ML20196J379
| ML20196J379 | |
| Person / Time | |
|---|---|
| Site: | Farley  |
| Issue date: | 06/30/1999 |
| From: | NRC (Affiliation Not Assigned) |
| To: | |
| Shared Package | |
| ML20196J366 | List: |
| References | |
| NUDOCS 9907070251 | |
| Download: ML20196J379 (11) | |
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NUCLEAR REGULATORY COMMISSION 2~
WASHINGTON. D.C. 2066tW001
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SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION RELATED TO WCAP-14750. " REACTOR COOLANT SYSTEM FLOW VERIFICATION I
- USING ELBOW TAPS AT WESTINGHOUSE 3-LOOP PRESSURIZED WATER REACTORS" SOUTHERN NUCLEAR OPERATING COMPANY. INC.. ET AL.
JOSEPH M. FARLEY NUCLEAR PLANT. UNITS 1 AND 2 i
DOCKET NOS. 50-348 AND 50-364
1.0 INTRODUCTION
l By letter dated November 26,1996 (Ref.1), Southern Nuclear Operating Company (SNC), the licensee for the operation of Joseph M. Farley Nuclear Plant, Units 1 and 2, submitted a Westinghouse Owners Group technical report, WCAP-14750, "RCS Flow Verification Using Elbow Taps at Westinghouse 3-Loop PWRs" for staff review. The technical (topical) report describes a methodology for using cold-leg elbow tap pressure differential (AP) measurements to verify reactor coolant system (RCS) flow.
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Technical Specifications (TS) Limiting Conditions for Operation (LCO) of Westinghouse-designed pressurized water reactors (PWRs) require utilities to maintain RCS flow greater than or equal to a specified minimum measured flow (MMF) rate during MODE 1 operation. This LCO MMF is an input value in the design basis transients safety analyses that use a statistical core design method (such as the Improved Thermal Design Procedure (ITDP) or the Revised Thermal Design Procedure (RTDP) (Ref. 2)) to demonstrate that the departure from nucleate boiling ratio (DNBR) limit is not violated during normal operation and anticipated operational occurrences (AOO). Surveillance Requirements (SR) require plant operators to read control board RCS flow indicators to verify RCS total flow is within its limit at least once per 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />.
( The SR also require plant operators to verify RCS total flow is within its limit using a precision heat balance at least once every 18 months.
In'the precision heat balance measurement, operators make calorimetric measurements on the steam generator (SG) secondary side using venturi meters to measure feedwater flow rates.
- The utility calculates RCS flow rate from the calorimetric measurements in conjunction with the enthalpy rise across the reactor vessel as indicated by the hot and cold-leg resistance temperature detectors (RTDs). Each hot leg has three thermowell RTDs installed around a cross-section to determine the bulk hot-leg temperature. However, Farley, Units 1 and 2, and p
_many other plants observed an iricreased het-leg temperature streaming phenomenon. Using low-leakage core loading pattems that result in core radial power distribution changes cause the phenomenon. _The increased temperature streaming causes the bulk hot-leg temperature, as measured by the three RTDs in each hot leg, to be erroneously hign. This results in the calculated RCS flow being lower than the actual value. Utilities expressed a need Enclosure 9907070251 990630 7
i PDR ADOCK 05000348 P
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. to use cold-leg elbow tap flow measurement as an alternate method to verify RCS flow because of the inherent limitation of the calorimetric-based method. The NRC approved using cold-leg elbow tap measurements for the 18-month RCS flow surveillance for McGuire Nuclear Station (Ref. 3), Catawba Nuclear Station (Ref. 4), and South Texas Project Electric Generating Station (Ref. 5).
As previously mentioned, WCAP-14750 describes a methodology for using cold-leg elbow tap AP measurements to measure RCS flow. The WCAP also describes how to apply the elbow tap AP methodology as an alternate method for satisfving the TS 18-month RCS total flow surveillance for the Westinghouse-designed 3-loop PWRs for which Farley Nuclear Plant is the lead plant.
