ML20080D985
| ML20080D985 | |
| Person / Time | |
|---|---|
| Site: | Oyster Creek |
| Issue date: | 12/29/1994 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20080D949 | List: |
| References | |
| NUDOCS 9501090296 | |
| Download: ML20080D985 (7) | |
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f WASHINGTON, D.C. 20066 4u01 SAFETY EVALUATION BY-THE'0FFICE OF NUCLEAR REACTOR REGULATION OYSTER CREEK TECHNICAL ~ SPECIFICATION REQUEST - CORE STABILITY REPLACEMENT OF EXISTING RECIRCULATION FLOW MONITORING ELECTRONICS GPU NUCLEAR CORPORATION i
OYSTER CREEK NUCLEAR GENERATING STATION DOCKET NO. 50-219
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1.0 INTRODUCTION
By letter dated October 9,1991, as supplemented April 27 1994, GPU Nuclear Corporation (GPUN/the licensee) requested an amendment to Facility Operating License No. DPR-16 to change the Oyster Creek Nuclear Generating Station (OCNGS) technical specifications (TSs) to establish additional requirements i
for the availability of Local Power Range Monitors (LPRMs) associated with the Average Power Range Monitor (APRM) system. The purpose of this change is to restrict the allowable number of out-of-service LPRM/APRM detectors to ensure the ability to detect and suppress power oscillations prior to exceeding'the i
Minimum Critical Power Ratio (MCPR) safety iimit. This request also identifies a lower bound MCPR operating limit for each cycle as identified in the Core Operating Limits Report (COLR).
t Additionally, by letter dated March 9,1994, the licensee submitted their planned modification to utilize Foxboro Specification 200 electronics in place of the existing recirculation flow monitoring electronics. The flow electronics are used to bias the APRM setpoinc for the reactor power instability trip.
2.0 DISCUSSION The APRM system consists of electronic equipment that averages the output signals from selected incore LPRM amplifiers and develops an output signal representative of the rated core thermal power. These LPRM signals are grouped together such that the resulting APRM signals provide coverage of expected power oscillations. The trip units associated with the APRM system actuate an automatic protective action when APRM signals exceed preset flow-biased values.
The APRM system consists of eight independent channels - two channels per core quadrant. Channels 1 through 4 are associated with Reactor Protection System (RPS) #1, and channels 5 through 8 are associated with RPS #2.
Each core quadrant is monitored by two APRM channels, each of which is associated with a different RPS. The two APRM channels in a given core quadrant utilize the 9501090296 941229 PDR ADOCK 05000219 P
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same four LPRM detector strings with RPS #1 APRM channel receiving inputs from A and C LPRM detectors, and RPS #2 APRM channel receiving inputs from B and D LPRM detectors.
Each APRM channel normally averages the inputs of eight LPRM detectors.
The quadrant-based APRM system provides automatic reactor protection by generating a flow-biased reactor trip signal based on the output of the individual APRM channels. At least one APRM channel in each of the two RPS must have a high flux or inoperative trip condition to produce a full reactor scram.
As a result of aging, the existing recirculation (recirc) flow monitoring electronics for determining flow biasing, have exhibited poor drift characteristics, calibration problems, and a lack of spare parts. These problems have reduced the reliable operating life of the electronics to less than 24 months.
Consecuently, the licensee proposes a modification to replace the existing eltetronics with state-of-the-art hardware manufactured by 1
Foxboro. This replacement includes 10 new flow transmitters (2 in each of the 5 recirc loops), and new electronics in the associated control room panels.
Control room equipment being replaced includes the 2 transmitter power supplies, square root converters for each flow transmitter, 4 summers, and the APRMs (2 flow converters and 2 power supplies).
Each division of the new recirc flow electronics converts the differential pressure (dp) in the five recirc loop venturis into an equivalent flow using a square root function.
Flow signals from Division 1 are used by the plant computer; signals from Division 2 are indicated on a control room panel.
The total flow in each division is calculated from the sum of the five division flows. The Division 1 total flow signal is provided to:
1) the flow recorder on Panel 3F, 2) the Division 2 flow converter (through an isolator) for comparison with the Division 2 total flow signal, and 3) the APRM 1, 2, 3 and 4 trip bias units.
The Division 2 total flow signal is provided to:
1) the total flow indicator on Panel 4F, 2) the Division I flow converter (through an isolator) for comparison with the Division I total flow signal, and 3) the APRM 5, 6, 7 and 8 trip bias units.
The electronics in each division provide the following trip functions:
1)
Voscale Trio - This half scram function is designed to initiate on high flow.
