Regulatory Guide 1.105: Difference between revisions

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{{Adams
{{Adams
| number = ML993560062
| number = ML13064A112
| issue date = 12/31/1999
| issue date = 11/30/1976
| title = Setpoints for Safety-Related Instrumentation
| title = Instrument Setpoints
| author name =  
| author name =  
| author affiliation = NRC/RES
| author affiliation = NRC/RES
Line 9: Line 9:
| docket =  
| docket =  
| license number =  
| license number =  
| contact person = Aggarwal S (301)415-6005
| contact person =  
| document report number = RG-1.105, Rev. 3
| document report number = RG-1.105, Rev 1
| document type = Regulatory Guide
| document type = Regulatory Guide
| page count = 7
| page count = 2
}}
}}
{{#Wiki_filter:U.S. NUCLEAR REGULATORY COMMISSION                                                                         Revision 3 December 1999 REGULATORY
{{#Wiki_filter:Revision ll U.S. NUCLEAR REGULATORY COMMISSION                                                                                                           November 1976 OFFICE OF STANDARDS DEVELOPMENT
                                    GUIDE
                                                                  REGULATORY GUIDE 1.105 INSTRUMENT SETPOINTS
                                    OFFICE OF NUCLEAR REGULATORY RESEARCH
                                                      REGULATORY GUIDE 1.105 (Draft was DG-1045)
                      SETPOINTS FOR SAFETY-RELATED INSTRUMENTATION


==A. INTRODUCTION==
==A. INTRODUCTION==
Criterion 13, "Instrumentation and Control," 1 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 monitor variables and systems and that controls be provided to maintain these variables and systems within prescribed operating ranges.
Protective instruments and alarms in nuclear power plants are provided with adjustable setpoints Criterion 13, "Instrumentation and Control," of                               where specific actions are either automatically in- Appendix A, "General Design Criteria for Nuclear                                   itiated, prohibited, or alarmed. For example, pres- Power Plants," to 10 CFR Part 50, "Licensing of                                     sure sensors typically are installed on main steam Production and Utilization Facilities," requires,                                   lines to measure steam pressure. These sensors in- among other things, that instrumentation be                                         itiate corrective action if the steam pressure decreases provided to monitor variables and systems and that                                 to the predetermined and preset value that would controls be Iprovided to maintain these variables and                               result, for example, from a steam line break. Set- systems within prescribed operating ranges.                                        points (e.g., pressure, differential pressure, flow, level, temperature, power, radiation level, time delay)
        Paragraph (cXl)(ii)(A) of §50.36, "Technical                                  correspond to certain provisions of technical Specifications," of 10 CFR Part 50 requires that,                                  specifications that have been incorporated into the where a limiting safety system setting is specified for a                          operating license by the Commission.


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.
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-                                    The single most prevalent reason for the drift of a ticipated before a safety limit is exceeded.                                       measured parameter out of compliance with a technical specification is the selection of a setpoint This guide describes a method acceptable to the                                that does not allow a sufficient margin between the NRC staff-for complying with the Commission's                                       setpoint and the technical specification limit to ac-
 
0 regulations with regard to ensuring that the instru-                                count for inherent instrument inaccuracy, expected ment setpoints in systems important to safety initially                            vibration, and minor calibration variations. In some are within and remain within the specified limits. The                             cascs, the setpoint selected was numerically equal to Advisory Committee on Reactor Safeguards has been                                  the technical specification limit and stated as an ab- consulted concerning this guide and has concurred in                              solute value, thus leaving no apparent margin for er- the regulatory position.                                                           ror. In other cases, the setpoint was so close to the upper or lower limit of the instrument's range that
Paragraph (c)(1)(ii)(A) of § 50.36, "Technical Specifications," of 10 CFR Part 50 requires, in part, 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 abnormal situation before a safety limit is exceeded. It also requires, among other things, that the licensee notify the NRC if the licensee determines that an automatic safety system does not function as required. The licensee is required to then review the matter and record the results of the review.
 
