ML18310A203

Appendix D Additions from RIS 2002-22 Supplement 1

ML18310A203
Person / Time
Issue date:	10/04/2018
From:	Mcginty T, Chris Miller NRC/NRO/DCIP/CIPB, Division of Inspection and Regional Support
To:
Govan T
References
RIS-2002-22
Download: ML18310A203 (21)

See also: RIS 2002-22

Text

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

OFFICE OF NEW REACTORS

WASHINGTON, DC 20555-0001

May 31, 2018

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 of operating licenses under Title 10 of the Code of Federal Regulations (10 CFR)

Part 50, Domestic Licensing of Production and Utilization Facilities.

All holders of combined licenses under 10 CFR Part 52, Licenses, Certifications, and Approvals

for Nuclear Power Plants.

INTENT

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

Summary (RIS) 2002-22, Use of EPRI/NEI Joint Task Force Report, 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, dated November 25, 2002 (Agencywide

Documents Access and Management System (ADAMS) Accession No. ML023160044).

RIS 2002-22 endorses Nuclear Energy Institute (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, issued March 2002 (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

referred to as digital I&C) in a consistent and comprehensive manner.

The RIS supplement clarifies 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 NEI 01-01, Sections 4 and 5 and Appendices A and B. 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 proposed digital I&C modification has a

sufficiently low1 likelihood of failure. This conclusion and the reasons for it should be

documented, as required by 10 CFR 50.59(d)(1), as part of the evaluations of proposed digital

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

are considered in the updated final safety analysis report (UFSAR) (e.g., single failures) and comparable to other

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

ML18143B633

RIS 2002-22 Supplement 1

Page 2 of 5

I&C modifications against some of the criteria in 10 CFR 50.59, Changes, tests, and

experiments.

Consistent with RIS 2002-22, this RIS supplement is intended to address digital modifications to

safety-related systems or components but may also be applied to modifications of non-safety

related systems or components at the discretion of the licensee. This RIS supplement is not

directed toward digital I&C replacements of the reactor protection system, the engineered safety

features actuation system, or modification/replacement of the internal logic portions of these

systems (e.g., voting logic, bistable inputs, and signal conditioning/processing) because

application of the guidance in this RIS supplement to such changes would likely involve

additional considerations. This RIS supplement provides guidance for addressing CCF in

10 CFR 50.59 evaluations. Other NRC guidance documents address potential CCFs of digital

I&C equipment.

This RIS supplement does not require any action or written response on the part of an

addressee.

BACKGROUND INFORMATION

By letter dated March 15, 2002, NEI submitted Electric Power Research Institute (EPRI)

TR-102348, Revision 1 (NEI 01-01), for the NRC staffs review. NEI 01-01 replaced the original

version of EPRI TR-102348, issued 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). On

November 25, 2002, the NRC staff issued RIS 2002-22 to notify addressees that it had

reviewed NEI 01-01 and was endorsing the report for use as guidance in designing and

implementing digital upgrades to nuclear power plant I&C systems.

Following the NRC staffs 2002 endorsement of NEI 01-01, holders of 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, issued November 2000 (ADAMS Accession No. ML003759710), which endorses

NEI 96-07, Guidelines for 10 CFR 50.59 Implementation, Revision 1, issued November 2000

(ADAMS Accession No. ML003771157).

NRC inspections of documentation for digital I&C plant modifications that some licensees

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

documentation of licensee engineering evaluations. In addition, NRC inspections identified

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

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

other than the 10 CFR 50.59 evaluations (e.g., 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 Staff Requirements

SECY-16-0070Integrated Strategy To Modernize the Nuclear Regulatory Commissions

Digital Instrumentation and Control Regulatory Infrastructure, dated October 25, 2016 (ADAMS

Accession No. ML16299A157), the NRC staff has engaged the public, NEI, and industry

representatives to improve the guidance for applying 10 CFR 50.59 to digital I&C-related design

RIS 2002-22 Supplement 1

Page 3 of 5

modifications as part of a broader effort to modernize the I&C regulatory infrastructure. The

integrated action plan developed to implement the SRM direction described the issuance of the

guidance in this RIS supplement as a near-term action to provide specific guidance for

documenting qualitative assessments that support a conclusion that a proposed digital I&C

modification will exhibit a sufficiently low likelihood of failure.

