ML18039A644

Discusses Results of Online Testing Conducted in Preparation for Enabling Sys.Encl 1 Provides Discussion of Oscillation Power Range Monitor Sensitivity Characteristics,Proposed Setpoint Range Revs & Associated Justifications

ML18039A644
Person / Time
Site:	Browns Ferry
Issue date:	12/15/1998
From:	ABNEY T E TENNESSEE VALLEY AUTHORITY
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
TVA-BFN-TS-354, NUDOCS 9812230196
Download: ML18039A644 (20)

Text

REGULAT~Y INFORMATION DISTRIBUTIOh YSTEM (RIDS)ACCESSION NBR:9812230196 DOC.DATE: 98/12/15 NOTARIZED:

NO DOCKET¹FACIL:50-260 Browns Ferry Nuclear Power Station, Unit 2, Tennessee 05000260 AUTH.NANP~~

'UTHOR AFFILIATION ABNEY,.T.E.

Tennessee Valley Authority RECIP.NAME RECIPIENT AFFILIATION Records Management Branch (Document Control Desk)

SUBJECT:

Discusses results of online testing conducted in preparation for enabling sys.Encl 1 provides discussion of oscil'lation power range monitor sensitivity characteristics, proposed setpoint range revs&.associated justifications.

DISTRIBUTION CODE: D030D COPIES RECEIVED:LTR ENCL SIZE: TITLE: TVA'acilities

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~i~I 7 dl Tennessee VaIley Authority, Post Office Box 2000, Decatur, Alabama 35609-2000 December 15, 1998 TVA-BFN-TS-354 U.S.Nuclear Regulatory Commission ATTN:.Document Control Desk Washington, D.C.20555 Gentlemen; In the Matter of Tennessee Valley Authority 10 CFR 50.4 Docket No.SO-260 BROWNS FERRY NUCLEAR PLANT (BFN)-UNIT 2-TECHNICAL SPECIFICATIONS (TS)CHANGE 354-OSCILLATION POWER RANGE MONITOR (OPRM)-RESULTS OF ONLINE TESTING On September 8, 1998, TVA submitted a change request to Unit 2 TS (TS-354)for enabling the OPRM for the next Unit 2 operating cycle (Spring 1999).This submittal provides the results of the online OPRM system testing conducted in preparation for enabling the system.These results are provided to facilitate staff review of TS-3S4.The OPRM system is part of the Power Range Neutron Monitoring (PRNM)system and was installed for trial operation on Unit 2 in September 1997 for Cycle 10 and on Unit 3 in September 1998 for Cycle 9 operation.

The OPRM module is designed to satisfy the TVA long-term solution r'egarding reactor stability referred to as Option III in NED0-31960, Supplement 1,"BWR Owners'roup Long;Term Stability.Solution Licensing Methodology." As discussed in the TS submittals for, the PRNM system installation (

Reference:

TS-353S1 submitted on April 11, 1997, and supplemented on March 13, 1998), the OPRM module is being operated in the"indicate only" mode for testing purposes during the current Unit 2 and 3 operating cycles.The purpose of this testing is to evaluate the adequacy of OPRM setpoint values and margins using the methodology described in Licensing Topical Report (LTR)NED0-32465-A,"Reactor Stability Detect and Suppress Solutions Licensing Basis Methodology for Reload Applications." 98l2230%96 98i2i5 t,~'PDR'DOCK 05000260," p PDR.i.'.

I~I C U.S.Nuclear Regulatory Commission Page 2'ecember 15, 1998 The subject OPRM testing is complete for Unit 2 and TVA is submitting the enclosed test conclusions for NRC Staff review in support of enabling the OPRM during the upcoming Unit 2 operating cycle.The online test data indicates that use of the least sensitive OPRM corner frequency and period tolerance setpoints specified by LTR NEDO-32465-A would result in a system that is too sensitive and, hence, susceptible to spurious alarms and trips.Also, TVA is lowering the maximum period setpoint to further reduce the chance of OPRM spurious alarms and trips.Based on BFN test results, an increased range for allowable corner frequency, ,period tolerance, and maximum period is presented and justified in this submittal.

These settings should substantially reduce the chance of spurious trips and alarms while allowing the OPRM to detect core wide and regional instabilities.

It is TVA's understanding that Plant Hatch has found it necessary to modify the corner frequency and period tolerance setpoints in a similar manner.Enclosure 1 provides a discussion of OPRM sensitivity characteristics, proposed setpoint range revisions, and associated justifications for deviation from LTR NEDO-32465-A values.General Electric (GE)has reviewed and concurred with the proposed setpoint range changes.Enclosure 2 contains a sampling of Unit 2 and 3 OPRM count data at various OPRM settings to demonstrate the margin to spurious alarms and trips when the revised OPRM setpoint ranges are utilized.As previously committed, BFN plans to fully enable the OPRM function following the next Unit 2 refueling outage which is scheduled to begin April ll, 1999.With the activation of the OPRM, TVA considers that commitments in the response (submitted September 9, 1994)to Generic Letter 94-02, Long-Term Solutions and Upgrade of Interim Operating Recommendations for Thermal-Hydraulic Instabilities in Boiling Water Reactors, are fulfilled for Unit 2.For Unit 3, activation of the OPRM will follow after the current cycle of operation.

