ML20072L213
| ML20072L213 | |
| Person / Time | |
|---|---|
| Site: | McGuire, Mcguire  |
| Issue date: | 06/28/1983 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20072L211 | List: |
| References | |
| NUDOCS 8307130118 | |
| Download: ML20072L213 (10) | |
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SAFETY EVALUATION REPORT RELATED TO AMENDNENT NO. 22 TO FACILITY OPERATING LICENSE NPF-9 AND TO AMENDMENT NO. 3 TO FACILITY OPERATING LICENSE NPF-17 DUKE paler C0'1PANY INTRODUCTI.0_N By letter dated November 23,1982, (Ref.1) Duke Power Company (the licensee) requested an amendment to the Technical Specifications, Appendix A of Operating License No. NPF-9 for the McGuire Nuclear Station Unit 1.
The amendment would revise the uncertainty in the measurement of the total reactor coolant system (RCS) flow rate from 3.5% to 1.7% plus 0.1% as stated in LCO 3.2.3.d, and as reflected in the minimum specified measured flow rate for acceptable operation identified in Figure 3.2 of the plant technical specifications.
Further, the surveillance requirements of Section 4.2.3.2 would be revised to indicate that the process computer readings or digital voltmeter measurements be used to confim acceptable operation, and Section 4.2.3.5 would be revised to indicate that the RCS flow rate shall be determined by precision heat balance at least once per 18 months.
BACKGROUND AND DISCUSSION A Technical Specification change requested by Duke Power Company by letter dated November 11, 1981, permitted operation of McGuire Unit 1 at RCS flow rates which i
are less than the design flow used in the safety analysis for power levels of 90%
or less. At that time, based on preliminary flow measurements, it became apparent that they might not be able to achieve the thermal design flow limits defined in Technical Specification 3/4.2.3. Since the Westinghouse Standard Technical Specifi-cations and the original McGuire Technical Specification require that the power level be reduced to 5% or less if the design flow used in the safety analyses is not satisfied, Duke requested that the Technical Specification minimum flow rate as a function of nuclear enthalpy rise factor be modified to permit operation at a reduced flow rate (95% of the original TS value) in conjunction with a reduced power level (90% of the licensed value). This change was approved and justified based on a staff study using W-3 correlation sensitivity factors showing that a 5%
reduction in flow requires a 4.32% reduction in power.
Since this sensitivity factor varies as a function of several parameters and since no safety analyses have been performed with initial flow lower than the original design flow, the licensee properly included some conservatism in its proposed power reduction as a function of flow rate resulting in the 90% power limit.
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' Specification 4.2.3.2 requires that RCS flow rate and nuclear enthalpy rise hot channel factors be determined to be within the acceptable operating region of Figure 3.2 prior to operation above 75% of rated thermal power af ter each fuel loading and at least once per 31 effective full power days thereaf ter.
Presently the RCS flow rate is detennined by elbow tap flow measurements in each of the four primary coolant loops.
The uncertainty in this method of deten11ning the total RCS flow rate is presently specified as 3.5%.
The licensee has proposed a change to the plant technical specifications whereby the RCS flow rate would be period-ically determined by a heat balance across the stea:n generators.
The RCS flow rate determined in this nanner would be used to calibrate the readings of RCS flow rate as measured by the elbow tap flow measurements for subsequent confirmation of an acceptable operating region defined by LC0 3.2.3.
As justification for this change the licensee provided an analysis to demonstrate that the uncertainty in measurement of the total RCS flow rate by the heat balance method does not exceed 1.7%.
EVALUATION The licensees submittal of November 23, 1982 provided an analysis of the uncertain-ties in determining total RCS flow rate using a heat balance across the steam gener-ator in each loop. A number of factors were considered which would contribute to the uncertainty in the determination of the total RCS flow rate.
For each factor, its effect on RCS flow was stated. Using the square root of the sum of the squares (RSS) method, the individual factors were conbined to detenaine the overall uncer-tainty in the RCS flow for each loop.
A condition for the validity of the RSS method is that each of the factors is independent.
These conditions are also applied to the determination of the uncertainty in the total RCS flow rate.
