05000400/LER-2009-001
| Docket Numbersequential Revmonth Day Year Year Month Day Year N/A 05000Number No. | |
| Event date: | 03-18-2009 |
|---|---|
| Report date: | 05-18-2009 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4002009001R00 - NRC Website | |
Energy Industry Identification System (EIIS) codes are identified in the text within brackets [ ].
I. DESCRIPTION OF EVENT
At the start of this event, Harris Nuclear Plant was in Mode 1 at 100% power. Plant average temperature was 588.8 degrees and plant pressure was 2235 psig. There were no additional structures, systems or components inoperable at the beginning of the event that contributed to the severity of the event.
On January 26, 2009, at 0910, performance of MST-E0075, "6.9 kV Emergency Bus, 1A-SA and 1B-SB undervoltage (Loss of Voltage) Channel Calibration," was authorized. This Maintenance Surveillance.Test (MST) provides compliance with Technical Specification (TS) 4.3.2.1 surveillance requirements for Limited Condition for Operation (LCO) 3.3.2 regarding the operability of applicable Engineered Safety Features Actuation System (ESFAS) instrumentation channels and interlocks.
The 6.9kV Emergency Bus [EK] 1A-SA undervoltage relays [27] 27-2/1729 and 27-3/1729 should trip within the Allowable Value parameters established by TS Table 3.3-4 (ESFAS Instrumentation Trip Setpoints), Function 9.a (Loss of Offsite Power). However, while performing MST-E0075 on January 26, 2009, the "as found" data for the 6.9kV Emergency Bus 1A- SA Primary undervoltage relays 27-2/1729 and 27-3/1729 were below the TS Allowable Value dropout voltage requirements, as follows:
27-2/1729 Contact 3-4 was 77.0 V, Contact 7-8 was 77.1 V, below TS Allowable Value of ?_ 78.2 V 27-3/1729 Contact 3-4 was 77.5 V, Contact 7-8 was 77.7 V, below TS Allowable Value of 78.2 V
II. CAUSE OF EVENT
The root cause of this event was the unavailability of information regarding the effect that a change in the starting input voltage applied to the relay coil would have on the corresponding dropout setting. When the starting input voltage applied to the relay coil was returned to 120 V for the 2009 performance of MST-E0075, the effects on the associated change in dropout setting value were not known.
A review of past calibration data found the Model 12NGV13 relays used for 6.9 kV Emergency Bus Primary and Reactor Coolant Pumps [P] undervoltage protection have been reliable and repeatable. Review of procedure revision history shows changes in the test methodology, related to improvements in available M&TE, and in procedure content over the years.
Changes in procedure content include the removal of some test parameters (input voltage) from the procedures due to their inclusion in the computer control program. Other critical test parameters (such as setpoints, time setting) are retained in the procedure.
During the initial development of MST-E0075 in 1998, the starting input voltage applied to the relay coil was changed from
==120 V to 95 V, as this voltage level was enough to "reset" the dropout contacts. When MST-E0075 was revised in 2007, the starting input voltage applied to the relay coil was changed back to 120 V, to reflect the nominal relay voltage under normal plant conditions. At that time, information was not available to indicate that the change in the relay coil voltage level would affect the dropout setting. After the 2009 calibration, this link between the relay coil voltage level and dropout setting was confirmed through bench testing, which verified that starting at a higher voltage resulted in a lower dropout.
III. SAFETY SIGNIFICANCE
There were no safety consequences impacting plant or public safety as a result of this event. The purpose of Emergency Bus 1A-SA "Loss of Voltage" undervoltage relay (also known as the primary undervoltage relay) is to isolate the bus from the preferred power source (offsite power) upon loss of electrical power and then to initiate load shedding and operation of the Emergency Diesel Generator [DG]. Although these actions would still have occurred with the Emergency Bus 1A-SA "Loss of Voltage" undervoltage relay calibrated such that it did not meet the TS Allowable Value dropout voltage requirement, the actions would have started approximately 1 ms later.
_ Potential Safety Consequences:
The only potential safety consequences are related to hoW the setpoint calibration error on the primary undervoltage relays could affect the timing of the diesel start signal on a loss of voltage. Based on the operating curves for NGV relays, a —1.5 Vcalibration error will result in a —1 ms delay. The delay time is based on a drop from 110% of nominal voltage (120 V) to 50% of the dropout voltage setting (-40 V). A. Harris Engineering Calculation allows a maximum setting of 1.12 seconds with a TS Table 3.3-4 Allowable Value of 1.5 seconds. The last performance of MST-E0075 was reviewed for both safety busses and the additional 1 ms delay would not have resulted in a TS violation or out of range time setting. The additional delay due to the calibration error did not have any significant effect on the protective features of the primary undervoltage relays.
IV. CORRECTIVE ACTIONS
The affected 1A-SA relays were recalibrated per MST-E0075 and the test was satisfactorily completed on January 26, 2009. Additionally, the same model 1B-SB relays were also checked. Since the extent of condition was Model 12NGV13 relays with Allowable Values defined in TS, additional UV relays will also be tested at a nominal 120 V starting point.
Process verification steps are being added to procedures performed with Pulsar software, Pulsennaster and Advanced Visual Test Software (ANTS), which will provide a listing with critical test parameters and description of the test being performed automatically by the Pulsar software. Setpoint changes for emergency bus primary. UV relays are being implemented to increase the margin between setpoint and Allowable Value.
V. PREVIOUS SIMILAR EVENTS
A review of LER and Action Requests over the past five years identified one similar Action Request (AR):
AR 231046, originated April 26, 2007 - Undervoltage Relays for the 1A-SA Bus were found out of calibration while protection remained operational. The apparent causes were determined to be design differences between relay types and effects on calibration methodologies, not identified until after the relays had been accepted and installed, and the failure of calibration procedures to forewarn users of the possibility of unintentionally affecting other contact pair setpoints when adjusting a target contact pair setpoint. The,potential link between the relay coil voltage level and dropout setting was not a factor in this 2007 report.
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