05000237/LER-2005-001
| Dresden Nuclear Power Station | |
| Event date: | 02-03-2005 |
|---|---|
| Report date: | 04-04-2005 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition |
| Initial Reporting | |
| ENS 41366 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2372005001R00 - NRC Website | |
Dresden Nuclear Power Station (DNPS) Units 2 and 3, are General Electric Company Boiling Water Reactors 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: 02-03-2005 Reactor Mode: 1 Mode Name: Power Operation Power Level: 95 percent Reactor Coolant System Pressure: 1000 psig
B. Description of Event:
On January 27, 2005, Crystal River Unit 3 reported to the NRC in Event Notification Report Number 41362, a design deficiency in a common metering circuit where a postulated single failure would result in the loss of all offsite and onsite alternating current (AC) power to both Divisions of safety related electrical distribution buses. The initial DNPS reviews focused on a spurious ground on the common circuitry. These initial reviews identified that grounds would not have an adverse impact on the circuitry. Similar reviews were conducted at other Exelon sites. On February 1, 2005, LaSalle Station identified that its circuitry was vulnerable to a single failure vulnerability that was due to spurious open circuits (Event Notification Report Number 41366). DNPS expanded the review of the design for open circuit single failure vulnerability.
at approximately 96 percent power, DNPS engineering personnel confirmed that single failure vulnerabilities existed on 4160 Volt Relaying and Metering transformers [XFMR] associated with the Unit Auxiliary Transformers (UATs) and Reserve Auxiliary Transformers (RATs) on both units.
Although the Relaying and Metering transformers were fully functional at that time, failure of the Relaying and Metering transformers circuitry would have caused the neutral overcurrent relay to trip and lockout the main, reserve and tie feed breakers. These combined protective relay trips would have acted to trip and lock out the circuit breakers supplying electrical power to buses 23 (33) and 24 (34), essentially isolating these buses from their normal and emergency power sources.
Emergency power from the Emergency Diesel Generators (EDGs) [DG] would have still been available to safety related buses 23-1 (33-1) and 24-1 (34-1), but the Containment Cooling Service Water (CCSW) [BI] system, which is fed from buses 23 (33) and 24 (34), would have remained without a power source. If this failure occurred during a Loss of Coolant Accident, then the CCSW pumps may not have been able to be started within the required time. This event was promptly reported to the NRC in accordance with 10 CFR 50.72(b)(3)(v)(B), "Any event or condition that at the time of discovery could have prevented the fulfillment of the safety function of structures or systems that are needed to remove residual heat," and 10 CFR 50.72(b)(3)(ii)(B), "Any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degrades plant safety." Subsequent investigations revealed that there was not a potential loss of safety function as a result of this event.
The single failure vulnerabilities were removed on February 4, 2005, at 0009 hours1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br />, and the effected equipment was declared operable.
This event is being reported in accordance with:
10 CFR 50.73(a)(2)(ii)(B), "Any event or condition that resulted in the nuclear power plant being in an unanalyzed condition that significantly degraded plant safety," as the single failure vulnerability defeated the single failure design of plant systems on Units 2 and 3, and 10 CFR 50.73(a)(2)(i)(B), "Any operation or condition which was prohibited by the plant's Technical Specifications," as the single failure vulnerability rendered Technical Specification equipment inoperable for a period of time that exceeded Technical Specification Allowed Outage Times on Units 2 and 3.
C. Cause of Event:
The root cause of this event was determined to be an existing latent design deficiency.
The original design of DNPS met the engineering standard practice of the time (the late 1960's and early 1970's) and was designed prior to the issuance of many of the Institute of Electrical and Electronics Engineers (IEEE) standards related to nuclear power stations. This design can also be found in non-nuclear facilities of similar vintage. The initial design contained the above described single failure vulnerability.
The purpose of interconnecting the transformers was to obtain total power reading from the UAT / RAT for totalization. These meters were a precursor to the revenue metering of today, and were used for internal utility accounting purposes. The transformers used for the totalizer were not required to be as accurate as those used for revenue metering. The preferred practice was to use separate transformers for metering and relaying; however, it was not uncommon, due to space limitations, cost, or retrofits, to use relaying transformers for metering and to interconnect them in this configuration.
D. Safety Analysis:
The safety significance of the event is minimal. The single failure to which the 4160 Volt Relaying and Metering transformers associated with the UATs and RATs on both units are vulnerable is not a typical or likely failure mode, and has not occurred historically at DNPS. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. Corrective Actions:
A temporary modification was installed to eliminate the identified single failure vulnerabilities.
The Corrective Action to Prevent Recurrence is currently in place as a result of the Configuration Change procedures used to install new designs at Exelon facilities. Separation and Station Single Point Vulnerability reviews, Exelon Human Performance Technical Rigor Standards, and failure mode and effect conditions are part of the preparation and review process contained in existing Exelon procedures.
A permanent modification to the circuitry to eliminate the single failure vulnerability will be designed and installed.
A preliminary DNPS engineering review for similar existing latent design deficiencies found no other similar deficiencies. A corrective action is in place for engineering to complete an extent of condition review of the alternating current, Emergency Diesel Generators, and direct current systems for latent design deficiency conditions similar to this event.
DNPS engineering will review this event as part of group discussions to raise awareness of common circuits and the potential to affect more than one train of equipment.
DNPS engineering will include a description of this event in continuing engineering training to raise awareness of common circuits and their potential to affect more than one train of equipment.
F. Previous Occurrences:
A review of DNPS Licensee Event Reports (LERs) identified no similar events.
G. Component Failure Data:
NA