2.0 EVALUATION The staff evaluation of WCAP-14750, as discussed in the ensuing sections, includes the following:
A generic evaluation of the appropriateness of the cold-leg elbow tap flow measurement.
The procedure for converting elbow tap AP measurements to RCS flow.
The best estimate hydraulics calculation for confirming RCS flow measurement.
The flow measurement uncertainty evaluation.
The recommended Staridard Technical Specifications (STS) changes to implement elbow tap measurement methodology.
A review of the process implementation for the Farley units.
2.1 Elbow Tap Flow Measurement Methodology 2.1.1 Elbow Tap Flow Measurement Utilities with Westinghouse plants, including Farley, Units 1 and 2, use cold-leg elbow tap flow meters and associated control board indication when pedorming surveillances to verify RCS flow every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. The purpose of the 12-hour surveillance is to verify that the full-power steady-state flow has not decreased below its limit during the fuel cycle. The principle of operation of an elbow meter is based on the centrifugal force of a fluid flowing through an elbow creating a AP between the outer and inner elbow radii. The relationship between the volumetric flow rate through an elbow, O, and AP between the pressure taps at the outer and inner elbow radii can be expressed as O = C APv2 The elbow meter coefficient C is a function of elbow bend and cross-section radii and is affected by pressure tap location, upstream and downstream piping, and other factors. The cold-leg elbow tap - flow element is not calibrated in advance in a laboratory. However, the measurement is typically normalized against the RCS flow rate established from the precision heat balance calorimetric flow measurement at the start of each fuel cycle. The cold-leg elbow taps are typically used to indicate relative changes in the RCS flow rather than to measure absolute value of the RCS flow. The cold-leg elbow tap AP RCS flow rate measurement also inputs to the low-flow reactor trip.
Figure 4-1 in WCAP-14750 shows the configuration of the Prairie Island, Unit 2, cold-leg elbow taps, which is a standard configuration used in other Westinghouse PWRs including the Farley units. The elbow taps are located in a plane 22.5 around the first 90a elbow turn in each of the
l (i
K 3-cold legs. Each elbow has three low-pressure taps spaced 15 apart on the inside pipe radius and one common high-pressure tap on the outside pipe radius. The pressure taps are
' connected to three differential pressure transmitters to obtain AP data. As the elbow taps in the
. cold legs are fixed, the elbow meter coefficients in each elbow tap configuration should remain unchanged. The topical report also cited an American Society of Mechanical Engineers
- (ASME) public,ation (Ref. 6) stating the t tests have demonstrated that elbow tap flow measurements have a high degree of iepeatability and are not affected by changes in the elbow surface roughne-)s.
To confirm elbow tap flow measurement repeatability, Section 4.1 of WCAP-14750 compares RCS flow measuremen:s using elbow taps with measurements using ultrasonic leading edge flow meters (LEFM) from the Hydraulic Test Program at Prairie Island Unit 2 (PI-2). The PI-2 Hydraulic Test program began in 1973 and the test data covered 11 years of plant operation, during which a significant change in system hydraulics was made. The data showed that the elbow tap measurements agree to within 0.3 percent of the LEFM flow measurements. Various processes or phenomena affecting the' elbow tap flow measurements were evaluated. These included fouling, erosion, upstream velocity distribution, and SG tube plugging and -
replacement. The Test Program concluded the following:
- 1. Fouling conditions are not present in the cold-leg elbow since there is no change in cross section to produce a velocity increase and ionization.
- 2. Stainless steel elbow surface erosion is unlikely, and the flow velocities are not large relative to the conditions that cause erosion.
- 3. The upstream velocity distribution, including the distribution in the elbow tap flow meter, remains constant so the elbow tap flow meter AP versus flow relationship does not change.
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' 4. The plenum velocity head approaching the outlet nozzle is small compared to the piping velocity head; tiierefore, SG tube plugging does not affect elbow tap flow measurement repeatability.
