05000323/LER-2001-002

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LER-2001-002,
Event date:
Report date:
3232001002R00 - NRC Website
v • d • e

I. Plant Conditions

Unit 2 was in Mode 5 (Cold Shutdown), during the tenth refueling outage (2R10).

II. Description of Problem

A. Background

The design of the vital 4-kV electrical system [EA] consists of 3 Vital Buses (F, G, and H), each capable of being powered from:

  • auxiliary power (normal power when connected to the 500-kV switchyard) [EL],
  • startup power (backup power from the 230-kV switchyard), or

An undervoltage condition on the vital bus causes the startup feeder breaker to close to power the bus. If startup power is not available, the undervoltage condition causes the associated EDG to start and power the bus.

Surveillance Test Procedure (STP) M-13H, "4-kV Bus H Non-SI Auto Transfer Test," verifies proper functioning of the auto-transfer of Bus H to startup power, and to the EDG on an undervoltage condition.

Specifically, the power supplies to Bus H include startup power via feeder breaker 52-HH-14 [BKR], and EDG 2-2 [DG].

Operating Procedure (OP) J-6A:IV, "4160 Volt Breaker Code Order," provides instructions for operation of 4-kV breakers. Attachment 9.3, "Place a 4160V Breaker in the TEST Position," specifies the steps required to place a breaker in TEST, including the hanging of an information tag in the control room on the breaker control switch. With the breaker in TEST, the breaker is fully lowered and disconnected from the bus, and the closing spring discharged. A mechanical block prevents operation of the breaker. Attachment 9.4, "Remove a 4160V Breaker From the TEST Position," specifies the steps to restore the functionality of a breaker in TEST, including removing the information tag in the control room on the breaker control switch.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) Diablo Canyon Unit 2 0 B. � Event Description On May 18, 2001, at 1445 PDT, operators prepared for the second midloop by walking down Bus H as part of the return to service for startup power. The startup feeder breaker was confirmed racked in and open.

At 1640 PDT, the startup feeder breaker was placed in the TEST position per part 1 of STP M-13H. The procedure places the breaker in TEST to block the auto transfer to startup power and allow testing of the auto start feature of EDG 2-2. Due to seismic concerns, procedures direct that the breaker cubicle door be closed when the breaker is in TEST. The switching procedure requires an information tag (lamacoid) be placed on the breaker control switch in the control room. However, operators could not recall actually placing a lamacoid on the startup feeder breaker control switch in the control room.

At 1750 PDT, part 1 of STP M-13H was successfully completed. The test was turned over to night shift for completion. However, the night shift crew was not aware that the Bus H startup feeder breaker was in TEST, and there was no information tag on the breaker control switch to indicate the TEST position. Various interruptions prevented the test from continuing promptly. The test was eventually put on hold for draining to midloop.

On May 19, 2001, after completing midloop operations, the night crew started part 2 of STP M-13H, which tests the auto-transfer of a vital bus to startup power. The procedure requires operators to verify auxiliary power is supplying power to the bus and startup power is available to Bus H.

Step 12.5.5 states, "Rack in or verify racked in Bus H Startup Feeder Breaker 52HH14." The breaker indications in the control room appeared normal, and since the breaker door was closed, the breaker also appeared normal. There was no information tag or lamacoid on the breaker or on the control switch in the control room indicating that the breaker was in TEST. EDG 2-2 was in manual as specified by the procedure to prevent an unnecessary auto start.

On May 20, 2001, at 0056 PDT, during the performance of part 2 of STP M-13H, operators initiated an undervoltage condition on Bus H, by opening the auxiliary feeder breaker. However, since the startup feeder breaker was still in TEST and EDG 2-2 was in manual to prevent its auto start, Bus H remained deenergized.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) Diablo Canyon Unit 2 0 On May 20, 2001 at 0100 PDT, operators implemented Appendix 7.2 of STP M-13H, "Contingency Action for Loss of Bus H Voltage During Auto-transfer to Startup." EDG 2-2 was returned to auto and it started and reenergized the bus. After determining the startup feeder breaker to Bus H was in TEST, operators paralleled auxiliary power to the bus and secured the EDG.

