ML18051A084

Clarification on Endorsement of Nuclear Energy Institute Guidance in Designing Digital Upgrades in Instrumentation and Control Systems

ML18051A084
Person / Time
Issue date:	03/01/2018
From:	Mcginty T, Chris Miller Division of Construction Inspection and Operational Programs, Division of Inspection and Regional Support
To:
Govan T
References
RIS-02-022, Supp 1
Download: ML18051A084 (22)

See also: RIS 2002-22

Text

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, D.C. 20555-0001

Month XX, 2018

DRAFT NRC REGULATORY ISSUE SUMMARY 2002-22, SUPPLEMENT 1

CLARIFICATION ON ENDORSEMENT OF NUCLEAR ENERGY INSTITUTE GUIDANCE IN

DESIGNING DIGITAL UPGRADES IN INSTRUMENTATION AND CONTROL SYSTEMS

ADDRESSEES

All holders and applicants for power reactor operating licenses or construction permits under

Title 10 of the Code of Federal Regulations (10 CFR) Part 50, Domestic Licensing of

Production and Utilization Facilities.

All holders of and applicants for a combined license, standard design approval, or

manufacturing license under 10 CFR Part 52, Licenses, Certifications, and Approvals for

Nuclear Power Plants. All applicants for a standard design certification, including such

applicants after initial issuance of a design certification rule.

All holders of, and applicants for, a construction permit or an operating license for non-power

production or utilization facilities under 10 CFR Part 50, including all existing non-power reactors

and proposed facilities for the production of medical radioisotopes, such as molybdenum-99,

except those that have permanently ceased operations and have returned all of their fuel to the

U.S. Department of Energy.

INTENT

The U.S. Nuclear Regulatory Commission (NRC) is issuing a supplement to Regulatory Issue

Summary (RIS) 2002-22, dated November 25, 2002 (Agencywide Documents Access and

Management System (ADAMS) Accession No. ML023160044). In RIS 2002-22, the NRC staff

endorsed Guideline on Licensing Digital Upgrades: EPRI TR-102348, Revision 1, NEI 01-01: A

Revision of EPRI TR-102348 to Reflect Changes to the 10 CFR 50.59 Rule, (Nuclear Energy

Institute (NEI) hereinafter NEI 01-01) (ADAMS Accession No. ML020860169). NEI 01-01

provides guidance for designing, licensing, and implementing digital upgrades and

replacements to instrumentation and control (I&C) systems (hereinafter digital I&C) in a

consistent and comprehensive manner.

The purpose of this RIS Supplement is to clarify RIS 2002-22, which remains in effect. The

NRC continues to endorse NEI 01-01 as stated in RIS 2002-22, as clarified by this RIS

Supplement. Specifically, the guidance in this RIS Supplement clarifies the NRC staffs

endorsement of the guidance pertaining to Sections 4, 5, and Appendices A and B of NEI 01-01.

This RIS Supplement clarifies the guidance for preparing and documenting qualitative

assessments, that can be used to evaluate the likelihood of failure of a proposed digital

modification, including the likelihood of failure due to a common cause, i.e., common cause

failure (CCF). Licensees can use these qualitative assessments to support a conclusion that a

ML18051A084

Draft RIS 2002-22 Supplement 1

Page 2 of 5

proposed digital I&C modification has a sufficiently low1 likelihood of failure. This conclusion,

and the reasons for it, should be documented, per 10 CFR 50.59(d)(1), as part of the

evaluations of proposed digital I&C modifications against some of the criteria in 10 CFR 50.59,

Changes, tests and experiments.

This RIS Supplement is not directed toward digital I&C upgrades and replacements of reactor

protection systems and engineered safety features actuation systems, since application of the

guidance in this RIS Supplement to such changes would likely involve additional considerations.

This RIS Supplement does not provide new design process guidance for addressing common

cause failure of the reactor protection systems and engineered safety features actuation

systems. Additional guidance for addressing potential common cause failure of digital I&C

equipment is contained in other NRC guidance documents and NRC-endorsed industry

guidance documents.

This RIS Supplement requires no action or written response on the part of an addressee.

BACKGROUND INFORMATION

By letter dated March 15, 2002, NEI submitted EPRI TR-102348, Revision 1 (NEI 01-01) for

NRC staff review. NEI 01-01 replaced the original version of EPRI TR-102348, dated

December 1993, which the NRC endorsed in Generic Letter 1995-02, Use of NUMARC/EPRI

Report TR-102348, Guideline on Licensing Digital Upgrades, in Determining the Acceptability

of Performing Analog-to-Digital Replacements Under 10 CFR 50.59, dated April 26, 1995

(ADAMS Accession No. ML031070081). In 2002, the NRC staff issued RIS 2002-22 to notify

addressees that the NRC staff had reviewed NEI 01-01 and was endorsing the report for use as

guidance in designing and implementing digital upgrades to nuclear power plant instrumentation

and control systems.

Following the NRC staffs 2002 endorsement of NEI 01-01, holders of construction permits and

operating licenses have used that guidance in support of digital design modifications in

conjunction with Regulatory Guide 1.187, Guidance for Implementation of 10 CFR 50.59,

Changes, Tests, and Experiments, dated November 2000 (ADAMS Accession

No. ML003759710), which endorsed NEI 96-07, Guidelines for 10 CFR 50.59 Implementation,

Revision 1, dated November 2000 (ADAMS Accession No. ML003771157).

NRC inspections of documentation for digital I&C plant modifications prepared by some

licensees using the guidance in NEI 01-01 identified inconsistencies in the performance and

documentation of licensee engineering evaluations. NRC inspections also identified

documentation issues with the written evaluations of the 10 CFR 50.59(c)(2) criteria. The term

engineering evaluation refers to evaluations performed in designing digital I&C modifications

other than the 10 CFR 50.59 evaluation, for example, evaluations performed under the

licensees NRC approved quality assurance program. This RIS Supplement clarifies the

guidance for licensees performing and documenting engineering evaluations and the

development of qualitative assessments.

