NUREG/IA-0463

 International

 Agreement Report

(Availability of) An International Report on

Safety Critical Software for Nuclear Reactors by 

the Regulator Task Force on Safety Critical 

Software (TF-SCS)

Prepared by:

Sushil Birla, Russell Sydnor

Norbert Carte, NRC Project Manager

Division of Engineering

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript completed: August 2014

Date Published: December 2015

Prepared as part of the technical exchange and cooperation between the U.S. Nuclear Regulatory Commission and the Task Force for 

Safety Critical Software in the Field of Nuclear Safety Research

Published by

U.S. Nuclear Regulatory Commission
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NUREG/IA-0463

 International

 Agreement Report

(Availability of) An International Report on

Safety Critical Software for Nuclear Reactors by 

the Regulator Task Force on Safety Critical 

Software (TF-SCS)

Prepared by:

Sushil Birla, Russell Sydnor

Norbert Carte, NRC Project Manager

Division of Engineering

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

Manuscript completed: August 2014

Date Published: December 2015

Prepared as part of the technical exchange and cooperation between the U.S. Nuclear Regulatory Commission and the Task Force for 

Safety Critical Software in the Field of Nuclear Safety Research

Published by

U.S. Nuclear Regulatory Commission

iii

ABSTRACT

The Regulator Task Force on Safety Critical Software (TF SCS) for nuclear reactors, which is 

comprised of seven international nuclear regulators and authorized technical-support 

organizations, and the U.S. Nuclear Regulatory Commission (NRC) have exchanged 

information seeking consistency in the technical basis for licensing reviews of software in 

safety-related digital instrumentation and control systems for nuclear power plants. The TF SCS 

has documented its consensus technical basis in the enclosed technical report and invited the 

NRC’s review and comments. Although the NRC is not a member of the TF SCS, the NRC staff 

have participated in meetings and provided comments on the TF SCS report. As a result, the

NRC collaborated with the TF SCS by reviewing and providing comments to improve the 

technical report documenting the TF SCS’s common positions. This NUREG includes the TF 

SCS report titled “Licensing of Safety Critical Software for Nuclear Reactors - Common Position 

of International Nuclear Regulators and Authorised Technical Support Organisations.” The NRC 

added an Appendix after the TF SCS report to assist NRC staff in using the information as a 

technical reference in any future changes to NRC’s regulatory guidance framework.

v

FOREWORD

This NUREG/IA report is a result of technical exchange between the Regulator Task Force on 

Safety Critical Software (TF SCS) for nuclear reactors and the U.S. Nuclear Regulatory 

Commission (NRC) seeking consistency in the technical basis for the licensing review of 

software in safety-related1 digital instrumentation and control systems for nuclear power plants. 

The TF SCS has developed common positions (CPs) on the technical basis for the licensing

reviews of safety-critical2 software for nuclear reactors. At the invitation of the TF SCS, the NRC 

reviewed the TF SCS’s technical report, provided comments for improvements and studied how 

the information could benefit the NRC. Publication will make this technical reference more 

readily accessible to NRC staff and stakeholders in the USA. 

The TF SCS is a group of experts from regulatory and safety authorities in Belgium, Canada, 

Finland, Germany, Spain, Sweden, and the United Kingdom as well as their technical-support 

organizations. The TF SCS technical report has evolved over the period of almost two decades.

Experts from these countries have developed these CPs based on their regulatory review 

experience. The TF SCS has also drawn on the results of relevant research activities in

European Union (EU) projects and has drawn from other literature cited in its list of references 

and an extensive bibliography. 

The major result of the work is the identification of common issues and a consistent technical

basis to address these issues in future reviews. Those items in which the TF SCS members 

have complete consensus are identified as CPs; those on which the TF SCS did not reach 

complete consensus are identified as recommended practices (RPs). These technical positions 

are targeted to address issues experienced in licensing reviews by TF SCS members. As such, 

these positions are intended to complement existing standards and guides rather than serve as 

“standalone, self-sufficient” guidance. TF SCS members use their report as a common 

authoritative reference, drawing on it as needed.

This NUREG/IA report includes (as is, without further editing by the NRC) the 2014 edition of 

the TF SCS’s report titled “Licensing of Safety Critical Software for Nuclear Reactors - Common 

Position of International Nuclear Regulators and Authorised Technical Support Organisations.” 

The scope of NRC’s review was limited to CPs (i.e., the third section of every chapter)

applicable to safety assessment of software for systems of the “highest grade (class) of safety”3

following deterministic approaches. Thus, the NRC review excluded Chapters 1.2, 1.8, 1.11, 

and 1.13; CPs elsewhere that refer to or depend on these chapters; and, in every chapter, the 

sections that concern Rationale, Issues, and Recommended Practices. The NRC added 

Appendix A to the NUREG/IA to inform NRC staff of these limitations. Where the TF SCS 

vocabulary differs from that used by the NRC, to avoid the potential for confusion, the 

 1 As defined by the NRC. 2 As this term is used by the TF SCS. 3 That is, systems that the NRC defines as “safety-related.”
vi

NRC-appended information provides a differentiation of how such vocabulary terms used by the 

TF SCS correspond to those used in the NRC’s commentaries.

The NRC is publishing this NUREG/IA because it considers the content a valuable technical 

reference that can be used for future improvements in NRC’s regulatory guidance and to inform 

current licensing reviews.
vii

CONTENTS

Page

ABSTRACT................................................................................................................................iii

FOREWORD ..............................................................................................................................v

ACKNOWLEDGMENTS.............................................................................................................ix

TF-SCS REPORT “LICENSING OF SAFETY CRITICAL SOFTWARE FOR NUCLEAR

REACTORS - COMMON POSITION OF INTERNATIONAL NUCLEAR REGULATORS 

AND AUTHORISED TECHNICAL SUPPORT ORGANISATIONS" ............................................ 1

APPENDIX A: Using TF SCS CPs in NRC Regulatory Guidance............................................A-1

A.1 Background...............................................................................................................A-1

A.2 Scope of NRC’s Review............................................................................................A-2

A.3 Scope Restrictions Due to Differences in Regulatory Frameworks............................A-2

A.4 Resolution of Vocabulary Differences .......................................................................A-3

A.5 Abbreviations Used in Appendix A............................................................................A-4

ix

ACKNOWLEDGMENTS

This NUREG/IA contains the entire report as produced and published by the Regulator Task 

Force on Safety Critical Software (TF SCS) for nuclear reactors. The authors of this NUREG/IA 

acknowledge that the enclosed report is the product of the co-authoring members of the 

TF SCS, as stated in the TF SCS report.

1

TF-SCS REPORT “LICENSING OF SAFETY CRITICAL SOFTWARE

FOR NUCLEAR REACTORS - COMMON POSITION OF

INTERNATIONAL NUCLEAR REGULATORS AND AUTHORISED 

TECHNICAL SUPPORT ORGANISATIONS”
Licensing of safety critical software

for nuclear reactors

Common position of

international nuclear regulators and

authorised technical support organisations

Bel V, Belgium

BfS, Germany

CNSC, Canada

CSN, Spain

ISTec, Germany

ONR, United Kingdom

SSM, Sweden

STUK, Finland

REVISION

2014
3

Foreword

This consensus document has been revised and improved by the Regulator Task Force on 

Safety Critical Software (TF SCS) several times since its original publication in 2000, in order 

to provide up-to-date practical guidance and consistent standards of quality in the regulatory 

review of safety critical software. TF SCS member organisations routinely use the document 

and recommend it to nuclear regulators and licensees throughout the world, for their reference 

and use.
4

Disclaimer

Neither the organisations nor any person acting on their behalf is responsible for the use which 

might be made of the information contained in this report.

The report can be obtained from the following organisations or downloaded from their 

websites:

Bel V, Subsidiary of the Federal Agency for 

Nuclear Control

Rue Walcourt, 148

B-1070 Brussels, Belgium

http://www.belv.be/

Institut für Sicherheitstechnologie (ISTec) 

GmbH 

Forschungsgelände 

D-85748 Garching b. München, Germany

http://www.istec-gmbh.de/

Federal Office for Radiation Protection

(BfS)

P.O. Box 100149

D-38201 Salzgitter, Germany

http://www.bfs.de/en/kerntechnik

Office for Nuclear Regulation (ONR)

Redgrave Court, Merton Road, 

Bootle, Merseyside, L20 7HS

United Kingdom

http://www.onr.org.uk

Canadian Nuclear Safety Commission 

(CNSC)

P.O. Box 1046, Station B, 280 Slater Street

Ottawa, Ontario, Canada K1P 5S9

http://www.cnsc-ccsn.gc.ca/

Strålsäkerhetsmyndigheten (SSM)

Swedish Radiation Safety Authority

SE-17116 Stockholm

Sweden

http://www.ssm.se

Consejo de Seguridad Nuclear (CSN)

C/ Justo Dorado 11

28040 Madrid

Spain

http://www.csn.es/

STUK • Radiation and Nuclear Safety 

Authority

Laippatie 4, P.O. Box 14

FIN-00881 Helsinki, Finland

http://www.stuk.fi/

© 2014 Bel V, BfS, CNSC, CSN, ISTec, ONR, SSM, STUK

Reproduction is authorised provided the source is acknowledged.
5

Contents

PART 1: GENERIC LICENSING ISSUES .....................................................................................................27

1.1 Safety Demonstration ................................................................................................................27

1.2 System Classes, Function Categories and Graded Requirements for Software ........................35

1.3 Reference Standards..................................................................................................................43

1.4 Pre-existing Software (PSW).....................................................................................................49

1.5 Tools..........................................................................................................................................53

1.6 Organisational Requirements.....................................................................................................59

1.7 Software Quality Assurance Programme and Plan....................................................................63

1.8 Security......................................................................................................................................67

1.9 Formal Methods.........................................................................................................................71

1.10 Independent Assessment............................................................................................................75

1.11 Graded Requirements for Safety Related Systems (New and Pre-existing Software)..............77

1.12 Software Design Diversity.........................................................................................................85

1.13 Software Reliability...................................................................................................................93

1.14 Use of Operating Experience.....................................................................................................99

1.15 Smart Sensors and Actuators...................................................................................................105

PART 2: LIFE CYCLE PHASE LICENSING ISSUES.................................................................................113

2.1 Computer Based System Requirements...................................................................................113

2.2 Computer System Architecture and Design.............................................................................117

2.3 Software Requirements, Architecture and Design ..................................................................121

2.4 Software Implementation.........................................................................................................127

2.5 Verification..............................................................................................................................135

2.6 Validation and Commissioning ...............................................................................................141

2.7 Change Control and Configuration Management....................................................................147

2.8 Operational Requirements.......................................................................................................153

References......................................................................................................................................................159

Executive Summary

Objectives

It is widely accepted that the assessment of software cannot be limited to verification and 

testing of the end product, ie the computer code. Other factors such as the quality of the 

processes and methods for specifying, designing and coding have an important impact on the 

implementation. Existing standards provide limited guidance on the regulatory and safety 

assessment of these factors. An undesirable consequence of this situation is that the licensing 

approaches taken by nuclear safety authorities and by technical support organisations are 

determined independently with only limited informal technical co-ordination and information 

exchange. It is notable that several software implementations of nuclear safety systems have 

been marred by costly delays caused by difficulties in co-ordinating the development and 

qualification process. 

It was thus felt necessary to compare the respective licensing approaches, to identify where a 

consensus already exists, and to see how greater consistency and more mutual acceptance 

could be introduced into current practices. Within this comparison, the term software also 

includes firmware and microcode.

This document is the result of the work of a group of regulator and safety authorities’ experts. 

The 2007 version was completed at the invitation of the Western European Nuclear 

Regulators’ Association (WENRA). The major result of the work is the identification of 

consensus and common technical positions on a set of important licensing issues raised by the 

design and operation of computer based systems used in nuclear power plants for the 

implementation of safety functions. The purpose is to introduce greater consistency and more 

mutual acceptance into current practices. To achieve these common positions, detailed 

consideration was paid to the licensing approaches followed in the different countries 

represented by the experts of the task force. 

The report is intended to be useful:

– to coordinate regulators’ and safety experts’ technical viewpoints in licensing practices, or 

design and revision of guidelines;

– as a reference in safety cases and demonstrations of safety of software based systems;

– as guidance for manufacturers and major I&C suppliers on the international market.
8

Document Structure

From the outset, attention focused on computer based systems used in nuclear power plants 

for the implementation of safety functions (ie the functions of the highest safety criticality 

level); namely, those systems classified by the International Atomic Energy Agency as “safety 

systems”. The common positions and recommended practices of this report therefore address 

“safety systems”,except certain common positions and recommended practices where “safetyrelated systems” are explicitly mentioned (in chapters 1.2 and 1.11), or an explict reference to 

“safety systems” implies a possible relaxation for other systems important to safety (these 

common positions and recommended practices are listed in 1.11.3.6).

In a first stage of investigation, the task force identified what were believed to be, from a 

regulatory viewpoint, some of the most important and practical issue areas raised by the 

licensing of software important to safety. In the second stage of the investigation, for each 

issue area, the task force strove for and reached: (1) a set of common positions on the basis for 

licensing and evidence which should be sought, (2) consensus on best design and licensing 

recommended practices, and (3) agreement on certain alternatives which could be acceptable.

The common positions are intended to convey the unanimous views of the Task Force

members on the guidance that the licensees need to follow as part of an adequate safety 

demonstration. Throughout the document these common positions are expressed with the 

auxiliary verb “shall”. The use of this verb for common positions is intended to convey the 

unanimous desire felt by the Task Force members for the licensees to satisfy the requirements 

expressed in the clause. The common positions are a common set of requirements and 

practices considered necessary by the member states represented in the task force. 

There was no systematic attempt, however, at guaranteeing that for each issue area these sets 

are complete or sufficient. It is also recognised that – in certain cases – other possible 

practices cannot be excluded, but the members felt that such alternatives will be difficult to 

justify.

Recommended practices are supported by most, but may not be systematically implemented 

by all of the members states represented in the task force. Recommended practices are 

expressed with the auxiliary verb “should”.

In order to avoid the guidance being merely reduced to a lowest common denominator of 

safety (inferior levelling), the task force – in addition to commonly accepted practices – also 

took care not to neglect essential safety or technical measures.
9

Background (history)

In 1994, the Nuclear Regulator Working Group (NRWG) and the Reactor Safety Working 

Group (RSWG) of the European Commission Directorate General XI (Environment, Nuclear 

safety and Civil Protection) launched a task force of experts from nuclear safety institutes with 

the mandate of “reaching a consensus among its members on software licensing issues having 

important practical aspects”. This task force selected a set of key issues and produced an EC 

report [4] publicly available and open to comments. In March 1998, a project called 

ARMONIA (Action by Regulators to Harmonise Digital Instrumentation Assessment) was 

launched with the mission to prepare a new version of the document, which would integrate 

the comments received and would deal with a few software issues not yet covered. In May 

2000, the NRWG approved a report classified by the EC under the category “consensus 

document” (report EUR 19265 EN [5]). After this publication, the task force continued to 

work on important licensing aspects of safety critical software that had not yet been addressed. 

At the end of 2005 when the NRWG was disbanded by the EC, the task force was invited by 

the WENRA association in 2007 to pursue and complete a revision of the report. The common 

positions and recommended practices of EUR 19265 [5] were included in the 2007 revision. 

The task force continued to work on missing and emerging licensing aspects of safety critical 

software, leading to new published revisions in 2010 and 2013. These have been further 

revised to produce the present edition.

The U.S. Nuclear Regulatory Commission (NRC) has, since 2009, participated in the 

meetings of the task force and provided input to this version of the report. Although the NRC 

has not endorsed this report for regulatory use by the NRC, it is publishing it as a technical 

report in NRC’s NUREG/IA series [20] because it considers the common positions a valuable 

technical reference for future improvements in its own regulatory guidance.

* * *

11

I INTRODUCTION

All government,

– indeed every human benefit and enjoyment,

every virtue and every prudent act –

is founded on compromise and barter.

(Edmund Burke, 1729-1797)

Objectives

It is widely accepted that the assessment of software cannot be limited to verification and 

testing of the end product, ie the computer code. Other factors such as the quality of the 

processes and methods for specifying, designing and coding have an important impact on the 

implementation. Existing standards provide limited guidance on the regulatory and safety 

assessment of these factors. An undesirable consequence of this situation is that the licensing 

approaches taken by nuclear safety authorities and by technical support organisations are 

determined independently and with only limited informal technical co-ordination and 

information exchange. It is notable that several software implementations of nuclear safety 

systems have been marred by costly delays caused by difficulties in co-ordinating the 

development and the qualification process. 

It was thus felt necessary to compare the respective licensing approaches, to identify where a 

consensus already exists, and to see how greater consistency and more mutual acceptance 

could be introduced into the current practices. Within this comparison, the term software also 

includes firmware and microcode.

This document is the result of the work of a group of regulator and safety authorities’ experts. 

The 2007 version was completed at the invitation of the Western European Nuclear 

Regulators’ Association (WENRA). The major result of the work is the identification of 

consensus and common technical positions on a set of important licensing issues raised by the 

design and operation of computer based systems used in nuclear power plants for the 

implementation of safety functions. The purpose is to introduce greater consistency and more 

mutual acceptance into current practices. To achieve these common positions, detailed 
12

consideration was paid to the licensing approaches followed in the different countries 

represented by the experts of the task force. 

The report is intended to be useful:

– to coordinate regulators’ and safety experts’ technical viewpoints in licensing practices, or 

design and revision of guidelines;

– as a reference in safety cases and demonstrations of safety of software based systems;

– as guidance for manufacturers and major I&C suppliers on the international market.

Scope

The task force decided at an early stage to focus attention on computer based systems used in 

nuclear power plants for the implementation of safety functions (ie the functions of the highest 

safety criticality level); namely, those systems classified by the International Atomic Energy 

Agency as “safety systems”. Therefore, recommendations of this report – except those of 

chapter 1.11 – address “safety systems” and not “safety related systems”.

The task force has not considered whether or not the common positions and recommended 

practices are applicable to safety-related systems, except for those in chapter 1.2 and 1.11 and 

listed in 1.11.3.6. In these cases, in the course of discussions, the task force came to the 

conclusion that specific practices could be dispensed with – or at least relaxed – for safety 

related systems. Reporting the possibility of such dispensations and relaxations was felt 

useful should there be future work on safety related systems. These practices are therefore 

explicitly identified as applying to safety systems. Some relaxations of requirements for 

safety related systems are also mentioned in chapter 1.2. All relaxations for safety related 

systems are restated in chapter 1.11.

The task force worked on the assumption that the use of digital and programmable technology 

has in many situations become inescapable. A discussion of the appropriateness of the use of 

this technology has therefore not been considered. Moreover, it was felt that the most difficult 

aspects of the licensing of digital programmable systems are rooted in the specific properties 

of the technology. The objective was therefore to delineate practical and technical licensing 

guidance, rather than discussing or proposing basic principles or requirements. The design 

requirements and the basic principles of nuclear safety in force in each member state are 

assumed to remain applicable. 
13

This report represents the consensus view achieved by the experts who contributed to the task 

force. It is the result of what was at the time of its initiation a first attempt at the international 

level to achieve consensus among nuclear regulators on practical methods for licensing 

software based systems.

This document should neither be considered as a standard, nor as a new set of European 

regulations, nor as a common subset of national regulations, nor as a replacement for national 

policies. It is the account, as complete as possible, of a common technical agreement among 

regulatory and safety experts. National regulations may have additional requirements or 

different requirements, but hopefully in the end no essential divergence with the common 

positions. It is precisely from this common agreement that regulators can draw support and 

benefit when assessing safety cases, licensee’s submissions, and issuing regulations. The 

document is also useful to licensees, designers, suppliers for issuing bids and developing new 

applications.

Safety Demonstration Approach

Evidence to support the safety demonstration of a computer based digital system is produced 

throughout the system life cycle, and evolves in nature and substance with the project. A 

number of distinguishable types of evidence exist on which the demonstration can be 

constructed. 

The task force has adopted the view that three basic independent types of evidence can and 

must be produced: evidence related to the quality of the development process; evidence 

related to the adequacy of the product; and evidence of the competence and qualifications of 

the staff involved in all of the system life cycle phases. In addition, convincing operating 

experience may be needed to support the safety demonstration of pre-existing software.

As a consequence, the task force reached early agreement on an important fundamental 

principle (see 1.1.3.1) that applies at the inception of any project, namely:

A safety plan1 shall be agreed upon at the beginning of the project between the 

licensor and the licensee. This plan shall identify how the safety 

demonstration will be achieved. More precisely, the plan shall identify the 

types of evidence that will be used, and how and when this evidence shall be 

produced.

 

1 A safety plan is not necessarily a specific document.
14

This report neither specifies nor imposes the contents of a specific safety plan. All the 

subsequent recommendations are founded on the premise that a safety plan exists and has been

agreed upon by all parties involved. The intent herein is to give guidance on the evidence set 

to be provided, its integration to demonstrate safety and the documentation for the safety 

demonstration and for the contents of the safety plan. It is therefore implied that all the 

evidence and documentation recommended by this report, among others that the regulator may 

request, should be made available to the regulator.

The safety plan should include a safety demonstration strategy. For instance, this strategy 

could be based on a plant independent type approval of software and hardware components, 

followed by the approval of plant specific features, as it is practised in certain countries.

Often this plant independent type approval is concerned with the analysis and testing of the 

non-plant-specific part of a configurable tool or system. It is a stepwise verification which 

includes:

– an analysis of each individual software and hardware component with its specified 

features, and

– integrated tests of the software on a hardware system using a configuration representative 

of the plant-specific systems and their environments.

Only properties at the component level can be demonstrated by this plant independent type 

approval. It must be remembered that a program can be correct for one set of data, and be 

erroneous for another. Hence assessment and testing of the plant specific software, integrated 

in the plant-specific system and environment, remains essential.

Licensing Issues: Generic and Life Cycle Specific

As described earlier, in a first stage, the task force selected a set of specific technical issue 

areas, which were felt to be of utmost importance to the licensing process. In a second stage 

phase, each of these issue areas was studied and discussed in detail until a common position 

was reached. 

These issue areas were partitioned into two sets: “generic licensing issues” and “life cycle 

phase licensing issues”. Issues in the second set are related to a specific stage of the computer 

based system design and development process, while those of the former have more general 

implications and apply to several stages or to the whole system lifecycle. Each issue area is 

dealt with in a separate chapter of this report, namely:
15

PART 1: GENERIC LICENSING ISSUES

1.1 Safety Demonstration

1.2 System Classes, Function Categories and Graded Requirements for Software

1.3 Reference Standards

1.4 Pre-existing Software (PSW)

1.5 Tools

1.6 Organisational Requirements

1.7 Software Quality Assurance Programme and Plan

1.8 Security

1.9 Formal Methods

1.10 Independent Assessment

1.11 Graded Requirements for Safety Related Systems (New and Pre-existing Software)

1.12 Software Design Diversity

1.13 Software Reliability

1.14 Use of Operating Experience

1.15 Smart Sensors and Actuators

PART 2: LIFE CYCLE PHASE LICENSING ISSUES

2.1 Computer Based System Requirements

2.2 Computer System Architecture and Design

2.3 Software Requirements, Architecture and Design

2.4 Software Implementation

2.5 Verification

2.6 Validation and Commissioning

2.7 Change Control and Configuration Management

2.8 Operational Requirements

This set of issue areas is felt to address a consistent set of licensing aspects right from the 

inception of the life cycle up to and including commissioning. It is important to note, 

however, that although the level of attention given in this document may not always reflect it, 

a balanced consideration of these different licensing aspects is needed for the safety 

demonstration.
16

Definition of Common Positions and Recommended Practices

Apart from chapter 1.3 which describes the standards in use by members of the task force, for 

each issue area covered, the following four aspects have been addressed:

– Rationale: technical motivations and justifications for the issue from a regulatory point of 

view;

– Description of the issue in terms of the problems to be resolved;

– Common position and the evidence required;

– Recommended practices.

Common positions are expressed with the auxiliary verb “shall”; recommended practices with 

the verb “should”. 

The use of the verb “shall” for common positions is intended to convey the unanimous desire 

felt by the Task Force members for the licensees to satisfy the requirement expressed in the 

clause. The use of “shall” and “requirement” shall not necessarily be construed as a 

mandatory condition incorporated in regulation.

These common position clauses can be regarded as a common set of requirements and 

practices in member states represented in the task force. The common position (set of clauses) 

for a particular issue is identified as necessary, but may not be sufficient or complete. It 

should also be recognised that – in certain cases – other possible practices cannot be excluded, 

but the members felt that such alternatives would be difficult to justify. 

Recommended practices are supported by most, but may not be systematically implemented 

by all of the members states represented in the task force. Some of these recommended 

practices originated from proposed common position resolutions on which unanimity could 

not be reached.

In order to avoid the guidance being merely reduced to a lowest common denominator of 

safety (inferior levelling), the task force – in addition to commonly accepted practices – also 

took care not to neglect essential safety or technical measures.
17

These common positions and recommended practices have of course not been elaborated in 

isolation. They take into account not only the positions of the participating regulators, but 

also the guidance issued by other regulators with experience in the licensing of computerbased nuclear safety systems. They have also been reviewed against the international 

guidance, the technical expertise and the evolving recommendations issued by the IAEA, the 

IEC and the IEEE organisations. The results of research activities on the design and the 

assessment of safety critical software by EC projects such as PDCS (Predictably dependable 

computer systems), DeVa (Design for Validation), CEMSIS (Cost Effective Modernisation of 

Systems Important to Safety) and by studies carried out by the EC Joint Research Centre have 

also provided sources of inspiration and guidance. A bibliography at the end of the report 

gives the major references that have been used by the task force and the consortium.

Historical Background

In 1994, the Nuclear Regulator Working Group (NRWG) and the Reactor Safety Working 

Group (RSWG) of the European Commission Directorate General XI (Environment, Nuclear 

safety and Civil Protection) launched a task force of experts from nuclear safety institutes with 

the mandate of “reaching a consensus among its members on software licensing issues having 

important practical aspects”. This task force selected a set of key issues and produced an EC 

report [4] publicly available and open to comment. In March 1998, a project called 

ARMONIA (Action by Regulators to Harmonise Digital Instrumentation Assessment) was 

launched with the mission to prepare a new version of the document, which would integrate 

the comments received and would deal with a few software issues not yet covered. In May 

2000, the NRWG approved a report classified by the EC under the category “consensus 

document” (report EUR 19265 EN [5]). After this publication, the task force continued to 

work on important licensing aspects of safety critical software that had not yet been addressed. 

At the end of 2005 when the NRWG was disbanded by the EC, the task force was invited by

WENRA in 2007 to pursue and complete a revision of the report. The common positions and 

recommended practices of EUR 19265 [5] were included in the 2007 revision. The task force 

continued to work on missing and emerging licensing aspects of safety critical software, 

leading to new published revisions in 2010 and 2013. These have been further revised to 

produce the present edition.
18

The experts, members of the task force, who actively contributed to this document are:

Belgium P.-J. Courtois, Bel V (1994- ) (Chairman 1994-2007)

A. Geens, AVN (2004-2006)

S. van Essche, Bel V (2011-2013)

Canada G. Chun, CNSC (2013- )

Finland M.L. Järvinen, STUK (1997-2003)

P. Suvanto, STUK (2003-2010)

M. Johansson, STUK (2010- )

Germany M. Kersken, ISTec (1994-2003)

E.W. Hoffman, ISTec (2003-2007)

J. Märtz, ISTec (2007- )

F. Seidel, BfS (1997- )

Spain R. Cid Campo, CSN (1997-2003)

F. Gallardo, CSN (2003- )

M. Martínez, CSN (2013- )

Sweden B. Liwång, SSM (1996- )

United Kingdom N. Wainwright, NII (1994-1999)

R. L. Yates, ONR (1999-2013) (Chairman 2007-2013)

M. Bowell, ONR (2007- ) (Chairman 2013- )

A consortium consisting of ISTec, NII, and AVN (chair) was created in March 1998 to give 

research, technical and editorial support to the task force. Under the project name of 

ARMONIA (Action by Regulators for harmonising Methods Of Nuclear digital 

Instrumentation Assessment), the consortium received financial support from the EC 

programme of initiatives aimed at promoting harmonisation in the field of nuclear safety. 

P.-J. Courtois (AVN), M. Kersken (ISTec), P. Hughes, N. Wainwright and R. L. Yates (NII)
19

were active members of ARMONIA. In the long course of meetings and revisions, technical 

assistance and support was received from J. Pelé, J. Gomez, F. Ruel, J.C. Schwartz, H. 

Zatlkajova from the EC, and G. Cojazzi and D. Fogli from JRC, Ispra. P. Govaerts (AVN) 

was instrumental in setting up the task force in 1994.

The task force acknowledges and appreciates the support provided by the EC and the Western 

European Nuclear Regulator Association (WENRA) during the production of some of the 

earlier versions of this work.

The U.S. Nuclear Regulatory Commission (NRC) has, since 2009, participated in the 

meetings of the task force and provided input to this version of the report. The NRC 

participants in task force meetings have included Dr. Steven Arndt, William Kemper, Norbert 

Carte, John Thorp and Dr. Sushil Birla. Additionally, Michael Waterman and Russell Sydnor

of the NRC staff have provided input. Although the NRC has not endorsed this report for 

regulatory use by the NRC, it is publishing it as a technical report in NRC’s NUREG/IA series 

[20] because it considers the common positions a valuable technical reference for future 

improvements in its own regulatory guidance. The NRC continues to participate in task force 

meetings to inform the task force of NRC technical positions on matters of interest, to better 

understand the technical views and common positions and to better inform the NRC of the 

task force’s position on technical positions of mutual interest.

* * *

21

II GLOSSARY

The following terms should be interpreted as having the following meaning

in the context of this document.

These terms are highlighted as a defined term the first time they are used in each chapter of 

this document.

