ML20205T055

Responses to NRC Questions on RTD Bypass Elimination Licensing Rept

ML20205T055
Person / Time
Site:	Robinson
Issue date:	10/31/1988
From:	WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP.
To:
Shared Package
ML14188B569	List: ML20205T055
References
WCAP-11890, WCAP-11890-ADD-01, WCAP-11890-ADD-1, NUDOCS 8811140092
Download: ML20205T055 (38)

Text

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WCAP-11890 ADDENDUM 1 WESTINGHOUSE CLASS 3 o

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RESPONSES TO NRC QUESTIONS ON RTD BYPASS ELIMINATION LICENSING REPORT OCTOBER, 1988 i

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l 8811140092 88110?

h PDR ADOCK 05000261 P

PDC

H. B. ROBINSON 2 RTD ~ 4 PASS ELIMINATION NRC QUESTIONS & RESPONSES 1.

In table 2.1-1 of Attachment 4 (WCAP-11889) of Reference 1, the response time parameters for the RCS temperature measurement are presented for both the existing RTD bypass system (old system) and the proposed system (new system). The new system, which has the bypass system removed, uses fast response themowell RTDs manufactured by WEED Instrteents Inc. Adding the times given in Table 2.1.1 for the components, the system response tir:e is t

5.0 seconds for tne old system (Rosemont RTD) and 4.75 seconds for the new system. However, a footnote explains that you cannot simply add all the response time parameters because of transfer functions whien have different forcing functions as inputs. Since it is a staff position that ti. 9TD response time and surveillance schedule (as it affects the overtemperature delta T and overpressure delta T) needs to be addressed in the plant Technical Specifications, the following information is needed for our evaluation:

QUESTION (a): khat RTD response time in seconds was used for the FSAR Chapter 15 accident analyses for the hot and cold les RTDs for transients depend'ng on overtemperature delta T (as perforced in the ANF-88-094 report) and overpower delta T trips?

RESPONSE: As presented in Table 2.1 of the Report ANF-88-094, the overtemperatuae deltaT reac'.or trip response time used in the accident analysis matches Taole 2.1-1 of WCAP-11889. This is an upper limit of a 4-second leg time constar.c in ccebination with a 0.75 second electro-mechanical delay.

In this respect, the equipment specification was directly incorporated in the accident analysis caluclations.

QUESTION: khat is ex,

..:d alue of the RTD system response time?

RESPONSE: The RTDs vill have a response tir.e of 4.0 seconds or less. The remaining channel electrcaics response time will be 0.75 seconds or less.

QUESTION: khat data supports these response time values?

RESPONSE

Ihe RTD response times were measured at the manufacturer utiliz.1g an industry standard plunge test-in water flcwing at 3 ft/sec.

The measured response times varied between 2.7 to 3.7 seconds.

All RTDs tested less than 4.0 seconds. When the RTIn are installed in the plant the in-situ responae times will be measured by the Loop Current Step hsponse (LCSR) test. "he higher fluid velocities and elevated temperatures in the plant should result in slightly better response times. The RTDs were e

tested inside ther=rwells from the production run to be used at HBR2.

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1.

(Continued)

QUESTION (b): If total system response time data is not.available, discuss the supporting dats for each of the caponents of the total response time and how a bounding value for the total response time is derived. Include each of the elements listed in Table 2.1-1 as well as a discussion of the affects on response time of the scoop and the gap t4 tween the scoop and the WEED RTD.

RESKWSE: De RTD data is discussed above. De electranics response time is typical of response times measured at other plants with analog electronics. While the conservatism nomally found in other plants electronics response times has been recoved, the value quoted in table 2.1-1 will bound the actual response time.

QUESTION: Discuss the effects on response time of the scoop and ti?e gap between the scoop and the WEED RTD.

RESPONSE: (

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There is no radial gap between the RTD and the themowell with the WEED design. Only the RdF themowell mounted RTre have this gap.

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2.

You have stated that your WEED RTD is similar to other WEED RTIs installed at other plants. However, it is noted that two other plants that are using WEED RTDs have quoted "total response" times that are 1.25 to 3 seconds longer than for the H. B. Robinson plant. Their combined "RTD/themowell" response times excluding electronics are 0.25 to 2.5 seconds lencer than e

for the H. B. Robinson plant.

j QUESTION: How do you account for the faster response times for the H.E.