This trip also results in a r 1 block, illumination of the UPSCALE and INOP lights on the flow converter module, and actuation of the APRM FLO BIAS OFF NORMAL annunciator alarm. The following conditions initiate an Upscale Trip:
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Total flow 1114% i 1% rated flow, Loss of power (trip is initiated but. status lights.do not
-illuminate due to power loss).
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Comoarator Trio - This rod block function is designed to detect a.
mismatch between divisions. This trip also results in the illumiaation of the COMP and INOP lights on the flow converter module, and actuation of the APRM FLO BIAS OFF NORMAL annunciator alarm. The following conditions initiate a Comparator Trip:
a Flow mismatch between divisions 210% 21% rated flow, Loss of power-(trip is initiated but status lights do not-illuminate-due to power. loss).
3)
Inon Trio - This scram function results in the same actions as the Upscale trip with the exception of the trip status indication.' If power is lost, the Inop light is the only indicator that illuminates due.to an Inop trip. An Inop Trip is initiated when the total flow voltage signal is below the 2.5v zero-flow level. This is an indication of a power supply or module failure.
The Upscale and Comparator Trips reset automatically when flow conditions return to normal. The status of the trips indicated at the flow converter module must be manually reset. The above trip functions i
can be tested using internal calibration signals.
Each division of the Foxboro electronics consists of two nests. Each nest i
contains an individual nest power supply. A power supply failure in the lower i
nest results in a fail safe Upscale and Comparator trip when the relays-deenergize. A total flow voltage signal (2.5 - 12.5V) from the upper nest is monitored by an alarm module in the lower nest.
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The enhanced system uses sensors that have the square root function incorporated into the flow transmitters, which reduces the number of modules' required in the control room. The new transmitters also allow simplified calibration of the control room electronics.
Test blocks have been added to the Foxboro electron:cs where needed to facilitate surveillance and calibration using external-test signals.
1 Additionally, the total flow signal between the divisions is provided through I
isolators to ensure separation between the two RPS divisions.
3.0 EVALUATION i
The staff's evaluation of the requested changes is discussed in this section.
The requested changes consist of TS changes that address operability of the i
LPRM/APRM system, and the previously described upgrade of the recirculation flow electronics that will improve the reliability of the flow biased APRM trip function.
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-i 3.1 Technical Soecification Chanaes The possibility of power oscillations caused by thermal-hydraulic instabilities in BWRs and the consequences of such events are addressed in Gentric Letter 86-02, "Long-Term Solutions to Thermal-Hydraulic Instabilities in Boiling Water Reactors," which requested licensees to examine each core reload and to impose operating limitations, as appropriate, to ensure t
compliance with General Design Criteria (GDC) 10 and 12. GDC 10 requires that the reactor core be designed with appropriate margin to assure that specified fuel ' design limits will not be exceeded during any_ condition' of normal operation, including the effects of anticipated operational occurrences.. GDC 12 requires assurance that power oscillations that can result in conditions that exceed specified acceptable fuel design limits are either not possible or can be reliably and readily detected and suppressed.
As a result of core-wide oscillations at a Boiling Water Reactor (BWR) in the U.S., the NRC staff questioned the adequacy of previous BWR core stability analyses and the ability of existing systems to detect and suppress large magnitude oscillations prior to violation of fuel design limits. More recent analyses performed by General Electric have demonstrated that for large magnitude oscillations, the potential exists for violation of the safety limit MCPR.
In response to this concern, GPUN performed plant-specific analyses to assess the capability of the OCNGS APRM system to respond to power.
oscillations.
The licensee requested TS changes to improve the availability of_ the LPRM/APRM detectors to detect and suppress power oscillations prior to exceeding core fuel design limits. The changes provide a more stringent requirement onthe availability of the LPRMs associated with the APRM system by restricting the allowable number of out-of-service LPRM/APRM monitors on the A and B levels.
Additionally, the changes provide for a minimum operating limit critical power i
ratio to address core stability concerns.
Section 3.1.B.1 of the existing OCNGS TS defines the minimum number of APRM channel inputs required to permit accurate average core power monitoring.
Specifications 3.1.B.2 and 3.1.C.1 further define the' distribution of the I
operable chambers to provide monitoring of local power changes that might be caused by a single rod withdrawal. TS Section 3.10.C identifies requirements associated with the MCPR during steady state power operations.
