Software for Computer Based Systems Important to Safety in Nuclear Power Plants
Issued by:
Issued on:
2004-12-08
Implemented on:
2005-01-01
Status:
VALID
Target Language:
English
File Format:
PDF
Word Count:
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1.1 General
1.1.1 Computer based systems are of increasing importance to safety in nuclear power plants as their use in plants is rapidly increasing. They are used both in safety related systems, such as some functions of the process control and monitoring systems, as well as in systems important to safety, such as reactor protection or actuation of safety facilities. The dependability of computer based systems important to safety is therefore of prime interest and shall be ensured.
1.1.2 With current technology, it is possible in principle to develop computer based instrumentation and control systems for systems important to safety that have the potential for improving the level of safety and reliability with sufficient dependability. However, their dependability can be predicted and demonstrated only if a systematic, fully documented and reviewable engineering process is followed. The development of software for computer based systems important to safety in nuclear power plants shall comply with the issued regulations, guides and standards dealing with software engineering and quality assurance.
1.2 Objective
The objective of the Safety Guide is to provide guidance on the collection of evidence and preparation of documentation to be used in the safety demonstration for the software for computer based systems important to safety in nuclear power plants, for all phases of the system life cycle.
1.3 Scope
1.3.1 The Safety Guide is applicable to systems important to safety as defined in relevant nuclear safety regulations. Since at present the reliability of a computer based system cannot be predicted on the sole basis of, or built in by, the design process, it is not allowed to agree systematically on any possible relaxation in the Safety Guide to apply to software for safety related systems. Whenever possible, software which applies only to safety systems and not to safety related systems are explicitly identified.
1.3.2 The Safety Guide relates primarily to the software used in computer based systems important to safety. Guidance on the other aspects of computer based systems, such as those concerned with the design and operation of the computer based system itself and its hardware, is limited to the issues raised by the development, verification and validation of software, and are beyond the scope of the Safety Guide.
1.3.3 The main focus of the Safety Guide is on the preparation of documentation that is used for an adequate demonstration of the safety and reliability of computer based systems important to safety.
1.3.4 The Safety Guide applies to all types of software: pre-existing software or firmware (such as an operating system), software to be specifically developed for the project, or software to be developed from a pre-existing equipment family of hardware or software modules. The issue of the use for safety functions of pre-existing or existing commercial software, on the development of which little information is available, is addressed in Annex A.
1.3.5 The Safety Guide is intended for use by those involved in the production, assessment and licensing of computer based systems, including plant system designers, software designers and programmers, verifiers, validators, certifiers and regulators, as well as plant operators.
2 Technical Considerations for Computer Based Systems
2.1 Characteristics of computer based systems
2.1.1 In relation to the assessment of safety and reliability, computer based systems have two basic properties. They are programmable and their hardware is based on discrete digital logic. As with other systems, hardware faults may be due to errors in design or manufacturing, but typically they result from wearing out, degradation or environmental processes, and are of a random nature. Software, the programmable part of the computer, does not wear out, but it may be affected by changes in the operating environment. Software faults may result from either bad or unclear specification of requirements (which gives rise to errors in the logical design or implementation) or errors introduced during the implementation phase or maintenance phase.
