implications of alarms and failures when determining what is to be alarmed and recorded. It might not be necessary, or practical to alarm and record every error or failure. Errors that are consistent with the expected normal behaviour of the communications technology or protocol need to be recorded, but need not be alarmed.
8.81. Checking of data communication should be done as a continuous process on each message sent and received by means of an automatic self-check function.
8.82. Features for the detection and correction of errors can be used to improve the reliability of signal transmission.
8.83. Self-checking methods should be appropriate for the use of the data and the frequency of demand for the safety functions being performed by the system.
8.84. Data communications technology should be chosen and suitably configured to ensure that it is capable of meeting, under all possible conditions of data loading, the design basis reliability and time response requirements.
8.85. The selection and use of more complex technology might offer functional advantages but might also introduce additional failure modes and validation difficulties.
8.86. Where data communication is used to execute operator actions, the design should ensure that the longest time from operating the control to receipt of confirmation of its action is acceptable to the operators.
8.87. Data links should be redundant to the extent necessary for systems supported by the data links to meet their design basis reliability requirements.
Communications features in safety systems
8.88. In systems performing safety functions, the processors that perform safety calculations or logic should be distinct and separate from those that perform communication handshaking or interrupt functions.
8.89. Often this recommendation is accomplished by using two processors that share data via carefully controlled access to shared memory. One processor is dedicated to performing the safety function and the other is dedicated to data communications tasks.
8.90. Separation of calculation and logic functions from communications and interrupt functions prevent errors in these later functions from disrupting the deterministic processing of safety calculations or logic functions. This separation, sometimes called buffering, ensures that faults and failures on the communication originating outside the safety division do not propagate to the processors that implement safety functions within the division, thus maintaining the integrity of the safety functions.