ML20141C633
| ML20141C633 | |
| Person / Time | |
|---|---|
| Site: | Paducah Gaseous Diffusion Plant |
| Issue date: | 06/19/1997 |
| From: | Polston S UNITED STATES ENRICHMENT CORP. (USEC) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GDP-97-1016, NUDOCS 9706250354 | |
| Download: ML20141C633 (7) | |
Text
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m United Sutes Enrichment Corporation e
Paducah Site OtTwe l
P.O. Ikin 1410 Paducah, KY 42001 Tel: 502 441.$803 Fax: 502 4415801 June 19,1997 United States Nuclear Regulatory Commission SERIAL: GDP 971016 Attention: Document Control I)esk Washington, DC 20555-001 Paducah Gaseous Diffusion Plant (PGDP) Docket No. 70-7001 - Event Report ER-97-10 Pursuant to 10 CFR 76.120(c)(2), enclosed is the required 30-day written report for the loss of the Criticality Accident Alarm System (CAAS) building horns and UF. detection alarms in C-310. This was initially reported on May 23,1997 (NRC No. 32381). Enclosure 2 is a list of commitments made in this report.
Should you require fuither information on this subject, please contact Bill Sykes at (502) 441-6796.
Sincerely,
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General Manager Paducah Gaseous Diffusion Plant SP:WES:JNil:mel Enclosures (2) cc:
NRC Region III NRC Senior Resident inspector, PGDP 1
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Docket No. 70-7001
' Enclosure 1 Page1of5 Event Report ER-97-10 BACKGROUND The Criticality Accident Alarm system (CAAS) is used for warning plant personnel of a criticality incident. The system is designed to detect gamma radiation and provide a distinctive, audible signal that will alert personnel to evacuate the areas that are potentially affected.
Building homs are used, in addition to local CAAS cluster horns, to provide an audible alarm for evacuation. Solenoid valves open to provide air to the building homs, operating from either 120 VAC or 125 VDC power from a local distribution within the building they support. In C-310 building, the building horn solenoids operate from 125 VDC power. Paducah Gaseous Diffusion Plant (PGDP) Technical Safety Requirements (TSR) Section 2.3.4.7b and Section 2.4.4.2b which covers C-310 building define a Limiting Condition for Operations (LCO) as follows:
LCO 2.3.4.7b/2.4.4.2b " Criticality accident alarm shall be operable (audible)."
Additionally, TSR Section 2.3.4.4 which covers the UF Release Detection System for UF.
6 condensers, accumulators and piped heated housing in C-310 building defines the following LCO:
LCO 2.3.4.4 "The high voltage ('old') UF. release detection heads monitoring the subject equipment shall be operable."
On May 18,1997,125 VDC power was lost to all loads on distribution panel 310-AC-B in C-310. This power is required to operate air solenoids on the C-310 building CAAS horns and annunciate UF leak detection alarms on the high-voltage system. These systems were rendered 6
inoperable due to this failure, when they were required to be operable by the TSRs described above. Additionally, this panel provides 125 VDC power to all audible and visual alarms in both the Area Control Room (ACR) and at the local cell panels and several other miscellaneous loads.
At the time of the loss of power, the inoperability of portions of the UF leak detection system 6
was recognized and appropriate LCO actions were initiated. However, the inoperability of the building CAAS horns was not recognized and no LCO actions were initiated for this system.
Operations personnel were not immediately aware that the CAAS building horns were fed from this panel.
Distribution panel 310-AC-B is supplied 125/250 VDC (250 VDC from one pole to another and 125 VDC from either of these poles to the supplied neutral) power by an automatic transfer
Dock.et No. 70-700I
' Enclosure 1 Page 2 of 5 switch. Normally, this power is supplied by a rectifier that converts 480 VAC to 125/250 VDC.
The transfer switch is designed to sense a loss of normal power and automatically transfer the distribution panel load to backup power supplied by a bank of batteries.
The sensing of normal power is accomplished by a transfer relay that is energized when normal power is available. Energizing this relay closes contacts on the relay that provides power to the normal coil of the transfer switch. This normal coil is energized and mechanically closes contactors that connect the rectifier to the distribution panel. During the loss of normal power, the relay is de-energized, opening the contacts to the normal coil and closing contacts that supply power to the emergency coil. This energizes the emergency coil, causing the emergency contactor to close and connect the backup battery power source to the distribution panel.
