Keywords

1 Introduction

With the rapid construction of China’s nuclear power project, and the promotion of the strategy of localization of nuclear power equipment as well as the “going out” with the nuclear power, the localization of safety digital control system (DCS), as the nerve center of the NPP, has also made a breakthrough. For example, the FirmSys, China’s first nuclear safety instrumentation and control I&C system that was independently developed by Chinese Guangdong Nuclear Power Group Co. Ltd has been successfully launched and used in nuclear power project. NicSys\( {\textregistered } \)8000N developed by CNNC Control Systems Engineering Ltd has also passed the independent engineering review of I&C systems [1]. As one of the necessary technologies to ensure the safety and reliability of the safety I&C systems in NPP, software V&V can effectively guarantee the software to meet the expected requirements of safety function and performance completely and correctly, that is to ensure that the software does not appear failure situation, and has received great attention in the field of nuclear power [2, 3]. In addition, it requires the I&C system products that implement safety functions in NPPs must conduct software V&V before launch [4].

The software V&V of digital safety I&C system for NPP runs through the whole process of software life cycle. It is used to conform that the activity output at each stage of the software life cycle can meet the requirements, and verify that the system can perform its expected function. The safety and reliability of software play an important role in ensuring system safety and avoiding heavy casualties and property losses so that there is an urgent need of the application of software safety and reliability analysis in the project [5]. While the independence, integrity and effectiveness of V&V activities are critical to ensure the safety and reliability of the safety digital control system software [6].

The software of safety I&C system in NPP is defined as the classification of safety according to the functional requirements [7]. The software V&V mainly refer to the requirements of IEEE 1012-2004, and perform the V&V tasks in accordance with the integrity Level 4 [8]. The activities of software V&V is divided into six major steps, namely: concept V&V, requirement V&V, design V&V, implementation V&V, integration test V&V, installation and checkout V&V. Because the requirement V&V is in the important position from software development outline design to detailed design, and considering the actual problem of huge amount of V&V activity input files, it is necessary to explore the activity of software requirements V&V, especially the V&V tasks included in the activity.

This article is organized as follows: the first part analyzes the content of the software requirements of safety I&C system of the NPP, combining with actual engineering to analyze the identification methods of software requirements, and proposes the software requirements evaluation method in the software requirements V&V; the second part analyzes the main indexes and key contents of the software requirement evaluation for the safety I&C system of NPP, and worked out the implementation steps for software requirements evaluation; the third part is combined with the software V&V activities of engineering practice, taking the practice of a nuclear power software V&V project as an example to illustrate the effectiveness of the software requirements evaluation method. At the same time, the challenges and directions of software requirements evaluation are summarized.

2 Software Requirement Identification of Safety I&C System in NPP

The software of safety I&C system in NPP is defined as the classification of safety according to the functional requirements. The software development process should meet the corresponding safety software requirements, and follow requirements of its product quality assurance system. According to the requirements of HAF 102-2004 and HAD 102/16-2004, for the safety digital I&C system software of NPP, the software V&V must be performed [9, 10].

According to the requirements of GB/T 13629-2008 and R.G. 1.168-2004, the safety I&C system NPP software should perform V&V in accordance with level 4 of integrity specified in IEEE 1012-2004 to check whether the software meets the requirements of relevant laws and regulations, user requirements and the requirements of I&C system of NPP [11, 12].

The software V&V NPP mainly includes six activities, namely: the concept V&V, the requirement V&V, the design V&V, the implementation V&V, the integration test V&V, the installation and checkout V&V.

The software concept V&V involves the design of system architecture and the analysis of system requirements, and puts forward the concrete solution to solve the users’ problems, and the system architecture assigns the corresponding system requirements for hardware, software, and users’ interface components. The software concept V&V activity verifies the correctness, accuracy and integrity of system requirements allocation, and make sure that the wrong assumptions are not entered into the program. The output of the software concept V&V activity, that is the system design transformed from system requirements assigned to the software, will be used as the input of software requirements V&V activity.

The software requirement V&V is to analyze the function and performance, the external interface of software, identification requirements, safety and security requirements, human engineering requirements, data definition requirements, the user documentation requirements of software, installation and acceptance requirements, user operations and implementation requirements, and user maintenance requirements, which is involved in the software of safety I&C system of NPP in order to ensure the correctness, integrity, accuracy, testability and consistency of software requirements.

The safety I&C system of NPP is a complex system engineering. For considerations of safety and reliability, the design and development of safety I&C system software in NPP requires to perform complex functional operations with simple program logic. Therefore, it is necessary to identify the software requirements before transforming the software requirements into detailed design and the subsequent code and the database structure. The relevant implementation standards, such as IEEE 1012-2004, only explicit that the software development should select the integrity level and complete the corresponding task according to function performed by the software, but there is no specific technical solution for the effective identification of software requirements. Through the engineering practice of CPR1000 nuclear power project, in order to identify software requirements effectively, it can be encircling the traceability analysis to make the inputs of software requirement V&V activity like requirements documents items and form the process records of software requirement analysis, providing the basis for subsequent analysis and evaluation of V&V tasks (Table 1).