Robinson plant? Does H.B. Robinson plant have a design difference to account for a faster RTD response time measurements than at other plants?

a RESPONSE: There are two other plants that utilize the same 4.0 second WEED

)f RTD that will be used at Robinson. Both of those plants elected to include excess response time margin in their safety analyses. One plant requested that the safety analysis asstne a 5-1/2 second RTD response time.

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Subsequent plant measurements indicated that, on averagt, the RTD response time was 3-1/3 secoMs. clearly significant response time margin was assigned to the RTIs. Robinson has elected to not include excess RTD l

response time margin in the analysis, and instead, will rely on the RTD's perfomance. The factory tests mentioned earlier showed that the slowest j

1 RTD was 3 7 seconds. Based on this information and the data free the plant mentior.ed above the Robinson RTD responses will be faster than the safety r

j analysis valve.

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Regarding the problem of drift of the RID response time identified in NURE-0809 (Reference 2).

QUESTION: Describe (a) the method (s) for checking RTD response time after installation, (b) the frequency of the checks and (c) the safety allowance or other methods to provide assurance that the response times do not drift outside acceptable limits between the required 18-month checks.

RESP 0tGE (a): The response time of the installed RTDs will be checked using the loop Current Step Response (LCSR) test prior to initial criticality.

RESPONSE (b): 2e RTDs will be checked on a refueling basis not to exceed every 18 months.

RESPONSE (c): The plant safety analysis has assmed an overall protection channel response time that has been shown to provide adequate protection frce design basis transients. The various elements which take up this assmed response time were chosen so as to bound the actual perfomance of each hardware constituent. When the channel response time is measured it can be compared to the assmed value and the difference will be the excess margin that exists.

De above surveillance schedule is designed to monitor RTDs and detemine if adverse trends may be developing. If such a trend were to exhibit itself with this model RID /themowell then at that time appropriate steps would be taken. The present surveillance interval is appropriate for identifying those trends and no further steps need to be taken at this tice.

5.

In sectier,1.2.1 of Attachment 4 of Reference 1 (WCAP-11889), it was stated I

that because of interference, one of the three RTDs in each of loops A and B will have a changed position anc will be located downstream frce the other RTDs. Also, as shown in Figure 1.2-3 of WCAP-11889, these RTDs are shown as not being placed in a scoop as the other RTib are.

QUESTION: Please provide the dimensions that locate both the downstream and circumferential location of these displaced RTIb in relation to the others.

RESNNSE: Since the the sutcittal of WCAP-11889 a plant outage has allowed for a detailed inspection of the loop piping and interferences.

It has i

been detemined that Loop B will not have any themowells relocated. All I

three RTD/themowells will be mounted in the existing scoops. On loop A it was detemined to move all three themowells out of the scoops and mount them 24 inches downstream. The themowells will maintain their original ciremferential spacing with one thermowell top dead center and the other tsr 120 degrees to either side.

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(Continued) f QUESTION: Will the immersion depth be the same as the others which are in the scoops?

RESPONSE: Yes. The RTD heat sensitive tip will be located at the same 6

radial position as the center flow hole of the scoop, but 24 inches downstream of the scoop location.

i QUESTION: What will be the relative affect of not having these RTts in the scoops, as the others are, oc the response time and accuracy of the reading?

l RESPONSE: The response time of the RID in the hot leg free stream should, t,

theoretically, be slightly faster than those located in the scoops.

However, data from another plant indicates that this slight advantage is not reflected in a faster measured response time with respect to the scoop mounted RTD/themowells.

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Since all three RTDs are being moved downstream together the accuracy of i

their cottbined measurement will be the sa:na as if they were all three still l

in the scoops. The flow perturbation caused by the scoop upstream of the i

new RTD location will have dissipated prior to reaching the RTD. It is expected the this mall perturbation will help mix the hot leg fluid and l

somewhat reduce the magnitude of the temperature f.treaming,

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1 QUESTION: What data supports the response time and accuracy of the RTDs in j

this configuration?

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l RESPONSE: The res;cnse time %a :~ rem another plant, as teentioned above, indicates that there is no systecutic diffe.'ence in response times between the RTD mounted in the scoops and those in the free stream. Since all three relocated RTru in Loop A are bein6 kept together the accuracy of their measurement will be the same as the three scoop mounted RTDs. The

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j data referred to in Section 2.2 of WCAP-11BS9 is therefore applicable to t

this configuration.

