05000323/LER-2004-001
| Diablo Canyon Unit 2 | |
| Event date: | 11-02-2004 |
|---|---|
| Report date: | 12-23-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 41168 | 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3232004001R00 - NRC Website | |
I. Plant Conditions
Unit 2 was in Mode 6 (Refueling), during the twelfth refueling outage (2R12) at the time of the event (November 2, 2004). Just prior to the event, 4kV vital bus G was powered from auxiliary power with startup power cleared for planned maintenance.
II. Description of Problem
A. Background
heat load was 9.5 MW. The Reactor Coolant System (RCS) was depressurized with the reactor vessel head removed and reactor refueling cavity filled. RCS temperature was 112 degrees F. Both Residual Heat Removal (RHR) Pumps were operable with RHR pump 2-2, powered from vital 4kV bus H, in service and pump 2-1, powered from vital 4kV bus G, in standby. All 3 Component Cooling Water (CCW) pumps, and both Auxiliary Saltwater pumps, were operable. This equipment lineup provided the normal active residual heat removal function to the core.
With the upper internals installed and the Refueling Water Storage Tank (RWST) level less than 80 percent due to filling of the reactor refueling cavity, the availability of passive residual heat removal function, required by the Plant's Outage Safety Plan (OSP) for defense-in-depth purpose, was not immediately available. This was recognized by the OSP as an acceptable special plant condition.
The design of the vital 4kV electrical system consists of 3 vital buses (F, G, and H), each capable of being powered from:
- auxiliary power (normal power when connected to the 500kV switchyard),
- startup power (backup power from the 230kV switchyard), or
- an onsite emergency diesel generator (EDG).
An undervoltage condition on the vital bus will first cause the auxiliary feeder breaker to open and then the startup feeder breaker to close to power the bus. If startup power is not available, the undervoltage condition will cause the associated EDG to start and power the vital bus.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) In preparation for EDG 2-1 4kV feeder cable replacement during the 2R12 outage, an as-found Post Modification Test (PMT) 21.46, "Diesel Generator 2-1 Phase Sequence Verification" was performed. The purpose of the test was to verify the phase sequence of the EDG 2-1 potential transformers (PT) against that of 4kV bus G PTs being fed from auxiliary power. The test consists of comparing the voltage of auxiliary power supplying bus G phases to EDG 2-1 phases through a temporary test setup consisting of two test transformers and a phasing panel with white lights. Any phase sequence variances would result in a potential difference across the applicable white lights; thereby, illuminating the lights associated with the out-of-sequence phases.
Performance of PMT 21.46 on DCPP, Unit 2, 4kV vital bus G was the first plant usage of this newly-configured test equipment.
For a graphical representation of the test equipment setup, refer to Figure 1.
EDG 2-1 PT Phasing Panel Bus "G" PT YT # 1 YT # 2 Lamp� Lamp Switches SwitchesXI�HI X3 0� 0� Lamp Test I ci Switches Connected to: Connected to Bus "G" PTEDG 2-1 PT
- Incorrect test equipment configurationCircuitry Circuitry Figure 1 LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) B.�Event Description On October 29, 2004, Pacific Gas & Electric (PG&E) Transmission/Substation Maintenance & Construction (T/SM&C) personnel configured the test equipment in preparation for the PMT 21.46. This involved wiring the two test transformers to the phasing panel with white test lights and white test light test switches.
On November 2, 2004, in preparation for performing PMT 21.46, the test equipment was connected to the secondary side of the EDG 2-1 PT circuitry, and the secondary side of the vital bus G PT circuitry. At 11:05 PST, a short circuit to ground caused the voltage of the secondary side of the vital bus G PT circuit to degrade and, in turn, actuated the two bus G second-level undervoltage relays to initiate the second-level auto-transfer scheme. This auto-transfer scheme resulted in load shedding from vital bus G and opening the auxiliary feeder breaker to de-energize the bus.
As designed, EDG 2-1 auto-started and loaded onto vital bus G. When EDG 2-1 auto-started, it energized the EDG's PT circuitry. With similar incorrect EDG-side test transformer connection to the EDG PT circuitry, another short circuit to ground was developed. Since the test equipment was connected to the downstream side of the EDG PT secondary circuitry fuse, the short circuit current resulted in blowing an EDG PT secondary side fuse.
The blown fuse affected the indicated voltage and frequency but did not affect the EDG actual voltage and frequency, which remained within specification.
The test equipment was disconnected and a visual inspection of the wiring in switchgear SHG5 and SHG12, which were associated with EDG 2-1 and the vital bus G PT circuitries, was performed for evidence of overheating and insulation degradation. The inspection results did not reveal any indication of damage.
At 17:08 PST on the same day, operators transferred vital bus G to auxiliary power and shut down EDG 2-1.
At 17:22 PST on the same day, the Shift Manager made a non-emergency event notification (EN #41168) to the NRC in accordance with 10 CFR 50.72(b)(3)(iv)(A).
After the shutdown of EDG 2-1, a more thorough inspection of the switchgear wiring was conducted with no adverse findings.
- LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event No inoperable plant structures, systems or components were involved in, or contributed to, this event. The vital bus protection, auto-transfer scheme, and EDG 2-1 operated as designed.
D. Other Systems or Secondary Functions Affected All other systems functioned as expected, and there were no secondary functions affected. The residual heat removal function was unaffected by this event.
E. Method of Discovery The event was self-revealing to licensed control room operators by annunciators indicating the bus G degraded bus voltage condition and EDG 2-1 auto-start and load onto vital bus G.
