05000498/LER-2006-001
| Docket Numbersequentialmonth Day Year Year Rev Mo Day Yearnumber No N/A | |
| Event date: | 03-28-2006 |
|---|---|
| Report date: | 05-25-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4982006001R00 - NRC Website | |
I.�DESCRIPTION OF EVENT
A. REPORTABLE EVENT CLASSIFICATION
This event is reportable pursuant to 10 CFR 50.73(a)(2)(i)(B). The South Texas Project (STP) determined that Standby Diesel Generator (SBDG) 13 was inoperable and actions had not been taken as required by Technical Specifications. The requirements of Technical Specification 3.8.1.1 were not met in that one of the three separate and independent standby diesel generators was not operable, and action 3.8.1.1.b was not implemented within the required time. Consequently, STP Unit 1 was in a condition prohibited by Technical Specifications.
B. PLANT OPERATING CONDITIONS PRIOR TO EVENT
STP Unit 1 was in Mode 1 at 100% power.
C. STATUS OF STRUCTURES, SYSTEMS, AND COMPONENTS THAT WERE INOPERABLE
AT THE START OF THE EVENT AND THAT CONTRIBUTED TO THE EVENT
No other inoperable structures, systems, or components contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT
At 2305 on March 25, 2006, SBDG 13 failed to reach 110% of rated load during performance of a 24-hour surveillance run. Actual performance was 103% of rated load. SBDG 13 was declared inoperable, but functional. Potential cause(s) for not reaching 110% load were initially thought to be either a defective speed controller or generator load sensor. On March 26, 2006, both of these components were replaced followed by a post-maintenance test run on March 27, 2006 at which time SBDG 13 again failed to reach 110% load. Troubleshooting performed on March 27, 2006, revealed the fuel linkage was in fact not at the required settings. The linkage was reset to the correct values and subsequent post-maintenance testing at 110% was successful. SBDG 13 was declared operable at 0304 on March 28, 2006.
The as-found installation of the fuel linkage limited the fuel flow to the engine such that an engine load of 110% could not be achieved. The fuel linkage had previously been adjusted during the SBDG 13 five-year overhaul in May 2005. The post-maintenance test (PMT) performed following this adjustment only required a test load of 100%, and did not include verification that the diesel generator was capable of achieving 110% of rated load. Neither the work documentation nor the inspection procedure required a load test at 110%.
Additional review determined that the incorrect setting of the linkage did not result from a properly set-up fuel rack becoming loose or misadjusted.
Only SBDG 13 was affected. This event resulted in no personnel injuries, no offsite radiological releases, and no damage to other safety-related equipment.
E. METHOD OF DISCOVERY OF EACH COMPONENT FAILURE, SYSTEM FAILURE, OR
PROCEDURAL ERROR
This condition, failure to reach 110% of load, was identified during a 24-hour surveillance test that is conducted at least every eighteen months in accordance with Technical Specifications.
IL EVENT-DRIVEN INFORMATION
A. SAFETY SYSTEMS THAT RESPONDED
No safety systems were required to respond during this event.
B. DURATION OF SAFETY SYSTEM INOPERABILITY
The fuel linkage had been adjusted during a five-year overhaul performed in May, 2005. The PMT performed following this adjustment did not include verification that the diesel generator was capable of achieving 110% of rated load. Consequently, SBDG 13 is considered to have been inoperable from May 28, 2005, until repairs were implemented and the diesel generator declared operable on March 28, 2006. Technical Specification action 3.8.1.1.b requires that an inoperable standby diesel generator be returned to operable status within 14 days or have the affected unit in at least hot shutdown within the next 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and in cold shutdown within the following 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. Inoperability in this case lasted for approximately 10 months.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
The Onsite Standby Power Supply Systems of Units 1 and 2 each consist of three independent, physically separated, standby diesel generators supplying power to three associated load groups. Rated load capacity of each SBDG is 5500 kW. The Onsite Standby Power Supply Systems of Units 1 and 2 operate independently of each other. Each standby diesel generator and load group of a particular unit is also physically separated and electrically independent from the other two standby diesel generators and their load groups.
Each train (i.e., Load Group) is independent but is not totally redundant; two trains are necessary to mitigate the consequences of a design-basis accident.
With SBDG 13 declared inoperable, Technical Specification action 3.8.1.1.b requires demonstration that the offsite AC sources are operable. This test confirmed that offsite AC power continued to be available.
�Similarly, Technical Specifications also require demonstration that the remaining two Unit 1 diesels are operable. Operability of SBDG 11 and SBDG 12 was confirmed at 100%.
Operability testing of SBDG 11 and SBDG 12 was initially performed at 100% of load, instead of 110%, for the following reasons:
- Meeting the Technical Specification requirements for operability did not require testing at 110% of maximum load.
- The cause of the SBDG 13 inoperability had not been identified at the time of operability tests of SBDG 11 and 12, and there was no indication of a potential common mode failure.
Following determination that the source of the problem was in the fuel linkage, SBDG 11 and SBDG 12 were tested at 110% of load. Unit 2 SBDG 21 was also tested at 110%. There was no need to test SBDG 22 and SBDG 23 at 110% because 24-hour surveillance runs had been performed on June 20, 2005, and October 20, 2005, that confirmed they could be operated at 110% of full load. There have been no fuel linkage maintenance activities on those diesels since then.
4 Initial tests of SBDG 13 determined it could provide output at 103% of its rated load of 5500 kW. The maximum steady-state load expected is 4000 kW. There was sufficient margin that, even with the reduced capacity of SBDG 13, the design requirements would continue to be satisfied.
Because SBDG 13 was still classified as being functional, there were no changes in core damage frequency or large early release frequency. However, in order to perform maintenance prior to surveillance testing on March 26, 2006, SBDG 13 was declared non functional from 2245 until 0000 on March 27. This unavailability for 75 minutes contributed a Conditional Core Damage Probability of 3.03E-9, which is very small when compared to the administrative limit of 1.0E-6.
This event resulted in no personnel injuries, no offsite radiological releases, and no damage to other safety-related equipment.
III. CAUSE OF THE EVENT
The root cause is insufficient barriers in place to ensure the SBDG is operable prior to returning it to service. The five-year diesel preventive maintenance procedure did not require verification to ensure the linkage as-left values are correct. The work package did not specify a 110% load test run following fuel linkage adjustment.
IV. CORRECTIVE ACTIONS
- The maintenance procedure has been revised to add verification that the linkage is in the correct position.
- Diesel generator maintenance requirements for the five-year overhaul will be revised to ensure the fuel system set-up is properly tested.
V. PREVIOUS SIMILAR EVENTS
0E16496 - (0E15542 Update) Diesel Generator Failed to Shutdown When Overspeed Actuated The apparent cause of this event is that the compounding effect of fuel linkage adjustments can misposition the fuel rack control shaft such that it will not sufficiently move the fuel pumps to the zero fuel position. Contributing causes are:
- Inadequate frequency of performing the overspeed trip test.
- Unclear instructions for checking overspeed trip mechanism operation.
Corrective action was to revise the diesel generator refueling frequency maintenance procedures.
Lessons from OE 16496 would not have precluded the operability concerns discussed in this Licensee Event Report.
STP has not had a similar event in which a diesel generator fuel linkage was improperly adjusted.
VI. ADDITIONAL INFORMATION
None.