1 For the full text of the General Design Criteria and other sections of the regulations cited in this guide, see 10 CFR Part 50,
      "Domestic Licensing of Production and Utilization Facilities."
Regulatory guides are issued to describe and make available to the public such information as methods acceptable to the NRC staff for implementing specific parts of the NRCs regulations, techniques used by the staff in evaluating specific problems or postulated accidents, and data needed by the NRC staff in its review of applications for permits and licenses. Regulatory guides are not substitutes for regulations, and compliance with them is not required. Methods and solutions different from those set out in the guides will be acceptable if they provide a basis for the findings requisite to the issuance or continuance of a permit or license by the Commission.
 
This guide was issued after consideration of comments received from the public. Comments and suggestions for improvements in these guides are encouraged at all times, and guides will be revised, as appropriate, to accommodate comments and to reflect new information or experience. Written comments may be submitted to the Rules and Directives Branch, ADM, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001.
 
Regulatory guides are issued in ten broad divisions: 1, Power Reactors; 2, Research and Test Reactors; 3, Fuels and Materials Facilities; 4, Environmental and Siting; 5, Materials and Plant Protection; 6, Products; 7, Transportation; 8, Occupational Health; 9, Antitrust and Financial Review; and 10, General.
 
Single copies of regulatory guides (which may be reproduced) may be obtained free of charge by writing the Distribution Services Section, U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by fax to (301)415-2289, or by email to DISTRIBUTION@NRC.GOV. Many regulatory guides are also available on the internet at NRCs home page at <WWW.NRC.GOV>.
 
This guide describes a method acceptable to the NRC staff for complying with the NRC's regulations for ensuring that setpoints for safety-related instrumentation are initially within and remain within the technical specification limits. The guide is being revised to endorse Part l of ISA-S67.04-1994,
"Setpoints for Nuclear Safety-Related Instrumentation." 2 This standard provides a basis for establishing setpoints for nuclear instrumentation for safety systems and addresses known contributing errors in the channel.
 
The information collections contained in this regulatory guide are covered by the requirements in
10 CFR Part 50, which were approved by the Office of Management and Budget, approval number 3150-
0011. The NRC may not conduct or sponsor, and a person is not required to respond to, a collection of information unless it displays a currently valid OMB control number.


==B. DISCUSSION==
==B. DISCUSSION==
Instrument setpoint uncertainty allowances and setpoint discrepancies have led to a number of operational problems. Operating experience indicates that setpoints for safety-related instrumentation may allow plants to operate outside the limiting conditions of operation specified in their technical specifications. Licensees have discovered conflicts between existing setpoints and engineering calculations. The causes for these setpoint discrepancies were problems with industry practices that led to errors in calibration procedures and a lack of understanding of the relationship of the setpoint to the allowable value. Additional problems noted included varying setpoint methodologies for engineering calculations, a lack of a consistent definition of allowable value between different setpoint methodologies, and improper understanding of the relationship of the allowable value to earlier setpoint terminology, procedures, and operability criteria. Further problems were noted when procedures (the setpoint process)
the instrument drift placed the setpoint beyond the instrument's range, thus nullifying the trip function.
(1) failed to provide an adequate margin between the instrument as-left criteria and the values (trip set point or allowable values) required per the technical specifications, (2) did not always reflect current design criteria, and (3) did not ensure that revised instrument loops were verified to the original design requirements or that instrument modifications were evaluated for their effect on setpoint calculations. It has also been noted that licensees do not typically verify whether setpoint calculation drift assumptions have remained valid for the system surveillance interval.


ISA-S67.04 was revised in 1987 to provide clarification and to reflect industry practic
Operating experience has shown that there is need                              Other causes for drift of a parameter out of confor- for guidance in the selection of required instrument                              mity with a technical specification have been in- accuracy and the settings that are used to initiate                                strumentation design inadequacies and questionable automatic protective actions and alarms.                                          calibration procedures.


====e. The term====
Abnormal Occurrence Reports submitted by The following terms are listed with the definitions operating utilities between January 1972 and June used in this guide:
"trip setpoint" was made consistent with the terminology used by the NRC staff.
  1973 record the most frequent abnormal occurrence as the drift of the protective instrument setpoint out- side the limits specified in the technical specifications.                             1. Instrument accuracy-the degree to which an indicated value conforms to an accepted standard


The standard was revised further in 1994. The effects of uncertainty allowances and discrepancies in setpoints, along with operational experience, were appropriately addressed during this revision of ISA-
* Lines indicate substantive changes from previous issue.                         value or a true value.
S67.04. This revision of the standard also reflects the Improved Technical Specification program (a cooperative effort between industry and the NRC staff) and reflect current industry practice. This standard provides a basis for establishing setpoints for nuclear instrumentation for safety systems and addresses known contributing errors in a particular channel from the process (including the primary element and sensor) through and including the final setpoint device.


