ML19326C701

Evaluation of Reactor Protection Sys Grounding Concern.

ML19326C701
Person / Time
Site:	Arkansas Nuclear
Issue date:	03/09/1978
From:	BABCOCK & WILCOX CO.
To:
Shared Package
ML19326C697	List: ML19326C696 ML19326C701
References
REF-PT21-78-037-000 NUDOCS 8004250512
Download: ML19326C701 (23)

Text

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EVALUATION OF RPS -

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. . . GROUNDING CONCER '. .

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[ This report documents the evaluation of a concern wherein it  !

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I was postulated that a loss of ground could cause the NI/RPS to i fail to perform-its intended function. The purpose of this report is to document the background and reasoning that led to r

the conclusion that this occurrence constitutes a substantial safety hazard as defined by 10 CFR Part 21. -

Identification

, The preliminary safety concern proposes the following ,

hypothesis: A Nuclear Instrumentation / Reactor Protection System q , {

I (NI/RPS) channel may experience a loss of ground to its instrument i

. 1 common without the loss of ground being evident. Given this condition, a single postulated failure in one channel can leave the Reactor Protection System in an unanalyzed condition. The concern exists for those plants that utilize ground as an active p return path. For B6W, the affected plants are: i Oconee 1, 2, 3 TMI 1, 2

. ANO-1 Rancho Seco Davis Besse 1

, Crystal River 3 '

Midland 1, 2 The concern is not applicable to our 145 FA, our 205 FA plants,

! or to Davis Besse.2 and 3.

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,., Analysis of Occurrence .

The concern was discovered while investigating the Davis I '

Besse ground system. A review of the RPS indicated that loss of ground will not cause a channel trip, as was previously.

assumed. Therefore, the potential exists for a loss of ground to occur _without being detected. To our knowledge, current

operating procedures do not call for periodic ground continuity checks. If a subsequent fault is imposed upon the system, then more than one channel might be adversely affected.

A detailed analysis of the event is not possible by-B6W, f because the individual ground system; vary from plant to plant '

and B6W does not kr$ow what ground continuity checks individual utilities may perform. However, a preliminary review indicates that without additional information to the contrary, this concern represents a substantial safety hazard as defined by 10 CFR Par't 21 since the concern has the potentia'. for a loss of safety function to the extent that there could be a major reduction in the degree of protection for a licensed facility, if ground is Iost and another fault occurs. -

It should be pointed cut, however, that loss-of-ground on the NI/RPS is believed to be a very improbable occurrence because

, of the multiple ground circuits which exist; e.g. each channel

',- connected to the ground bus, each cabinet interconnected through h nultiple bolted joints, etc. In addition, loss-of-ground by itself will not prevent the RPS from tripping.

e Corrective Action It is recommended that utilities institute a periodic test of the NI/RPS to assure that ground 'has not been lost, t

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Reportability .

This concern is ' reportable as a Substantial Safety Hazard as defined by 10 CFR Fart 21 for B4W 177 FA plants.

It does not affect 145 FA plants, 205 FA plants or Davis

} Besse 2 and 3, which utilize RPS-II, since RPS-II does not utilize ground as an active return path. .

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' ENCLOSURE 2

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4 177 FA PUM 1;I/RPS GROUh'D PROELDI DISCUSSION AND

( RECOW.DDED TEST SCHD.E i

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1.0 LIMITATION '

This report and its recom=endations are limited to the Babcock and Wilcox 880 system Nuclear Instrumentation / Reactor Protection System (NI/RPS) as applied to the B&W 177 FA NSS. .

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2.0 PURPOSE The purpose of this report is three-fold:

2.1 To explain a potential proble= that exists' in the unlikely event of loss of connection of th,e plant ground system to gag or more NI/RPS channel instrument co= mons. .

2.2 To reco==end, to the operators of 177 FA plants, expansion of the routine 4 NI/RPS test progra= to include checks designed to ensure the connection of the NI/RPS instrument ec==on ground to the plant t r o t- c' system _ -

2;3 To explain the basic philosophy of a ground connection test scheme that can be adopted by the operators of 177 FA plants to check the

~ ground connection.

3.0 _TFf PROBLLM -

3 T>e purpose of this section is to pre [ent an analysis which vill serve to illustrate the effects of loss of ground to instru=ent common. The analysis

, addresses a plant containing dual ground syste=s: (1) instrument ground

, sand (2) station ground. It is recognized that ground systems and methods

' vary frem plant to plant. The data and analysis methods contained herein should serve to assist the individual plant operator in the assessment of the particular plant ground system and the impact of loss of ground connection.

71gure 1 illustrates the method of conveying trip signals between redundant channels of the NI/RPS. Each channel contains a -15 y power supply whose positive terminal is referenced to the instrument common in that channel.

e Each channel contains four relays--KF, KC, KH, and KJ. The function of r . .