In response to staff requirements memorandum (SRM)-SECY-16-0070 Integrated Strategy to

Modernize the Nuclear Regulatory Commissions Digital Instrumentation and Control Regulatory

1 NEI 01-01, Page 4-20, defines sufficiently low to mean much lower than the likelihood of failures that are

considered in the UFSAR (e.g., single failures) and comparable to other common cause failures that are not

considered in the UFSAR (e.g., design flaws, maintenance errors, calibration errors).

Draft RIS 2002-22 Supplement 1

Page 3 of 5

Infrastructure (ADAMS Accession No. ML16299A157), NRC staff has engaged the public,

including NEI and industry representatives, to improve the guidance for applying 10 CFR 50.59

to digital I&C-related design modifications as part of a broader effort to modernize I&C

regulatory infrastructure. Making available the guidance in this RIS Supplement is described as

a near-term action in the integrated action plan to provide specific guidance for documenting

qualitative assessments concluding that a proposed digital I&C modification will exhibit a

sufficiently low likelihood of failure.

Applicability to Non-Power Reactor Licensees

The examples and specific discussion in this RIS Supplement and other guidance referenced by

this RIS Supplement (i.e., NEI 01-01 and original RIS 2002-22) primarily focus on power

reactors. Nonetheless, licensees of non-power production or utilization facilities (NPUFs) may

also use the guidance in RIS 2002-22 and apply the guidance in this RIS Supplement to

develop written evaluations addressing the criteria in 10 CFR 50.59(c)(2). In particular, NPUF

licensees may use the guidance to prepare qualitative assessments that consider design

attributes, quality measures, and applicable operating experience to evaluate proposed digital

I&C changes to their facilities as described in Sections 4, 5, and Appendix A of NEI 01-01.

However, certain aspects of the guidance that discuss the relationship of other regulatory

requirements to 10 CFR 50.59 may not be fully applicable to NPUFs (e.g., 10 CFR Part 50,

Appendix A and B are not applicable to NPUFs).

SUMMARY OF ISSUE

In general, digital I&C modifications may include a potential for an increase in the likelihood of

equipment failures occurring within modified SSCs, including common cause failures. In

particular, digital I&C modifications that introduce or modify identical software within

independent trains, divisions, or channels within a system, and those that introduce new shared

resources, hardware, or software among multiple control functions, may include such a

potential. A qualitative assessment can be used to support a conclusion that there is not more

than a minimal increase in the frequency of occurrence of accidents or in the likelihood of

occurrence of malfunctions (10 CFR 50.59(c)(2)(i) and (ii)). A qualitative assessment can also

be used to support a conclusion that the proposed modification does not create the possibility of

an accident of a different type or malfunction with a different result than previously evaluated in

the UFSAR (10 CFR 50.59(c)(2)(v) and (vi)).

For digital I&C modifications, an adequate basis for a determination that a change involves a

sufficiently low likelihood of failure may be derived from a qualitative assessment of factors

involving system design features, the quality of the design processes employed, and an

evaluation of relevant operating experience of the software and hardware used (i.e., product

maturity and in-service experience). A licensee may use a qualitative assessment to document

the factors and rationale for concluding that there is an adequate basis for determining that a

digital I&C modification will exhibit a sufficiently low likelihood of failure. In doing so, a licensee

may consider the aggregate of these factors. The attachment to this RIS Supplement provides

a framework for preparing and documenting qualitative assessments and engineering

evaluations.

In addition, this RIS Supplement clarifies the applicability of some aspects of the NRC policy

described in Item II.Q of SRM/SECY 93-087, Policy, Technical, and Licensing Issues

Pertaining to Evolutionary and Advanced Light Water Reactor (ALWR) Designs, (ADAMS

Draft RIS 2002-22 Supplement 1

Page 4 of 5

No. ML003708056), in regard to the application of 10 CFR 50.59(c)(2) criteria for digital I&C

modifications.

BACKFITTING AND ISSUE FINALITY DISCUSSION

This RIS Supplement clarifies but does not supersede RIS 2002-22, and includes additional

guidance regarding how to perform and document qualitative assessments for digital I&C

changes under 10 CFR 50.59.

The NRC does not intend or approve any imposition of the guidance in this RIS Supplement,

and this RIS Supplement does not contain new or changed requirements or staff positions that

constitute either backfitting under the definition of backfitting in 10 CFR 50.109(a)(1) or a

violation of issue finality under any of the issue finality provisions in 10 CFR Part 52. Therefore,

this RIS Supplement does not represent backfitting as defined in 10 CFR 50.109(a)(1), nor is it

otherwise inconsistent with any issue finality provision in 10 CFR Part 52. Consequently, the

NRC staff did not perform a backfit analysis for this RIS Supplement or further address the issue

finality criteria in 10 CFR Part 52.

FEDERAL REGISTER NOTIFICATION

The NRC will publish a notice of opportunity for public comment on this draft RIS in the Federal

Register.

CONGRESSIONAL REVIEW ACT

This RIS is a rule as defined in the Congressional Review Act (5 U.S.C. §§ 801-808). However,

the Office of Management and Budget has not found it to be a major rule as defined in the

Congressional Review Act.

PAPERWORK REDUCTION ACT STATEMENT

This RIS provides guidance for implementing mandatory information collections covered by

10 CFR Part 50 that are subject to the Paperwork Reduction Act of 1995 (44 U.S.C. 3501 et.

seq.). This information collection was approved by the Office of Management and Budget

(OMB) under control number 3150-0011. Send comments regarding this information collection

to the Information Services Branch, U.S. Nuclear Regulatory Commission, Washington, DC

20555-0001, or by e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of

Information and Regulatory Affairs, NEOB-10202, (3150-0011) Office of Management and

Budget, Washington, DC 20503.

Public Protection Notification

The NRC may not conduct or sponsor, and a person is not required to respond to, a request for

information or an information collection requirement unless the requesting document displays a

currently valid OMB control number.