Availability: The ability of an item to be in a state to perform a required function under given 

conditions at a given instant of time or over a given time interval, assuming that the required 

external resources are provided. (IEC 60050-191-02-05)

Note: This term is also used, when quantification is implied, to refer to the mean of the 

instantaneous availability under steady-state conditions over a given time interval. (Where 

instantaneous availability is the probability that an item is in a state to perform a required 

function under given conditions at a given instant of time, assuming that the required external 

resources are provided.). (IEC 60050-191-11-06)

Category (safety-): One of three possible assignments (safety, safety related and not important 

to safety) of functions in relation to their different importance to safety.

Channel: An arrangement of interconnected components within a system that initiates a single 

output. A channel loses its identity where single output signals are combined with signals 

from other channels such as a monitoring channel or a safety actuation channel. (See IEC 

61513 and IAEA Safety Guide NS-G_1.3)

Class (safety-): One of three possible assignments (safety, safety related and not important to 

safety) of systems, components and software in relation to their different importance to safety.

Commissioning: The onsite process during which plant components and systems, having been 

constructed, are made operational and confirmed to be in accordance with the design 

assumptions and to have met the safety requirements, the performance criteria and the 

requirements for periodic testing and maintainability.

Common cause failure: Failure of two or more structures, systems or components due to a 

single specific event or cause. (IAEA Safety Guide NS-G-1.3)

Common position: Requirement or practice unanimously considered by the member states 

represented in the task force as necessary for the licensee to satisfy. The use of the term 

“requirement” shall not necessarily be construed as a mandatory condition incorporated in 

regulation.
22

Completeness: Property of a formal system in which every true fact is provable.

Complexity: The degree to which a system or component has a design or implementation that 

is difficult to understand and verify. (IEC STD 100-2000)

Note: This document does not use the term complexity in the context of metrics.

Component: One of the parts that make up a system. A component may be hardware or 

software and may be subdivided into other components. (IEC 61513, 3.9)

Computer based system (in short also the System): The plant system important to safety in 

which is embedded the computer implementation of the safety/safety related function(s).

Computer system architecture: The hardware components (processors, memories, I/O 

devices) of the computer based system, their interconnections, physical separation and 

electrical isolation, the communication systems, and the mapping of the software functions on 

these components.

Consistency: Property of a formal system which contains no sentence such that both the 

sentence and its negation can be proven from the assumptions.

Dangerous failure: Used as a probabilistic notion, failure that has the potential to put the 

safety system in a hazardous or fail-to-function state. Whether the potential is realised may 

depend on the channels of the system architecture; in systems with multiple channels to 

improve safety, a dangerous hardware failure is less likely to lead to the overall dangerous or 

fail-to-function state. (IEC 61508-4, 3.6.7) 

Diversity: Existence of two or more different ways or means of achieving a special objective.

(IEC 60880 Ed. 2 [12])

Diversity design options/seeking decisions: Choices made by those tasked with delivering 

diverse software programs as to what are the most effective methods and techniques to 

prevent coincident failure of the programs.

Equivalence partitioning: Technique for determining which classes of input data receive 

equivalent treatment by a system, a software module or program. A result of equivalence 

partitioning is the identification of a finite set of software functions and of their associated 

input and output domains. Test data can be specified based on the known characteristics of 

these functions.

Error: Manifestation of a fault and/or state liable to lead to a failure.

Failure: The termination of the ability of a structure, system or component to perform a 

required function. (IEEE STD 100)

Failure mode: The physical or functional manifestation of a failure. (ISO/IEC/IEEE 24765)
23

Fault: Defect or flaw in a hardware or software component. (IEEE STD 100 definition 13)

Formal methods, formalism: The use of mathematical models and techniques in the design 

and analysis of computer hardware and software.

Functional diversity: Application of diversity at the functional level (for example, to have trip 

activation on both pressure and temperature limit). (IEC 60880 Ed.2 (3.18) [12] and 

IEC 61513 (3.23) [15])

Functional requirement: Service or function expected to be delivered.

Graded requirement: A possible assignment of graded or relaxed requirements on the 

qualification of the software development processes, on the qualities of software products and 

on the amount of verification and validation resulting from consideration of what is necessary 

to reach the appropriate level of confidence that the software is fit for purpose to execute 

specific functions in a given safety category.

Harm: Physical injury or damage to the health of people, either directly or indirectly, as a 

result of damage to property or the environment. (IEC 61508-4, 3.1.1)

Hazard: Potential source of harm. (IEC 61508-4, 3.1.2)

I&C: instrumentation and control.

Licensee safety department: A licensee’s department, staffed with appropriate computer 

competencies independent from the project team and operating departments appointed to 

reduce the risk that project or operational pressures jeopardise the safety systems’ fitness for 

purpose.

NPP: nuclear power plant.

Non-functional requirement: Requirement that specifies a property of a system or of its 

element(s) in addition to their functional behaviour.

Note 1: See 1.1.1 for further detail and example properties.

Note 2: ISO 25010:2011 uses the term “quality requirement”, defined as a requirement for the 

corresponding intrinsic quality property to be present.

pfd: probability of failure on demand.

Plant safety analysis: Deterministic and/or probabilistic analysis of the selected postulated 

initiating events to determine the minimum safety system requirements to ensure the safe 

behaviour of the plant. System requirements are elicited on the basis of the results of this 

analysis.

Pre-existing software (PSW): Software which is used in a NPP computer based system

important to safety, but which was not produced by the development process under the control 
24

of those responsible for the project (also referred to as “pre-developed” software). “Off-theshelf” software is a kind of PSW.

Probability of failure: A numerical value of failure rate normally expressed as either 

probability of failure on demand (pfd) or probability of dangerous failure per year (eg 10-4 pfd 

or 10-4 probability of dangerous failure per year).

Programmed electronic component: An electronic component with embedded software that 

has the following restrictions:

– its principal function is dedicated and completely defined by design;

– it is functionally autonomous;

– it is parametrizable but not programmable by the user.

These components can have additional secondary functions such as calibration, autotests, 

communication, information displays. Examples are relays, recorders, regulators, smart 

sensors and actuators.

PSA: Probability safety assessment.

QRA: Quantitative risk assessment.

Recommended practice: Requirement or practice considered by most member states 

represented in the task force as necessary for the licensee to satisfy.

Regulator: The regulatory body and/or authorised technical support organisation acting on 

behalf of its authority. 

Reliability: Continuity of proper service. Reliability may be interpreted as either a qualitative 

or quantitative property.

Reliability level: A defined numerical probability of failure range (eg 10-3 > pfd >10-4). 

Reliability target: Probability of failure value typically arising from the plant safety analysis

(eg PSA/QRA) for which a safety demonstration is required.

Requirement specification: Precise and documented description or representation of a 

requirement.

Risk: Combined measure of the likelihood of a specified undesired event and of the 

consequences associated with this event.

(Nuclear) Safety: The achievement of proper operating conditions, prevention of accidents or 

mitigation of accident consequences, resulting in protection of workers, the public and the 

environment from undue radiation hazards. (IAEA Safety Glossary)

Note: In this document nuclear safety is abbreviated to safety.
25

Safety demonstration: The set of arguments and evidence elements which support a selected 

set of claims on the safety of the operation of a system important to safety used in a given 

plant environment.

Safety integrity level (SIL): Discrete level (one out of a possible four), corresponding to a 

range of values of the probability of a system important to safety satisfactorily performing its 

specified safety requirements under all the stated conditions within a stated period of time.

Safety plan: A plan, which identifies how the safety demonstration is to be achieved; more 

precisely, a plan which identifies the types of evidence that will be used, and how and when 

this evidence shall be produced. A safety plan is not necessarily a specific document.

Safety related systems: Those instrumentation and control systems important to safety that are 

not included in safety systems (IAEA safety guide NSG 1.3).

Safety system: A system important to safety provided to assure the safe shutdown of the 

reactor and the heat removal from the core, or to limit the consequences of operational 

occurrences and accident conditions. (IAEA safety guide NSG 1.3)

Security: The prevention of unauthorised disclosure (confidentiality), modification (integrity) 

and retention of information, software or data (availability).

Shall: Conveys unanimous consensus by the Task Force members for the licensees to satisfy 

the requirement expressed in the clause.

Should: Conveys that the practice is recommended, ie is supported by most Task Force 

members but may not be systematically implemented by all.

Smart sensor/actuator: Intelligent measuring, communication and actuation devices 

employing programmed electronic components to enhance the performance provided in 

comparison to conventional devices.

Software architecture, software modules, programs, subroutines: Software architecture refers 

to the structure of the modules making up the software. These modules interact with each 

other and with the environment through interfaces. Each module includes one or more 

programs, subroutines, abstract data types, communication paths, data structures, display 

templates, etc. If the system includes multiple computers and the software is distributed 

amongst them, then the software architecture must be mapped to the hardware architecture by 

specifying which programs run on which processors, where files and displays are located and 

so on. The existence of interfaces between the various software modules, and between the 

software and the external environment (as per the software requirements document), should be 

identified.
26

Software maintenance: Software change in operation following the completion of 

commissioning at site.

Software modification: Software change occurring during the development of a system up to 

and including the end of commissioning.

Soundness: Property of a formal system in which every provable fact is true.

SQA: Software quality assurance.

Synchronisation programming primitive: High level programming construct, such as for 

example a semaphore variable, used to abstract from interrupts and to program mutual 

exclusion and synchronisation operations between co-operating processes (see eg [5]).

System: When used as a stand-alone term, abbreviation for computer based system.

Systems important to safety: Systems which include safety systems and safety-related systems. 

In general, all those items which, if they were to fail to act, or act when not required, may 

result in the need for action to prevent undue radiation exposure (IAEA safety guide NSG 

1.3).

Transformation tool: A tool such as a code generator or compiler, that transforms a work 

product text at one level of abstraction into another, usually lower, level of abstraction.

Validation: Confirmation, through the provision of evidence, that a product satisifies its

specific intended use or application. (Adapted from ISO/IEC/IEEE 9000:2008(E))

Note 1: The product may be a set of requirements or may be a computer-based system.

Note 2: Validation of a set of requirements will include a demonstration of its correctness, 

completeness, consistency and unambiguity.

Verification: Confirmation that a product satisfies its specified requirements. (Adapted from 

ISO/IEC/IEEE 24765:2010(E))

Note: The specified requirements are usually the output of a previous phase or phases. 

V&V: verification and validation.

* * *
27

PART 1: GENERIC LICENSING ISSUES

1.1 Safety Demonstration

“Sapiens nihil affirmat quod non probet.”

1.1.1 Rationale

Standards and national rules reflect the knowledge and consensus of experts; the fulfilment of 

these requirements may not be sufficient to assure safety in all cases. They usefully describe 

what is recommended in fields such as requirements specification, design, verification, 

validation, maintenance, operation, etc. and contribute to the improvement of safety 

demonstration practices.

However, the process of approving software for safety and safety related functions is far from 

trivial, and will continue to evolve. Reviews of licensing approaches showed that, except for 

procedures, which formalise negotiations between licensee and licensor, no systematic method 

is defined or in use in many member countries for demonstrating the safety of a software 

based system.

A systematic and well-planned approach contributes to improving the quality and costeffectiveness of the safety demonstration. The benefit can be at least three-fold:

– To allow the parties involved to focus attention on the specific safety issues raised by the 

system and on the corresponding specific system requirements as defined in chapter 2.1

that must be satisfied;

– To allow system requirements to be prioritised, with a commensurate allocation of 

resources; 

– To organise system requirements so that the arguments and evidence are limited to what is 

deemed necessary and sufficient by the parties involved. 

A safety demonstration addresses the properties of a particular system operating in a specific 

environment. These properties include safety and security. It is therefore specific and carried 

out on a case-by-case basis, and not once and for all. This does not mean, however, that the 

demonstration could be performed “à la carte” with a free choice of means and objectives.
28

The safety functions for a system depend on properties in addition to the functional behaviour 

of the system hardware and software. Hence, claims that safety functions are satisfied need to

be supported by claims that the system and its functions have particular properties. These are 

properties such as reliability, availability, maintainability, testability, usability, accuracy and 

performance (eg accuracy, timing constraints, cycle and required response times in relation to 

rates of change of the plant parameters).

1.1.2 Issues Involved

1.1.2.1 Various approaches are possible

There are several approaches offered to a licensee and a regulator for the demonstration of the 

safety of a computer based system. The demonstration may be conditioned on the provision 

of evidence of compliance with a set of agreed rules, laws, standards, or design and 

assessment principles (rule-based approach). It also may be conditioned on the provision of 

evidence that certain specific residual risks are acceptable, or that certain safety properties are 

achieved (goal based approach). Any combination of these approaches is of course possible. 

For instance, compliance with a set of rules or a standard can be invoked as evidence to 

support a particular system requirement. A safety demonstration may be multi-legged, 

supported by many types of evidence.

None of these approaches is without problems. The law-, rule-, design principle- or standardcompliance approach often fails to demonstrate convincingly by itself that a system is safe 

enough for a given application, thereby entailing licensing delays and costs. A multi-legged 

approach may suffer from the same shortcomings. By collecting evidence in three different 

and orthogonal directions, which remain unrelated, one may fail to convincingly establish a 

system property. The safety goal approach requires ensuring that the initial set of goals, which 

is selected, is complete and coherent.

1.1.2.2 The plant-system interface cannot be ignored

Most safety requirements are determined by the application in which the computer and its 

software are embedded. Many pertinent arguments to demonstrate safety – for instance the 

provision of safe states and safe failure modes – are not provided by the computer system 

design and implementation, but are determined by the environment and the role the system is 

expected to play in it. Guidance on the safety demonstration of computer based systems often 

concentrates on the V&V of the detailed design and implementation and pays little attention to 

a top-down approach starting with the environment-system interface.
29

1.1.2.3 The system lifetime must be covered

Safety depends not only on the design, but also, and ultimately, on the installation of the 

integrated system, on operational procedures, on procedures for (re) calibration, (re) 

configuration, maintenance, even for decommissioning in some cases. A safety case is not 

closed before the actual behaviour of the system in real conditions of operations has been 

found acceptable. The safety demonstration of a software based system therefore involves 

more than a code correctness proof. It involves a large number of various claims spanning the 

whole system life cycle, and well known to application engineers, but often not sufficiently 

addressed in the computer system design.

Besides, as already said in the introduction, evidence to support the safety demonstration of a 

computer based system is produced throughout the system life cycle, and evolves in nature 

and substance with the project.

1.1.2.4 Safety Demonstration

The key issue of concern is how to demonstrate that the system requirements as defined in 

chapter 2.1 have been met. Basically, a safety demonstration is a set of arguments and 

evidence elements that support a selected set of safety claimsof the operation of a system 

important to safety used in a given plant environment. (see Figure 1 below and eg [2]). 

Claims identify functional and/or non-functional requirements that must be satisfied by the 

system. A claim may require the existence of a safe state, the correct execution of an action, a 

specified level of reliability or availability, etc... The set of claims must be coherent and as 

complete as possible. By itself, this set defines what the expected properties of the system are. 

Claims can be decomposed and inferred from sub-claims at various levels of the system 

architecture, design and operations. Claims may coincide with the computer based system 

requirements that are discussed in chapter 2.1. They also may pertain to a property of these 

requirements (completeness, coherency, soundness) or claim an additional property of the 

system that was not part of the initial requirements, as in the case of COTS for instance. 
30

Claims and sub-claims are supported by evidence components that identify facts or data, for 

which there is confidence beyond any reasonable doubt in the axiomatic truth of these facts 

and data, without further evaluation, quantification or demonstration. Such a confidence 

inevitably requires a consensus of all parties involved to consider the evidence as being 

unquestionable. As already stated in the introduction, a number of distinguishable and 

independent types of evidence exist on which the demonstration can be constructed: evidence 

related to the quality of the development process; evidence related to the adequacy of the 

product specifications and the correctness of its implementation, and evidence of the 

competence and qualifications of the staff involved in all of the system life cycle phases. 

Convincing operating experience may be needed to support the safety demonstration of preexisting software (see Chapter 1.14). These types of evidence may not be merely juxtaposed. 

They must be organised so as to achieve the safety demonstration.

An argument is the set of evidence components that support a claim, together with a 

specification of the relationship between these evidence components and the claim. 

Figure 1: Claim, arguments and evidence structure

claim

Sub-claim conjunction

Argument

Sub-claim

evidence

Sub-claim

evidence

evidence

evidence

 Sub-claim

evidence

inference
31

1.1.2.5 System descriptions and their interpretations are important

Safety is a property, the demonstration of which – in most practical cases – cannot be strictly 

experimental and obtained by eg testing or operational experience – especially for complex 

and digital systems. For instance, safety does not only include claims of the type: “the class X 

of unacceptable events shall occur less than once per Y hours in operation”. It also includes or 

subsumes the claim that the class of events X is adequately identified, complete and 

consistent. Thus, safety cannot be discussed and shown to exist without using accurate 

descriptions of the system architecture, of the hardware and software design of the system 

behaviour and of its interactions with the environment, and using models of postulated 

accidents. These descriptions must include unintended systems behaviour, be unambiguously 

understood and agreed upon by all those who have safety case responsibilities: users, 

designers and assessors. This is unfortunately not always the case. Claims – although usually 

based on a huge engineering and industrial past experience – may be only poorly specified. 

The simplifying assumptions behind the descriptions that are used in the system and 

environment representations and for the evaluation of safety are not always sufficiently 

explicit. As a result one should be wary of attempts to “juggle with assumptions” (ie to argue 

and interpret system, environment and/or accident hypotheses in order to make unfounded 

claims of increased safety and/or reliability) during licensing negotiations. A need exists in 

industrial safety cases for more attention to be paid to the use of accurate descriptions of the 

system and of its environment.

It is worth noting that the software itself is the most accurate available description of the 

behaviour of the computer. 

1.1.3 Common Positions

1.1.3.1 A safety plan shall be agreed upon at the beginning of the project between the 

licensor and the licensee. A safety plan is not necessarily a specific document.

1.1.3.2 The licensee shall identify all software used in systems important to safety 

(including pre-existing software, firmware in programmable field devices etc). All software 

identified shall be covered by a safety plan.

1.1.3.3 The licensee shall produce a safety plan as early as possible in the project, and shall 

make this safety plan available to the regulator.

1.1.3.4 The safety plan shall identify how the safety demonstration will be achieved.

1.1.3.5 The safety plan shall define

– the activities to be undertaken in order to demonstrate that the system is adequately safe

for its intended use and environment,
32

– the organisational arrangements needed for the demonstration (including independence of 

those undertaking the safety demonstration activities) and

– the programme (the activities and their inter-relationships, allocated resources and time 

schedule) for the safety demonstration.

1.1.3.6 The plan shall identify the types of evidence that will be used, its logical 

organisation, and how and when this evidence shall be produced. The following three 

different types of evidence shall be considered:

– evidence related to quality of the development process

– evidence related to adequacy of the product

– evidence of the competence and qualification of the staff involved in all of the system life 

cycle phases.

1.1.3.7 The safety plan shall be implemented by the licensee.

1.1.3.8 If the safety demonstration is based on a claim/evidence/argument structure, then 

the safety plan shall identify the claims that are made on the system, the types of evidence that 

are required, the arguments that are applied, and when this evidence shall be produced. 

1.1.3.9 The safety demonstration shall identify a complete and consistent set of 

requirements that need to be satisfied. These requirements shall address at least:

– The validity of the functional and non-functional system requirements and the adequacy of 

the design specifications; those must satisfy the plant/system interface safety requirements 

and deal with the constraints imposed by the plant environment on the computer based 

system;

– The correctness of the design and the implementation of the embedded computer system 

for ensuring that it performs according to its specifications;

– The operation and maintenance of the system to ensure that the safety claims and the 

environmental constraints will remain satisfied during the whole lifetime of the system. 

This includes claims that the system does not display behaviours unanticipated by the 

system specifications, or that potential behaviours outside the scope of these specifications 

will be detected and their consequences mitigated.

1.1.3.10 The licensee shall make available all evidence identified in the safety 

demonstration.

1.1.3.11 When upgrading an old system, with a new digital system, it shall be demonstrated 

that the new system preserves the existing plant safety properties, eg considering timing 

constraints, delays and response times etc. 
33

1.1.3.12 If a claim/evidence/argument structure is followed, the safety demonstration shall 

accurately document the evidence that supports all claims, as well as the arguments that relate 

the claims to the evidence.

1.1.3.13 The plan shall precisely identify the regulations, standards and guidelines that are 

used for the safety demonstration. The applicability of the standards to be used shall be 

justified, with potential deviations being evaluated and justified. 

1.1.3.14 When standards are intended to support specific claims or evidence components, 

this shall be indicated. Guarantee shall also be given that the coherence of the regulations, 

standards, or guidelines that are used is preserved. 

1.1.3.15 If a claim/evidence/argument structure is followed, the basic assumptions and the 

necessary descriptions and interpretations, which support the claims, evidence components, 

arguments and relevant safety requirements (eg related to incident or accident scenarios, 

performance constraints etc), shall be precisely documented in the safety demonstration. 

1.1.3.16 The system descriptions used to support the safety demonstration shall accurately

describe the system architecture, system/environment interface, interaction, constraints and 

assumptions, the system design, the system hardware and software architecture, and the 

system operation and maintenance. 

1.1.3.17 The safety plan and safety demonstration (including all supporting evidence) shall 

be subject to configuration management, change control and impact analysis, and available to 

the regulator. The safety plan and safety demonstration shall be updated and maintained in a 

valid state throughout the lifetime of the system.
34

1.1.4 Recommended Practices

1.1.4.1 A safety claim at the plant-system interface level and its supporting evidence can be 

usefully organised in a multi-level structure. Such a structure is based on the fact that a claim 

for prevention or for mitigation of a hazard or of a threat at the plant-system interface level 

necessarily implies sub-claims of some or all of three different types: 

– Sub-claims that the functional and/or non-functional requirement specifications of how the 

system has to deal with the hazard/threat are valid, 

– Sub-claims that the system architecture and design correctly implement these 

specifications, 

– and sub-claims that the specifications remain valid and correctly implemented in operation 

and through maintenance interventions. 

The supporting evidence for a safety claim can therefore be organised along the same 

structure. It can be decomposed into the evidence components necessary to support the 

various sub-claims from which the safety claim is inferred. 

* * *
35

1.2 System Classes, Function Categories and

Graded Requirements for Software

1.2.1 Rationale

Software is a pervasive technology increasingly used in many different nuclear applications. 

Not all of this software has the same criticality level with respect to safety. Therefore not all 

of the software needs to be developed and assessed to the same degree of rigour. 

Attention in design and assessment must be weighted to those parts of the system and to those 

technical issues that have the highest importance to safety.

This chapter discusses the assignment of categories to functions and of classes to systems, 

components and software in relation to their importance to safety. We also consider the 

assignment of “graded requirements” for the qualification of the software development 

process, of the software products of these processes, and on the amount of verification and 

validation necessary to reach the appropriate level of confidence that software is fit for 

purpose.

To ensure that proper attention is paid to the design, assessment, operation and maintenance of 

the systems important to safety, all systems, components and software at a nuclear facility 

should be assigned to different safety classes. Graded requirements may be advantageously 

used in order to balance the software qualification effort. 

Thus levels of software relaxations must always conform and be compatible with levels of 

safety. The distinction between the safety categories applied to functions, classes applied to 

systems etc. and graded requirements for software qualification does not mean that they can be 

defined independently from one another. The distinction is intended to add some flexibility. 

Usually, there will be a one to one mapping between the safety categories applied to functions, 

classes applied to systems etc. and the graded requirements applied to software design, 

implementation and V&V.

1.2.1.1 System Classification

This document focuses attention on computer based systems used to implement safety 

functions (ie the functions of the highest safety criticality level); namely, those systems

classified by the International Atomic Energy Agency as “safety systems”. The task force 
36

found it convenient to work with the following three system classes (cf. IAEA NS-R-1 and 

NS-G-1.3):

– safety systems

– safety related systems

– systems not important to safety.

The three system classes have been chosen for their simplicity, and for their adaptability to the 

different system class and functional category definitions in use in EUR countries and 

elsewhere.

The three-level system classification scheme serves the purpose of this document, which is 

principally focusing on software of the highest safety criticality (ie the software used in safety 

systems to implement safety functions). Software of lower criticality is addressed in chapter 

1.11, and elsewhere when relaxations on software requirements appear clearly practical and 

recommendable.

The rather simple correspondence between the three system classes above, the IAEA system 

classes and the IEC 61226 function categories can be explained as follows. Correspondence 

between the IEC function categories and the IAEA system classes can be approximately 

established by identifying the IAEA safety systems class with IEC 61226 category A, and 

coalescing categories B and C of the IEC and mapping them into IAEA safety related systems

class. This is summarized in Table 1 below.

Table 1: Correspondence between categories, classes and graded requirements

Categories of functions Classes of systems Examples of graded requirements 

for software

IEC61226 This document IAEA and this document IEC and this document

A Safety functions

B Safety related 

functions

C Safety related 

functions

Functions not important to safety

Safety systems

Safety related systems

Safety related systems

Systems not important to 

safety

– IEC 60880; 

– Table 2 and Table 3 (chapter 1.11) 

in this document;

– IEC 62138 chapter 6; 

– Table 2 and Table 3 (chapter 1.11) 

in this document;

– IEC 62138 chapter 5;

– Table 2 and Table 3 (chapter 1.11) 

in this document.

----
37

1.2.1.2 Graded Requirements

The justification for grading software qualification requirements is given by common position

1.2.3.10, namely the necessity to ensure a proper balance of the design and V&V resources in 

relation to their impact on safety. Different levels of software requirements are thus justified 

by the existence of different levels of safety importance, and there is indeed no sense in having 

the former without the latter. In this document, we use the term “graded requirements” to 

refer to these relaxations. Graded requirements should of course not be confused with the 

system and design requirements discussed in chapter 2.1.

This document does not attempt to define complete sets of software requirement relaxations. It 

does, however, identify relaxations in requirements that are found admissible or even 

recommendable, and under which conditions. These relaxations are obviously on software 

qualification requirements, never on safety functions.

It is reasonable to consider relaxations on software qualification requirements for the 

implementation of functions of lower safety category, although establishing such relaxations is 

not straightforward. The criteria used should be transparent and the relaxations should be 

justifiable.

Graded licensing requirements and relaxations for safety related software are discussed in 

chapter 1.11 where an example of classes and graded requirements is given (see recommended 

practice 1.11.4.2).

1.2.2 Issues Involved

1.2.2.1 Identification and assignment of system classes, function categories and 

graded requirements

Adequate criteria are needed to define relevant classes and graded requirements for software 

in relation to importance to safety. At plant level the plant is designed into systems to which 

safety and safety related functions are assigned. These systems are subdivided, in sufficient 

detail, into structures and components. An item that forms a clearly definable entity with 

respect to manufacturing, installation, operation and quality control may be regarded as one 

structure or component. Every structure and component is assigned to a system class or to the 

class “not important to safety”. Identification of the different types of software and their roles 

in the system is needed to assign the system class and corresponding graded requirements.

In addition to the system itself, the support and maintenance software also need to be assigned 

to system classes with adequate graded requirements. 
38

Software tools also have an impact on safety (see chapter 1.5 and the common positions 

therein). The safety importance of the tools depends on their usage: in particular, on whether 

the tool is used to directly generate online executed software or is used indirectly in a 

supporting role, or is used for V&V.

1.2.2.2 Criteria

Adequate criteria are needed to assign functions to relevant categories in relation to their 

importance to safety and, correspondingly, to identify adequate software graded requirements.

When developing the criteria for graded requirements for the qualification of software 

components it is not sufficient to assess the function in the accomplishment of which the 

component takes part or which it ensures. The impact of a safety or safety related function 

failure during the normal operation of the plant or during a transient or an accident must also 

be considered. In particular, attention must be paid to the possible emergence of an initiating 

event that could endanger nuclear safety and the prevention of the initiating event's 

consequences.

In addition, the following factors may have an impact on the graded requirements imposed on 

a software component and should be taken into account: 

– availability of compensatory systems,

– possibilities for fault detection,

– time available for repair,

– repair possibilities,

– necessary actions before repair work,

– the reliability level that can possibly be granted to the component by the graded 

requirements of the corresponding class. 

1.2.2.3 Regulatory approval

Safety categorisation, classification and a scheme of graded requirements for the qualification 

of software can be used as a basis to define the need for regulatory review and regulatory 

requirements. 
39

1.2.3 Common Position

System Classes

1.2.3.1 The importance to safety of a computer based system and of its software is 

determined by the importance to safety of the functions it is required to perform. 

Categorisation of functions shall be based on an evaluation of the importance to safety of 

these functions. This functional importance to safety is evaluated by a plant safety analysis

with respect to the safety objectives and the design safety principles applicable to the plant.

1.2.3.2 The consequences of the potential failure modes of the system shall also be 

evaluated. This evaluation is more difficult because software failures are hard to predict. 

Assessment of risk associated with software shall therefore be primarily determined by the 

potential consequences of the system malfunctioning, such as its possible failures of 

operation, misbehaviours in presence of faults, or spurious operations (system failure

analysis).

1.2.3.3 The classes associated with the different types of software or components of the 

system must be defined in the safety plan at the beginning of the project.

1.2.3.4 For a smart sensor/actuator, the licensee shall define the safety class, which shall be 

the same as the class of the system in which the smart sensor/actuator is embedded unless 

justified otherwise.

1.2.3.5 Any software performing a safety function, or responsible within a safety system for 

detecting the failure of that system, is in the safety systems class.

1.2.3.6 As a general principle, a computer based system important to safety and its software 

is a safety related system if its correct behaviour is not essential for the execution of safety 

functions (eg corresponding to IEC 61226 category A functions – see Table 1 above)

1.2.3.7 Common position 1.12.3.11 is applicable.

1.2.3.8 Any software generating and/or communicating information (for example, to load 

calibration data, parameter or threshold modified values) shall be of at least the same class as 

the systems that automatically process this information, unless

– the data produced is independently checked by a diverse method, or

– it is demonstrated (for example using software hazard analysis) that the processing 

software and the data cannot be corrupted by the generating/communicating software. 

