05000368/LER-2012-001
| Arkansas Nuclear One - Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3682012001R00 - NRC Website | |
A. Plant Status At the time of the subject event, Arkansas Nuclear One Unit-2 (ANO-2) was at approximately 100% power. All structures, systems, and components that were needed to mitigate, reduce the consequences of, or limit the safety implications of the event were available.
B. Event Description
On August 08, 2012 at 0823 CDT, Arkansas Nuclear One Unit-2 (ANO-2) tripped from approximately one hundred percent power due to degrading main condenser vacuum. The 2C-5B Condenser Vacuum pump [SH] [P] had been secured for oil level checks, with the 2C-5A Condenser Vacuum Pump operating to maintain condenser vacuum. Ambient conditions were such that condenser vacuum was within the capacity of one vacuum pump. Operations placed the 2C-5B handswitch in pull-to-lock to allow a cooldown to ambient conditions for routine oil level checks as recommended by the vendor technical manual.
The ANO-2 Condenser Vacuum System consists of two vacuum pumps designed to maintain vacuum on the two main steam condensers. Each ANO-2 condenser vacuum pump is designed with five air operated valves. There are two suction valves and two air ejector bypass valves piped from the main condensers, and one valve to provide motive air flow to the air ejectors. This motive air path is normally aligned to atmosphere. Normally, when the vacuum pump is secured, one solenoid valve de-energizes to relieve air and close the two suction valves, and one solenoid valve de-energizes to relieve air and close the motive air valve.
During this event, both solenoid valves failed to reposition when the 2C-5B Vacuum Pump was secured, resulting in the two suction valves and the motive air valve (aligned to atmosphere) remaining open, establishing a flowpath from atmosphere through the vacuum pump and into the main condensers. Condenser pressure indication in the control room did not begin to indicate the rising condenser pressure for approximately sixteen to twenty seconds due to the location of the air ingress path in relation to the location of the condenser pressure transmitters.
Approximately thirty seconds after 2C-5B was secured, the control room received a high main condenser pressure alarm, at which time 2C-5B was removed from pull-to-lock and restarted.
By the time the condenser pressure transmitters sensed the pressure increase and actuated the high condenser pressure alarm, the volume of air that had been drawn into the main condensers was beyond the capacity of the condenser vacuum system to prevent a main turbine trip. Approximately twenty-two seconds after the high condenser pressure alarm was received, the main turbine automatically tripped on high exhaust hood pressure, resulting in a subsequent automatic reactor trip on high pressurizer pressure. Condenser vacuum began to recover approximately eleven seconds after the trip. The Emergency Feedwater Actuation System [BA] actuated for the "A" Steam Generator due to water level trending slightly below setpoint, and the Emergency Feedwater System was used to feed the steam generators until Auxiliary Feedwater was aligned.
After investigation of the event and corrective maintenance on the 2C-5B solenoid valves, ANO-2 was reconnected to the electrical grid on August 09, 2012.
which were installed in an environment outside of designed temperature limits, resulting in a heat related failure. The vacuum pump solenoid valves are located inside an enclosed cabinet which is mounted as part of the vendor supplied pump skid. The cabinet doors are normally closed and the enclosure is not ventilated to allow heat generated by the solenoids to escape.
The vendor tech manual indicates that the solenoid valves were suited for ambient conditions between 32 and 125 degrees F. The internal cabinet temperature near the valve bodies was measured at approximately 150 degrees F, with solenoid coil temperatures well over 200 degrees F. The internal elastomers were not rated for these temperatures and resulted in valve binding or sluggishness when the solenoids were de-energized.
D. Corrective Actions
The solenoid valves were replaced prior to plant startup. Future planned corrective action to prevent recurrence of the root cause is expected to include a modification to change the solenoid valve location to allow proper heat dissipation. Additionally, a modification is under consideration that would install a switch or button to verify proper operation of the solenoids and suction valves prior to securing a vacuum pump.
E. Safety Significance Evaluation Systems and components required to shutdown the reactor, maintain safe shutdown conditions, remove residual heat, and control the release of radioactive material were available and performed as required. No safety limits were exceeded and the loss of external load and/or main turbine trip is an analyzed condition listed in ANO-2 Safety Analysis Report. The reactor scram due to high pressurizer pressure as a result of the main turbine trip did not pose any safety risk to the general public, nor create any unanalyzed nuclear, industrial, or radiological safety issues.
F. Basis For Reportability This event is reported pursuant to 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).
10 CFR 50.73(a)(2)(iv)(B): "Reactor protection system (RPS) including: reactor scram or reactor trip" and "PWR auxiliary or emergency feedwater system.
G. Additional Information
10CFR 50.73(b)(5) states that this report shall contain reference to "any previous similar events at the same plant that are known to the licensee." NUREG-1022 reporting guidance states that term "previous occurrences" should include previous events or conditions that involved the same underlying concern or reason as this event, such as the same root cause, failure, or sequence of events.
A review of the ANO corrective action program and Licensee Event Reports for the previous three years revealed no relevant similar events.
Energy Industry Identification System (EllS) codes and component codes are identified in the text of this report as p0q.