ML20205C405

Safety Evaluation Supporting Facility Restart Following 860601 Feedwater Transient.Licensee Action Plan,Supplemented by Listed Actions,Adequate Basis for Restart & short-term Operation.Supporting Drawings & Matls Encl

ML20205C405
Person / Time
Site:	LaSalle
Issue date:	08/07/1986
From:	Office of Nuclear Reactor Regulation
To:
Shared Package
ML20205C386	List: ML20205C381 ML20205C405
References
IEB-86-002, IEB-86-2, NUDOCS 8608120323
Download: ML20205C405 (15)

Text

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UNITED STATES J .ma naem T

j NUCLEAR REGULATORY COMMISSION

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....+ l SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION SUPPORTING THE RESTART OF LASALLE COUNTY STATION UNIT 2 COMMONWEALTH EDISON COMPANY DOCKET NO. 50-374 l

1.0 INTRODUCTION

l On June 1, 1986, La Salle County Station Unit 2 was operating at 95 percent power when a feedwater transient occurred which One of four caused levelreactor 3 trip channels vessel level to drop toresulting actuated the levelin3 atrip halfsetpoint.The scram. operators were able to restore level to the normal operating range and continue plant operation. Subsequent investigation determined that the reactor vessel level had dropped below the ,

level 3 trip setpoint and a reactor scram had not occurred. The licensee l declared an " Alert, shut the plant down, and r.otified the NRC. ,

Further investigation detennined that the event was due to a failure of the reactor vessel level 3 switches to trip at their set level. A special level  ;

)

drop test was perfonned to check the actual setpoints of each level 3 switch while at operating temperature and pressure. With the reactor at 950 psig, the reactor water was slowly lowered out of the nonral operating range down to the trip point of each level 3 switch. The results were erratic with the switches tripping at levels between 3.9 and 10.2 inches versus their calibr- i ated setpoint of 13.5 inches.  !

The investigation also revealed similar anomalous behavior of the same type of switch used to actuate the emergency core cooling system, primary contain-ment isolation system, and other engineered safety feature systems as illustrated below:

1. It was determined that the minimum flow valves for the emergency core cooling system pumps would not open at the proper setpoints. For example, the switch for the high pressure core spray system pump was calibrated to actuate at 1300 gpm but did not actuate until flow decreased to 530 gpm.

2. As a result of observations of the level drop test, it was discovered that one of two switches, used to initiate the automatic depressuri-zation system due to low reactor water level, failed to function. i l

Calibration of the switch showed that the setpoint had shifted nonconservatively by 25 inches. With this shift of setpoint and the relative location of the instrument taps, sufficient differential pressure could not have been produced to actuate the switch. The switch was disassembled and inspected. Rust was found inside the  ;

switch bearing assembly.

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The investigation, conducted by the licensee, identified two collateral primary causes for the nonconservative behavior of the Static-0-Ring (50R) switches:

1. Rusted cross shaft bearing and

2. Deformation of 0-ring seals j The first cause, " Rusted cross shaft bearing," resulted in the functional failure of one of two SOR switches used to initiate the automatic depres-surization system. Initial calibration of the switch showed that the A second calibration a few hours later setpoint had shifted 25 also showed setpoint shift. inches. Based on these calibration findings, the licensee concluded that the " Rusted cross shaft bearing" cause for degraded switch performance can be detected through a nonconservative shifting of the calibration setpoint. In order to detect this shift, the licensee has proposed an augmented surveillance program. This augmented surveillance program is discussed in Reference 2.

The second cause, " Deformation of 0-Ring Seals," was identified by the SOR switch manufacturer. When the 0-ring seals are subjected to operational pressures over a period of time, they change. shape such that a greater i

frictional force is exerted in opposition to the rotation of the cross shaft.

Thus, the switch trip setpoint shifts with the change of pressure from the "0" psig calibration level to the operational pressure level (1000 psig for most applications). The licensee has determined through testing by the switch r.anfacturer that maximum shift of the calibrated setpoint occurs during the first 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> period of operation and that negligible shifts occur i beyond this time.

The switches used in safety system applications at the La Salle County (Station are a differential pressure type switch manufactured by Static-0-Ring 50R) t Incorporated. These switches are shown pictorially in Attachments 1 and 2 of this report; and a discussion on the operation of the switch is made in ,

Section III of the licensee's submittal of July 18, 1986 (Reference 2).

