Regulatory Guide 1.105

Instrument Setpoints

ML13064A112
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Issue date:	11/30/1976
From:	Office of Nuclear Regulatory Research
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RG-1.105, Rev 1
Download: ML13064A112 (2)

U.S. NUCLEAR REGULATORY

COMMISSION

OFFICE OF STANDARDS

DEVELOPMENT

REGULATORY

GUIDE 1.105 INSTRUMENT

SETPOINTS Revision ll November 1976

A. INTRODUCTION

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)

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.

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.

• 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.

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.

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.

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.

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

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 na and siting .atitrus Review 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 of an instrument over a period of time.3. Margin-the difference between a limiting con-dition and an operating condition.

4. Range-the region within which a quantity is measured, received, or transmitted.

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.

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.

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

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.

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.

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.

D. IMPLEMENTATION

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