ML18064A580

LER 94-012-01:on 940427,w/plant in Cold Shutdown,Determined That Internal Ground in Thermal Margin Monitor Caused Nonconformance W/Rps Design Basis.Mod Implemented to Effectively Isolate Grounded Monitor from Pressure Loop

ML18064A580
Person / Time
Site:	Palisades
Issue date:	01/17/1995
From:	Hass K, Roberts W CONSUMERS ENERGY CO. (FORMERLY CONSUMERS POWER CO.)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-94-012-01, LER-94-12-1, NUDOCS 9501250114
Download: ML18064A580 (10)

Text

.. consumers Power *

• Kurt M. Haas

. Plant Safety and Licensing Director

• POWERING

/llllCHIGAN-S PIUIGllESS Palisades Nuclear Plant: 27780 Blue Star Memorial Highway, Covert, Ml 49043 January 17, l 995 Nuclear Regulatory Commission*

Document Control Desk Washington, DC 20~55 DOCKET 50-255. - LICENSE DPR~ PALISADES PLANT LICENSEE EVENT REPORT 94:-012 THERMAL MARGIN MONITOR. INTERNAL GROUND -

REACTOR PROTECTIVE SYSTEM (RPS) DESIGN BASIS - SUPPLEMENTAL REPORT LiGensee Event Report (LER)94-012, Supplement 1, is ~ttached. This supplement includes a revis~d analysis of the event sectiori, as well as an extension of our estimated completion date for upgrading the Thermal Margin Monitor to the end of the refueling outage following the 1995 refueling outage. * * *

• This event was originally reported to the NRC as a condition outside the de~ign basis of the plant, per 10CFR50:73{a){2){ii){B).

• COMMITMENT

SUMMARY

The following commitments 'were part of the original* LER and are still active':*

The-Fai)ure modes and affects analysis for the RPS will be updated to reflect the mbdifications performed to the RPS and the TMM as a result rif

. this event report.

The schedule for co~pleti~g the ~odifications to the Thermal Margin Monitors to remove the internal ground resistance from the TMM, has been

• changed from the 1995 refueling outage to the ~nd of the refueling following the 19~5 refueling. *

• 9501250114 950117 PDR ADOCK 05000255 " ~ ..

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A CMS' ENERGY COMPANY .

2 After the grounds are removed the need for th~ circuit isolation devices

.added as a result of .this event to the inputs and outputs of the TMM will be reevaluated.

The following are new commitments from the supplement:

A procedure will be developed to test RPS and TMM signal circuits for grOUDdS. .

A m.emo wi 11 be issued to all department heads, members of the PRC and Management Review Board emphasizing the need to recognize situations where.

the plant is not in com~liance with design basis.,

Kurt MHaas Plant Safety and Licensing Minager CC Administrator, Region III, USNRC NRC Resident Inspector - Palisades.

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NRC*Form 366 U.S. NUCLEAR REGULATORY COMMISSION.

(9-831" APPROVED OMBNO. 3160-0104 EXPIRES: 8/31 /86 LICENSEE EVENT REPORT (LERI .

FACILITY NAME 111 DOCKET NUMBER 121 PAGE 131 Palisades Plant o 5 o* o o 2 5 5 TITLE 141_ Thermal M13rgin Monitor Internal Ground - Reactor Protective System (RPS) Design Basis - Supplemental Report EVENT DATE 161 LER NUMBER 161 REPORT DATE 16) OTHER FAC1UTIES INVOLVED 18)

REVISION _FACILITY NAMES MONTH DAY YEAR YEAR . NUMBER MONTH

• DAY . YEAR NIA 0 6 0 0 0 0 4 2 7 9 4 9 4 0 1 2 o. 0 1 7. 9 5 NIA 0 6 0 0 0 THIS REPORT IS SUBMITTED PURS.UANT TO THE REQUIREMENTS OF10 CFR I: fCMclc on. f>!'tnOIY of 1M fo/JowingJ (11)

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• LICENSEE CONTACT FOR THIS LER (121

.NAME TELEPHONE NUMBER AREA CODE William L Roberts, -St~ff Licensing Engineer 6 6 7 6 4 8 9 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT 1131 .