The differential pressure switches are calibrated at atmospheric pressure without removing them from the system by use of a test rig consisting of two bottles, each containing water and air or nitrogen connected to the differential pressure switch with one bottle on either side of the diaphragm.

Because the significant shifts of the 50R differential pressure switch calibration trip setpoints occur at operating pressures of approximately 1000 psig over a 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> period, the use of existing procedures to perform all switch calibrations and checks which are performed at atmospheric pressure (i.e., initial calibration, pre-operational testing, periodic functional testing, and periodic calibrations) could not detect the setpoint i

)

i shift of these switches.

i l

l In its submittals of July 18, 21, 23, and 24, 1986, the licensee has proposed a plan for testing and monitoring operation of Staff the switches to support evaluation restart of this plan and short term operation of La Salle Unit 2.

is discussed in Section 2.1 of this SER.

In order to address the generic implications of these setpoint shifts, the staff issued IE Bulletin No. 86-02, " Static-0-Ring Differential Pressure 18, 1986, to all power reactor facilities holding Switches," La Salle County an operating license (OL) or a construction permit (CP).

dated July Station Unit 2 submitted its responses to the bulletin by letter dated July 25, 1986. Staff evaluation of the licensee response to the bulletin is discussed in Section 2.2 of this SER.

2.0 EVALUATION 2.1 Licensee's Plan for Testing and Monitoring of S0R Switches to Support Restart and Short Term Operation of La Salle Unit 2 I

The purpose of this SER is to evaluate the licensee's proposed short term actions to change switch calibration set points for operational parameters associated with S0R switches in the more conservative direction to compensate for anomalous switch behavior. Hardware changes and/or continued use of SOR type switches with their associated operational constraints for the longer term (beyond the next scheduled refueling outage) will be the subject of a future safety evaluation report.

At a July 18, 1986 meeting in Bethesda, the La Salle County Station licensee (with representatives from SOR Inc.) presented the results of their investiga-tion into the June 1,1986, setpoint shift of SOR switches. Subsequent to this meeting, the licensee provided submittals, in which it discussed the investigative efforts expended to identify the root cause of the anomalous switch behavior, and described its short term plan and actions it will implement to compensate for switch behavior and to monitor and test the switches to assure safe operation of the plant to support restart and short ters operation of La Salle Unit 2.

The licensee's investigative process to determine the root cause of the anomalous 50R switch behavior as described in Reference 2 appears to be sufficiently comprehensive. The discussion provided on 0-ring deformation is clear and reasonable; however, the theory on causes of switch failure ,

I due to rusting may not fully explain the phenomena for observed switch failure. Therefore, additional investigation and testing is being conducted by the switch manufacturer in this area. When the results of this additional investigation and testing become available, they will be evaluated by the staff and the results of the staff evaluation will be included in a future report.

. 4-The licensee's investigative program to determine measures that could be implemented to compensate for anomalous switch behavior included seven (7) tests and test procedures which are described on pages V-4 through V-12 in Reference 2. Representative switch samples were subjected to each test to establish the impact on switch perfonnance. Results of these tests indicated that test No. 1, " Static Pressure Cycling" test, and test No. 2, " Time Related Effects" test results showed the most adverse impact on switch per-fomance. The static pressure cycling test, which was performed for all 50R differential pressure switches, involved the calibration of each switch per station calibration procedures. Each switch was then cycled at zero static pressure, and at elevated static pressure to determine the repeatability of performance of first actuations. The time related effects test involved subjecting several switches to elevated static pressure for periods of 4, 24, 48, and 72 hour8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> periods and then cycling each to detemine the first actua-tion point. Successive first actuation cycling following each time / pressure period was compared to determine whether the setpoint drift increased with time. Results of this test indicated that the significant setpoint shifts occurred between 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and that shifts beyond 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> were negligible. The information derived from the testing program was used to characterize 50R switch operation, to establish bounds for static pressure shifts and switch actuation repeatability, and to develop an operational monitoring and test program (Reference 5).

To confirm the data derived from the La Salle test program, 50R Inc. is per-forming tests in parallel over a longer period of time. In the 50R Inc. test program, several different models of SOR switches are being exercised at intervals of everyday, every two weeks, every four weeks, and every six weeks, to determine the magnitude and direction of setpoint shift over time.