Unit 3 OPRM TS will be submitted separately.

The test results provided in this submittal apply to both Units 2 and 3.In the referenced TS-354 submittal, TVA requested that the revised Unit 2 TS be approved by March 1, 1999, and be made effective by the end of the Unit 2 Cycle 10 outage (currently scheduled May 5, 1999).This schedule will allow enabling of the OPRM for cycle 11 operation.

U.S.Nuclear Regulatory Commission Page 3'ecember 15, 1998 If the Staff has.any questions concerning this submittal, please contact me at (256)729-2636 for assistance.

S'ncerely, ney Manager of'and Ind stry so.ng ffairs Enclosures cc (Enclosure Mr.Harold O.Christensen, Branch Chief U.S-.Nuclear Regulatory Commission Region II 61 Forsyth Street, S.W.Suite 23T85 Atlanta, Georgia 30303 NRC Resident Inspector Browns Ferry Nuclear Plant 10833 Shaw Road Athens, Alabama 35611 Mr.L.Raghavan, Project Manager U.S.Nuclear Regulatory Commission One White Flint, North 11555 Rockville Pike Rockville, Maryland 20852

ENCLOSURE 1 TENNESSEE VALLEY AUTHORITY BROWNS FERRY NUCLEAR PLANT (BFN)UNIT 2 AND 3 OPRM CORNER FREQUENCY~'PERIOD TOLERANCE I AND MAXIMUM PERIOD PROBLEM D I SCUSS ION I PROPOSED SETPOINT RANGE REVIS IONS AND ASSOCIATED JUSTIFICATIONS BACKGROUND INFORMATION LTR NED0-32465-A,"Reactor Stability Detect and Suppress Solutions Licensing Basis Methodology for Reload Applications," describes the analytical basis for the Option III long term stability solution.The licensing basis for this solution is the period based algorithm (PBA)which relies on the fact that the local power range monitors (LPRMs)can be used to distinguish between thermal/hydraulic instabilities and stable reactor operation.

During normal, steady-state, reactor operation, LPRM signals are comprised of a broad range of frequencies that are typically present in a boiling water reactor (BWR).These LPRM signals become more coherent displaying a characteristic frequency in the 0.3 to 0.7 Hertz (Hz)range with the onset of thermal/hydraulic instability.

The PBA uses this difference in LPRM signal coherence to detect instabilities.

Specifically, the OPRM combines signals from four LPRMs per OPRM cell and determines each successive pair of OPRM cell maxima and minima.If the maxima/minima have a frequency in the range of 0.3-0.7 Hz, the oscillation is considered to be a single period confirmation.

The PBA then evaluates a subsequent maxima/minima to determine if it falls within the specified frequency range.Xf so, the PBA continuous period confirmation (CPC)counter is increased by one.This process continues until a maxima/minima is'found to be outside the specified frequency range at which time the CPC counter is reset because the maxima/minima frequency is no longer indicative of instability.

The CPC count prior to resetting is termed the maximum continuous period confirmation (MCPC)count.

LTR NEDO-32465-A (Section 3.4.1)describes the acceptable range of values for two OPRM parameters, period tolerance (c)and corner frequency (f).Both of these parameters can be independently adjusted to tune the OPRM to each plant's unique LPRM noise characteristics.

Within the ranges defined for these parameters, the OPRM will provide sufficient CPCs to detect thermal/hydraulic instabilities prior to reaching the PBA amplitude trip setpoint (S).The ranges presented in LTR NEDO-32465-A were based upon testing the PBA using data taken with analog LPRM processing hardware for single LPRM signals from several different plants.Data was taken with a 50 millisecond sample rate during stable and unstable reactor operation.

A range for each OPRM setpoint value was defined to ensure that the OPRM is sensitive enough to detect an instability as it develops at low amplitudes while allowing utilities the flexibility to adjust the OPRM response to their plant's noise characteristics during steady-state operation.

The adjustments to account for noise characteristics are necessary to avoid spurious alarms and reactor scrams.Normal operational LPRM signals are viewed by the OPRM as a distribution of MCPCs.The OPRM is tuned based upon the MCPC distribution under plant operating conditions which have significant stability margin (i.e., near or at rated conditions).

Based upon tuning criteria proposed by GE, the BFN OPRM selected setpoints as discussed below provide more than adequate sensitivity.