At 0331 PDT, PG&E made an 8-hour nonemergency report for the valid unplanned system actuation in accordance with 10CFR50.72(b)(3)(iv)(A).

C. Inoperable Structures, Components, or Systems that Contributed to the

Event

None

D. Other Systems or Secondary Functions Affected

All other systems functioned as expected, and there were no secondary functions affected.

E. Method of Discovery

Licensed utility operators initiated the auto start of EDG 2-2 when the startup feeder breaker failed to power the bus as expected during the test, and therefore, "discovered the event." A non-licensed utility operator discovered the startup feeder breaker in TEST when he opened the breaker cubicle.

F. Operator Actions

Operators implemented the contingency plan in STP M-13H Appendix 7.2 by placing EDG 2-2 in auto, thus allowing it to start on the undervoltage signal and power the bus.

G. Safety System Responses

When EDG 2-2 was placed in auto, it responded as designed by starting and powering the bus. Loads automatically sequenced onto the bus as designed.

LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) OF 2001 5 3 0 6 0 0 2 0 0 3 2 0 0 0 Diablo Canyon Unit 2 5 III. � Cause of the Problem

A. Immediate Cause

The immediate cause of the system actuation was that the startup feeder breaker was left in TEST and, therefore, would not close to power the bus on the undervoltage signal. This necessitated the unplanned start and loading of EDG 2-2.

B. Root Cause

The root cause was inadequate equipment status control. Contrary to requirements of OPJ-6A:IV, the startup feeder breaker was left in TEST with no control board information tag to indicate the breaker status. The non-licensed operator notified the senior control operator by telephone that the breaker was in TEST and asked him to hang the information tag, per standard practice rather than personally going to the control room.

The control operator does not recall specifically hanging the tag.

C. Contributory Cause Part 1 of STP M-13H did not contain a step to remove the startup feeder breaker from the TEST position. Part 2 usually follows part 1 and contains a step to verify the startup feeder breaker is racked in. While this and similar tests have been performed several times during refueling outages without this status control problem, the tests should specifically remove the startup feeder breaker from the TEST position per OP J-6A:IV.

IV. � Analysis of the Event In Modes 5 and 6, core cooling is provided by the residual heat removal (RHR) system [BP]. Either RHR pump is capable of providing adequate heat removal to prevent core boiling. The RHR pumps are powered from Vital Buses G and H.

During the time the breaker was in TEST, the minimum time to boil the core was 70 minutes. This is based on midloop conditions. At the time of the event, after midloop, the core was being cooled via the RHR pump of Vital Bus G. When Vital Bus H was deenergized for 4 minutes, core cooling was unaffected.

Additionally, the EDG 2-2 functioned as designed by starting and picking up the vital bus when it was returned to automatic as directed by procedures. TS 3.4.8, "RCS Loops — Mode 5, Loops Not Filled," requires that one RHR train be in operation and the other train be operable. Additionally, both RHR trains can be shutdown for up to 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> provided the core outlet temperature is maintained at LICENSEE EVENT REPORT (LER) TEXT CONTINUATION FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) least 10 degrees F below saturation temperature, reactor coolant system (RCS) boron concentration is not reduced, and the RCS water volume is not reduced.

During the event, one RHR train remained in service. Furthermore the criteria listed above for shutdown of both trains was met.

Therefore, the event:

  • was not considered risk significant;
  • did not adversely affect the health and safety of the public;
  • was not a Safety System Functional Failure; and

V A Corrective Actions

A. Immediate Corrective Actions

Operators implemented STP M-13H contingency steps to restore power to Bus H within 4 minutes by placing EDG 2-2 in auto, allowing it to start and power Bus H.

B. Corrective Actions to Prevent Recurrence

1. Operating Procedure OP J-6A:IV will be revised to require verification that information tags are hung on the control switches when 4-kV breakers are placed in the TEST position.

2. The STP M-13 series of procedures will be revised. Part 1 will specify removing startup feeder breakers from the TEST position in accordance with OP J-6A:IV.

3. Similar STPs will be reviewed for additional equipment status control for 4-kV breakers.

VI. A Additional Information

A. Failed Components

None

B. Previous Similar Events

None

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