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{{Adams
{{Adams
| number = ML13064A112
| number = ML003740318
| issue date = 11/30/1976
| issue date = 02/28/1986
| title = Instrument Setpoints
| title = (Task IC 010-6), Instrument Setpoints for Safety-Related Systems
| author name =  
| author name =  
| author affiliation = NRC/RES
| author affiliation = NRC/RES
Line 10: Line 10:
| license number =  
| license number =  
| contact person =  
| contact person =  
| document report number = RG-1.105, Rev 1
| document report number = RG-1.105, Rev 2
| document type = Regulatory Guide
| document type = Regulatory Guide
| page count = 2
| page count = 4
}}
}}
{{#Wiki_filter:U.S. NUCLEAR REGULATORY  
{{#Wiki_filter:Revision 2* 0 ,; U.S. NUCLEAR REGULATORY  
COMMISSION
COMMISSION  
OFFICE OF STANDARDS
February 1986 SREGULATO
DEVELOPMENT
RY GUIDE OFFICE OF NUCLEAR REGULATORY
REGULATORY  
RESEARCH REGULATORY  
GUIDE 1.105 INSTRUMENT  
GUIDE 1.105 (Task IC 010-5)INSTRUMENT  
SETPOINTS Revision ll November 1976
SETPOINTS  
FOR SAFETY-RELATED
SYSTEMS


==A. INTRODUCTION==
==A. INTRODUCTION==
Criterion  
Criterion  
13, "Instrumentation and Control," of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50, "Licensing of Production and Utilization Facilities," requires, among other things, that instrumentation be provided to monitor variables and systems and that controls be Iprovided to maintain these variables and systems within prescribed operating ranges.Paragraph (cXl)(ii)(A)  
13, "Instrumentation and Control," of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50, "Domestic Licensing of Production and Utilization Facilities," requires, among other things, that instrumentation be provided to moni tor variables and systems and that controls be provided to maintain these variables and systems within prescribed operating range
of §50.36, "Technical Specifications," of 10 CFR Part 50 requires that, where a limiting safety system setting is specified for a variable on which a safety limit has been placed, the setting be so chosen that automatic protective action will correct the most severe abnormal situation an-ticipated before a safety limit is exceeded.This guide describes a method acceptable to the NRC staff-for complying with the Commission's regulations with regard to ensuring that the instru-ment setpoints in systems important to safety initially are within and remain within the specified limits. The Advisory Committee on Reactor Safeguards has been consulted concerning this guide and has concurred in the regulatory position.
 
====s. Criterion ====
20, "Protection System Functions," of Appendix A to 10 CFR Part 50 requires, among other things, that the protection system be designed to initiate operation of appropriate systems to ensure that specified acceptable fuel design limits are not exceeded.
 
Paragraph (cXl)(ii)(A)  
of § 50.36, "Technical Specifi cations," of 10 CFR Part 50 requires that, where a limiting safety system setting is specified for a variable on which a safety limit has been placed, the setting be so chosen that automatic protective action will correct the most severe abnormal situation anticipated without exceeding a safety limit. It also requires the licensee to notify the NRC of any automatic safety system mal functions, to review the matter, and to record the results of the review. Setpoints that exceed technical specification limits are considered a malfunction of an automatic safety system.  This guide describes a method acceptable to the NRC staff for complying with the Commission's regulations for ensuring that instrument setpoints are initially within and remain within the technical specification limits. The Advisory Committee on Reactor Safeguards has been consulted concerning this guide and has concurred in the regulatory position.
 
iMe substantial number of changes in this revision thas made it Impractical to indicate the changes with lines in the margin.Any information collection activities mentioned in this regulatory guide are contained as requirements in 10 CFR Part 50, which provides the regulatory basis for this guide. The information collection requirements in 10 CFR Part 50 have been cleared under OMB Clear ance No. 3150-0011.


==B. DISCUSSION==
==B. DISCUSSION==
Operating experience has shown that there is need for guidance in the selection of required instrument accuracy and the settings that are used to initiate automatic protective actions and alarms.Abnormal Occurrence Reports submitted by operating utilities between January 1972 and June 1973 record the most frequent abnormal occurrence as the drift of the protective instrument setpoint out-side the limits specified in the technical specifications.
Revision I to Regulatory Guide 1.105, "Instrument Setpoints," was published in November 1976 in response to the large number of reported instances in which instrument setpoints in safety-related systems drifted outside the limits specified in the technical specifications.