The term "trip setpoint" is retained in ISA-S67.04-1994. However, Figure 1 in ISA-S67.04-1994 (for convenience, this figure has been reproduced as Figure 1 in this guide) has been revised to depict region "E," "a region of calibration tolerance." The calibration tolerance uncertainties depicted by region
USNRC REGULATORY GUIDES                                        Comments should be sent to the Secretary of the Commission. U.S Nuclear Regulatory Guides are issued to describe and make available to the public          Regulatory Commission. Washington. D.C. 2058r. Attention Docketing and methods acceptable to thc NRC staff of implementing specific parts of the           Service Section.
2 Copies may be obtained from the Instrument Society of America, 67 Alexander Drive, Research Triangle Park, NC
20779.


2
Commission's regulations. 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:
  cents. Regulatory Guides are not substitutes for regulations, and compliance        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.              3. Ere        and na      siting            .    atitrusReview
                                                                                      4. Environmental and Siting            9. Antitrust Review Comments and suggestions for improvements in these guides are encouraged            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. Washington. D C
  review.                                                                            20565. Attention: Director. Office of Standards Development.


"E" should be defined and accounted for in the licensees setpoint methodology. A trip setpoint value identified to be outside region "E" regardless of direction requires readjustment to satisfy the setpoint methodology and uncertainties identified in Figure 1 (acceptable as-left condition). It should be noted that this standard does not define "nominal" trip setpoint. The trip setpoint as depicted in Figure 1 is consistent with the term "nominal" trip setpoint as shown about a defined calibration tolerance band.
2. DrIft-a change in the input-output relationship            3. The range selected for the instrumentation of an instrument over a period of time.                         should encompass the expected operating range of the process variable being monitored to the extent
    3. Margin-the difference between a limiting con-           that saturation does not negate the required action of dition and an operating condition.                             the instrument.


Figure 1 of the standard provides setpoint relationships for nuclear safety-related setpoints. The figure denotes relative position and not direction, but it should be noted that the uncertainty relationships depicted by Figure 1 do not represent any one particular method (direction, combination, or relationship of uncertainty groupings) for the development of a trip setpoint or allowable value.
4. Range-the region within which a quantity is                4. The accuracy of all setpoints should be equal to measured, received, or transmitted.                            or better than the accuracy assumed in the safety analysis, which considers the ambient temperature
    5. Safety limit-a limit on an important process            changes, vibration, and other environmental condi- variable that is necessary to reasonably protect the in-       tions. The instruments should not anneal, stress tegrity of physical barriers that guard against un-            relieve, or work harden under design conditions to controlled release of radioactivity.                            the extent that they will not maintain the required ac- curacy. Design verification of these instruments
    6. Setpoint-a predetermined level at which a                should be demonstrated as part of the instrument bistable device changes state to indicate that the             qualification program recommended in Regulatory quantity under surveillance has reached the selected            Guide 1.89, "Qualification of Class IE Equipment value.                                                          for Nuclear Power Plants."
    7. Span-the algebraic difference between the up-              5. Instruments should have a securing device on per and lower limits of the range.                              the setpoint adjustment mechanism unless it can be demonstrated by analysis or test that such devices
    8. Technical specification limit-the limit                  will not aid in maintaining the required setpoint ac- prescribed as a license condition on an important              curacy and minimizing setpoint changes. The secur- process variable for safe operation.                            ing device should be designed so that it can be secured or released without altering the setpoint and
    9. Systems important to safety-those systems
                                                              .should be under administrative control.