Applicability to 10 CFR Part 50 Licensees other than Power Reactors

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

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

reactors. Other licensees may also use the guidance in RIS 2002-22 and apply the guidance in

this RIS supplement to develop written evaluations that address the criteria in

10 CFR 50.59(c)(2). In particular, these licensees may use the guidance to prepare qualitative

assessments that consider design attributes, quality of the design process, and operating

experience to evaluate proposed digital I&C modifications to their facilities as described in

NEI 01-01, Section 4, Section 5, and Appendices A and B. However, certain aspects of the RIS

guidance discuss regulatory requirements that may not fully apply to these licensees (e.g.,

Appendix B, Quality Assurance Criteria for Nuclear Power Plants and Fuel Reprocessing

Plants).

SUMMARY OF ISSUE

In general, implementation of digital I&C technology can provide dependability and safety

benefits. Notably, digital technology can be designed to provide continuous diagnostic

information to plant operators on the integrity of its internal systems operation and its

availability. However, implementation of digital I&C technology may introduce potential hazards

such as software CCF or failures introduced as result of interconnectivity.

Some hazards, such as software CCF, may be addressed through a qualitative assessment.

A qualitative assessment can be used to support a conclusion that a proposed digital I&C

modification will not result in 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 updated final safety analysis report

(10 CFR 50.59(c)(2)(v) and (vi)). These conclusions can be satisfied if a proposed digital I&C

modification has a sufficiently low likelihood of failure.

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

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

factors such as design attributes, the quality of the design processes used, and an evaluation of

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

maturity and inservice experience). The licensee may use a qualitative assessment to

document the factors and rationale for concluding that an adequate basis exists for determining

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

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 including approaches for addressing interconnectivity hazards.

RIS 2002-22 Supplement 1

Page 4 of 5

BACKFITTING AND ISSUE FINALITY DISCUSSION

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

guidance on how to perform and document qualitative assessments and supporting engineering

evaluations 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 published a notice of opportunity for public comment on a previous draft of this RIS in

the Federal Register on July 3, 2017 (82 FR 30913). Subsequently, the NRC published a

second notice of opportunity for public comment on the revised RIS in Volume 83 of the Federal

Register, page 11154, on March 14, 2018 (83 FR 11154). As noted in the March 14, 2018

Federal Register Notice (FRN), The NRC staff engaged in multiple communications with the

public and stakeholders and continued internal discussions about the RIS. As a result of these

efforts, the NRC has substantially rewritten the RIS. Due to the extensive nature of these

revisions, and in light of this additional opportunity for comment, the NRC is not directly

responding to each comment received in the previous comment period.

The NRC received comments from seven commenters as a result of the March 14, 2018

FRN. The NRC considered all comments, some of which resulted in changes to the RIS. The

staff will document the evaluation of these comments and the resulting changes to the RIS in a

memorandum that will be made publicly available in ADAMS at a later date.

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 (OMB) 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.).

OMB approved this information collection 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.

RIS 2002-22 Supplement 1

Page 5 of 5

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.

CONTACT

Please direct any questions about this matter to the technical contacts or the Project Manager

listed below.