* Lines indicate substantive changes from previous issue.Protective instruments and alarms in nuclear power plants are provided with adjustable setpoints where specific actions are either automatically in-itiated, prohibited, or alarmed. For example, pres-sure sensors typically are installed on main steam lines to measure steam pressure.
Using the method described in Revision 1 to Regulatory Guide 1.105 and additional criteria on establishing and maintaining setpoints, Subcommittee SP67.04, Setpoints for Safety-Related Instruments in Nuclear Power Plants, under the Nuclear Power Plant Standards Committee of the Instrument Society of America (ISA) has developed a standard containing minimum requirements to be used for establishing and maintaining setpoints of individual instrument channels in safety-related systems. This stan dard is ISA-S67.04-1982, "Setpoints for Nuclear Safety Related Instrumentation Used in Nuclear Power Plants."e*
Some key terms used throughout ISA-S67.04-1982 are not defined or have unclear applications.


These sensors in-itiate corrective action if the steam pressure decreases to the predetermined and preset value that would result, for example, from a steam line break. Set-points (e.g., pressure, differential pressure, flow, level, temperature, power, radiation level, time delay)correspond to certain provisions of technical specifications that have been incorporated into the operating license by the Commission.
For cun venience, the following information is provided:
(1) the definition of the term "safety limit" is contained in 1 50.36 of 10 CFR Part 50, (2) the term "allowable value" as used in the standard is consistent with the usage in the bases sections of the Standard Technical Specification (STS),***
(3) the term "upper setpoint "Copies are available from the Instrument Society of America, P.O. Box 12277, Research Triangle Park, North Carolina 27709.  ***NUREG-0103, Revision 4, "Standard Technical Specifica tions for Babcock and Wilcox Pressurized Water Reactors";
NUREG 0123. Revision 3, "Standard Technical Specifications for General Electric Boiling Water Reactors (BWR/S)";
NUREG-0212, Revision 2, "Standard Technical Specifications for Combustion Engineering Pressurized Water Reactors";
and NUREG-0452.-
Revision 4. "Stan dard Technical Specifications for Westinghouse Pressurized Water Reactors." Copies of NUREG-series documents may be purchased from the Superintendent of Documents, U.S. Goverrunent Print.  ing Office, Poet Office Box 37082, Washihston, DC 20013-7082.


The single most prevalent reason for the drift of a measured parameter out of compliance with a technical specification is the selection of a setpoint that does not allow a sufficient margin between the setpoint and the technical specification limit to ac-count for inherent instrument inaccuracy, expected vibration, and minor calibration variations.
USNRC REGULATORY
GUIDES Written comments may be suomitted to the Rules and Procedures Branch, DRR ADM, U.S. Nuclear Regulatory Commission, Regulatory Guides are Issued to describe and make avallable to the Washington, oC 20555.  public methods acceptable to the NRC staff of Implementing specific parts of the Commission's regulations, to delineate tech niques used by the staff In evaluating specific problems or postu- The guideo are issuea in tne following tun broad divisions:
lated accidents or to provide guidance to applicants.


In some cascs, the setpoint selected was numerically equal to the technical specification limit and stated as an ab-solute value, thus leaving no apparent margin for er-ror. In other cases, the setpoint was so close to the upper or lower limit of the instrument's range that the instrument drift placed the setpoint beyond the instrument's range, thus nullifying the trip function.Other causes for drift of a parameter out of confor-mity with a technical specification have been in-strumentation design inadequacies and questionable calibration procedures.
Regulatory Guides are not substitutes for regulations, and compliance with 1. Power Reactors 6. Products them Is not required.


The following terms are listed with the definitions used in this guide: 1. Instrument accuracy-the degree to which an indicated value conforms to an accepted standard value or a true value.0 USNRC REGULATORY
Methods and solutions different from those set 2. Research and Test Reactors 7. Transportation Out in the guides will be acceptable if they provide a basis for the 3. Fuels and Materials Facilities
GUIDES Comments should be sent to the Secretary of the Commission.
5. Occupational Health findings requisite to the Issuance or continuance of a permit or 4. Environmental and Siting 9. Antitrust and Financial Review license by the Commission.