(Common position 2.8.3.3.4 restates this position for the loading of calibration data.)

1.2.3.9 Any software processing information for classified displays or recorders shall be of 

at least the same class as the display or the recorder. 
40

Graded Requirements

1.2.3.10 The licensee shall have criteria for grading requirements for different types of 

software or components to ensure that proper attention is paid in the design, assessment, 

operation and maintenance of the systems important to safety.

1.2.3.11 Notwithstanding the existence of graded requirements, the licensee shall determine 

which development process attributes are required and the amount of verification and 

validation necessary to reach the appropriate level of confidence that the system is fit for 

purpose.

1.2.3.12 If requirement relaxations (graded requirements) are allowed for the qualification of 

certain specific types of software, these relaxations shall be justified and precisely defined in 

the safety plan, at the start of the project.

1.2.3.13 The graded requirements shall take into account developments in software 

engineering.

1.2.3.14 The definition of graded requirements can result from deterministic analysis, 

probabilistic analysis or from both of these methods. The adequacy of the graded 

requirements shall be justified, well documented and described in the safety plan. 

1.2.3.15 The graded requirements shall coherently cover all software lifecycle phases from 

computer based system requirement specifications to decommissioning of the software.

1.2.3.16 The graded requirements (defined in section 1.2.1.2) that are applicable to safety 

systems include all “shall” requirements of the IEC 60880, ref. [11].

1.2.3.17 The risk of common cause failure shall remain a primary concern for 

multitrain/channel systems utilising software. Requirements on defence against common 

cause failure, on the quality of software specifications, and on failure modes, effects and 

consequence analysis (FMECA) of hardware and software shall not be relaxed without careful 

consideration of the consequences.

1.2.4 Recommended Practices

System Classes

1.2.4.1 The functional criteria and the category definition procedures of IEC 61226 are 

recommended for the identification of the functions, and hence systems, that belong to the 

most critical class safety systems and that – with the exception of chapter 1.11 – are under the 

scope of this document. The examples of categories given in the annex A of this IEC standard 

are particularly informative and useful. 
41

1.2.4.2 It may be reasonable to consider the existence of a back-up by manual intervention 

or by another item of equipment in the determination of the requirements applied to the design 

of a safety system.

Graded Requirements 

1.2.4.3 Relaxations and Exceptions: 

For reasons already explained above, the definition of distinct requirements per class and of 

exceptions and relaxations for classes of lower importance to safety is difficult when software 

is involved.

However, if reliability targets are used, exceptions to the common position 1.2.3.16 above can 

be acceptable. For example, if reliability targets derived from a probability assessment are 

used, certain IEC 60880 requirements can be relaxed for software of class safety system for 

which the required reliability is proven to be low, ie for example in the range where the 

probability of failure on demand is between 10-1 and 10-2. An example of such relaxations can 

be applicable to air-borne radiation detectors used as an ultimate LOCA (Loss of Coolant 

Accident) detection mechanism for containment isolation in the event of a failure of the 

protection system to detect this LOCA.

Relaxations to the standard IEC 60880 and to its requirements for computer based systems of 

the safety related systems class are dealt with in chapter 1.11. 

1.2.4.4 In all cases, a system failure analysis should be performed to assess the 

consequences of the potential failures of the system. It is recommended that this failure 

analysis be performed on the system requirement specifications, rather than at code level, and 

as early as possible so as to come to an agreement with the licensee on graded requirements.

1.2.4.5 Only design and V&V factors which affect indirectly the behaviour of programmed 

functions should be subject to possible relaxations: eg validation of development tools, QA 

and V&V specific plans, independence of V&V team, documentation, and traceability of 

safety functions in specifications.

1.2.4.6 In agreement with 1.11.2.3 (safety related system class equipment), a software 

based system intended to cover less frequent initiating event demands can be assigned 

adequate less stringent graded requirements if and only if it is justified by accident analysis.

* * *

43

1.3 Reference Standards

“Cuando creíamos que teníamos todas las respuestas, 

de pronto, cambiaron todas las preguntas”

Mario Benedetti

This document does not endorse industrial standards. Standards in use vary from one country 

and one project to another. 

Many regulators do not “endorse” industrial standards in their whole. As a matter of fact, 

many national regulations ask for the standards used in a project design and implementation to 

be specified and their applicability justified, with potential deviations being evaluated and 

justified (see common position 1.1.3.13 in chapter 1.1)

The applicable standards mentioned below in sections 1.3.1 and 1.3.2 are standards which are 

or will be used by members of the Task Force as part of their assessment of the adequacy of 

safety systems.

A system is regarded as compliant with an applicable standard when it meets all the 

requirements of that standard on a clause by clause basis, sufficient evidence being provided 

to support the claim of compliance with each clause.

However, in software development, compliance with an applicable standard may be taken to 

mean that the software is compliant with a major part of the standard. Any non-compliance 

will then need to be justified as providing an acceptable alternative to the related clause in the 

standard.

1.3.1 Reference standards for the software of safety systems in Belgium, 

Canada, Finland, Germany, Spain, Sweden and UK

At the time of writing this revision, the only common reference standard, in addition to this 

consensus document, for the software of safety systems in all states represented in the task 

force is the IEC 60880 (1986) standard: “Software for computers in the safety systems of 

nuclear power stations” 1st edition [11].
44

1.3.2 Additional reference standards and guidelines for safety systems in 

the member states represented in the task force

Belgium

– ANSI/IEEE-ANS-7-4.3.2, 2003 revision: “IEEE criteria for Digital Computers in Safety 

Systems of Nuclear power generating Stations” as endorsed by revision 2 of US NRC 

Regulatory Guide 1.152: “Criteria for programmable digital computer system software in 

safety related systems of NPPs”.

– IEC 61226, Edition 2 (2005) “Nuclear power plants – Instrumentation and control systems 

important to safety – Classification.”

– IAEA NS-G-1.1 “Software for Computer-Based Systems Important to Safety in Nuclear 

Power Plants.”

These are the standards applicable in most cases, but are not mandated by the regulator. 

Standards are selected on a case-by-case basis and the selection has to be justified in the safety 

demonstration.

Canada

– RD-337 Design of New Nuclear Power Plants, CNSC, November, 2008.

– RD/GD-369 Licence Application Guide, Licence to Construct a Nuclear Power Plants, 

CNSC, August 2011.

– IAEA Safety Standards Series No. NS-G-1.3, Instrumentation and Control Systems 

Important to Safety in Nuclear Power Plants.

– IAEA Safety Standards Series No. NS-G-1.1, Software for Computer Based Systems 

Important to Safety in Nuclear Power Plants.

– IEC 61513 Nuclear power plants - Instrumentation and control for systems important to 

safety - General requirements for systems, 2011.

– IEC 61226 Nuclear power plants - Instrumentation and control systems important to safety 

- Classification of Instrumentation and control functions, 2009.

– IEEE Std. 7-4.3.2-2010 IEEE Standard Criteria for Digital Computers in Safety Systems 

of Nuclear Power Generating Stations.

– CSA Standard N290.14-07 Qualification of pre-developed software for use in safetyrelated instrumentation and control applications in nuclear power plants, 2007.
45

Finland

– Guide YVL 1.0 Safety criteria for design of nuclear power plants, Helsinki 12.1.1996.

– Guide YVL 2.0 Systems design for nuclear power plants, Helsinki 1.7.2002. 

– Guide YVL 2.1 Nuclear power plant systems, structures and components and their safety 

classification, Helsinki 26.6.2000. 

– Guide YVL 5.5 Instrumentation systems and components at nuclear facilities, Helsinki 

13.9.2002. 

Germany

– RSK Guidelines, Chapter 7.6, amended August 1996.

– DIN IEC 61226, 3rd edition (2009).

– DIN IEC 62138 (2004).

– DIN IEC 60880 (2007).

– KTA. 3503, November 2005, Type testing of electrical modules for the safety related 

instrumentation and control system.

– VDI/VDE 3527 Design criteria serving to ensure independence of I & C safety functions, 

July 2002.

– VDI/VDE 3528 Requirements of repetition parts and criteria for their use in the 

instrumentation and control to safety in nuclear power plants , Part 1, Aug. 2011.

Spain

– UNESA CEN-6 “Guía para la implantación de sistemas digitales en centrales nucleares”,

Revision 0, May 2002.

– UNE 73-404-91. “Garantia de la Calidad en los Sistemas Informaticos Aplicados a 

Instalaciones Nucleares”.

– CSN 10.9 Safety Guide. “Garantia de Calidad de las Aplicaciones Informaticas 

Relacionadas con las Instalaciones Nucleares”.

– ANSI/IEEE-ANS 7-4.3.2. “IEEE Criteria for Digital Computers in Safety Systems of 

Nuclear Power Generating Stations”, as endorsed by USNRC Regulatory Guide 1.152 

“Criteria for Programmable Digital Computer System Software in Safety Systems of 

NPPs”.

– RSK Guidelines, Chapter 7.6, amended August 1996.
46

– CSN Safety Instruction IS-21 on the requierements applicable to modifications at nuclear 

power plants.

– CSN Safety Instruction IS-27 on general nuclear power plant criteria.

– CSN Safety Instruction IS-26 on basic nuclear safety requierements applicable to nuclear 

instalations.

Sweden

– IAEA Safety Guide NS-G-1.1

– IEC 61226

– IEC 61513

The above standards are applicable in most cases, but are not mandated by the regulator. 

Standards are selected on a case-by-case basis and this selection shall be fully justified in the 

safety demonstration.

UK

– NS-TAST-GD-046 Nuclear Safety Technical Assessment Guide “Computer Based 

Systems”.

– IAEA Safety Guide NS-G-1.1 “Software for Computer Based Systems Important to Safety 

in Nuclear Power Plants”.

– IEC 61226:2009 (3rd edition) “Nuclear power plants - Instrumentation and control 

important to safety - Classification of instrumentation and control functions”.

– IEC 61513:2011 “Nuclear Power Plants – Instrumentation and control for systems 

important to safety – General requirements for systems”.

1.3.3 Related Standards, Guidelines and Technical Documents

Guides and standards are continuously in evolution. At the time of writing this report, the

International Atomic Energy Agency (IAEA) and the IEC have issued a number of relevant 

reference documents.

In 2002 and 2000, the International Atomic Energy Agency (IAEA) published the safety 

guides entitled “Instrumentation and control systems important to safety in nuclear power 

plants” (NS-G-1.3) and “Software for Computer-Based Systems Important to Safety in 

Nuclear Power Plants” (NS-G-1.1), both giving guidance on the evidence to be provided at 

each phase of the software lifecycle to demonstrate the safety of the computer based system. 
47

The intention is to assist the licensing process by the provision of evidence. Recently, these 

two safety guides have been reviewed in order to be merged in a new draft IAEA safety guide 

“Design of Instrumentation and Control Systems for Nuclear Power Plants” (DS 431). 

In addition, the IAEA produced and published technical documents concerning distinct safety 

and licensing aspects of digital systems for nuclear installations, such as “Safety Assessment 

of Computerized Protection Systems” (IAEA-TECDOC-780, 1994), “ESRS Guidelines for 

Software Safety Reviews: Reference document for the organization and conduct of 

Engineering Safety Review Services (ESRS) on software important to safety in nuclear power 

plants” (IAEA Services Series No. 6, 2000), “Harmonization of the licensing process for 

digital instrumentation and control systems in nuclear power plants” (IAEA-TECDOC-1327, 

2002), “Solutions for Cost Effective Assessment of Software Based Instrumentation and 

Control Systems in Nuclear Power Plants” (IAEA TECDOC Series No. 1328, 2002), and the 

Technical Report 397 “Quality Assurance for Software important to safety which addresses 

graded requirements for software.”

The IEC 61513 standard “Instrumentation and control for systems important to safety –

General requirements for systems” (2001) covers the system aspects of computer based I&C. 

It acts as a link document for a number of standards, including IEC 60880 “Nuclear power 

plants – Instrumentation and control systems important to safety – Software aspects for 

computer based systems performing category A functions” (second edition 2006), IEC 60987 

“Nuclear power plants – Instrumentation and control important to safety – Hardware design 

requirements for computer-based systems” (second edition 2007), and IEC 61226 

“Classification of instrumentation and control functions” (second edition 2005). The IEC 

61513 standard contains the top level requirements on system functions, architecture and I&C 

system design which are of particular importance to the software specification. The 3rd edition 

was published in 2011 in order to take into account new developments in software 

engineering. 

The second edition of IEC 61226 contains criteria to rank I&C functions in accordance with 

their significance to safety. It comprises main requirements on I&C functions which are 

attached to three safety categories (A, B, and C). The third edition (2009) defines the nonhazardous stable state and specifies the criteria to assign safety functions to categories in more 

detail.

The second edition of IEC 60880 (2006), comprises both the first edition issued 1986 and its 

supplement issued 2000 with updated requirements covering the whole software life cycle 

(described in IEC 61513). The new edition also contains informal annexes on different special 

software qualification aspects such as defence against common cause failures, tools for 

software development and qualification, as well as requirements on pre-existing software. 
48

The IEC 62138 standard “Software aspects for computer-based systems performing category 

B or C functions” (2004) contains graded requirements for software implementing category B 

and C functions (in the terminology of IEC 61226).

The IEC 62340 standard “Nuclear power plants – Instrumentation and control systems 

important to safety – Requirements for coping with common cause failure” (2007) contains, 

amongst others, distinct requirements on software, eg linked to software robustness, tolerance 

against postulated latent failure software errors, as well as software maintenance and security. 

The second edition of the IEC 60987 standard contains reduced system and project level 

requirements. It also contains additional requirements on topics such as pre-developed 

hardware equipment from a generic platform, new function-based statements on the single 

failure criteria, as well as guidance on the use of advanced hardware designs with embedded 

micro-codes.

In the mid 1980’s IEC set up two working groups to produce a standard on programmable 

electronic systems. This international standard is now known as IEC 61508 and is entitled 

“Functional Safety: Safety-related systems”. The standard covers the determination of safety 

integrity levels (SILs) and the means of achieving these levels in terms of requirements on 

I&C systems, hardware and software. The standard contains seven parts covering, for 

example, management, hardware and software requirements. Based on the SILs, part 3 

contains graded requirements on the software qualification. 

IEC regards IEC 61508 as a generic standard with the intention that, using IEC 61508 as a 

basis, the various industry sectors will produce their own specific standards. This has been 

recognised by the IEC working group producing the nuclear standards (see above) and is being 

accommodated. The second editions of IEC 61508 Part 1 “General Requirements” and Part 3 

“Software Requirements” were published in 2010.

More information on the current work and publications of the IAEA and IEC can be found on 

their websites:

– http://www-pub.iaea.org/MTCD/publications/series1.asp

– http://www.iec.ch/ourwork/iecpub-e.htm

* * *
49

1.4 Pre-existing Software (PSW)

1.4.1 Rationale

The inclusion of appropriately verified pre-existing software (PSW) components into a 

computer based system may not only be beneficial for productivity but, if applied correctly,

may also increase confidence that the system is safe. The benefit stems from the premise that 

PSW components have often been used in many applications, and their operating experience, 

when assessable and representative, can be taken into account. Reusable software components

that are adequately specified, verified and proven for safety critical applications in other 

industries may be available and reusable in the nuclear industry. Proper application of such 

components, supported with appropriate justification, may reduce the assessment effort. 

Licensees may wish to make use of such software, along with its previous assessment and 

with contributing evidence such as independent verification records and documented operating 

experience.

This guidance on pre-existing software components is intended to enable their use in a safetycritical system without complete re-engineering of the PSW components or compromising 

system safety. Examples of PSW components are: real-time operating system services; device 

drivers or interfaces to hardware devices; mathematical libraries; timing monitors; and 

application logic subroutines.

1.4.2 Issues involved

1.4.2.1 The functional and non-functional behaviour of the PSW is often not clearly 

specified and documented. 

1.4.2.2 It is doubtful that evidence will be available to demonstrate that the PSW has been 

developed and produced in accordance with a defined safety life cycle such as outlined in IEC 

60880.

1.4.2.3 The documentation and data on operational experience of the PSW are often not 

adequate enough to provide the evidence which would be required to compensate for the lack 

of knowledge on the PSW product and on its development process.

1.4.2.4 As a result of issues 1.4.2.1, 1.4.2.2 and 1.4.2.3, acceptance criteria and procedures 

of investigation for demonstrating fitness for purpose of PSW for a specific application may 

be difficult to put in place.
50

1.4.2.5 The operational experience related to the PSW may not be in exact correspondence 

with that of the intended application. Therefore, software paths of unknown quality may be 

invoked by the application.

1.4.3 Common Position

1.4.3.1 The functions that have to be performed by the PSW components, and the other 

properties allocated to them, shall be clearly and unambiguously specified.

1.4.3.2 Evidence shall be provided that the requirement specifications allocated to the PSW 

comply with the safety requirements of the system, and may include the results of a system 

hazard and failure analysis. Special attention shall be paid to possible side-effects and to 

failures that may occur at the interfaces between the PSW and the user and/or other software 

components.

1.4.3.3 The PSW components to be used shall be clearly identified including their code 

version(s).

1.4.3.4 The interfaces through which the user or other software invokes PSW items shall be 

clearly identified and thoroughly validated. Evidence shall be given that there is no other 

means of accessing the PSW module, even inadvertently.

1.4.3.5 The PSW shall have been developed and shall be maintained according to good 

software engineering practice and QA standards appropriate to its intended use.

1.4.3.6 For safety systems, the PSW shall be subjected to the same assessment (analysis 

and review) of the final product (not of the production process) as new software developed for 

the application. If necessary, reverse engineering shall be performed to enable the full 

specification of the PSW to be evaluated.

1.4.3.7 If modifications of PSW components are necessary, the design documentation and 

the source code of the PSW shall be available.

1.4.3.8 The information required to evaluate the quality of the PSW product and of its 

assessment and development processes shall be available; this information shall be sufficient 

to assess the PSW to the required level of quality.
51

1.4.3.9 For acceptance the following actions shall be taken: 

1.4.3.9.1 Verify that the functions performed by the PSW meet all of the requirements 

expressed in the safety system requirement specifications and in other applicable software 

specifications.

1.4.3.9.2 Verify that the PSW functions that are not required by the safety system

requirement specifications cannot be invoked and adversely affect the required functions, 

for example through erroneous inputs, interruptions, and misuses.

1.4.3.9.3 Verify that, if PSW functions are omitted, this omission has no adverse impact 

on safety. 

1.4.3.9.4 Perform a compliance analysis of the PSW design against the applicable 

standards requirements (eg IEC 60880).

1.4.3.9.5 The PSW functions intended for use shall be validated by testing. The tests 

may include tests performed by the vendor.

1.4.3.10 If credit is given to feedback experience in the licensing process, sufficient 

information on operational history and failure rates shall be available. Feedback experience 

shall be properly evaluated on the basis of an analysis of the operating time, defect reports and 

release history of systems in operation. This feedback experience shall also be based on use of 

the PSW under evaluation in identical operational profiles. This operating experience shall be 

based on the last release except if an adequate impact analysis shows that previous experience 

based on unchanged parts of the PSW is still valid because these parts have been unaffected 

by later releases.

1.4.3.11 Defects that are found during the validation of the PSW shall be analysed and taken 

into account in the acceptance procedure.
52

1.4.4 Recommended Practices

1.4.4.1 Operational experience may be regarded as statistically based evidence 

complementary to validation, or to the verification of system software (operating systems, 

communication protocols, standard functions).

1.4.4.2 Data for the evaluation of the credit that can be given to feedback experience should

be collected, in terms of site information and operational profiles, demand rate and operating 

time, error reports and release history.

Site information and operational profile data should include:

– Configuration of the PSW;

◦ Functions used;

◦ Types and characteristics of input signals, including the ranges and, if needed, rates of 

change;

◦ User interfaces;

◦ Number of systems.

– Demand rate and operating time data should include:

◦ Elapsed time since first start-up;

◦ Elapsed time since last release of the PSW;

◦ Elapsed time since last severe error (if any);

◦ Elapsed time since last error report (if any);

◦ Types and number of demands exercised on the PSW.

– Error reports should include:

◦ Descriptions and dates of errors, severity;

◦ Descriptions of fixes.

– Release history should include:

◦ Dates and identifications of releases;

◦ Descriptions of faults fixed, functional modifications or extensions;

◦ Pending problems.

The data referenced above should be recorded along with the identification of the release of 

the PSW and its associated configuration.

1.4.4.3 The PSW functions should be prevented from being used in ways that are different 

from those which have been specified and tested, through the implementation of preconditions, locking mechanisms or other protections.

* * *
53

1.5 Tools

1.5.1 Rationale

The use of appropriate software tools can increase software product quality by improving the 

reliability of the software development process and the feasibility of establishing the product’s 

fitness for purpose. The use of automated tools reduces the amount of clerical effort required, 

and hence the risk of introducing errors in the development process. Tools can automatically 

check for adherence to rules of construction and standards, generate proper records and 

consistent documentation in standard formats, and support change control. This assures 

process reliability which is important for achieving product quality.

Different types of tools can be used that affect the correctness of the software:

– Requirements engineering tools used for: eliciting, capturing, specifying and modelling 

(unambiguously representing) requirements - including design constraints and 

assumptions; generating test cases; and assisting traceability.

– Design tools for transforming requirement specifications and constraints into architectural 

and detailed design models.

– Transformation tools such as code generators and compilers, that transform a work 

product at one level of abstraction into another, usually lower, level of abstraction.

– Verification and validation tools such as test environments, computer equipment for 

periodic testing, static code analysers, test coverage monitors, theorem proving assistants 

and simulators.

– Tools to check for adherence to rules of construction and standards.

– Tools to generate proper records and consistent documentation in standard formats.

– Tools to support change control, version control and configuration management.

– Service tools used to produce, modify, display and output software objects in design and 

assessment. They include graphic editors, text editors, text formatters, communication 

drivers, print spoolers, window arrangement software, etc.

– Infrastructure tools such as development operating systems, public tool interfaces and 

version control tools. These tools are potential sources of common errors, as they may 

interfere with other tools or corrupt intermediate software products.
54

1.5.2 Issues Involved

1.5.2.1 The production of software may be adversely affected by the use of tools in several 

ways. For example, transformation tools may introduce faults by producing corrupted 

outputs; and verification tools may fail to reveal faults that are already present, or may be 

unable to reveal certain types of faults. In addition, faults introduced by tools may lead to 

common cause failures.

1.5.2.2 Those using the tool require competencies that may be new and cognitive 

capabilities that may not be well-understood. The assumptions built into the tool or 

limitations on its application, eg its valid input range, may not be explicit or even determined.

1.5.2.3 Often, work products from different lifecyle phases are in different languages and 

tool environments, and their semantics are not completely defined. Tools transforming these 

work products will then rely on assumptions about the source and target languages. With 

multiple transformation stages, the uncertainties combine in unknown ways.

1.5.2.4 Where tools are used for single programs or software components, the integration 

of these components sometimes requires manual changes to the tool outputs. Verification of 

these changes may be outside of the normal tool output verification process. Furthermore, 

manual changes and their verification may require knowledge about the tool’s operation or 

other outputs that is hard to acquire.

1.5.2.5 It is not easy to determine the required level of tool qualification. Essentially, it 

will depend on the safety class of the system for which the tool is being used, on the 

consequence of an error in the tool, on the probability that the fault introduced by the tool will 

cause a safety significant error in the software being developed, and on the possibilities of the 

error remaining undetected by other tools or verification steps. Qualification standards and 

procedures are not always mature. Manual verification of the tool output is difficult and 

introduces additional uncertainties.

1.5.2.6 In the current state of the art in the certification and qualification of tools, 

complementary evidence, such as independent checks of the tool outputs, is also necessary.
55

1.5.3 Common Position

1.5.3.1 The qualification requirements for all tools used for the development, operation and 

maintenance of the software shall be determined and the requirements of this section 1.5.3

applied accordingly (see 1.5.1 for example tools). The available information shall be 

sufficient to assess the appropriateness of the tools.

1.5.3.2 The qualification requirements for an individual tool will be a function of its impact 

on the target software, and of how other tools or processes may or may not detect faults 

introduced by that tool. The combined set of tools shall be qualified to a level that preserves 

the required properties of the target software.

1.5.3.3 The manufacturer’s quality assurance documentation of tools used in the 

development of safety system software shall be available to the regulator.

1.5.3.4 If a different version of the tool on which the safety demonstration depends is 

considered for use, then the changes to the tool shall be analysed for their impact on the safety 

demonstration.

1.5.3.5 An analysis shall be applied to the transformation tools used in the software 

development process to identify the nature of the faults they can introduce in the target 

software, and their consequences.

1.5.3.6 For safety system software, only tools that are validated against the requirements 

defined in common position 1.5.3.2 here above shall be used. Safety system software 

produced by a transformation tool shall be subjected to verification, and to validation on the 

target system. The use of a transformation tool for safety system software without further 

review of its output shall not be accepted unless special justification is provided.

1.5.3.7 For safety system software, only tools that are validated against the requirements 

defined in common position 1.5.3.1 here above shall be used. Safety system software 

produced by a transformation tool shall be subjected to verification, and to validation on the 

target system. 

1.5.3.8 The use of a transformation tool for safety system software without further review 

of its output shall not be accepted unless special justification is provided. In particular, this 

justification shall include evidence

– that only input data that is syntactically valid can be accepted by the tool for 

transformation, and 

– that the tool preserves the semantics of valid input to the last output stage of the 

transformation.
56

1.5.3.9 The vendor of a tool shall maintain and update the tool experience feedback and 

inform users of all anomalies discovered (including those discovered by users).

1.5.3.10 The strategy for tool use and validation shall be defined in the software quality plan.

1.5.3.11 The programming techniques which are used in combination with transformation 

tools shall comply with the relevant requirements of chapter 2.4.

1.5.4 Recommended Practices

1.5.4.1 To validate a tool, the following possibilities are recommended:

– the output of a tool can be verified by inspection;

– the behaviour of the output of the tool can be verified by running the output on the target 

system or on a simulator of this system;

– the tool itself can be certified as processing correctly all valid statements and combinations 

thereof contained in the definition of a programming language;

– the tool can be validated by appropriate profiles and a sufficient amount of previous 

operational experience.

1.5.4.2 When formal methods are applied manually, they require the involvement of well 

trained humans. In addition, application of these techniques is likely to be affected by 

mistakes. Therefore these formal methods should be supported by tools.

1.5.4.3 The tools should be incorporated into an integrated project support environment to 

ensure proper control and consistency. This environment should include suitable data 

dictionaries which check for inconsistencies in the data base, ensure the correctness of values 

of items entered and enable data to be interrogated. In addition, it should support the 

exporting and importing of data to and from other sources.

1.5.4.4 A software tool should be independent from the operating system so as to minimise 

the consequences of having to use new versions of this system.
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1.5.4.5 Tools of reasonable maturity should be used to support the process of specification 

and design, namely:

– tools for editing and type-setting;

– tools for syntax-checking, data type-checking and consistency-checking;

– tools to prove properties of specifications and discharge proof obligations in refining 

specifications;

– tools to animate formal specifications;

– tools to derive usable software components automatically from formal specifications;

– configuration control tools to relate specifications to development and to keep track of the 

level of verification of each step;

– Static and dynamic analysis tools to complement code inspections and walkthrough 

techniques.

1.5.4.6 Reverse engineering tools should be used to validate the output of transformation 

tools or the output of manual programming processes.

* * *

59

1.6 Organisational Requirements

1.6.1 Rationale

Well structured and effective organisations, ie licensees, their subcontractors and suppliers, 

are regarded as necessary in the management of all aspects of the safety system lifecycle to 

reduce the faults introduced into a system, and to ensure that those faults which are found are 

handled properly. Throughout the lifecycle, clear delineation of responsibilities and effective 

lines of communication plus proper recording of decisions will minimise the risk of incorrect 

safety system behaviour. An additional important aspect of an effective management system 

is the development of a safety culture which, at all levels within the organisation, emphasises 

safety within the organisation. By the use of managerial supervisory and individual practices 

and constraints, the safety culture sustains attention to safety through an awareness of the risk 

posed by the plant and on the potential consequences of incorrect actions.

1.6.2 Issues involved

Safety Culture

1.6.2.1 If staff is not made aware of the risks posed by the plant and the potential 

consequences of those risks then project pressures, inattention to detail and slipshod work may 

result in the safety system not being fit for purpose. More significantly, this unfitness may not 

be revealed. However, it has to be recognised that although a safety culture is necessary, it is 

not regarded, on its own, as sufficient for developing a safety system that is fit for purpose.

Delineation of Responsibility

1.6.2.2 If responsibilities are not clearly defined then potentially unsafe activities or system 

aspects may not be given their due attention resulting in an increased plant risk.

Staffing Levels 

1.6.2.3 Inadequate staffing levels may result in a drastic deterioration in the safety culture 

due to staff being overworked and demoralised. Thus, safety issues may not be explored 

properly and inadequate designs may result.
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Staff Competencies

1.6.2.4 If staff do not have the appropriate level of training and experience for the tasks 

they are undertaking within the system lifecycle then it is likely that errors will be introduced. 

In addition, errors may not be detected and the design may not have the fault tolerance 

required to prevent failure of the computer-based safety system.

Project Pressures

1.6.2.5 Project pressures, both financial and temporal, might mean that the safety 

provisions are inadequate, or that there is insufficient attention to detail, resulting in a safety 

system that is not fit for purpose.

1.6.3 Common Position 

1.6.3.1 Only companies with demonstrable capabilities and competence in the development 

of safety systems shall be used in all stages of the development life-cycle. 