- 5. SG replacement will have no impact on the elbow tap flow coefficient since replaced SG configuration is the same, and the same difference in plenum and nozzle velocity heads
. will exist.
i The elbow taps at Farley were not calibrated and the elbow meter coefficients were not determined. ; However, RCS flow measurements using elbow taps were normalized against the
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precision heat balance flow measurements at the start of each fuel cycle. The staff concludes the since the elbow meter coefficients remain constant, the relative changes of flow rate i
through the cold-leg elbows can be correlated with the relative changes in the elbow tap AP.
2.1.2 Elbow Tap Flow Measurement Procedure Section 4.2 of WCAP-14750 describes the procedure for determining the RCS flow from elb::w tap AP measurements. This procedure relies on the total baseline calorimetric flow (BCF),
which is based on.the calorimetric flow measurements from early fuel cycles. The future cycle flow (FCF) will be determined from the BCF multiplied by the elbow tap flow ratio (R),
Section 4.2 of WCAP-14750 defines the elbow tap flow ratio (R) by the following:
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' 1 R = (K/B)*, where:
f K is the " future cycle elbow tap total flow coefficient" = AP, x vg
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B is the " baseline elbow tap total flow coefficient" = AP,x vs v is the specific volume of the coolant WCAP-14750, Section 4.2 indicated in the paragraphs under " Baseline Calorimetric Flow" that the BCF would be based on either (1) one baseline cycle such as the first fuel cycle, or (2) the average of multiple cycles with consistent measurements (i.e., the calorimetric measurements of the averaged cycles are adjusted for the effects of known changes in system hydraulics to the same hydraulic configuration as the baseline cycle). SNC,in response to a staff question (Question 4, Ref. 7), provided the criteria to be used in choosing the calorimetric flow measurements from early cycles for averaging and indicated that using only one calorimetric measurement was the preferred procedure. In response to a further staff request for guidelines in defining the baseline flow from one baseline cycle or the average of multiple cycles (Question 1 Ref. 8), SNC revised the paragraphs of " Baseline Calorimetric Flow." These new paragraphs provide a revised procedure for determining the BCF, including the acceptance criteria in the choice of early cycle calorimetric flow measurements, and determining the BCF from the chosen cycle data. The staff reviewed the revised procedure for defining the BCF and found it to be acceptable. SNC indicated that they will use the revised procedure in place of the existing procedure in WCAP-14750, Section 4.2.
The baseline and future cycle " flow coefficients" B and K, are calculated based on the average AP from all cold-leg elbow taps. For each individual elbow tap, the elbow meter coefficient C in the elbow meter equation would be constant. Therefore, the ratio of the volumetric flow rates through the elbow tap between two fuel cycles can be expressed in terms of the square root of the AP ratio. The AP ratio would be the same for the three elbow taps in the same cold leg, barring measurement uncertainties. The staff questioned (Questbn 2b, Ref. 7) whether it would be appropriate to define the elbow tap flow ratio (R) based on the average of the square root of the AP ratios from all elbow taps, rather than the AP average. In response, SNC calculated R based on the average of the square root of the AP ratios and compared that with R calculated using the AP average method described in WCAP-14750, Section 4.2. SNC used Farley Unit 1 and 2 indicated transmitter AP values for each fuel cycle to do this. The results show insignificant difference between the two calculations. The staff, therefore, concludes that using average AP is acceptable.
SNC also asserted, in response to a staff question (Questions 2c,2d, Ref. 7), that there is no need to include an additional allowance to the future cycle flow ratio R to account for the AP ratio distribution among the elbow taps using an one-sided tolerance limit to provide a 95 percent probability at 95 percent confidence level. The overall RCS flow determination procedure based on the elbow tap AP measurements includes the following:
- 1. A calculation of the future cycle flow ratio R based on determining the ratio (between the future and baseline cycles) of the average indicated AP values.
- 2. A separate comparison with the predicted system flow to account for the system hydrau:ic effects such as SG tube plugging.
- 3. A separate uncertainty calculation to account for the flow measurement uncertainties.