The licensee determined that the APRM channel response is more sensitive to the availability of the A and B level LPRM detectors (bottom half of the core) than to the C and D level LPRM detectors (top half of the core). APRM channel response is significantly improved if it can be assumed that at least one channel (per RPS system) responding to regional oscillations has no more than one A level (or B level) detector out of service. The. licensee's TS change request for TS Sections 3.1.B.1, 3.1.B.2 and 3.1.C.1 places additional and more restrictive operability requirements on the number of allowed I
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' out-of-service LPRM/APRM detectors on the A and B levels. The licensee states that this will ensure the availability of LPRM/APRM detectors -in order to provide a sufficient response to global as well as regional oscillations to prevent violation of the MCPR limit.
The proposed change to TS Section 3.10.C places a lower bound on the MCPR for each cycle, as identified in the COLR. The lower bound limit is required to provide sufficient margin to ensure the MCPR limit is not exceeded during global and regional power oscillations. The new proposed limit shall be greater than or equal to 1.47.
The proposed TS changes requested by the licensee address the requirements of GDC 10 and 12, and the guidelines of GL 86-02 for detection and suppression of power oscillation and are, therefore, acceptable.
3.2 Enhanced Recirculation Flow Electronics The enhanced APRM electronics are part of the safety-related Class IE plant protection system.
Therefore, the staff reviewed the system design against the applicable GDC and IEEE Standards for safety-related instrumentation and control systems as indicated in the Standard Review Plan (SRP), NUREG-0800.
To ensure that the enhanced APRM system will perform its intended function (s) under accident conditions, the staff reviewed the equipment for (1) independence, (2) environmental qualification, (3) seismic qualification and (4) maintenance and testing.
3.2.1 Independence The staff reviewed the equipment design using the criteria of IEEE Standard 384-1981, "IEEE Standard Criteria for Independence of Class IE Equipment and Circuits." The safety-related components receive power from dedicated Class IE power supplies, and interface with non-Class IE equipment through qualified isolation devices.
The staff determined that the methods of isolating the Class IE components from the non-Class IE components ara consistent with IEEE Standard 384-1981 and are, therefore, acceptable.
3.2.2 Environmental Oualification The licensee used IEEE Standard 323-1974, " Standard for Qualifying Class lE Equipment for Nuclear Power Generating Stations" to qualify the new APRM equipment to the same environmental limits specified for tht. equipment being replaced.
The new equipment was designed for use in areas that have a mild environment.
The equipment is located in an environmentally qualified zone that is considered a mild environment during normal operations. The equipment is not required to operate during conditions that produce a harsh environment.
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' The new olectronics are qualified for 104*F (which includes an estimated 19'F tempergture rise in the cabinet),14.7 psia,100% relative humidity, and 7.9X10 Rad. These conditions bound the environmental conditions for the locations in which the equipment is to operate. Consequently, the staff finds that the environmental qualification of the equipment meets the intent of IEEE Standard 323-1974 and is, therefore, acceptable.
3.2.3 Seismic Oualification The APRM modification replaces the existing flow transmitters and control room electronics with equipment that the licensee has committed to qualify as seismic Category 1.
Supports and mounting of this equipment will be in accordance with the Seismic Qualification Utilities Group (SQUG) Generic Implementation Procedure (GIP) for seismic verification of nuclear plant equipment. The staff finds the seismic qualification to be acceptable.
3.2.4 Periodic Maintenance and Site Acceptance Testina The control room electronics i. iae Foxboro system do not require changes in access or space allocations. Al ess to this equipment requires removal of a transparent cover plate that serves to protect the equipment.
Routine preventative maintenance activities will be performed in accordance with the licensee's maintenance procedures, which have been prepared in accordance with the manufacturer's recommendations.
The new recirc flow monitoring system will undergo site acceptance testing to verify operability. This will include simulation of inputs and verification of outputs. The testing will include the following:
1)
Verification of proper flow signals (individual and total loop flow) based on actual or simulated flow input signals, i
2) verification of proper flow indication (flow indicators, recorder, and plant computer),
3) verification of proper trip functions at the appropriate setpoints and upon loss of power or downscale indication (Inop), and 4) proper operation of the flow biased rod block and scram functions within the APRMs.
i The staff finds the scope of the periodic maintenance and site acceptance test activities to be in accordance with the guidelines of the Standard Review Plan and, therefore, acceptable.
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4.0 CONCLUSION
Based on the above, the staff concludes that the design changes related to the upgrade of the existing LPRM/APRM equipment, and the associated changes to the TSs to address core power oscillations meet the requirements of GDC 10 and 12 for control of core power oscillations, and the criteria of IEEE 384, and 323 and Seismic Qualification Utility Group (SQUG) procedures for independence, environmental qualification and seismic qualification and are, therefore, acceptable.
Principal Contributor:
M. Waterman Date: December 29, 1994 9
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