Section 4 of ISA-S67.04-1994 states that the safety significance of various types of setpoints for safety-related instrumentation may differ, and thus a less rigorous setpoint determination method may be applied for certain functional units and limiting conditions of operation (LCOs). A setpoint methodology can include such a graded approach. However, the grading technique chosen by the licensee should be consistent with the standard and should consider applicable uncertainties regardless of the setpoint application. Additionally, the application of the standard, using a "graded" approach, is also appropriate for non-safety system instrumentation for maintaining design limits described in the Technical Specifications. Examples may include instrumentation relied on in emergency operating procedures (EOPS), and for meeting applicable LCOs, and for meeting the variables in Regulatory Guide
that are necessary to ensure (1) the integrity of the reactor coolant pressure boundary, (2) the capability              6. The assumptions used in selecting the setpoint to shut down the reactor and maintain it in a safe              values in regulatory position I and the minimum condition, or (3) the capability to prevent or mitigate        margin with respect to the limiting safety system set- the consequences of accidents that could result in             tings, setpoint rate of deviation (drift rate), and the potential offsite exposures comparable to the                  relationship of drift rate to testing interval (if any)
1.97, "Instrumentation for Light-Water-Cooled Nuclear Power Plants To Assess Plant and Environs Conditions During and Following an Accident." 3 The industry consensus standard ANSI/ANS-10.4-1987, "Guidelines for the Verification and Validation of Scientific and Engineering Computer Programs for the Nuclear Industry," provides helpful information on the qualification of setpoint methodology software.
guideline exposures of 10 CFR Part 100, "Reactor                should be documented.


ISA-S67.04-1982 has been used by licensees for setpoint methodology and instrument drift evaluations. ISA-S67.04-1994 provides limited guidance on drift evaluations and uncertainty term development for the evaluation of an instrument surveillance interval. The
Site Criteria."
3 Single copies of regulatory guides, both active and draft, may be obtained free of charge by writing the Office of Administration, Attn: Reproduction and Distribution Services Section, U.S. Nuclear Regulatory Commission, Washington, DC 20555, or by fax to (301)415-2289, or by email to <DISTRIBUTION@NRC.GOV>. Copies are also available for inspection or copying for a fee from the NRC Public Document Room at 2120 L Street NW.,
Washington, DC; the PDRs mailing address is Mail Stop LL-6, Washington, DC 20555; telephone (202)634-3273;
fax (202)634-3343.


3
==D. IMPLEMENTATION==
 
S a fe ty L im it A n a lytica l L im it N o te :    T h is fig u re is in te n d e d to p ro vid e re la tive p o sitio n a n d n o t to im p ly d ire ctio n .
                              A                                                    C
                                                                                              A llo w a b le V a lu e (L S S S )
                                                                                      B
                                                                                                  T rip E                                        S e tp o in t (L S S S )
                                D
                                                N o rm a l A. A llo w a n ce d e scrib e d in p a ra g ra p h 4 .3 .1 B. A llo w a n ce d e scrib e d in p a ra g ra p h 4 .3 .1 C. R e g io n w h e re ch a n n e l m a y b e d e te rm in e d in o p e ra b le D. P la n t o p e ra tin g m a rg in E. R e g io n o f ca lib ra tio n to le ra n ce (a cce p ta b le a s le ft co n d itio n )
  d e scrib e d in p a ra g ra p h 4 .3 .1 Figure 1. Nuclear Safety-Related Setpoint Relationships
                                                    4
 
(Reproduced from ISA-67.04-1994)
                5
 
staff has generally accepted drift evaluations based on statistical prediction techniques. However, significant variability has been observed in licensees surveillance interval evaluations with regard to drift, setpoint methodology, and completeness. The following concerns were identified during the NRC staff review, but they have been resolved during the development of ISA-S67.04-1994.
 
+        Limited instrument drift data were included in the licensee setpoint study.
 
+        Drift data account for all data points from a surveillance calibration (i.e., nine-point check) as independent data, but inadequate justification is provided for this assumption. Drift data points also included interim calibrations.
 
+        A large number of data points was provided for a limited number of instruments.
 
+        Flawed outlier analysis resulted in valid data being removed from the data set.
 
+        Drift dependency on time was assumed to be negligible over the interval selected, and inadequate justification was provided when extrapolating to an extended surveillance interval (e.g., 24 months).
+        Setpoint methodology assumes normal distribution of data when such an assumption was not verified.
 
+        Instrumentation evaluations (historical, maintenance, drift) were incomplete.
 
+        Drift projections, including those based on regression analyses, may not account for penalties for uncertainty projection (extended surveillance interval-drift) beyond the time range for the data collected.
 