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MANUFAC* REPORTABLE MANUFAC, . R~PORTA8LE CAUSE

• SYSTEM COMPONENT TURER.*

• TO NPROS. CAUSE SYSTEM COMPONENT TUR ER TO NPRDS SUPPLEMENTAL REPORT EXPECTED 1141 MONTH DAY YEAR EXPECTED SUBMISSION YES Uf yes. comp/eta EXPECTED SUBMISSION DATE) . DATE (161

.; ABSTRACT (Limii ~ 1400 &paces, i.e., approximately nfttten .utg1_.Spke typewritttt~ lines) (16)

On. April i°7;" 1994,. with the plant iri. cold shutdown, it was determined that an internal ground in the Thermal Margin Monitor (TMM) causes a non-conformance with the Reactor Protective System (RPS) design basis. The TMM is part of the RPS and the RPS is designed. to operate as ari

. ungrounded system. The ground path in the TMM *wa~ connected to the pressurizer pressure

• .'measurement loop, the Nuclear Instrumentation (NI) System and the Primary Coolant System (PCS) ,

temperature inpµts. A second ground in the pressurizer pressure loop or in the PCS temperatlJre.

• transmitter loop could adversely affect the RPS accuracy and reliability.

• The cause of this event is an internal ground in the TMM design which was.not identified at the time of installation and the failure to recognize that this internal ground put the *RPS outside 'of the *1 FSAR design basis in that the RPS is designed to be ungrounded.

• I*--

Modifications to effectively isolate the internal ground in the TMM from the pressurizer pressure

.circ:uitry and PCS temperature circuits were completed prior to plant heat-up. 1*

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-LICENSEE EVENT REPORT (LERI TEXT CONTINUATiON

. r FACILITY NAME (1) DOCKET NUMBER !21 . LER NUMBER !31 PAGE (41 SEQUENTIAL REVISION YEAR NUMBER NUMBER Palisades Plant 0600026694-0 2 - 0 . Q 2 OF Q 8 EVENT DESCRIPTION:

.

• On April 27, 1994, the plant was in cold shutdown. During a re-:-evaluation of the Thermal Margin Monitor (TMM), PY-0102A (8, C & D), and its effects on the operability of the Reactor .

• Protective System* (RPS), it was determined that a resistance path to ground internal to the

• TMM, caused a non-conformance with the design basis as stated in the Palisades FSAR section 7.2.7, "Effects of Failures". FSAR section 7.2~7, Analog Portion, ite_m 2 states that "Shorting the [signal] leads to an ungrounded voltage source, has no effect since the signal circuit is *

. ungrounded." Section 3, states "Single grounds on the signal circuit have no effect. Double grounds would tend t<;> cause the channel to fail in the safe direction"; .Similar statements are found in the FSAR section 7.2.7, "Logic Portion~" items 7 and 1l. .. '

. The FSAR section 1.2. 7 item 2 statement indicates the RPS analog signal circuits are designed to operate ungrounded. Despite this design requirement, the TMM h*ad an internal ground path ..

from the 24 volt de power supply through a 10K ohm resistor to ground. This ground path was

. 'initially fol.ind connectec,i from the TMM through the Thermal Margin/Low Pressure (TM/LP) bi-

~table trip unit to the pressurizer pressure* measurement loop, where an additional ground _in the

• ' . pressurizer pressure loop could establish a ground loop which may have a non-conservative

• influence on the pressurizer pressure signaJs of the RPS.

• After finding the inte_rnal (as-designed)TMM ground and realizing the potential impact on the pressurizer pressure measurement loop, _all four channel.s of the pressurizer pressure related equipment were conservatively declared inoperable pending *further investigation~ Although monthly surveillance by the l&C department would discover a second (unintehtional) ground on*

• ttie pressurizer pressure loop, the Operations department does not have a means to readily detect the second ground. The results of l&C testing, in response to this event, by inducing actual grounds. in the pr~ssurizer pressure loop identified the potential for causing non-conservative shifts in some loop signals or setpoints.