The licensee's program for identifying the problems associated with the 50R switch and the testing program to develop measures to compensate for anomalous switch behavior are acceptable to the staff.

l The licensee's proposed corrective actions for the short term for monitoring and testing of 50R differential pressure switches is discussed in Reference i 5 and is also included as Attachment 3 to the SER. Attachment 3 also includes a list of 50R switches installed at La Salle, and a table which provides pro-posed new calibration setpoints for each SOR switch application.

The licensee's corrective actions plan consists of eight (8) elements with completion dates for each element. The actions include: flow testing to verify ECCS minimum flow switch setpoints (completed); reactor water level j drop tests for level 3 switches at startup and shutdown for the next refueling outage (scheduled for December 1986); completion of calibration procedures (completed); completion of recalibration of switches with new setpoints, (prior to startup); implementation of increased surveillance (after startup);

establishment of acceptance limits for switches (completed); submittal of La Salle Unit 2, Final SOR Report (completed); and completion of evalua-tion of alternative level sensing instruments to possibly replace 50R switches (by 1/1/87).

_ . ___ _ _ m-. - --a

The staff concludes that the licensee's corrective action plan and schedule supplemented by the actions described below, provide an adequate basis for restart and short tem operation of La Salle Unit 2 until the next refueling outage (scheduled for December,1986). The staff discussed the supplemental actions with the licensee; and the licensee agreed to them via a telecon on July 31, 1986. The licensee confimed this in a letter dated August 1,1986.

The supplemental actions are as follows:

A. The licensee's plan to conduct reactor water level drop tests to verify level 3 switch setpoints during startup after the current outage and The results during shutdown for the next refueling outage is acceptable.

from the water level drop tests which are conducted at operating pressure for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> should demonstrate stability of the trip setpoints and i should confirm that the switches operate within the La Salle Technical Specification limits. l 1

l B. The licensee's plan for defining an action limit and a reject limit for each switch application is acceptable except that when any 50R differ- l ential pressure switch calibration check indicates a change which equals or exceeds the reject limit,~the licensee shall declare the component and the associated channel inoperable and take the appropriate actions  :

required by the La Salle County Station Unit 2 Technical Specifications, and the licensee shall report the incident in accordance with the require- l j

ment of 10 CFR 50.72 and 10 CFR 50.73.

C. The licensee's increased surveillance program (Reference 5) provides for calibration of the 50R Differential Pressures Switches at atmospheric pressure to the new trip setpoints (Attachment 3) which compensate, in a conservative direction, for shifts due to pressure and time, and perfoms periodic checks of the new trip setpoint for each switch at atmospheric pressure. The increased surveillance program, conducted at atmospheric pressure, should demonstrate stability of the trip setpoint, and confirm that the switches operate within the technical specification limit. The increased surveillance program, supplemented by the actions described below, is acceptable to the staff.

1. The licensee shall continue performing its monthly channel func-tional test in accordance with the LaSalle Technical Specification.

2. If one channel of a system of redundant channels (or similar equip-ment in redundant safety systems) is found to have an actual trip point that is outride the Technical Specifications or otherwise unacceptable for reliable system operation, then the redundant channels (or similar equipment) shall be tested as soon thereafter as practical.

In addition, if the licensee is contemplating long term use of the SOR dif-ferential pressure switches, or other mechanical devices, it should consider providing the means for makir,g local channel checks by use of a differential pressure gauge or other equivalent instrument display.

2.2 Licensee Response to IE Bulletin 86-02 " Static-0-Ring Differential Pressure Switche:;"

In response to IE Bulletin 86-02, the licensee has taken certain actions to assure that systems using SOR 102 and 103 differential pressure switches operate reliably and has submitted the infomation for the staff to review by letter dated July 25, 1986. This infomation is the same as that dis-cussed in paragraph 2.1 of the SER. Our evaluation of the licensee's sub-mittal in response to IE Bulletin 86-02 follows.

The licensee installed 59 SOR series 102 and 103 switches at La On June Salle Unit 1,1986, 2, as part of an environmental qualification modification.

certain SOR differential pressure switches failed to actuate at the calibrated trip setpoints. Subsequent test results showed that a number of the switches would not have actuated at their existing setpoints under nomal operating conditions. The licensee has taken short term corrective actions and has developed a program (see paragraph 2.1) to improve the performance of these switches. The setpoints of the differential pressure switches have been revised to provide additional margin for static pressure shift and l repea tability. These revisions will ensure that the 50R Differential Pressure Switches will actuate at the required trip setpoints. Switches l l

that do not meet the static shift and repeatability acceptance limits l will be replaced. To provide assurance that the switches will function pro- l perly, the ECCS minimum flow switches have been tested at their new setpoints )

during system operation and the reactor level drop tests will be performed at '

pressures of 950, 500 and 0 psig to verify the level 3 switchThesetpoints during calibration startup and during shutdown for the next refueling outage.

procedure is being revised to record the first actuation of the switch as the "as found" setpoint and to eliminate the effects of cycling. The j surveillance frequency will be increased if a switch setpoint exceeds its action limit. If its setpoint exceeds the rejection limit or the action limit on two consecutive surveillance tests, the switch will be replaced.