U.S Nuclear Regulatory Guides are issued to describe and make available to the public Regulatory Commission.
5. Materials and Plant Protection
10. General This guide was Issued after consideration of comments received from Copies of issued guides may be purchased at the current Government the public. Comments and suggestions for Improvements In these Printing Office price. Information on current GPO prices may be guides are encouraged at all times, and guides will be revised, as obtained by contacting the Superintendent of Documents, U.S.  appropriate, to accommodate comments and to reflect new Informa- Government Printing Office. Post" Office Box 37082 Wasnhngton, tlon or experience.


Washington.
DC 20013-7082.


D.C. 2058r. Attention Docketing and methods acceptable to thc NRC staff of implementing specific parts of the Service Section.Commission's regulations.
telephone
(202)275-2060
or (202)27h-2171.


to delineate techniques used by the staff in evalu-ating specific problems or postulated accidents, or to provide guidance to appli- The guides are issued in the following ten broad divisions:
limit" as used in Figure 1 of the standard is the same as "trip setpoints" as used in the aforementioned STSs in that drift above the "upper setpoint limit" (standard)
cents. Regulatory Guides are not substitutes for regulations, and compliance
or "trip setpoint" (STSs) requires readjustment.
1. Power Reactors 6. Products with them is not required.


Methods and solutions different from those set out in 2. Research and Test ReactOrs 7. "ransportation the guides will be acceptable if they provide a basis for the findings requisite to 2. Resea and Materials Facilitis Occupational Health the issuance or continuance of a permit or license by the Commission.
Paragraph
4.3 of the standard specifies the methods for combining uncertainties in determining a trip set point and its allowable values. Typically, the NRC staff has accepted 95% as a probability limit for errors. That is, of the observed distribution of values for a particular error component in the empirical data base, 95% of the data points will be bounded by the value selected.


3. Ere na and siting .atitrus Review 4. Environmental and Siting 9. Antitrust Review Comments and suggestions for improvements in these guides are encouraged
If the data base follows a normal distribution, this corres ponds to an error distribution approximately equal to a "two sigma" value. Section 6 requires that "software qualification" be documented.
6. Materials and Plant Protection
10. General at all times, and guides will be revised, as appropriate, to accommodate com-ments and to reflect new information or experience.


This guide was revised as a Copies of published guides may be obtained by written, request indicating the result of substantive comments received from the public and additional staff divisions desired to the U.S. Nuclear Regulatory Commission.
Although there is no generally accepted definition in the nuclear industry for software qualifica tion, the industry has used ANSI/IEEE-ANS-7-4.3.2-1982, "Application Criteria for Programmable Digital Computer Systems in Safety Systems of Nuclear Power Generating Stations," for verification and validation of computer software used in safety-related systems. Regulatory Guide 1.152, "Criteria for Programmable Digital Com puter System Software in Safety-Related Systems of Nuclear Power Plants," endorses this standard.


Washington.
Some of the considerations in documenting setpoint drift are (1) the degree of redundancy of the channels for which the allowable limits have been exceeded, (2) the type of instrument, including the instrument's designed accuracy, function, and plant identification number, (3) the allowable value in the technical specifi cations, (4) the "as left" setpoint from prior surveillance, (5) the measured setpoint, (6) the amount of adjustment in the reported occurrence and the current "as left" setpoint, and (7) the history of previous testing and the amount of any drift and adjustment in previous testing.C. REGULATORY
POSITION ISA-$67.04-1982, "Setpoints for Nuclear Safety Related Instrumentation Used in Nuclear Power Plants,' establishes requirements acceptable to the NRC staff for ensuring that instrument setpoints in safety-related systems are initially within and remain within the technical specification limits. The last section of ISA S67.04-1982 lists additional standards that are referenced in other sections of the standard.


D C review. 20565. Attention:
Those referenced standards not endorsed by a regulatory guide (or incor porated into the regulations)
Director.
also contain valuable information and, if used, should be used in a manner consistent with current regulations.


Office of Standards Development.
==D. IMPLEMENTATION==
The purpose of this section is to provide information to applicants and licensees regarding the NRC staff's plans for using this regulatory guide.  Except in those cases in which the applicant or li censee proposes an acceptable alternative method for complying with specified portions of the Commission's regulations, the methods described in this guide will be used by the NRC staff in the evaluation of instru ment setpoints for safety-related systems with respect to the technical specification limits for the following nuclear power plants: 1. Plants for which the constructi6n permit is issue after February 1986.  2. Plants for which the operating license applica tion is docketed 6 months or more after February 1986.  3. Plants for which the applicant or licensee vol untarily commits to the provisions of this guide.1.105-2 VALUE/IMPACT
STATEMENT


2. DrIft-a change in the input-output relationship of an instrument over a period of time.3. Margin-the difference between a limiting con-dition and an operating condition.
===1. BACKGROUND ===
The most common cause of a setpoint in a safety related system being out of compliance with plant .technical specifications has been the failure to allow for a sufficient margin to account for instrument inaccura cies, expected environmental drift, and minor calibration variations.