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f 6.

Provide the value of the latest indicated RCS flow measuranent for the H.

j B. Robinson Unit 2 plant in Ib/hr and gpm and also the value for the

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thermal design flow (TDF) in Ib/hr and gp:.

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QUESTION: What is the current flow measurunent uncertainty valus?

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RESPONSE: The results of the latest precision calorimetrics RCS flow measurenent are:

WITH ALI4WANCE FOR l

NOMINA!,

MEASURDOiT UNCERTAINTY l

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103.8 x 10 lb/hr (1-0.0187') =

101 9 X 10 lb/hr 273,400 gpm (1-0.0187') =

270,250 gun

• Measurement Uncertainty t

From Tables 15.2.1-1, 15.4.2-1. and 15.4.3 1 in ANF-88-094, the i

corresponding themal design flow is:

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97.29 X 10 lb/hr # 550.2 F j

With a density of 46.85 lb/ft3, this translates to:

6 (97.29 X 10 lb/hr)(1 ft /46.85 lb)(7.48 gal /ft )(1 hr/60 min.) :

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3 258,900 spn j

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In Section 31 of WCAP-1 TBS 9, there is reference to using standard j

Westinghouse methodolegy previously approved for the flow measurecent i

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uncertainty analysis.

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QUESTION: Please provide the reference for the cethodology and also

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indicate if the H. B. Robinson analysis has any deviations from this l

l Lethodology.

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RESPONSE: he Westinghouse methodology used to calculate RCS Flew Measurecent Uncertainty has been in use for many years and was first j

definitively noted in a Westinghouse letter to the NRC with respect to the l

I use of the Improved nemal resign Procedure (ITIP) on D. C. Cook Unit 2; r

l Westinghouse letter NS-TMA-1806, te E. G. Case (NRC), from T. M. Anderson (Westinghouse), 5/30/78. Subsequent to this sutaitt31, the methodology was used for all ITDP RCS Flow Measurecent Uncertainty calculations, e.g.,

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McGuire, Catawba, Ginna, Point Beach, By:aon and Braidwood. The methodology t

i was explicitly reviewed and approved by the Staff for McGuire in an NRC

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letter; to H. B. Tucker (Duke Power) from E. G. Adensam (NRC), 6/23/83.

j Modifications were made to this methodology to specifically address the use j

s of three Het Leg RTDs and Hot Leg Streaming with the calculations perfomcd for RTD Bypass Elimination. De results of these calculations were f

i autaitted on the dockets for the following plants (most of which are

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currently in operation with this plant change); McGuire, Catawba (WCAP-11308), Bryon/Braidwood (WCAP-11323), Millstone Unit 3 (WCAP-11273),

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Salem (WCAP-11580), and South Texas Project (WCAP-11273 for the protection

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syste:c setpoints). he Westinghouse cethodology is consistent with that L

q outlined in NURED/CR-3659, "A Mathematical Model for Assessing the J

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RESPONSE: (Continued)

Uncertainties of Instru=entation Measurements for Pcwer and Flow of PWR Reactors". The calculational methodology used to perform the calculations for this para eter for H. B. Robinson has no deviations from that presented for the above noted plants.

8.

A flow measurement uncertainty analysis was presented in WCAP-11889 for the

h. B. Robinson plant in which the uncertainties of the new WEED RTCs was l

factored into the analysis. However, the effect of the uncertainty due to

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the readings from cold leg elbow taps was omitted in arriving at the final uncertainty value.

It is understood that verification is cade of the RCS flew by a calori=etric heat balance perfomed after return to full power operation following a refueling shutdown. At that time, the cold leg elbow tap pressure drop indicatien is taken as the corresponding flow reading.

Because the cold led elbow tap reading is subject to additional drift and other uncertainties, it is usual (as perfomed for your Shearon Harris plant) to include these additional uncertainties in the flew measurecent uncertainty analysis.

QUESTION: Plesse provide a flow measurement uncertainty that includes the l

effect of cold leg elbow tap uncertainty. Also, no cention was made of the uncertainty for feedwater venturi fouling.