Draft RIS 2002-22 Supplement 1

Page 5 of 5

CONTACT

Please direct any questions about this matter to the technical contact(s) or the Lead Project

Manager listed below.

Timothy J. McGinty, Director Christopher G. Miller, Director

Division of Construction Inspection Division of Inspection and Regional Support

and Operation Programs Office of Nuclear Reactor Regulation

Office of New Reactors

Technical Contacts: David Rahn, NRR Wendell Morton, NRR

301-415-1315 301-415-1658

e-mail: David.Rahn@nrc.gov e-mail: Wendell.Morton@nrc.gov

Norbert Carte, NRR David Beaulieu, NRR

301-415-5890 301-415-3243

e-mail: Norbert.Carte@nrc.gov e-mail: David.Beaulieu@nrc.gov

Duane Hardesty, NRR

301-415-3724

email: Duane.Hardesty@nrc.gov (Specifically for non-power reactors)

Project Manager Contact: Tekia Govan, NRR

301-415-6197

e-mail: Tekia.Govan@nrc.gov

Note: NRC generic communications may be found on the NRC public Web site,

http://www.nrc.gov, under NRC Library/Document Collections.

Attachment: Qualitative Assessment and Engineering Evaluation Framework

Qualitative Assessment and Engineering Evaluation Framework

1. Purpose

Regulatory Issue Summary (RIS) 2002-22 provided the U.S. Nuclear Regulatory Commission

(NRC) staffs endorsement of Nuclear Energy Institute (NEI) Guidance document NEI 01-01,

Guideline on Licensing Digital Upgrades: EPRI TR-102348, Revision 1, NEI 01-01: A Revision

of EPRI TR-102348 To Reflect Changes to the 10 CFR 50.59 Rule. NEI 01-01 provides

guidance for implementing and licensing digital upgrades, in a consistent, comprehensive, and

predictable manner, as well as guidance in performing qualitative assessments of the

dependability of digital instrumentation and control (I&C) systems.

The purpose of this attachment is to provide supplemental clarifying guidance to licensees to

ensure that, if qualitative assessments are used, they are described and documented

consistently, through an evaluation of applicable qualitative evidence. Following the guidance in

RIS 2002-22 and NEI 01-01, as clarified by the guidance in this RIS Supplement, will help

licensees document qualitative assessments in sufficient detail that an independent third

party can verify the judgements, as stated in NEI 01-01. While this qualitative assessment is

used to support the Title 10 of the Code of Federal Regulations (10 CFR) 50.59, Changes tests

and experiments, evaluation, it does not provide guidance for screening and it does not

presume that all digital modifications screen in.

NEI 01-01 uses the terms qualitative assessment and dependability evaluations

interchangeably. Within this document only the terms qualitative assessment and sufficiently

low2 are used in conjunction with performance of 10 CFR 50.59 evaluations. The term

dependability evaluation is used in the context of engineering evaluations, which are not

performed or documented as part of a 10 CFR 50.59 evaluation, but engineering evaluations

are performed in accordance with the licensees NRC quality assurance program in developing

digital I&C modification.

If a qualitative assessment determines that a potential failure (e.g., software common cause

failure (CCF) has a sufficiently low likelihood, then the effects of the failure do not need to be

considered in the 10 CFR 50.59 evaluation. Thus, the qualitative assessment provides a

means of addressing software CCF. In some cases, the effects of a software CCF may not

create a different result than any previously evaluated in the updated final safety analysis report

(UFSAR).

Sections 2 and 3 of this attachment provide acceptable approaches for describing the scope,

form, and content of the type of a qualitative assessment described above. Section 4 of this

attachment provides acceptable approaches for engineering evaluations that may be used in

performing and documenting a qualitative assessment.

2

NEI 01-01, Page 4-20, defines sufficiently low to mean much lower than the likelihood of failures that are

considered in the UFSAR (e.g., single failures) and comparable to other common cause failures that are not

considered in the UFSAR (e.g., design flaws, maintenance errors, calibration errors).

Attachment

Draft RIS 2002-22 Supplement 1, Attachment

Page 2 of 17

2. Regulatory ClarificationApplication of Qualitative Assessments to Title 10 of the

Code of Federal Regulations, Section 50.59

When a licensee decides to undertake an activity that changes its facility as described in the

updated final safety evaluation report, the licensee first performs the engineering and technical

evaluations in accordance with plant procedures. If the licensee determines that an activity is

acceptable through appropriate engineering and technical evaluations, the licensee enters the

10 CFR 50.59 process. The regulations in 10 CFR 50.59 provide a threshold for regulatory

review, not a determination of safety, for the proposed activities. In addition, 10 CFR 50.59

establishes the conditions under which licensees may make changes to the facility or

procedures and conduct tests or experiments without prior NRC approval.

Evaluations must address all elements of proposed changes. Some elements of a change may

have positive effects on SSC failure likelihood while other elements of a change may have

adverse effects. As derived from the guidance in NEI 96-07, positive and negative elements

can be considered together if they are interdependent. This means that if elements are not

interdependent, they must be evaluated separately.

2.1 Likelihood

Properly documented qualitative assessments may be used to support a conclusion that a

proposed digital I&C modification has a sufficiently low likelihood of failure, consistent with the

UFSAR analysis assumptions. This conclusion is used in the 10 CFR 50.59 written evaluation

to determine whether prior NRC approval is required.

Qualitative Assessment

The determination that a digital I&C modification will exhibit a sufficiently low likelihood of failure

can be derived from a qualitative assessment of factors involving system design attributes, the

quality of the design processes employed, the operating experience with the software and

hardware used (i.e., product maturity and in-service experience). Documenting the qualitative

assessment includes describing the factors, rationale, and reasoning (including engineering

judgement) for determining that the digital I&C modification exhibits a sufficiently low likelihood

of failure.

The determination of likelihood of failure may consider the aggregate of all the factors described

above. Some of these factors may compensate for weaknesses in other areas. For example,

for a digital device that is simple and highly testable, thorough testing may provide additional

assurance of a sufficiently low likelihood of failure that helps compensate for a lack of operating

experience.