4. Range-the region within which a quantity is measured, received, or transmitted.
For example, in some cases, the trip setpoint selected was numerically equal to the allowable value and stated as an "absolute value," thus leaving no apparent margin for drift. In other cases, the trip setpoint was so close to the upper or lower limit of the range of the instrument that instrument drift placed the setpoint beyond the range of the instrument, thus nullifying the trip function.


5. Safety limit-a limit on an important process variable that is necessary to reasonably protect the in-tegrity of physical barriers that guard against un-controlled release of radioactivity.
Other general causes for a setpoint being out of conformity with the technical specifications have been instrument design inadequacies and questionable calibration procedures.


6. Setpoint-a predetermined level at which a bistable device changes state to indicate that the quantity under surveillance has reached the selected value.7. Span-the algebraic difference between the up-per and lower limits of the range.8. Technical specification limit-the limit prescribed as a license condition on an important process variable for safe operation.
Revision I to Regulatory Guide 1.105, "Instrument Setpoints," was issued in November 1976 in response to the large number of instances reported in Licensee Event Reports (LERs) of setpoints drifting outside the limits specified in the technical specifications.


9. Systems important to safety-those systems that are necessary to ensure (1) the integrity of the reactor coolant pressure boundary, (2) the capability to shut down the reactor and maintain it in a safe condition, or (3) the capability to prevent or mitigate the consequences of accidents that could result in potential offsite exposures comparable to the guideline exposures of 10 CFR Part 100, "Reactor Site Criteria." C. REGULATORY
Revision I provided general guidance for (1) specifying setpoints (by considering instrument drift, accuracy, and range) and (2) having a securing device for the set point adjustment mechanism.
POSITION The following are applicable to instruments in systems important to safety: i. The setpoints should be established with suf-ficient margin between the technical specification limits for the process variable and the nominal trip setpoints to allow for (a) the inaccuracy of the instru-ment, (b) uncertainties in the calibration, and (c) the instrument drift that could occur during the interval between calibrations.


2. All setpoints should be established in that por-tion of the instrument span which ensures that the ac-curacy, as required by regulatory position 4 below, is maintained.
The method described in Revision I to Regulatory Guide 1.105 has been incorporated into an Instrument Society of America Standard, ISA-$67.04-1982, "Set points for Nuclear Safety-Related Instrumentation Used in Nuclear Power Plants." Revision 2 to Regulatory Guide 1.105 was developed to use the guidance of ISA-$67.04-1982.


Instruments should be calibrated so as to ensure the required accuracy at the setpoint.3. The range selected for the instrumentation should encompass the expected operating range of the process variable being monitored to the extent that saturation does not negate the required action of the instrument.
This revision provides more specific guidance on establishing and maintaining setpoints in response to the needs that were apparent from (1) a continuing large number of reportable occurrences and (2) the licensing review of methodology for specifying allowable values and trip setpoints.


4. The accuracy of all setpoints should be equal to or better than the accuracy assumed in the safety analysis, which considers the ambient temperature changes, vibration, and other environmental condi-tions. The instruments should not anneal, stress relieve, or work harden under design conditions to the extent that they will not maintain the required ac-curacy. Design verification of these instruments should be demonstrated as part of the instrument qualification program recommended in Regulatory Guide 1.89, "Qualification of Class IE Equipment for Nuclear Power Plants." 5. Instruments should have a securing device on the setpoint adjustment mechanism unless it can be demonstrated by analysis or test that such devices will not aid in maintaining the required setpoint ac-curacy and minimizing setpoint changes. The secur-ing device should be designed so that it can be secured or released without altering the setpoint and.should be under administrative control.6. The assumptions used in selecting the setpoint values in regulatory position I and the minimum margin with respect to the limiting safety system set-tings, setpoint rate of deviation (drift rate), and the relationship of drift rate to testing interval (if any)should be documented.
2. VALUE/IMPACT
ASSESSMENT
2.1 General ISA-S67.04-1982 is considered state-of-the-art meth odology for specifying and reviewing technical specifica tions on allowable values and trip setpoints, and mem bers of the industry have incorporated this standard into their internal procedures.


