05000220/LER-2006-002
| Nine Mile Point Unit 1 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2202006002R00 - NRC Website | |
I. Description of Event
On June 12, 2006, with Nine Mile Point Nuclear Station (NMPNS) Unit 1 reactor plant in the Startup mode at 0% power, 5 psig and 192 degrees Fahrenheit, reactor water level increased to the high level trip setpoint when plant operators placed the Feedwater system in the long path recirculation configuration to support chemistry sampling. This resulted in a valid Main Turbine Trip and actuation of the High Pressure Coolant Injection (HPCI) initiation logic.
HPCI is a mode of operation of the condensate and feedwater system. The HPCI mode of feedwater operation consists of selected equipment, including the condensate surge and storage tanks, main condenser hotwell, two condensate pumps, condensate demineralizers, two feedwater booster pumps, feedwater heaters, two motor-driven feedwater pumps, an integrated control system, and all associated piping and valves. To ensure a continuous, uninterrupted supply of high pressure feedwater to the reactor, HPCI is auto-initiated by reactor vessel low level, a turbine trip, or excessive flow through an individual feedwater pump. At the time of the event, during the plant start up, HPCI was not required to be operable with reactor pressure less than 110 psig.
As a regular practice, the feedwater system is placed in the long path recirculation configuration during plant startup to filter reactor feedwater and support collecting a chemistry sample prior to initiating high feedwater flows to the Reactor.
The Feedwater system remained lined up for long path recirculation from the plant shutdown activities and required the opening of only one valve to initiate the flow path.
When the operator slowly cracked open the valve to place the Feedwater system in long path recirculation configuration, no changes in reactor level, feedwater flow, or feedpump bypass valve position were observed. When the valve was opened further (approximately 20%), a level rise greater than anticipated occurred. Level reached the reactor high level setpoint, resulting in a Main Turbine Trip and actuation of the HPCI initiation logic, per design.
Due to the operating conditions during plant start up, the feedwater booster pumps were not required to be in service at the time of the event and the control switches were in pull-to-lock. Without the booster pumps running, the feedwater pumps' start permissive circuitry is not satisfied. Therefore, though the HPCI start logic was actuated, injection did not occur and was not required.
The source of water that caused the high level condition in the reactor came from the elevated portions of the feedwater system that gravity-drained to the reactor vessel (at —5 psig) when the FCV was opened.
II. Cause of Event
The cause of this event is inadequate implementation of previous operating experience. Similar level control problems occurred in the past without the lessons learned incorporated into operating procedures to warn operators of possible consequences.
A contributing cause was an inadequate pre-job brief. The pre-job brief did not adequately consider the potential consequences of reactor water level control issues.
III. Analysis of Event
A review of plant data showed that reactor water level rose approximately 7" from 81"-83" to a high of 90". Hotwell temperature at the time of the event was 68.5 degrees with reactor water temperature at —192 degrees. The reactor was subcritical. There was no change in indicated reactor power or in reactor coolant temperature as a result of the water that gravity-drained to the reactor vessel (at 5 psig) from the elevated portions of the feedwater system.
The plant impact associated with the rise in level was annunciation and circuit actuation of the HPCI auto initiation logic from a Turbine Trip signal, as designed. No components started, no water flowed to the reactor, no challenges to operators or the plant other than alarm response actions and reset of the turbine to support plant startup. At the time of the event, during the plant start up, HPCI was not required to be operable with reactor pressure less than 110 psig.
Based on the above, the event did not pose a significant threat to the health and safety of the public or plant personnel.
IV. Corrective Actions
A. Action Taken to Return Affected Systems to Pre-Event Normal Status Immediate actions were taken to place reactivity maneuvers on hold, reset HPCI, and restore reactor water level to the operating band. Due to plant conditions, no further actions were required.
B. Action Taken or Planned to Prevent Recurrence NOTE: There are no NRC regulatory commitments in this Licensee Event Report.
To prevent recurrence, the applicable operating procedures will be revised to establish the proper sequence for placing the Feedwater system in the long path recirculation configuration during plant startup and a precaution will be added to alert operators about potential reactor water level control problems. In addition, Unit 1 licensed operators will be briefed on this event.
V. Additional Information
A. Failed Components:
None
B. Previous similar events:
Similar level control events with the plant in cold shutdown and the reactor vented to atmosphere have occurred without resulting in a turbine trip (reactor water level did not rise excessively or the turbine trip was not reset) and therefore did not actuate HPCI logic. For these events operators failed to incorporate lessons learned into operating procedures to warn operators of possible consequences.
In 1997, two similar events occurred where HPCI logic was actuated when unplanned reactor water level increases initiated a turbine trip and actuated HPCI logic. The cause of the first event on July 11, 1997, was attributed to component failure of valves not involved in this recent event. The cause of the second event on December 7, 1997, was attributed to operator error in that operating parameters were not adequately monitored, allowing reactor water level to increase to the high level trip point. Because the specific causes of the three events are different, previous corrective actions were not designed to prevent this recent event.
C. Identification of components referred to in this Licensee Event Report:
Components� IEEE 805 System ID�IEEE 803A Function Feedwater System/Booster Pump� SJ Feedwater Isolation Valve� SJ� ISV Feedwater Flow Control Valve� SJ� FCV High Pressure Coolant Injection (HPCI) System� BJ HPCI Initiation Logic� JG Reactor Vessel� RPV Main Turbine/Supervisory Control� JJ� TRB