1.6.3.2 The licensee, its suppliers and subcontractors shall each provide evidence of the 

following:

1.6.3.2.1 Safety Culture: 

A written safety policy shall be available demonstrating a commitment to a safety culture 

which enhances and supports the safety actions and interactions of all managers, personnel 

and organisations involved in the safety activities relating to the production of the safety 

system.

1.6.3.2.2 Delineation of Responsibility: 

The responsibilities of, and relationships between, all staff and organisations involved in 

all aspects of the safety system lifecycle shall be documented and clearly understood by all 

concerned.

1.6.3.2.3 Staffing Levels:

It shall be demonstrated, for example by means of appropriate work planning techniques, 

that levels of staffing are adequate to ensure that the safety system’s fitness for purpose is 

not put in jeopardy through lack of staff effort.

1.6.3.2.4 Staff Competencies: 

Staff shall have the appropriate level of training and experience for the tasks they are 

undertaking within the safety system lifecycle. Suitable evidence shall be available for 

inspection that demonstrates that safety proficiency is achieved and maintained.
61

1.6.3.3 Before a system is taken into operation, the licensee should document the methods 

to be employed (change control, configuration management, maintenance, data entry etc.) to 

ensure that the required level of integrity of the safety system will be maintained throughout 

its operational life.

1.6.3.4 Project programme recovery strategies shall only be undertaken provided the 

requirements of the safety system remain achievable.

1.6.4 Recommended Practices

1.6.4.1 Adequate procedures should be in place for controlling contract documentation 

between customers and suppliers to ensure that all specification and safety-critical contractual 

requirements are properly controlled.

1.6.4.2 The licensee should provide evidence that he is monitoring adequately the safety 

aspects of any contract involving a safety system.

1.6.4.3 Project/Operational Pressures:

A licensee safety department staffed by personnel with the appropriate computer competencies 

and independent from the project team (for plant under construction or modification) or from 

the operating department, should be appointed to reduce the probability (through regular 

monitoring of the activities associated with the safety system) that project or operational 

pressures will jeopardise the safety systems fitness for purpose. This department should 

ensure that the appropriate standards, procedures and personnel are used at all stages of the 

software lifecycle to meet the safety requirements of the computer-based system. This 

department should be independent from all project and operational staff such as designers, 

implementers, verifiers, validaters, and maintainers.

1.6.4.4 Regular progress reports from the licensee safety department or from an appropriate 

body within the licensee’s organisation should be available for inspection throughout the 

project lifecycle.

* * *

63

1.7 Software Quality Assurance Programme and Plan

1.7.1 Rationale

It is widely accepted that the quality of software cannot be guaranteed by inspection or by 

testing of the product alone. In general, the application of a good quality assurance system 

provides essential evidence of the quality of the end product. Software is no different: 

confidence in the safety of the final software based system increases with the use of quality 

assurance techniques. This is particularly true for any non-trivial software product since it 

cannot be exhaustively tested and interpolation between test inputs is not possible due to the 

digital nature of the system (a situation that does not pertain to analogue systems).

Additionally, software quality assurance (SQA) aids the development of a convincing safety 

demonstration because it is a structured and disciplined method of ensuring that there is 

sufficient auditable evidence.

1.7.2 Issues Involved

1.7.2.1 It is recognised that the reliability of a computer-based safety system cannot be 

demonstrated by testing. Therefore, the demonstration of safety has to depend to some degree 

on the quality of the processes involved. The major strength of a SQA plan and programme is 

that it ensures the procedures and controls are in place to govern and monitor the system 

lifecycle thus providing an auditable record of the production process and of the operational 

life of the system.

1.7.2.2 The SQA programme and plan do not ensure that the design solution is the most 

appropriate unless there are specific requirements in the specification for design reviews by 

appropriate personnel.

1.7.2.3 SQA audits, because of their sampling nature do not always detect non-compliance 

with procedures and standards.

1.7.2.4 In the majority of cases, the more complex the system and the larger the project, the 

more important SQA becomes, because of the ensuing increase in the functional and 

organisational complexity.
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1.7.3 Common Position

1.7.3.1 The Software quality assurance shall cover all aspects of the system lifecycle.

1.7.3.2 A reviewable quality assurance plan and programme for the software aspects of the 

safety system, covering all stages of specification, design, manufacture, V&V, installation, 

commissioning, operation and maintenance shall be produced at the beginning of the project. 

This shall form part of the evidence that the project is being properly controlled. SQA will 

ensure that the functional behaviour is traceable throughout the system lifecycle and that the 

behaviour has been adequately demonstrated through testing to the required standards.

1.7.3.3 The properties addressed in the specification of the non-functional requirements

(see 1.1.1 and 2.1.3.2.2) shall be considered in the SQA programme and plan through a 

specific statement on how conformance with the requirements is to be established.

1.7.3.4 The SQA programme and plan shall include a description of the organisational 

structure, staff competencies and adequate procedures covering activities such as defect

reporting and corrective action, computer media control (production and storage), testing, 

supplier control. Record keeping and documentation shall be available to demonstrate 

adequate control of life cycle activities.

1.7.3.5 The SQA programme and plan shall make reference to standards and written 

procedures to control the activities described in 1.6.3.3. These standards and procedures shall 

also act as the focus for periodic, independent audits to confirm compliance (this is considered 

to be an important aspect, since well documented audits with corrective action notices fully 

resolved provide a valuable source of evidence on the adequacy of the lifecycle processes).

1.7.3.6 The SQA programme and plan shall be accessible to the regulator for review and 

acceptance, if required. 

1.7.3.7 There shall be SQA audits of compliance with the agreed SQA plan. A justification 

shall be provided for the frequency of SQA audits to be performed by the specialist software 

auditors. 

1.7.3.8 The performance of appropriate reviews shall be required by the SQA process to 

ensure that due consideration is given to the completeness and correctness of documents.

1.7.3.9 SQA shall ensure that the tests are systematically defined and documented to 

demonstrate that adequate test coverage has been achieved and that the tests are traceable to 

the system requirement specifications. When faults are found, SQA shall ensure that 

procedures are in place for addressing those faults, and also for ensuring that corrections are 

properly implemented.
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1.7.4 Recommended Practices

1.7.4.1 It is highly recommended that an agreement be reached at the beginning of the 

project between the licensee and the licensor on the SQA programme and plan.

1.7.4.2 Whenever possible, audits should be performed using aids and tools which assist in 

identifying compliance with standards and procedures, and record anomalies and corrective 

action notices. 

1.7.4.3 The standard IEEE Std 730.1-2002, “Standard for Software Quality Assurance 

Plans” is recommended.

* * *

67

1.8 Security

1.8.1 Rationale

In the context of this document, the objective of information and system security is to 

guarantee and preserve the safety of computer-based systems by preventing security incidents 

or by minimising their impacts. Security seeks to prevent unauthorised accesses to 

information, software and data in order to ensure that three attributes are met, namely:

– the prevention of disclosures that could be used to perform mischievous, malicious or 

misguided acts which could lead to an accident or an unsafe situation (confidentiality), 

– the prevention of unauthorised modifications (integrity), 

– the prevention of unauthorised withholding of information, data or resources that could 

compromise the delivery of the required safety function at the time when it is needed 

(availability). 

All systems important to safety in a licensed nuclear installation, be they conventional or 

computer-based, need to be protected from unauthorised access since such access could result 

in a system mal-functioning due to either intentional or unintentional interference. The 

consequence of such interference could be that an accident might be triggered or the system 

might fail to perform its intended protective action. The restriction of physical access through 

the use of locked doors and key interlock systems has been the traditional method, and still 

remains a major contributor to preventing unauthorised access to systems important to safety. 

However, with the introduction of computer based systems important to safety, there are now 

a number of security issues that are unique to the technology due to its programmability and 

communication facilities, and which need to be addressed.

In addition, there are other computer systems such as maintenance scheduling systems, fuel 

burn-up calculators and other computer codes which are used off-line in support of operational 

safety, computers used in support of emergency arrangements and dosimetry record systems, 

that could indirectly affect plant or individual health and safety, and that must be considered.
68

1.8.2 Issues Involved

1.8.2.1 So far as is reasonably practicable, there should be no significant increase in overall 

plant risk from computer based systems important to safety in a nuclear installation due to:

– intentional or reckless interference or misuse of these systems,

– well intentioned, but misguided, use by authorised persons of these systems,

– the security provisions themselves.

1.8.2.2 Information can be stored in computer central memories or mass storage devices,

can be printed, and can be transmitted by various networks or movable media. This means 

that there is the potential for intentional or unintentional impairments and misuses by 

company employees or members of the public, authorised or not. Additionally, specific 

threats also exist from hackers, viruses, software logic bombs, software time bombs, Trojan 

horses, and other similar mechanisms.

1.8.2.3 Interfaces and data exchanges between systems important to safety and off-line 

maintenance systems are a security issue to be taken into account. Loading parameter values 

or calibration data and periodic testing equipments are examples of the use of such interfaces. 

1.8.2.4 Information has to be protected by taking into account the different storage and 

communication media which are used, not only at the plant but also at a supplier site.

1.8.3 Common Position

The licensee of a nuclear installation shall adopt, as a minimum the following requirements.

1.8.3.1 General organisation

1.8.3.1.1 For all computer based systems important to safety, as a baseline, the security 

provisions shall incorporate the standard security measures that are regarded as good 

commercial practice against threats coming from other computer based systems, unauthorised 

access, viruses, software logic bombs, Trojan horses, and other similar mechanisms.

1.8.3.1.2 The security provisions which address the procedures and the environment of the 

design and development of a computer based system important to safety shall be 

commensurate with the class of the target system and the maximum consequences of a 

successful cyber attack on this system in terms of plant safety.

1.8.3.1.3 The procedures and equipment for loading software into a system important to 

safety shall be included in any security evaluation.
69

1.8.3.1.4 Where systems important to safety interface to computer-based support systems, 

for instance for the updating of calibration data or plant maintenance activities, then, as a 

minimum, good security practices shall be observed. Examples of such practices are: user 

identification and password protection, the installation of virus protection software (if 

relevant), and the standard physical restriction of access to only authorised persons.

1.8.3.1.5 Access shall be restricted (for example through the use of password protection and 

key lock systems) to only those parts of a system to which a person has authority.

1.8.3.1.6 A document on security shall be drawn up so as to identify the security risks, 

threats and vulnerabilities of the safety system. This document shall be used to derive the 

security design requirements of the computer-based safety system.

1.8.3.1.7 In this document allocation of information security responsibilities shall be 

defined. In particular, the licensee shall define security requirements that its contractors and 

service providers have to satisfy.

1.8.3.1.8 Computer security incidents shall be reported to the regulatory authority.

1.8.3.1.9 Special security attention shall be taken during vulnerable phases, for example, on 

the occasion of software or hardware changes.

1.8.3.1.10 For safety systems, security specialists in association with the safety specialists 

shall evaluate the adequacy of the security provisions.

1.8.3.2 Physical security measures

1.8.3.2.1 For safety systems, security threats shall be avoided by design where possible.

1.8.3.2.2 The safety system software (programs and fixed data, including operational data) 

shall be held in suitable fixed read only memory. A systematic comparison of this read only 

memory code with the source code shall be performed for the purpose of detecting 

unauthorised code.

1.8.3.2.3 No inward data transmission link into the safety system shall be allowed except the 

direct connection required by the specified local maintenance and testing equipment. These 

systems and other specified support systems shall have restricted access, for example, through 

the use of passwords and key locks.

1.8.3.2.4 Direct links from the safety system to equipment outside the plant shall be 

prohibited.
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1.8.4 Recommended Practices

1.8.4.1 To provide basic security protection, for all plant computer based systems, standard 

security measures should be implemented that are regarded as good practice against threats

and vulnerabilities.

1.8.4.2 All changes of constants and parameters should be automatically recorded in a 

secure manner for subsequent auditing.

1.8.4.3 All authorised accesses should be recorded and all security attacks should be 

reported (automatically where possible) and investigated.

1.8.4.4 Education on the security policy should be imposed to all persons involved.

1.8.4.5 Constant vigilance is required. The security policy should address the security issue 

through appropriate training programmes and specifically targeted campaigns.

* * *
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1.9 Formal Methods

“Saber y saberlo demonstrar es valer dos veces”

Baltasar Gracián

1.9.1 Rationale

“Formal methods” is a general and rather confusing term which is used to refer to various 

methods for modelling systems, expressing specifications, programming, analysis, proof, 

notations, V&V, and even design and code generation. In this document, the term will be 

used to refer to the use of mathematics and logic to describe systems and designs in a way that 

makes their mathematical or logical analysis possible. The terms correctness, consistency, 

soundness, and completeness are used in this chapter with their classical meaning in 

mathematical logic.

In practice such methods offer the possibility of establishing precise and unambiguous 

specifications, thus helping in a – sometimes not easy – understanding of specifications. The 

availability of such precise descriptions enables analysis, verification, and sometimes proof. 

Another effect of these methods is to increase the possibility of more systematically or even 

automatically refining the specifications (levels of design), towards executable or high-level 

language code. The latter possibility, however, is often confused with the automatic 

generation of code (compilation) from high level application oriented programming languages.

Precise models of system specifications also allow animation. Animations may be used for 

checking certain aspects of the requirements and may provide a reference to test the final 

software. Mathematical methods may also be used for the validation of system requirements, 

as they can be used to show that certain properties are consequences of the specification, ie 

that programs which satisfy the specification will necessarily have these properties.

As a general remark, it is useful to remember that system design, verification and validation

are human activities which necessarily imply the use of abstract representations and 

descriptions. We cannot apprehend reality without the use of mental models. In particular, 

safety can only be discussed and anticipated as an (abstract) property of models of system 

structure and behaviour, of its interactions with the environment and of accident scenarios. 
72

1.9.2 Issues Involved

1.9.2.1 Potential benefits of formal methods may include:

– more systematic verification of system specifications for consistency and completeness;

– provision of more rigorous strategies for decomposition and design; 

– more systematic refinement of the system specifications towards descriptions from which 

source or object code can be generated;

– demonstration of syntactic correctness of software specifications;

– verification of a piece of code using mathematical proof or rigorous analysis techniques to 

show that it is a complete and correct translation of its specification;

– validation by using mathematical proof techniques to derive properties from the 

specification (e. g. proving that an unsafe state is not reachable);

– evidence that a module with a formally specified and verified behaviour can be 

incorporated into a larger software system in such a way that certain properties, eg safety, 

remain satisfied.

1.9.2.2 A useful introduction to formal methods and their use for the certification of critical 

systems can be found in [22].

1.9.2.3 Unfortunately, mathematical analysis has important limitations, which make its use 

problematic in practice. Each type of analysis can cover only specific aspects as no theory 

currently can combine all the features and properties found in a real system. In addition, 

mathematical analysis cannot establish the correctness of a system requirement specification, 

nor the intentions behind it. 

1.9.2.4 Moreover, when used inappropriately, formal methods may be dangerous as:

– their lack of legibility may lead to difficulties in understanding and verification, especially 

for plant specific application software which must be understood by different branches of 

engineering,

– no method is universal, and the impossibility of expressing some types of requirements 

important to safety (e. g. non functional requirements or aspects of real time behaviour) 

may lead to incompleteness or inconsistencies,

– they might be used by insufficiently trained personnel or without support of adequate 

tools.

1.9.2.5 In addition, the training effort required by the use of a formal method can be 

disproportionate to the benefits that can be expected.
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1.9.3 Common Position

1.9.3.1 No credit can be taken in a safety demonstration for the use “per se” of a formal

method without due consideration being given to the specific evidence brought in by this use, 

and to its contribution to the safety demonstration of the system.

1.9.3.2 Whatever (combination of) method(s) and notation(s) is used to describe the system 

requirements, this description shall be based on a definition of the system boundaries and on a 

systematic capturing of the functional and non-functional requirements of the system. These 

boundaries, requirements and associated properties shall be explicitly, unambiguously and 

accurately documented (see common position 2.1.3.2).

1.9.3.3 Selection of methods and tools with respect to their intended application for formal 

descriptions and mathematical analysis shall be justified. The justification shall be in 

accordance with the safety demonstration (see 1.1.3).

1.9.3.4 There shall be objective evidence of a successful use of the formalisms and 

methods used in an application with comparable properties.

1.9.3.5 The procedures and constraints for using the formal methods and tools shall be 

documented.

1.9.3.6 Any limitations of the formal descriptions, methods and tools used, and resulting 

descriptions, shall be explicitly documented. For example, any limitation in describing and 

reasoning about non-functional requirements, use of resources, or time critical events shall be 

stated.

1.9.3.7 The formalisms and the methods used for specifying the system requirements shall 

be unambiguous and understandable by all technical staff involved.

1.9.3.8 The formal description of the system requirements shall be validated against the 

results of a prior plant safety analysis, and of other relevant analyses at the plant level (see 

common position 2.1.3.1).
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1.9.4 Recommended Practices

1.9.4.1 Criteria for the choice of formal methods

1.9.4.1.1 Training courses and textbooks on the formalisms and methods in use should be 

available.

1.9.4.1.2 Tools should enable formal descriptions to be checked for consistency. Tools 

should enable the use of animation, so as to aid the checking of completeness.

1.9.4.1.3 The tools associated with the formal method should provide means for producing 

the necessary documentation for the project, including the ability to add natural language 

comments to the notation to aid understanding.

1.9.4.2 Verification when applying formal methods

1.9.4.2.1 Logical reasoning to justify properties of correctness, consistency and completeness 

should support the formal descriptions, and where appropriate mathematical analysis should 

be used to verify properties of input, output and dynamic behaviour of the items described. 

Checks on formal descriptions comparable to the checks performed by a good compiler on a 

program should also be performed when possible. 

1.9.4.2.2 Development methods based on application oriented graphical languages generally 

offer a means for connecting and configuring a set of predefined components in logic 

diagrams. For these methods, the relations between these predefined components in the logic 

diagrams should be verified to show that they satisfy a set of syntactical and semantical rules. 

For this purpose, simulation and/or animation tools should be incorporated.

1.9.4.2.3 Some development methods supported by a graphical notation, for example those 

which incorporate a structured design approach, offer top-down consistency checks between 

levels, type checking and automatic code generation. For these methods, additional 

verification (eg testing, inspections, static analysis) should still be performed in order to detect 

semantic faults.

* * *
75

1.10 Independent Assessment

1.10.1 Rationale

For a software-based safety system an independent assessment of the system is essential to 

provide the degree of confidence in the design process, in the product and in the personnel 

involved. The independent assessment ordered by the supplier of the system or by the licensee

can be regarded as an important part of the evidence in the safety demonstration. Independent 

assessment can also be an essential activity during modification of the system. 

Its purpose is to provide an objective view on the adequacy of the system and its software 

which is – as much as possible – independent of both the supplier and the user (licensee).

1.10.2 Issues Involved

1.10.2.1 Ensuring that the independent assessors are independent of project pressures,

which – depending on circumstances – may come from the development team, the supplier or 

from the user, in order that assessors are able to comment freely and frankly without being 

adversely influenced by these project pressures.

1.10.2.2 Ensuring that independence does not deprive the assessors from access to all 

relevant information and from the level of competence and familiarity with the system which 

is necessary to make an efficient analysis.

1.10.2.3 Making sure that the findings of the independent assessment are properly addressed 

and recorded, and, where changes are proposed, these are handled through the change 

management process.

1.10.2.4 Independent assessment may be generic or targeted to a given application. In the 

first case it may allow components to be pre-qualified and re-used in applications, the 

independent assessment of which may become easier. 

1.10.3 Common Position

1.10.3.1 The system and its safety demonstration shall be subjected to a documented review 

by persons who are:

– Competent (commensurate to the nature and complexity of the system);
76

– Organisationally independent of the supplier(s) of the system (and of its safety 

demonstration), and

– Not responsible for – or not involved in – the development, procurement and production 

chain of the system.

1.10.3.2 The area and depth of assessment shall be carefully considered and proposed in 

each case by the independent assessors, in order to be commensurate with the objective of 

confirming (to a high level of confidence) that the delivered/modified system will satisfy its 

safety requirements. 

1.10.3.3 Independent assessment of the product shall be undertaken on the final version of 

the software. The independent assessment should also involve examination of process 

activities and the resources on which these activities depend, eg people, tools, documentation.

1.10.3.4 Where the independent assessment reveals that certain functional or non-functional 

requirements or features of the system have not been adequately implemented, ensured or 

understood, the independent assessor shall identify and document them for response by the 

development team.

1.10.3.5 Safety questions raised by the independent assessors shall be resolved; the

assessor’s comments on the responses provided shall be a documented component of their 

final reporting.

1.10.3.6 The scope, the criteria and mechanisms of the independent assessment activity shall 

be the subject of an independent assessment plan. This plan shall be formulated as soon as 

possible in the project evolution, so that the necessary servicing of the associated activities can 

be properly accommodated and resourced as part of the management of the overall project 

programme. 

1.10.3.7 The independent assessment shall be agreed upon by all parties involved 

(regulator/licensee/supplier), so that the appropriate resources can be made available at the 

agreed time. 

1.10.3.8 The tools and techniques used by the independent assessor shall, where practicable, 

be different from those used in the development process and the system’s safety 

demonstration. Where it is necessary to use the same or similar tools the independent assessor 

shall recognise this in the independent assessment plan and propose a different approach to 

guard against a common failure of both the independent assessment and the development team 

to reveal faults within the software. 

* * *
77

1.11 Graded Requirements for Safety Related Systems

(New and Pre-existing Software)

1.11.1 Rationale

1.11.1.1 Software is increasingly used in many different nuclear applications. Not all of this 

software has the same criticality level with respect to safety. Three classes of systems have 

been defined and discussed in chapter 1.2: safety systems, safety-related systems and systems 

not important to safety.

1.11.1.2 It is recognised that for safety related systems’ software the requirements for 

demonstrating an adequate level of safety can be reduced.

1.11.1.3 Simplicity is required for safety systems. Safety related systems can be more 

complex. For these latter systems less information may be available on the development 

process and on the product. In certain cases, it might be possible to compensate for this lack 

of information – typical for pre-existing software of safety related systems – by using evidence 

provided by functional testing and adequate operational feedback. 

1.11.1.4 For the operational feedback, it is important to determine which data are to be 

collected, and how they must be collected and evaluated.

1.11.1.5 The requirements on safety related system software can sometimes be relaxed, 

based on the results of a failure mode analysis of the system, or on arguments or conditions of 

use which show that the reliability required from the software is low.

1.11.2 Issues Involved 

1.11.2.1 The safety classification of a computer based system and of its software is 

determined by the importance to safety of the functions it is implementing. 

1.11.2.2 The quality of a piece of software can neither be quantified, nor tailored to demand. 

Graded requirements are therefore not intended to define concessions and relaxations allowing 

lower quality standards of design and development for certain classes of software. As 

mentioned in common position 1.2.3.11, they are intended to determine which development 

process attributes are adequately required, and the amount of verification and validation

necessary to reach the appropriate level of confidence that the system is fit for purpose. It is 

therefore essential to identify and assess the importance to safety of the software as well as 
78

any graded requirement scheme intended to be used, as accurately and as early as possible in a 

new project.

1.11.2.3 As a general principle, and as already stated in common position 1.2.3.6, a 

computer based system important to safety and its software is a safety related system if its 

correct behaviour is not essential for the execution of safety functions (see also recommended 

practice 1.2.4.6).

1.11.3 Common Position

1.11.3.1 The safety class of a particular computer based system shall be determined 

according to the principles of common position 1.2.3.1. Graded requirements as defined in 

issue 1.11.2.2 cannot be invoked in the safety demonstration of a computer based system of 

any class without a justification for the use and adequacy of these graded requirements.

1.11.3.2 Exceptions to requirements for safety systems can be made for safety related 

systems, but must be justified. 

1.11.3.3 In order to evaluate the possibility of relaxing certain requirements of the safety 

demonstration, as a minimum, the consequences of the potential modes of failures of the 

computer based system shall be evaluated. For instance, a failure mode analysis may show 

that certain relaxations are possible, when failures of the system can be anticipated and their 

effects can be detected and corrected in time by other means. 

1.11.3.4 More generally, for safety related systems, the specific requirements of IEC 61226, 

2nd ed. section 7 can be taken into consideration. The relaxations to the prescriptions of IEC 

60880 and to quality assurance that are allowed by these sections shall be justified.

1.11.3.5 The common position 1.2.3.17 is applicable.

1.11.3.6 All common positions and recommended practices of this document which are 

explicitly stated for safety systems are either not enforced, or are enforced and can be relaxed 

on safety related systems. These positions and practices are those mentioned in:

Chapter 1.2 on system classes, function categories and graded requirements for software:

– Common position 1.2.3.16

– Recommended practice 1.2.4.3

Chapter 1.4 on pre-existing software:

– Common position 1.4.3.6
79

Chapter 1.5 on tools:

– Common positions 1.5.3.2, 1.5.3.3, 1.5.3.6

Chapter 1.6 on organisational requirements:

– Common positions 1.6.3.2.1, 1.6.3.2.2, 1.6.3.2.3

– Recommended practice 1.6.4.3

Chapter 1.8 on security:

– Common positions 1.8.3.1.6, 1.8.3.1.10 and all common positions in 1.8.3.2

Chapter 2.2 on computer system design:

– Common positions 2.2.3.6, 2.2.3.12

Chapter 2.4 on coding and programming directives:

– Recommended practices 2.4.3.1, 2.4.3.4.1, 2.4.3.4.2, 2.4.4.3.4

Chapter 2.5 on verification:

– Recommended practice 2.5.3.5.1

Chapter 2.6 on validation:

– Recommended practice 2.6.4.1

Chapter 2.7 on change control and configuration management:

– Common positions 2.7.3.1.2, 2.7.3.3.2, 2.7.3.3.3, 2.7.3.3.6, 2.7.3.4.3, 2.7.3.4.5

– Recommended practices 2.7.4.1, 2.7.4.2

Chapter 2.8 on operational requirements:

– Common positions 2.8.3.3.4, 2.8.3.4.2, 2.8.3.4.3, 2.8.3.5.1

– Recommended practices 2.8.4.2.1, 2.8.4.3
80

1.11.4 Recommended Practices

1.11.4.1 As mentioned in practice 1.2.4.4, it is recommended that the failure analysis 

required in common position 1.11.3.3 is performed on the system specifications, rather than at 

code level, and as early as possible in the project so as to reach an agreement between the 

licensee and the regulator on graded requirements and/or the requirements for the 

demonstration of safety.

Graded requirements for safety related systems

1.11.4.2 Design and V&V requirements addressing factors which affect indirectly the 

behaviour of programmed functions may be subject to possible relaxations: eg validation of 

development tools, QA and V&V specific plans, independence of V&V team, documentation, 

and traceability of safety related functions in specifications.

1.11.4.3 Table 2 and Table 3 give an example of classes and graded requirements which 

could be used in the demonstration of the safety of computer based safety related systems. 

Graded requirements are given for pre-existing software (Table 2) and for newly developed 

software (Table 3). In each case safety related system graded requirements are compared to

safety system graded requirements.

It is important to note that these example tables are not meant to provide a complete listing of 

safety related system and safety system graded requirements. 

Some of the principles on which these tables are founded are expressed in the rationale 

paragraph 1.11.1.3. Safety related systems can be more complex than safety systems, and less 

information may be available on their development process and on their software. In certain 

cases, it might be possible to compensate for this lack of information – typical of pre-existing 

software of safety related systems – by provision of evidence resulting from restrictions as to 

their conditions of use, an analysis of failures and consequences, functional testing and 

adequate operational feedback.

1.11.4.4 The following standards include sets of graded requirements:

– IEC 62138 includes graded requirements for the qualification of software executing 

functions of the categories B and C defined in IEC 61226;

– IEC 61508 includes graded requirements for different safety integrity levels.
Table 2: EXAMPLE OF COMPARATIVE CLASSES OF GRADED REQUIREMENTS FOR PRE-EXIS

(continued on next page)

Systems not important to safety Safety related systems 

Standards

– Recognised codes and guides – IEC 60

require

princip

Quality 

assurance

– Professional level – QA plan (generic) covering the whole life 

cycle

– Evidence of structured design approach

– Configuration and software maintenance

management after installation

– QA pla

coverin

– Structu

– Review

– Eviden

– Config

manag

Safety 

assessment

– Analysis of impact on safety of the 

software component potential failures

–

countermeasures

software CCF; defence and mitigation 

Analysis of risk and consequences of 

– Analys

softwa

–

softwa

Analys

counte

Development

– Complete, clear, non-ambiguous 

and auditable specifications

– Complete, clear, non-ambiguous and 

auditable specifications

–

in the specifications

Identifiability of safety related functions 

– Autocontrol functions (HW + SW) ∈

specifications

– Design/coding limiting faults

– Validated tools 

– Compl

auditab

– Identif

related 

–

the dev

Tracea

– Autoco

specifi

–

autoco

Demon

– Design

– Valida
82

Table 2: EXAMPLE OF COMPARATIVE CLASSES OF GRADED REQUIREMENTS FOR PRE-EXIS

(continued from previous page)

Systems not important to safety Safety related systems 

Verification/

validation

– Supplier’s V&V

– Site commissioning

– V&V plan

– Verification (chapter 2.5) at the end of 

development

– Independent validation (chapter 2.6)

– Site commissioning

– V&V p

– Verific

each d

– Indepe

– Indepe

– Site co

Documentation – Auditable documentation demonstrating 

satisfaction of requirements

– Integra

Operational 

feedback

– Relevant and documented operational 

feedback

– Releva

feedba
Table 3: EXAMPLE OF COMPARATIVE CLASSES OF REQUIREMENTS FOR THE DEVELOPMENT 

(continued on next page)

Systems not important to safety Safety related systems 

Standards

– Recognised codes and guides – IEC 880

requirem

principl

Quality 

assurance

– Professional level – QA plan (generic) covering the whole 

life cycle

– Evidence of structured design approach

– Evidence of experience of staff

– Configuration and software maintenance 

management after installation

– QA plan 

life cyc

– Structur

– Review

– Evidenc

– Configu

manage

Development

– Complete, clear, non-ambiguous 

and auditable specifications

– Standard tools 

– Complete, clear, non-ambiguous and 

auditable specifications

–

in the specificat

Identifiability of safety related functions 

ions 

– FMCA of hardware and software

–

software

Analysis of risk and consequences of 

CCF; defence and mitigation 

countermeasures

– Autocontrol functions (HW + SW) ∈

specifications

– Design/coding methods limiting faults 

– Validated tools

– Comple

auditab

– Identifia

function

–

develop

Traceab

– FMCA 

–

softwar

Analysi

counter

– Autocon

specific

–

autocon

Demons

–

(eviden

Design/

– Validat
84

Table 3: EXAMPLE OF COMPARATIVE CLASSES OF GRADED REQUIREMENTS FOR THE DEVELOPME

(continued from previous page)

Systems not important to safety Safety Related Systems 

Verification/

validation

– Supplier’s V&V

– Site commissioning

– V&V plan

– Verification (chapter 2.5) at the end of 

each development phase

– Independent validation (chapter 2.6)

– Site commissioning

– Specific 

– Verifica

each de

– Indepen

– Indepen

– Site Co

Documentation – Auditable documentation demonstrating 

satisfaction of requirements

– Integrat

Operational 

feedback

– Relevant and documented operational 

feedback for support software, libraries 

and other re-used software

– Relevan

feedbac

and oth
85

1.12 Software Design Diversity

“Parempi virsta väärää kuin vaaksa vaaraa.”