Subsequent circui~ analysis and: testing performed .on May 16, 1994 on the TMM inputs and outputs revealed that the TMM 1OK ohm ground* is also connected from the TMM to the PCS*

instrumentation and the Nuclear Instrumentation (NI) systems .. In the PCS instrumentation

• system, the ground is conne.cted to the cold leg and the hot leg average temperature instruments, wher.e an additional (unintentional) ground couid establish a ground loop which may have a non-conservative influence on the temperature inputs to the TMM. Tt)e

• temperature inputs are used in the TM/LP trip setpolnt calculation and in the Variable high power *

• • trip calculation. * * * *

• In the Nuclear Instrumentation (NI) System, the TMM internal ground is connected to.the upper, lower, and upper-plus-lower power range detector signals~ Testing performed on the NI signals indicates that an additional ground in these circuits has no effect on the NI system or on. the.

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NRC. Form 38SA U.S. NUCLEAR REGULATORY COMMISSION

• . (9-83) APPROVED OMB NO. 3160-0104 .

EXPIRES: 8/31186 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION

• FACILITY NAME ill DOCKET NUMBER 12) LER NUMBER (3) PAGE (4)

SEQUENTIAL REVISIOtj YEAR NUMBER NUMBER Palisades Plant 0 5 0 0 0 2 5 5 9 4. - 0 2 - 0 Q 3 OF Q 8 RPS. This is due to.the presence of buffer isolators located in the NI system in each of the NI signals. Operations does not have a method to detect a second ground on the temperature loops or on the NI systems.

CAUSE OF THE EVENT:

The cause of this event is a signal common to earth ground in the Thermal Margin Monitor design which was not identified at the time the TMM's were installed. The cause of the event is also a failure to recognize that the presence*of the internal grounds in the TMM placed the RPS outside qf the FSAR design basis in that the RPS is designed to. be ungrounded.

ANALYSIS OF THE EVENT:

One of the RPS trip parameters is the Thermal Margin/Low Pressure (TM/LP) calculated signal.

. ,In 1988, the original analog TM/lP calculators. were replaced by digital Thermal Margin -

! Monitors.* The TMM function is to prevent .reactor conditions from violating a minimum ..

departure from *nucleate boiling ratio (DNBR) by a continuously compµted function of core power, reactor coolant maximum inlet temperature:* core coolant system pressure and axial shape index. The TMM provides a signal to the TM/LP bi-.stable trip unit which also receives an input signal from the pressurizer pressure measurement loop. The TM/LP bi-stable trip unit will generate ala.rm and trip.signals 'based on the comparison of the trip setpoint input from the

• TMM and the pressure signal from the pressurizer pre_ssure measurement loop.

In the course of troubleshooting a July 3, 1991 RPS trip (LER 91-012, dated 8/2/91 ), it' was concluded that the source of power which energized the test portion of the bi-stable dual coil relays and contributed to the event, came from the TMM 24 volt de power supply.* As a corrective action for the July 3, 1991 reactor trip event, the TMM vendor was contacted regarding a potential signal common to earth ground in the TMM. The vendor indicated there

• was none. Because *of a continuing in_dication ofa ground path,-investigation by Palisades plant continued and eventually revealed the existence of a 1OK ohm resistor which provided an .

internal ground path through the resistor to the power supply in the TMM, through setpoint indicator PIA-0102A (for Channel A), an.d to the signal common for the setpoint voltage*

transmitted to the.TM/LP bi-stable trip unit. This signal propagated through the TM/LP bi-stable to the signal common of the pressurizer pressure measurement loop .. Thus, if a short develops in the pressurizer pressure* loop, the ground loop would be fed by the 24 volt de power supply

• in the TMM. It was determined that all four RPS channels were similarly affected.

The vendor was contacted regarding the removal of the 1OK ohm resistor which provided the

• ground path. The vendor did not support its removal because its removal would present the potential for a shock hazard to personnel near the chassis and the possibility of degradation of

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• U.S. NUCLEAR REGULATORY COMMISSION

. 19-83) APPROVED OMB NO. 3160,0104 EXPIRES: 8/31 /86 LICENSEE EVENT REPORT (LER) TEXT* CONTIN.UATION

. . FACILITY NAME 111 DOCKET NUMBER 121

• LER NUMBER 131 PAGE 14f SEQUENTIAL . REVISION YEAR NUMBER NUMBER Palisades Plant

