05000313/LER-2001-001
| Docket Number | |
| Event date: | 01-05-2001 |
|---|---|
| Report date: | 02-05-2001 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| 3132001001R00 - NRC Website | |
APPROVED BY OMB NO. 3150-0104 EXPIRES 5/31/95 ESTIMATED � BURDEN � PER � RESPONSE � TO COMPLY WITH THIS � INFORMATION COLLECTION REQUEST: � 50.0 HRS.
FORWARD � COMMENTS � REGARDING BURDEN ESTIMATE � TO THE � INFORMATION � AND � RECORDS MANAGEMENT BRANCH (MNBB 7714), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC � 20555-0001, AND TO THE PAPERWORK REDUCTION � PROJECT � (3150-0104), � OFFICE � OF MANAGEMENT � AND � BUDGET, � WASHINGTON, DC � 20503.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Arkansas Nuclear One - Unit 1 05000313 A. Plant Status At the time of this event, ANO-1 was at approximately 72 percent power with a power reduction in progress at 10 percent per minute.
B. Event Description
On January 5, 2001, at approximately 0848, the reactor was manually tripped due to excessive hydrogen leakage from the main generator (TB].
The main generator hydrogen cooling system [TK] is utilized to maintain generator hydrogen pressure at approximately 70 psig. A small change in the normal leak rate was noticed on the morning of January 4, 2001 at approximately 0445. A small increase in leakage was observed that night at approximately 2300. Safety barriers were posted and access was controlled into the affected areas of the turbine building. Turbine building ventilation [VK] was maximized in order to accelerate the dispersion of the hydrogen gas and Industrial Safety personnel were dispatched to provide continuous monitoring for explosive mixtures. Operations personnel established plant shutdown criteria based on observed increases in the generator hydrogen leakage or detection of any explosive mixture of hydrogen and oxygen.
Operations and Engineering personnel continued to search for the source of the leakage.
A significant increase in the hydrogen leak rate occurred at approximately 0830 on January 5, 2001, and a rapid plant shutdown from 100 percent power was initiated at 0845 based on the pre-determined shutdown criteria. As plant power was reduced the control room received a report from personnel near the generator that the leak rate had increased and a significant loss of generator hydrogen was occurring. At 0848, operators manually tripped the reactor from approximately 72 percent power to prevent overheating of the main generator and to allow rapid depressurization of the generator through the roof vent.
All post trip plant responses were normal. There were no engineered safety feature (ESF)(JE] actuations and the plant proceeded to a stable hot shutdown condition. Immediately following the reactor trip the main generator was vented and purged with carbon dioxide. No explosive mixtures of hydrogen and oxygen were detected during the event.
The source of the hydrogen leak was subsequently discovered to be a small bore hydrogen vent line (1/2 inch) connected to the main generator. Field inspections found that one of the pipe supports in the vicinity of the pipe failure was missing both nuts on one of its two bolts allowing the pipe to move freely within the support. The vent pipe was found severed near the generator connection flange approximately 3 feet from the pipe support.
Following completion of corrective maintenance to replace the piping and repair the pipe support, the reactor was restarted and tied to the grid at 1853 on January 7, 2001.
APPROVED BY OMB NO. 3150-0104 EXPIRES 5/31/95 ESTIMATED � BURDEN � PER � RESPONSE � TO COMPLY WITH THIS � INFORMATION COLLECTION REQUEST: � 50.0 HRS.
FORWARD � COMMENTS � REGARDING BURDEN ESTIMATE � TO THE � INFORMATION � AND � RECORDS MANAGEMENT BRANCH (MNBB 7714), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC � 20555-0001, AND TO THE PAPERWORK REDUCTION � PROJECT � (3150-0104), � OFFICE � OF MANAGEMENT � AND � BUDGET, � WASHINGTON, DC � 20503.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Arkansas Nuclear One - Unit 1 05000313
C. Root Cause
Investigation into the cause of the failure identified a pipe support bolting configuration that was not appropriate for the conditions that were present.
The affected piping is designed to be supported by two friction block supports. Each support is secured using two bolts with double nuts on each bolt. The friction block support was found to contain the two bolts; however, both nuts on one of the bolts were missing. This allowed excessive pipe movement within the support. One of the nuts was found directly under the support and upon further inspection the evidence indicated that both of the nuts had been installed but had recently come off of the bolt. There were no anti-loosening methods noted, such as Loc-Tite or staking. Field inspections suggest that resonant frequency in this section of piping caused the double nut arrangement on this particular support to loosen and eventually fall off the bolt. Higher than normal main turbine [TA] vibration during the current operating cycle contributed to this failure. As both nuts loosened, the pipe was no longer properly restrained which altered the pipe structural frequency resulting in increased piping vibration amplitude. The higher vibration caused the pipe failure.
Visual inspection of the break area indicates that the time between crack initiation and a complete through-wall crack was relatively short, possibly a matter of hours. This indicates a high-cycle type of fatigue failure.
D. Corrective Actions
Repairs to the generator vent line and post maintenance leak testing were completed. The degraded pipe support was repaired and reinforced with staked bolted connections to reduce the probability of a recurrence of the failure.
Leak checks were performed on similar generator hydrogen supply system piping and structural inspections were performed on system piping, pipe supports, and flanges that are located in the vicinity of the failure. Adjacent pipe supports were also reinforced with staked bolted connections. A balance shot was performed on the main turbine which reduced vibrations.
E. Safety Significance
The main generator and its supporting hydrogen cooling system are not required to be operable in order to achieve safe shutdown of the plant. Safety systems operated as designed following the manual reactor trip and the plant was safely taken to a stable hot shutdown condition. The Reactor Protection System [JC] functioned properly in response to the manual trip signal and there were no actuations or conditions warranting actuation of any ESF systems. Therefore, this event was determined to be of minimal safety significance.
APPROVED BY OMB NO. 3150-0104 EXPIRES 5/31/95 ESTIMATED � BURDEN � PER � RESPONSE � TO COMPLY WITH THIS � INFORMATION COLLECTION REQUEST: � 50.0 HRS.
FORWARD � COMMENTS � REGARDING BURDEN ESTIMATE � TO THE � INFORMATION � AND � RECORDS MANAGEMENT BRANCH (MNBB 7714), U.S. NUCLEAR REGULATORY COMMISSION, WASHINGTON, DC � 20555-0001, AND TO THE PAPERWORK REDUCTION � PROJECT � (3150-0104), � OFFICE � OF MANAGEMENT � AND � BUDGET, � WASHINGTON, DC � 20503.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6) PAGE (3) Arkansas Nuclear One - Unit 1 05000313 F. Basis for Reportability The failure of the main generator vent line and resulting loss of generator hydrogen necessitated a manual reactor trip. Therefore, this event is reportable pursuant to 10CFR50.73(a)(2)(iv) as a manual actuation of the Reactor Protection System.
This event was also reported to the NRC Operations Center in accordance with 10CFR50.72(b)(2)(ii) at 1125 CST on January 5, 2001.
G. Additional Information
There have been no previous similar conditions reported by ANO as Licensee Event Reports (LERs).
The manufacturer of the failed pipe support is not known.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].