Old Finnish proverb

1.12.1 Rationale

Identical redundancy is effective in guarding against random hardware failures. However, a 

common cause failure can be a systematic failure, ie a failure that can only be eliminated by 

changes in specification, design, implementation and configuration. Diversity may be 

introduced to provide protection against common cause failures. Decisions are required as to 

the way in which the required diversity will be achieved. Attention has also to be given to the 

reliability that can be claimed following the introduction of diversity and the demonstration 

required to support such claims. This chapter focuses on software design diversity issues 

arising from the use of computer based systems.

1.12.2 Issues Involved

1.12.2.1 Systems’ Architecture Software Considerations

When designing systems important to safety to provide plant functions such as control and 

shut down protection, important decisions have to be made as to what is an acceptable 

systems’ architecture. This section addresses systems’ architecture issues that are likely to 

influence the need for and nature of software design diversity. 

One approach typically adopted during architecture design, to protect against the possibility of 

common cause failure, is to consider the use of multiple, possibly diverse, systems. Also 

consideration of the need for defence in depth such that a failure in one layer is compensated 

for in the overall systems’ architecture may lead to the need for diverse, possibly software 

based, systems.

The number of systems, components or channels required, the degree of diversity between 

them, the selection of the technology for each of them, and possibly the apportionment of 

reliability targets, have to be addressed. One approach that may be adopted for one-out-of two 

systems where one of them is a computer based system is to employ a simple non-computer 
86

based secondary system. Where multiple computer based systems, channels or components are 

employed, the issue of software diversity has to be considered and this is discussed further 

below. 

1.12.2.2 Independence

The computer system architecture design should ensure that the required independence is 

maintained between systems, components or channels.

Many factors may affect and compromise independence; for example factors relating to the 

conceptual design, the programming language, practice, coding style, tools used for 

implementation, physical separation etc. Total independence, however, seldom exists; it is 

therefore extremely hard (usually impossible) to demonstrate, and so should not be required.

The objective is to prevent, as far as possible, defects that are potential common causes of 

unsafe failures. To this purpose, a discussion, common positions and recommendations on 

computer system architecture including use of physical separation and electrical isolation etc 

can be found in chapter 2.2 on computer system design. It is at the systems’ architecture level 

that functions are assigned to the diverse systems, components or channels and an example of 

the use of functional diversity at this level is discussed below. 

The design of the systems’ architecture should employ the technique of defence in depth (eg to 

ensure overlapping defences such that a failure in one system is mitigated elsewhere). An 

example of this defence in depth would be a protection system providing protective action in 

case of a control system failure. In this example software might be used in both the control 

and protection systems; and the potential for common cause failures due to the software would 

require analysis. More generally the potential for common cause software failures across all 

defence barriers utilising computer based systems should be analysed.

Decisions taken as to the devices used at the plant level (eg sensors and actuators) will also 

require careful consideration since modern instrumentation is increasingly utilising software 

components (eg smart sensors and actuators). For example, the use of the same smart

sensor/actuator type across diverse systems (eg primary and secondary protection systems and; 

control and protection systems) will introduce an element of common software, the 

implications of which should be fully analysed.

1.12.2.3 Hardware and software intrinsic quality

The correctness and completeness of the hardware and software implementation constitute the 

primary defence against coincident failures. The evidence obtainable to support claims made 
87

for correctness and completeness may not, in some cases, be sufficient (ie may not be 

commensurate with the confidence required in the implementation). Diversity is a second 

classical line of defence to prevent coincident failures within the system.

The use of diverse channels, subsystems or components, therefore, neither compensates nor is 

an excuse for using hardware or software of inadequate quality. Diversity is intended, like 

defence in depth, to bring additional evidence on the absence of coincident failures, and not be 

a substitute for a lack of evidence that the hardware and software of the diverse components 

satisfy their own specifications.

Diverse unacceptable channels or components never make an acceptable system. By way of 

examples, a channel or component would be unacceptable if any the following were the case:

– it cannot be shown to satisfy its specification,

– there is insufficient evidence to justify its claimed reliability,

– there is insufficient evidence to justify adequate independence from all other components 

and/or channels claimed as diverse, with respect to potential causes of coincident failures,

– all anticipated unsafe failure modes will not always be detected, or

– it is not always able to properly take over control when any of the other diverse channels 

or components fail.

Depending on the application, other conditions for acceptability may apply.

1.12.2.4 Software diversity

Software diversity can be introduced to overcome the problem of coincident software failures 

in redundant parts of a computer based system. Software diversity is introduced by different 

means, N version techniques being the most common. Since such computer based systems

tend to be complex, arguments of simplicity and correctness typically used to support 

hardware or procedural diversity may not be applicable. N version techniques typically 

involve the independent creation of programs for the diverse computer based systems. Those 

building the programs typically work independently of each other with no direct 

communication. The teams can either be given complete freedom on the production methods 

or methods can be imposed (forced diversity). The expectation is that the faults introduced by 

the teams will be diverse and coincident failures across the computer based systems will be 

reduced. Unfortunately there is no precise way of determining the benefit that such diversity

delivers. There are many different sources of potential coincident software failures and 

research in this field shows that statistical independence cannot always be assumed, see 

references [16], [18], [23]. Then, positive correlation of failures cannot be ruled out and this 

will lead to worse reliability than suggested by assumptions of statistical independence (eg
88

simple multiplication of each of the reliabilities for a one out of two systems’ architecture). As 

a result, safety case arguments as to the reliability achieved by the use of software diversity are 

required.

1.12.2.5 Software Reliability Figure

For a discussion on the reliability figure to be used following introduction of software 

diversity, see chapter 1.13 on software reliability.

1.12.2.6 Diversity Design Options and Seeking Decisions

Options must be taken as to what are the most effective methods and techniques to ensure 

diversity of failure behaviour across diverse software programs. These choices are termed 

diversity seeking decisions. Current practice is founded on engineering judgment, which 

identifies what are considered to be the best means to force diversity with some limited 

support from research. The possibility of providing further scientific support to these 

judgments is an open research issue. The guidance provided in this document is, therefore, 

predominately based on engineering judgment. Examples of methods to force diversity

include specification of different functions, use of different development environments, tools, 

languages, design and coding methods, algorithms and V&V methods etc. The intent is to 

present the development teams with different specifications, requirement methods and 

techniques such that coincident failures are unlikely. Another important but potentially 

limiting factor is the need to ensure the software has the greatest simplicity compatible with 

the design intent.

Amongst the various methods to force diversity the use of functional diversity is generally 

accepted as a particularly important measure (eg [1]; and [12], §13.4.1 which specifically 

recommends functional diversity implementation unless it is impossible or inappropriate; also 

see eg [23]). Functional diversity typically involves the use of different parameters in each 

system responsible to achieve the same safety objective (eg response to a plant event requiring 

reactor trip is based on temperature measurement in one system and pressure in the other). The 

best means of implementing functional diversity has to be considered and this might include 

ensuring that at least one system utilises a simple function (eg temperature level trip) where 

the other(s) employ functions involving non trivial calculations. Functional diversity may also 

be used within a computer based system (ie across different channels or components utilising 

common hardware) but the means of implementing functional diversity and benefit that this 

approach delivers will need to be carefully considered (eg while the application software 

might be different the impact of identical components such as operating system software will 

have to be addressed).
89

1.12.2.7 Common Cause Failure Analysis

While diversity design options intend to utilise diverse software components, confidence is 

required that this objective has been satisfied. A common cause failure analysis of the diverse 

software components and their development arrangements should be performed to provide the 

required confidence. This analysis should address the potential for and implications of the use 

of common modules, algorithms and code across (and within multi-channel) systems. Further 

guidance on common cause failure analysis can be found in [12].

1.12.3 Common Positions

1.12.3.1 The decision by the licensee to use diversity, the type of diversity or the decision 

not to use diversity shall be justified.

1.12.3.2 Where a justification not to use diversity is based on the use of particular measures 

or techniques, it shall be shown how these techniques avoid the potential for common cause 

failure.

1.12.3.3 The specification of any diverse component shall be demonstrated to be adequate in 

the context of the intended application.

1.12.3.4 Diversity shall not be used as a substitute for a lack of evidence that the hardware 

and software of the diverse components satisfy their own specifications.

1.12.3.5 Software satisfying the requirements of part 2 of this document shall be used in 

each system employing software diversity.

1.12.3.6 The plant design and the computer system architecture shall ensure that the 

necessary independence is maintained between the systems, components or channels for 

which diversity is claimed.

1.12.3.7 Functional diversity shall be used whenever possible in the implementation of 

systems, components or channels intended to be diverse.

1.12.3.8 The design of computer based systems utilising software diversity shall be justified 

in the context of the particular application, against current best practice which includes but is 

not limited to: 

– functional diversity,

– technology diversity,

– independent teams for specifying, decomposing and deriving requirements,

– independent development teams with no direct communication,
90

– independent teams for performing verification and validation,

– different allocation of requirements to software components,

– different allocation of software components to hardware components,

– different timing and different order of execution,

– simplicity of software design and implementation,

– different description/programming languages and notations,

– different development methods,

– different development platforms, tools and compilers,

– different operating systems,

– different hardware,

– diverse verification and validation.

1.12.3.9 The safety demonstration provided for diverse computer based systems, 

components or channels shall include an analysis of any potential common mode or common 

cause failures. This demonstration shall include consideration of the techniques for diversity 

noted above. A specific justification shall be given as to the impact on reliability and potential 

for common cause failure arising from any commonalities (eg identical software components, 

compilers, V&V methods etc.). For each major component of executable target software, the 

justification shall pay special attention to the possibility of potential common causes of failure 

specifically presented by this component, and show with adequate and specific analysis and 

evidence that the corresponding risks are acceptable.

1.12.3.10 The systems and software architecture design shall have the minimum complexity

commensurate with the design requirements.

1.12.3.11 When functional diversity is required, the diverse software based systems shall be 

associated with the same class, unless appropriate justification is provided (see eg [13]).

1.12.3.12 When functional diversity is required by deterministic arguments of the safety 

demonstration, the software of the diverse systems shall be subjected to the same graded 

requirements.

1.12.3.13 The design of application software – which is usually new software being 

developed – shall be structured so as to preserve the functional diversity designed at plant 

level.
91

1.12.4 Recommended Practices

1.12.4.1 Where diverse safety systems are required, and one is computer based, 

consideration should be given to implementing the second one using a simple non-computer 

based system. 

1.12.4.2 The reliability target assigned to any combination of computer based systems (eg

one out of two protection configuration), components or channels, should be demonstrably 

conservative having given full consideration to the factors that could lead to coincident 

failures.

1.12.4.3 Where a claim is made that very high reliability has been achieved through software 

diversity then it must be shown that dissimilar means have been employed in all aspects of the 

development lifecycle. Any divergence from this should cause the claim to be down rated.

1.12.4.4 Since research is active in this area, the position should be reviewed to ensure best 

use is made of emerging scientifically based advice, in particular, to determine claim limits on 

diverse computer based systems, components or channels produced by means of software 

diversity techniques.

1.12.4.5 The application software processing the input data of a same safety function in 

redundant trains can be implemented by code with adequate protections against coincident 

failures, eg with variations in names, in memory locations, in execution sequences or by other 

software diversity methods (see section 1.12.2.3 in this chapter).

* * *

93

1.13 Software Reliability

Note: This chapter is only applicable where reliability targets (expressed as probabilities of 

failure) have been set for the computer based systems.

1.13.1 Rationale

1.13.1.1 Traditional deterministic approaches for safety demonstration are supported by the 

use of system classification discussed elsewhere in this document. This approach typically 

requires the highest international standards to be applied to safety systems. However, use of 

probabilistic techniques (eg PSA and QRA) may lead to reliability targets (expressed as 

probabilities of failure) being set for I&C systems important to safety. This chapter is only 

applicable where such targets have been set for the computer based systems. Within this 

chapter references to reliability or probability of failure refer to software unless stated 

otherwise. It should also be recognised that software reliability is only one issue of 

dependability (see 1.1.1).

1.13.1.2 Consideration has to be given to (i) what is the best reliability that can be claimed 

for the software of computer based systems, and (ii) use of graded requirements to support 

different reliability levels. It is widely recognised that there is no way of precisely measuring 

the actual reliability of a computer based system. Reliability estimates are usually obtained by 

expert judgment based on operational experience and/or best engineering practices. 

1.13.2 Issues Involved

1.13.2.1 Quantitative and Qualitative Assessment

Qualitative assessment involves using engineering/expert judgement to determine the 

reliability that can be claimed following application of defined software engineering 

requirements. Quantitative assessment uses techniques that provide a numerical estimate of 

the reliability of the developed software (eg software statistical testing). The use of qualitative 

or quantitative approaches alone or some combination of the two needs to be determined.

The determination of acceptable sets of graded requirements including justification of 

appropriate techniques and measures to support different reliability levels is a key issue 

requiring resolution. For instance the IEC 61508 standard [14] addresses this issue through 
94

the use of four safety integrity levels (SILs) with recommendations on software techniques and 

measures applicable for each SIL (the higher the SIL the more onerous the 

techniques/measures). For example, techniques and measures that have to be considered for 

satisfaction of the detailed software design and development requirements include formal 

methods, defensive programming and modular approaches (IEC 61508-3:1998). It should be 

noted that current approaches are dominated by qualitative requirements. However, the use of 

Software Statistical Testing (SST) techniques offers the possibility of a quantitative 

demonstration (see below).

1.13.2.2 Claim Limitation

Claims of ultra-high software reliability cannot be demonstrated with current techniques [18]. 

Therefore, a decision has to be made as to the limit that can be claimed for a computer based 

system. A review of nuclear sector standards [13] and [8] shows that claims of lower than 

10-4 probability of failure on demand for a computer based system are required to be treated 

with caution. There are additional problems in determining claim limits when multiple 

computer based systems are associated with the same plant initiating event fault (eg multiple 

protection systems) arising from, for example, common cause failures and these issues are 

discussed further below and in chapter 1.12 on diversity.

1.13.2.3 Multiple Safety/Safety Related Functions

Where a computer based system implements multiple safety and/or safety related functions it 

is possible that different reliability targets are assigned to the different functions. This might 

suggest that different approaches could be taken to the design of the software implementing 

these functions. Such a claim would require a demonstration of independence between the 

functions (ie to demonstrate that a worse reliability target function could not prevent the 

correct operation of a better reliability target function). Without such a demonstration the best 

individual function reliability target (and hence the more onerous requirements) should be 

applied to all of the software.
95

1.13.2.4 Balance safety system versus safety related system

The deterministic approach generally allows no relaxation of requirements within a class but 

does across classes. Hence higher standards are usually required for a safety system than a 

safety related system. Using relaxations based on assigned reliability targets alone might lead 

to the situation where the safety system requirements are less onerous than those for safety 

related systems (eg as a result of requiring better reliability for a safety related control system 

than a protection system) and such situations should be viewed with caution. Computer based 

safety related systems such as control systems tend to be more complex than protection 

systems (usually implementing simple shutdown algorithms) and as a result it is recognised as 

good practice to keep reliability targets for such control systems modest.

1.13.2.5 Software Statistical Testing 

The use of software statistical testing (SST) provides the potential to derive an estimate of 

demonstrated system reliability. There are two main models used to estimate reliability, one 

of which uses classical [3] and the other Bayesian statistics [19]. For example, using the 

classical approach we can determine the number of required tests (representative operational 

transients randomly selected from the input space) to support a reliability claim at a desired 

confidence level (eg of the order of 50,000 tests with no failure for a 10-4 pfd demonstration to 

99% confidence). Similar test figures can be derived using the Bayesian approach. For SST 

to be valid it has to be shown that a number of assumptions hold, which are typically: 1) tests 

reflect the actual operating conditions of the software, 2) the tests are independent, 3) any 

occurring failure would be detected by the oracle, and no failures of the SST method and tools 

are actually detected during the SST. Research into SST is an open area with progress being 

made into code coverage related techniques and component based approaches to facilitate 

reuse. As a result those intending to use SST should carry out a review of SST research to 

ensure that the steps needed to generate a convincing statistical reliability demonstration are 

fully understood.

Generation of the test data for SST may be both difficult and time consuming. As noted 

above the amount of test data required for higher reliability demonstrations is not 

insignificant. Additionally the time to undertake the test runs and analyse the results has to be 

factored into the project timescales. It is important that the test runs are truly representative of 

the plant operational behaviour under the anticipated fault conditions (eg plant parameter data 

such as temperature and pressure vary in the way expected during demands for plant 

protection). This might require the use of plant fault simulation software (which will also 

require validation) to generate representative test data. However, given the representative 

nature of the tests to actual plant behaviour the use of SST provides an important reliability

demonstration. 
96

The role that SST plays in the overall safety demonstration has to be considered, for example, 

whether to employ as part of the software design and implementation verification or as a 

confidence check during commissioning. In addition the use of reliability growth models may 

be considered since they might also provide useful guidance on the reliability.

1.13.2.6 Determination of Reliability Values when Software Diversity is Employed

A reliability claim intended to be supported by the use software diversity has to be justified. 

For example, where two diverse computer based systems in a one out of two protection 

configuration have been introduced to provide defence against plant faults the probability of 

failure for the combination is often taken to be the product of the two system reliabilities. This 

is, however, a particular case of statistical independence; the actual value could fall between 

that of the more reliable system and zero (both systems never fail on the same demand), see eg

[16], [17], [18]. 

Thus, the use of software diversity (see chapter 1.12) presents particular challenges when 

attempting to determine the probability of failure of diverse systems. Attention, therefore, 

should focus on those conditions that support claims of statistical independence for software 

versions. At present there is no precise method to guarantee the achievement of statistical 

independence: demonstrations are based on qualitative arguments of best engineering practice 

(eg such as use of diversity seeking decisions) supported by examination of bounding cases 

and use of conservatism.

1.13.2.7 Assignment of Reliability Targets when Software Diversity is Employed

The assignment of reliability targets to systems employing software diversity shall be 

appropriate given the need to ensure software of the highest possible quality (ie up to the 

defined common cause failure limitation) is used in each system to reduce the chance of 

residual errors. In particular, claims of high reliability involving multiple low reliability

computer based systems shall not be allowed (eg usage of four 10-1 pfd systems as opposed to 

a single 10-4 pfd system).

1.13.3 Common Positions – Applicable when reliability targets are used

1.13.3.1 Reliability claims for a single software based system important to safety of lower 

than 10-4 probability of failure (on demand or dangerous failure per year) shall be treated with 

extreme caution.

1.13.3.2 The safety plan shall identify how achievement of the reliability targets will be 

demonstrated.
97

1.13.3.3 The sensitivity of the plant risk to variation of the reliability targets shall be 

assessed.

1.13.3.4 Software of the highest possible quality (ie up to the defined common cause failure 

limitation) shall be used in each system employing software diversity. Claims of high 

reliability involving multiple low reliability as opposed to a single high reliability computer 

based systems shall not be allowed.

1.13.3.5 The reliability expected from each executable software component (operating 

system, library, application software, intelligent drivers, sensing and actuation devices, 

communication protocols, etc.), shall be defined and shown to satisfy these targets. See 

common position 2.3.3.1.8. 

1.13.4 Recommended Practices

1.13.4.1 If the software is required to meet a reliability target, schemes using graded 

requirements assigned to reliability levels should be used during the development and 

assessment processes. An example of such a scheme would be four reliability levels (eg

level 4 – 10-4

, 3 – 10-3, 2 – 10-2, 1 – 10-1) with best use made of existing standards such as [14] 

and [8] to determine the graded requirements applicable at each level. Another approach is the 

assignment of a probability of failure figure to the class relaxations shown elsewhere in this 

document.

1.13.4.2 The reliability target assigned to systems of higher safety importance should 

normally be better (ie lower probability of failure) than that assigned to systems of lower 

safety importance.

1.13.4.3 The demonstration requirements applicable to the function with the best reliability 

target (ie lowest probability of failure) of all of the functions implemented within a single 

computer based system should be taken to apply to all of the software unless a demonstration 

of independence between the functions and their implementations is provided.

1.13.4.4 Reliability claims for complex computer based systems such as control systems 

should be kept low so as to ease the safety demonstration process. The necessary risk 

reduction should be provided elsewhere (eg within low complexity protection systems). 

1.13.4.5 Where practicable, SST techniques should be used to support the safety 

demonstration. Consideration should also be given to the use of other reliability models (eg

reliability growth) that provide a quantification of the expected reliability. 
98

1.13.4.6 In the case of a replacement or upgrade, the software may be required to have a 

reliability target that is at least as good as that of the system being replaced. Under 

appropriate conditions – in particular adequate hardware redundancy – it may be acceptable to 

compare levels of common mode failure probabilities, ie to require a level of common mode 

failure probability for the computer based system that is equivalent to or less than the level for 

the hardware being replaced. 

* * *
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1.14 Use of Operating Experience

Über die Antwort des Kandidaten Jobses

Geschah allgemeines Schütteln des Kopfes.

Der Inspektor sprach zuerst hem! hem!

Drauf die anderen secundum ordinem.

Wilhelm Busch, “Bilder zur Jobsiade”, 

Wiedensahl (Hanover), 1872

1.14.1 Rationale

1.14.1.1 This chapter addresses requirements for collection and use of operating experience 

data. By “operating experience”, we mean a collection of information on how a piece of 

software has been performing in service. 

1.14.1.2 Data on operating experience may be collected in different contexts and for 

different purposes. Data may be collected on systems, components, and tools of different 

safety classes.

1.14.1.3 Chapter 1.4 “Pre-existing Software (PSW)” and chapter 1.11 “Graded 

Requirements for Safety Related Systems (New and Pre-existing Software)” consider, among 

other factors, the contribution of operating experience to the validation of pre-existing 

software. This chapter deals with the operating experience in a more general context in order 

to determine how it can provide useful evidence.

1.14.1.4 The monitoring and recording of operational experience data may serve different 

useful purposes, in particular: 

– following the classical engineering and scientific approach, to take advantage of the past 

experience to improve the understanding of systems and of their failure modes, which is 

necessary to exercise expert and engineering judgment,

– to ensure, for example, through periodic safety reviews, that the safety case remains valid,

– to improve system design and their methods of construction, verification and validation,

– to obtain information on the stability and the maturity of software,
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– to support quantitative claims of reliability and availability based on statistical data, 

– to provide qualitative confidence in some specific safety properties of similar designs 

under similar profiles of usage,

– to provide insight, after installation and operation, into the appropriateness of the software 

qualification process,

– to provide a contribution to the qualification of software development and validation tools.

1.14.2 Issues Involved

1.14.2.1 Operational experience alone does not give enough evidence for the justification of 

the safety of a safety system or even a safety related system. On the other hand a system that 

has been used for a long time faultlessly in several installations may offer in practice, with 

proper documentation and under certain conditions, substantial supporting evidence. 

1.14.2.2 Traditionally, I&C operating experience is recorded and evaluated only if 

disturbances and failures occur. However, information about the I&C system’s correct 

performance during various plant operating modes may provide additional operational 

experience data.

1.14.2.3 There are many diverse industrial applications of digital I&C systems. Thus, 

various operating experience data may be available from non-safety applications. Regarding 

safety applications, however, special qualification requirements have to be met. The fulfilment 

of these requirements is difficult to demonstrate and in general, no credit can be taken from 

the operational experience of non-safety applications unless very stringent conditions are met.

1.14.2.4 The acceptability and usability of operating experience data is dependent on several 

factors, in particular:

1.14.2.4.1 Software configuration management

Configuration management allows effective assessment of whether the product’s operating 

experience is valid in the presence of changes, such as additions to functionality or 

correction of faults. Uncontrolled changes to the executable code may invalidate the use of 

operating experience.

1.14.2.4.2 Effectiveness and relevance of problem reporting

In-service problems should be reported together with information about the experienced 

operating and environmental conditions, and problems observed should be recorded in a 

way which allows identification of the affected items within the products configuration 

management system. 
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1.14.2.4.3 Relevance of the product service operating profile and environment

An analysis should show that the software will perform the same function in the proposed 

new application as it performed during previous applications. Aspects to compare might 

be the parameter input and output ranges, data rates as well as performance and accuracy 

requirements. For instance, operating experience is not applicable to software functions 

that were not exercised in the previous applications. 

1.14.2.4.4 Impact of maintenance and changes

Maintenance activities contribute to operating experience. As a main aspect of the root 

cause analysis after a distinct event, the possible impact of earlier maintenance activities 

like periodical testing under special test conditions or software upgrading have to be 

analysed. The issue is to properly identify the contribution resulting from:

– incorrect design or execution of maintenance activities, or

– incorrect design of the I&C system.

1.14.2.5 The objectives of operational experience data collection cannot be satisfied unless 

the involved parties accept the required resource commitments and confidentiality agreements. 

Licensees may not be required to communicate to the regulator data associated to events that 

are not related to safety. The issue of concern is that the potential impact on safety of an event 

may vary from one application to the other. 

1.14.2.6 With regard to tool certification, the relevance of operating experience is also 

dependent on the above-mentioned issues, in particular on adequate configuration 

management, stability and maturity of the tool.

1.14.3 Common Position

1.14.3.1 Detected faults and failures that may affect safety shall be analysed by the licensee 

and reported to the regulator. In particular, the potential for common cause failure, the 

relevance to other software based systems important to safety and the impact and the 

necessary improvements and corrective actions to the design, to the software development and 

the V&V and qualification processes shall be assessed.

1.14.3.2 Operating experience data shall be collected by the licensee in a systematic manner, 

for instance by means of a computerised database. For safety related events the data record 

shall comprise at a minimum the general data to identify the event (contextual and historical 

information) and as much data as possible to reproduce the event. 

1.14.3.3 Arguments and evidence shall be provided to confirm that no safety significant 

faults have been omitted during the data collection activities.
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1.14.3.4 Whenever credit is to be sought from operating experience in a safety case, the 

objectives and criteria for collecting operating experience data shall be identified and shown 

to be adequate in relation to the safety properties for which credit is sought.

1.14.3.5 Credit can be sought from the operating experience of previous hardware/software 

installations provided that the following conditions are satisfied:

1.14.3.5.1 The credit is considered as being of no other nature than that which would be 

sought from factory or site integrated tests; and not, in particular, as evidence for quality of 

design or for proper functionality.

1.14.3.5.2 The value of the credit as an acceptance or complementing factor is judged 

against the criteria stated in IEC 60880 Ed. 2 (2006) [12];

1.14.3.5.3 The operating experience data are demonstrably relevant, ie they are shown to 

satisfy the conditions of relevance of common positions 1.14.3.4, 1.14.3.6 and 1.14.3.7;

1.14.3.5.4 The set of implemented functions and the input profiles (input parameters, data 

ranges and rates) of the system/component under consideration are demonstrably the same 

as those of the installations on which data are collected;

1.14.3.5.5 When only parts of a software package for which operating experience credit is 

sought are used, analysis of the collected data shall show their validity for the intended 

application and the unused parts of the software must be shown to have no impact on 

safety. 

1.14.3.6 Relevant operating experience data shall at least include:

– meaningful information on the occurrences, the severity of the effects and the actions 

taken to correct or mitigate all detected faults, anomalies and failures,

– information on malfunctions detected in other applications,

– information on system and software configuration and version, on system operational 

conditions and on application environment profiles; this information must be shown to be 

consistent with the configuration, version, conditions and profile of the I&C system under 

consideration.

Where applicable, the above information shall be given separately for the distinct operating 

modes of the system in relation to the different operating modes of the plant, eg full power, 

shutdown.

1.14.3.7 When evaluating the relevance of operating experience, the impact of configuration 

management and maintenance activities shall be taken into account (see in particular 

paragraph 1.14.2.4.4 above and common position 2.7.3.3 addressing software maintenance, ie 

software changes after installation).
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1.14.3.8 Operating experience data on a system/component that are obtained from other 

relevant installations of that system/component must be maintained, updated and made 

available to the licensee and to the regulator.

1.14.4 Recommended practices

1.14.4.1 For different systems designed and qualified on a unique common platform, the 

operating experience may be coherently evaluated by the vendor to increase the knowledge 

about their performance according to the common platform requirements. The extent to 

which the operating experience can be applied more widely should be determined through 

comparison of the operating profiles of the various systems. The analysis should determine 

how the platform functions have been exercised by each operating profile.