==D. IMPLEMENTATION==
Further, paragraphs
The purpose of this section is to provide informa-tion to applicants and licensees regarding the staffs plans for utilizing this regulatory guide.Except in those cases in which the applicant proposes an acceptable alternative method for com-plying with specified portions of the Commission's regulations, the method described herein will be used in the evaluation of submittals in connection with construction permit applications docketed after December 15, 1976.If an applicant wishes to use this regulatory guide in developing submittals for applications docketed on or before December 15, 1976, the pertinent portions I of the application will be evaluated on the basis of this guide.1. 105-2}}
50.73(a) and (b) of 10 CFR Part 50 define when an LER is required and what is to be included in an LER, respectively.
 
2.2 Value The value to NRC operations and industry- is that there would be (1) a systematic method for specifying and reviewing technical specifications on allowable values and trip setpoints, (2) more sophisticated methods for specifying technical specifications, (3) a reduction in setpoint readjustments, (4) less chance for unwarranted reactor shutdown, and (5) fewer LERs and other report able occurrences from the allowable limits of setpoints being exceeded.
 
2.3 Impact The impact would be minimal as ISA-$67.04-1982 represents current industry practice that has been codified in a national consensus standard.1.105-3 UNITED STATES NUCLEAR REGULATORY
COMMISSION
WASHINGTON, D.C. 20555 OFFICIAL BUSINESS PENALTY FOR PRIVATE USE, $300 FIRST CLASS MAIL POSTAGE & FEES PAID USNNCI WASH. D.C.  PERMIT No. G-67}}


{{RG-Nav}}
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Revision as of 02:29, 21 September 2018

(Task IC 010-6), Instrument Setpoints for Safety-Related Systems
ML003740318
Person / Time
Issue date: 02/28/1986
From:
Office of Nuclear Regulatory Research
To:
References
RG-1.105, Rev 2
Download: ML003740318 (4)
v • d • e


Revision 2* 0 ,; U.S. NUCLEAR REGULATORY

COMMISSION

February 1986 SREGULATO

RY GUIDE OFFICE OF NUCLEAR REGULATORY

RESEARCH REGULATORY

GUIDE 1.105 (Task IC 010-5)INSTRUMENT

SETPOINTS

FOR SAFETY-RELATED

SYSTEMS

A. INTRODUCTION

Criterion

13, "Instrumentation and Control," of Appendix A, "General Design Criteria for Nuclear Power Plants," to 10 CFR Part 50, "Domestic Licensing of Production and Utilization Facilities," requires, among other things, that instrumentation be provided to moni tor variables and systems and that controls be provided to maintain these variables and systems within prescribed operating range

s. Criterion

20, "Protection System Functions," of Appendix A to 10 CFR Part 50 requires, among other things, that the protection system be designed to initiate operation of appropriate systems to ensure that specified acceptable fuel design limits are not exceeded.

Paragraph (cXl)(ii)(A)

of § 50.36, "Technical Specifi cations," of 10 CFR Part 50 requires that, where a limiting safety system setting is specified for a variable on which a safety limit has been placed, the setting be so chosen that automatic protective action will correct the most severe abnormal situation anticipated without exceeding a safety limit. It also requires the licensee to notify the NRC of any automatic safety system mal functions, to review the matter, and to record the results of the review. Setpoints that exceed technical specification limits are considered a malfunction of an automatic safety system. This guide describes a method acceptable to the NRC staff for complying with the Commission's regulations for ensuring that instrument setpoints are initially within and remain within the technical specification limits. The Advisory Committee on Reactor Safeguards has been consulted concerning this guide and has concurred in the regulatory position.

iMe substantial number of changes in this revision thas made it Impractical to indicate the changes with lines in the margin.Any information collection activities mentioned in this regulatory guide are contained as requirements in 10 CFR Part 50, which provides the regulatory basis for this guide. The information collection requirements in 10 CFR Part 50 have been cleared under OMB Clear ance No. 3150-0011.

B. DISCUSSION

Revision I to Regulatory Guide 1.105, "Instrument Setpoints," was published in November 1976 in response to the large number of reported instances in which instrument setpoints in safety-related systems drifted outside the limits specified in the technical specifications.