The entire process assures a conservative RCS flow surveillance verification because of the conservative uncertainties to the baseline calorimetric flow measurement and plant process
5-computer indication and a one-sided acceptance criterion for flow measurements confirmation with best estimate calculations. The staff reviews of the best estimate flow confirmation and the flow measurement uncertainty calculation are discussed in Sections 2.1.3 and 2.2.1 of this report, respectively. Based on the above, the staff concludes that the elbow tap RCS flow measurement procedure described in WCAP-14750, Section 4.2 with the revised BCF l
procedure is acceptable for se at Farley, Units 1 and 2.
2.1.3 Best Estimate Flow Confirmation The elbow tap flow measurement procedure includes a requirement that utilities are to perform f
a best estimate (BE) hydraulics analysis to confirm the future total RCS flow determined from j
the elbow tap flow measurement. The BE RCS flow calculation, described in Section 5 of j
WCAP-14750, is based on the flow resistance of various components in the reactor coolant loops and the reactor coolant pump performance characteristics. Therefore, changes in the RCS flow rate can be evaluated based on system hydraulic changes in the plant (e.g., plugging i
and sleeving of SG U-tubes, reactor coolant pump wear, and changes in the fuel design).
The BE hydraulic analysis confirmation procedure specifies that utilities are to compare the elbow tap flow ratio (R) to an estimated future cycle flow ratio (R'). R is based on the elbow tap AP measurements as previously discussed. R'is the ratio of the estimated future cycle RCS flow to the estimated initial baseline cycle flow based on the flow analysis of known RCS hydraulics changes, such as SG tube plugging or fuel design changes. If the measured R is greater than (1.004 x R'), R will be limited to (1.004 x R'). The multiplier 1.004 applied to R'is a measure to provide an allowance of 0.4 percent for elbow tap flow measurement repeatability.
The 0.4 percent repeatability value was determined by combining appropriate instrument uncertainties for two different cycle measurements of RCS flow at 100 percent rated thermal power using all of the cold-leg elbow tap channels. SNC provided their derivation of the 1
0.4 percent flow repeatability value for Farley, Units 1 and 2 in response to staff requests for additional information (Question #3, Ref. 7, and Question #3, Ref. 9). The repeatability allowance is implicitly included in the elbow tap flow measurement uncertainty calculations because all of the instrument uncertainties included in the repeatability derivation are common with those in the elbow tap flow measurement uncertainty calculations. The licensee states that since the elbow tap flow measurement uncertainty includes this repeatability allawanco, the measured flow ratio R can be 0.4 percent higher than the estimated flow ratio R' and still define a conservative flow.
The BE RCS flow analysis employs an RCS flow calculational procedure developed by Westinghouse in 1974. The procedure uses BE values of the RCS component flow resistance and pump performance with no margins applied, so the resulting flow calculations define a true best estimate of the actual flow. In the analysis, the RCS loop (which consists of the reactor vessel, reactor coolant piping, and the SGs) flow resistance is used in conjunction with the reactor coolant pump head-flow performance to define individual loop and total RCS flows. The component hydraulic design data and hydraulic coefficients are determined from analyses of test data. The reactor vessel (consisting of the reactor core, vesselinternals and vessel nozzle) flow resistance is determined from the AP measurements of a full-size fuel assembly hydraulic test and hydraulic model test data for each type of reactor vessel. The reactor coolant piping flow resistance combines the resistance of the hot-leg, crossover-leg and cold-leg piping. The flow resistance is based on analyzing the effects of upstream and downstream components on
. i elbow hydraulic loss coefficients, using the results of industry hydraulic tests. The flow resistance is defined in five parts: inlet nozzle, tube inlet, tubes, tube outlet, and outlet nozzle.
Section 5.1 of WCAP-14750 indicates that numerous component flow resistance tests and analyses (including the overall flow resistance confirmed by the Prairie Island, Unit 2 HydrwUcs Test Program) have confirmed that this hydraulic analysis procedure has an uncertainty of 2 percent flow. This indicates that actual flow is expected to be within 2 percent of the calculated BE flow, Utilities have used this hydraulics analysis procedure to estimate RCS flows at all Westinghouse plants, including Farley, Units 1 and 2.
