05000237/LER-2007-002
| Docket Number Sequential Revmonth Day Year Year Month Day Year N/A N/Anumber No. | |
| Event date: | 05-04-2007 |
|---|---|
| Report date: | 07-03-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2372007002R00 - NRC Website | |
Dresden Nuclear Power Station (DNPS) Unit 2 is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A.� Plant Conditions Prior to Event:
Unit: 02� Event Date: 5-4-2007 Reactor Mode: 1� Mode Name: Power Operation Power Level: 100 percent Reactor Coolant System Pressure: 1000 psig
Description of Event:
On May 4, 2007, at 2305 hours0.0267 days <br />0.64 hours <br />0.00381 weeks <br />8.770525e-4 months <br /> (CDT), with Unit 2 at approximately 100 percent power, DNPS control room personnel noted decreasing reactor water level and manually scrammed the reactor. All controls rods inserted into the core. The decreasing reactor water level was due to a loss of feedwater transient caused by an unexpected closure of all Unit 2 Condensate Prefilter System valves. Prior to this event, technicians were replacing a failed Condensate Prefilter System [SD] Central Processing Unit (CPU) card. The High Pressure Coolant Injection System [BJ] was utilized to restore reactor water level. During the event, the Main Steam Isolation Valves [V] closed, the Reactor Recirculation Pumps [AD] were tripped and the Unit 2 and 3 Emergency Diesel Generators [EK] automatically started as designed.
An ENS call was made on May 5, 2007, at 0113 hours0.00131 days <br />0.0314 hours <br />1.868386e-4 weeks <br />4.29965e-5 months <br /> (CDT) for the above-described event. The assigned ENS event number was 43346.
Unit 2 was restored to the electrical grid at 1638 hours0.019 days <br />0.455 hours <br />0.00271 weeks <br />6.23259e-4 months <br /> (CDT) on May 8, 2007.
This event is being reported in accordance with 10 CFR 50.73(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).
The event included the manual actuation of the Reactor Protective System.
C.� Cause of Event:
The root cause of the unexpected closure of all Unit 2 Condensate Prefilter System valves and the subsequent loss of feedwater transient was a vendor latent software deficiency that caused the valves to close when the new Condensate Prefilter System CPU card was energized.
Pre-operational testing in 2001 of the Unit 2 Condensate Prefilter System identified the system was not operating as expected as the system periodically initiated auto-bypassing the Condensate Prefilters from power supply perturbations. DNPS requested the software contractor to perform a software revision to alleviate the power supply perturbation effects. Based on the Exelon request, a contractor programmer performed a logic change which implemented the "0-1" logic for Condensate Prefilter System valve alignment. This revision utilized registers for valve position with zero for closed and non-zero for open. The revised software retained a memory of the valve positions prior to the momentary power interruption, and upon power restoration, sends a signal to align the valves to the last-known position, The original intent of the software revision performed as designed since implementation in 2001 as demonstrated by the periodic soft reboots to reset the CPU.
The investigation of this event identified the programmer implementation of the "0-1" logic for valve alignment introduced an unrecognized latent software deficiency under which all valves could be sent a closed signal. The latent software deficiency was not apparent during the post-modification testing and normal system operation, including CPU re-boots. The condition only manifests when the all CPU registers contain a "0" resulting from a lack of previous valve positions in the CPU memory. All registers would contain a "0" when a new CPU card is installed. The vendor was unaware of the existence of the software deficiency until discovered during the post-analysis of this event.
Condensate Prefilters were bypassed. Further investigation identified the Unit 2 Condensate Prefilter System CPU card had failed. DNPS initiated work to replace the CPU card. This was a first time evolution, as the Condensate Prefilter System CPU card had not been replaced since initial system startup. Upon energizing the CPU card, all the Condensate Prefilter System valves were given a closed signal due to the new CPU registers all containing a "0." The closure of all Condensate Prefilter System valves caused a loss of feedwater transient and the resulting need to manually scram the reactor.
An extent of condition review identified that the Unit 3 Condensate Prefilter System and the Station Black Out Diesel (SBO) Generator are affected by a similar latent software deficiency. Corrective actions from this event will address the Unit 3 Condensate Prefilter System. Based on the SBO being in a standby mode, any risk associated with online replacement of the CPU is minimal, as it would only be replaced during standby operation followed by an operability startup and testing. Failures under this condition have no operational impact on a system required for plant operation.
Safety Analysis:
The safety significance of the event is minimal. All control rods inserted as a result of the scram and all systems responded as required. A loss of feedwater event is an analyzed transient in the DNPS Updated Final Safety Analyses Report in Section 15.2.7, "Loss of Normal Feedwater Flow," and is classified as a moderate frequency event. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
Corrective Actions:
The corrective actions to address this event include the following actions.
DNPS has proceduralized the requirements for performing Condensate Prefilter System troubleshooting, maintenance and system restoration.
DNPS will develop a design specification for the Condensate Prefilter valve CPU software.
DNPS will evaluate a configuration change to the Condensate Prefilter System software that opens the bypass valves on low feedwater flow.
Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) for the last three years did not identified any LERs associated with latent software deficiency.
.�Component Failure Data:
NA