+        Instrument application and process or installation variables were not evaluated.
 
+        The uncertainties assumed for instrumentation, including primary elements, were subsequently not verified or controlled through surveillance testing, qualification, or maintenance programs.
 
+        The acceptability of pooling generic drift data with plant-specific data or weighing the data according to the source of the data was not justified.
 
+        All available applicable data were not utilized in the analysis.
 
Section 4.3 of ISA-S67.04-1994 states that the limiting safety system setting (LSSS) may be the trip setpoint, an allowable value, or both. For the standard technical specifications, the staff designated the allowable value as the LSSS. In association with the trip setpoint and limiting conditions for operation (LCOs), the LSSS establishes the threshold for protective system action to prevent acceptable limits being exceeded during design basis accidents. The LSSS therefore ensures that automatic protective action will correct the abnormal situation before a safety limit is exceeded. A licensee, with justification, may propose an alternative LSSS based on its particular setpoint methodology or license.
 
The standard provides for the accounting of measurement and test equipment (MTE) uncertainties, but MTE criteria are not specifically identified within the standard. Criteria XI and XII in Appendix B to
                                                        6
 
10 CFR Part 50 provide requirements for quality regarding testing. Regulatory Guide 1.118, "Periodic Testing of Electric Power and Protection Systems," 3 provides guidance on periodic surveillance testing.
 
Part II, "Methodologies for the Determination of Setpoints for the Nuclear Safety-Related Instrumentation," of ISA-S67.04-1994 is not addressed by this regulatory guide.


==C. REGULATORY POSITION==
==C. REGULATORY POSITION==
Conformance with Part 1 of ISA-S67.04-1994, "Setpoints for Nuclear Safety-Related
The purpose of this section is to provide informa- The following are applicable to instruments in             tion to applicants and licensees regarding the staffs systems important to safety:                                    plans for utilizing this regulatory guide.
                    2 Instrumentation," with the following exceptions and clarifications, provides a method acceptable to the NRC staff for satisfying the NRC's regulations for ensuring that setpoints for safety-related instrumentation are established and maintained within the technical specification limits.
 
1.        Section 4 of ISA-S67.04-1994 specifies the methods, but not the criterion, for combining uncertainties in determining a trip setpoint and its allowable values. The 95/95 tolerance limit is an acceptable criterion for uncertainties. That is, there is a 95% probability that the constructed limits contain
95% of the population of interest for the surveillance interval selected.
 
2.        Sections 7 and 8 of Part 1 of ISA-S67.04-1994 reference several industry codes and standards. If a referenced standard has been incorporated separately into the NRC's regulations, licensees and applicants must comply with that standard as set forth in the regulation. If the referenced standard has been endorsed in a regulatory guide, the standard constitutes a method acceptable to the NRC staff of meeting a regulatory requirement as described in the regulatory guide. If a referenced standard has been neither incorporated into the NRC's regulations nor endorsed in a regulatory guide, licensees and applicants may consider and use the information in the referenced standard if appropriately justified, consistent with current regulatory practice.
 
3.        Section 4.3 of ISA-S67.04-1994 states that the limiting safety system setting (LSSS) may be maintained in technical specifications or appropriate plant procedures. However, 10 CFR 50.36 states that the technical specifications will include items in the categories of safety limits, limiting safety system settings, and limiting control settings. Thus, the LSSS may not be maintained in plant procedures. Rather, the LSSS must be specified as a technical-specification-defined limit in order to satisfy the requirements of
10 CFR 50.36. The LSSS should be developed in accordance with the setpoint methodology set forth in the standard, with the LSSS listed in the technical specifications.
 
4.        ISA-S67.04-1994 provides a discussion on the purpose and application of an allowable value. The allowable value is the limiting value that the trip setpoint can have when tested periodically, beyond which the instrument channel is considered inoperable and corrective action must be taken in accordance with the technical specifications. The allowable value relationship to the setpoint methodology and testing requirements in the technical specifications must be documented.
 
==D. IMPLEMENTATION==
The purpose of the section is to provide information to applicants and licensees regarding the NRC
staff's plans for using this regulatory guide.
 