SNC stated, in response to a staff question (Question #8, Ref. 9), that comparing the elbow tap flow measurement results to a BE flow model predicted value is for the purpose of a cross check and does not provide a direct input to verify the safety analyses RCS flow assumption.
The best estimate flow analysis defines the expected change in flow for a new cycle. If the g
elbow tap measured flow is greater than the BE flow by more than the repeatability uncertainty for the elbow taps, then the more conservative (smaller) of the two values is used to define the RCS flow for the cycle. The staff finds that the BE hydraulic analysis will be used merely to confirm the elbow tap flow measurement and will not change the TS surveillance requirerr.ent for a flow measurement and is, therefore, acceptable for use at Farley, Units 1 and 2.
2.2 Elbow Tap Flow Measurement Licensing Considerations Plant TSs require utilities to verify the LCO RCS MMF through calorimetric measurements performed every 18 months at the beginning of each fuel cycle and qualitative verification thereafter using the installed flow instrumentation every 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> to assess potential degradation. The LCO MMF limit and the low-RCS flow reactor trip setpoint are inputs to the safety analyses to demonstrate that the DNBR limit is not exceeded during normal operation and anticipated transients. The RCS flow measurement uncedainty associated with the low-flow reactor trip is accounted for in the reactor trip setpoint allowance. The flow measurement uncertainty associated with the MMF surveillance is accounted for in the safety analyses either deterministically or statistically. In the deterministic safety analyses, the initial RCS flow is assumed to be the thermal design flow which is the MMF minus the measurement uncertainty. In the statistical method (e.g., the ITDP or RTDP), the initial RCS flow is assumed to be the MMF with the flow measurement uncertainty accounted for statistically in the DNBR safety limit.
Section 7.0 of WCAP-14750 discusses licensing considerations associated with applying the elbow tap AP measurement as an alternate method for performing the 18-month RCS flow surveillance currently performed with the precision calorimetric measurement. An evaluation must be made on the flow measurement uncertainty and its impact on the existing safety analyses as the RCS flow measurement uncertainty will likely increase with the elbow tap AP methodology. An increase in the flow uncertainty beyond those currently established in the low-RCS flow trip setpoint allowance or the LCO MMF uncertainty assumed in the safety analyses will require a revision to the TS to reflect the uncertainty changes. The increase the flow uncertainty will also require a new safety analyses to demonstrate that the flow uncertainty and l'
during normal operation and anticipated transients.
the MMF assumed in the safety analyses will not result in the CNBR safety limit being exceeded l
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. 2.2.1 Flow Measurement Uncertainties Appendix B of WCAP-14750 provides a sample elbow tap flow measurement safety evaluation (SE) for the Westinghouse-designed 3-loop PWRs. The sample SE includes a discussion of the uncertainty calculation and a proposed STS markup associated with using the elbow tap AP method. With the elbow tap methodology, the RCS elbow tap AP measurements are correlated to the precision calorimetric measurements performed during an earlier fuel cycle when the hot-leg streaming effects were minimal. The RCS flow measurement uncertainties include uncertainties associated with the following:
calorimetric measurement of the RCS total flow for the baseline cycle a
AP transmitters plant process computer indication for the current cycle RCS flow measurements using a
the cold-leg elbow taps The sample evaluation stated that the uncertainty calculation performed for this method of flow j
measurement is consistent with the methodology described in NUREG/CR-3659 (Ref.10), with the only significant difference being assuming correlation to previously performed RCS flow calorimetrics. This is accounted for by adding certain instrument uncertainties previously considered to be zerced out by the assumption of normalization to a calorimetric performed each cycle.
The calculations account for the plant instrumentation, test equipment, and procedures which were in place at the time the calorimuric was performed. The calculations also include additionalinstrumentation drift uncertainties to reflect the correlation between the elbow tap AP measurements and the calorimetric flow measurements. Uncertainty calculations are performed for the indicated RCS flow (computer) and the RCS low-flow reactor trip.
