05000331/LER-2007-004
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No. 05000 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor |
| 3312007004R00 - NRC Website | |
I. Description of Event:
On February 24, 2007, while the plant was shutdown and in Mode 5 for Refueling Outage 20, a severe winter storm brought freezing rain, ice and high winds to the DAEC grid area, causing degraded voltage conditions on essential busses. The DAEC was performing Core Alterations and the B SBDG was being manually run for post maintenance testing. The A Residual Heat Removal (RHR) Pump was operating in Shutdown Cooling mode. Time to boil was calculated as 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br /> and 48 minutes. Both Fuel Pool Cooling Pumps were in service providing cavity and fuel pool cooling, since the Fuel Pool Gates were removed and Condition A.
The DAEC switchyard is supplied by six offsite circuits of which two are 345KV (Hazelton and Tiffin/Hills) and four are 161KV (Fairfax, 6th Street, Dysart/Vinton and Hiawatha). Due to severe weather conditions, the Hazelton line was lost at 1654, the Tiffin/Hills line was lost at 1714. At 1755, the A SBDG automatically started on bus undervoltage. The A SBDG did not load since the power remained available from the startup transformer. The B SBDG was already manually started for regulator tuning following governor replacement. Core Alterations were secured at 1756 based on indications that the grid was becoming unstable. At 1757 a full scram occurred due to loss of B RPS and Neutron Monitoring System Trip on the A RPS, and Groups 1 through 5 isolations (excluding the Main Stream Isolation Valves) occurred, resulting in a loss of Shutdown Cooling. At the time of the scram, all control rods were inserted in cells containing fuel, therefore, there was no rod movement.
At 1820, bus degraded voltage conditions caused both running SBDGs to load onto their respective essential busses. Fuel Pool Cooling, General Service Water (GSW) and B Control Building Chiller were lost due to load shedding. At 1825, grid voltage recovered, however subsequent conversations with the grid operator indicated that the area grid, including the DAEC switchyard, was isolated from most of the offsite power sources and only the 6 Street and Hiawatha 161KV lines remained powered into the DAEC switchyard. All work activities in the power block were suspended at 1835. The essential busses remained loaded to the SBDG.
Shutdown Cooling was restored to operation at 1826. GSW was restored to operation at 1844. At 1850, the B Control Building Chiller and Fuel Pool Cooling system operation commenced. The RPS scram signal was reset at 1912.
Grid repair and recovery allowed the essential bus 1A4 power supply to be transferred from the B SBDG to the Startup Transformer at 1148 on February 25, 2007. The essential bus 1A3 power supply was transferred from the A SBDG to the Startup Transformer at 0049 on February 26, 2007.
This event was reported to the NRC as an 8 hour9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> event under 10 CFR 50.72(b)(3)(iv)(B), Any event or condition that results in valid actuation of: (1) Reactor protection system (RPS) including reactor scram and reactor trip, (2) General containment isolation signals affecting containment isolation valves in more than one system or multiple main steam isolation valves (MSIVs), and (8) emergency ac electrical power systems; 10CFR 50.72(b)(3)(v), Any event or condition that could have prevented the fulfillment of the safety function of structures or systems that are needed to: (A) Shut down the reactor and maintain it in a safe shutdown condition, and (B) Remove residual heat.
FACILITY NAME (1) � DOCKET NUMBER (2) � LER NUMBER (8) � PAGE (3) I � REVISION 05000331 � YEAR NUMBER � NUMBER Duane Arnold Energy Center Condition for Operation for one Control Building Chiller subsystem inoperable.
Ii. Assessment of Safety Consequences:
This report is being submitted pursuant to 10CFR50.73(a)(2)(iv)(A), 10CFR50.73(a)(2)(v)(A) and 10CFR50.73(a)(2)(v)(B).
In this event, disturbance of the offsite electrical grid resulted in automatic loading of the SBDGs and in temporary loss of Shutdown Cooling. However, significant margin existed throughout the event to the unacceptable consequence of uncovering spent fuel due to boiling of the water in which it is submerged.
A large volume of water covered the fuel in the reactor vessel and in the spent fuel pool, and the rate of decay heat production was relatively low since 21 days had elapsed since the reactor had been shutdown. Therefore, a long period of time would be required for this body of water to start to boil (estimated to be approximately one and one-half days) in the event that cooling systems failed to operate.
Although the Shutdown Cooling system was out of service for approximately one-half hour, there was no noticeable change in reactor coolant temperature during this time.
Momentary loss of power to essential busses 1A3 and 1A4 caused both Shutdown Cooling and Spent Fuel Pool Cooling pumps to trip. Power was available to both systems however, upon connection of the SBDGs to their respective essential buses. Therefore, the water in both the reactor and spent fuel pool were maintained near their initial temperatures.
This event did not result in a Safety System Functional Failure.
III. Cause of Event:
An investigation was completed under Apparent Cause Evaluation (ACE) 1697. The cause of this event was a severe winter storm that brought snow, ice accumulation and high winds to the area. This storm caused extensive damage to the area including damage to the electrical grid.
IV. Corrective Actions:
There are no corrective actions related to the degraded voltage event because the causes, severe weather conditions, are outside the control of the DAEC.
V. Additional Information:
Previous Similar Occurrences:
Transformer was inadvertently de-energized. At the time of the event, the plant was in cold shutdown with essential power being supplied by the Standby Transformer due to the Startup Transformer being out of service for maintenance.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Duane Arnold Energy Center 05000331 The cause of this event was determined to be a personnel error by a non-nuclear utility system protection technician who failed to block trip signals during a breaker failure relay test. The task being performed was not covered by an approved procedure.
ENS System and Component Codes:
DA — Fuel Pool Cooling and Purification System EA — Medium-Voltage Power System (601V through 35kV) EK — Emergency Onsite Power Supply System FK — Switchyard System JM — Containment Isolation Control System VI — Control Building/Control Complex Environmental Control System BO — RHR Shutdown cooling JC — RPS KG - GSW Reporting Requirements:
This report is being submitted under 10CFR50.73(a)(2)(iv)(A), 10CFR50.73(a)(2)(v)(A) and 10CFR50.73(a)(2)(v)(B).