Using the method described in Revision 1 to Regulatory Guide 1.105 and additional criteria on establishing and maintaining setpoints, Subcommittee SP67.04, Setpoints for Safety-Related Instruments in Nuclear Power Plants, under the Nuclear Power Plant Standards Committee of the Instrument Society of America (ISA) has developed a standard containing minimum requirements to be used for establishing and maintaining setpoints of individual instrument channels in safety-related systems. This stan dard is ISA-S67.04-1982, "Setpoints for Nuclear Safety Related Instrumentation Used in Nuclear Power Plants."e*

Some key terms used throughout ISA-S67.04-1982 are not defined or have unclear applications.

For cun venience, the following information is provided:

(1) the definition of the term "safety limit" is contained in 1 50.36 of 10 CFR Part 50, (2) the term "allowable value" as used in the standard is consistent with the usage in the bases sections of the Standard Technical Specification (STS),***

(3) the term "upper setpoint "Copies are available from the Instrument Society of America, P.O. Box 12277, Research Triangle Park, North Carolina 27709. ***NUREG-0103, Revision 4, "Standard Technical Specifica tions for Babcock and Wilcox Pressurized Water Reactors";

NUREG 0123. Revision 3, "Standard Technical Specifications for General Electric Boiling Water Reactors (BWR/S)";

NUREG-0212, Revision 2, "Standard Technical Specifications for Combustion Engineering Pressurized Water Reactors";

and NUREG-0452.-

Revision 4. "Stan dard Technical Specifications for Westinghouse Pressurized Water Reactors." Copies of NUREG-series documents may be purchased from the Superintendent of Documents, U.S. Goverrunent Print. ing Office, Poet Office Box 37082, Washihston, DC 20013-7082.

USNRC REGULATORY

GUIDES Written comments may be suomitted to the Rules and Procedures Branch, DRR ADM, U.S. Nuclear Regulatory Commission, Regulatory Guides are Issued to describe and make avallable to the Washington, oC 20555. public methods acceptable to the NRC staff of Implementing specific parts of the Commission's regulations, to delineate tech niques used by the staff In evaluating specific problems or postu- The guideo are issuea in tne following tun broad divisions:

lated accidents or to provide guidance to applicants.

Regulatory Guides are not substitutes for regulations, and compliance with 1. Power Reactors 6. Products them Is not required.

Methods and solutions different from those set 2. Research and Test Reactors 7. Transportation Out in the guides will be acceptable if they provide a basis for the 3. Fuels and Materials Facilities

5. Occupational Health findings requisite to the Issuance or continuance of a permit or 4. Environmental and Siting 9. Antitrust and Financial Review license by the Commission.

5. Materials and Plant Protection

10. General This guide was Issued after consideration of comments received from Copies of issued guides may be purchased at the current Government the public. Comments and suggestions for Improvements In these Printing Office price. Information on current GPO prices may be guides are encouraged at all times, and guides will be revised, as obtained by contacting the Superintendent of Documents, U.S. appropriate, to accommodate comments and to reflect new Informa- Government Printing Office. Post" Office Box 37082 Wasnhngton, tlon or experience.

DC 20013-7082.

telephone

(202)275-2060

or (202)27h-2171.

limit" as used in Figure 1 of the standard is the same as "trip setpoints" as used in the aforementioned STSs in that drift above the "upper setpoint limit" (standard)

or "trip setpoint" (STSs) requires readjustment.

Paragraph

4.3 of the standard specifies the methods for combining uncertainties in determining a trip set point and its allowable values. Typically, the NRC staff has accepted 95% as a probability limit for errors. That is, of the observed distribution of values for a particular error component in the empirical data base, 95% of the data points will be bounded by the value selected.

If the data base follows a normal distribution, this corres ponds to an error distribution approximately equal to a "two sigma" value. Section 6 requires that "software qualification" be documented.

Although there is no generally accepted definition in the nuclear industry for software qualifica tion, the industry has used ANSI/IEEE-ANS-7-4.3.2-1982, "Application Criteria for Programmable Digital Computer Systems in Safety Systems of Nuclear Power Generating Stations," for verification and validation of computer software used in safety-related systems. Regulatory Guide 1.152, "Criteria for Programmable Digital Com puter System Software in Safety-Related Systems of Nuclear Power Plants," endorses this standard.

