Regulatory Guide 1.105

Instrument Setpoints

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

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."

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.

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