05000461/LER-2009-003
| Clinton Power Station, Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(D), Loss of Safety Function - Mitigate the Consequences of an Accident |
| 4612009003R00 - NRC Website | |
CONTINUATION SHEET
� � �1. FACILITY NAME 2. DOCKET 6. LER NUMBER 3. PAGE �REV � � � YEAR NUMBER NO.�Clinton Power Station, Unit 1 05000461 2�OF�4
PLANT OPERATING CONDITIONS
Unit: 1 Event Date: 9/24/07 Event Time: 1700 hours0.0197 days <br />0.472 hours <br />0.00281 weeks <br />6.4685e-4 months <br /> Central Daylight Time Mode: 1 (Power Operation) Reactor Power: 92.4 percent
DESCRIPTION OF EVENT
On 9/23/09, the Division 3 Emergency Diesel Generator [EK] [DG] was inoperable and unavailable due to scheduled maintenance.
At about 1100 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.1855e-4 months <br />, during performance of preventive maintenance to calibrate Division 3 DG control panel [PL] auto restart relay [2] K-8A, technicians discovered that the relay did not have the correct time delay range. The specified time delay range for this relay is20-200 seconds. The installed relay had a time delay range of 1.5 - 15 seconds. Further investigation revealed that field flash time delay relay K-32 in the same panel, should be a 1.5 to 15 second time delay relay but was instead a 20 to 200 second time delay relay.
Thus, the two relays were in swapped locations. The technicians examined the other relays in the panel, and verified the other relays were correct. The swapped relays were removed and reinstalled in their correct locations, and preventive maintenance and post maintenance testing confirmed proper operation of both relays in the Division 3 DG control panel.
The Division 3 DG assembly is an onsite standby power source for the safety related 4 kilovolts 1C1 bus [BU] and is a dedicated power source for the High Pressure Core Spray (HPCS) system [BG]. The DG starts automatically on loss of coolant accident (LOCA) signal (i.e. low reactor water level signal or high drywell pressure signal). In the event of a Loss of Offsite Power (LOOP), the HPCS system electrical loads are automatically connected to the Division 3 DG in sufficient time to respond to a design basis accident (DBA) such as a LOCA and provide for injection of coolant into the reactor vessel [RPV].
The safety design function of the HPCS system is to maintain reactor vessel inventory after small breaks that do not depressurize the reactor vessel. The HPCS also provides spray cooling heat transfer during breaks in which uncovering of the reactor core is calculated.
Relay K-8A is an automatic restart relay that bypasses diesel engine shutdown relay K-8 for 80 seconds.
When the diesel engine is shutdown, relay K-8 prevents the engine from restarting for 50 seconds. Relay K 8A is energized at the same time as relay K-8 and allows an automatic restart of the engine from a bus under voltage (LOOP) or a LOCA signal. The 30-second difference for the K-8 and K-8A relays was to account for relay drift. Relay K-32 is a five-second time delay for flashing the generator field.
Swapping the K-8A relay with the K-32 relay had two negative affects. With relay K-8A in the K-32 position, the field flashed for 80 seconds instead of five seconds which damaged current limiting resistor R-12 and a portion of the associated wiring. The resistor performed its function and was damaged in the process due to the extended flashing time. However, since the initiation of the field flash and minimum duration was not impacted, this condition did not impact the ability of the DG to meet minimum start requirements. The current limiting resistor is in the circuit to limit the voltage and current applied to the generator exciter field to approximately 70% of what is normally seen at full speed no-load. The generator and exciter were cleaned and inspected in response to this event and no abnormal conditions were noted. The damaged resistor was � inspected and found to be functional within the expected resistance band. The damaged resistor and wiring were repaired and the field flash function was tested with satisfactory results.