It is a staff position that a 0.1% venturi fculing penalty should be added unless the venturi tubes are cleaned at each refueling before the precision heat balance is made and that the tetal flew teasurement uncertainty should also be included in the plant Technical Specifications.

The uncertainty factor can be placed either in a section addressing F delta H or a section pertaining to LUB parameters.

RESPONSE: Table 1 notes the instrument uncertainty breakdown for the Cold Leg Elbew Tap indication of RCS Flow with the use of the plant process

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cocputer. The 0.1% venturi fouling penalty has been added.

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The calibration of the RTos is perforced before the calorimetric heat balance at each refueling. It is understood that this is by a cross l

calibraticn method in which it is assur.ed that variation occurs in a random f

manner frun the original calibration from the manufacturer. Therefore, the mean value is assumed to be the correct value.

However, over a 40 year r

lifg, the RTDs may drift in one direction. One manufacturer has indicated a 1 F drift in five years. It is noted that several references indicate that although platinum RTDs are quite stable, there is evidence of drift (Refs. 2, 3 and 4).

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CU.JTION: How is the original calibration accuracy of the RTD established? How will you be able to tell if there is unacceptable drift l

in ene direction and what steps will be then taken?

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RESPONSE: The recommended means for confiming installed RTD accuracy is il N through RTD cross calibration testing, whereby each RTD is compared to the average of all RTDs at several isothemal temperatures during plant heatup or cooldown. Evaluation of the test data supports confirmation of RTD l

accuracy, recalibration to the plateau average temperatures, or RTD replacement, as necessary. Cross calibration will be perfonced followin3 installation of new RTDs to check for calibration shifts resulting from shipping, storage, or installation, and at refueling outages to confim l

stability and to check for drift.

1 Cross calibration requires the detemination of accurate plateau average l

temperatures, which requires that calibration errors be random, and not i

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systemmatic. This is considered the case for vendor supplied

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calibrations.

In addition, Westinghouse experience suggests that RTD drift j

is random, if it occurs. Therefore, the existence of drift does not j

degrade the plateau average temperature accuracy. Consequently, RTDs that

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l have drifted significantly will be detected and realigned to the average,

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or replaced if gross stability is in question.

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The Westinghouse position on RTL drift is based on experience, in

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conjunction with the limited literature published on this subject. In several cross calibration test data evaluations the absence of drift was i

j confirna through comparison of existing installed RTEe to newly installed

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1 unita. The literature, most notably References 3 and 5, indicate that i

drift is generally small, can be positive and negative, and, when extreme, l

can be detected and corrected through cross calibration testing.

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calculations is judged to bound any unanticipated drift. In Westinghouse i

j judgement, RTDs that exceed this allowance are of questionable stability f

I and should be considered for replac(nent.

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10. Your new method of obtaining the hot leg temperature differs from the i

previous method. In the previous method, the flow in each hot Ing was j

sampled frce the scoops in the pipe cross section. This sampled flow was I

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measured in a mixing plentn to ettain an average tereperature value. Your l

present teethod replaces the sampled flow from a scoop with a single f

temperature measurertent of the scoop flow which is used to be equivalent to l

l the fomer sampled flow value.

QUESTION: Please indicate how you plan to check and confirm the accurac of this new hot leg average temprature measurercent method (new system) y 1

f, against the fomer RTD bypass method (old systas). The staff should be infomed of the results of such a test, a

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10. RESPONSE: 'Ihe hot leg temperature measured with the themowell RTDs can be compared with the hot leg temperature previously measured with the RTDs in the bypass system by ccr: paring delta Ts (normalized to full power) measured before and after the modification. Since there are uncertainties in both RTD system measurements, and differences in streaming patterns from cycle to cycle, it is not likely that the two measurements will be exactly the same. Any difference between the two delta T's considered to be an indication of the error in both measurments rather than an indicatio of an error in just the new thermowell RTD measurment. The staff will be informed of the results of this analysis.

12. In WCAF-118F,9 you have discussed the method for detecting failed RTCs thich may go offseale or fail gradually.

QUESTION: What is the amount of temperature deviatioi in degrees that will cause Tavg and delta T alarms? What is the frequency of channel checks?

RESPONSF- ?:: the present channel configuration Tavg is calculated by adding Thot and Teold and dividing the sum by two. With the deviation alare set at two degrees F the alarm will enunciate when a Thot RID has shifted four degrees F.