The staff has evaluated the licensee's submittal in response to IE Bulletin 86-02 " Static-0-Ring Differential Pressure Systems" and has concluded that the licensee has taken short term corrective actions to assure that systemsThere-using 50R 102 and 103 differential pressure switches operate reliably.

fore, the staff finds the short tem actions taken by the licensee are at least equivalent to the actions required by IE Bulletin 86-02 and are, there-fore, acceptable for La Salle Unit 2.

3.0 CONCLUSION

3.1 Licensee Actions to Support Restart and Short Tem Operation of La Salle Unit 2 Until Shutdown for the Next Refueling Outage Based on our review of sebmittals dated July 18, 21, 23, and 24, 1986, and the licensee's comitment to implement actions to assure reliable SOR pressure switch actuation at the required trip setpoints as described

! l in Table 5, " Corrective Action" (Reference 5), the test results of SOR l 1

switches reviewed to date and the calibration of all SOR switches to new calibration trip setpoints which should assure reliable safe operation of the SOR switches, we conclude that the licensee's action plan *upplemented by the actions described below provides an adequate basis for the restart and short tem safe operation of the La Salle County Station Unit 2 until the next refueling outage (scheduled for December, 4

1986). The supplemental actions have been discussed with, and agreed, to by the licensee during a telecon on July 31, 1986. The licensee The confim to perform these actions in a letter dated August 1,1986.

supplemental actions are:

1. When any 50R differential pressure switch calibration setpoint check indicates a change which equals or exceeds the reject limit, the licensee shall declare the component and the associated channel inoperable and take the appropriate actions required by the La Salle County Station Unit 2 Technical Specifications, and the licensee shall report the incident in accordance with the requirements of 10 CFR 50.72 and 10 CFR 50.73.

2. The increased frequency of surveillance calibration testing progra.,

which is conducted at atmospheric pressore, and the level drop tests, i

which are conducted at operating pressure for 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and which are to be performed during restart and during shutdown for the next refuelinc outage, shall demonstrate stability of the trip setpoint, and shall confirm that the switches operate within the La Salle Technical Specifi-cation limits.

3. The licensee shall continue performing its monthly Channel Functional Test, perfomed in accordance with the La Salle Technical Specification.

4. If one channel of a system of redundant channels (or similar equipment in redundant safety systems) is found to have an actual trip point that l i

is outside the Technical Specifications or otherwise unacceptable for reliable system operation, then the redundant channels (cr similar equipment) shall be tested as soon thereafter as practical.

5. The licensee shall submit, for staff review, test results acquired through its periodic and increased surveillance calibration testing j

program and from the SOR Inc. long term testing of the 50R Differential Pressure Switch, as the data become available, or when the La Salle Unit 2 shuts down for the next refueling outage.

3.2 Licensee Response to IE Bulletin 86-02 " Static-0-Ring Differential Pressure Switches" The staff has evaluated the licensee's submittal which responded to IE Bulletin 86-02 and has concuded that the licensee has taken appropriate measures for the short term to assure that the systems using SOR 102 and i 103 switches operate reliably. Therefore, the staff finds the short term actioris taken by the licensee are at least equivalent to the actions required by lE Bulletin 86-02 and are, therefore, 6cceptable for La Salle Ur.it :.

TABLE OF REF_ERENCE,5

1. Letter from Region III to Coni.onwealth Edison dated June 19, 1986 regarding confirmatory action to resolve problem prior to startup.

2. July 18, 1986, letter from M.S. Turbak to H. R. Denton transmitting this Draft " Report of Investigation of SOR Differential Pressure Switches."

3. Letter to Denton from Turbak, dated July 21, 1986, regarding the validity of 24 hour2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> static pressure tests.