/RA/ (Paul G. Krohn for) /RA/

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

e-mail: Duane.Hardesty@nrc.gov (specifically for nonpower 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 Failure Analysis

QUALITATIVE ASSESSMENT AND FAILURE ANALYSIS

1. Purpose

Regulatory Issue Summary (RIS) 2002-22, NRC Regulatory Issue Summary 2002-022: Use of

EPRI/NEI Joint Task Force Report, 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, dated November 25, 2002 (Agencywide Documents Access and

Management System (ADAMS) Accession No. ML023160044), endorses Nuclear Energy

Institute (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,

issued March 2002 (ADAMS Accession No. ML020860169). NEI 01-01 provides guidance on

implementing and licensing digital upgrades and provides guidance on performing qualitative

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

NEI 96-07, Guidelines for 10 CFR 50.59 Implementation, Revision 1, issued November 2000

(ADAMS Accession No. ML003771157), acknowledges that qualitative assessments may be

used to address some of the criteria in Title 10 of the Code of Federal Regulations (10 CFR)

50.59, Changes, tests, and experiments. This attachment provides supplemental clarifying

guidance on one acceptable approach for performing qualitative assessments of digital I&C

modifications. 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. Although a qualitative assessment can be used to support a 10 CFR 50.59

evaluation, this RIS supplement does not provide guidance for screening nor does it presume

that all digital modifications screen in.

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

interchangeably. Within this RIS supplement attachment the terms qualitative assessment

and sufficiently low1 are used in conjunction with the performance of 10 CFR 50.59

evaluations. The term dependability evaluation is used in the context of engineering

evaluations. Engineering evaluations are not part of a 10 CFR 50.59 evaluation but may

provide technical supporting information for a 10 CFR 50.59 evaluation. Engineering

evaluations are performed in accordance with the licensees U.S. Nuclear Regulatory

Commission (NRC)-approved quality assurance program in developing digital I&C

modifications.

If a qualitative assessment determines that a potential failure (e.g., software CCF) has a

sufficiently low likelihood, the 10 CFR 50.59 evaluation does not need to consider the effects of

the failure. Thus, the qualitative assessment provides a means of addressing potential failures

to support a 10 CFR 50.59 evaluation. In some cases, the effects of a potential failures may not

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

(UFSAR).

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

are considered in the updated final safety analysis report (UFSAR) (e.g., single failures) and comparable to other

common-cause failures (CCFs) that are not considered in the UFSAR (e.g., design flaws, maintenance errors,

calibration errors).

Attachment

RIS 2002-22, Supplement 1, Attachment

Page 2 of 16

2. Regulatory ClarificationApplication of Qualitative Assessments to 10 CFR 50.59

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

UFSAR, the licensee 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.

These evaluations must address all elements of proposed changes. Some elements of a

change may have positive effects on the likelihood of structure, system, and component (SSC)

failures, whereas other elements of a change may have negative effects. As derived from the

guidance in NEI 96-07, positive and negative elements can be considered together if they are

interdependent. Elements that are not interdependent must be evaluated separately.

2.1 Qualitative Assessment

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. The 10 CFR 50.59 written evaluation uses this conclusion to

determine whether prior NRC approval is required.

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 such as system design attributes, the

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

software and hardware used (i.e., product maturity and inservice 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, a

digital device that is simple and highly testable, thorough testing coupled with an analysis

demonstrating untested states are accounted for in the proposed application, may provide

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

operating experience.

Sufficiently Low Outcome

One approach for a qualitative assessment employs the concept of a sufficiently low likelihood

of failures. The use of this concept results in two possible outcomes of the sufficiently low

determination: (1) failure likelihood is sufficiently low and (2) failure likelihood is not

sufficiently low. NEI 01-01, Section 4.3.6, states that sufficiently low means much lower than

the likelihood of failures that are considered in the UFSAR (e.g., single failures) and comparable

to other CCFs that are not considered in the UFSAR (e.g., design flaws, maintenance error,

calibration errors). This sufficiently low threshold is not interchangeable with that used for

distinguishing between events that are credible or not credible. The threshold for

RIS 2002-22, Supplement 1, Attachment

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determining whether an event is credible is whether it is as likely as (i.e., not much lower

than) the malfunctions already assumed in the UFSAR.

If a qualitative assessment determines that a potential failure (e.g., software CCF) has a

sufficiently low likelihood, the 10 CFR 50.59 evaluation does not need to consider the effects of

the failure. Thus, the qualitative assessment provides a means of addressing potential failures

to support a 10 CFR 50.59 evaluation.