With relay K-32 in the K-8A position, replacing the 80-second automatic restart relay with a 5-second relay resulted in an inability of the diesel to automatically restart and remain sealed in for approximately 45 seconds (50 seconds for the K-8 shutdown timer minus 5 seconds for relay K-32 being in the K-8A position). Consequently, the diesel would not have been capable of powering the HPCS loads following a LOOP/LOCA signal after manual shutdown of the engine. The diesel generator is analyzed to automatically start and load, energize the HPCS system pump [P] motor [MO], and provide power to initiate opening of the HPCS injection valve [INV] in less than 23 seconds after the LOOP/LOCA initiation signal. Further, the HPCS injection valve would not have opened, and the HPCS pump would not have been at design flow in less than 47 seconds as analyzed after a LOOP/LOCA initiation signal. This issue would not have affected the HPCS safety function if offsite power was available at the time of a LOCA signal.
A review of equipment history identified that relays K-8A and K-32 were pre-calibrated and replaced on 9/24/07 at about 1700 hours0.0197 days <br />0.472 hours <br />0.00281 weeks <br />6.4685e-4 months <br /> during the last Division 3 DG maintenance outage. At this time the relays were installed in the wrong locations.
No other inoperable equipment or components directly affected this event.
Issue Report 969157 was initiated to investigate and correct this issue.
CAUSE OF EVENT
The evaluation of this event concluded that the cause of this event was inadequate attention to detail and failure to comply with written instructions.
Maintenance procedures require replacement parts to be compared to the old parts and verified to be like-for like prior to installation, and allow the use of substitute materials only after the proper evaluation and authorization has been obtained. Further, the supervisor of the work is required to obtain required parts as specified in the work package and ensure the parts are like-for-like or evaluated for use prior to replacement.
The individuals installing the relays in 2007 demonstrated inadequate attention to detail and did not recognize that they were installing relays that had different model numbers from the relays that had been removed. One contractor installed the relays and one utility employee verified the installation. Neither of the individuals involved in installing and verifying the relays recalls the method used to verify the relays were 'like for like' prior to installation. The two relays were pre-calibrated about two weeks prior to installation and tagged for identification and then installed in September 2007 to replace existing installed relays. The individuals that completed the pre-installation calibration of the relays may have entered incorrect identification inform'ation on the material tags affixed to the relays; however, the individuals installing the relays have the responsibility to ensure they are installing correct parts.
In each of the work orders used to install these relays in 2007, the technicians recorded the correct serial numbers of the relays in the preparation tasks, but listed the wrong serial numbers in the installation task; however, this discrepancy was not recognized or documented as a deficiency. This discrepancy was reproduced in the electronic work order completion comments for each work order and again not recognized.
The supervisory review of work completion did not catch this discrepancy during closeout review of the work orders.
SAFETY ANALYSIS
This event is reportable under the provisions of 10 CFR 50.73(a)(2)(v)(D) due to a condition that could have prevented fulfillment of the HPCS safety function to mitigate the consequences of an accident.
This event had low safety significance due to the very low probability of a LOOP event occurring during the short duration (45 seconds) of unavailability of the Division 3 DG immediately following each of the 36 times the DG was shut down from surveillance runs while the relays were in swapped locations from 9/24/07 to 9/23/09. During each of the 45-second periods following each of the 36 manual shutdowns of the Division 3 DG during the period the relays were in the swapped locations, offsite power was available. Additionally, a review of the 36 DG runs identified that no events requiring the Division 3 DG and HPCS to perform their LOOP or LOCA functions occurred while the relays were in the swapped positions. Further, following the DG runs while the relays were in the swapped positions, the Reactor Core Isolation Cooling (RCIC) system [BN] and the other Emergency Core Cooling Systems (ECCS) and their power sources were available to mitigate an accident in the event that a LOOP/LOCA would have occurred.
CORRECTIVE ACTION
Model work orders for time delay relays in the Division 3 DG control panel will be revised to include specific instructions to record "old" and "new" model numbers to reinforce and ensure the correct model is installed and to add a verification job step to compare as-found and as-left model numbers.
A briefing will be provided to the Electrical Maintenance on this event, reinforcing expectations for parts verification and 'like for like' requirements.
Training on parts verification activities will be included in dynamic learning activities for 2010.
PREVIOUS OCCURRENCES
None
COMPONENT FAILURE DATA
No components failed during this event.
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