DESCRIPTION OF EVENT At approximately 1825 on May 18,1997, operators in the Area Control Room of C-310 heard a
" pop" from an alarm panel in the ACR. Immediate investigation revealed the loss ofpower to audible and visual alarms in the building. Testing was conducted to determine if this unavailability of alarm power extended to the high-voltage UF leak detection system. This 6
alarm was discovered to also be inoperable and the front line manager responsible for the C-310 building directed the initiation of LCO actions on this system.
Further investigation by Operations, Maintenance, and Engineering personnel revealed that all 125 VDC power supplied by the normal rectifier had apparently been lost. Several attempts to reset the rectifier supplying this power yielded only brief, intermittent return of the power. At approximately 1950, the 480 VAC breaker supplying the rectifier was opened. At this time, the automatic transfer switch connected the distribution panel to the C-310 battery bank.
Subsequent troubleshooting by Maintenance and Engineering revealed a sus's...J problem with the control relay in the transfer switch. (Control relay manufactured by Square 6 Model 7001 Class, type RO-11.) Discovery that the building CAAS homs were inoperable during the period of the outage occurred on May 22,1997, during planning activities associated with the replacement of this control relay. At that time, problem report PR-EN-97-2713 was generated to document the CAAS inoperability. A notification was made to the NRC Headquarters Operations office at 0646 on May 23,1997, pursuant to 10 CFR 76.120 (c)(2). On May 23, 1997, the control relay of the transfer switch was replaced under work package R-9703321-07.
(Completed Action No.1) An attempt to return the normal power to service following this repair led to the discovery of degraded fuses on the output of the rectifier. The fuses were replaced and normal power was restored to all 125 VDC loads in the early moming hours of May 24,1997.
Docket No. 70-7001
' Enclosure 1 Page 3 of 5 CAUSES OF THE EVENT, Both the replaced control relay and the replaced rectifier fuses have been inspected by Engineering personnel as part of this investigation. The contacts on the control relay responsible for energizing the backup power coil exhibited a large amount of visible oxidation. Additionally, these contacts did not pass a bench continuity test when closed. The rectifier fuses were examined closely and found to have loose and corroded caps. Additionally, the internal element of one fuse was found pitted.
While the exact sequence olevents in the failure of the 125 VDC power cannot be definitively stated, the following is the most likely scenario based on the above component examinations and interviews with Operations, Maintenance, and Engineering personnel involved in the response to and resolution of the incident. At least one of the two replaced rectifier fuses lost continuity under load due to its degraded condition. The transfer switch control relay was de-energized by this interruption in normal power from the rectifier and the backup power contacts on the relay closed. Due to oxidation, electrical connection was not made between the contacts. The degraded fuse (s) was able to intermittently conduct and caused the control relay to cycle the backup power contacts during attempts to reset the rectifier. The backup power contacts of the control relay made electrical connection when Operations opened the 480 VAC breaker to the rectifier. This allowed the transfer switch to connect the battery room to the 125 VDC load.
a.
Direct Cause The initiator of the event was the loss of power attibuted to the degraded condition of the fuse which could not continuously maintain continuity. The direct cause of the event was the inability to achieve an electrical connection between the emergency power contacts of the relay, due to excessive oxidation.
- b. Root Cause The root cause of this failure is inadequate performance of preventive maintenance which ensures the oxidation buildup does not reach a level which prevents electrical continuity at the contacts. The Preventive Maintenance (PM) database contains a task to conduct an " Area Control Room (ACR) DC Power Switch Test." This task is applicable to C-310, as well as C-331, C-333, C-335, and C-337. This test is scheduled to be conducted annually; however, the last test for C-310 was completed on August 15,1995. The test which should have been completed in C-310 during August 1996 was not done and the delinquent status of this task has been listed on monthly notices of delinquency which are issued to managers via the PM database. The primary purpose of the DC Power Switch Test is to verify the ability to switch over to backup power; however, a secondary benefit is achieved as the flow of current through the contacts helps to reduce the buildup of oxidation. If this test had been completed on schedule, this unexpected loss of DC power which occurred on May 18 may have been
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Dock.et No. 70-7001
' Enclosure 1 Page 4 of 5 j
l avoided. If failure had occurred during a scheduled test, normal power could have been l
restored and an extended outage averted.
Corrective Action Nos. I and 2 will inspect the current condition of the transfer relay and the associated fuses to the rectifier of the DC power transfer systems in C-331, C-333, C-335, and C-337 and will identify and correct deficiencies, as required. Corrective Action No. 3 will evaluate and determine the preventive maintenance requirements for the transfer relay, including evaluating the adequacy of the frequency of the DC Power Switch Test. Review of the PM database indicated that the annual "ACR DC Power Switch Test" was also delinquent in C-335 building. All other buildings had met their annual DC power switch test requirement. Completed Action No. 2 reflects the completion of that testing requirement for C-335.
