05000339/LER-2005-001
| Document Numbersequential Revisionmonth Day Year Year Month Day Year | |
| Event date: | 08-05-2005 |
|---|---|
| Report date: | 09-28-2005 |
| 3392005001R00 - NRC Website | |
FACILITY NAME (1) � DOCKET 1LER NUMBER 6) PAGE (3) 1.0 DESCRIPTION OF THE EVENT On August 5, 2005, at 2227 hours0.0258 days <br />0.619 hours <br />0.00368 weeks <br />8.473735e-4 months <br /> North Anna Unit 2 experienced an automatic Reactor trip from 100 percent power during a severe thunderstorm. A lightning strike was the direct cause of this event. The lightning strike caused an Overtemperature Delta T (OTDT) Unit 2 Reactor Trip signal without an actual overtemperature condition. The reactor trip initiated a turbine trip. The control room team responded to the reactor trip in accordance with procedure 2-E-0, Reactor Trip or Safety injection. All Engineered Safety Feature (EIIS System-JE) equipment responded as designed. The post trip response progressed smoothly and within five minutes the Operations crew transitioned to 2-ES 0.1, Reactor Trip without Safety Injection. Initially, Reactor Coolant System (RCS) (EIIS System-AB) pressure decreased to approximately 1947 psig and subsequently returned to normal operating pressure. RCS temperature stabilized at approximately 547 degrees F and Pressurizer level stabilized at 31 percent.
Prior to the reactor trip a spike increase on the "A" loop hot-leg and cold-leg temperatures was noted by the Control Room team. The "B" loop cold-leg temperature also spiked.
There was no change in "B" loop hot-leg temperature, or in "C" loop temperatures. The spikes lasted for approximately 1.1 seconds before returning to normal operating values.
As a result of the spikes, the "A" loop overtemperature delta-T (OTDT) reactor trip setpoint decreased and the "A" loop delta-T increased above the OTDT setpoint. The "B" loop OTDT setpoint decreased. Although the "B" loop delta-T decreased it still exceeded the OTDT reactor trip setpoint. The logic for a OTDT reactor trip is two out of three loops, and the reactor tripped due to "A" and "B" loop delta-T exceeding their respective loop's OTDT reactor trip setpoint. The Unit 2 "Cu loop temperature instruments and all three Unit 1 loops were unaffected during this lightning event.
2.0 SIGNIFICANT SAFETY CONSEQUENCES AND IMPLICATIONS This event posed no significant safety implications since the reactor protection functioned to trip the reactor safely. All Engineered Safety Feature equipment responded as designed. Therefore, the health and safety of the public were not affected by this event.
A non-emergency 4-hour report was made to the NRC Operations Center at 0020 hours2.314815e-4 days <br />0.00556 hours <br />3.306878e-5 weeks <br />7.61e-6 months <br /> on August 6, 2005, in accordance with 10 CFR 50.72 (b)(2)(iv)(B). During this call an 8 hour report was also made in accordance with 10 CFR 50.72(b)(3)(iv)(A) due to actuation of the Auxiliary Feedwater System (EIIS System-BA). This event is reportable pursuant to 10 CFR 50.73 (a)(2)(iv)(A) for a condition that resulted in automatic actuation of the reactor protection system and the AFW system.
NRC FORM 36M (7-2001) NRC FORM 366Al U.S. NUCLEAR REGULATORY COMMISSION (7-2001) FACILITY NAME (1) DOCKET LER NUMBER 6) PAGE (3) 3.0 CAUSE The lightning strike caused an Overtemperature Delta T (OTDT) Unit 2 Reactor Trip signal without an actual overtemperature condition. The "A" and "B" loop delta-T exceeded their respective loop's OTDT reactor trip setpoint.
Event investigation identified two previous lightning strikes which caused instrumentation spiking on the Unit 2 "A" and "B" loops while not affecting the Unit 2 "C" loop temperature instruments nor any of the three Unit 1 loops. A lightning strike on September 17, 1998, resulted in a Unit 2 automatic reactor trip from the same initiating reactor trip signal. A lightning strike on July 28, 2003, did not result in a unit trip but did result in similar instrumentation fluctuations on one channel.
Although the spurious signals were the result of a lightning strike, inadequate grounding was determined to be the cause of the instrument spikes resulting in the reactor trip. The Unit 2 spare T - hot and T — cold narrow range RTD shields were not grounded at the terminal boards in Protection Cabinets Channel 1 & 2 (2-EI-CB-51 and 2-EI-CB-52).
Drawings show that the shield on these spare RTD's should be connected to ground in the process racks.
4.0 IMMEDIATE CORRECTIVE ACTION(S) The Control Room team responded to the reactor trip in accordance with procedure 2-E-0, Reactor Trip or Safety injection. The post trip response progressed smoothly and within five minutes the Control Room team transitioned to 2-ES-0.1, Reactor Trip without Safety Injection.
5.0 ADDITIONAL CORRECTIVE ACTIONS Work Orders were generated to correct improper spare RTD grounds. The remaining T hot and T — cold RTD shields were checked in both units and were found properly grounded.
All six protection cabinets have been inspected for correct spare RTD ground wire configurations. The only cabinets identified were 02-EI-CB-51 & 52. These were the only channels that exhibited protection channel spikes on their Plant Computer System (PCS) printouts during the lightning strike.
FACILITY NAME (1) DOCKET LER NUMBER (6) PAGE (3) 6.0 ACTIONS TO PREVENT RECURRENCE Installation of the spare RTD grounds should prevent instrumentation spikes causing the OTDT fluctuations.
7.0 SIMILAR EVENTS LER Report No. 50-339/98-004-00, dated October 6, 1998, documents an automatic reactor trip due to a lightning strike. The initiating reactor trip signals were the same for both events.
8.0 ADDITIONAL INFORMATION Unit 1 was not affected by this event.