• 0600026694 -o 2 - 0 0 4 OF 0 8 An engineering evaluation of the acceptability of the signal common to ground connection in the TMM and the ability of the RPS to satisfy FSAR 7.2.7, "Effects o.f Failures" was performed in response to Palisades plant corrective action E-PAL-91-014K and was completed July _23,

• 19.93. _The evaluation considered the ability of the RPS to perform its safety function given the TMM short to ground ar:id the verification that an ~nintentional short to ground (single failure)

.. *would not cause* an unnecessary RPS actuation or safety function. The evaluation concluded

.. that for single shorts to ground in the pressurizer pressure loop, the short would only affect one channel out of four and that the short would be detectable on the setpoint indicator of the -

• TMM. Monthly surveillance by the l&C department compare the displayed signal. value to the

• TMM calculated signal and would detect a short on the pressurizer pressure loo*p by a deviation in the two values. At that time, it was concluded the RPS did not violate. its. Technical

• Specification requirements and was therefore considered operable. .

In April 1994, du~ing reviews of equipment for operabilitY in consid.eration for hea*~-up from cold

• • . shutdown following an outage, the ability of the RPS to satisfy the design req'uirements* in FSAR section 7.2.7, given the TMM internal ground and the potential for additional grounds on the pressurizer pressure measurement loop, was re-evalL1ated .. On April27, 1994, it.was decided that all four *channels of the RPS pressurizer pressure measurement loops including the TMM-s

• should conservatively be declared inoperable pending further investigatio.n because unintehtiona-1 grounds on the pressurizer pressure loops were not readily detectible by the Operations*

department .. This was deemed the prudent arid conservative course of action considering the *

' .. *enhanced sensitivity to issues as part of the overall plant performance improvement plat\...

Additional evaluation and testing confirmed that for certain postulated grounds in the pressurizer pressure signal loops, the ground pa_th established through the TMM grounding resistor could .

adversely affect some pressurizer pressure trip signals.

There were three opportunities where the non-conformance with the RPS design requirements _ I* .

for an ungrounded system could have been recognized. First, it could have been- recognized and .-I tes.ted fpr during the initial installation of the TMM modification (FC-62Sh

• Second; during . l

- replacement of the RPS trip relay coils in FC-888 following a July 3, 1991 RPS trip, a thorough - I design review of FC:-888 could have identified the non-conformance.* Third, an opportunity was I missed during resolution of the corrective action for the July 3, 1991 RPS trip where removal of I-

• the TMM ground was considered. I

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During the TMM* installation, the vendor's statement that the TM Ms were ungrounded was I

.. accepted at face value and no additional testing or investigation was initiated to verify this I design criteria. Had this been identified as a critical design characteristic, as required by the I current modification procedures, verification of the un'grounded criteria would have been. I required at the time of installation. .I I

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NRC'Form 366A U.S. NUCLEAR REGULATORY COMMISSION

,l&c631. APPROVED OMB NO. 3160-0104 EXPIRES*: 8/31 /BS

. LICENSEE EVENT REPORT (LERI TEXT CONTINUATION FACILITY NAME 111 DOCKET NUMBER 121 LER NUMBER 131 PAGE 141 SEQUENTIAL REVISION YEAR NUMBER NUMBER Palisades Plant 0 6 0 0 0 2 6 *5 9 4 - 0. 2 - 0 Q 5 OF Q 8 During .the design review of /FC-888, the resolution *focused on the design of the trip relay coils I and on 'preventive maintenance to monitor the relay coif performance .*and not on the entire TMM I

.- : design. The review relied heavily on a vendor letter discussing the design and construction of . I the re.placement relays* and. concluded that the *design was acceptable.

• I I

When removal of the TMM ground connection was not supported by the vendor, an analysis *

• I was submitted which documented a justification to use the TMMs with the grounds in place. I*

The analysis conclude.d that a second ground in the TM/LP pressure trip circuits of the RPS I would'be a single failure. It also concluded that the single failure would only affect one. of the I four channels and, therefore, since it would neither initiate a false protective action (reactor trip) I nor prevent a proper protective action, it was deemed an acceptable configuration. It was not I concluded. at this time that *the existing configuration did not comply with the associated I.

licensing desi.gn basis. This conclusion was not reviewed by the corrective a.ction review .bo*ard

. prior to closeout of the activity.* * *

,I.