1.14.4.2 The analysis of a distinct event (see 1.14.2.4.4 and 1.14.3.1) needs to assess 

whether the main design principles and qualification requirements were met during the course 

of the event. In this evaluation, the vendor should identify and analyse the main design 

principles and qualification requirements item by item. The evaluation result should make 

clear whether the event was caused by a system internal fault or by external influences like

manual actions, eg as part of maintenance, calibration or testing activities.

1.14.4.3 Appropriate data for the analysis may include documentation of the design, the 

development process for the system [8], the verification results, the system architecture, and 

information about associated resources and about malfunctions detected in other relevant 

applications.

1.14.4.4 If the intention is to use the operating experience for reliability analysis, special data 

like the time of occurrence, elapsed time for repair and maintenance, processor and data 

communication load, etc. should be collected.

1.14.4.5 The system supplier should have an organisation in place to allow users to 

exchange information on their respective operating experience and be kept informed of the 

system/component evolution.

* * *

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1.15 Smart Sensors and Actuators

1.15.1 Rationale

1.15.1.1 Conventional sensors/actuators are becoming unavailable and are being replaced by 

smart sensors/actuators. Smart sensors/actuators contain microprocessors and the use of 

firmware and software in these microprocessors presents challenges to the nuclear industry, 

particularly when they are used in safety systems.

1.15.1.2 Smart sensors/actuators support sensing and/or control functions and provide 

computation and communication facilities. Smart sensors/actuators typically have the 

following hardware: a microcontroller for computation, a small RAM for dynamic data, one or 

more flash memories that hold the program code and long-lived data, an analogue-digital 

converter, one or more sensors, a communication interface and a power source.

1.15.1.3 An advantage of these smart sensors/actuators is the modular fashion in which they 

can be connected to central processing units. As one might expect, many products and 

variants are available. For example, some smart sensors/actuators contain digital signal 

processor chips while others support wireless communication.

1.15.1.4 Most smart sensors/actuators have a minimal operating system that handles 

interrupts and performs simple task scheduling; sometimes features such as priority 

scheduling, logging to files on flash or emulating virtual memory are added. The application 

software layer, using these basic capabilities, implements the application specific 

functionality, which of course varies considerably.

1.15.1.5 As far as licensing is concerned, the software of smart sensors/actuators presents 

many aspects similar to those discussed in chapters 1.4 and 1.11 for pre-existing software 

(PSW). Thus, several issues, common positions and recommended practices discussed in 

these chapters apply to the use of smart sensors/actuators.
106

1.15.2 Issues Involved

1.15.2.1 In addition to the issues discussed in section 1.4.2, the following more specific 

problematic aspects need to be addressed.

1.15.2.2 The demonstration that smart sensors/actuators are fit for use in a nuclear safety

application is not straightforward. High reliability of smart sensors/actuators can only be 

demonstrated by means of an independent assessment with full access to the design 

documentation. A user of a smart sensor/actuator, however, would not usually have access to 

the design documentation. The supplier would not normally allow other parties to review its 

design documentation and as a result it is not usually produced for that purpose. 

1.15.2.3 Access to design documentation presents the main challenge to nuclear 

operators/licensees. In practice, this access is hindered by several factors such as:

– need to protect the suppliers intellectual property rights;

– the potential revelation of anomalies in the suppliers’ processes and products

– time and effort required from the manufacturer;

– the argument that previous certification (if available) should suffice; 

– the relatively small size of the nuclear market which typically means that nuclear licensees 

have limited influence on smart sensor/actuator suppliers.

1.15.2.4 Smart sensor/actuator suppliers produce their products to a quality that they regard 

as appropriate but this cannot take into account the requirements of specific use such as those 

of nuclear plant applications. Since the supplier does not know the context of use, the supplier 

cannot show that the instrument reliability is sufficient for the intended application.

1.15.2.5 There is currently no nuclear sector standard specifying the design documentation 

for smart sensors/actuators, nor even the issues that should be addressed. However, some 

smart sensor/actuator suppliers claim compliance to the IEC 61508 standard [14] but there 

remains no generic mechanism to satisfy those users who require independent evidence to 

support a functional claim of high reliability.

1.15.2.6 Assessments will be performed on a specific version of a smart sensor/actuator and 

the validity of the assessment may be challenged if the supplier changes its product (eg the 

software version). As a result a process will need to be in place to ensure that the installed 

smart sensors/actuators are the same versions as those subjected to assessment. In addition a 

supplier might upgrade a product from a conventional device to a smart device without the 

end user being aware of this change. Again a process will need to be in place to ensure such 

changes are detected and the smart sensor/actuator subjected to assessment. 
107

1.15.2.7 The use of smart sensors/actuators often implies a drive to distributed intelligence. 

It is often thought that this move will increase reliability. This is however not necessarily the 

case. The use of distributed intelligence replaces one central data acquisition system and 

simple field sensors with sensors integrating complete acquisition and communication systems 

in less benign and possibly aggressive environments, hence potentially reducing the reliability 

of the whole system. 

1.15.2.8 When multiple copies of an instrument are used across the plant, the potential for 

common cause failure both within and across systems is another risk that must be addressed.

1.15.2.9 Engineering competence is another hurdle to the successful implementation of 

distributed systems. Conventional 4-20mA systems are well understood, easily tested and 

adequately addressed by current technician skills. Digital signalling requires new skills and 

understanding, for example, leaving aside the topic of wireless communications, consider the 

problem of demonstrating that the software has high integrity when Fieldbus communications 

are employed. Fieldbus contains most levels of the Open Systems Interconnection basic 

reference model architecture and the volume and complexity of its software is large (eg one 

smart sensor that has been analysed by the nuclear industry was found to contain 16k lines of 

code for the implementation of the sensor functionality and another 100k lines of code for the 

implementation of the Fieldbus communications).

1.15.2.10 As a consequence of all these issues, a pragmatic assessment approach is needed 

which accounts for scenarios where design information is lacking by making use of 

compensating evidence (addressing for instance specific operational conditions, failure modes

and past operating experience) coupled with specific independent (ie from the smart 

sensor/actuator supplier) confidence building activities.

1.15.2.11 The approach needs to be compatible with nuclear safety standards and current 

research, and needs to be graded by importance to safety. 

1.15.3 Common Position

Functionality and Categorisation

1.15.3.1 The make, model, version and application of the smart sensor/actuator subjected to 

assessment shall be clearly identified and it shall be confirmed that the type testing applies (ie 

that the smart sensor/actuator subject to previous tests was fully representative of the smart 

sensor/actuator being assessed). The licensee shall ensure that the installed smart 

sensors/actuators are the same versions as those subjected to assessment. 

1.15.3.2 The licensee’s procurement arrangements shall ensure that technology changes 

from conventional sensor/actuator to smart sensor/actuator are recognised.
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1.15.3.3 The licensee shall demonstrate that the smart sensor/actuator has the functional and 

performance properties appropriate for the intended application.

1.15.3.4 The smart sensor/actuator properties necessary to satisfy the plant application 

requirements shall be identified and documented by the licensee. This documented 

identification is essential for two reasons: (i) in its absence, one would not know what the 

safety properties to be justified are; and (ii) the identification helps to circumscribe the 

evidence which is needed. (See also 1.2.3.4, and 1.13 if applicable).

Failure Analysis

1.15.3.5 The potential failure modes of the smart sensors/actuators shall be identified. Their 

consequences on the safety of the plant shall be identified and shown to be acceptable (see 

1.1.3.4). Special attention shall be paid to the interface between smart sensors/actuators and 

other components to which they are connected and to the possible side-effects that may occur 

at these interfaces.

1.15.3.6 When multiple use of smart sensors/actuators is employed (eg in redundant 

configurations), the risk of common cause failure is of special concern and shall be analysed. 

Note that failures due to software are not random and that replacing redundant hardware 

devices by redundant embedded software systems does not necessarily provide the reliability 

of the original design. 

Assessment and Qualification

1.15.3.7 The smart sensor/actuator’s production process shall be compared to that of an 

applicable safety standard. The applicability of the safety standard shall be justified. Any 

gaps revealed by the comparison exercise shall be addressed by compensating activities and/or 

arguments (see also 1.4.3.9.4). If any of the safety standard clauses are not used then the 

omissions shall be evaluated and justified.

1.15.3.8 The licensee shall undertake a programme of independent (ie from the smart 

sensor/actuator supplier) confidence building activities appropriate to the safety requirements.

(These activities might include commissioning tests, static analysis, statistical testing and 

analysis of operating experience.) The requirement for confidence building measures and their 

extent may require agreement between the licensee and regulator. 

1.15.3.9 For safety systems, the smart sensors/actuators software implementation that 

realises the properties mentioned in common position 1.15.3.4 above shall be subjected by the 

licensee to the same assessment (analysis, review and testing) of the final product (not of the 

production process) as new software developed for the application. If necessary, reverse 

engineering shall be performed. Should this assessment not be feasible in whole or in part, 
109

complementary and compensating valid evidence shall be sought in compliance with common 

position 1.15.3.10 below.

1.15.3.10 The compensating activities and/or arguments (eg additional tests and independent 

verification and validation) required to compensate for the gaps identified by the comparison 

activity (see common position 1.15.3.7 above) shall be determined and justified by the 

licensee in relation to the:

– specific nature of the missing evidence;

– properties identified under common position 1.15.3.4; and

– potential failure modes of the smart sensor/actuator identified under common position 

1.15.3.5.

1.15.3.11 The compensating activities and/or arguments shall be shown to be adequate to 

address specific evidence deficiencies and to bring the specific credit needed to support the 

properties identified under common position 1.15.3.4. Sufficient evidence shall be available 

to justify:

– the quality of the production process (quality and verification and validation plans, 

pedigree and experience of designers and supplier, etc);

– the quality of the product (test and verification coverage, etc);

– the acceptability of the smart sensor/actuator in the light of its operational experience as

witnessed by product returns and failure notices etc. (documentation, etc);

– that specific risks (eg common cause failure, unused existing software, flooding 

(overloading), etc) are not a concern.

1.15.3.12 When the software of the candidate smart sensor/actuator is qualified for its 

intended use, the safe envelope and limits within which it is acceptable to use the candidate 

smart sensor/actuator shall be identified and documented. Limits can include requirements on 

procedures for installation, periodic inspections, maintenance, training of staff, or assumptions 

on the environment and system in which the smart sensor/actuator is used, etc.

1.15.3.13 The assessments required here above shall be undertaken for each smart 

sensor/actuator based on the make, model, version, configuration and intended application 

(see also 1.4.3.3). Reliance should not be based on a generic assessment unless this can be 

shown to be adequate for the specific application. Evidence from a previous qualification or a 

generic pre-qualification shall be given careful consideration as to the limits of its 

applicability to the current application.
110

Documentation and Other Requirements

1.15.3.14 The licensee shall ensure that access to all information required to support the 

safety case is available to those with a need to have such access (eg licensee, regulator and 

authorised technical support organisations). This information shall be sufficient to assess the 

smart sensor/actuator to the required level of quality. For high integrity applications this could

include access to source code (see also 1.4.3.8 and 1.15.4.1).

1.15.3.15 For smart sensors/actuators embedded in safety systems, the licensee shall have full 

access to information on the software production process (see 1.15.4.1 for those cases where 

this access is limited).

1.15.3.16 Common positions 1.4.3.5, 1.4.3.7, 1.4.3.9.1, 1.4.3.9.2, 1.4.3.9.3, 1.4.3.9.5, 

1.4.3.10, 1.4.3.11 shall apply without change to smart sensors/actuators.

1.15.3.17 More generally, the following common positions apply:

Chapter 1.1 on safety demonstration:

– Common positions 1.1.3.1 to 1.1.3.11 (note that references to “system” in these common 

positions should be interpreted as meaning “smart sensor/actuator”)

Chapter 1.10 on independent assessment:

– Common positions 1.10.3.1 to 1.10.3.8

Chapter 1.11 on graded requirements for safety-related systems (new and pre-existing 

software):

– Common position 1.11.3.3

Chapter 1.12 on software design diversity:

– Common positions 1.12.3.1 to 1.12.3.13

Chapter 1.14 on use of operating experience:

– Common positions 1.14.3.1 to 1.14.3.8.
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1.15.4 Recommended Practices

1.15.4.1 The recommended practices of chapter 1.4 apply.

1.15.4.2 Recommended practices 1.15.4.3 to 1.15.4.5 are only applicable where reliability 

targets have been set for the computer based systems2.

1.15.4.3 Common position 1.15.3.7 above requires access by the licensee to the production 

process of the smart sensor/actuator. If reliability targets are used, different levels of access to 

the source code can be catered for using a pragmatic graded approach dependent on the 

required reliability. If the required reliability for the software in a safety system or safety 

related system is proven to be sufficiently low, ie where the probability of failure on demand 

is greater (ie less onerous) than 10-2

, access to the source code may not be required. 

1.15.4.4 If reliability targets are used and a probability of failure on demand of 10-2 is 

required, access to and analysis of the source code and circuit information is expected. 

However, this may not always be possible and typically, three options are available, presented 

here in order of preference:

a access to source code obtained:

◦ assess in accordance with the common positions shown above,

◦ apply static and dynamic analysis to the firmware.

b access to source code not obtained:

◦ present evidence to show attempts to access data have been unsuccessful and that there 

are no dumb or alternative justified smart sensor/actuators are available, 

◦ assess in accordance with the common positions shown above, 

◦ perform statistical testing.

c diverse instruments used (and access to source code not obtained), each of which meet the 

requirements for systems with a probability of failure on demand greater than 10-2

:

◦ provide a justified worst case estimation of the probability of failure on demand of 

each diverse instrument,

◦ assess each smart sensor/actuator as previously described, providing compensating 

evidence where gaps are identified,

◦ provide explicit arguments of diversity (see chapter 1.12),

◦ audit the manufacturers to support the diversity arguments.

 

2 These recommended practices are not applicable in Germany.
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1.15.4.5 If reliability targets are used and a probability of failure on demand of 10-3 is 

required, all common positions, including analysis of the source code and circuit information, 

shall be rigorously applied.

* * *
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PART 2: LIFE CYCLE PHASE LICENSING 

ISSUES

2.1 Computer Based System Requirements

Även en liten tuva kan stjälpa ett stort lass

(Old Swedish proverb)

2.1.1 Rationale

The development of the computer based system requirements (for short called hereafter 

system requirements) assumes that certain activities have already been completed. It is 

assumed that the system concept of the facility and associated systems has already been 

established and that the associated analyses have resulted in certain requirements being placed 

on the computer based system. In addition, national regulations will also impose certain 

functional or performance requirements on the computer-based system.

The system requirements are derived from the plant safety analyses and the applicable 

regulations. They are considered here as being either functional (eg the application functions 

to be implemented on the computer) or non-functional (qualities expected from the computer 

implementation such as reliability level, accuracy, time response). They should be 

implementation independent and should deal with the computer based system as a black box. 

While the system requirements are expected to be decoupled from their implementation to the 

extent feasible, the non-functional requirements may also be supported with design and 

implementation constraints imposed for various justifiable reasons

The elicitation of the system requirements is a critical step in the design. They are the result of 

co-operative work between experts of different disciplines and, as such, are prone to 

ambiguities, misunderstandings, inconsistencies and defects. Experience shows that these 

defects may have an important impact on all subsequent stages of the design, and can be 

difficult to correct at later stages.
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2.1.2 Issues Involved 

The system requirements and their documentation must be understandable to all parties 

involved, including those in charge of writing the computer system hardware and software 

specifications, the suppliers, and those involved in verification, validation and licensing 

activities. At the same time the description must be unambiguous and accurate, and this 

implies a certain degree of formalism. A compromise which preserves safety must be made 

between these two requirements which are in conflict [21].

2.1.3 Common Position

2.1.3.1 The system requirements shall be validated, ie their correctness, consistency,

completeness and absence of ambiguities shall be established in relation to the results of a 

prior plant safety analysis, and other relevant analyses at the plant level, in accordance with 

1.1.3.5, 1.1.3.6, 1.1.3.9 and 1.1.3.16. The traceability of the system requirements to the 

relevant analysis shall be documented.

2.1.3.2 The system requirements documentation shall at least cover the following items: 

2.1.3.2.1 A precise definition of the system boundaries, that is of the system interface to 

the plant and of its man machine interface. It shall in particular include the documentation 

of the restrictions and constraints placed on the system by compatibility requirements with 

other existing systems in the plant, by physical and natural environment constraints, by 

protection against propagation of non-safety system failures.

2.1.3.2.2 A specification of system requirements – functional and non-functional

(see1.1.1) – required from the system.

2.1.3.2.3 The results of hazard and failure analyses performed at the system concept 

level. In particular, the results of the analysis shall include required responses to 

hazardous conditions at the system interface with the plant such as sensor failures and 

input variables being out of range. Potential incorrect system outputs shall be identified, 

their impact on the plant and environment evaluated, and the presence of adequate safety 

defences confirmed.

2.1.3.2.4 A precise definition of the input variables to be monitored by the system 

(process variables, operator signals) and of the output variables to be controlled (output to 

actuators and indicators), together with a specification of the valid ranges of these 

variables. Criteria to validate the values of the input variables shall be specified. In some 

cases, the validation criteria need to include limits on the rate of change of a variable.
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2.1.3.2.5 The specification of special engineered features where there is a need to change 

specific system parameters such as calibration constants for operational reasons.

2.1.3.2.6 The results of the validation required in 2.1.3.1. 

2.1.3.2.7 All modes of operation (eg power operations, refuelling, post accident 

monitoring) and human interactions (eg operation, maintenance, tests).

2.1.3.3 The documentation of the system requirements and of their demonstration of 

correctness, completeness and consistency (requirement validation), shall be made available to 

the regulator as early as possible in the project.

2.1.3.4 The system should facilitate verificationthat the final control elements have 

achieved the desired state in the required time. Correct completion of a safety function shall 

be monitored and detection of a violation against the monitoring criteria shall be notified to 

the plant operator.

2.1.4 Recommended Practices

2.1.4.1 When practical, the system requirements specifications should be animated or 

simulated.

2.1.4.2 The specifications of the system validation tests should be derived from the system 

requirements.

* * *

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2.2 Computer System Architecture and Design

2.2.1 Rationale

The means by which the computer system architecture is demonstrated to be adequately safe 

and to meet the computer based system requirements (functional and non-functional) must be 

addressed at this stage. 

The methodical and systematic derivation of the requirement specifications for this 

architecture is important for the safety demonstration. It is not a trivial task, and it must be 

addressed carefully and properly. To a large extent, the design process is an empirical and 

heuristic process based on past experience gained in previous designs. New designs are often 

driven by technological considerations (use of a new, more efficient or more reliable 

technology) or by commercial objectives (better configurability, easier maintenance).

For an adequate safety demonstration, the computer system architecture that is proposed 

should be traceably derived from the system requirements with documented arguments for the 

choices being made.

Executable software components can be of different types: operating system, library, 

application software, intelligent drivers, sensing and actuation devices, communication 

protocols, man-machine interfaces, etc. These components do not contribute in the same way 

to the safety of the whole system. Defence in depth principles should be applied to the design 

of these different components: a component (for example the application software) can be 

designed so as to mitigate potential unsafe failures of the components it is using.

2.2.2 Issues Involved

2.2.2.1 The computer system architecture design has to be shown to result from a 

consideration of alternatives and from a mapping (systematic, structural or formal) of the 

system requirements onto the selected solution for:

– Hardware or ASICs (application specific integrated circuits),

– Purchased software,

– Software to be developed,

– Software to be produced by configuring pre-developed software.
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2.2.2.2 At this stage, the specification documents typically need to incorporate additional 

requirements relating to: 

– Equipment qualification (EQ); electromagnetic interference (EMI) and radio-frequency 

interference (RFI) immunity,

– Communication systems, multiplexers, network protocols,

– Fault detection and auto-supervision,

– HW/SW fault tolerance mechanisms,

– Testability, In service testing and maintenance, auto-testing

– Systems outside the computer based system boundaries which require design decisions (eg

instrumentation requiring software compensation or filtering),

– Human interfaces,

– Security,

– Constraints on downstream lifecycle activities and work products. 

2.2.3 Common Position

2.2.3.1 The computer system architecture, with its centralised versus distributed 

organisation, and its derivation and allocation of functions to either hardware or software shall

be demonstrated to result from the system requirements and from the added requirements 

mentioned above in issue 2.2.2.2.

2.2.3.2 The principles of redundancy, diversity, physical separation, electrical isolation and 

independence between safety functions, safety related functions and functions not important to 

safety shall be applied to the design of the computer system architecture. The contribution 

made by each principle to meeting the requirements – including design criteria such as the 

single failure criterion – shall be documented. In particular, the non-functional requirements, 

including reliability, shall be explicitly addressed.

2.2.3.3 The boundaries between components associated with different safety classes shall 

be well defined. Failure propagation across the boundaries from a component of lesser 

criticality to a component of higher criticality shall be prevented.

2.2.3.4 The design shall achieve the required accuracy and response times to satisfy the 

system accuracy and real-time performance requirements. In particular, attention shall be paid 

to the cycle times, signal sampling and processing functions that will be implemented.
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2.2.3.5 It shall be confirmed that the design includes an explicit, verifiable specification for 

the order of execution and timing inter-relationships, especially considering multiple 

concurrent physical processes, inter-process synchronization and shared resources.

2.2.3.6 The design shall ensure that failure of a part of a redundant safety system shall not 

adversely affect other redundant or common parts of this safety system.

2.2.3.7 The system shall be designed so that all hardware faults which may affect safety or 

reliability can be detected, by auto testing or by periodic testing, and are notified. In the 

presence of faults, the system must move to a subset of states which can be demonstrated to 

satisfy the system safety requirements. This shall be true even in the presence of an external 

fault like a loss of electrical power to the system or to its input/output devices.

2.2.3.8 Computer system architecture failure and hazard analyses shall demonstrate that:

– the possible failure modes of the architecture that may compromise the safety functions 

have been taken into account, and

– adequate exception handling mechanisms and hazard mitigating functions have been 

included in the design.

2.2.3.9 It shall be ensured that the use of these fault tolerant, exception handling and hazard 

mitigating mechanisms is appropriate and that they do not introduce unnecessary complexity.

2.2.3.10 The computer system architecture shall be designed so that it is periodically testable 

in service without degradation of reliability. Test periodicities shall be derived from the 

system reliability and availability requirements. Hardware faults shall be postulated, even 

those that will remain undetected; these faults shall be taken into account in the consideration 

of the required hardware redundancy and tests periodicities.

2.2.3.11 The computer system architecture shall be designed to support the common 

positions concerning verification and validation (see chapters 2.5 and 2.6).

2.2.3.12 The computer system architecture shall be designed to support the common 

positions concerning security (see chapter 1.8), particularly those concerning communications 

and connectivity between systems and their associated equipment.

2.2.3.13 For safety systems, the common position 2.7.3.3.2 is applicable.

2.2.3.14 The evidence of satisfying the requirements of the above common positions shall be 

documented and made available to the licensor as early as possible in the project.
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2.2.3.15 It shall be confirmed that the system architecture ensures that

– all system states are analysed to identify those that are hazardous,

– the safe state space of the system is defined,

– the system components are constrained to those whose safe state space is observable, and 

departure from it detectable.

2.2.4 Recommended Practices

2.2.4.1 Fault Tolerance and Recovery

All fault tolerant architectural mechanisms should be justified against the following 

principles:

2.2.4.1.1 Fault tolerance is the capability to mitigate the effect of errors, so as to prevent 

them from turning into failures that jeopardise system safety. It requires detection, control 

and decision mechanisms, and interconnections. The efficiency of fault tolerant 

mechanisms depends on the knowledge of the faults to be protected against. Also, it 

works best when reliable components of design are employed, when the error rate is 

already low, and the fault mechanisms well identified. In addition it must be recognised 

that a fault tolerant mechanism can become a source of failures in its own right. These 

potential failures should be identified, and their impact on the fault tolerance capabilities 

should be assessed. 

2.2.4.1.2 Fault avoidance through the use of highly reliable design elements is therefore 

a prerequisite of fault tolerance. If a fault can be identified and avoided, it should be 

designed out first before fault tolerant mechanisms are considered. Thus, fault tolerance 

should not be used to compensate for deficiencies in the design. It should be used only if 

the deficiencies are well understood, unavoidable and specific of a fault type.

2.2.4.2 The use of features not specified by the manufacturer, for example undeclared 

instructions or clock rates, should be prohibited unless their use is fully justified.

* * *
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2.3 Software Requirements, Architecture and Design

2.3.1 Rationale

Software development is typically an iterative process that consists of three stages: software 

requirements definition, software architecture and design, and implementation (ie the 

production of code).

The software requirements are the subset of the system and computer system architecture

requirements and properties (see chapter 2.1), or requirements, properties and constraints 

derived from these, which are allocated to software during system design (see chapter 2.2), 

and will be implemented as computer programs. It is important that all requirements are 

allocated to or transformed into hardware or software requirements completely and correctly.

Software design must meet all the design requirements and use a simple modular structure 

clearly understood for implementation and testing. It must be clear enough for people who did 

not produce the requirements (reviewers and regulators) to trace the origin of the software 

requirements and verify their allocation and transformation.

The software used in a computer based system can be from different origins (see eg reference 

[9]):

– New Software: software created specifically for the application, with complete 

documentation;

– Existing accessible software: typically software from a similar application that is to be 

reused and for which complete documentation is available, including the code and an 

unambiguous characterisation of former application environments;

– Existing proprietary software: typically a commercial product or software from another 

application that meets all or some of the current application requirements but for which 

little documentation is available;

– Configurable software: typically software that already exists but is configured for the 

specific application using simple parameter values and/or an application oriented specific 

input language. 

These different types of software are essentially produced by two distinct processes:

– the classical development process through refinement stages of specifications and design;

– development by means of code generating tools, which receive as input a high level 

language application oriented description of the system requirements. 
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The choice between these two approaches depends on the tools and resources available, and 

must, in particular, take into account trade-offs between design and tool qualification (see 

chapter 1.5). However, in all cases, programming in a given computer language, and the use 

of tools always take place to some extent.

2.3.2 Issues Involved

2.3.2.1 Often, software requirements are not written as they should be. They should be 

written so that they can be understood and used by many different categories of people:

software designers, computer system engineers, regulators and, sometimes, process engineers 

and safety engineers. They should be written in a form that is independent of the particular 

implementation, and which essentially aids understanding of how the requirements are derived 

from the system and safety requirements [21].

2.3.2.2 Often, software is not adequately structured. Software design is the division of the 

software into a set of interacting modules, and the description of those modules and of their 

interfaces. It is important to design so that the result is well structured and understandable to 

the implementers, maintainers, testers, and regulators. The design should demonstrably cover 

all software requirements and should not contain any unnecessary item.

2.3.2.3 The demonstration that the code is a correct implementation of the software 

specifications is also a major issue. 

2.3.2.4 Despite all best endeavours to produce fault free software through good design 

practices and thorough testing, there is always the potential for unforeseen error conditions to 

arise. Therefore the technique of incorporating error checking (which may be based on formal

assertions) into software is regarded as a sound policy. This technique is known as defensive 

programming. It should cover both internally and externally arising exceptions, without 

adding unnecessary complexity to the software.

2.3.2.5 The code usually cannot be produced correctly at once in a single version. 

Therefore, change requests and modifications have to be carefully controlled.

2.3.2.6 Very often, changes are not anticipated properly, the architecture does not localise 

their impact and it is difficult to fully analyse the impact of the change. A software change is 

more likely to be conceived and implemented correctly if the system’s software is well 

structured, modular, well documented and containing features which aid changes.
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2.3.3 Common Position

2.3.3.1 Software Functional and Non-Functional Requirements

2.3.3.1.1 The software requirements shall be demonstrated to satisfy the system and 

architecture requirements, both functional and non-functional (see 2.1.3.2.2). The 

requirement documentation shall describe the design correctly and completely, and shall be 

understandable unambiguously by the community of its users (typically from different 

disciplines and backgrounds), eg implementers, maintainers, verifiers, reviewers and 

regulators.

2.3.3.1.2 The software requirements document shall define the interfaces between the 

software and the following: the operator, the instrumentation (sensors and actuators), the 

computer hardware and other systems with which communication links exist.

2.3.3.1.3 The software requirements shall address the results of the computer system failure

and hazard analyses (see common position 2.2.3.8). The software requirements shall include 

the necessary supervision features to detect and report hardware failures. They shall also 

include the necessary supervision features for the software. In accordance with common 

position 2.8.3.1.2, the adequacy of automatic self supervision in combination with periodic 

testing during operation shall be demonstrated.

2.3.3.1.4 Defensive programming techniques shall be employed where known states and 

ranges of variable and parameter values can be predicted. For example, where the state of a 

valve can be either open or closed, a check shall be made that, after a finite change-over time, 

one state or the other is achieved. This should be done by checking open AND not closed, or 

closed AND not open. In the case of parameter ranges, if the output of a computation returns 

an out of range value then an exception shall be notified (see also 2.1.3.2.3, 2.1.3.2.4, 2.2.3.7, 

2.2.3.8 and 2.2.3.9).

2.3.3.1.5 Requirements shall include an explicit and verifiable specification for the order of 

execution and timing inter-relationships, especially considering multiple concurrent physical 

processes, inter-process synchronization and shared resources. It shall be confirmed that 

constraints are included to ensure analysability and verifiability.

2.3.3.1.6 The software shall be designed so that, where practicable, the correct operation of 

the plant actuator selection and addressing system is verified. The design shall also include 

means for verifying that the final actuated equipment has achieved the desired state in the 

required time.

2.3.3.1.7 The computer system shall employ a hardware watchdog of demonstrably adequate 

reliability. This watchdog shall be serviced by a program operating at the base level of 

priority so as to detect problems such as programs locked in loops or deadlocked.
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2.3.3.1.8 If reliability targets are set for the computer based system, then the common 

positions and recommended practices of chapter 1.13 on reliability are applicable. The level 

of reliability required from the software shall be explicitly stated, with the understanding that 

the achievement of a software reliability level is less demonstrable than other requirements. 