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2 these relays is to sense the trip state of the redundant channels of the

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i NI/RPS. Each channel contains a channel trip relay KE which is energized 1 1

while the channel is not tripped. Therefore, contacts KE are closed and open to convey a trip. Relays KF, KG, KH, and KJ in each channel are,  !

l therefore, normally energized when all channels are in a non-tripped 1 I

state. If the relay KE in a che.nnel de-energizes (trips), a relay in _

each channel vill be de-energized. The normal current path of relays KF, KG, KH, and KJ involves the ground system outside the NI/RPS cabinets.

This ground dependency is depicted by the heavy arrows on Figure 1 which illustrate the path whereby KG1 is normaily' energized by the Channel A i

power supply through a KE contact in Chansc1 B. This is typical of the paths involved for all interchannel relays.

3.1 Loss of Ground to a Channel The purpose of this section is to analyze the effects of loss of connection of the plant ground to the instrument common of one NI/RPS channel.

3.1.1 Sieplified Circuit. Figure 2 is a simplified drawing of the interchannel-crip arrangement under the conditions where the instrument co==en to instrument ground connection to one channel is lost or disconnected. Here the channels are identified as V, X, Y, and Z, rather than A, B, C, and D.

This allows this one figure to illustrate all combinations -

of loss of ground. In this equivalent circuit normally:

A. Relays KWX, KWY, and KWZ are in parallel with a. total resistance equivalent to the value of one-third the value  !

of the resistance of one relay coil.

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B. Power supplies X, Y, and Z are in parallel by virtue of sero potentisi difference between their negative terminals.

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.* 3 C. la view of 3.1.1.B above, relays KXW, KYW and KZW are

._ in perallel with a total resistance equal to one-third the value of the resistance of one relay coil.

• 3.1.2 Electrical Equivalent Circuit. Figure 3 is an electrical equivalent circuit of Figure 2 taking into account the

. conclusions of 3.1.1. A, B, and C. It should be noted that the equivalent circuit is a bridge, where the voltage A to B is determined by the degree of balance existing in the bridge.

It is obvious that the potential A to 3 cannot be predicted due to the tolerance of power supply potentials (+ 0.65% V) and relay coil resistance (L 10% ohms). ' The potential A to B is calculated in Section 4.0 in several cases for illustration purposes. Inasmuch as the output potential of bridge circuits

, is a ratio phenomenon, the engineer can take great liberties in the assumptions and still achieve accurate results.

3.1.3 Effects on Interchanne'l Trip Relay Arrangement. The purpose of this section is to analyze the effects of the loss of ground to the instrument com=en bus of a single channel on the interchannel trip relay arrangement illustrated in Figure 1.

As explained in Sections 3.1.1 and 3.1.2, the equivalent circuit utilized for this analysis is shown in Figure 3. The reader vill recognize the equivalent circuit as a bridge where

, the voltage A to B is dependent upon the degree of balance of the bridge which is determined by the balance of the power supplies and equality of the resistance of the relay coils l

making up the equivalent resistances Rei and Re2' 3.1.3.1 Balanced System. If one assumes that both power supply potentials are equal and that all relay coil

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,, That is, El=E2 and Rei = Re2 In this case, the i potential A to B is zero and so A and B are electrically the same point. The system should act realize that the ground at A has been lost.

3.1.3.2 Unbalanced Systen. There are an infinite number of combinations of values whereby the system can be 4

unbalanced with an attendant number of values of potential between A and B. Since relay coil tolerance is much looser than power supply regulation, this section analyzes for effects of relay coil resistance variation. Unbalance in power supply voltages would have si=ilar but smaller effects.

, This section evaluates the voltage A to B assuming that one relay coil resistance is in error ten percent. .

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A-3 1 Rey where Rey *(E1 fE} 2 .

ke y" Rey- + Rey ,

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= .32144(30) 9.6432 Rc y .32144 + .333

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= 14.735 Rey + Rc2 c . .. .

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= 15 - 14.735 - 0.26496 V

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, 5 3.2 Conclusions i

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Extrapolation of the data and analysis methods of Sections 3.1.3.1' f

and 3.1.3.2 can lead to the following conclusions relative to loss

. of ground to an NI/RPS channel. i

. A. Loss of Cround Connection

. (a) May not be self. annunciating ,,

1 (b) May not be disclosed during routine NI/RPS testing .