SAFETY SIGNIFICANCE:

With the internal ground in the TMM, testing has demonstrated the potential for an additional

• single ground on the pressurizer pressure loop to cause a non~conservative shift ih the pressure, signal or setpoint in that loop. All postulated ground paths establish current flow through the

.* setpoint indicator of the TMM and are therefore detectable. Testing h~s als9 demonstrated the potential for an additional: gro_und to cause a* non-conservative shift in the temperature input .

signals to the TMM which affect the the~mal power calculation.

. For .<?larification, the following. interpretation of the FSAR section 7.2. 7 items 2, 3; 7 and 11 regarding the terms "grounds" and "no effect" is provided. *

• _ An* "un_grounded" circuit is interpreted to mean a circuit in which the impedance of the

• .circuit as measured to ground, is at a sufficiently high level such that with a addition of a single* ground on the circuit there will be no significant effect on the instrument loop~ II* ,

• :**.~no significant effect. : . ".is interpreted to mean that-a slngle ground (in addition to the*

• circuit impedance of the circuit-to-ground) results in. either no change in the circuit signal level, a change in the signal level in a conservative direction or a change in the signal level that is within the tolerances of the loop accuracy analyses. * * * *-* -~

Conversely, a "significant effect" is one where the ch.ange to the signal circuit is in a non-conservative direction and is outside the tolerances of the instrument loop accuracy analysis which could then impact the FSAR chapter 14 Safety Analyses.

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. FACILITY f<IAME 11 l DOCKET NUMBER 121 LER NUMBER. 131 PAGE 141 SEQUENTIAL REVISION YEAR

• NUMBER NUMBER Palisades Plant 0600026694-0 2 - 0 0 6 OF 0 8 CORRECTIVE ACTION*:

The corrective action process ha~ been substantially upgraded in sev~ral respects. All I conditions affecting installed plant equipment are now reviewed for the effect on equipment I operability and a reportability review is conducted for all conditions identified.* A riew screening I group has been established to identify those conditions which ate significant and need plant I.

. management attention and *detailed root cause analysis. There i$ increased management I involvement in the corrective action process. * *I

• A modification has been implement~d to effectively isolate the grounded TMM frmn the I

. ungroundedpressurizer f?ressure loop of the ~PS through the installation of an isolation device located external to the T.MM. * *

. On May 1.6, 1994, during the engineering of the modification to install Isolation between the

• . TMM signc;il and the TM/LP trip unit, an analysis revealed an additional ground path from the internal TMM 1OK ohm ground to. the Nuclear. Instrument inputs .and the Primary Coolant

'system temperature inputs to *the .TMM'.' Testing performed on May 19, 1994 revealed that an additional gro1,Jnd inserted in .the temperature transmitter. loop could. adversely affect the

.* temperature signals but that an additional ground inserted in the _NI input circuits had no effects on the TMM .nor on the NI circuits.

.. A second modification was i11itiated which increases the impedance between the internal ground I connect'ion from the TMM to the temperature signal instrument loops. This was accomplished.

• I by installing three operational a.mplifier input devices internal to the TMM in plac~ of jumpers in*:

the NI -input circuits. Identical input devices are currently in use in the TMM temperature input circuits; however; jumpers in the NI input circuits pr.ovide the ground path back to the

, temperature circuits. Th.e input d~vices will increase signal commcm~to-ground resistance .to ari acceptable value. Analysis and testing of the temperature instrument circuits reveals that with the input devices installed on the NI inputs and an* additional ground on the temperature circuits, there is .no .~ffect on the operation of the TMM or on the temperature .instruments.

