05000306/LER-2001-002
| Docket Number | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| Initial Reporting | |
| 3062001002R00 - NRC Website | |
EVENT DESCRIPTION
On April 16, 2001, the Prairie Island D6 emergency diesel generator' (EDG) exceeded the Allowed Outage Time (AOT) of Technical Specification 3.7.B.1, which allows one EDG to be inoperable for up to seven days. D6 was declared out of service at 0930 on April 9, 2001, for a 24-hour surveillance run.
That run was terminated by operators when D6 engine2 2 developed high crankcase pressure.
On April 16, 2001, Prairie Island requested a Notice of Enforcement Discretion (NOED) to extend the D6 AOT to ten days to allow completion of the necessary repair and testing to restore D6 operability.
The NRC gave verbal approval of the NOED at about 0800 on April 16, 2001.
The D6 Engine 2 Cylinder Bl piston, piston rings, and cylinder liner were replaced. Following a post- maintenance break-in run and a 24-hour post-maintenance test, another borescopic investigation of Cylinder B1 and Cylinder Al was performed. Some unusual indications were identified by the second borescopic inspection of Cylinder B1. D6 was operated for another 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and a third borescopic inspection was performed. This inspection indicated that Cylinder B1 did not show indication that the unusual indications were signs of degradation. A fast-start test was conducted and D6 was declared operable at 2331 on April 17, 2001.
CAUSE OF THE EVENT
The root cause evaluation of the event (exceeding the AOT of Technical Specification 3.7.B.1) is ongoing, however, the root cause team has identified a number of time delays, that if eliminated, could have prevented this event from exceeding the 7-day AOT, these include:
1. A delay in performing the first borescope inspection — there were not enough engineering resources available and the plant staff did not think there was a major engine problem and, thus, did not immediately increase staffing levels and isolate D6 for borescoping.
2. 0-ring availability — warehouse inventory system did not accurately reflect stock level.
3. During maintenance, plant staff experienced difficulties installing the connecting rod for one cylinder — plant staff elected to wait for the SACM technical representative to arrive before completing the installation.
4. SACM Technical Representative availability — Prairie Island maintenance personnel have limited experience with these types of repairs and elected to wait for the SACM technical representative to arrive from France.
5. The sequence of post-maintenance testing (including borescoping) could have been more efficient.
1 (EIIS Component Identifier: DG) 2 (EIIS Component Identifier: EMG) The cause of the high crankcase pressure was blow-by in one cylinder of one of D6's two diesel engines. Subsequent events on D5 and D6 determined the cause of the blow-by. This root cause will be described in detail in LER 2-01-03.
ANALYSIS OF THE EVENT
Prairie Island requested and received a NOED from the NRC to get enforcement discretion for extending the Allowed Outage Time for the D6 EDG from seven to ten days. Prairie Island did not request a Technical Specification change. The D6 EDG was out of service for longer than allowed by the AOT of Prairie Island Technical Specification 3.7.B.1. Thus, this event is reportable per 10CFR 50.73(a)(2)(i)(B), operation prohibited by Technical Specifications.
Loss Of Safety Function and Other Performance Indicator Impact Because subsequent events have a bearing on the discussion of loss of safety function, whether this event represents a loss of safety function will be discussed in LER 2-01-03.
This event did affect the Safety System Unavailability Performance Indicator for Unit 2 Emergency AC Power. The impact on planned and unplanned unavailability of this event is the amount of time the D6 was unavailable beginning on April 9, 2001 when D6 was removed from service for the 24-hour surveillance and ending on April 17, 2001 when D6 was declared operable (approximately 200 hours0.00231 days <br />0.0556 hours <br />3.306878e-4 weeks <br />7.61e-5 months <br /> of unavailability for D6). Also included is about one hour of unavailability on D5 for Technical Specification required testing.
Because subsequent events have a bearing on the discussion of fault exposure time, the impact this event had on fault exposure time will be discussed in LER 2-01-03.
Significance Determination To support the request for NOED, Prairie Island completed a risk assessment for allowing the AOT of Technical Specification 3.7.B.1 to be extended from seven days to ten days. The risk assessment determined the risk to be very low. Prairie Island did not use the entire additional three days that were granted in the NOED, thus, the NOED risk assessment bounds the risk associated with this event (specifically, the exceeding of the 7-day AOT of Technical Specification 3.7.B.1).
Subsequent Issues with D5 and D6 Following the restoration of D6 operability, subsequent issues arose which called the operability of D5 and D6 into question. The analysis of these issues will be included in LER 2-01-03.
CORRECTIVE ACTION
1. The D6 EDG has been repaired and tested. D6 was declared operable on April 17, 2001.
2. The exceeding of the 7-day AOT is being evaluated in a Condition Report (GEN 20013515) in the Prairie Island Corrective Action process. Corrective actions to prevent recurrence will be identified by the Condition Report.
FAILED COMPONENT IDENTIFICATION
One of the diesel engines on the D6 EDG exhibited high crankcase pressure during a 24-hour surveillance run. The high crankcase pressure was attributed to blow-by in Cylinder B1 of Engine 2 of the D6 EDG. The cylinder liner, piston, and rings were replaced. D6 was manufactured by SACM.
Additional investigation of the blow-by will be discussed in LER 2-01-03.
PREVIOUS SIMILAR EVENTS
No previous similar EDG issues have been reported in the last three years.