Qualitative Assessment Outcome

There are two possible outcomes of the qualitative assessment: (1) failure likelihood is

sufficiently low, and (2) failure likelihood is not sufficiently low. Guidance in NEI 01-01,

Section 4.3.6, states, sufficiently low means much lower than the likelihood of failures that are

considered in the UFSAR (e.g., single failures) and comparable to other common cause failures

that are not considered in the UFSAR (e.g., design flaws, maintenance error, calibration errors).

This sufficiently low threshold is not interchangeable with that for distinguishing between

events that are credible or not credible. The threshold for determining whether an event is

Draft RIS 2002-22 Supplement 1, Attachment

Page 3 of 17

credible or not is whether it is as likely as (i.e., not much lower than) malfunctions already

assumed in the UFSAR.

Likelihood Thresholds for 10 CFR 50.59(c)(2)(i), (ii), (v), and (vi)

A key element of 10 CFR 50.59 evaluations is demonstrating whether the modification

considered will exhibit a sufficiently low likelihood of failure. For digital modifications,

particularly those that introduce software, there may be a potential increase in likelihood of

failure. For redundant SSCs, this potential increase in the likelihood of failure creates a similar

increase in the likelihood of a common cause failure.

The sufficiently low threshold discussions have been developed using criteria from NEI 96-07,

Revision 1, and NEI 01-01. They are intended to clarify the existing 10 CFR 50.59 guidance

and should not be interpreted as a new or modified NRC position.

Criteria

Although it may be required by other criteria, prior NRC approval is not required by 10 CFR 50.59(c)(2)(i), (ii), (v), and (vi) if there is a qualitative assessment outcome of sufficiently low, as

described below:

10 CFR 50.59(c)(2)(i)

Does the activity result in more than a minimal increase in the frequency of occurrence of an

accident previously evaluated in the UFSAR?

Sufficiently low threshold - The frequency of occurrence of an accident is directly

related to the likelihood of failure of equipment that initiates the accident (e.g., an

increase in the likelihood of a steam generator tube failure has a corresponding increase

in the frequency of a steam generator tube rupture accident). Thus, an increase in

likelihood of failure of the modified equipment results in an increase in the frequency of

the accident. Therefore, if the qualitative assessment outcome is sufficiently low, then

there is a no more than a minimal increase in the frequency of occurrence of an accident

previously evaluated in the UFSAR.

10 CFR 50.59(c)(2)(ii)

Does the activity result in more than a minimal increase in the likelihood of occurrence of a

malfunction of a structure, system, or component (SSC) important to safety3 previously

evaluated in the UFSAR?

Sufficiently low threshold - The likelihood of occurrence of a malfunction of an SSC

important to safety is directly related to the likelihood of failure of equipment that causes

a failure of SSCs to perform their intended design functions4 (e.g., an increase in the

3 NEI 96-07, Revision 1, Section 3.9, states, Malfunction of SSCs important to safety means the failure of SSCs to

perform their intended design functions described in the UFSAR (whether or not classified as safety-related in

accordance with 10 CFR [Part] 50, Appendix B).

4

The term design functions, as used in this RIS Supplement, conforms to the definition of design functions in NEI 96-07, Revision 1.

Draft RIS 2002-22 Supplement 1, Attachment

Page 4 of 17

likelihood of failure of an auxiliary feedwater (AFW) pump has a corresponding increase

in the likelihood of occurrence of a malfunction of SSCs-the AFW pump and AFW

system). Thus, the likelihood of failure of modified equipment that causes the failure of

SSCs to perform their intended design functions is directly related to the likelihood of

occurrence of a malfunction of an SSC important to safety. Therefore, if the qualitative

assessment outcome is sufficiently low, then the activity does not result in more than a

minimal increase in the likelihood of occurrence of a malfunction of an SSC important to

safety previously evaluated in the UFSAR.

10 CFR 50.59(c)(2)(v)

Does the activity create a possibility for an accident of a different type than any previously

evaluated in the UFSAR?

Sufficiently low threshold-NEI 96-07, Revision 1, Section 4.3.5, states, Accidents of a

different type are limited to those that are as likely to happen as those previously

evaluated in the UFSAR. Accidents of a different type are caused by failures of

equipment that initiate an accident of a different type. If the outcome of the qualitative

assessment of the proposed change is that the likelihood of failure associated with the

proposed activity is sufficiently low, then there are no failures introduced by the activity

that are as likely to happen as those in the UFSAR that can initiate an accident of a

different type. Therefore, the activity does not create a possibility for an accident of a

different type than any previously evaluated in the UFSAR. If the qualitative assessment

determines that a potential failure (e.g., software CCF) does not have a sufficiently low

likelihood, then the effects of this failure need to be considered in the 10 CFR 50.59

evaluation.

10 CFR 50.59(c)(2)(vi)

Does the activity create a possibility for a malfunction of an SSC important to safety with a

different result than any previously evaluated in the UFSAR?

Sufficiently low threshold - NEI 96-07, Section 4.3.6, states, malfunctions with a

different result are limited to those that are as likely to happen as those in the UFSAR.

A malfunction of an SSC important to safety is an equipment failure that causes the

failure of SSCs to perform their intended design functions. If the outcome of the

qualitative assessment of the proposed change is that the likelihood of failure associated

with the proposed activity is sufficiently low, then there are no failures introduced by the

activity that are as likely to happen as those in the UFSAR. Therefore, the activity does

not create a possibility for a malfunction of an SSC important to safety with a different

result than any previously evaluated in the UFSAR. If the qualitative assessment

determines that a potential failure (e.g., software CCF) does not have a sufficiently low

likelihood, then the effects of this failure need to be considered in the 10 CFR 50.59

evaluation using methods consistent with the plants UFSAR.

3. Qualitative Assessments

The NRC staff has determined that proposed digital I&C modifications having the characteristics

listed below are likely to result in qualitative assessment outcomes that support a sufficiently low

likelihood determination:

Draft RIS 2002-22 Supplement 1, Attachment

Page 5 of 17

1. Digital I&C modifications that:

a) Do not create a CCF vulnerability due to the integration of subsystems or

components from different systems that combine design functions that were

not previously combined within the same system, subsystem, or component

being replaced.