Since there are four loop flow rates which are sumned to obtain the total RCS flow rate, the uncertainty of the individual flow rate is reduced by one over the square root of four to obtain the uncertainty in the total RCS flow rate.
Therefore, particular attention was given in this review to the condition that each of the factors considered in the uncertainty analysis is independent.
Since the overall uncertainty in total RCS flow rate is stated as 1.7%, an uncertainty value of 0.05% was used as a lower bound in assessing the significance of any individual factor considered in the uncertainty analyset.
The 0.05% value for the bound of significance is arrived at as follows:
If in the RSS analysis that produced the result of 1.7% an additional term with a value of 0.05%
is considered, this would increase the result by an increment of only 0.001%, i.e.,
to the value of 1.701%.
Therefore, it is concluded that a conservative basis exists for the use of 0.05% as a bound of significance, and that this would be true even if additional tenns of this magnitude or less were neglected. As an example, OFFICE >
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. feedwater pressure is a measured parameter which is used to determine both feedwater flow and feedwater enthalpy for each loop. Although the analysis treats the unce-tainty in feedwater flow and enthalpy as being independent, both are dependent apon the uncertainty in the measurement of feedwater pressure.
Since the uncertainties in feedwater pressure expressed in percent of RCS flow for both feedwater flow and enthalpy add up to less than 0.05"., it is concluded that this interdependency has a negligible effect on the loop RCS flow uncertainty.
A lack of ino.pendence may occur for factors which are cominon to all coolant loops.
Since the uncertainty in total RCS flow is one half that of individual flow measure-ments, an uncertainty value of 0.025% was used as a bound in assessing the signif t-cance of any dependent factor which could impact total RCS flow.
The basis for this conclusion is arrived at in a manner similar to that cited above except the value of the bound of significance on loop dependent parameters is one half that used for dependent parameters within a loop. As an example, feedwater flow is dependent upon the measurenent of differential pressure across the feedwater flow nozzles in each loop. However, the same differential pressure measuring instrument is used to deter-mine feedwater flow in each loop.
Thus, the uncertainty in RCS flow rate is not independent between loops.
However, since the uncertainty in RCS flow rate for each loop due to this measurement is less than 0.025"., it is concluded that this interde-pendency has a negligible effect on total RCS flow uncertainty.
For the secondary side of the heat balance the determination of three major param-eters is required. They are feedwater flow, feedwater enthalpy and main steam enthalpy. The plant has feedwater flow nozzles which were installed in a section of feedwater pipe, and as a unit, flow calibrated in a laboratory. The licensee provided data (Ref. 2) to substantiate the uncertainty in the flow coefficient based on the accuracy of the flow calibration at Alden Research Laboratories.
The staff concludes that this infonnation provides adequate justification for the uncertainty associated with this factor of the uncertainty in feedwater flow.
However, the staff did question the validity of the assumption that the characteristics of the flow nozzles do not change over the life of the plant. Of particular concern, was the potential that fouling of the feedwater flow nozzles could result in a bias which would result in an increase in calculated RCS flow for each 1000 as well as the total RCS flow rate. The licensee, therefore, revised this analysis (Ref. 3) to include consideration of an uncertainty of 0.1". due to feedwater flow nozzle fouling. The basis for this number is that monitoring and trending of various plaat performance parameters are expected to reveal fouling of this magnitude. The staff does not have sufficient information to confirm that a bias of 0.1% can be detected by this means.
However, the staff judges that the program being used by Duke Power Company for maintaining water chemistry has, from experience, been demonstrated to be excellent; and significant fouling, if it should occur at all, would not be expected to occur for many years. The staff expects that the licensee will maintain appropriate records at the plant site of its monitoring and trend analysis for this effect that can be audited by the NRC.
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4-The staff questioned the uncertainty in the measurenent of the differential pressure across the feedwater flow nozzles since it was a very low nunber.
The licensees pro-vided specification data (Ref.4) to support the accuracy of this inscrument.
Based on this data and the fact that the RCS flow uncertainty due to this measurenent is more than an order of magnitude below the lower bound of significance noted above, we find the accuracy value of the differential pressure neasurement instrument used for feedwater flow to be acceptable.