BFN-SPECIFIC INFORMATION Based on OPRM data collected during Unit 2 Cycle 10 and Unit 3 Cycle 9, it is apparent that the OPRM is too sensitive when the least sensitive setpoints defined in Table 3-1 of LTR NEDO-32465-A are used (i.e., period tolerance of 100 milliseconds and corner frequency of 2.5 Hz).However, the design of the PRNM system allows the OPRM period tolerance and corner frequency to be set to less sensitive values than those defined in the LTR.BFN testing indicates that the OPRM more closely meets the GE tuning criteria under normal operating conditions if a period tolerance of 50 milliseconds and corner frequency of 3.0 Hz is utilized.The following factors contribute to the OPRM function being more sensitive than originally anticipated'or the BFN installation:

(1)The plant data used to develop the OPRM detection algorithm had a sample frequency of 50 milliseconds.

The BFN PRNMS provides LPRM sample data every 25 milliseconds.

This sampling rate tends to increase OPRM sensitivity.

E1-2 4l C' (2)BFN Unit 2 and 3 noise characteristics differ from those of the reference plants used to test the detection algorithm.

Specifically, the PRNM.system has improved accuracy, noise immunity, and LPRM signal filtering.

The additional LPRM filtering tends to increase OPRM sensitivity thus producing higher MCPC counts when the plant is operating with a large stability margin.The maximum oscillation period (T)is the largest expected period which the OPRM would sense if a reactor instability was present.For example, if the time between successive LPRM signal maxima/minima is greater than T , the oscillation is not indicative of an anticipated reactor instability.

LTR NEDO-32465-A (Section 4.3.2.4)states that studies of actual instability events indicate that the expected period is approximately 1.8 to 2.0 seconds.However, it is desirable to consider an oscillation frequency range between 0.3 and 0.7 Hz for conservatism.

This corresponds to a T value ofŽ3.30 seconds.The OPRM design allows this parameter to be set in the range of 3.0 to 5.0 seconds.A review of the online test data indicates that Tshould be set at its lower design limit of 3.0 seconds to further ensure adequate sensitivity while balancing the need to avoid spurious OPRM alarms and trips.Based on LTR NEDO-32465-A (Figure 4-5), setting Tequal to 3.0 seconds does not significantly alter the probability of detecting core instability while helping to minimize the possibility of spurious OPRM alarms and trips.CONCLUSION The OPRM is fully expected to produce enough MCPCs to exceed the alarm and trip setpoints (N, and N~)if a thermal/hydraulic instability should occur.Allowing TVA to set the corner frequency, period tolerance, and maximum period down to 3.0 Hz, 50 milliseconds, and 3.0 seconds, respectively, provides acceptable OPRM sensitivity based upon the foregoing discussions.

These setpoint values are slightly outside the ranges described in LTR NEDO-32465-A which were based on data from a few plants with different power monitoring system designs.The proposed setpoint range changes provide margin to spurious alarms and trips during stable reactor operation and do not compromise the ability of the OPRM to detect instabilities and initiate an automatic reactor scram prior to violating the minimum critical power ratio (MCPR)safety limit.El-3 0

ENCLOSURE 2 TENNESSEE VALLEY AUTHORITY BROWNS FERRY NUCLEAR PLANT (BFN)UNIT 2 AND 3 SAMPLE OPRM COUNT'ATA II~I IP U2 OPRM DATA (NON-POWER UPRATE CONDlTIONS)

OPRM Core Core'Chan Pow'er Flow Corner Peiiod Tmax Freq Toler Continuous Period Confirmation CPC Counts.()6 7 8 9 10 3 1 1 1 1 1 1 100 100 100 100 100 100 95 95 95 95 95 95 1.0 1;0 1 2 1.2 1.4 1.4 3.0 3.0 3.0 3.0 3;0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 50 50 50 50 50 50 8520 8425 5596 5218 2962 2854 1917 2046 1189 1298 721 702 397 437 197 254 47 47 48 64 10 6 11 9 1 128 33 98 34 1 2 3 4 1 2 3 1 2 3 4 1 2 3 1 2 3 1 2 3 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 100 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 95 1.4 1.41.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1~4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1~4 3.0 3.0 3.0 3;0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3.0 3,0 3.0 3.0 3.0 3.0 3.0 2.0 2.0 2.0 2;0 2.0 2.0 2.0 2.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 3.0 3'3;0 3.0 3.0 3.0 3.0 3.0 100 100 100 100 50 50 50 50 100 100 100 100 50 50 50 50 100 100 100 100 50 50 50 50 4065 4045 3980 3960 4878 4955 4916 4797 3392 3446 3403 3396 4029 3986 3894 3826 2973 2976 2908 2883 3404 3530 3362 3253 1461 1467 1446 1399 1141 1093 1114 1150 1147 1186 1174 1117 878 922 924 925 1014 1015 982 933 769 699 685 691 183 170 175 172 98 82 98 92 140 111 116 142 98 94 84 64 82 104 117 86 46 61 61 63 66 55 52 43 5 20 23 19 54 44 50 31 9 1.2 14 8 26 29 33 21 10 15 9 12 12 10 15 13 2 0 1 13 10 13 6 0 2 3 1 1 6 5 6 1 4 1 1 2 6 1 jjz,'jg$(giCrSi'(}'irkjgjiQ izg E2-2 Cl U3 OPRM DATA (POWER UPRATE CONDITIONS)

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