05000251/LER-2006-002

LER-2006-002, Intermediate Range High Flux Trip Setpoint Exceeded Technical Specification Allowable Value

Docket Number
Event date:	12-04-2006
Report date:	3-29-2007
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
2512006002R00 - NRC Website

FACILITY. NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Turkey Point Unit 4 05000251 2006 -� 002 00

DESCRIPTION OF THE EVENT

On December 4, 2006, prior to Low Power Physics Testing on Unit 4 for Cycle 23, the intermediate range detector [IG, DET] setpoints for amps equivalent to 25% power trip and 20% rod stop were questioned and found to be non-conservative for N-35.

Subsequent investigation determined that the event occurred when the Intermediate Range (IR) detector was changed from a model using an aluminum housing to one using a titanium housing in the refueling outage prior to Unit 4 Cycle 22. The new detector was installed and new setpoints were calculated. Plant engineering personnel failed to recognize that the installed setpoint value exceeded the Technical Specification (TS) Table 2.2-1 Allowable Value of 31%.

Unit 4 was in Mode 3 at the time of discovery in preparation for low power physics testing. Operations requested verification that the installed Intermediate Range trip setpoints were set to less than or equal to 25% power. A reactor engineer reviewed operating data from Cycle 22, expecting to find that the installed setpoints had been less than 25% of the measured current at 100% power. Instead, it was discovered that the installed setpoint for N-35 had been 34.6% of the 100% power current. Since this non-conservative setpoint had been used as an input to the Cycle 23 setpoint, it was determined that the Cycle 23 setpoint was also non-conservative.

BACKGROUND

Turkey Point has transitioned from aluminum housings to titanium housings for the Nuclear Instrumentation System (NIS) source range/intermediate range detectors to provide better corrosion resistance.

CAUSE OF THE EVENT

The primary cause of the event was determined to be inadequate change management (a process weakness) with respect to changing the detector housing from aluminum to titanium for the IR detectors. This design change affected the operating characteristics of the detector in the as-installed condition that were not recognized by the vendor or the plant staff. In particular, the vendor assessment of the design change to titanium housing did not identify any operational impacts. The vendor should have identified the need to use the neutron sensitivity value for the detector inside the housing. A contributing cause was inadequate procedures. The procedure for determining trip setpoints was not specific regarding which neutron sensitivity values should be used and which previous cycle setpoints should be used when calculating IR trip setpoints for a new operating cycle. Nor was there a procedural requirement to ensure the installed trip setpoint was less than or equal to the Technical Specifications allowable value of 31%.

ANALYSIS OF THE EVENT

The intermediate range (IR) NIS detectors N35 and N36 are designed for multiple (about 7) decade use and have a compensated ion chamber. The detector current is a function of the detector characteristics and the neutron and gamma flux leakage from the core, which is proportional to reactor power. The primary FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Turkey Point Unit 4 05000251 2006 -�002 00 function of the IR NIS detectors is to provide a relative indication of reactor power when transitioning from the source range through the power range. The IR NIS channels provide a High Flux Reactor Trip at an IR reset at 70 percent of the setpoint detector current, decreasing. This trip is required by Technical Specifications, but no credit is taken for this trip in the safety analysis.

TS Table 2.2-1 Allowable Value for the intermediate range neutron flux is " intermediate range trip is intended to mitigate the consequences of a rod cluster control assembly (RCCA) bank withdrawal accident from sub-critical conditions. This event is defined as an uncontrolled addition of reactivity to the reactor core caused by withdrawal of one or more RCCA banks, resulting in a power excursion, which would have non-conservatively occurred late for N-35 only. The trip logic for intermediate range is 1 out of 2, such that the N-36 setpoint would have generated a reactor trip prior to exceeding 31%.

For Cycle 22 (approx. April 18, 2005), Engineering generated the intermediate range detector setpoints based on the installation of two new intermediate range (N-35 and N-36) detectors. These setpoints were calculated using the new detector's neutron sensitivity and ratioing it to the previous detector's neutron sensitivity. For Cycle 22, the setpoints were adjusted at 100% power and a comparison of the pre- versus post high flux setpoint showed a 55% reduction in the N-35 setpoint and a 7% increase in the N-36 setpoint.