The two reaaining measured parameters for detennining feedwater flow are feedwater temperature and pressure.
Feedwater temperature is used to detennine both feedwater flow and enthalpy.
Since the uncertainty on loop RCS flow due to temperature exceeds the lower bounds of significance, the licensee's revised analysis (Ref.3) accounts for the fact that temperature uncertainty dependence exists in loop RCS flow uncer-tainty.
The uncertainty values for feedwater flow and enthalpy due to temperature are summed to account for this dependence on RCS loop flow.
The same readout instru-ment is used to measure feedwater tenperature in each loop.
This interdependance has not been included in the analysis, however since the readout instrument uncer-tainty does not significantly exceed the lower bounds of significance, this inter-dependence may be neglected.
The dependence of feedwater pressure as a single measurement which affects the uncertainty in feedwater flow and enthalpy was addressed in the example related to lower bounds of significance noted previously.
With regard to the magnitude of the encertainty in the measure 1ent of feedwater pressure and tenperature, the staff concludes that reasonable values are used.
On April 25, 1983, the staff net with the licensee to discuss the uncertainty analysis and to review data taken from a heat balance to determine RCS flow. At this time it was noted that a portion of the feedwater to each steam generator enters the upper steam generator nozzle.
This flow does not pass through the nain feedwater nozzles and is measured separately.
In that this flow is about 2 to 3 percent of the total feed to the steaa generator it is significant to the heat balance and uncertainty analysis.
The licensee's revised analysis (Ref. 3) includes the uncertainty in RCS loop flow based on a 5% uncertainty in measuring this auxil-1ary flow rate. Also, steam generator blowdown flows were included with the same accuracy of measurenent.
The staff finds these values reasonable.
The enthalpy of steam at the outlet of the steam generator is determined based on the measurement of pressure.
Since the uncertainty in RCS flow due to the steam pressure measurement is three orders of magnitude less than the lower bounds of significance, its uncertainty is not of consequence.
Consideration was given to moisture carry-over to the main steam lines as it would have an effect on the cal-culated thennal output of the steam generator. Based on a canparison of the value of the uncertainty in moisture carry-over to the design value of moisture carry-over for these stean generators, the staff concludes that a conservative treatment of this factor was used in this analysis.
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5-The primary side heat balance includes consideration of thermal input to the RCS from the reactor coolant pumps.
This value is calculated by the process computer based on measurements of reactor coolant pump voltage and current which are pro-4 vided as hard wired inputs to the computer. The uncertainty analysis includes a single value of the component error and RCS flow uncertainty for reactor coolant i
pump thermal power. This uncertainty in RCS flow is approxinately at the lower tounds of significance and is considered a reasonable value.
The heat losses from primary and secondary piping are estimated values.
The uncer-tainty in this factor is conservatively taken at one half of its value. This effect on RCS flow uncertainty is approximately at the lower bounds of significance and is considered a reasonable value. The effect of charging and letdown to and from the primary coolant loops was included in the revised uncertainty analysis (Ref.
3).
The uncertainty in measuring the flows and charging temperature appear to be reasonable values.
Since their effect on RCS flow uncertainty is approximately at the lower bounds of significance, they are considered reasonable values.
The final consideration in the primary side is the detennination of the hot and cold leg enthalpy.
The values are based on measurnents of hot and cold leg temperature and pressurizer pressure.
The temperatures of the hot and cold legs are deternined by direct resistance read-ings of the RTO's located in the bypass manifolds that receive a sample of the reactor coolant flow. While the pressure of the reactor coolant sample at the hot and cold leg manifolds should be approximately equal, the staff estimates that the pressure at the manifolds could be lower than the pressurizer pressure by approxi-mately 20 psi. Since this single pressure measurement is used to calculate the RCS flow in each loop, the error in total RCS flow rate would be of the same magni-tude. The staff estimate of this error is about 6 times the lower bounds of signifi-l cance for interdependent factors affecting total RCS flow.
As a consequence the licensee has stated that he will correct the pressurizer pressure measurement for static and dynamic head when determining hot and cold leg enthalpy at the RTD mani-folds.