7
 
Except in those cases in which an applicant or licensee proposes an acceptable alternative method for complying with specified portions of the NRC's regulations, the methods described in this guide will be used in the evaluation of submittals in connection with applications for construction permits, operating licenses, and combined licenses. It will also be used to evaluate submittals from operating reactor licensees who voluntarily propose to initiate system modifications if there is a clear nexus between the proposed modifications and this guidance.


8
i. The setpoints should be established with suf-              Except in those cases in which the applicant ficient margin between the technical specification            proposes an acceptable alternative method for com- limits for the process variable and the nominal trip            plying with specified portions of the Commission's setpoints to allow for (a) the inaccuracy of the instru-        regulations, the method described herein will be used ment, (b) uncertainties in the calibration, and (c) the        in the evaluation of submittals in connection with instrument drift that could occur during the interval          construction permit applications docketed after between calibrations.                                          December 15, 1976.


VALUE/IMPACT STATEMENT
2. All setpoints should be established in that por-            If an applicant wishes to use this regulatory guide tion of the instrument span which ensures that the ac-          in developing submittals for applications docketed on curacy, as required by regulatory position 4 below, is          or before December 15, 1976, the pertinent portions    I
        A draft value/impact statement was published with the draft proposed Revision 3 of this guide when it was published for public comment (DG-1045, October 1996). No changes were necessary, so a separate value/impact statement for the final guide has not been prepared. A copy of the draft value/impact statement is available for inspection or copying for a fee in the NRCs Public Document Room at 2120 L
maintained. Instruments should be calibrated so as to          of the application will be evaluated on the basis of ensure the required accuracy at the setpoint.                   this guide.
Street NW., Washington, DC under task DG-1045.


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Revision as of 13:56, 21 November 2019

Instrument Setpoints
ML13064A112
Person / Time
Issue date: 11/30/1976
From:
Office of Nuclear Regulatory Research
To:
References
RG-1.105, Rev 1
Download: ML13064A112 (2)
v • d • e


Revision ll U.S. NUCLEAR REGULATORY COMMISSION November 1976 OFFICE OF STANDARDS DEVELOPMENT

REGULATORY GUIDE 1.105 INSTRUMENT SETPOINTS

A. INTRODUCTION

Protective instruments and alarms in nuclear power plants are provided with adjustable setpoints Criterion 13, "Instrumentation and Control," of where specific actions are either automatically in- Appendix A, "General Design Criteria for Nuclear itiated, prohibited, or alarmed. For example, pres- Power Plants," to 10 CFR Part 50, "Licensing of sure sensors typically are installed on main steam Production and Utilization Facilities," requires, lines to measure steam pressure. These sensors in- among other things, that instrumentation be itiate corrective action if the steam pressure decreases provided to monitor variables and systems and that to the predetermined and preset value that would controls be Iprovided to maintain these variables and result, for example, from a steam line break. Set- systems within prescribed operating ranges. points (e.g., pressure, differential pressure, flow, level, temperature, power, radiation level, time delay)

Paragraph (cXl)(ii)(A) of §50.36, "Technical correspond to certain provisions of technical Specifications," of 10 CFR Part 50 requires that, specifications that have been incorporated into the where a limiting safety system setting is specified for a operating license by the Commission.

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- The single most prevalent reason for the drift of a ticipated before a safety limit is exceeded. measured parameter out of compliance with a technical specification is the selection of a setpoint This guide describes a method acceptable to the that does not allow a sufficient margin between the NRC staff-for complying with the Commission's setpoint and the technical specification limit to ac-

0 regulations with regard to ensuring that the instru- count for inherent instrument inaccuracy, expected ment setpoints in systems important to safety initially vibration, and minor calibration variations. In some are within and remain within the specified limits. The cascs, the setpoint selected was numerically equal to Advisory Committee on Reactor Safeguards has been the technical specification limit and stated as an ab- consulted concerning this guide and has concurred in solute value, thus leaving no apparent margin for er- the regulatory position. ror. In other cases, the setpoint was so close to the upper or lower limit of the instrument's range that

B. DISCUSSION

the instrument drift placed the setpoint beyond the instrument's range, thus nullifying the trip function.

Operating experience has shown that there is need Other causes for drift of a parameter out of confor- for guidance in the selection of required instrument mity with a technical specification have been in- accuracy and the settings that are used to initiate strumentation design inadequacies and questionable automatic protective actions and alarms. calibration procedures.

