05000446/LER-2008-001
| Document Number03 16 2008 2008 001 00 05 12 2008 N/A 05000 | |
| Event date: | 03-16-2008 |
|---|---|
| Report date: | 05-12-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 4462008001R00 - NRC Website | |
I. DESCRIPTION OF THE REPORTABLE EVENT
A. REPORTABLE EVENT CLASSIFICATION
10CFR50.73(a)(2)(iv)(A); "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B).
B. PLANT OPERATING CONDITIONS PRIOR TO THE EVENT
On March 16, 2008, CPNPP Unit 2 was in Mode 1, operating at 100% power.
C. STATUS OF STRUCTURES, SYSTEMS, OR COMPONENTS THAT WERE INOPERABLE AT THE START OF
THE EVENT AND THAT CONTRIBUTED TO THE EVENT
There were no structures, systems, or components that were inoperable at the start of the event that contributed to the event.
D. NARRATIVE SUMMARY OF THE EVENT, INCLUDING DATES AND APPROXIMATE TIMES
On March 16, 2008, CPNPP Unit 2 was in Mode 1 operating, at 100% power. At 1123 hours0.013 days <br />0.312 hours <br />0.00186 weeks <br />4.273015e-4 months <br />, Operators (utility, licensed),in the Unit 2 control room received alarms that indicated Main Condenser B vacuum [EIIS: (SH)(COND)] was less than 24 inches on Channels 1, 2 and 3 and slowly trending down. By verifying alternate indications, the Operators found that condenser vacuum was actually stable at 28 inches. At 1137 hours0.0132 days <br />0.316 hours <br />0.00188 weeks <br />4.326285e-4 months <br />, Operators received an automatic "low vacuum" trip on the Main Turbine [EllS: (TA)(TRB)]. The turbine trip initiated an automatic reactor trip. All control rods fully inserted, and all three Auxiliary Feedwater pumps [EIIS: (BA)(P)] started as expected as a result of the reactor trip. All systems responded normally during and following the reactor trip.
E. THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM FAILURE, OR PROCEDURAL OR
PERSONNEL ERROR
Operators (utility, licensed) in the Unit 2 Control Room received alarms that indicated Main Condenser B vacuum was less Pan 24 inches on Channels 1, 2 and 3 and trending down.
II. COMPONENT OR SYSTEM FAILURES
A. FAILURE MODE, MECHANISM, AND EFFECT OF EACH FAILED COMPONENT
Not applicable — there were no component failures associated with this event.
B. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE
Not applicable - there were no component failures associated with this event.
C. SYSTEMS OR SECONDARY FUNCTIONS THAT WERE AFFECTED BY FAILURE OF COMPONENTS WITH
MULTIPLE FUNCTIONS
Not applicable - there were no component failures associated with this event.
� ■
D. FAILED COMPONENT INFORMATION
Not applicable - there were no component failures associated with this event.
III. ANALYSIS OF THE EVENT
A. SAFETY SYSTEM RESPONSES THAT OCCURRED
Both Motor Driven Auxiliary Feedwater Pumps and the Turbine Driven Auxiliary Feedwater Pump started as expected as a result of the reactor trip.
B. DURATION OF SAFETY SYSTEM TRAIN INOPERABILITY
Not applicable — there was no safety system train inoperability that resulted from this event.
C. SAFETY CONSEQUENCES AND IMPLICATIONS OF THE EVENT
This event is bounded by the CPNPP Final Safety Analysis Report (FSAR) accident analysis which assumes conservative initial conditions which bound the plant operating range and other assumptions which reduce the capability of safety systems to mitigate the consequences of the transient.
This event is bounded by the analysis of the turbine trip presented in Section 15.2.3 of the CPNPP FSAR. The analysis uses conservative assumptions to demonstrate the capability of pressure relieving devices and to demonstrate core protection margins. The event of March 16, 2008, occurred at 100 percent reactor power, and all systems and components functioned as designed.
Based on the above, it is concluded that the health and safety of the public were unaffected by this condition and this event has been evaluated to not meet the definition of a safety system functional failure per 10CFR50.73(a)(2)(v).
IV.C CAUSE OF THE EVENT The cause of this event was a sheared common vacuum instrument sensing line to the Main Condenser 2B due to the sensing line being designed and installed without adequate flexibility. The failure was ultimately the result of a combination of residual stress induced by condenser movement and cyclic stress due to vibration.
As a part of the Turbine Generator (TG) Protection System Reliability Digital Upgrade Project, the existing TG mechanical hydraulic control system was replaced with a new digital control system during the eleventh refueling outage on Unit 1 and the ninth refueling outage on Unit 2. A common vacuum source to each Main Condenser shell was installed to supply three transmitters with a 2/3 trip logic. This resulted in a passive single point failure vulnerability for a loss of the common sensing line. During this event, loss of the Main Condenser shell B common sensing line due to inadequate flexibility and high vibration caused all three transmittqrs to sense a low condenser vacuum, and this led to a low vacuum trip on the Main Turbine and an automatic reactor trip.
CNRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER Enclosure to TXX-08071
V. CORRECTIVE ACTIONS
Immediate corrective actions included repair of the sheared vacuum instrument sensing line and modification of the supports on the Unit 1 and 2 sensing lines to add flexibility to the lines. These support modifications reduced the bending moments, which in turn reduced the chronic stresses that led to the failure of the sensing line on March 16, 2008.
To reduce the probability of similar events occurring in the future, the passive single point failure vulnerability was addressed on Unit 2 during the tenth refueling outage by removing the common line and installing separate sensing lines to the transmitters. As a part of this modification, the condenser pipe segments were also modified to reduce vibration.
As a part of the CPNPP Corrective Action Program, a similar modification to remove the common line and install separate, sensing lines will also be performed on the Unit 1 Main Condensers during the next refueling outage.
Engineering will issue a lessons learned on this event and will also review other similar passive single point failure vulnerabilities in both units.
VI. PREVIOUS SIMILAR EVENTS
There have been no previous similar reportable events at CPNPP in the last three years.
�NRC FORM 366A (9-2007) PRINTED ON RECYCLED PAPER