The uncertainty methodology of NUREG/CR-3659 uses a statistical uncertainty combination technique (i.e., those groups of components which are statistically independent are statistically combined, and those errors which are not independent are combined arithmetically to form independent groups, which can then be statistically combined). As the elbow tap AP measurements were correlated to the calorimetric measurements of the baseline cycles, the overall RCS flow measurement uncertainty is a statistical combination of the baseline cycle calorimetric measurement and elbow tap measurement uncertainties. This uncertainty calculation method has been accepted in connection with the ITDP and RTDP evaluations.
Appendix A of WCAP-14750 contains sample uncertainty calculations performed using Farley-specific inputs. The calculations include the following uncertainties:
BCF measurement instrumentation uncertalaties flow calorimetric sensitivities a
overall calorimetric flow measurement uncertainties cold-leg elbow tap flow rneasurement uncertainties for the process computer a
low-flow reactor trip uncertainties The staff evaluation of the Farley-specific uncertainty calculation is addressed in Section 2.3.2 of this SE.
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, 2.2.2. Modifications To Standard Technical Specifications
? Section 7.3 of WCAP-14750 recommends that, for the improved STS, the LCO and the surveillance requirements include two different DNB flow requirements - one to be used if a precision calorimetric measurement is performed and one to be used with an elbow tap AP measurement. The applicable Bases section will also require revisions to include a description of the elbow tap AP method of flow measurement. Attachment 1 to Appendix B of WCAP-14750_ provides a sample improved STS markup. TS Table 3.3.1-1, " Reactor Trip System Instrumentation," ltem 10, " Reactor Coolant Flow - Low" trip function will be modified with the trip setpoint and allowable value consistent with the safety analysis limit assumption and the elbow tap AP flow measurement uncertainty for the low-flow reactor trip, item c of
" LCO 3.4.1, and both SR 3.4.1.3 and 3.4.1.4 of the 12-hour and 18-month RCS flow verification will be modified to allow for the use of eithe'r the precision heat balance method or the elbow tap AP measurement method with a respective MMF limit specified for each method. Each of the limit values contains a flow measurement uncertainty calculated for the precision calorimetric measurement or the elbow tap AP measurement. Basis B 3.4.1 will also be modified to reflect -
the alternate methods of precision calorimetric and elbow tap AP measurements for the RCS flow verification and their respective measurement uncertainties. The staff finds these recommended TS modifications to be acceptable.
2.3 Farley implementation of Cold-leg Elbow Tap Flow Measurement Procedure 2.3.1 RCS Flow Performance Evaluation Section 6.0 of WCAP-14750 describes the evaluation of Farley, Units 1 and 2 RCS flow performance. For each unit, the evaluation includes the following:
determining the baseline cycle calorimetric flow (BCF) determining the elbow tap total flow coefficient (B) e evaluating the elbow tap flow ratio (R) based on the current cycle elbow tap flow coefficient calculated from tne elbow tap AP measurements A best estimate RCS flow prediction is also made based on the known hydraulic changes.
SNC provided the analytical model, including the RCS hydraulic network diagram, and
' component flow resistance values in response to a staff request for additional information (Question 3c, Ref. 7). ' The analyses determined the baseline cycle 1 initial startup flows of both units based on the baseline hydraulic designs. Hydraulic changes during subsequent cycles, including pump impeller smoothing, steam generator plugging, and fuel design changes, are modeled to determine best estimate flow rates of various cycles. The BE flow prediction is used to confirm the elbow tap measured RCS flow. The evaluation process follows the procedure described in Section 4.2 of WCAP-14750 and is, therefore, acceptable with the exceptions discussed in the following paragraphs.
Section 6.1 discussed the evaluation of the BCFs for Farley, Units 1 and 2. Tables 6.1 and 6.2, respectively, provide the Farley, Units 1 and 2, early cycle calorimetric flow measurement data and the calculated BCF values for Farley, Units 1 and 2, respectively. This BCF evaluation was I
based on the original BCF determination procedure described in Section 4.2 of WCAP-14750.