The modifications to: ( 1) provide electrical isolation between the TMM signal and the TM/LP trip

• unit to effectively separate the ground in the TfV!M pressurizer pressure circuitry ahd, (2) provide increased electrical resistance* between the TMM temperature in pl.its and the TMM unit to effectively separate the grounded TMM froni the ungrounded temperature circuits; were I

. completed before plant heat-up. . . ' . . r A future modification will be pursued which will remove the internal resistance ground from the TMM, thus restoring the RPS and the Tl\t!M to an ungrounded state. This modification is .I currently scheduled for installation during the refueling outage following the 1995 refueling I

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.(9-83) APPROVED OMB NO. 3160-0104 EXPIRES: 8/31/86 LICENSEE EVENT REPORT (LERI TEXT CONTINUATION

• FACILITY NAME 11) .DOCKET NUMBER 121 LER NUMBER (3) PAGE 141 SEQUENTIAL REVISION YEAR NUMBER NUMBER Palisades Plant 0600026694-0 2 - 0 Q 7 OF Q 8 outage. The TMM vendor has indicated .. that this is a complex modification and the engineering I anddesign changes needed to remove the internal ground cannot be completed in time for the .I*

1995 refueling outage. I The modifications installed prior to commencing plant heat-up effectively isolate the grounded -I TMM from the ungrounded RPS. Raising the impedance of the ground in the TMM effectively *1 restores the deficient condition to the design and licensing bases.

I.- PRIOR TO PLANT HEAT-UP:

Actions A-D were accomplished pri_or to plant heat-up.

, A. Isolate the TM/LP Pressure trip signal circuits from the internal TMM impedanc_e.

B. Isolate the Primciry Coolant System temperature and Nuclear Instrumentation System Power Range input signals to the TMMs by isolating those circuits from the_ internal TMM ground or raising the impedance level of the ground to an acceptable level ..

C.. Verify that all of the TMM inputs ano outputs are effectively ungrounded after the completion of modifications and perform testing to verify the modified loops, the TMMs and. the affected portions of the RPS function properly. .

D. *Test e_ach Tl\J1M to validate: that circuit isolcition modifications do not effect the TMMs safety function._

11. TO PREVENT RECURRENCE:

A. Upda.te the RPS' Failure Modes Effects Analysis to reflect the modifications performed to the RPS and the TMM *as a result of this Event Report.

B. Electrical and l&C engineering personnel have been provided with a summary of this .I event report.

C. The RPS Design Basis Document has been updated to include the work completed. on I, the system and to clarify what an "ungrounded" signal is.

• I D. This event report has been incorporated into the appropriate accredited training programs.

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NRC Form 366A U.S. NUCLEAR REGULATORY.COMMISSION

'. 19-B31 APPROVED OMB NO.* 3160-0104 EXPIRES: 8/31/86 .

. LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME 111 DOCKET NUMBER 121 ._ . LER_NliMBER 131 PAGE 14)

SEQUENTIAL REVISION YEAR NUMBER NUMBER Palisades Plant 0500025694-0 2 . - 0 Q 8 OF Q 8 E. The original commitment was to change the Nuclear Engineering and Construction I Organization (NECO) Guidelines to ensure that all modifications are designed and I.

. tested appropriately to ensure that isolation and separation requirements are I maintained. This action was riot taken as described, as it is not reasonable to test for I separation and isolation. Separation is a factor of cable and device location: I Engineering design and review are adequate to determine correct device location. . I Isolation is also a design function. Testing of the isolator is done at the time of * *1 procurement if adequa.te design documentation does not exist.

• The existing design* *- I procedures reference the appropriate design documents which spesify the* * *I requirements for adequate electrical circuit separation and isolation. 1.

• F. Change the. FSAFl t_o Clarify the definition of "ungrounded" and the "effects of a* *

. ground" in sectio~ 7 .. 2. 7. . . . .

.* G. Modify the TMMs to remove the internal ground and then reevaluate the need for*

. circuit isolation devices added by this event. report to the inputs and outputs of t.he TMM.

H.. It has been determined that additional surveillance procedures on. the added TMM I isolation devices will not be wr.itten, as an existlng Technical Specification . I*

Surveillance procedure (Ml-2A) tests the isolators as part of the signal loop test .

. I. *. It. was determined that the need exists to periodicaliy monitor unisolated TMM and 1*

RPS circuits for the presence of grounds. A new action has been initiated to develop I a procedure to test RPS and TMM signal circuits for grounds. I J. A memo has been provided _to all l&C technicians on the importan~e of, and methods

• to detect, ..

grounds on. floating systems. . .. *

.K. Provide a memo summarizing th.e event to all department heads, members of the PRC*

and Management Review Board, emphasizing the ne.ed to recognize situations where the plant is not in compliance with design basis.-*

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