Note: Integration, as used in this RIS supplement refers to the process of

combining software components, hardware components, or both into an overall

system, or the merger of the design function of two or more systems or

components into a functioning, unified system or component. Integration also

refers to the coupling of design functions (software/ hardware) via bi-directional

digital communications. Modifications can result in design functions of different

systems being integrated or combined either directly in the same digital device or

indirectly via shared resources, such as bi-directional digital communications or

networks, common controllers, power supplies, or visual display units. Such

integration could be problematic because the safety analysis may have explicitly

or implicitly modeled the equipment performing the design functions that would

be integrated on the basis that it is not subject to any potential source of common

cause failure.

b) Do not create a CCF vulnerability due to new shared resources (such as

power supplies, controllers, and human-machine interfaces) with other design

functions that are (i) explicitly or implicitly described in the UFSAR as

functioning independently from other plant design functions, or (ii) modeled in

the current design basis to be functioning independently from other plant

design functions.

c) Do not affect reactor trip or engineered safety feature initiation/control logic or

emergency power bus load sequencers.

2. Digital I&C modifications that maintain the level of diversity, separation, and

independence of design functions described in the UFSAR. A change that reduces

redundancy, diversity, separation or independence of USFAR-described design

functions is considered a more than minimal increase in the likelihood of malfunction.

3. Digital I&C modifications that are sufficiently simple (as demonstrated through 100

percent testing or a combination of testing and input/output state analysis); or

demonstrate adequate internal diversity.

3.1 Qualitative Assessment Categories

Consistent with the guidance provided in NEI 01-01, this attachment specifies three general

categories of characteristics: design attributes, quality of the design process, and operating

experience. Qualitatively assessing and then documenting these characteristics separately, by

category, and in the aggregate provides a common framework that will better enable licensees

Draft RIS 2002-22 Supplement 1, Attachment

Page 6 of 17

to document qualitative assessments in sufficient detail that an independent third party can

verify the judgements.

Table 1 provides acceptable examples of design attributes, quality of the design processes, and

documentation of operating experience. This listing is not all inclusive nor does the qualitative

assessment need to address each specific item.

3.1.1 Design attributes

NEI 01-01 Section 5.3.1 states:

To determine whether a digital system is sufficiently dependable, and therefore

that the likelihood of failure is sufficiently low, there are some important

characteristics that should be evaluated. These characteristics, discussed in

more detail in the following sections include: Hardware and software design

features that contribute to high dependability (See Section 5.3.4). Such

[hardware and software design] features include built-in fault detection and failure

management schemes, internal redundancy and diagnostics, and use of software

and hardware architectures designed to minimize failure consequences and

facilitate problem diagnosis.

Consistent with the above-quoted text, design attributes of a proposed modification can prevent

or limit failures from occurring. A qualitative assessment describes and documents hardware

and software design features that contribute to high dependability. Design attributes focus

primarily on built-in features such as fault detection and failure management schemes, internal

redundancy and diagnostics, and use of software and hardware architectures and facilitate

problem diagnosis. However, design features external to the proposed modification (e.g.,

mechanical stops on valves) may also need to be considered.

Many system design attributes, procedures, and practices can contribute to significantly

reducing the likelihood of failure (e.g., CCF). A licensee can account for this by deterministically

assessing the specific vulnerabilities through postulated failure modes (e.g., software CCF)

within a proposed modification and applying specific design attributes to address those

vulnerabilities (see Table 1). An adequate qualitative assessment regarding the likelihood of

failure of a proposed modification would consist of a description of: (a) the potential failures

introduced by the proposed modification, (b) the design attributes used to resolve identified

potential failures, and (c) how the chosen design attributes and features resolve identified

potential failures.

Diversity is one example of a design attribute that can be used to demonstrate an SSC modified

with digital technology is protected from a loss of design function due to a potential common

cause failure. In some cases, a plants design basis may specify diversity as part of the design.

In all other cases, the licensees need not consider the use of diversity (e.g., as described in the

staff requirements memorandum on SECY 93-087) in evaluating a proposed modification.

However, diversity within the proposed design, and any affected SSCs is a powerful means for

significantly reducing the occurrence of failures affecting the accomplishment of design

functions.

Draft RIS 2002-22 Supplement 1, Attachment

Page 7 of 17

3.1.2 Quality of the Design Process

Section 5.3.3 of NEI 01-01 states:

For digital equipment incorporating software, it is well recognized that

prerequisites for quality and dependability are experienced software engineering

professionals combined with well-defined processes for project management,

software design, development, implementation, verification, validation, software

safety analysis, change control, and configuration control.

Consistent with the guidance provided in NEI 01-01, Quality Design Processes means those

processes employed in the development of the proposed modification. Such processes include

software development, hardware and software integration processes, hardware design, and

validation and testing processes that have been incorporated into the development process.

For safety-related equipment this development process would be documented and available for

referencing in the qualitative assessment for proposed modifications. However, for

commercial-grade-dedicated or non-safety related equipment documentation of the

development process may not be readily available. In such cases, the qualitative assessment

may place greater emphasis on the design attributes included and the extent of successful

operating experience for the equipment proposed.

Quality of the design process is a key element in determining the dependability of proposed

modifications. Licensees employing design processes consistent with their NRC-approved

quality assurance programs will result in a quality design process.

When possible, the use of applicable industry consensus standards contributes to a quality

design process and provides a previously established acceptable approach (e.g., Institute of

Electrical and Electronics Engineers (IEEE) Standard 1074-2006, IEEE Standard for

Developing a Software Project Life Cycle Process, endorsed in Regulatory Guide 1.173,

Developing Software Life Cycle Processes for Digital Computer Software Used in Safety

Systems of Nuclear Power Plant). In some cases, other nuclear or non-nuclear standards also

provide technically justifiable approaches that can be used if confirmed applicable for the

specific application.

