05000260/LER-2001-002

LER-2001-002,

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2602001002R00 - NRC Website

NAME

Steve Austin, Licensing Engineer TELEPHONE NUMBER (Include Area Code) (256) 729-2070 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

CAUSE SYSTEM COMPONENT MANUFACTURER REPORTABLE TO

NPRDS x

• .s.

TO NPRDS

SUPPLEMENTAL REPORT EXPECTED (14) EXPECTED

YES

(If yes, complete EXPECTED SUBMISSION DATE).

X DATE (15)

NO

ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) (16) On July 2, 2001, TVA confirmed that a potential 10 CFR Part 21 condition related to the use of non- conservative parameters associated with calculation of Oscillation Power Range Monitor (OPRM) upscale trip setpoint was applicable to BFN. At approximately 1100 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.1855e-4 months <br /> Central Daylight Time the OPRM upscale trip function was declared inoperable. Technical Specification (TS) Actions 3.3.1.1.1.1, and 3.3.1.1.1.2 were entered on both Units 2 and 3. TS Action 3.3.1.1.1.1 requires initiation of alternate method to detect and suppress thermal-hydraulic instability oscillations within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Action 3.3.1.1.1.2 requires that the OPRM upscale trip be returned to service in 120 days. The OPRM upscale trip function has been inoperable on Unit 2 since May of 1999, and on Unit 3 since May of 2000. Hence, TVA is reporting this event pursuant to 10 CFR 50.73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's Technical Specifications. The root cause of this event is the original analysis performed by General Electric (GE) that defines the generic Delta Critical Power Ratio/Initial Versus Oscillation Magnitude curve was inadequate. GE identified a non-conservative deficiency for the high peak bundle power-to-flow ratios in the generic DIVOM curves. GE has proposed the development and licensing of a revised methodology for determining OPRM upscale trip setpoint. However, the revised methodology approval is not expected to meet the TS Action 120 day time limit. Subsequently, TVA proposed a TS change to delete TS Action 3.3.1.1.1.2. This change has been approved for Unit 2.

I. PLANT CONDITION(S)

was shutdown and defueled.

II. DESCRIPTION OF EVENT

A. Event:

On June 29, 2001, General Electric (GE) notified TVA of a potential 10 CFR Part 21 condition related to the use of non-conservative parameters associated with the calculation of Oscillation Power Range Monitor (OPRM) [IG] upscale trip setpoints. On July 2, 2001, based on additional information from GE, TVA confirmed that the condition was applicable to BFN and at approximately 1100 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.1855e-4 months <br /> Central Daylight Time (CDT), the OPRM upscale trip function was declared inoperable. Technical Specification (TS) Action 3.3.1.1.1.1, which requires, within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />, initiate alternate method to detect and suppress thermal-hydraulic instability oscillations, was entered. Required Action 3.3.1.1.1.2 further stipulates the return of the OPRM upscale trip to service within 120 days. If the 120-day completion time cannot be met the unit shall be in mode 2 in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />. TS Actions 3.3.1.1.1.1 and 3.3.1.1.1.2 were taken on both Units 2 and 3.

At approximately 1340 hours0.0155 days <br />0.372 hours <br />0.00222 weeks <br />5.0987e-4 months <br /> CDT the plant operators commenced performance of 3-SR-3.3.1.1.I, Core Thermal Stability for Unit 3 and at approximately 1400 hours0.0162 days <br />0.389 hours <br />0.00231 weeks <br />5.327e-4 months <br /> CDT, plant operators commenced performance of 2-SR-3.3.1.1.I, Core Thermal Stability for Unit 2. These surveillances ensure the reactor thermal power and flow are within appropriate parameter limits to prevent uncontrolled power oscillations. The surveillances were completed satisfactorily by 1410 hours0.0163 days <br />0.392 hours <br />0.00233 weeks <br />5.36505e-4 months <br />.

The OPRM module of the GE Power Range Neutron Monitoring (PRNM) [IG] was installed to satisfy TVA's long-term solution regarding reactor core stability. Browns Ferry has implemented the long-term stability solution designated as Option III in NEDO-31960, Supplement 1, "BWR Owners' Group Long-Term Stability Solution Licensing Methodology", the OPRM Upscale Trip function of the Power Range Neutron Monitoring PRNM. The trip function was enabled in May of 1999 on Unit 2 and in May of 2000 on Unit 3. The OPRM Upscale Trip function provides protection from exceeding the fuel MCPR Safety Limit in the event of thermal-hydraulic power oscillations.

Because this condition has existed on Unit 2 since May of 1999, and on Unit 3 since May of 2000, TVA is reporting this event in pursuant to 10 CFR 50.73(a)(2)(i)(B), as any operation or condition which was prohibited by the plant's Technical Specifications.

B. Inoperable Structures, Components, or Systems that Contributed to the Event:

None.

FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) C. Dates and Approximate Times of Major Occurrences:

June 29, 2001 July 2, 2001, at 1100 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.1855e-4 months <br /> CDT GE notified TVA of a potential 10 CFR Part 21 condition related to the use on non- conservative parameters associated with the calculation of the OPRM upscale trip points.

Based on additional information from GE, WA concluded that the 10 CFR Part 21 condition was applicable to BFN and entered the required TS Action.