2.3.3.2 Software Architecture and Design

2.3.3.2.1 The software architecture and design shall satisfy the architecture and design 

requirements.

2.3.3.2.2 The software architecture shall be organised on a modular basis in a manner which 

minimises the potential for design faults and also facilitates verification by an independent 

team. The use of hierarchical modularisation, encapsulation and information hiding is 

recommended such that the design of a module is restricted to a small clearly defined set of 

functions that requires only minimum interaction with other functions and minimises the 

impact of changes. The interfaces between the various modules shall be simple, completely 

identified and documented.

2.3.3.2.3 The separation between safety functions, safety related functions and functions not 

important to safety, which is defined by the computer based system requirements, shall be 

maintained in the software design and its achievement demonstrated.

2.3.3.2.4 Documented evidence shall be given that the supervision features mentioned in 

common position 2.3.3.1.3 maintain the system in a safe state.

2.3.3.2.5 The design shall constrain the implementation so that the execution of the software 

is verifiable and predictable.

2.3.3.2.6 Library modules shall benefit from an adequate level of defensive programming at 

the application level, in particular with respect to the “within range” validation of their input 

parameter values, the detection of anomalies and the generation of safe outputs.

2.3.3.2.7 The design of application software – which is usually new software being 

developed – shall be shown to be defensive. In particular, it will be shown to properly react to 

the potential failure modes identified for the system it makes use of, according to the 

requirements of common positions 2.2.3.8 and 2.3.3.1.3.

2.3.3.2.8 The software shall be designed to support the common positions concerning 

verification (see chapter 2.5), for instance by minimising retention of state information 

between system execution cycles and by maximising independence between software inputoutput paths (see common positions 2.4.3.3.1 and 2.4.3.3.6 on coding).
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2.3.3.2.9 All system software (as opposed to the application software), including the runtime 

environment etc, shall ensure correct execution of application functions under the full range of 

possible system operating conditions.

2.3.4 Recommended Practices

2.3.4.1 Functional and non-Functional Software Requirements

2.3.4.1.1 The software requirement specifications should be based on a functional model of 

the system to be implemented, as for example, a finite state machine model or an input/output 

relation model (see, for example [10]).

2.3.4.1.2 For better traceability and verifiability, the naming and encoding of data structures, 

input, output and internal variables should be meaningful in the system environment (for 

example, names as set point, temperature, trip, should be used). Names should be used 

consistently throughout the whole software.

2.3.4.1.3 For each relevant software output, a safe state should be indicated that can be used 

in case of a detectable but not correctable error.

2.3.4.1.4 Assertions aiding fault detection should be inserted into the code, and the system 

should be designed and implemented to be as fault tolerant as possible. For example, data 

errors occurring in inputs and on transfer between modules should be trapped and alarmed; 

instrument readings should indicate when they are off scale.

2.3.4.2 Software Architecture and Design

2.3.4.2.1 Module interfaces should be simple, and such that the consequences of expected 

changes should be confined to a single or to a small number of modules. 

2.3.4.2.2 Modules should be defined in a way which makes it possible to reason about and 

test a module part (program) in isolation from other module parts and from other modules. 

The contextual information which is required for these activities should be part of the module 

documentation.

2.3.4.2.3 The interface between the system software and the application software should be 

completely defined and documented.

* * *

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2.4 Software Implementation

2.4.1 Rationale

Software implementation is the phase which follows the design phase. It is the process –

typically using a programming language – of transforming the software specification into a set 

of instructions executable by computer.

Well structured programs employing good programming practices will, in the main, be more 

dependable than those that do not follow these practices, in particular, when coding is 

performed manually. A careful selection of the language features and program constructs that 

are used also makes the software easier to test.

Programming directives and coding seek to ensure that good programming practices are used, 

that the formats of the code and of its documentation are consistent throughout the system and 

that they are well structured and understandable.

2.4.2 Issues Involved

2.4.2.1 The instruction sets of digital computers allow, through the use of branch 

instructions (including computed branches), index registers (used as address pointers) and 

indirect addressing, extreme complexity to be introduced into the code (both through poor 

structuring or the addition of unnecessary functionality). This complexity in coding can result 

in the introduction of faults during the coding phase and when making software changes. It 

also makes the detection, during the verification and validation phases, of any fault introduced 

more difficult.

2.4.2.2 To address these problems, the code needs to be verifiable, statically analysable and 

contain no unknown or undefined states. Hence, strict coding rules need to be defined and 

enforced. These coding rules should impose a stringent discipline on the writing of the safety 

system software and ensure that code legibility is given priority over ease of writing.
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2.4.3 Common Position

2.4.3.1 The detailed recommendations given in appendix B of IEC 60880 for the coding of 

software shall be followed. For the implementation of safety system software, all exceptions 

to these recommendations shall be duly justified. However, the recommendations contained 

in this section 2.4.3 shall be followed under all circumstances.

2.4.3.2 A document containing the coding directives (including design constraints in 

accordance with 2.3, coding rules, tool configuration restrictions and use of predefined 

functions) that are enforced on the programmers to support the system properties required (eg

verifiability, static analysability, absence of unknown and undefined states etc) shall be 

produced before the start of the coding phase. This document shall be available to all those 

with a need for such information (eg regulator and third parties).

2.4.3.3 Coding

2.4.3.3.1 The programming languages that are used shall have a rigorously defined syntax 

and semantics or they shall be demonstrably restricted to a safe language subset.

2.4.3.3.2 The code shall be traceable to and verifiable against the software requirements. If 

verification is planned to include human inspections, the code shall be readable, adequately 

commented, and understandable. 

2.4.3.3.3 The code of the different programs of a module shall be displayed in specific 

sections of the module documentation, together with the textual information necessary to 

verify the correctness of the program to the module requirements.

2.4.3.3.4 The code shall be written in a style and with language features and program 

constructs that support the common positions concerning verification (see chapter 2.5), for 

example by reducing the number of dynamic test cases required. 

2.4.3.3.5 The code shall be designed so as to facilitate static analysis, testing and readability.

2.4.3.3.6 In particular, the code shall – as much as possible – run in a direct and fixed 

sequence pattern, ie:

– Computed branching shall be prohibited. Branching into loops, modules or subroutines

shall be prohibited.

– Dynamic instruction changes shall be prohibited.

– Interrupts shall be avoided unless they lead to a significant simplification. Where 

interrupts are used, their usage and masking during time and data critical operations shall 

be proven correct and shall be well documented. The use of high-level synchronisation 

programming primitives shall preferably be used to deal with interrupts. The hardware 
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and software shall be designed so that every interrupt is either serviced or explicitly 

masked.

– Recursion – not amenable to static analysis – and re-entrancy shall be avoided unless they 

are shown to produce simpler code than by other means.

– Nesting of loops shall be limited to a specified maximum depth for safety system software, 

and modification of loop control variables within the loops shall be prohibited.

– All alternatives in switch or case statements shall be explicitly covered by the code. 

Appropriate actions shall be implemented to deal with default conditions.

– Indirect addressing shall be used carefully so that computed addresses shall be easy to 

identify.

2.4.3.3.7 Dynamic storage allocation shall be prohibited. 

2.4.3.3.8 Module size shall not exceed a limit specified for the system without justification.

2.4.3.3.9 Unnecessary code compacting, programming tricks and unnecessary optimisations 

which make the code less understandable, less amenable to static analysis and more difficult 

to test shall be avoided.

2.4.3.3.10 To facilitate static analysis and be independent from particular compiler 

conventions on typing, operations involving different types of variables shall be avoided 

unless conversions appear explicitly.

2.4.3.4 Subroutines

2.4.3.4.1 For safety system software, a maximum shall be specified for the depth of nested 

subroutines (eg to avoid stack overflows).

2.4.3.4.2 To reduce complexity, avoid potential coding mistakes, and facilitate static 

analysis, failure mode analysis and testing, subroutines shall

– communicate with their environment exclusively via their parameters;

– for safety system software, have only one entry point and return from only one point.

2.4.3.5 Data structures and addressing

2.4.3.5.1 Variables and data shall be explicitly declared and assigned. Explicit initialisation 

of all variables shall be made before their first use. A variable and its identifier, if not local to 

a subroutine, shall not have more than one meaning.
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2.4.3.5.2 In order to facilitate code reading, static analysis, failure mode analysis and testing, 

distinctly different symbolic names shall be used to represent constants, variables, modules, 

procedures and parameters.

2.4.3.5.3 To facilitate static analysis, failure mode analysis and testing, only explicit 

mechanisms such as mutual exclusion or message passing programming primitives shall be 

used to access data shared by concurrent processes. Equivalence statements used to specify the 

sharing of storage units by two or more entities in a program unit shall be prohibited.

2.4.3.5.4 In agreement with 2.4.3.3, arrays shall have a fixed, pre-defined length. The 

number of dimensions in every array reference shall be equal to the number of dimensions in 

its corresponding declaration. To facilitate static analysis, failure mode analysis and testing, 

dynamic computation of indexes shall be prohibited.

2.4.3.5.5 Arithmetic expressions shall reflect the equations they represent. 

2.4.3.5.6 Arithmetic expressions shall be executed in binary fixed-point arithmetic, unless 

the use of floating-point arithmetic can be demonstrated to contribute to safety.

2.4.3.5.7 The mapping of real continuous variables into fixed point variables shall be 

explicit and documented in the code, and its accuracy justified. If different mappings are used, 

their mutual compatibility shall be justified.

2.4.3.5.8 If execution in floating-point arithmetic is necessary, the hardware and software 

used to implement floating point calculations and functions shall be suitably qualified.

2.4.3.6 Defensive Programming

2.4.3.6.1 If complicated calculations are necessary, the program shall be written so that –

where feasible – a simple function or assertion will provide an exception check and back-up 

action.

2.4.3.6.2 In addition to requirements 2.2.3.8 and 2.3.3.1.4, defensive coding shall be applied 

by programmers with the objective of maintaining program behaviour within its 

specifications, including the following:

– known states and program properties, and parameter ranges shall be verified;

– variable values that can cause faulty operations shall be detected, (for example, division 

by 0).

2.4.3.6.3 Preventing overflow and underflow of assigned memory (eg in the case of arrays, 

lists, queues, buffers) shall be statically addressed through strong data types and type 

checking. If static prevention cannot be guaranteed, dynamic checking shall be performed.
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2.4.3.7 Language and compiler requirements

2.4.3.7.1 The programming language used to write the code shall be restricted to a safe 

subset. In particular, a high level, strongly typed, programming language that prohibits the 

unsafe mixing of variable types is preferred.

2.4.3.7.2 The use of assembly language or machine language shall be limited to the code 

needed to interface with hardware. If used for other purposes, properties such as verifiability 

and traceability shall be confirmed, and the usage justified.

2.4.3.7.3 The compiler of the programming language shall satisfy the applicable common 

positions of section 1.5.3. In particular, the defect detection capabilities of the compiler shall 

be clearly defined and verified to the extent that the safety demonstration depends on these 

capabilities.

2.4.3.7.4 The same compiler version shall be used for all compiled parts of the software

running on the same processor.

2.4.3.7.5 If a new version of the compiler has to be used, the software affected by the change 

shall be re-tested unless it can be demonstrated that the machine code has not changed.

2.4.3.7.6 Any compiler code optimisation shall be provably correct. The same optimisation 

options shall be used on all the software compiled by the same compiler.

2.4.3.8 Operating Systems

2.4.3.8.1 In support of common position 2.3.3.2.5, the behaviour of the operating system 

shall be predictable, analysable and verifiable (see also 2.4.3.8.3), for example by ensuring 

that:

– the organisation of the operating system into tasks, including the allocation of services to 

tasks, is as simple as possible, clearly identified and documented;

– the task generation should take place statically in a linear sequence in the initialising phase 

(no dynamic task creation and deletion);

– the task priorities are statically assigned;

– no dynamic memory allocation is used;

– the system includes all the self diagnosis features necessary to detect its own failures;

– use of interrupts is restricted to cases where this simplifies the design and there are no 

unanalysed conditions (see also recommended practice 2.4.4);

– the absence of overflow is demonstrated;
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– the absence of overflow and underflow for stacks, lists, queues and buffers, and the 

absence of deadlocks and starvation are demonstrated.

2.4.3.8.2 In support of 2.3.3.2.5, it shall be demonstrated that all platform services are 

available when needed with the necessary capacity.

2.4.3.8.3 If the requirements of common position 2.4.3.8.1 are not satisfied by a complete 

operating system, a properly selected and configured subset of the operating system shall be 

used in conjunction with the specific application. The specification shall cover the qualified 

subset and configuration, the environment including the application for which it is qualified 

and any other constraints under which the operating system was qualified.

2.4.3.9 Support Software and Tools

2.4.3.9.1 All support software and tools used in the production and testing of code 

(compilers, assemblers, linkers, loaders, operating systems, configuration control systems, 

static analysers, test harnesses, coverage analysers, etc.) shall be qualified or otherwise 

suitably justified, and shown to comply with the requirements of section 1.5.3.

2.4.3.10 Documentation

2.4.3.10.1 The source code shall include commentaries to a level of detail that is consistent 

throughout the whole software. Commentaries shall be consistent with the documentation of 

the software requirements and of the design. They shall be a sufficient complement to this 

documentation to make the code readable and understandable.

2.4.3.10.2 In particular, commentaries, related to the code of every software module or 

program, shall contain all the complementary information necessary for an independent and 

stand alone review and verification of this code against its specification.

2.4.3.10.3 Commentaries shall also include any specific information required for maintenance

of the code, in particular, its history and the designers’ names.

2.4.3.10.4 If a tool has generated the code, the common positions in this section 2.4.3.10

apply to the tool inputs.
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2.4.4 Recommended Practices

2.4.4.1 Coding

2.4.4.1.1 If a requirement specification language is used, it should have a well defined 

syntax and semantics.

2.4.4.1.2 The coding phase should start only after the verification team has approved the 

design phase.

2.4.4.1.3 Evidence should be provided that structured programming techniques have been 

followed.

2.4.4.1.4 Branching out of loops should be avoided, except in the case of exceptions .

2.4.4.2 Data structures and addressing

2.4.4.2.1 The use of global variables within subroutines should be avoided.

2.4.4.2.2 The symbolic names and types of variables, constants, modules, procedures and 

parameters should be meaningful at the application level.

2.4.4.3 Language and compiler requirements

2.4.4.3.1 For application software, it is strongly recommended that a problem-oriented 

language rather than machine-oriented language be used.

2.4.4.3.2 Assembler and machine code insertions into high level language code should not 

be allowed without justification.

2.4.4.3.3 Automatic generation of source code by validated tools is preferable to manual 

writing. If a generation tool is used, it shall comply with the recommendations of chapter 1.5.

2.4.4.3.4 The compiler should have been previously and widely used in comparable 

applications, and should comply with the recommendations of chapter 1.5. For safety system 

software, the use of compiled code optimisation options should be avoided.
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2.4.4.4 Operating systems

2.4.4.4.1 When the evidence required in 2.4.3.8.1 for using interrupts cannot be provided, 

other techniques can be used to support satisfaction of timing constraints (see 2.4.3.8.2) and 

correct order of execution. Examples of evidence to be provided for monitoring hardware 

such as sensors and actuators, and for confirming interrupt-avoiding provisions are:

– correct identification of rates of change in the phenomena to be monitored,

– for monitoring a continuously varying phenomenon, correct identification of the sampling 

interval that characterises the monitored variable with accuracy,

– for reacting to sporadic events, eg sudden change, correct identification of the minimum 

time between arrival of successive events, based on the physics of the event-generating 

process,

– evidence of definition of commensurate periodic tasks,

– evidence of adequate periodicities of the tasks to achieve cyclic, deterministic timing 

behaviour,

– provision of an explicit, verifiable specification for the order of execution and timing 

inter-relationships, especially considering multiple concurrent physical processes, interprocess synchronisation and shared resources,

– provision of an explicit, verifiable specification for the detection of the occurrence of 

faults and the existence of faulted states, including the required monitoring interval (or 

sampling period) and timing relationships,

– avoidance of hardware that does not support the above monitoring and fault detection,

– provision of an explicit, verifiable specification for the mechanisms to recover from faults.

* * *
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2.5 Verification

2.5.1 Rationale

In order to obtain sufficient evidence that the software implementation complies with the 

functional and non-functional requirements, appropriate verification procedures have to be 

established and implemented at well-defined milestones in the course of the software life 

cycle process.

Most of the commonly used software development life-cycle models include a verification

step after each software development phase to ensure that all requirements of the previous 

development phase have been implemented completely, consistently and correctly. 

There is a wide spectrum of verification methods from which ideally an optimal and well 

balanced subset has to be selected. 

Some of the main methods currently used for software verification are

– Reviews, walks-through, audits: Manual or tool supported measures, particularly used to 

verify the phases of requirement specification and design specification;

– Static analysis: Source code verification, mainly with respect to design and coding 

standards, symbolic execution and data-flow; 

– Testing: Verification of the executed object code with respect to software correctness 

(note: In the framework of validation, tests will be performed on target hardware and 

under target environmental conditions). The IEC 60880 standard [12] lists the most 

common software testing methods. (See eg also [22].)

2.5.2 Issues Involved

2.5.2.1 Demonstration of software correctness and of its contribution to reliability:

2.5.2.1.1 As already stated in chapter 2.2, executable software components can be of 

different types: operating system, library, application software, intelligent drivers, sensing 

and actuation devices, communication protocols, man-machine interfaces, etc. These 

components do not contribute in the same way to the safety of the whole system. Specific 

evidence and methods of validation and verification are necessary to show that these 

components comply with the safety requirements of the system.
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2.5.2.1.2 Software behaviour can be highly sensitive to “small” programming, coding or 

even typing mistakes. Further, the functionality of software based systems is usually more 

complex than that of their analogue counterparts. Therefore it is difficult to estimate how 

and to what extent the software may affect the overall system reliability.

2.5.2.1.3 Although validation is an important final step to obtain evidence of the 

software correctness, experience shows that it must be complemented by the results of 

well-defined verification prior to the final validation.

2.5.2.1.4 A formal proof of correctness is sometimes possible for certain requirements 

that can be modelled and analysed by formal and mathematical techniques. In practice, 

this will not be possible for the overall software system which may include an operating 

system, communication protocols, etc.

2.5.2.1.5 The digital nature of a computer based system prevents advantage being taken 

of extrapolation and interpolation during the determination of the types and number of 

tests needed to demonstrate compliance with the requirements. This limitation obliges the 

verifier to carefully justify the amount of testing performed in relation to the required 

system reliability.

2.5.2.2 There is a wide spectrum of verification methods and tools. A selected set of these 

must be planned and shown to be appropriate to the lifecycle phase to which they are to be 

applied, and to the safety claims that are to be made.

2.5.2.3 Recent experience shows that the verification of safety critical software may be 

costly in time and resources. A verification plan therefore has to be carefully established early 

in a project in order to gain the maximum safety benefit from the expenditure of this time and 

resources. 

2.5.2.4 A detailed understanding of the relationship between verification results and 

software requirements is necessary in order to obtain evidence that the software fulfils its 

requirements and demonstrate the absence of undesired functionality and behaviour.

2.5.2.5 Confidence in verification results depends also on organisational measures; 

therefore those in charge of the verification are required to have the necessary qualifications, 

independence and knowledge of the system.

2.5.2.6 Some further issues such as the verification of pre-existing software and use of 

formal methods and tools are separately discussed in chapters 1.4, 1.5 and 1.9. 
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2.5.3 Common Position

2.5.3.1 Selection of Verification Methods and Tools

2.5.3.1.1 At the end of each phase of the life cycle (requirements, design, coding, hardwaresoftware integration), the output of the phase shall be verified against its input specifications.

2.5.3.1.2 The scope and depth of analysis and verification that is planned for the end of each 

phase shall be determined and justified in terms of the evidence it is intended to contribute to 

the demonstration of safety. This justification shall be documented, and made accessible to 

the regulator.

2.5.3.1.3 For each major component of target executable software (operating system, library, 

application software, intelligent drivers, sensing and actuation devices, communication 

protocols, etc.), as far as reasonably practicable, the absence of potentially unsafe remaining 

defects that can cause the loss of a safety function of the system shall be demonstrated.

2.5.3.1.4 In addition, different verification methods shall be combined in order to achieve a 

sufficient coverage of the functional and the non-functional requirements. The coverage 

achieved by this combination shall be determined and justified. This justification will be 

accessible to the regulator.

2.5.3.1.5 Every software module shall be verified by executing the code to a degree of 

coverage that complies with the common positions in section 2.5.3.3.

2.5.3.1.6 The verification of the system integration shall include testing by executing the 

code.

2.5.3.2 Verification Planning

2.5.3.2.1 A verification plan which specifies the verification steps, their schedule, the 

procedures, the persons in charge, the contents of the verification reports, and the follow-up 

for detected faults and anomalies will be established and made accessible to the regulator early 

in the project.

2.5.3.2.2 The verification plan shall cover in detail all distinct software development phases 

as well as the different levels of system integration and software architecture (module, 

program, and subroutine).

2.5.3.2.3 The verification plan shall address the criteria, strategy and equipment that are 

necessary for each verification step. Furthermore the verification plan shall clearly justify and 

define the intended test coverage.
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2.5.3.3 Coverage

2.5.3.3.1 The range of values and all the discontinuity and boundary neighbourhoods of 

every input variable shall be tested. Attention shall be paid to the use of equivalence 

partitioning.

2.5.3.3.2 All modes of system operations shall be considered during the design of test cases.

2.5.3.3.3 Each distinct test at system or module level shall monitor all outputs of the system 

or module.

2.5.3.3.4 All interfaces (between modules, and between programs), and all communication 

protocols shall be tested.

2.5.3.3.5 Fault tolerant and exception mechanisms (eg divided by zero, out of range values, 

overflows) shall be tested. If this is unfeasible, evidence of their correct behaviour shall be 

provided by other means.

2.5.3.3.6 Test specifications shall ensure that all lines of code, and – at system integration –

all module calls are exercised, and coverage analysis shall justify the coverage of these tests.

2.5.3.3.7 Data structures and declared constant values shall be verified for correctness.

2.5.3.3.8 Operations on – and access to – all data items shall be exercised, and the coverage 

of these tests shall be justified.

2.5.3.3.9 All time critical sections of code shall be tested under realistic conditions, and the

coverage of these tests shall be justified.

2.5.3.3.10 The result of calculations performed by software shall be verified against precalculated values.

2.5.3.3.11 The adequacy of commentaries and code documentation shall be verified.

2.5.3.3.12 Compliance with the appropriate project design and coding directives and 

standards shall be verified.
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2.5.3.4 Traceability and Documentation

2.5.3.4.1 Test plans, procedures and results shall be documented. The expected test results 

shall also be documented, together with the method for their derivation. This documentation 

should be accessible prior to the execution of the tests.

2.5.3.4.2 Test results shall clearly indicate which tests met expectations and which did not.

2.5.3.4.3 All anomalies revealed by the verifications shall be recorded and investigated. The 

results and follow-up of these investigations shall be reported and made accessible to the 

regulator. If software changes are necessary, change requests shall be issued and processed 

according to the procedures of chapter 2.7.

2.5.3.4.4 Using the documentation, it shall be possible to trace each verification result back 

to the associated functional and non-functional requirement(s). For this purpose appropriate 

comments shall be contained in the source code documentation.

2.5.3.5 Independent Verification

2.5.3.5.1 For safety system software, the planned verification shall be designed and 

performed by personnel that are independent from the team that designs and produces the 

software. This requirement, however, does not apply to the verification of the hardwaresoftware integration phase.

2.5.3.5.2 All communications and interactions between the verification team and the design 

team, which may have a significant bearing on the verification results, shall be recorded in 

writing.

2.5.3.5.3 All verification activities shall be comprehensively documented to enable auditing.

2.5.3.5.4 Personnel conducting the tests shall have received proper training in the use of the 

applicable test tools and methods.
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2.5.4 Recommended Practices

2.5.4.1 In order to limit the high effort and costs of verification that may be involved, an 

optimised ratio between static analysis techniques and testing should be found.

2.5.4.2 By using development tools, some of the neighbouring development steps of the 

classical software life-cycle model can be combined – for instance the steps from design 

specification to software implementation. In such cases, a special verification effort should be 

dedicated to the qualification of the applied tools (see chapter 1.5) and of the target object 

code.

2.5.4.3 In addition to the requirements of IEC 60880, the application of verification 

concepts based on formal methods – eg symbolic code execution and formal code verification

– and automated transformation tools are recommended. 

2.5.4.4 As a precaution against the introduction of erroneous or unwanted code –

introduced for example by translation tools – the machine code should be translated into a 

form suitable for comparison with the specifications (reverse engineering). This process can 

be carried out either manually or using software aids.

2.5.4.5 Anomalies only should be indicated and reported by the verification team; they 

should not recommend design changes.

* * *
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2.6 Validation and Commissioning

2.6.1 Rationale

Validation and commissioning are processes that demonstrate through testing that the system

will perform its intended functions. These processes of the safety demonstration are regarded 

as essential since the current analysis techniques do not enable the correctness of a system to 

be fully demonstrated.

More precisely, validation in this consensus document is regarded as the test and the 

evaluation of the integrated computer based system hardware and software to demonstrate 

compliance with its functional and non-functional requirements specifications. 

Commissioning is regarded as the onsite process during which plant components and systems, 

having been constructed, are made operational and confirmed to be in accordance with the 

design assumptions and to have met the safety requirements and the performance criteria. For 

the purposes of this Consensus Document it will be assumed that commissioning follows the 

principal stages given in the IAEA Safety Guide No 50-SG-D4 [7], “Commissioning 

Procedures for Nuclear Plants”, namely: pre-operational testing; cold performance testing; hot 

performance testing; loading of fuel and sub-critical testing; start-up to initial criticality phase; 

low power tests.

2.6.2 Issues Involved

2.6.2.1 The digital nature of a computer based system precludes the use of extrapolation 

and interpolation in the determination of the type and number of tests needed to demonstrate 

compliance with the requirements. This limitation obliges the validater to carefully justify the 

amount of testing performed in relation to the required system reliability. As for verification

tests, use may have to be made of equivalence partitioning and boundary conditions. There is 

also the problem of demonstrating that inaccessible features (those not readily testable at the 

validation stage) have been tested satisfactorily at the commissioning stage. Finally, the level 

of input simulation for an acceptable demonstration has to be shown to be adequate.

2.6.2.2 As already stated in chapter 2.2, executable software components can be of 

different software types. These components do not contribute in the same way to the safety of 

the whole system. Specific evidence and methods of validation and verification are necessary 

to show that these components comply with the safety requirements of the system.
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2.6.2.3 During the production of the software, there are occasions when the designer 

recognises the need for an “added feature”. It is important that these added features are 

reflected in the appropriate requirement specifications and that they are included in the 

validation tests.

2.6.2.4 Since it is neither safe nor reasonably practicable to test the behaviour of a safety 

system using real accident scenarios on the plant, this aspect of the system has to be tested 

using simulated scenarios which represent the dynamic behaviour of the plant. The derivation 

of representative scenarios can be problematical.

2.6.2.5 There is a possibility that the assumptions made about the behaviour of a device 

connected to the system may be incorrect – and, therefore, that the simulation model might be 

inadequate. This possibility leads to the need for a justification of the correctness of the 

simulation of all devices – in terms of input and output – during validation and 

commissioning.

2.6.2.6 It is important that the design team does not influence the validation activity since 

this could be a source of common cause failure due to inappropriate test specifications or 

incorrect interpretation of results. Therefore, there should be some agreed level of 

independence between these two teams. This independence, however, should not deprive the 

validation team from acquiring sufficient knowledge of – and familiarity with – the system.

2.6.2.7 There is also the problem of determining the degree of test coverage required and 

the adequate level of simulation at each of the commissioning phases mentioned in the 

rationale section 2.6.1.

2.6.3 Common Position

2.6.3.1 Validation

2.6.3.1.1 The computer system validation shall be conducted in accordance with a formal 

validation plan. 

2.6.3.1.2 The plan shall identify the intended coverage of the validation and justify it with 

respect to the requirement of common position 2.1.3.1.

2.6.3.1.3 The plan shall identify acceptance criteria, techniques, tools, test cases and 

expected results.

2.6.3.1.4 Validation tests shall be performed on target hardware and under representative 

environmental conditions.

2.6.3.1.5 The test programme shall encompass all modes of operation (covering particularly 

maximum loadings) and ranges of variables under conditions which are as representative of 
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the operating environment as possible, including relevant normal operations, anticipated 

operational occurrences and accident conditions.

2.6.3.1.6 Testing shall be conducted in a disciplined and orderly manner and in accordance 

with IEC 60880 [12]. During every test, the status of all outputs shall be monitored to ensure 

that unintentional changes do not occur. The proposed tests shall exercise the software across 

the full range of the requirement specifications paying particular attention to boundary 

conditions and making justified use of equivalence partitioning.

2.6.3.1.7 Timing conflicts and bus contention problems are of importance for validation 

because they cannot be tested comprehensively in earlier phases. These aspects shall be 

thoroughly tested. Adequacy of spare CPU time, scan times and data transfer rates shall be 

demonstrated.

2.6.3.1.8 It shall be ensured that the requirements specifying the fault, exception and failure

mitigating mechanisms (see common positions 2.2.3.6 and 2.2.3.7), and the requirements 

which specify the system behaviour in the presence of abnormal inputs and conditions (see 

common position 2.1.3.2.4), are correctly implemented.

2.6.3.1.9 The system shall also be subjected to appropriate validation tests to check that the 

set of system requirements is complete, consistent and correct.