Quality standards should not be confused with quality assurance programs or procedures.

Quality standards are those standards which describe the benchmarks that are specified to be

achieved in a design. Quality standards should be documents that are established by

consensus and approved by an accredited standards development organization. For example,

IEEE publishes consensus-based quality standards relevant to digital I&C modifications and is a

recognized standards development organization. Quality standards used to ensure the

proposed change has been developed using a quality design process do not need to be solely

those endorsed by the NRC staff. The qualitative assessment document should demonstrate

that the standard being applied is valid for the circumstances for which it is being used.

3.1.3 Operating Experience

Section 5.3.1 of NEI 01-01 states, Substantial applicable operating history reduces uncertainty

in demonstrating adequate dependability.

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Consistent with the above-quoted text, relevant operating experience can be used to help

demonstrate that software and hardware employed in a proposed modification have adequate

dependability. The licensee may document information showing that the proposed system or

component modification employs equipment with significant operating experience in nuclear

power plant applications, or in non-nuclear applications with comparable performance standards

and operating environment. The licensee may also consider whether the suppliers of such

equipment incorporate quality processes such as continual process improvement, incorporation

of lessons learned, etc., and document how that information demonstrates adequate equipment

dependability.

Operating experience relevant to a proposed digital I&C change may be credited as part of an

adequate basis for a determination that the proposed change does not result in more than a

minimal increase in the frequency of occurrence of initiating events that can lead to accidents or

in more than a minimal increase in the likelihood of occurrence of a malfunction of an SSC

important to safety previously evaluated in the UFSAR. Differences may exist in the specific

digital I&C application between the proposed digital I&C modification and that of the equipment

and software whose operating experience is being credited. In all cases, however, the

architecture of the referenced equipment and software should be substantially similar to that of

the system being proposed.

Further, the design conditions and modes of operation of the equipment whose operating

experience is being referenced also needs to be substantially similar to that being proposed as

a digital I&C modification. For example, one needs to understand what operating conditions

(e.g., ambient environment, continuous duty, etc.) were experienced by the referenced design.

In addition, it is important to recognize that when crediting operating experience from other

facilities, one needs to understand what design features were present in the design whose

operating experience is being credited. Design features that serve to prevent or limit possible

common cause failures in a design referenced as relevant operating experience should be

noted and considered for inclusion in the proposed design. Doing so would provide additional

support for a determination that the dependability of the proposed design will be similar to the

referenced application.

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Table 1Qualitative Assessment Category Examples

Categories Examples for Each Category

Design * Design criteriaDiversity (if applicable), Independence, and Redundancy.

Attributes * Inherent design features for software, hardware or architectural/network

Watchdog timers that operate independent of software, isolation devices,

segmentation of distributed networks, self-testing, and self-diagnostic

features.

• Basis for identifying that possible triggers are non-concurrent.

• Sufficiently simple (i.e., enabling 100 percent testing or comprehensive

testing in combination with analysis of likelihood of occurrence of

input/output states not tested).

• Failure state always known to be safe, or at least the same state as allowed

by the previously installed equipment safety analysis.

Quality of * Justification for use of industry consensus standardsfor codes and

the Design standards not endorsed by the NRC.

Process * Justification for use of other standards.

• Use of Appendix B vendors. If not an Appendix B vendor, the analysis can

state which generally accepted industrial quality program was applied.

• Use of Commercial Grade Dedication processes per guidance of EPRI TR-

106439, Annex D of IEEE 7-4.3.2, and examples within EPRI TR-107330.

• Demonstrated capability (e.g., through qualification testing) to withstand

environmental conditions within which the SSC is credited to perform its

design function (e.g., EMI/RFI, Seismic).

• Development process rigor (adherence to generally-accepted commercial or

nuclear standards.)

• Demonstrated dependability of custom software code for application

software through extensive evaluation or testing.

Operating * Wide range of operating experience in similar applications, operating

Experience environments, duty cycles, loading, comparable configurations, etc., to that

of the proposed modification.

• History of lessons learned from field experience addressed in the design.

• Relevant operating experience: Architecture of the referenced equipment

and software (operating system and application) along with the design

conditions and modes of operation of the equipment should be substantially

similar to those of the system being proposed as a digital I&C modification.

High volume production usage in different applicationsNote that for

software, the concern is centered on lower volume, custom, or

user-configurable software applications. High volume, high quality

commercial products with relevant operating experience used in other

applications have the potential to avoid design errors.

• Experience working with software development tools used to create

configuration files.

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3.2 Qualitative Assessment Documentation

The U.S. Nuclear Regulatory Commission endorsed guidance for documenting 10 CFR 50.59

evaluations to meet the requirements of 10 CFR 50.59 (d) is provided in both NEI 96-07,

Revision 1 in Section 5.0, Documentation and Reporting and NEI 01-01, Appendix B. Both of

these documents reiterate the principles that documentation should include an explanation

providing adequate basis for the conclusion so that a knowledgeable reviewer could draw the

same conclusion.

Considerations and conclusions reached while performing qualitative assessments supporting

the evaluation criteria of 10 CFR 50.59, are subject to the aforementioned principles. In order

for a knowledgeable reviewer to draw the same conclusion regarding qualitative assessments,

details of the considerations made, and their separate and aggregate effect on any qualitative

assessments need to be included or clearly referenced in the 10 CFR 50.59 evaluation

documentation. References to other documents should include the document name and location

of the information within any referenced document.

If qualitative assessment categories are used, each category would be discussed in the

documentation including positive and negative aspects considered, consistent with the

examples provided in Table 1. In addition, a discussion of the degree to which each of the

categories was relied on to reach the qualitative assessment conclusion would be documented.

4. Engineering Evaluations

4.1 Overview

This section describes approaches that could be used for conducting and documenting

engineering evaluations. completed in accordance with the licensees NRC approved quality

assurance program. The term engineering evaluation refers to evaluations performed in

designing digital I&C modifications. These evaluations are performed under the licensees NRC

approved quality assurance program. These engineering evaluations may include, but are not

limited to discussion of compliance with regulatory requirements and conformity to the UFSAR,

regulatory guidance, and design standards.