2.2 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, the likelihood of failure may potentially increase. For

redundant SSCs, this potential increase in the likelihood of failure may create an increase in the

likelihood of a CCF.

The NRC has used the criteria from NEI 96-07, Revision 1, and NEI 01-01 in its discussions of

sufficiently low threshold. These discussions are intended to clarify the existing 10 CFR 50.59

guidance; licensees should not interpret such discussions as a new or modified NRC position.

Criteria

For this RIS supplement, the outcome of the qualitative assessment is focused on a single

likelihood threshold of sufficiently low that encompasses and satisfies the individual likelihood

thresholds of 10 CFR 50.59(c)(2)(i), (ii), (v), and (vi). Although it may be required by other

10 CFR 50.59 criteria, prior NRC approval is not required by10 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 the

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, there

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

previously evaluated in the UFSAR.

RIS 2002-22, Supplement 1, Attachment

Page 4 of 16

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 an SSC important to safety2 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 functions3 (e.g., an increase in the

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

in the likelihood of occurrence of a malfunction of SSCsthe 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 the

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

assessment outcome is sufficiently low, 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, the activity does not introduce any failures 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, the 10 CFR 50.59 evaluation needs to consider the effects of this failure.

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

Does the activity create a possibility for a malfunction of an SSC that is 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 that 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 that is 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

2 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).

3 The term design functions, as used in this RIS supplement, conforms to the definition of design functions in

NEI 96-07, Revision 1.

RIS 2002-22, Supplement 1, Attachment

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with the proposed activity is sufficiently low, the activity does not introduce any failures

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

from any other 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, the 10 CFR 50.59 evaluation needs to consider the effects of this failure using

methods consistent with the plants UFSAR.

In some cases, the effects of a possible software CCF may not create a different result than any

previously evaluated in the UFSAR.

3. Qualitative Assessments

The activities listed below are examples of digital I&C modifications that licensees can likely

implement without prior NRC approval using properly documented qualitative assessments.

• replacement of analog relays (including timing relays) with digital relays

• replacement of analog controls for safety-related support systems such as chiller

(heating, ventilation, and air conditioning) systems and lubricating oil coolers

• replacement of analog controls for emergency diesel generator supporting systems and

auxiliary systems such as voltage regulation

• installation of circuit breakers that contain embedded digital devices

• replacement of analog recorders and indicators with digital recorders and indicators

• digital upgrades to non-safety related control systems

The evaluation of these proposed modifications is expected to be straightforward if they have no

interconnectivity across channels, systems, and divisions; and they do not reduce the

redundancy, diversity, separation, or independence4 of their UFSAR-described design functions.

However, digital modifications that involve networking, combining design functions from different

systems; interconnectivity across channels, systems, and divisions; or shared resources merit

careful review to ensure that such modifications incorporate appropriate design attributes so

that reductions in the redundancy, diversity, separation, or independence of UFSAR-described

design functions are not introduced.

Combining different design functions within digital modifications can result in combining design

functions of different systems either directly in the same digital device or indirectly through

shared resources, such as implementation of bidirectional digital communications or networks,

common controllers, power supplies, or a multifunction display and control station. Shared

resources (such as bidirectional communications, power supplies, controllers, and multifunction

display and control stations) introduced by digital modifications may also reduce the

redundancy, diversity, separation, or independence of UFSAR-described design functions.

4 NEI 96-07, Section 4.3.2, explains that a change that reduces system/equipment redundancy, diversity,

separation, or independence requires prior NRC approval because it would result in more than a minimal

increase in the likelihood of occurrences of a malfunction of a SSC important to safety.

RIS 2002-22, Supplement 1, Attachment

Page 6 of 16

3.1 Qualitative Assessment Factors

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

(1) design attributes, (2) quality of the design process, and (3) operating experience.