The staff noted the fact that the single measurement of pressurizer pressure is a dependent variable when assessing the uncertainty on hot and cold leg enthalpy. The revised analysis treats uncertainties in hot and cold leg enthalpy due to pressure uncertainties as interdependent. Further since the original analysis considered the uncertainty in RCS flow as a single factor due to pressure, the staff investigated the sensitivity of flow uncertainty due to a change in hot and cold leg enthalpy and found that one is about 4 to 5 times more sensitive to a small change in pressure than the other.
In the revised analysis the sensitivity of RCS flow uncertainty per psi change in pressure was stated to be equal for both hot and cold leg enthalpy. When questioned on this matter the licensee stated that the value given reflects the change in flow uncertainty when both hot and cold leg enthalpy vary by the same amount. How-ever, the analysis assigns this value to both hot and cold leg enthalpy. Thus, one OFFICE)
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- l would conclude that twice the required uncertainty was used to determine pressure effects on hot and cold leg enthalpy for loop RCS flow.
However, the same pressure neasurement is used for all loops and a dependence exists which was not included in the analysis.
Since the total RCS flow uncertainty is one half of loop flow uncer-tainty, the conservatism of a factor of 2 in loop flow uncertainty is eliminated when the pressure dependence on total RCS flow uncertainty is accounted for. Finally the revised analysis stated a value for the pressure measurenent uncertainty which was about 50f greater than the original analysis. When questioned on this point the i
licensee stated that this calculation was based on a full range pressure channel l
rather than the narrower span pressurizer pressure measurement channels used by the protection system.
The licensee noted that the original accuracy for pressure i
neasurenent should be used.
Further since there are four of these channels, a minimum of three should always be available.
The revised analysis uses the average i
of three channels, with the uncertainty in this average being reduced by a factor of one over the square root of three. While the staff questions the validity of this approach, due to the potential for biased errors, the effect borders on the lower i
bounds of significance.
1 The temperatures of the hot and cold legs are determined by resistance measurements of two RTD's installed in each nanifold. The staff questioned the basis for the uncertainty in the resistance measurement as provided by the licensee (Ref. 4) which was expressed in terms of the measurement span of instruments not used in this i'
neasurement.
In the revised analysis (Ref.3), the licensee provided the basis for the uncertainty in resistance measurements based on the specifications of the equipment used to nake this measurement. The instrument accuracy is stated as
+0.075 ohms. Althougn data provided by the licensee on a previous heat balance Tndicates that resistance readings were only recorded to the nearest 0.1 ohms, the licensee has stated that subsequent reading will be recorded to the nearest 0.01 1
ohm. The staff concludes that the added precision in resistance readings is appro-priate in light of the instruments specified accuracy and readout capability.
The same readout instrument is used to measure the resistance of all RTD's.
This dependence was not accounted for in the analysis, however the difference in RCS flow uncertainty for hot and cold leg temperature measurements is about at the lower bounds of significance. Thus the staff finds that this interdependence may be neglected.
i The sample of the primary coolant for the hot leg RTD manifold is obtained by three probes mounted in the 29 inch diameter hot leg pipe. The probes sample the reactor coolant to a depth of seven inches to obtain a sample which is representative of the average hot leg temperature.
Test data from other Westinghouse plants, where temperature gradients were measured across the hot leg, were used with conservative values of sample probe flows as a basis to establish a temperature uncertainty due to this measurement configuration. Based on the discussion of the considerations included on temperature streaming and its effect on hot leg temperature measurement, the staff concludes that a reasonable uncertainty for temperature streaming effects has been included in the analysis.
It should be noted that this effect is the dominant. consideration in the uncertainty analysis and accounts for about two thirds of the uncertainty in total RCS flow rate.
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, Measurements in the primary and secondary loops required for the heat balance are performed by taking 12 readings of each parameter over a period of about one hour.
For feedwater flow, 36 readings are taken. The calculation of RCS flow is based on the average value of each neasured parameter over the one hour period.