In addition, these engineering evaluations may include discussions of: a) the performance of

deterministic failure analyses, including analysis of the effects of digital I&C failures at the

component-level, system-level, and plant-level; b) the evaluation of defense-in-depth; and c) the

evaluation of the proposed modification for its overall dependability. The qualitative

assessment framework discussed in the previous sections of this attachment may rely, in part,

on the technical bases and conclusions documented within these engineering evaluations.

Thus, improved performance and documentation of engineering evaluations can enable better

qualitative assessments.

One result of performing these evaluations is to provide insights as to whether a proposed

digital I&C design modification may need to be enhanced with the inclusion of different or

additional design attributes. Such different or additional design attributes would serve to

prevent the occurrence of a possible CCF or reduce the potential for a software CCF to cause a

loss of design function.

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These approaches are provided for consideration only. They do not represent NRC

requirements and may be used at the discretion of licensees.

4.2 Selected Design Considerations

During the design process, it is important to consider both the positive effects of installing the

digital equipment (e.g., elimination of single-point vulnerabilities (SPVs), ability to perform signal

validation, diagnostic capabilities) with the potential negative effects (e.g., software CCF).

Digital I&C modifications can reduce SSC independence. Reduction in independence of design

functions from that described in the USFAR would require prior NRC approval.

4.2.1 Digital Communications

Careful consideration of digital communications is needed to preclude adverse effects on SSC

independence. DI&C-ISG-04, Revision 1, Highly-Integrated Control Rooms - Communications

Issues (Agencywide Documents Access and Management System Accession Number

ML083310185) provides guidance for NRC staff reviewing digital communications. This ISG

describes considerations for the design of communications between redundant SSCs, echelons

of defense-in-depth5 or SSCs with different safety classifications. The principles of this ISG or

other technically justifiable considerations, may be used to assess non-safety related SSCs.

4.2.2 Combining Design Functions

Combining design functions of different safety-related or non-safety related SSCs in a manner

not previously evaluated or described in the UFSAR could introduce new interdependencies and

interactions that make it more difficult to account for new potential failure modes. Failure of

combined design functions that: 1) can effect malfunctions of SSCs or accidents evaluated in

the UFSAR; or 2) involve different defense-in-depth echelons; are of significant concern.

Combining previously separate component functions can result in more dependable system

performance due to the tightly coupled nature of the components and a reduction in complexity.

If a licensee proposes to combine previously separate design functions in a safety-related

and/or non-safety related digital I&C modification, possible new failures need to be carefully

weighed with respect to the benefits of combining the previous separately controlled functions.

Failure analyses and control system segmentation analyses can help identify potential issues.

Segmentation analyses are particularly helpful for the evaluation of the design of non-safety

related distributed networks.

4.3 Failure Analyses

Failure analysis can be used to identify possible CCFs in order to assess the need to further

modify the design. In some cases, potential failures maybe excluded from consideration if the

failure has been determined to be implausible as a result of factors such as design

features/attributes, and procedures. Modifications that employ design attributes and features,

5

As stated in NEI 01-01, Section 5.2, A fundamental concept in the regulatory requirements and expectations for

instrumentation and control systems in nuclear power plants is the use of four echelons of defense-in-depth: 1)

Control Systems; 2) Reactor Trip System (RTS) and Anticipated Transient without SCRAM (ATWS); 3) Engineered

Safety Features Actuation System (ESFAS); and 4) Monitoring and indications.

Draft RIS 2002-22 Supplement 1, Attachment

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such as internal diversity, help to minimize the potential for CCFs. Sources of CCF, could

include the introduction of identical software into redundant channels, the use of shared

resources; or the use of common hardware and software among systems performing different

design functions. Therefore, it is essential that such sources of CCF be identified, to the extent

practicable, and addressed during the design stage as one acceptable method to support the

technical basis for the proposed modification.

Digital designs having sources of CCF that could affect more than one SSC need to be closely

reviewed to ensure that an accident of a different type from those previously evaluated in the

UFSAR has not been created. This is particularly the case when such common sources of CCF

also are subject to common triggers. For example, the interface of the modified SSCs with

other SSCs using identical hardware and software, power supplies, human-machine interfaces,

needs to be closely reviewed to ensure that possible common triggers have been addressed.

A software CCF may be assessed using best-estimate methods and realistic assumptions.

Unless already incorporated into the licensees UFSAR, best-estimate methods cannot be

used for evaluating different results than those previously evaluated in the UFSAR.

4.4 Defense-in-Depth Analyses

NEI 01-01 describes the need for defense-in-depth analysis as limited to substantial digital

replacements of reactor protection system and ESFAS. A defense-in-depth analysis for

complex digital modifications of systems other than protection systems may also reveal the

impact of any new potential CCFs due to the introduction of shared resources, common

hardware and software, or the combination of design functions of systems that were previously

considered to be independent of one another. Additionally, defense-in-depth analysis may

reveal direct or indirect impacts on interfaces with existing plant SSCs. This type of analysis

may show that existing SSCs and/or procedures could serve to mitigate effects of possible

CCFs introduced through the proposed modification.

4.5 Dependability Evaluation

Section 5.3.1 of NEI 01-01 states that a digital system that is sufficiently dependable will have a

likelihood of failure that is sufficiently low. This section describes considerations that can be

used to determine whether a digital system is sufficiently dependable.

The dependability evaluation relies on some degree of engineering judgment to support a

conclusion that the digital modification is considered to be sufficiently dependable. When

performing a dependability evaluation, one acceptable method is to consider: (1) inclusion of

any deterministically-applied defensive design features and attributes; (2) conformance with

applicable standards regarding quality of the design process for software and hardware; and (3)

relevant operating experience. Although not stated in NEI 01-01, judgments regarding the

quality of the design process and operating experience may supplement, but not replace the

inclusion of design features and attributes.