Qualitatively assessing and documenting these factors separately, and in the aggregate, will

enable licensees to document qualitative assessments in sufficient detail that an independent

third party can verify the judgements. Note that design attributes and the quality of the design

process are interrelated (i.e., the quality of the design process assures the proper

implementation of design attributes). As a result, these two factors will always be essential

elements of a qualitative assessment. Operating experience in most cases can serve to

compensate for weakness in the other two factors. This guidance applies to modifications of

safety-related systems or components but may also be applied to modifications of non-safety

related systems or components at the discretion of the licensee.

Table 1 provides acceptable examples of design attributes, quality of the design process, 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 the following:

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.

Design attributes of a proposed modification can prevent or limit failures from occurring.

Design attributes focus primarily on built-in features such as fault detection and failure

management schemes, internal redundancy, and diagnostics within the integrated software and

hardware architecture. However, design features external to the proposed modification

(e.g., mechanical stops on valves or pump speed limiters) may also 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 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 of the likelihood of failure of a proposed

modification would describe the potential failures that the proposed modification could introduce

and the specific design attributes incorporated to resolve the identified potential failures. It

would also explain how the chosen design attributes and features resolve the identified potential

failures.

RIS 2002-22, Supplement 1, Attachment

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Diversity is one example of a design attribute that licensees can use to demonstrate that an

SSC modified with digital technology is protected from a loss of design function caused by a

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

design. In other cases, licensees do not need to consider the use of diversity in evaluating a

proposed modification. Diversity within the proposed design can be a powerful means for

significantly reducing the occurrence of failures that affect the accomplishment of design

functions.

3.1.2 Quality of the Design Process

NEI 01-01, Section 5.3.3, states the following:

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.

Such processes include software development, hardware and software integration processes,

system design, and validation and testing processes that have been incorporated into

development. For safety-related digital equipment composed of integrated hardware and

software, 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 digital equipment comprising integrated hardware and software,

documentation of the development process may not be as extensive. 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.

The quality of the design process is a key element in determining the dependability of the

proposed modifications. 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 (Std.)

1074-2006, IEEE Standard for Developing a Software Project Life Cycle Process, which is

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 nonnuclear standards can also provide technically justifiable approaches for use if

they apply to the specific application.

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

Quality standards are those standards that 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 is a recognized standards development organization that publishes consensus-based

quality standards that are relevant to digital I&C modifications. Quality standards used to

ensure that a quality design process was used to develop the proposed change need not be

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

demonstrate that the standard being applied is valid under the circumstances for which it is

being used.

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For non-safety related SSCs, adherence to generally accepted commercial standards may be

sufficient. The qualitative assessment should list the generally accepted commercial industry

standards used in development of the equipment. If NRC-endorsed industry standards were

applied during the design or manufacturing process, or both, for non-safety related equipment,

these standards may be documented in the qualitative assessment to provide additional

evidence of quality.

3.1.3 Operating Experience

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

demonstrating adequate dependability.

Relevant operating experience can be used to help evaluate and demonstrate that integrated

software and hardware equipment employed in a proposed modification has adequate

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

component modification uses equipment with significant operating experience in nuclear power

plant applications or in nonnuclear 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 and

incorporation of lessons learned and document how that information demonstrates adequate

equipment dependability.

Differences may exist in the specific digital I&C application between the proposed digital I&C

modification and that of the integrated hardware 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 proposed system.

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

experience is being referenced also need to be substantially similar to that of the proposed

digital I&C modification. For example, analysts need to understand the operating conditions

(e.g., ambient environment, continuous duty) experienced by the referenced equipment and

software. In addition when crediting operating experience from other facilities, it is important to

understand which design features were present in the design whose operating experience is

being credited. Design features that serve to prevent or limit possible CCFs in a design that is

referenced as relevant operating experience should be documented 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 1 Qualitative Assessment Factors Examples

Factors Examples for Each Factor

Design * Defense-in-depth, diversity, independence, and redundancy (if applicable)

Attributes

• Inherent design features for integrated software and hardware or

architectural/network (e.g., watchdog timers that operate independent of

software, isolation devices, segmentation of distributed networks,

self-testing, and self-diagnostic features)

• Nonconcurrent triggers

• Sufficiently simple (see NEI 01-01, Section 5.3.1)

• Testability (e.g., highly testable)

• Resolution of the possible failures identified in the failure analysis

Quality of Safety-Related Equipment:

the Design

Process * Use of industry consensus standards shown to be applicable

• Use of other standards shown to be applicable

• Use of Appendix B vendors

If an Appendix B vendor is not used, the analysis can state which generally

accepted industrial quality program was applied.