Abnormal Occurrence Reports submitted by The following terms are listed with the definitions operating utilities between January 1972 and June used in this guide:

1973 record the most frequent abnormal occurrence as the drift of the protective instrument setpoint out- side the limits specified in the technical specifications. 1. Instrument accuracy-the degree to which an indicated value conforms to an accepted standard

  • Lines indicate substantive changes from previous issue. value or a true value.

USNRC REGULATORY GUIDES Comments should be sent to the Secretary of the Commission. U.S Nuclear Regulatory Guides are issued to describe and make available to the public Regulatory Commission. Washington. D.C. 2058r. Attention Docketing and methods acceptable to thc NRC staff of implementing specific parts of the Service Section.

Commission's regulations. 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:

cents. Regulatory Guides are not substitutes for regulations, and compliance 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. 3. Ere and na siting . atitrusReview

4. Environmental and Siting 9. Antitrust Review Comments and suggestions for improvements in these guides are encouraged 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. Washington. D C

review. 20565. Attention: Director. Office of Standards Development.

2. DrIft-a change in the input-output relationship 3. The range selected for the instrumentation of an instrument over a period of time. should encompass the expected operating range of the process variable being monitored to the extent

3. Margin-the difference between a limiting con- that saturation does not negate the required action of dition and an operating condition. the instrument.

4. Range-the region within which a quantity is 4. The accuracy of all setpoints should be equal to measured, received, or transmitted. or better than the accuracy assumed in the safety analysis, which considers the ambient temperature

5. Safety limit-a limit on an important process changes, vibration, and other environmental condi- variable that is necessary to reasonably protect the in- tions. The instruments should not anneal, stress tegrity of physical barriers that guard against un- relieve, or work harden under design conditions to controlled release of radioactivity. the extent that they will not maintain the required ac- curacy. Design verification of these instruments

6. Setpoint-a predetermined level at which a should be demonstrated as part of the instrument bistable device changes state to indicate that the qualification program recommended in Regulatory quantity under surveillance has reached the selected Guide 1.89, "Qualification of Class IE Equipment value. for Nuclear Power Plants."

7. Span-the algebraic difference between the up- 5. Instruments should have a securing device on per and lower limits of the range. the setpoint adjustment mechanism unless it can be demonstrated by analysis or test that such devices

8. Technical specification limit-the limit will not aid in maintaining the required setpoint ac- prescribed as a license condition on an important curacy and minimizing setpoint changes. The secur- process variable for safe operation. ing device should be designed so that it can be secured or released without altering the setpoint and

9. Systems important to safety-those systems

.should be under administrative control.

that are necessary to ensure (1) the integrity of the reactor coolant pressure boundary, (2) the capability 6. The assumptions used in selecting the setpoint to shut down the reactor and maintain it in a safe values in regulatory position I and the minimum condition, or (3) the capability to prevent or mitigate margin with respect to the limiting safety system set- the consequences of accidents that could result in tings, setpoint rate of deviation (drift rate), and the potential offsite exposures comparable to the relationship of drift rate to testing interval (if any)

guideline exposures of 10 CFR Part 100, "Reactor should be documented.

Site Criteria."

D. IMPLEMENTATION

C. REGULATORY POSITION

The purpose of this section is to provide informa- The following are applicable to instruments in tion to applicants and licensees regarding the staffs systems important to safety: plans for utilizing this regulatory guide.

i. The setpoints should be established with suf- Except in those cases in which the applicant ficient margin between the technical specification proposes an acceptable alternative method for com- limits for the process variable and the nominal trip plying with specified portions of the Commission's setpoints to allow for (a) the inaccuracy of the instru- regulations, the method described herein will be used ment, (b) uncertainties in the calibration, and (c) the in the evaluation of submittals in connection with instrument drift that could occur during the interval construction permit applications docketed after between calibrations. December 15, 1976.

2. All setpoints should be established in that por- If an applicant wishes to use this regulatory guide tion of the instrument span which ensures that the ac- in developing submittals for applications docketed on curacy, as required by regulatory position 4 below, is or before December 15, 1976, the pertinent portions I

maintained. Instruments should be calibrated so as to of the application will be evaluated on the basis of ensure the required accuracy at the setpoint. this guide.

1. 105-2


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