D. Other Systems or Secondary Functions Affected

None.

E. Method of Discovery

GE notified TVA of possible issues involving the calculations used to determine the OPRM upscale trip set points.

F. Operator Actions

None.

G. Safety System Responses

None.

III. � CAUSE OF THE EVENT

A. Immediate Cause

Reload safety analysis may not be adequate to protect against violation of the Safety Limit Minimum Critical Power Ratio (MCPR) for a limiting instability event.

B. Root Cause

The original analysis performed by GE that defines the generic DIVOM curve was inadequate.

GE identified a non-conservative deficiency for the high peak bundle power-to-flow ratios in the generic Delta Critical Power Ratio/Initial Versus Oscillation Magnitude (DIVOM) curves specified in NEDO-32464-A, Reactor Stability Detect and Suppress Solutions Licensing Basis Methodology for Reload Applications, August 1996. Specifically, GE evaluations have shown that the generic DIVOM curves are not always conservative for plants which have implemented Option III stability trip system.

C. � Contributing Factors None.

IV. � ANALYSIS OF THE EVENT Browns Ferry has implemented the long-term stability solution designated as Option III in NEDO-31960, Supplement 1, "BWR Owners' Group Long-Term Stability Solution Licensing Methodology" the OPRM Upscale Trip function of the Power Range Neutron Monitoring PRNM. The trip function was enabled in May of 1999 on Unit 2 and May of 2000 on Unit 3. The OPRM Upscale Trip function provides protection from exceeding the fuel MCPR Safety Limit in the event of thermal-hydraulic power oscillations.

The OPRM is a subsystem of the Average Power Range Monitor system (APRM) which is designed to detect reactor core thermal hydraulic instability and provide readout, alarms, and trips associated with an instability event. The OPRM receives input signals from the Local Power Range Monitors (LPRMs) [IG] within the reactor core. An Upscale Trip is initiated if oscillatory changes in the neutron flux are detected.

The Upscale Trip function is required to be operable when the plant is in a region of power-flow operation where actual thermal-hydraulic oscillations might occur. When the reactor is operating in regions of the power/flow map, where it has been determined that unstable power oscillations cannot occur, the OPRMS trip outputs are automatically bypassed. Each OPRM channel provides an oscillation trip enable alarm that indicates when the reactor has entered the operating region where instability can occur and the trip is no longer bypassed.

A cycle specific Option III stability analysis is performed for each reload core to determine the appropriate OPRM setpoint. The analysis considers both steady state startup operation and the case of a two recirculation pump trip from rated power. The resulting stability based Operating Limit MCPRs as a function of OPRM setpoint are reported in the Supplemental Reload Licensing Report. The actual OPRM setpoint is selected such that required margin of the MCPR Safety Limit is provided without stability being a limiting event.

If the OPRM trip function should become inoperable, alternate methods to detect and suppress oscillations may be implemented for a limited period of time in accordance with the TSs. Procedures require the use of the control room instrumentation and process computer [ID] outputs to determine the operating conditions to be plotted on the power flow map. By plotting the reactor power versus core flow, the operator can demonstrate reactor power is not operating under conditions which may lead to power oscillations.

V. � ASSESSMENT OF SAFETY CONSEQUENCES Operating procedures do not allow operation of BFN in regions of the Power Flow Map where thermal-hydraulic instability are known to occur. An abnormal operating event, such as an inadvertent dual reactor recirculation pump [AD] runback would have to occur to enter the region. If such an event were to occur, procedures dictate required actions.

Based on the above, and considering that the OPRM would have to suppress instability oscillations during a very limited set of circumstances, it is concluded that there is no adverse impact on safety as a result of this event.

VI. CORRECTIVE ACTIONS

A. Immediate Corrective Actions

TVA declared the OPRMs for Unit 2 and Unit 3 inoperable and the TS required actions were implemented including an alternate method of detecting and surpressing thermal-hydraulic oscillations. Although inoperable, the OPRM upscale trip capacity is functional and continues to provide a measure of automatic protection even though it does not meet the licensing criteria for MCPR safety limit protection.

B. Corrective Actions to Prevent Recurrence

As a result of the issues leading to the noncompliance described in this report, GE will issue interim revised setpoint calculations for plants which have declared the OPRM inoperable. TVA will evaluate these revised setpoints for implementation at BFN1. GE has proposed the development and licensing of a revised methodology for determining OPRM upscale trip setpoint. However, GE's proposal is expected to exceed the 120 day time limit allocated in TS Action 3.3.1.1.1.2.

To address TS Action 3.3.1.1.1.2, return the OPRM upscale trip to service within 120 days, TVA proposed a TS change to delete this requirement. TVA has received the NRC Safety Evaluation that deletes TS Action 3.3.1.1.1.2 for Unit 2, and anticipates receiving the Unit 3 Safety Evaluation prior to expiration the TS Action 3.3.1.1.1.2 time limit.

A. Failed Components

None.

B. Previous LERs on Similar Events None.

C. Additional Information

None.

1. � This action is not a regulatory commitment.

D. Safety System Functional Failure:

This event did not result in a safety system functional failure in accordance with NEI 99-02.

VIII. COMMITMENTS

None.

Energy Industry Identification system (EIIS) system and component codes are identified in the TEXT with brackets (i.e., [XX]).