• Use of commercial-grade dedication processes in accordance with the

guidance in EPRI TR-106439, Guideline on Evaluation and Acceptance of

Commercial-Grade Digital Equipment for Nuclear Safety Applications,

dated October 1, 1996.

• Use of commercial-grade dedication processes in accordance with the

guidance in Annex D to IEEE Std. 7-4.3.2, IEEE Standard Criteria for Digital

Computers in Safety Systems of Nuclear Power Generating Stations, and

with the examples in EPRI TR-107330, Generic Requirements

Specification for Qualifying a Commercially Available PLC for

Safety-Related Applications in Nuclear Power Plants

• Documented capability through qualification testing or analysis, or both, to

withstand environmental conditions within which the SSC is credited to

perform its design function (e.g., electromagnetic interference,

radio-frequency interference, seismic activity)

• Demonstrated dependability of custom software code for application

software through extensive evaluation or testing

Non-safety Related Equipment:

• Adherence to generally accepted applicable commercial standards

• Procurement or manufacturer documentation, or both, showing that design

specifications are met or exceeded for equipment being replaced

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• Verification of design requirements and specifications

Operating * Operating experience in similar applications, operating environments, duty

Experience cycles, loading, and comparable configurations to that of the proposed

modification

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

• Referenced relevant operating experience should be equipment similar to

that being proposed in the digital I&C modification.

- Architecture of the referenced equipment and software (operating

system and application)

- Design conditions and modes of operation

- Widely used high-quality commercial products with relevant operating

experience used in other applications

For software, limited use, custom, or user-configurable software

applications can be challenging.

- Experience with software development tools used to create configuration

files

3.2 Qualitative Assessment Documentation

NEI 96-07, Revision 1, Section 5.0, and NEI 01-01, Appendix B, provide NRC-endorsed

guidance for documenting 10 CFR 50.59 evaluations to meet the requirements of 10 CFR 50.59(d). 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 that support

the 10 CFR 50.59 evaluation are subject to the aforementioned principles. For a knowledgeable

reviewer to draw the same conclusion regarding qualitative assessments, the 10 CFR 50.59

evaluation documentation needs to include and clearly reference details of the considerations

made and their separate and aggregate effects on any qualitative assessments. References to

other documents should include the document name and location of the information within any

referenced document (e.g., section or page numbers or both).

If qualitative assessment factors are used, the documentation would discuss each factor,

including the positive and negative aspects considered, consistent with the examples provided

in Table 1. In addition, the documentation would discuss the degree to which each of the

categories was relied upon to reach the qualitative assessment conclusion.

4.0 Engineering Evaluations: Failure Analysis

Consistent with NEI 01-01, Sections 4.4.2 and 5.1, the failure analysis provides insights needed

to determine whether the proposed change could reduce redundancy, diversity, separation, or

independence, any of which is considered to result in more than a minimal increase in the

likelihood of the occurrence of malfunctions. In addition to failures caused by software, it is

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important to note that other effects of a digital modification could create new results of

malfunctions (e.g., combining functions, creating new interactions with other systems, changing

response time). The design should address these other effects. For example, if previously

separate functions are combined in a single digital device, the failure analysis should consider

whether single failures that could previously have affected only individual design functions can

now affect multiple design functions. Where potential interconnectivity hazards are identified,

the incorporation of appropriate design attributes should address these hazards. It is important

to carefully document the analysis and resolution of potential hazards to ensure that future plant

design changes appropriately address these hazards.