Table 1. Software requirements items
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Before the software requirements V&V starts the first is to clear the inputs of the activity, which is including the criterion documents and object documents. The criterion documents are the upstream input for software requirements V&V, that is the output of system concept design document of the concept V&V. While the object documents is the output software requirement analysis document of the requirements V&V. Through the item of requirement item for each input document of the software requirement V&V to perform the requirements of tasks. In the process of implementation of the V&V activity, if there is an anomaly in the input document, the opinion is “to be discussed”, corresponding to record the anomaly and organize the list, anomaly submitting them to the design and development parties for correction.

Through items of the requirement of input document of the software requirement V&V, it can effectively meet the requirements of V&V tasks, which is involving in huge amount of documents, drawings. And it also can identify the software requirements and discover the anomaly, ensure the safety and reliability of safety I&C system in NPP.

3 Software Requirement Evaluation of Safety I&C System in NPP

The software of safety I&C system in NPP is defined as the classification of safety according to the functional requirements NPP. The software requirement V&V need to be performed according to the integrity Level 4 in IEEE 1012-2004. The minimum task V&V involved in requirements V&V mainly includes: traceability analysis, software requirements evaluation, interface analysis, critical analysis, hazard analysis, risk analysis, safety analysis, configuration management evaluation, the generation of system test plan of verification and validation and acceptance test plan of verification and validation. In the software requirements identification of previous chapter, according to traceability analysis and interface analysis, document review and traceability analysis should be carried out firstly for requirement items. The result of identification and analysis of software requirements items can serve as a reference for software requirements evaluation and to evaluate the requirement document of software design and development phase combining with the index of software requirement evaluation.

The software requirement V&V undertakes the system design of the system requirements for the outline design software, and carries on detailed design and program implementation so that software requirements can be correctly identified. After the effective identification of software requirements, the identified software requirements and their analysis results should be evaluated. The purpose of the evaluation is assurance the correctness, consistency, completeness, accuracy, readability and testability of the software requirements. The corresponding evaluation indexes and their evaluation contents are shown in Table 2.

Table 2. Software requirements evaluation indexes and contents
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Evaluate the target content through the software requirements listed above, evaluate the requirements items that have been identified by entry division of software requirements. In the V&V project of nuclear power engineering software, refer to the exception grading principle of GJB2786A-2009, the identified abnormal items are divided into 5 grades according to the possible severity of the system failure consequences, namely “fatal”, “critical”, “important”, “general” and “suggestion” [13]. The requirement V&V can identify the existing abnormal items and risk items through the process of identification, analysis and grading evaluation, the evaluation results can also provide reference for other V&V tasks, such as hazard risk analysis.

4 The Project Practice of Software V&V

Take the software V&V project of safety of instrument control system prototype of CPR1000 autonomous NPP as an example, using the software requirements identification and evaluation method proposed in this paper to evaluate the requirement of software in the requirement document so as to illustrate the effectiveness of the method. In document review and traceability analysis of requirements V&V, form 585 requirements check entries through the entry division of requirement document, 10 typical exception items are found, 1 of these is the “critical” issues in the software development process, the other 9 are the “important” issues that have not been reflected in the downstream software requirements document that from the upstream requirements of software in system design. Abnormal problems are summarized in Table 3.

Table 3. Software requirements items evaluation
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What needs to be explained is that, the implementation process and evaluation results described above are not only applicable to the requirements V&V in the software development life cycle, the other five verification and validation mentioned above are also apply. In this way, the whole process of software V&V development from requirement analysis, outline design, detailed design and transformation implementation to final integration test and installation test is formed. The entry analysis of software requirements is beneficial to keep the correctness, consistency, completeness and accuracy in the whole life cycle of software. If software development can be considered from the began of whole life cycle, and achieve the best balance between development cost and the precision of software requirements entry, it will have a significant effect on the improvement of the quality of software.

5 Conclusion

Software requirement evaluation is one of the important V&V tasks of the requirement V&V in the whole life cycle of software development. Because it takes part the system architecture and requirements allocation for software outline design, and carry the detailed design and implementation of the software, in addition, the requirements V&V is in the field of the V&V project of nuclear power software. Therefore, the amount of input files involved has great practical challenges. How to evaluate software requirements efficiently is the key technology problem in the requirement V&V.

In this paper, by referring to the relevant requirements of the implementation standard of safety critical software of IEEE 1012-2004, combined with the engineering practice of V&V project of nuclear power software, summarize and refine the effective method of software requirement evaluation – the entry of software requirement. In the process of requirements V&V, according to the index content of software requirement evaluation, make grading evaluation of the requirement items in input documents, so as to get the design quality of software development from system design to the requirement analysis of software.

The software requirements evaluation method proposed in this paper can be applied to document requirement evaluation of all software V&V during the whole life cycle of software development, it can solve the problem of unclear or messy design of the documents requirement in the process of software development, which is beneficial to the abnormal traceability of the design and development, operation and maintenance of the safety of I&C system in NPPs, it also provides technical reference for the entry of software requirements and the traceability mapping of full life cycle in automation tools.