05000255/LER-2008-005
| Docket Numbersequential Revmonth Day Year Year Month Day Yearnumber No | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| 2552008005R00 - NRC Website | |
unidentified primary coolant system [AB] (PCS) leakage had exceeded the Technical Specification (TS) limit of one gallon per minute. Limiting Condition of Operation (LCO) 3.4.13, Condition A, was entered while the planned shutdown continued. All required actions for the condition were completed. The event was reported as a four-hour non-emergency report, in accordance with 10 CFR 50.72(b)(2)(i), as the initiation of a plant shutdown required by TS, because if the event had occurred with the plant at normal operation, a shutdown would have been required. The plant entered Mode 5 on August 6, 2008, at 1025 hours0.0119 days <br />0.285 hours <br />0.00169 weeks <br />3.900125e-4 months <br />.
The cause of the leakage was determined to be letdown relief valve [RV;CB] RV-2006, which lifted when a second charging pump [P;CB] was started. The relief valve opened as a result of inadequate response of the letdown system pressure indicating controller [PIC;CB]. Due to the pressure sensitivity of the system, if the pressure indicating controller is unable to respond fast enough to reduce the pressure at the inlet of the letdown heat exchanger [HX;CB], over-pressurization protection of the heat exchanger is accomplished by the lifting of the relief valve to the quench tank [TK;AB].
RV-2006 was replaced during the planned shutdown. In addition, an engineering study is being completed for a modification to slow the associated orifice [OR] stop valve [V;CB] response to an open/close signal, and allow the associated pressure indicating controller to adequately control letdown pressure.
This is reportable in accordance with 10 CFR 50.73(a)(2)(i)(A) as the completion of a plant shutdown required by TS.
DESCRIPTION OF SYSTEM OPERATION
Pressure indicating controller (PIC)-0202 (letdown system pressure controller) is programmed to anticipate and reduce pressure spikes that occur when any of the three PCS letdown valves are opened or closed. Opening or closing of these valves is sensed by PIC-0202, which alters the position of backpressure regulating valves CV-2122 or CV-2012. This alteration of pressure provides a nominal inlet pressure to the letdown heat exchanger, E-58. PIC-0202 is designed to maintain letdown pressure above saturation pressure downstream of the letdown orifices to prevent flashing of letdown fluid. If PIC-0202 is unable to respond fast enough to alter the pressure at the inlet to E-58, over-pressurization protection of E-58 is accomplished by lifting RV-2006.
The letdown pressure controller interfaces with the pressurizer level program through the orifice valve operation. When pressurizer level is low, orifice stop valves are closed to increase pressurizer level. The stop valves open very quickly. When an orifice valve opens, a pressure surge is sent down the letdown line, which is sensed by the pressure transmitter (PT-0202). The backpressure control valves react to limit the pressure build-up in the lines. Due to the pressure sensitivity of the system, the controller does not have enough time to react to the pressure signal and reposition the back pressure valves. As a result, PIC-0202 monitors the position of the orifice stop valves to send a pre-determined signal to CV-2012 to start responding. This is a very sensitive tuning sequence since tenths of a second changes in PIC-0202 signal output to CV-2012 can result in poor system performance.
CAUSE OF THE EVENT
When the charging pump started, the system pressure increased. RV-2006 lifted due to improper coordination between the backpressure regulating valves and orifice stop valve response. The stop valves operate too quickly for the letdown pressure controller to anticipate the pressure surge, and the response of the letdown backpressure valves is too slow to respond.
RV-2006 setpoint repeatability issues allow the valve to open at a much lower system pressure, which magnifies the letdown system pressure control weaknesses.
CORRECTIVE ACTIONS
RV-2006 was replaced during the planned shutdown. In addition, an engineering study is being completed for a modification to slow the orifice stop valve response to an open/close signal and allow PIC-0202 to adequately control letdown pressure.
ASSESSMENT OF SAFETY CONSEQUENCES
The safety consequences of this event were minimal. RV-2006 relieved to the quench tank, as designed. All TS required actions met completion times.
PREVIOUS SIMILAR EVENTS
This is a long-standing design issue that has occurred several times in the past.