For proposed designs that are more complex or more risk significant, the inclusion of design

features and attributes that: serve to prevent CCF, significantly reduce the possible occurrence

of software CCF, or significantly limit the consequences of such software CCF, should be key

considerations for supporting a sufficiently dependable determination. Design features

Draft RIS 2002-22 Supplement 1, Attachment

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maximizing reliable system performance, to the extent practicable, can also be critical in

establishing a basis for the dependability of complex or risk significant designs.

Section 5.1.3 of NEI 01-01 states that Judgments regarding dependability, likelihood of failures,

and significance of identified potential failures should be documented. Depending on the

SSCs being modified and the complexity of the proposed modification, it may be challenging to

demonstrate sufficient dependability based solely upon the quality of the design process

and/or operating history. Engineering judgments regarding the quality of the design process

and operating experience may supplement, but not replace the inclusion of design features and

attributes when considering complex modifications.

Figure 1 of this attachment provides a simplified illustration of the engineering evaluations

process described in Section 4 of this attachment.

4.6 Engineering Documentation

Documentation for a proposed digital I&C modification is developed and retained in accordance

with the licensees design engineering procedures, and the NRC-approved QA program. The

documentation of an engineering evaluation identifies the possible failures introduced in the

design and the effects of these failures. It also identifies the design features and/or procedures

that document resolutions to identified failures, as described in NEI 01-01, Section 5.1.4. The

level of detail used may be commensurate with the safety significance and complexity of the

modification in accordance with licensees procedures.

Although not required, licensees may use Table 2 of this attachment to develop and document

engineering evaluations. Documentation should include an explanation providing adequate

bases for conclusions so that a knowledgeable reviewer could draw the same conclusion.

Draft RIS 2002-22 Supplement 1, Attachment

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Draft RIS 2002-22 Supplement 1, Attachment

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Table 2Example - Engineering Evaluation Documentation Outline

to support a Qualitative Assessment

Topical Area Description

Step 1- Describe the full extent of the SSCs to be modifiedboundaries of the

Identification design change, interconnections with other SSCs, and potential

commonality to vulnerabilities with existing equipment.

• What are all of the UFSAR-described design functions of the

upgraded/modified components within the context of the plant

system, subsystem, etc.?

• What design function(s) provided by the previously installed

equipment are affected and how will those design functions be

accomplished by the modified design? Also describe any new

design functions that were not part of the original design.

• What assumptions and conditions are expected for each associated

design function? For example, the evaluation should consider both

active and inactive states, as well as transitions from one mode of

operation to another.

Step 2Identify Consider the possibility that the proposed modification may have

potential failure introduced potential failures.

modes and * Are there potential failure modes or undesirable behaviors as a

undesirable behavior result of the modification? A key consideration is that

undesirable behaviors may not necessarily constitute an SSC

failure, but a misoperation. (e.g., spurious actuation)

• Are failures including, but not limited to, hardware, software,

combining of functions, shared resources, or common

hardware/software considered?

• Are there interconnections or interdependencies among the

modified SSC and other SSCs?

• Are there sources of CCF being introduced that are also subject

to common triggering mechanisms with those of other SSCs not

being modified?

• Are potential failure modes introduced by software tools or

programmable logic devices?

Step 3Assess the * Could the possible failure mode or undesired behavior lead to a

effects of identified plant trip or transient?

failures * Can the possible failure mode or undesired behavior affect the

ability of other SSCs to perform their design function?

• Could the possible failure mode of the SSC, concurrent with a

similar failure of another SSC not being modified but sharing a

common failure and triggering mechanism affect the ability of the

SSC or other SSCs to perform their design functions?

• What are the results of the postulated new failure(s) of the

modified SSC(s) compared to previous evaluation results

described in the UFSAR?

Step 4Identify What actions are being taken (or were taken) to address significant

appropriate identified failures?

• Are further actions warranted?

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Table 2Example - Engineering Evaluation Documentation Outline

to support a Qualitative Assessment

Topical Area Description

resolutions for each * Is re-design warranted to add additional design features or

identified failures attributes?

• Is the occurrence of failure self-revealing or are there means to

annunciate the failure or misbehavior to the operator?

Step 5 * Describe the resolutions identified in Step 4 of this table that

Documentation address the identified failures.

• Describe the conformance to regulatory requirements, plants

UFSAR, regulatory guidance, and industry consensus standards

(e.g., seismic, EMI/RFI, ambient temperature, heat contribution).

• Describe the quality of the design processes used within the

software life cycle development (e.g., verification and validation

process, traceability matrix, quality assurance documentation,

unit test and system test results).

• Describe relevant operating history (e.g., platform used in

numerous applications worldwide with minimal failure history).

• Describe the design features/attributes that support the

dependability conclusion (e.g., internal design features within the

digital I&C architectures such as self-diagnostic and self-testing

features or physical restrictions external to the digital I&C

portions of the modified SSC), defense-in-depth (e.g., internal

diversity, redundancy, segmentation of distributed networks, or

alternate means to accomplish the design function).

• Summarize the results of the engineering evaluation including the

dependability determination.

Draft RIS 2002-22 Supplement 1, Attachment

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DRAFT NRC REGULATORY ISSUE SUMMARY 2002-22, SUPPLEMENT 1, CLARIFICATION

ON ENDORSEMENT OF NUCLEAR ENERGY INSTITUTE GUIDANCE IN DESIGNING

DIGITAL UPGRADES IN INSTRUMENTATION AND CONTROL SYSTEMS DATE: Month

XX, 2018

OFFICE NRR/DIRS/IRGB/PM NRR/PMDA OE/EB OCIO

NAME TGovan LHill JPeralta DCullison

DATE 02/16/2018 01/18/2018 01/22/2018 01/17/2018

OFFICE NRR/DE/EICB/BC NRR/DIRS/IRGB/LA NRR/DIRS/IRGB/PM NRR/DIRS/IRGB/BC

NAME MWaters ELee TGovan HChernoff

DATE 03/01/2018 02/22/2018 03/01/2018 03/01/2018