05000298/LER-2006-003, Regarding Both Diesel Generators Inoperable Due to Voltage Regulator Design Results in Loss of Safety Evaluation
| ML061710100 | |
| Person / Time | |
|---|---|
| Site: | Cooper  |
| Issue date: | 06/15/2006 |
| From: | Minahan S Nebraska Public Power District (NPPD) |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| NLS2006049 LER 06-003-00 | |
| Download: ML061710100 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat |
| 2982006003R00 - NRC Website | |
text
H Nebraska Public Power District Always there when you need us 50.73(a)(2)(v)
NLS2006049 June 15, 2006 U.S. Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001
Subject:
Licensee Event Report No. 2006-003-00 Cooper Nuclear Station, NRC Docket No. 50-298, DPR-46 The purpose of this correspondence is to forward a Licensee Event Report.
- Sincerely, Stewart B. Minahan General Manager of Plant Operations
/cb Enclosure cc:
Regional Administrator w/enclosure USNRC - Region IV Cooper Project Manager w/enclosure USNRC - NRR Project Directorate IV-]
Senior Resident Inspector w/enclosure USNRC - CNS NPG Distribution w/enclosure INPO Records Center wv/enclosure SORC Administrator wv/enclosure SRAB Administrator w/enclosure CNS Records w/enclosure COOPER NUCLEAR STATION P.O. Box 98 / Brownville, NE 68321-0098 Telephone: (402) 825-3811 / Fax: (402) 825-5211 www.nppd.com
Abstract
On April 21, 2006, during a drawing review, Engineering discovered that the diesel generator (DG) control circuitry caused a diesel generator connected (aligned) to the offsite power grid to be inoperable. A records review determined that on one occasion, March 23, 2004, DG #1 had been inoperable for maintenance. During this same time period, a surveillance test was performed that resulted in DG #2 being aligned to the grid and thus being inoperable. Both DG's inoperable constitutes a loss of safety function and is reportable under 10 CFR 50.73(a)(2)(v)(B) as an inability to remove residual heat.
This was caused by a procedure change in late 1998 that, based on inaccurate information, allowed a DG to be considered operable while aligned to the grid. In subsequent years the procedure change process has been revised to require more detailed and extensive reviews of procedure changes.
Immediate actions were to identify procedures that include aligning a DG to the offsite power grid during surveillance testing. These procedures have either been revised to require declaring the DG inoperable or already contained a requirement to declare the DG inoperable when aligned to the grid.
Further corrective actions include additional changes to the procedure revision process, reviewing a sample of other procedure changes that used that same process for accuracy, and modifying the DG control circuitry to no longer require declaring a DG inoperable when aligned to the grid.
NRC FORM 366 (6-2004)
(If more space is PLANT STATUS At the time of the event, Cooper Nuclear Station (CNS) was in Mode 1 at approximately 100% power.
BACKGROUND During surveillance testing, when the diesel generator (DG) is aligned parallel to the grid, the DG voltage regulator droop setting is not bypassed upon receipt of an emergency signal from, for example, a loss of voltage or a loss of coolant accident (LOCA). Because the voltage regulator remains in droop mode, and because existing analysis of the condition described above does not bound the condition, the DG is inoperable by definition.
Due to voltage regulator droop mode operation, prior to 1998, the DG's were declared inoperable whenever they were aligned parallel to the offsite power grid (grid) during surveillance tests. To reduce the accumulation of out-of-service time related to the inoperability requirement, a modification to enhance the DG circuitry was undertaken in early 1998. Because the initial work scope was insufficient to accomplish the original goal, a second work scope was proposed and approved. The modification was completed according to the second work scope but the need to declare the DG inoperable when aligned to the grid remained.
The key difference between the various DG operating mode responses is the logic of the unit, which for a LOCA (regardless if a concurrent loss of offsite power exists or not), causes most of the logic to appropriately change to an Emergency Start mode. Specifically, the following actions occur upon receipt of an Emergency Start Signal:
The Non-Essential DG trips are bypassed.
The Digital Reference Unit (speed input device) reverts to 60.0 Hz (removes the Engine Speed Droop function).
The Engine Speed Governor reverts to a "high responsiveness" mode (the governor has the capability to operate in two modes. The two modes can be used to provide greater stability when operating in parallel to the grid (i.e., it is less responsive to step changes).
In contrast to these three logic changes, one other required change does not occur. This latter change relates to the voltage droop function. This function is enabled anytime the DROOP/PARALLEL switch is placed in the PARALLEL position, which is required for operating in parallel with the grid. The voltage droop function allows for stability while operating in parallel with the grid. This function is not disabled in response to an Emergency Start Signal.
While some circuitry changes were accomplished in 1998, the original need to declare the DG's inoperable remained. Consequently, all procedure changes resulting from the completion of control circuitry enhancement modification still contained this requirement. Six weeks after the modification was closed out and new procedures had been implemented, new procedure change requests (PCR's) were submitted that removed the inoperability requirement in DG surveillance tests. Because the new PCR's were processed as instant changes, there was no review by anyone directly involved in the modification.
Under the procedure change process in use at that time an instant change was required to be a "non-intent" change, meaning that the procedure change would not change the intent of the procedure. This included what the procedure could be used to accomplish, the level of nuclear safety, or the design or licensing basis.
NRC FORM 366 (1-2001)
(If more space is required, use additional copies of (If more space is required, use additional copies of Form 366A) information in the justification section of the procedure change request was not challenged and compared to the base documents. Reviewing work completion records would have identified the use of original scope information instead of work completion records to justify the procedure change.
CORRECTIVE ACTION
The following corrective action has been completed.
- 1. CNS procedures related to DG that include aligning the DG to the grid during surveillance testing were identified. These procedures have either been revised to require declaring the DG inoperable when aligned in parallel with the grid, or already contained the requirement to declare the DG inoperable.
The following corrective actions have been entered into the CNS corrective action program.
- 1. The procedure change process will be revised to provide a list of conditions that meet the requirements of "non-intent."
- 2. A sample of non-administrative instant procedure changes implemented using the instant change process that was in use between 1998 and 2000 will be reviewed to ensure that the information upon which the change was based is accurate and the instant change met the requirements to qualify as an instant change.
- 3.
Determine the availability of the DG's between December 12, 1998 and April 21, 2006. Determine the impact on performance indicators reported to the Nuclear Regulatory Commission during the same period.
- 4. The DG control circuitry will be modified so that it is not necessary to declare a DG inoperable when it is aligned parallel to the grid during DG surveillance testing.
PREVIOUS EVENTS A review of CNS LERs since 2000 was conducted. There were no reportable events similar to this event.
NRC FORM 366 (1-2001)
I ATTACHMENT 3 LIST OF REGULATORY COMMITMENTS© Correspondence Number: NLS2006049 The following table identifies those actions committed to by Nebraska Public Power District (NPPD) in this document. Any other actions discussed in the submittal represent intended or planned actions by NPPD. They are described for information only and are not regulatory commitments. Please notify the Licensing Manager at Cooper Nuclear Station of any questions regarding this document or any associated regulatory
commitments
COMMITMENT
COMMITTED DATE
COMMITMENT
NUMBER OR OUTAGE None
.4.
- - 4
.5.
4
.4.
4
.4.
4
.4.
4 4-4 I.
- 4.
4 I
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