The latent error in the process occurred at this time, when Reactor Engineering did not recognize this difference and the fact that the trip setpoint for Unit 4 N-35 was non-conservative (above TS allowable value). A review of the Unit 4 operating history revealed that N-35 was recalibrated once the unit reached 100% equilibrium conditions.

REPORTABILITY

A review of the reporting requirements of 10 CFR 50.72 and 10 CFR 50.73 and NRC guidance provided in "Event Reporting Guidelines," (NUREG-1022, Rev. 2) was performed for the subject event. As a result of this review, the event is considered reportable in accordance with 10CFR50.73(a)(2)(i)(B) in that two technical specifications were not met:

1. Turkey Point Technical Specification Table 2.2-1 Allowable Value for the intermediate range neutron flux is " 34.6% of RTP for N-35.

2. The unit ascended in power in violation of TS 3.3.1 and its action statements a and b for having one less than the minimum channels operable for the intermediate range detectors. At the time the incorrect setpoint was not recognized.

APPLICABILITY: MODES 1 and 2 DOCKETFACILITY NAME (1) LER NUMBER (6) PAGE (3)NUMBER (2)

ANALYSIS OF SAFETY SIGNIFICANCE

Based on the analysis described below, it is concluded that the health and safety of the public were not affected by this event.

The purpose of this intermediate range trip is to mitigate the consequences of a rod cluster control assembly (RCCA) bank [AA, ROD] withdrawal accident from sub-critical conditions. This event is defined as an uncontrolled addition of reactivity to the reactor core caused by withdrawal of one or more RCCA banks, resulting in a power excursion. Such a transient could be caused by a malfunction of the reactor control rod drive systems or due to operator error.

Should a continuous RCCA bank withdrawal accident occur, the transient will be terminated by one or more of the following automatic features of the Reactor Protection System (RPS).

- Source Range High Neutron Flux - Intermediate Range High Neutron Flux - Power Range High Neutron Flux (Low Setting) The analysis of this accident conservatively assumes a reactor trip initiated by the Power Range High Neutron Flux (low setting, two-of-four power channels). Other RPS functions available to terminate this event as discussed above include:

- Source Range High Neutron Flux trip - Intermediate Range High Neutron Flux trip However, credit is not taken for these trips. Therefore, having the setpoint for one of the Intermediate Range Detectors set at 34.6% would be of little safety significance since it is a backup to the Power Range trip.

Note that there is no accident analysis that takes credit for the Intermediate Range trip as the primary reactor protection. In addition, a review of the Unit 4 operating history revealed that N-35 was recalibrated once the unit reached 100% equilibrium conditions. So the condition of a high IR detector setpoint on one of the two IR detectors occurred only for a limited time frame following the Beginning of Cycle 22 startup.

CORRECTIVE ACTIONS

Immediate corrective actions were completed as follows:

1. New IR detector currents (setpoints) were recalculated and re-installed prior to initial criticality in Unit 4 Cycle 23.

2. New IR detector currents at 48% power and again at 100% equilibrium conditions were calculated and re-calibrated as required.

3. Unit 3 was assessed to ensure it was not impacted with acceptable conclusions.

Additional corrective actions including the following are identified in Condition Report 2006-35513:

DOCKETFACILITY NAME (1) LER NUMBER (6) PAGE (3)NUMBER (2) 2006� 00 1. Revise the plant procedure for Nuclear Instrumentation System Setpoint and Calibration Predictions for a New Cycle Startup to address: (a) which specific setpoint should be used for the start of the previous cycle calculation; (b) enhance the description of when specific multiplier factors are used; (c) specific directions regarding the applicable neutron sensitivity values to be used; (d) validation performed by comparing the calculated beginning of cycle setpoints with the previous cycle power history; and (e) validating the neutron sensitivity of each detector against the vendor acceptance criteria.

2. Revise the plant procedure for Nuclear Instrumentation Channel Check and Calibration to address the required actions if the calculated setpoint exceeds the Technical Specification Table 2.2-1 Allowable Values of 31%.

3. Reactor Engineering to follow-up actions with the vendor to confirm correct use of the neutron sensitivity for the particular detector assemblies.

ADDITIONAL INFORMATION

An internal and external Operating Experience review was performed. None of the conditions specifically match the event described in this event report.

second component function identifier (if appropriate)].

FAILED COMPONENTS IDENTIFIED:�N/A SIMILAR EVENTS: None