05000254/LER-2005-001
| Docket Numbersequential Revmonth Day Year Year Day Year Quauatiesd Cit Nuclear Power I Unit 2Month Ies Stat On, Number No. 05000265 | |
| Event date: | 02-03-2005 |
|---|---|
| Report date: | 04-01-2005 |
| 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 41362 | 10 CFR 50.72(b)(3)(ii)(B), Unanalyzed Condition |
| 2542005001R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Quad Cities Nuclear Power Station Unit 1 05000254 NUMBER NUMBER (If more space is required, use additional copies of NRC Form 366A)(17)
PLANT AND SYSTEM IDENTIFICATION
General Electric - Boiling Water Reactor, 2957 Megawatts Rated Core Thermal Power Energy Industry Identification System (EIIS) codes are identified in the text as [XX].
EVENT IDENTIFICATION
4160 Volt Relaying and Metering Single Failure Vulnerability
A. CONDITION PRIOR TO EVENT
Unit: 1 Event Date: February 3, 2005 Event Time: 1819 hours0.0211 days <br />0.505 hours <br />0.00301 weeks <br />6.921295e-4 months <br /> -Reactor Mode: 1 Mode-Name: Power Operation Power .Level: 085% Unit: 2 ,Event Date: February 3, 2005 Event Time: 1819 hours0.0211 days <br />0.505 hours <br />0.00301 weeks <br />6.921295e-4 months <br /> Reactor Mode: 1 Mode.Name: Power,',Operation Power Level: 085% Power Operation (1) - Mode switch in the RUN position with average reactor coolant temperature at any temperature.
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 such that a postulated single failure would result in the loss of all offsite and onsite AC power to both divisions of safety related distribution buses. The initial Quad Cities' 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). Quad Cities Nuclear Power Station expanded the review of the design for an open circuit single failure vulnerability.
On February 3, 2005, at 1819 hour-6 (CST), with Unit 1 at approximately 85 percent power and Unit 2 at approximately 85 percent power, Quad Cities Engineering personnel confirmed that a single failure vulnerability existed on 4160 Volt Relaying and Metering transformers associated with the Unit Auxiliary Transformers (UATs) [XFMR] 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 could have caused the neutral overcurrent relay to trip and lock out the main, reserve and cross-tie feed breakers. These combined protective relay trips would have acted to trip and lock out the circuit breakers supplying electrical feeds to Buses 13 (23) and 14 (24) [EA], essentially isolating these buses from their normal and emergency power FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Quad Cities Nuclear Power Station Unit 1 05000254
- - NUMBER0 NUMBER 2005 (If more space is required, use additional copies of NRC Form 366A)(17) sources. Emergency power from the Emergency Diesel Generators (EDGs) [EK] would have been available to safety related Buses 13-1 (23-1) and 14-1 (24-1) [EB], but the Residual Heat Removal Service Water (RHRSW) system [BI], which is fed from Buses 13 (23) and 14 (24), would have remained without a power source. If this failure occurred during a Loss of Coolant Accident, then the RHRSW pumps may not have been able to start within the required ten minutes. These events were promptly reported to the NRC in accordance with 10CFR50.72(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 10CFR50.72(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 have since revealed that there was not a potential loss of safety function as a result of these events.
The single failure vulnerabilities were removed on February 4, 2005, at 0152 hours0.00176 days <br />0.0422 hours <br />2.513228e-4 weeks <br />5.7836e-5 months <br />, and the affected equipment was declared operable.
These events are 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 vulnerabilities defeated the single failure design of plant systems, 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 Required Action Times.
C.O CAUSE OF EVENT The root cause of these events was determined to be an existing latent design deficiency. Additionally, this design deficiency was not identified in design reviews performed for subsequent modifications, which allowed the systems to continue to be deficient for a common single failure.
The original design 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 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 power readings 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.
FACILITY NAME (1)
- DOCKET NUMBER (2) , LER NUMBER (6 (If more space is required, use additional copies of NRC Form 366A)(17) Quad Cities has performed reviews of and modifications to the plant's electrical distribution system since initial operation. An evaluation of these opportunities that could have previously discovered the above described single failure vulnerabilities concluded that the discovery was unlikely.
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 were vulnerable is not a typical or likely failure mode, and has not occurred historically at Quad Cities. Therefore, the consequences of this event had minimal impact on the health and safety of the public and reactor safety.
E. CORRECTIVE ACTIONS
The Corrective Action to Prevent Recurrence is currently in place in the Configuration Change procedures used to install new designs at Exelon facilities.
Separation and Station Single Point Vulnerability reviews and failure mode and effect conditions are part of the preparation and review process contained in existing Exelon procedures. The existing Exelon Human Performance Technical Rigor Standards would also mitigate this situation.
Immediate Corrective Actions:
Temporary modifications were installed to eliminate the single failure vulnerability on both units.
Corrective Actions to be completed:
A permanent modification to the circuitry to eliminate the single failure vulnerability will be designed and installed.
The AC, DC, and EDG systems will be reviewed for latent design deficiency conditions similar to this event.
This event will be reviewed as part of a briefing/discussion to raise awareness of common circuits and the potential to affect more than one train of equipment.
A description of this event will be included in continuing engineering training to raise awareness of common circuits and the potential to affect more than one train of equipment.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Quad Cities Nuclear Power Station Unit 1 05000254 NUMBER NUMBER (If more space is required, use additional copies of NRC Form 366A)(17)
F. PREVIOUS OCCURRENCES
A review of Quad Cities Nuclear Power Station LERs identified no similar events.
G. COMPONENT FAILURE DATA
There were no component failures associated with this issue.