4. Letter to Denton from Turbak, dated July 23, 1986, Transmitting Daily and Two Week Test Data.

5. Letter to Denton from Turbak, dated July 24, 1986, Transmitting the Executive Sunnery.

6. Letter to Denton from Turbak, Response to I&E Bulletin 86-02, dated July 25, 1986.

7. Letter to Denton fron Allen, dated August 1,1986, Transmitting Commitment to Supplemental Actions.

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a. New setpoints includ'.ng: static pressure shif t.

repeatability margin and drift sargin,

b. New calibration anthods including: tha *ns-peund" setpoint will be the first actuatism and during calibration the switch will be cycled from the appropriate 0% or lon% of differential pressure s,an to the setpoint. ,

. "Ar-Fet .d* se t;oint ac eptanea l'. .its wi'_1 be included 1.:: the p-:caduras, and actions will be l defined for each 1*mit. T.e limits and actions will be the following:

( .; Action Limit (a) Except main steam Line sigh Flow.

13% of ad',ustatie rar.ga gram new calibrat'.cn setecint.

(b) For main steam the high ficw this limit was 1.!X repeatatiLi:7 of the ass variatie swice. i= servica.

Repeatability was calcu*ated for each switch to heur4 in of the data with a 93% canf*conce level.

gel If this lim *: is escaeded increase survei1*,ance frequencias for the switsh. T.e nest surte*11ancs viii be perf=rmed at ea same interval as the *.ast suriet Lance '

wi:hi . This limit.

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. RU. Gatto *k7 ' 86 12: 57 HRC MESSAGE gvPj.e7 CENTER BETHE5DR MD ,

2 calibration procedure revisions (continued)

(c) *As-Found" setpoint acceptance limits (continued)

(2) tojoctton Limit -

(s) 2, (24 of Adjustable tante + Tech. Spec.

Margin for drift)

(b) t For mainsteam line high flow this limit is 5

(Repeatability + Tech. Spec. drift) for the most variable switch in service.

Repeatability was calculated for each switch to 95% of the data with a 95%

confidence level.

(c) If this limit is exceeded the switch will be rejected.

5. Complete recalibration of switches with revised artpcists a .d revised procedures. Prior to startup from aurrect cutage.  !;

6. Implement Increased Surveillance hfter Startup

4. Categories from current-outage.

1.

Level 3 4switches startep. (6 switches) weeks after startup, t -sonths The Level gfter 3sta switches will be calibrated 2 weks after 4th month, the tavel 3 switches will remain ga a qua and 4 months after startup. after the ly frequency, schedule asswuss no preeless ector with the llelts as Gescribed aseve. tote that this .,

2.

Main switches 5 team will to Line Break4S.

ca11brated itches weeks (16 after switches) - At least 4 of the Main Stees Line (M5L) startup.

of the MEL sultches will be calibrated S veeks after startap.Of the tamaining 12 switches at least 4.

t least a of the MEL switches will be talthroled 11 weeks after startup.Of the Theremaining maalema

• Switches.

inierval for each ladividual switch will te limit 44 to a giortarly frequency .

3. Ramalains tches C31 switches) - A sasyle runstaing tches Leperealettel 1/3) will model nueers) repetsentative of the runsleing 1/3 will be es trated 24monthsuseks efter after starte. Of the -

tches, wul runnining sultches 9pprealante) be call ted starte. The 3 months efter startu lacividual switch will be timited te a quarterly frequency.p. The maalaue laterval for each be chaser.. where possible, to include a seeling of earlous switch sacel neeers.The representative sangles w "1.

cc..plete evaluatien of alternative level sensing ins tri.,=e..ts to replace scP.

1-1-87

a. Review Requirements

b. Review vendor environmental qualification data.

c. Review vender performance test data, .

d. Recessend Technically acceptable alternatives.

e. Cc=plete approval.

preliminary conceptual design and obtain reviews and General description of key features effecting design installation, operation and maintenanca and project plan.

f. Initiate detailed conceptual design.

. s.. .

hvis109.888 RUG.97 '86 12:57 HRC MESSAGE CENTER BETHE5DR 7/e5/86 MD onset.t.d [1 s astatish acceptance timits for new son switches. The Purchase Order with SOE will be revised to require tests snailar to

• betpoint characterization tests tactuding a 34 hour3.935185e-4 days <br />0.00944 hours <br />5.621693e-5 weeks <br />1.2937e-5 months <br /> test and to require switcinas -

perform within the static shift and repeatabt11ty limits.

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