In the proposed new configuration, Thot is calculated using the input of three RTDs. Since the contribution of each RTD is divided by three in this averaging process a single Thot RTD will have to shift three times the original four degrees (twelve degrees F) to trigger an alarm. Even though a single RTD must shift further to trigger an alarm, the contro11ng function (Tavg) has still caly noved two degrees F.

The same 1ctic can be applied to relta T.

Ecita T is calculated by subtracting Teold from Thot. Again the influence of a single Thot RTD is only one third of the total. Therefore a single Thot RTD must shift six degrees bercre the two degree deviation alarm is triggered. Channel checks are performed on a shift basis.

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TABLE 1 COLD LED ELBOW TAP FLOW UNCERTAINTl INSTRUMENT UNCERTAINTIES PPA =

+a,c PEA :

SCA =

SPE :

STE SD RCA =

PATEN RTE RD ID =

A/D RIOT:

BIAS:

FLOW CAL.' BIAS FLOW CALCRIFITRIC INSTRUMD;T SPAN 120.0 % r1M

+3,C sit 0LE LOOP ELBOW TAP FLOW UNC =

5 F16 N LOOP EL30W TAP FLOW UNC

=

N LOOP RCS FLOW UNCERTAINTY (WITHOUT BIAS VALUES)

N LDOP RCS FLCM UNCERTAIhTY (WITH BIAS VALUES)

=

N LOOPE RCS FLCW UNCERTAINTY (WITH BIAS VALUES)

WITH VENTURI FCU,ING FD'ALTY i

i RESPONSE TO NRC QUESTIONS 4 AND 11 4.

In Section 3 of Attachment 3 (ANF-88-094), you state that three FSAR Chapter 15 limiting DNBR events were found which trip on the overtemperaeure delta T reactor protection feature. These aret (1) loss of load-MDNB0 case, (2) uncontrolled control rod bank withdrawal from full power, and (1) dropped full length RCCA. The accompanying transient plots show a number of parameters (power, temperature volume, etc.) vs time.

Please also provide plots of DNBR vs time for these accidents.

RESPONS t Plots of HDNBR vs TIME for the three events were transmitted to NRC on August 26, 1988 by lett( Scrial NLS-88-203.

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QUESTION:

Also, discuss why overpower delta T trip related events are not impacted by the change in RTD response time.

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RESPONSE

Changes in the RTD response time have not resulted in changes I

to the overpower delta-t (OPAT) reactor trip because the accident analysis in Chapter 15 of the UFSAR does not take credit for this trip.

J The ANT analysis never mentions OPAT and it is not used to actuate reactor 4

scram in any calculations for the H. B. Robinson plant.

Since the OPAT trip is not in the accident analysis, there is nothing to change.

4 As illustrated in 'JFSAR Table 15.0.7-1 and Technical Specification I

figure 2.1-1, relsance is placed on the high flux reactor trip (instead of f

i OPAT) to limit core power. The OPAT trip is considtred to be redundtnt to the high setting of the power range high neutron flux in performing this i

I function. The high flux reactor trip uses the four excore neutron I

detectors to measure core power whereas the OPAT trip uses the temperature l

difference between the fluid in the hot leg and cold leg piping.

11. In Tables 15.2.1-2,

'.5.4.2-2, and 15.4.3-2 of ANF-88-094, you tabulace the I

sequence of times for three transient events. Please locate in these time i

sequences the total RTD response time as it affects the overtemperature delta T trip.

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RESPONSE

This is a difficult request because the OTAT trip setpoint j

itself is variable.

If the setpoint did correspond to a fixed temperature i

difference between the hot and cold legs, then it would be easy to specify when the trip condition was reached in the fluid.

Instead, since the average temperature contributes to defining the OTAT satpoint, the RTD response time is also part of determining the e, rip condition.

Presently Tables 15.2.1-2, 15.4.2-2, and 15.4.3-2 of ANF-88-0,4 identify the time that the control rods are free to fall.

Lumping all the delay at t

the end of the channel or at the last of the trip function (e.g., che time it takes the bistable to physically break the circuitt the time require (

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for the voltage to decay in the control rod drive mechanism coils) would j

put the trip condition being reached in the signal-to-setpoint comparatot at 0.75 seconds before the beginning of control rod motion.