- c. Contributing Cause A contributing cause of this event is the lack of enforcement in controlling delinquent PM tasks. Recently, the PM database tracking system added the requirement for a signed deferral form for any PM task being deferred which is categorized as "Q" or "AQ-NCS." However,
this transfer relay is categorized as a non-safety item; therefore no deferral signatures or written justifications are required. A delinquency list is provided monthly to group managers for their review. Corrective Action No. 4 will address lack of enforcement on PM maintenance tasks.
1 An additional concern associated with this event is that there was inadequate, available documentation def' ming the systems that were impacted by the loss of power at that panel.
Operations failed to execute the action steps of the CAAS LCOs, since they were not aware that the CAAS building horns were fed from this panel. Corrective Actions No. 5 and No. 6 address this by issuing off-normal procedures for loss of DC Power in C-310, C-331, C-333, C-335, and C-337.
COMPLETED CORRECTIVE ACTION
- 1. On May 23,1997, Maintenance replaced the control relay of the transfer switch in C-310.
- 2. On June 10,1997, Operations completed the ACR DC Power Switch Test in C-335 building.
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Doc);et No. 70-7001 Page 5 of 5 CORRECTIVE ACTIONS PLANNED
- 1. By July 31,1997, Engineering will inspect, with the assistance of Electrical Maintenance, all DC power transfer systems in C-331, C-333, C-335, and C-337. The inspection will determine the current condition of the transfer relay and the associated fuses to the rectifier of the DC power transfer systems and will identify deficiencies. A copy of the findings will be provided to Reliability Engineering.
- 2. By August 29,1997, Operations will develop a schedule to correct deficiencies, as required, identified in Corrective Action No.1.
- 3. By September 30,1997, Reliability Engineering will evaluate (taking into account the results of Corrective Action No.1 ) and determine the preventive maintenance requirements for the transfer relays in C-310, C-331, C-333, C-335, and C-337. This evaluation will also include determining the adequacy of the frequency of the DC power switch test.
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- 4. By August 15,1997, Operations will ensure a system of communication of PM status / clear i
ownership and approval of PM deferral exists.
- 5. By September 30,1997, Operations will issue an off-normal procedure for loss of DC power in C-310.
- 6. By September 30,1997, Operations will issue off-normal procedures for loss of DC power in C-331, C-333, C-335, and C-337.
EXTENT OF EXPOSURE OF INDIVIDUALS TO RADIATION OR TO RADIOACTIVE MATERIALS There was no exposure to individuals to radiation or to radioactive materials during the time that the CAAS building horns were inoperable.
LESSONS LEARNED I
l The performance of adequate preventive maintenance is important to reduce unexpected failures.
A technical evaluation of the impact of any deferred PM tasks is needed to determine the potential operational impact.
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t Docket No. 70-7001 Page1of1 ER-97-10 List of Commitments COMPLETED CORRECTIVE ACTION 1.
On May 23,1997, Maintenance replaced the control relay of the transfer switch in C-310.
2.
On June 10,1997, Operations completed the ACR DC Power Switch Test in C-335 building.
CORRECTIVE ACTIONS PLANNED 1.
By July 31,1997, Engineering will inspect, with the assistance of Electrical Maintenance, all DC power transfer systems in C-331, C-333, C-335, and C-337. The inspection will determine the current condition of the transfer relay and the associated fuses to the rectifier of the DC power transfer systems and will identify deficiencies. A copy of the findings will be provided to Reliability Engineering.
2.
By August 29,1997, Operations will develop a schedule to correct deficiencies, as required, identified in Corrective Action No.1.
3.
By September 30,1997, Reliability Engineering will evaluate (taking into account the results of Corrective Action No.1 ) and determine the preventive maintenance requirements for the transfer relays in C-310, C-331, C-333, C-335, and C-337. This evaluation will also include determining the adequacy of the frequency of the DC power switch test.
4.
By August 15,1997, Operations will ensure a system of communication of PM l
status / clear ownership and approval of PM deferral exists.
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5.
By September 30,1997, Operations will issue an off-normal procedure for loss of DC power in C-310.
6.
By September 30,1997, Operations will issue off-normal procedures for loss of DC power in C-331, C-333, C-335, and C-337.