Some of the considerations in documenting setpoint drift are (1) the degree of redundancy of the channels for which the allowable limits have been exceeded, (2) the type of instrument, including the instrument's designed accuracy, function, and plant identification number, (3) the allowable value in the technical specifi cations, (4) the "as left" setpoint from prior surveillance, (5) the measured setpoint, (6) the amount of adjustment in the reported occurrence and the current "as left" setpoint, and (7) the history of previous testing and the amount of any drift and adjustment in previous testing.C. REGULATORY

POSITION ISA-$67.04-1982, "Setpoints for Nuclear Safety Related Instrumentation Used in Nuclear Power Plants,' establishes requirements acceptable to the NRC staff for ensuring that instrument setpoints in safety-related systems are initially within and remain within the technical specification limits. The last section of ISA S67.04-1982 lists additional standards that are referenced in other sections of the standard.

Those referenced standards not endorsed by a regulatory guide (or incor porated into the regulations)

also contain valuable information and, if used, should be used in a manner consistent with current regulations.

D. IMPLEMENTATION

The purpose of this section is to provide information to applicants and licensees regarding the NRC staff's plans for using this regulatory guide. Except in those cases in which the applicant or li censee proposes an acceptable alternative method for complying with specified portions of the Commission's regulations, the methods described in this guide will be used by the NRC staff in the evaluation of instru ment setpoints for safety-related systems with respect to the technical specification limits for the following nuclear power plants: 1. Plants for which the constructi6n permit is issue after February 1986. 2. Plants for which the operating license applica tion is docketed 6 months or more after February 1986. 3. Plants for which the applicant or licensee vol untarily commits to the provisions of this guide.1.105-2 VALUE/IMPACT

STATEMENT

1. BACKGROUND

The most common cause of a setpoint in a safety related system being out of compliance with plant .technical specifications has been the failure to allow for a sufficient margin to account for instrument inaccura cies, expected environmental drift, and minor calibration variations.

For example, in some cases, the trip setpoint selected was numerically equal to the allowable value and stated as an "absolute value," thus leaving no apparent margin for drift. In other cases, the trip setpoint was so close to the upper or lower limit of the range of the instrument that instrument drift placed the setpoint beyond the range of the instrument, thus nullifying the trip function.

Other general causes for a setpoint being out of conformity with the technical specifications have been instrument design inadequacies and questionable calibration procedures.

Revision I to Regulatory Guide 1.105, "Instrument Setpoints," was issued in November 1976 in response to the large number of instances reported in Licensee Event Reports (LERs) of setpoints drifting outside the limits specified in the technical specifications.

Revision I provided general guidance for (1) specifying setpoints (by considering instrument drift, accuracy, and range) and (2) having a securing device for the set point adjustment mechanism.

The method described in Revision I to Regulatory Guide 1.105 has been incorporated into an Instrument Society of America Standard, ISA-$67.04-1982, "Set points for Nuclear Safety-Related Instrumentation Used in Nuclear Power Plants." Revision 2 to Regulatory Guide 1.105 was developed to use the guidance of ISA-$67.04-1982.

This revision provides more specific guidance on establishing and maintaining setpoints in response to the needs that were apparent from (1) a continuing large number of reportable occurrences and (2) the licensing review of methodology for specifying allowable values and trip setpoints.

2. VALUE/IMPACT

ASSESSMENT

2.1 General ISA-S67.04-1982 is considered state-of-the-art meth odology for specifying and reviewing technical specifica tions on allowable values and trip setpoints, and mem bers of the industry have incorporated this standard into their internal procedures.

Further, paragraphs

50.73(a) and (b) of 10 CFR Part 50 define when an LER is required and what is to be included in an LER, respectively.

2.2 Value The value to NRC operations and industry- is that there would be (1) a systematic method for specifying and reviewing technical specifications on allowable values and trip setpoints, (2) more sophisticated methods for specifying technical specifications, (3) a reduction in setpoint readjustments, (4) less chance for unwarranted reactor shutdown, and (5) fewer LERs and other report able occurrences from the allowable limits of setpoints being exceeded.

2.3 Impact The impact would be minimal as ISA-$67.04-1982 represents current industry practice that has been codified in a national consensus standard.1.105-3 UNITED STATES NUCLEAR REGULATORY

COMMISSION

WASHINGTON, D.C. 20555 OFFICIAL BUSINESS PENALTY FOR PRIVATE USE, $300 FIRST CLASS MAIL POSTAGE & FEES PAID USNNCI WASH. D.C. PERMIT No. G-67


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