2.6.3.1.10 All time critical sections of code shall be tested under realistic conditions, and the 

coverage shall be justified. Where calculations are performed in the software their result shall 

be checked against pre-calculated values.

2.6.3.1.11 Appropriate tests shall be done to verify the fault tolerance of the system. These 

shall include a series of tests that demonstrate that the system is tolerant of input sets that are 

outside its operational input space. The chosen set of tests shall be justified in terms of its 

appropriateness to the operational profile.

2.6.3.1.12 All tests shall be fully documented and analysed, and each test shall be traced to 

the associated system requirement specification. This documented analysis shall be used to 

demonstrate the test coverage achieved.

2.6.3.1.13 The expected outputs shall be stated in the test procedures before the tests are 

undertaken. In each test, all outputs shall be monitored, and any anomaly investigated and its 

resolution fully documented.

2.6.3.1.14 All changes to either software or test specifications shall be subjected to change 

control procedures that comply with the recommendations of chapter 2.7.
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2.6.3.1.15 Features added during the design process shall be fully tested. These shall be 

traced to the changes in the system or design requirement specifications to demonstrate the 

consistency, completeness and correctness.

2.6.3.1.16 The validity of simulated inputs shall be justified and tests shall demonstrate the 

correct operation of simulated items.

2.6.3.1.17 The system shall be tested using a simulation of the accident scenarios. These tests 

shall be based on an analysis of the plant transients induced by postulated initiating events. 

The number of tests executed shall be sufficient to provide confidence in the system safety.

2.6.3.1.18 The validaters shall be independent from the team producing the requirement 

specifications and the software. All communications and interactions with significance to 

validation results shall be recorded in writing. The validation team shall review the 

specifications for correctness and completeness, and shall devise and implement the test 

strategy that demonstrates that the system meets its requirements.

2.6.3.1.19 All validation activities shall be comprehensively documented to enable auditing. 

The validation team shall indicate anomalies only; they shall not recommend design changes.

2.6.3.1.20 The system operation and maintenance manuals shall be validated, as far as 

possible, during the validation phase.

2.6.3.2 Commissioning

2.6.3.2.1 Commissioning tests shall ensure that both the system and the plant safety 

requirements are complete and correct.

2.6.3.2.2 The number and type of commissioning tests shall be justified. The omission of 

any validation test from the commissioning tests shall be justified.

2.6.3.2.3 The validation of the system operational and maintenance manuals shall, if 

necessary, be completed during the commissioning phase.

2.6.3.2.4 On-site testing of the full system (software and hardware) over an extended period 

(the duration will depend on system complexity) shall take place. During this period the 

system shall be subjected to routine test and maintenance activities. A log shall be kept of any 

revealed faults, and appropriate actions shall be taken in agreement with chapter 2.7.
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2.6.4 Recommended Practices

2.6.4.1 For safety systems, the system reliability should be estimated using statistical 

testing. The applied tests should be randomly selected from the operational input space and 

their number should be based upon the safety system required reliability and confidence level.

2.6.4.2 The validation plan shall be defined as soon as possible during the project.

2.6.4.3 The traceability of the tests to the requirement specifications should be based on a 

requirement matrix, that contains for every software requirement a reference to the tests which 

cover this requirement, and to the lifecycle phase (verification step, validation, 

commissioning) where they have been performed.

* * *

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2.7 Change Control and Configuration Management 

2.7.1 Rationale

2.7.1.1 Changes to the software of safety systems in a nuclear plant have the potential to 

affect significantly the safety of that plant if incorrectly conceived or implemented. Any 

alteration to the software of these systems at any phase in the system lifecycle, whether due to 

an enhancement or a need to adapt to a changing environment (resulting in a modification to 

the system requirements) or to correct a defect must be conceived and implemented at least to 

the same standards as the original implementation. This point is particularly important in the 

case of changes made after delivery. Furthermore, there should be procedures in place to 

ensure that the effects of such changes, on all parts of the system, are assessed to reduce to 

acceptable levels the potential for faults to be introduced.

2.7.1.2 A software-based safety system is formed from many different items of software 

and hardware and includes many documents that describe these items. Hence it is important 

that there is a full index of the items involved in the construction of a system, and that the 

status of each item (including changes made) is established and tracked so that faults are not 

introduced due to the incorrect versions of these items being used. A well ordered 

configuration management system provides the means to ensure this, as well as playing a 

significant role in a properly managed change control process.

2.7.2 Issues Involved

2.7.2.1 Software changes, as with any other changes, can occur either during the 

development of a system up to and including the end of commissioning (usually classed as 

“software modifications”), or in operation following the completion of commissioning at site 

(usually classed as “software maintenance”).

2.7.2.2 The international standards IEC 60880 [12] and IEC 61513 [15] contain the 

necessary elements for an acceptable software change process. However, for regulatory 

purposes and for safety systems there is a need to add an additional dimension to the change 

process. This additional dimension results from the need for: (i) an analysis of the effect of 

the change on safety; (ii) the provision of documentary evidence that the change has been 

conceived and implemented correctly; and (iii) a process of independent review and approval 

of the change. The reviewing process described in common position 2.7.3 is a clarification for 

licensing purposes of the requirements of the standard.
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2.7.2.3 The need for a software change has to be carefully evaluated because the 

consequences of a change are not always easily anticipated. For example, other faults may be 

introduced.

2.7.2.4 Consideration should be given during the design phase to software maintainability 

since this attribute will reduce the number of faults introduced by any software change 

process.

2.7.2.5 If the configuration management is inadequate then earlier versions of software and 

documentation may be inadvertently incorporated into the current version of the system or 

documentation. These earlier version items may include faults or properties that are unsafe 

when used in the current version.

2.7.2.6 For changes enacted post delivery to site (particularly during commissioning and 

operation), there is a need to ensure continuity of a rigorous regime of software change 

control.

2.7.3 Common Position

2.7.3.1 General

2.7.3.1.1 The software change procedure and documentary process shall apply to all 

software elements of the system including its documentation. This procedure shall apply 

equally to system functionality enhancements, environmental adaptations (resulting in a 

modification to the system requirements), changes in the elements (including hardware) on 

which the software depends and corrections of implementation defects. 

2.7.3.1.2 An appropriate software architecture (see chapter 2.3 for guidance on software 

design features which aid maintainability) and a suitable software configuration management 

system (see section 2.7.3.4) shall be used during the lifecycle process in order to maintain 

safety. For safety systems, no distinction shall be drawn between major and minor software 

changes since a wrongly implemented change could affect safety. 

2.7.3.1.3 Once a software or documentation item has been approved, ie has been placed 

under configuration control (usually following its initial verification and placing in the 

software/documentation library for release), any changes to this item shall be controlled by a 

procedure containing the elements given in the following paragraphs 2.7.3.1.4, 2.7.3.1.5 and

2.7.3.1.6.

2.7.3.1.4 The software change procedure and the configuration management system shall 

include an adequate problem reporting and tracking system.
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2.7.3.1.5 The software change procedure shall contain the following basic elements:

– the identification and documentation of the need for the change;

– the analysis and evaluation of the change request, including: a description of the design 

solution and its technical feasibility; and its effect on the safety of the plant and on the 

software itself;

– the impact analysis of each software change: for each software change, the implementers

of the change shall produce a software impact analysis containing a short description of 

the change plus the relationship with the hardware and software items, a list of software 

parts affected by the change and the extent to which the functional and non-functional 

requirements remain satisfied. Particular attention shall be applied to the interfaces 

between modified and unmodified software. Objective evidence that the full impact of 

each software change has been considered by the implementer for each software part 

affected shall be provided. As a minimum this shall consist of a summary description of 

the change to be implemented, plus documentary evidence of the effect of the change on 

other software parts. The software impact analysis shall also list data items (with their 

locations – scope of the data item) that are affected by the change plus any new items 

introduced and the relationship of the software items with the affected or introduced data 

items.

– the implementation (consistent with the standards employed in the original production 

process), verification, validation (as appropriate) and release of the changed software or 

document item. The V&V phases may make use of the software impact analysis to 

perform regression testing.

– The requirements of IEC 60880 [11] sections 9.1, 9.2 and 9.3 shall apply.

2.7.3.1.6 Faults shall be analysed for cause and lack of earlier detection. Any generic 

concern shall be rectified and a report produced.

2.7.3.1.7 Change control and configuration management tools shall be qualified according to 

1.5.3.

2.7.3.2 Software Modification

2.7.3.2.1 A correction to a wrongly implemented software change, if found at the stage of 

site testing (ie following installation of the software on the plant), shall be processed as though 

it were a new software change proposal. 

2.7.3.2.2 The commissioning team shall use the software impact analysis and the factory 

V&V report to develop its own test specification and site commissioning test schedule.
150

2.7.3.3 Software Maintenance

2.7.3.3.1 All software changes in operation following the completion of commissioning at 

site (called software maintenance in this document), shall be controlled by procedures which 

meet the requirements of this section 2.7.3.3. 

2.7.3.3.2 For safety systems, program and fixed data (including operational data) shall be 

held in read only memory (ROM) so that they cannot be changed on-line either intentionally 

or due to a software malfunction.

2.7.3.3.3 For safety systems, software maintenance changes shall be tested on the computer 

system installed at site. Any divergence from this shall be justified in the test documentation.

2.7.3.3.4 Proposed software changes shall be analysed and reviewed for effect on safety, by 

suitably qualified and experienced staff – eg manufacturers (suppliers), system designers, 

safety analysts, plant and operational staff – that are independent from those persons 

proposing, designing and implementing the change. The analysis and review shall consider 

manufacturers (suppliers) V&V and independent assessment reports, as well as test 

specifications and test reports, or other such documentation as appropriate to the change being 

proposed. The results of the analysis and review shall be documented and shall include a

recommendation for approval, or rejection of the change from the safety perspective.

2.7.3.3.5 Only software approved for release shall be installed at site.

2.7.3.3.6 Before the start of site testing of any changes to the software of a safety system, the 

test specification shall be reviewed by independent reviewers.

2.7.3.3.7 Before permitting the operational use of software following a change, an updated 

and checked version of the system and safety demonstration documentation (including any 

routine test schedules) shall be available, fully reflecting the changes that have been made. 

Compliance with this requirement shall be confirmed by independent review.

2.7.3.3.8 The independent reviews shall be documented.

2.7.3.4 Configuration Management

2.7.3.4.1 All the items of software including documents, data (and their structures) and 

support software shall be covered by a suitable, readily understood and fully documented 

configuration management system (CMS) throughout the lifecycle. An item of software or 

documentation shall be accessible only to those who have a legitimate need (eg to persons 

responsible for its design and verification) until it is approved and under configuration control.
151

2.7.3.4.2 A formal procedure shall be set up for version control and the issuing of correct 

versions. Provision shall be made for informing all relevant personnel, including the regulator 

(in accordance with regulatory expectations), of pending changes and approved modifications. 

2.7.3.4.3 All software copies, including any pre-existing software that is being used, shall be 

clearly and uniquely labelled with at least title, version number and creation or acquisition 

date. The identification and version number shall be included in the code so that this can be 

checked. For safety systems, changes made, approval status, and the names of authors, 

reviewers and approvers, shall be included in the source code listing or explicitly documented.

2.7.3.4.4 After delivery of the software and its documentation, the same level of 

configuration management shall be maintained at the site where the delivered software is 

stored.

2.7.3.4.5 A configuration audit shall be performed on the safety system software prior to 

loading to establish that the correct items and versions have been included in the system. 

Following system loading, the loaded version shall be verified as being uncorrupted.

2.7.4 Recommended Practices

2.7.4.1 Any software item intended to be used as part of a safety system and that has not 

been subjected to the above software change process should be assessed on the basis of 

chapter 1.4. 

2.7.4.2 Where a pre-existing software item (qualified to chapter 1.3) is introduced into a 

safety system as part of the change process, its impact on the system should be evaluated as 

appropriate to the same level as described by common position 2.7.3. 

2.7.4.3 It is always good practice to make only one software change at a time within the 

same system unless changes are shown to be independent. Exceptions to this practice are 

permissible but once significant quantities of nuclear material have been introduced into a 

plant (eg fuel loaded into a nuclear reactor) then there should be no more than one software 

change being implemented on a safety system at any one time.

2.7.4.4 To aid backward traceability, the configuration management system should 

maintain lists of items, and their changes, that were included in previous baselines. These lists 

should include names of individuals and organisations to whom configuration items have been 

distributed.

* * *

153

2.8 Operational Requirements

2.8.1 Rationale

If the processes that support and maintain the safety systems of nuclear installations during 

their operational use are not of a sufficiently high integrity then they may jeopardise the safety 

system's fitness for purpose. For example, the accuracy of calibration data, or the process of 

loading data that are required for core limits' calculation, or the conversion of electrical signals

into engineering units can have a significant effect on the behaviour of the safety system; and 

could, in fact, lead to a failure to trip. Also, the incorrect operation of automatic test 

equipment could result in a fault remaining un-revealed in the tested system with the potential 

for spreading out, even over redundant trains during later operation.

Proper operators’ training, as well as the development of a periodic test plan, is necessary in 

order to ensure correct and safe plant operation.

2.8.2 Issues Involved

2.8.2.1 Periodic Testing

As a part of a safety system, the computer based system needs to undergo periodic testing in 

order to verify maintenance of its basic functional capabilities.

2.8.2.2 Generation of Calibration Data

A computer-based safety system on a nuclear installation is dependent for its correct operation 

on a large amount of calibration data that varies during the operational life of the installation 

due to fuel burn-up, equipment failure and planned outages. These data are derived from a 

variety of sources often involving both manual and automatic activities. As mentioned above, 

mistakes in these data could adversely affect the integrity of the safety system. For instance, a 

mistake in the core flux mapping readings (either in terms of instrument readings or positional 

measurements) could prevent the reactor's core limits calculator from tripping the reactor on a 

reactivity event. Also, wrong calibration data could mean that an instrument is generating an 

improper reading resulting again in a failure to actuate safety equipment.
154

2.8.2.3 Loading of Calibration Data

Even when the data is correctly generated there is the potential for faults to be introduced into 

the safety system due to incorrect entry of that data. Data entry may be either manual or 

electronic (either via a data link or from magnetic media). Manual data entry has all the usual 

problems of human error (transposition mistakes, misreading and incorrect addressing). 

Equally, when data is entered electronically, there is the possibility of data corruption eg due 

to programming faults or electronic noise.

2.8.2.4 Self-Supervision Functions and Automatic Test Equipment

Digital computer technology has the capability to perform extensive self supervision functions 

as well as routine testing of safety systems by means of test equipment. When all trains are 

subjected to identical supervision or tests via identical software, there is the potential for 

common cause failure of the safety system due to a design defect in the supervision functions 

or the tester.

2.8.2.5 Plant Component Test and Corrective Maintenance Equipment

Certain activities of maintenance on redundant items of equipment are allowed whilst the 

reactor is at power. This may involve the replacement of an output card of a guardline, the 

renewal of cabling between the guard-line and the plant item or the repair of devices 

controlled by the computer.

When equipment is replaced while the reactor is at power, there is the potential for causing the 

reactor protective system to spuriously actuate or to place the reactor into an unanticipated 

state. This type of event should be addressed.

2.8.3 Common Position

2.8.3.1 Periodic Testing

2.8.3.1.1 A program for safety systems periodic tests, that includes applicable functional 

tests, instruments checks, verification of proper calibration and response time tests, and 

maintenance of all associated records, shall be defined and implemented. The tests shall 

verify periodically the basic functional capabilities of the system, including basic safety and 

safety related functions, major functions not important to safety, and special testing used to 

detect failures unable to be revealed by self-supervision or by alarm or anomaly indications. 

Periodic testing shall not adversely affect the intended system functions. 
155

2.8.3.1.2 It shall be demonstrated and documented that the combination of the provisions of 

automatic self-supervision and periodic testing during operations (cf. common positions

2.2.3.7 and 2.2.3.10) cover all postulated faults.

2.8.3.1.3 The operator interface shall include the means to allow the operator to easily obtain 

confirmation that the computer system is alive and that the information produced on displays 

and screens is properly refreshed and updated (cf. also common position 2.1.3.4).

2.8.3.2 Generation of Calibration Data

The calibration data shall be generated according to the safety and reliability requirements of 

the I&C function to which they are assigned. An appropriate quality assurance procedure shall 

be established.

2.8.3.3 Loading of Calibration Data

2.8.3.3.1 Where data is loaded manually there shall be a read-back facility that requires the 

operator to confirm the data entered before he/she is allowed to proceed to the next item. 

2.8.3.3.2 On completion of the data entry there shall be a printout of all data entered. 

Procedures shall be in place to require that printout to be checked by an independent party 

before the guardline is put back into service.

2.8.3.3.3 Where the data is entered electronically, that set of data shall be covered by a 

checksum which will enable the data to be verified. In addition, the data shall be read back

and verified automatically against the original. Printouts of the data entered and that held in 

the guardline shall be provided and retained for auditing purposes.

2.8.3.3.4 The software for loading the calibration data and monitoring data values shall be of 

the same integrity as the safety system. If this is not the case then it shall be demonstrated by 

means of a software hazard analysis that the safety system software and data cannot be 

corrupted by the loading/display software.

2.8.3.3.5 A full test of the guardline shall be performed following a data change so as to 

demonstrate its correct operation.
156

2.8.3.4 Equipment for Periodic Testing

2.8.3.4.1 The test equipment software shall be qualified to a level in accordance with its 

intended use.

2.8.3.4.2 So as to avoid common mode failures, constant and fixed parameter values to be 

used by the testing equipment shall not be obtained from the computer of the safety system, 

but eg from the system or the computer design requirement specifications.

2.8.3.4.3 On termination of the testing operations, the test equipment shall restore the safety 

system to its original status.

2.8.3.4.4 The test equipment shall be subjected to periodic calibration checks.

2.8.3.5 Equipment for Plant Component Testing and Corrective Maintenance

2.8.3.5.1 Integrated means of testing plant items shall be engineered as part of the safety 

system. Any additional tool shall be qualified according to 1.5.3. Engineered test systems and 

their user interfaces shall be designed to standards commensurate with their duty. They shall 

provide a read-back facility, with confirmation by the operator, of the plant item selected 

before it is activated for test. Such test equipment shall restrict the activation to one plant item

at a time. A plant item shall be returned to its original entry status before another item is 

selected.

2.8.3.5.2 It shall not be possible to connect this type of test equipment to more than one 

guardline at any one time.

2.8.3.5.3 Bypass of actuation signals for maintenance of output devices and components

shall be indicated by alarm and be recorded as unavailable.

2.8.3.6 Operator Training

2.8.3.6.1 Instrumentation and control specialists shall follow a training program that 

addresses safety system performances, operation and maintenance during normal and 

abnormal reactor operation.

2.8.3.6.2 Operator training shall be conducted on a training system which is equivalent to 

the actual hardware/software system. Training facilities for each operator interface device 

shall be provided by the training system. A computer simulator may be used for this purpose.
157

2.8.3.7 Operational Experience

Operational experience shall be recorded and cover all operating situations. The records shall 

be appropriately structured and analysed so as to allow data on the system functional and non 

functional behaviour to be derived over given periods of time. The records shall be made 

accessible to the regulator. The documentation of the operating experience shall comply with 

the requirements of common positions 1.14.3.1 and 1.14.3.2. It shall be assured that the safety 

demonstration remains valid in the light of the operating experience. The common positions 

of sections 2.7.3.1, 2.7.3.2, 2.7.3.3 are applicable.

2.8.3.8 Requirements to support inspection

2.8.3.8.1 The licensee shall facilitate effective inspection by the regulator and make available, 

and provide on request, all the information necessary for the regulator to carry out its 

inspection program. This program may depend on the applied technology and comprise 

activities at different points in the development, operation and maintenance of the systems 

important to safety. Activities for software-based systems may include random observations 

during operation, specific inspections in the case of an event or modification, and planned 

inspections to accompany periodic testing.

2.8.3.8.2 In particular, the licensee shall make available and provide on request all the 

information necessary to comply with the common positions of this document. The table 

below highlights some of the common positions that are particularly relevant to inspection.

Inspection topic Common position

Safety plan 1.1.3.3

System upgrades 1.1.3.11

Assessing pre-existing software 1.4.3.1 and 1.4.3.4

Tool qualification 1.5.3.2

Operating experience 1.14.3.1 to 1.14.3.8

Software change procedure 2.7.3.1.5

Version control procedure 2.7.3.4.2

Periodic testing 2.8.3.1.1
158

2.8.4 Recommended Practices

2.8.4.1 Periodic Testing

The quality of the computer equipment and software used for periodic testing functions should

comply with the recommendations for tools mentioned in Chapter 1.5 (see in particular 

common position 1.5.3.2).

2.8.4.2 Generation of Calibration Data

2.8.4.2.1 Unless the calibration data is generated totally automatically via a system that has 

been developed to the same standards as the safety systems then there should be a diverse

means for the production of the data or an independent verification of the data. This diverse 

means of verification may be either manual or automatic but it should be demonstrated that 

the combination of the principal and the diverse means of verification forms a sufficiently 

high integrity process so as not to degrade the safety system. All calculations should be 

documented and retained for future audit.

2.8.4.2.2 The principal means for the generation of the data should be computer assisted so 

as to reduce the potential for the introduction of human error. The software should be 

produced to high quality industrial standards. Verification check points should be introduced 

at convenient points.

2.8.4.3 Loading of Calibration Data

The process of loading and changing the calibration data (eg in the case of changing the 

operational mode) should be computer assisted using pre-calculated and fixed stored 

calibration data sets. The software for loading calibration data should be to the same 

standards as the safety system software.

* * *
159

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* * *
163

Vous dépendez, dans une affaire qui est juste et importante, 

du consentement de deux personnes. L’un vous dit:”J’y donne les mains pourvu qu’un tel 

condescende”; et ce tel condescend 

et ne désire plus que d’être assuré des intentions de l’autre.

Cependant rien n’avance; les mois, les années s’écoulent inutilement:

”Je m’y perds, dites-vous, et je n’y comprends rien;

il ne s’agit que de faire qu’ils s’abouchent et qu’ils se parlent.”

Je vous dis moi, que j’y vois clair, et que j’y comprends tout: 

ils se sont parlé.

La Bruyère, Les caractères de la cour, (1688, 1st version – 1694, 8th and last version)

A-1

APPENDIX A: Using TF SCS CPs in NRC Regulatory Guidance

The purpose and scope of this appendix is to assist U.S. Nuclear Regulatory Commission 

(NRC) users4 in understanding and interpreting the Common Positions (CPs) of the Regulator 

Task Force on Safety Critical Software for nuclear reactors (TF SCS) in the context of the 

NRC’s regulatory and guidance framework and deriving value5 from these CPs through future

improved guidance and to support judgment, when needed, in licensing reviews.

A.1 Background

The TF SCS common positions are based on TF SCS members’ regulatory experiences during 

a period of almost two decades. These experiences are very valuable. At the request of the TF 

SCS, the NRC reviewed parts of the TF SCS’s CP document chosen by the TF SCS to try to 

identify changes needed for the NRC to support the common positions. The parts of the 

document chosen by the TF SCS were the third sections (titled “Common Positions”) of each 

chapter6

. The TF SCS considered the NRC’s review comments and adjusted those CPs where 

it was agreed that the changes would improve the technical consistency between the TF SCS 

and the NRC. The NRC has begun to cite specific CPs in its technical work (e.g., RIL-1101,

“Technical Basis to Review Hazard Analysis of Digital Safety Systems”).

In many cases, NRC reviewers analyzed specific differences by understanding the underlying 

issues that the TF SCS CPs were addressing, identifying broader classes of similar issues, and 

formulating correspondingly generalized CPs. This appendix explains the remaining differences 

and proposes approaches by which the NRC could derive value:

1. Where the NRC’s existing regulatory and guidance framework is different from the TF 

SCS’s regulatory framework, the NRC-appended information in Section A.3 below 

discusses the differences and scope restrictions of the TF SCS’s CPs.

2. Where the TF SCS vocabulary differs from that used by the NRC or where terms have 

multiple meanings across different usage contexts, to avoid the potential for confusion, 

the NRC-appended information in Section A.4 below provides a differentiation of how 

such vocabulary terms used by the TF SCS correspond to those used in the NRC’s 

commentaries in this appendix.

 4 NRC staff, engaged in revision of the NRC’s standard review plan, regulatory guides, or standards cited 

therein. 5 By, for example, exercising judgment in regulatory review or assimilating in future revisions to regulatory 

guidance. 6 NRC reviewers also read the first section (“Rationale”) and second section (“Issues”) only in order to 

understand the context of the third section.
A-2

A.2 Scope of NRC’s Review

The initial scope of NRC’s review was limited to the parts of the TF SCS’s CP document chosen 

by the TF SCS (i.e., the third sections of Chapters 1.1 through 1.15 and 2.1 through 2.8) and 

excluded general introductory information (e.g., Chapter 1 and, in other chapters, the first 

section, “Rationale”; the second section, “Issues”; and the fourth section, “Recommended 

Practices”). On further examination, the NRC also excluded certain other chapters and 

paragraphs from the scope, as identified and explained below.

A.3 Scope Restrictions Due to Differences in Regulatory Frameworks

Differences in regulatory frameworks affect TF SCS CPs in four main categories.

A.3.1 Differences Resulting from Safety Classes and Graded Requirements

Although some TF SCS CPs apply to systems of different safety classes, the NRC’s regulations

do not identity corresponding safety classes. Therefore, NRC’s review was limited to CPs 

concerning the highest criticality level of safety classes. Specifically, Chapters 1.2 and 1.11 are 

excluded from the scope of the review.

A.3.2 Differences in Security-Related Regulations

In the NRC regulatory framework, “Security” reviews are performed under another part of the 

regulation that is different from the part for safety reviews. Thus, security-related CPs are 

excluded from the review scope. Specifically, Chapter 1.8 and subsections dealing with the 

applicability of other CPs to security (e.g., 2.1.3.2.2 and 2.3.3.1.1) are excluded from the scope 

of the review.

A.3.3 Scope Restrictions Concerning Probabilistic Safety Assessment

NRC’s regulatory review of digital safety systems is based on deterministic criteria. Therefore,

Chapter 1.13 and reliability-dependent clauses (e.g., subsections 1.5.3.1, 1.5.3.2, 1.6.3.4, 

1.10.3.2, 1.15.3.5, 1.15.3.8 through 1.15.3.8.12, 2.1.3.2.2, 2.6.3.1.15, and 2.7.3.1.5) are 

excluded from the scope of the review.

A.3.4 Information Available for Licensing Review, Especially Under 10 CFR Part 52

For CPs in Chapter 2.4, a code review would be needed to verify that the CP has been satisfied. 

However, code is not required to be submitted for NRC review under Title 10, “Energy,” of the 

Code of Federal Regulations (10 CFR) Part 52, “Licenses, Certifications, and Approvals for 

Nuclear Power Plants,” although the NRC might observe samples of the code during audit or 

inspection procedures to demonstrate proper software implementation.

However, the NRC can derive benefit from the concepts in Chapter 2.4 by understanding the 

issue being addressed in each CP and formulating7 an implementation-constraining guideline, in 

such a way that:

 7 Such repurposing does not imply deficiency in the TF SCS CP for TF SCS purposes.
A-3

1.1. Compliance with the guideline can be checked during licensing reviews and included in 

the licensing basis.

1.2. Compliance with the guideline can be checked during reviews for license amendments.

A.4 Resolution of Vocabulary Differences

The scope of the discussion on vocabulary is limited to this document.

Cases where there are differences from a TF SCS term to an NRC term (so that the NRC can 

derive value to improve its regulatory guidance) are described below.

A.4.1 Safety Classes

The TF SCS CP definitions of “Systems important to safety,” “Safety related systems,” and 

“Safety system” do not align with definitions in NRC regulation; thus, the terms are not directly 

usable by the NRC8

. For example, the TF SCS report defines “safety related systems” as:

Those instrumentation and control systems important to safety that are not included in 

safety systems.

However, U.S. 10 CFR 50.2, “Definitions,” defines “safety-related” as follows:

Safety-related structures, systems and components means those structures, systems 

and components that are relied upon to remain functional during and following design 

basis events to assure:

(1) The integrity of the reactor coolant pressure boundary9

(2) The capability to shut down the reactor and maintain it in a safe shutdown 

condition; or

(3) The capability to prevent or mitigate the consequences of accidents which could 

result in potential offsite exposure comparable to the applicable guideline 

exposures set forth in § 50.34(a)(1) or § 100.11 of this chapter, as applicable.

A.4.2 Usage of the Term “Requirement”

With respect to the TF SCS definition of the term “common position” (“Requirement or practice 

unanimously considered by the member states represented in the task force as necessary for 

the licensee to satisfy”), the NRC uses the term “requirement” to describe a mandatory condition 

in the regulations that is intended to be technology-independent and stable over a long period. 

Therefore, the TF SCS’s CPs, characterized through “shall” statements, should be mapped into 

additional guidance to improve technical consistency or to augment or clarify existing guidance. 

Precedents for such mapping exist in NRC regulatory guides citing IEEE standards; the latter 

typically include clauses called requirements, characterized using the modal verb “shall.”

 8 This is not an issue specific to the TF SCS. European standards and guides drive the difference. 

Harmonization is a long-term goal for the NRC, IAEA, IEC, and IEEE and goes beyond the scope of this 

effort. 9 Also defined in 10 CFR 50.2.
A-4

A.4.3 Cases for Which a Differentiation Could Not Be Found

For cases for which a differentiation could not be found at this time, to ensure freedom from 

confusion, the corresponding information was excluded from the scope of review and 

consideration for NRC’s guidance. Specifically, Sections 1.2, 1.11, and 1.13 were excluded from 

the scope of the review.

A.5 Abbreviations Used in Appendix A

IEC International Electrotechnical Commission

IEEE Institute of Electrical and Electronic Engineers

ISO International Standards Organization

CP common position

NRC Unites States Nuclear Regulatory Commission