As discussed in Section 2.1.2 of this SE, this BCF determination procedure has been replaced with a reviseo procedure described in the licensee's response to Question 1, Reference 8. In 3,
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,. the same document, SNC also provided a revised calculation of the BCFs using the revised BCF procedure. SNC states in the footnote that applicable information in Section 6 and Appendix D of WCAP-14750, which summarizes the elbow tap measurement procedure, will be i
revised to reflect these changes. The staff has reviewed and found the revised BCFs for Farley, Units'1 and 2, to be acceptable.
2.3.2. Uncertainty Evaluation i
i Appendix A of WCAP-14750 contains uncertainty calculations which were performed using Farley-specific inputs. Tables A-1, A-2, and A-3, respectively provide the values of the baseline calorimetric flow measurement instrumentation uncertainties, flow calorimetric sensitivities, and
, calorimetric flow measurement uncertainties. Tables A-4 and A-5, respectively, provide the cold-leg elbow tap flow measurement uncertainties for the process computer and low-flow reactor trip uncertainties. The uncertainties for a calorimetric measurement or the elbow tap measurement consist of uncertainties from all components in the measurement channel.
These include non-instrument-related measurement errors (such as temperature stratification of a fluid in a pipe) and instrument-related errors (such as errors due to metering devices; calibration accuracles of sensors, process rack, and readout devices; drift; temperature and pressure effects, etc.). These uncertainty components are combined to derive a channel statistical allowance using the statistical combination technique consistent with the methodology described in NUREG/CR-3659 (Ref.10).
Table A-4 in WCAP-14750 shows an overall RCS flow uncertainty of 2.3 percent for the process computer. Table A-5 shows the calculated channel statistical allowance for the reactor trip function is lower than the total allowance of 4 percent flow span assumed for the low-flow reactor trip function. In response to a staff question (Question 3b, Ref. 7), SNC siserted that the uncertainty input values relative to 1) the reference accuracy,2) pressure and temperature effects,3) calibration accuracy for sensors and process racks, and 4) sensor and rack drift magnitudes, are 2a (standard deviation) or better. Therefore, the overall uncertainty for RCS flow utilizing the cold-leg elbow tap methodology and used for the RTDP analyses is a 95/95
' probability / confidence value.
SNC considered drift of the instruments and process racks. SNC did this since they do not normalize cold-leg elbow tap measurement against a precision heat balance flow measurement i
at the beginning of each fuel cycle. In response to a staff question (Question 5, Ref. 7), SNC explained that they included sufficient drift allowances in the instrument uncertainties shown in l
' Tables A-4 and A-5 for the surveillance interval. The staff concludes that this explanation is acceptable.
q 2.3.3 Farley Technical Specification Modifications i
The current Farley TS SR 4.2.5.2 requires cNC to determine that the RCS total flow is within its i
limit by measurement at least once per 18 months. The SR does not specify the method to measure RCS flow. TS Bases 3/4.2.5, "DNB Parameters," states that the 18-month surveillance of the total RCS flow rate is a precision measurement that verifies the RCS flow requirement at the beginning of each fuel cycle. Therefore, there is no need to change the -
Farley TS to implement RCS flow measurement using the cold-leg elbow tap AP measurements, except for modifying the Bases.
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. Attachment 1 to Appendix C of WCAP-14750 contains markups of proposed changes to Farley TS Bases 3/4.2.5. The changes reflect using cold-leg elbow tap AP measurement as an alternate method for the 18-month RCS flow surveillance. The change references WCAP-14750 regarding the method to correlate the flow indication channels with selected J
precision calorimetrics. In their response to Question 4 in Reference 8, SNC also revised the Bases to state that indicated total RCS flow rate is based on a measurement using two indication channels per loop and an uncertainty of 2.4 percent flow. The fiow uncertainty value, which includes 0.1 percent flow uncertainty for feedwater venturi fouling, is consistent with the calculated elbow tap flow measurement uncertainty of 2.3 percent shown in Table A-4 of WCAP-14750 for the process computer. The staff finds that the changes to the TS Bases are acceptable.