Section 4 of this RIS supplement is intended to emphasize key areas of consideration for

identifying CCF vulnerabilities in the failure analysis to be addressed and documented in the

final design and to support a qualitative assessment.

4.1 Failure Analysis

Failure analysis can be used to identify possible CCF vulnerabilities and assess the need to

further modify the design. In some cases, design features and attributes could be used to

preclude potential failures from further consideration. Modifications that use design attributes

and features such as internal diversity or segmentation help to minimize the potential for CCFs.

Similarly, backup capabilities offered by other systems can address identified failures. Sources

of CCF vulnerabilities could include the introduction of identical software into redundant

channels, the use of shared resources, or the use of common hardware and software across

interconnected systems that perform different design functions. Another key consideration is

that undesirable behaviors may not necessarily constitute an SSC failure but rather a

misoperation (e.g., spurious actuation, erroneous control). Therefore, identifying sources of

CCF to the extent practicable and addressing them during the design process is essential and

constitutes an acceptable method for supporting the technical basis for the proposed

modification.

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

closely reviewed to ensure that an accident of a different type or a malfunction with a different

result 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 that use identical hardware and

software, power supplies, or human-machine interfaces needs to be closely reviewed to ensure

that possible common triggers have been addressed.

Unless the licensees UFSAR already incorporates best estimate methods, it cannot use such

methods to evaluate different results than those previously evaluated in the UFSAR. Use of

best estimate methods in 10 CFR 50.59 evaluations is limited to the subject of the previous

use of such methods in the UFSAR.

The failure analysis can also reveal potential sources of CCF introduced through software

development and configuration tools. Careful consideration should also be given to individual

programmable logic devices or user-configurable devices as potential sources of CCFs.

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Digital Communications

Careful consideration should be given to digital communications to preclude effects on SSC

independence within the failure analysis. Digital communications may introduce interactions

resulting in new types of failure modes. Digital modifications that introduce digital networks, or

interconnectivity across channels and divisions or between different systems should incorporate

appropriate design attributes so that reductions in redundancy, diversity, separation, or

independence of UFSAR-described design functions are not introduced. Adherence to

applicable industry consensus standards (e.g., IEEE 7-4.3.2, IEEE Standard Criteria for Digital

Computers in Safety Systems of Nuclear Power Generating Stations) can provide technically

justifiable design attributes to address potential failure modes of modifications with digital

communications, including modifications to non-safety related SSCs.

Combining Design Functions and Shared Resources

Failure analysis may address the combination of design functions for different safety-related or

non-safety related SSCs in a manner not previously evaluated or described in the UFSAR

because such combinations could introduce new interdependencies and interactions that make

identifying new potential failure modes more difficult. Of significant concern is the failure of

combined design functions or within shared resources that (1) can affect malfunctions of SSCs

or accidents evaluated in the UFSAR or (2) involve different defense-in-depth echelons.

Combining previously separate component functions can result in more dependable system

performance because of the tightly coupled nature of the components and the reduction in

complexity. If a licensee proposes to combine previously separate design functions or introduce

shared resources in a safety-related or non-safety related digital I&C modification, it needs to

carefully weigh possible new failures. Failure analysis and control system segmentation

analysis can help identify potential issues. A segmentation analysis is particularly helpful for the

evaluation of the design of non-safety related distributed networks.

Defense-in-Depth Analysis

The use of defense-in-depth design principles provides a means for addressing identified CCF

vulnerabilities. NEI 01-01 describes the need for a defense-in-depth analysis as limited to

substantial digital replacements of the reactor protection system and engineered safety features

actuation system. However, a defense-in-depth analysis may be used to evaluate the

capabilities of any digital modifications to reveal the impact of any new potential CCFs caused

by 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, a 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 or procedures could

serve to mitigate effects of possible CCF vulnerabilities introduced through the proposed

modification.