(c) Does not, ia itself, negate the ability of the NI/RPS 'to ,

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perform its protective function , ,

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3. Due to Conclusion A, a channel may lose 11.4 rument common ground and re=ain in that posture for an extended period of time. During this time:

(a) the instru=ent coc=on in the f aulted channel is not fixed at station ground potential by a lov ohnic resistance while:

(b) the instrument commons of the remaining channels are fixed

• se station ground potential by lov ohnic resistance; therefore:

(c) the potential between the instrument common in the faulted channel and the instrument commons in the remaining channels is normally fixed by bridge balance conditions--see Figure 3--

and:

(d) the potential at the instrument common of the faulted channel can be forced to abnormal values by the introduction of foreign fault voltage sources.

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C. Single Failure ' .*,, ,,

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Given the foregoing conclusions, it is possible to postulate single failures in a channel (which has lost ground to its instrument _ ,

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- i common) which may have unacceptable effects on the' redundant -

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channels. -

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6 A significant example is the het short of primary power to the

, instrument c,emon of the affected channels. ,

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4.0 TEST PHILOSOPHY '

. 1 Conclusion C of Section 3.2 mik. ft_ imperative ehme nn-rater er ivi FA plants examine their grcund system /NI/R.DS intar#=ra te ~a'4e* uheth=e

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the single failure potentical exists ir their installation.

If the answer is in the affirmative, a test program must be instituted.

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The objective of the test program would be to move the loss of. ground out of the undetectable region to a detectable failure and thereby minimize the sin;1e failure threat (Section 3.2.C). .

The purpose of this section is to examine the theory involved in a proposed test method. There is no intent to imply that the method proposed here is necessarily the only or best method that can be devised. In addition, the metted may not find acceptance in or be practical in all plants for a variety of reasons including variations in plant grounding methodology.

It is hoped, however, that the methods discussed may assist the user in the development of methods that best suit the installation.

4.1 Ground Check with Ground System Intact Figure 4 is a duplicate of Figure 2 but with all grounds intact and a test DVM connected. Figure 2 is explained in earlier sections of this report. The analysis is presented in Figure 6A.

KWX, WY, WZ, 51, S2, S3, and WPS reside in the channel under test.

The DVM negative lead is connected to the negative fifteen volt DC bus of the channel under test while a screw terminal is available to which the DVM positive lead is connected. Consultation of the NI/RPS schematics will easily identify the connection. points involved.

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. 7 Note that with the ground system intact, the DVM is connected across the channel W power supply and will indicate the value of voltage to which the power supply is' set to regulate ( 415 volts DC).

In the routine of testing channel W, switches $1, S2, and S3 opened are to confirm the ability of channel W to respond to a trip generat'ed by channels X, Y, and Z. A trip of channels X, Y, and Z is normally signaled by opening contacts T1, T2, and T3, respectively.

Were one to observe the reading of the DVM vhile opening and closing 51, S2, and S3, no change in the indicated value would be noted in as much as the DVM is indicating the output voltage of channel W power supply. No change in the DVM reading upon opening and closing S1, S2, and S3 indicates a connection from the NI/RPS instrument consnan to the plant ground system.

4.2 Cround Check with Channel W Ground Open Figure 5111urtrates the same circuit as Figure 4 except that the channel W instrument common ground is completely open. The analysis is presented in Figure 63. The digital voltmeter is connected as-before. .

1 Inasmuch as the channel W instrument common is no longer connected ,

to ground, the DVM is now indicating .the voltage across the parallel '

combination of KWX, KWY, and KWZ rather than the output of the W power supply. Refer to Figure 3--the DVM is now measuring the voltage across Rey. Since Rel C$ Re2 andlE gs E2, the voltmeter vill indicate approximately fifteen volts. Were the circuit perfectly balanced, it would indicate power supply potential. , . . . . , _ ,

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8 Refer to Figure 5 and open S1 thereby interrupting the current

,, through KWX. Refer to Figure 3. Rey is now the equivalent value of two relays in parallel while Reg is the equivalent value of three -

relays in parallel. 2e1 135 ohms while S1 is open and 90 ohns. .

while S1 is closed Re2 - 90 ohms. Ratio and proportion calculations yield an indicated DVM value of 18 velts DC while Si is open and 15 volts while 51 is closed.

NOTE:

Ref er to Section 4.1--vith the ground system intact, no change in the DVM indication was expected while S1 was opened and closed.

With the Channel W ground path open, a delta voltage of 3 volts vould be observed while S1 is opened and closed. A significant change in DvM indicated value is therefore indicative of an open ground fault.

. 4.3 Cround Check with Channel W Cround Containing Some Resistance Inas=uch as some or all NI/RPS channels have many ties between their instrument commons and the plant ground system (main connection, cable shields, cabinet mounting bolts), this section vill analyze to determine the expected delta voltage on opening and closing S1 with some resistance between the channel V instrument common and, plant ground. -

Analysis is presented in Figures 6C, 6D, and 6E to demonstrate the arpected change in DVM indication when the channel W ground path contains 2,1, and 0.5 ohms, respectively. Opening and closing 51 produces a change in indication of approximately 100 millivolts when "

, , , ground path resistance is two ohms, approximately 50 millivolts for a ground path resistance of one ohm, and approximately 5 millivolts when ground path resistance is one tenth of one ohm.