3.0 CONCLUSION
The staff has reviewed the cold-leg elbow tap RCS ficw measurement methodology, including the following items:
)
correlation of the indicated APs to the baseline calorimetric RCS flow rate the flow measurement uncertainty calculation a
proposed improved STS changes Farley-specific uncertainty calculation TS changes associated with implementing the elbow tap flow measurement Based on its review of the technical bases regarding the cold-leg elbow tap RCS flow measurement procedure and the measurement uncertainty calculation provided in SNC's submittal, the staff finds WCAP-14750 acceptable for SNC to reference in licensing actions.
The staff notes that this acceptance is based, in part, on the revisions described in the response to Question 1, Reference 8, regarding the procedures for determining the baseline calorimetric flow, as well as the BCF calculations for Farley, Units 1 and 2.
Principal Contributor: Y. Hsii Date: June 30, 1999 I
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REFERENCES 1.
Letter, D. N. Morey (Southern Nuclear Operating Company) to U.S. Nuclear Regulatory Commission, " Joseph M. Farley Nuclear Plant, Request for NRC Review of WCAP-14750,
'RCS Flow Verification Using Elbow Taps at Westinghouse 3-Loop PWRs',"
November 26,1996. (Proprietary and Non-Proprietary documents are available) 2.
WCAP-11397-P-A, " Revised Thermal Design Procedure," Westinghouse Electric Corporation, April 1989. (Proprietary and Non-Proprietary documents are available) i 3.
Letter from Victor Nerses (USNRC) to T. C. McMeekin (Duke Power Company), " Issuance of Amendments - McGuire Nuclear Station, Units 1 and 2, Reactor Coolant System (RCS)
Flow Rate measurement (TAC Nos. M88659 and M88660)," January 12,1995.
4.
Letter from R. E. Martin (USNRC) to D. L. Rehn (Duke Power Company), " Issuance of Amendments - Catawba Nuclear Station, Units 1 and 2 Reactor Coolant System (RCS)
Flowrate measurement (TAC Nos. M88480 and M88658)," February 17,1995.
5.
Letter, Thomas W. Alexion (USNRC) to William T. Cottle (STP Nuclear Operating Company)," South Texas Project, Units 1 and 2 - Issuance of Amendments, Re: Reactor Coolant System Flow Monitoring (TAC Nos. M99245 and M99246)," April 19,1999.
6.
" Fluid Meters, Their Theory and Application," 6th Edition, Howard S. Bean, ASME, New York,1971.
j 7.
Letter, D. N. Morey (Southern Nuclear Operating Company) to U. S. Nuclear Regulatory Commission, " Joseph M. Farley Nuclear Plant, Response to Request for Additional information Related to WCAP-14750, 'RCS Flow Verification Using Elbow Taps At Westinghouse 3-Loop PWRs'," February 2,1999. (Proprietary document is not publicly available) 8.
Letter, D. N. Morey (Southern Nuclear Operating Company) to U. S. Nuclear Regulatory Commission," Joseph M. Farley Nuclear Plant, Response to Request for Additional Information Related to WCAP-14750, 'RCS Flow Verification Using Elbow Taps At Westinghouse 3-Loop PWRs'," June 7,1999. (Proprietary and Non-Proprietary documents are available) 3.
Letter, D. N. Morey (Southern Nuclear Operating Company) to U. S. Nuclear Regulatory Commission," Joseph M. Farley Nuclear Plant, Response to Request for Additional Information Related to WCAP-14750, 'RCS Flow Verification Using Elbow Taps At Westinghouse 3-Loop PWRs'," October 1,1997.
- 10. NUREG/CR-3659, PNL-4973, "A Mathematical Model for Assessing the Uncertainties of Instrumentation Measurements for Power and Flow of PWR Reactors," February 1963.
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