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In order to specifically illustrate the effect of the RTD time response, a new and dif ferent item is included in the attached ve-sion of these time saquence tables. The additional line item fdentifies the time that trip conditions would be reached. The new value is based on the assumption of an infinitely fast, perfectly prompt, or instantaneous temperature indic tion. As such, the new entry is somewhat nonphysical, i.e.,

hypothetical. Plots showing time sequences for the three transients are enclosed.

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.y ENCLOSURE TIME SEQUENCE TABLES (547FJ5(/le%)

TABLE 15.2.1-2 LOSS OF EXTERNAL LOAD EVENT SEQUENCE Event Time (sec.)

i Turbine Trip 0.0 Pressuriser PORVs Open 4.7 Steam Line Safety Valves Open 6.1 Pressurizer Safety Valves Open 8.8 OTAT Reactor Trip Conditions Reached

• 11.1 Peak Pressure 15.0 Reactor Scram (Begin Rod Insertion) 15.5 Peak Power

'5.5 Minimum DNBR 16.3 Peak Core Average Temperature 16.7 Peak Steam Dome Pressure 19.9 For the hypothetical case of an infinitely fast temperature sensor (for illustrative purposes only).

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I UNCONTROLLED ROD ASSEMBLY WITHDRAW /.L EVENT SEQUENCE Event Time (sse.)

Uncontrolled RCCA Bank Withdrawal Begins 0.0 Overtemperature AT Setpoint Reached (for 23.0 the Hypothetical Case of no Sensor Time Lag)*

Overtemperature AT Setpoint Reached 26.5 Scrar Results in Rod Motion 27.2 Minimum DNBR occurs 27.4 l

• For illustrative purposes only.

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TABLE 15.4.3-2 3ROPPED FULL LENGTH RCCA (HANUAL) EVENT SEQUENCE With Turbine Runback l

Event Time (sec.)

Dropped RCCA Fully in 0.0 Turbine Runback Begins 0.0 Turbine Runback Reaches Low Load Limit 9.0 l

Pressuriser PORVs Open 16.1 I

Peak Pressuriser Pressure 17.0 OTAT Reactor Trip Conditions Reached

• 57.0 Reactor Scram and Rods Begin to Fall 61.1 (Overtemperature AT)

Peak Core Power Level 0.0 Steam Generator Safety Valves Open 57.5 Minimum DNBR Occurs 61.2 For the hypothetical case of an infinitely fast temperature sensor (for illustrative purposes only).

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1 The attached figures are a better illustration of the 4-second RTD time tag.

i Figures A through F are provided for each of three eventst Loss of Load, Uncontrolled RCCA Bank Withdrawal, and RCCA Drop.

Figure A is a reproduction of Figure 15.X.X-2 in ANF-88-094.

It is included as a reference or baseline.

Figures B and C are cold and hot leg temperatures with and without the r

4-second lag. Whereas the "base" or original plot represents the fluid l

condition in the accident analysis calculations, the values with the 4-second lag time constant are a conservatively slow represintation of the signal that the RTD passes to the remainder of the instrument channel.

For each figure, the nonlagged temperature may be regarded as input to the RTD vs the lagged temperature as the RTD output.

In other words, Figures B and C show how "actual" temperatures would relate to "indicated" temperatures.

t Figure D shows the difference between the lagged and nonlagged AT signal.

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lagged AT represents the instrument signal that is comparr* with the OTAT trip setpoint. Figure D for loss of load is a good illustration of how a "lag" is different than a "delay".

Whereas a "delay" is a simple one-for-one shift in time, the "lag" acts to smooth the shape of the signal.

The tagged averdge temperature shown in Figure E represents "T" in the Technical Specification equation.

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For example, in loss of load the indicated Tava is lower than the average fluid temperature. This, in turn, decreases the (T-T') error signal and the setpoint is not lowered as much as it would have been in the case without a 4-second lag applied to Tava.

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Figure F is the plot of legged and nonlagged AT with the addition of the trip setpoint. The trip setpoint is also shown separately as functions of a lagged l

and nonlagged average temperature. The "lagged" AT and "lagged" setpoint intersect 0.75 seconds before the control rods begin to fall on reactor i

scram. The intersection of the "nontagged" AT and "nonlagged" setpoint 111ustrate when reactor trip conditions would be reached if there was no time lag introduced by the sensor.

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