F. Operator Actions Operators immediately stopped the PMT and allowed EDG 2-1 to continue to power vital bus G, while maintenance and engineering performed a visual inspection of the associated switchgear for damage to determine if it was acceptable to return bus G to auxiliary power and secure EDG 2-1.
G. Safety System Responses Prior to the event, vital bus G was supplied by auxiliary power, with startup power cleared for planned maintenance. Vital bus G was being prepared to be cleared for maintenance. The equipment required for this mode of operation was in-service on the other two vital buses (vital bus F and vital bus H).
During auto-start and loading of EDG 2-1 onto vital bus G, the other two vital buses were not affected and remained operable on auxiliary power with the associated operable EDG 2-3 (vital bus F) and EDG 2-2 (vital bus H) in standby. Equipment on these two vital busses provided the necessary decay heat removal function.
During auto-start and loading of EDG 2-1 onto vital bus G, equipment on vital bus G, which is designed for decay heat removal, responded as designed. CCW Pump 2-2, which was in standby before the event, LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) Diablo Canyon Unit 2 0 5J 0 DOCKET NUMBER (2) 0 0 3 3 2004 LER NUMBER (6) 0 0 1 0 0 6 1 OF 8 started on the transfer to EDG 2-1 as designed. Auxiliary Saltwater Pump 2-2, which was in service before the event, tripped when receiving the load shed signal and auto-restarted after the EDG 2-1 output breaker was closed to re-energize vital bus G. RHR pump 2-1 remained in standby as designed. This is because this undervoltage auto bus transfer is a non engineered safety feature and does not require auto-start of the RHR pump.
Unit 1 was unaffected and continued to operate in Mode 1 (Power Operation) at 100 percent power.
III.�Cause of the Problem
A. Immediate Cause
Personnel errors by both the performer and independent verifier resulted in an incorrect configuration of the test equipment wiring. The wiring error resulted in a short circuit to ground on the secondary side of the vital bus G PT circuit through the test equipment. This resulted in the actuation of the two vital bus G second-level undervoltage relays to initiate an auto transfer of vital bus G from auxiliary power to EDG 2-1.
B. Root Cause
The temporary test equipment, typically used for this phase sequence testing at Diablo Canyon Power Plant (DCPP) in the past, was not available. Discreet components were then used by PG&E T/SM&C personnel to construct new phase sequence test equipment. This new test equipment was incorrectly constructed. In the test setup, the two test transforiners should have been configured in a delta-wye manner, with the primary (delta) side connected to the PT circuitry fused for the PT circuit protection, and the secondary (wye) side grounded for lamp test. In the actual test setup, the two test transformers were incorrectly configured in a wye-delta manner, with the primary (wye) side connected to the PT circuitry grounded and no fuse protection. The secondary (delta) side of the two test transformers were fused and connected to the phase panel with white lights. This test equipment configuration did not allow the needed protection for the test setup circuitry nor the PT circuitry.
However, bench testing of this test equipment setup did not identify this wiring mistake.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) The root cause of this event was due to inadequate independent verification of the T/SM&C personnel to control activities associated with the configuration of the test equipment.
C.�Contributory Cause Bench testing of the incorrectly configured test equipment did not simulate the actual bus PT configuration. This resulted in not being able to detect the mis-wiring of the two test transformers.
There was a missed opportunity at the beginning of the test when an On The-Spot-Change (OTSC) to the test procedure to allow for an alternate termination location was being processed. During this time period, a discussion of the wiring of the test equipment shown in the drawing of the test procedure was raised, but was not pursued far enough to discover the wiring mistake.
IV. Assessment of Safety Consequences
There were no significant safety consequences as a result of this event. Prior to the event, vital bus G was being prepared to be cleared for maintenance. As a result, equipment which was required to provide decay heat removal was powered from the other two vital buses, and there was no loss of decay heat removal function during this event.
Therefore, the event is not considered risk significant and it did not adversely affect the health and safety of the public.
V. Corrective Actions
A.�Immediate Corrective Actions 1. The low-voltage side of the PT wiring was inspected for overheating and insulation degradation. The inspection results revealed no indications of damage.
2. A post-test setup review was conducted on the test equipment, which consisted of two test transformers and a phasing panel. The test setup between the test transformer and the station ground was found not to have been connected properly. The test set up was then reconfigured properly. The configuration was verified to comply with PMT 21.46.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) 3. Following an evaluation by the DCPP System engineer of the acceptability of powering vital bus G by auxiliary power, the bus was transferred and EDG 2-1 was shut down.
B.ACorrective Actions to Prevent Recurrence 1. DCPP Administrative Procedure AD7.ID6, "Nuclear Generation/Contractor Interface" will be revised to require supplemental (non-site permanent) personnel to have a pre-job brief in human performance error reduction techniques prior to performing work on sensitive plant equipment.
2. PMT 21.46 has been revised to require:
a. The test director to conduct a pre-job brief on error reduction techniques. This briefing includes discussion of where the work could adversely impact plant equipment.
b. The test director and T/SM&C technician to review the test procedure, sequence, and expected results together.
c. The configuration of this piece of test equipment be controlled.
The test panel was bench tested to ensure the proper configuration and was successfully used for the "as left" check.
3. A phase test panel will be built and controlled as plant maintenance and test equipment (M&TE). This will eliminate the reliance on using test equipment that DCPP does not own or maintain for similar phase sequence check in the future.
VI.AAdditional Information
A. Failed Components
None
B. Previous Similar Events
None