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4.2 Failure Analysis Resolution and Documentation

The licensee must develop and retain documentation for a proposed digital I&C modification in

accordance with its NRC-approved quality assurance program, including design engineering

procedures. The documentation of failure analysis identifies the possible vulnerabilities

introduced in the design and the effects of failures resulting from such vulnerabilities. In

addition, the documentation identifies the design features and procedures that resolve identified

failures, as described in NEI 01-01, Section 5.1.4. The level of detail used should be

commensurate with the safety significance and complexity of the modification in accordance

with licensee procedures.

Licensees may, but need not, use the table below in developing and documenting the failure

analysis. Documentation should explain how adequate bases preclude or limit failures so that

a knowledgeable reviewer could reach the same conclusion.

Table 2 Example: Failure Analysis Resolution and Documentation

Topical Area Description

Step 1 * Describe the scope and boundaries of the proposed activity, including

Identification interconnections and commonalities with other SSCs.

• List the UFSAR-described design function(s) affected by the

proposed change.

• Describe any new design functions performed by the modified design

that were not part of the original design.

• Describe any design functions eliminated from the modified design

that were part of the original design.

• Describe any previously separate design functions that were

combined as part of the activity.

• Describe any automatic actions to be transferred to manual control.

• Describe any manual actions that are to be transferred to automatic

control.

• Describe the expected modes of operation and transitions from one

mode of operation to another.

Step 2Failure Mode * Provide a comparison between the failure modes of the new digital

Comparison equipment and the failure modes of the equipment being replaced.

• If the failure modes are different, describe the resulting effect of

equipment failure on the affected UFSAR-described design

function(s). Consider the possibility that the proposed modification

may have introduced potential failures:

- Describe the effects of identified potential failure modes or

undesirable behaviors, including, but not limited to, failure

modes associated with hardware, software, combining

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Table 2 Example: Failure Analysis Resolution and Documentation

Topical Area Description

functions, use of shared resources, software tools,

programmable logic devices, or common hardware/software.

- Describe the potential sources of CCFs being introduced that

are also subject to common triggering mechanisms with those

of other SSCs that are not being modified.

• Explain how identified potential failures are being resolved (see

NEI 01-01, Section 5.1.4.).

Step 3 Based on the qualitative assessment factors provided in Table 1, is the new

Determination of digital equipment at least as reliable as the equipment being replaced?

Equipment

Dependability and

CCF Likelihood

Step 4 IF the results of Step 3 indicate that the new digital equipment is at least as

Assessment of dependable as the equipment being replaced or that the level of

Equipment dependability is determined acceptable:

Dependability and

CCF Likelihood * Document the bases for the conclusion.

Results * Continue to Step 5.

IF not, consider modifying the design or rely on existing design function

backup capabilities.

Step 5 Summarize the results and overall conclusions reached. Discuss the effect

Documentation of the proposed activity, if any, on applicable UFSAR-described design

functions. Discuss the differences in equipment failure modes and the

associated effects of different failure modes on applicable UFSAR-described

design functions. Describe the incorporation of design attributes to resolve

potential CCF vulnerabilities.

Examples of supporting documents include the following:

• Applicable codes and standards applied in the design

• Equipment environmental conditions (e.g., ambient temperature,

electromagnetic interference, radio-frequency interference, seismic

activity)

• Quality design processes used (e.g., Subpart 2.7 of Part II of

American National Standards Institute/American Society of

Mechanical Engineers NQA-1, Quality Assurance Program

Requirements for Nuclear Power Plants)

• Commercial-grade dedication documentation, such as described in

EPRI TR-106439 (if applicable)

• Failure modes and effects analysis (if applicable)

• Software hazard analysis (if applicable)

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Table 2 Example: Failure Analysis Resolution and Documentation

Topical Area Description

• Critical digital reviews, such as described in EPRI TR-1011710,

Handbook for Evaluating Critical Digital Equipment and Systems (if

applicable)

• Documentation of equipment operating experience

Step 6Application Apply engineering conclusions to the 10 CFR 50.59 evaluation questions.

of Failure Analysis

Conclusions to the

10 CFR 50.59

Evaluation Criteria

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