On examination of the data from a previous heat balance calculation, the staf f noted that feedwater flow is one variable for which variations as large as a 10 percent change in measured feedwater flow occurred. As a means to assess the uncertainty in RCS flow due to these variations, the licensee calculated the standard deviation in ear.h set of the 36 readings of feedwater flow. From this, the average of all sets was obtained and divided by the square root of 36 to give a standard deviation of averages.
The uncertainty due to data scatter was then assessed, at a 95% confidence level, to be twice the standard deviation of averages.
Similarly the uncertainty in RCS flow was detemined for other measured parameters.
Since the RSS method was used to com-bine uncertainties, the effect on loop RCS flow uncertainty was calculated by the staff to add only 0.07% uncertainty to the loop flow uncertainty of 3.02%.
The staff concludes that the treatment of fluctuations in process parameters in this nanner do not significantly impact the final result.
i On the basis of the detemination of loop RCS flow by the heat balance calculations, the elbow tap flow measurements for the primary coolant system are calibrated to provide subsequent measurements of RCS flow as required by plant technical specifi-cations. Additional uncertainty is factored into the total RCS flow rate obtained by the heat balance calculation to account for uncertainties associated with the elbow tap flow measurements.
Since these instruments provide signals to the protection system, uncertainties in these instruments have previously been ar.alyzed by Westinghouse in establishing the basis for protection system set points.
- However, in the assessment of RCS flew uncertainty, certain factors have been excluded based on conditions under which the plant is operating when the normalization process takes place, in contrast to conditions existing when considered in the set point methodology analysis.
In addition credit is taken for a set of components as a group which are 4
calibrated more frequently than assumed in set point analysis.
The net effect is that each elbow flow neasurement channel has an uncertainty of about 1%. Asstsning that two of the three elbow flow measurement channels will always be available, the uncertainty with two measurements is taken as one over the square root of two of that for one channel. Finally it was assumed that each loop neasurement is independent. This results in a further reduction in total RCS flow uncertainty by a factor of one over the square root of four. Where each of the various factors which contributes to the uncertainty of the elbow flow measurement channels are truly independent and random, this is a valid process for evaluating uncertainties.
However, where bias errors may be introduced due to dependent conditions, this approach is non-conservative.
Examples would be process conditions of tenperature and pressure, environnental con-ditions, the same test equipment used to calibrate components common to all channels,
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and the like.
Because a strong case has not been made that these factors are truly independent, the staff has considered that a value of about 0.7%, being about halfway between the accuracy value of one channel and that of eight independent channels, would be a more appropriate basis for consideration. However, this value combined with a total heat balance uncertainty of about 1.S% using RSS is still within the proposed technical specification limit of 1.7% total RCS flow uncertainty.
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In addition, for all anticipated operational occurrences (A00), the minimum DNBR has been determined to be no less than 1.4 compared to the ONBR limit of 1.3 for the W-3 correlati on. Sensitivity studies have shown that the effect of RC flow on DNBR is about 1.5% DNBR per 1% of RC flow reduction.
Therefore, the reduction in DNBR margin due to reduced flow will be less than 3%, which is not sufficient to cause the DNBR limit of 1.3 to be violated even if the initial fitw asstraed for analyzed A00s is reduced by 2%.
Likewise, a 2% reduction in initial flow assumed for accident l
transients would not result in violation of design or licensing limits.
Therefore, we conclude that the proposed McGuire Technical Specification change reducing flow l
measurement uncertainity from 3.5% to 1.8% will have no significant safety concern.
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l ENVIRONt' ENTAL CONSIDERATION:
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We have detemined that the amendment does not authorized a change in effluent types or total amounts.
Furthemore, we have detemined that this amendment will pemit 4
operation at the full power level reviewed and evaluated in our Final Environmental l
J State'ent (NOREG-0063) dated April 1976.
Therefore, we have detemined that this l
amendment will not result in any significant environmental impact different from that previously evaluated.
Having made this detemination, we have further concluded that j
the amendment involves an action which is insignificant from the standpoint of environ-nental impact, and pursuant to 10 CFR 951.5(d)(4), that an environmental impact state-ment or negative declaration and environmental impact appraisal need not be prepared in connection with the issuance of this amendment.