From the foregoing, the proposed test method would appear to be able to discern small changes in ground path resistance. " "~~~"

5. 0 TEST PROCRN! AND RECO*0!ENDATIONS R&W recommends that O

the operators of 177 FA. plants formulato and institute q_ m- -w w - ,- -,- ,- -- - - - - - , y

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a test program which will, on a periodic basis, verify the connection of l

the instrument common bus in each HI/RPS channel to the plant ground system. It is further recommended that the operator develop the program based on the methods described and analyzed in Section 4 or, utilizing this data, institute a program of the operator's ewu devising. We, further, reco==end the following:

'5.1 Frequency We understand that a NI/RPS channel is tested each week on a rotational basis with each channel being subjected to test every four weeks.

1 We recommend that the ground test be integrated into the test routine ]

1 of each channel. This will verify ground continuity, of each channel, every four weeks.

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Data extracted free IEEE-500 indicates that there are approximately seven open f ailures per 10 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> for a link consisting of a copper conductor terminated at each end. Since each of the four NT/RPS channels is provid'ed with a ground link, there are possibly twenty-eight open failures per 106 hours0.00123 days <br />0.0294 hours <br />1.752645e-4 weeks <br />4.0333e-5 months <br /> or approxi=ately one per four years.

5.2 Bench Mark Testing ,

Refer to Section 4.3--the proposed test scheme is sensitive to

, small changes in ground link resistance. In addition, the case presented

• in Figure 6A vill not be attainable due to ground lead resistance.

That is, with the ground system normal, some delta E may be noted as S1 is opened and closed.

~. Also, since other ground paths may exist due to signal return lines,

. shields, etc., the case presented in Tigure 63 may not be attainable.

That is, with the primary ground path open, a delta E less than' three r . _ . . ....;_. . ... . .... .

.- 10 volts may be noted as S1 is opened and closed.

For the foregoing reasons, it is reco== ended that an initial I

test be conducted on each NI/RPS channel to establish a bench mark

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for future tests.

A. Prepare for test (connect DVM, etc.)

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B. Note the DVM indication with S1 in its normal position (S1 isspring loaded in nor=al position)

C. Operate Si to the open position. The DVM indicated value should remain essentially the same as in B above. Note: The readings attained in B and C will be essentially equal if the ground path resistance approaches zero oh=s. If the delta E step B to C ,

is appreciable, the ground system' v111 have to be inspected to ascertain that it is normal.

D. Record the value obtained in step C. Note: If the succeeding steps are completed successfully, the delta E step B to C will

, serve as the future bench mark minimum values.

E. Disconnect the primary ground path. Nith 51 in its normal position, the DVM indication should be essentia11y'the same as that obtained in B above.

F. Operate Si to the open position and note the DVM indicated value. The delta E step E to F should be appreciably larger than that obtained in step D. The larger delta E obtained in this step, af ter primary ground was disconnected, verifies that the ground was complete and intact during steps A, B, C, and D.

G. Reconnect the primary ground.

i E. Repeat steps A, B, C, and D and note that the recorded values are repeated. This step ensures that the primary ground was properly reconnected.

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11 5.3 Routine Testint A. Connect the DV11 B. With S1 in its normal position, note the DVM indicated value.

, C. With 51 in its open position, note the DVM indicated value.

D. Note the delta E step 3 to C. Compare to the bench mark delta E step 5.2.D--if these are approximately equal, the primary ground is intact. If the delta E approaches the value obtained in steps 5.2.F a possible ground fault exists. Investigation and possible repairs vill be required to resolve and correct the probics.

E. Af ter any corrective action to the ground system, steps A through D should be repeated.

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I. MEETING NOTICE DISTRIBUTION .

i -

. ORB #4 l ,,

DhetFi'i2'.

NRC POR

• L PDR -

l ORBi4 Reading NRR Reading E. G. Case Y. Stello 3  !

I i

L D. Eisenhut

6. urimes T. Carter

A. Schwencer .

D. Ziemann .

G. Lear R. Reid L. Shao i R. Baer W. Butler B. Grimes Project Manager -

Attorney, OELD

• OI&E(3) -

l B. Faulkenberry, I&E l' R. Ingram ,

Receptionist, Bethesda .

[ R. Fraley, ACRS (16) -

Meeting Notice File

,; Principal Staff Participants (R. Martin, H. George) y e

9 e

4 e

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4