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FINAL N0 SIGNIFICANT HAZARDS CONSIDERATION (SHC) DETERMINATION i
The Commission made a proposed detemination that the amendment involves no SHC which was published in the Federal Register (48 FR 2717) on June 13, 1983, and consulted i
with the State of North Carolina.
No public comments were received and the State of i
North Carolina did not have any conments.
Based on the Commission's final review and the absence of State and Pubile comments, the Commission has made a final determin-j ation that the amendment involves no significant hazards consideration.
I CONCLUSION The staff has reviewed the reduction in flow uncertainty now proposed by the licensee and has concluded that changing the flow uncertainty to 1.7% plus 0.1% as stated in LC0 3.2.3.d from the previous value of 3.5% has been adequately justified on the basis l
of physical factors (all volatile.:hemistry treatment of reactor coolant and accuracy of instrument components) and statistical methods used in the computation of overall i
j measurement uncertainty.
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. The nost dominant factor in the uncertainty in the measurement of RCS flow by the heat balance method is the uncertainty associated with the hot leg temperature streaming error.
Likewise any error in its assumed value would have a larger impact on the results of this analysis.
Based on the infonaation presented, the staff concludes that an acceptable case has been made to address this uncertainty.
The conditions under which the RSS method of combining uncertainties is valid have been closely examined.
In that the total RCS flow rate uncertainty is assessed to be one half of that of the loop flow uncertainty, this is a very significant con-sideration.
In sone cases where dependency exists between factors affecting either or both RCS loop flow and total RCS flow, the effects have been judged to be non-significant.
The licensee revised his analysis to address dependent factors where we considered it appropriate.
Therefore, we conclude that interdependencies which impact the validity of the RSS method have been either appropriately con-sidered in the analysis or, based on staff judgement, do not impact the proposed technical specification limiting uncertainty.
The revised analysis included the effects of fouling of the feedwater flow nozzles.
The licensees basis for including 0.1 percent uncertainty in total RCS flow rate for this effect was based on the ability to detect changes of this magnitude by a program that trends changes in plant neasurements. Since the staff cannot confirm the capa-bility of the trending program to reveal changes of this magnitude, it expects that the licensee will maintain appropriate records at the plant site of its monitoring and trend analysis for this effect that can be audited by the NRC.
The staff con-cludes, however, that the use of the additional 0.1% uncertainty due to potential feedwater venturi fouling is acceptable until more conclusive data are available.
In conclusion the staff finds the proposed changes to the plant technical specifica-tions based on a 1.7% uncertainty in total RCS flow rate determined by the primary side elbow taps which are calibrated periodically by precision heat balance to be acceptable and based on the considerations discussed above, that: (1) this amendment does not involve a significant increase in the probability or consequences of an accident previously evaluated, does not create the possibility of an accident of a type different from any evaluated previously and does not involve a significant
.aduction in a margin of safety. On this basis the staff concludes that this amend-nent does not involve a significant hazards consideration, (2) there is reasonable assurance that the health and safety of the public will not be endangered by oper-ation in the proposed manner, and (3) such activities will be conducted in compli-ance with the Commission's regulations and the issuance of these amendments will not be inimical to the common defense and security or to the health and safety of the pubitc.
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. REFERENCES (1)
Duke Power Company Letter from Hal B. Tucker to Harold R. Denton, Director, NRR, NRC dated November 23, 1982.
(2)
Duke Power Company Letter from Hal B. Tucker to Harold R. Denton Director, NRR, NRC dated March 14, 1983.
(3)
Duke Power Company Letter from Hal B. Tucker to Harold H. Denton Director, NRR, NRC dated April 27, 1983.
(4)
Duke Power Company Letter from Hal B. Tucker to Harold R. Denton Director, NRR, NP.C dated March 28, 1983.
Date: June 28, 1983 Principal Contributors:
T. fl. Dunning, Instrumentation and Control Systems Branch, DSI Y. Hsii, Core Performance Branch, DSI F. Anderson, Standardization and Special Projects Branch, DL R. Birkel. Licensing Branch No. 4, DL H. Thadani, Licensing Branch No. 4. DL d
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