05000354/LER-2007-001
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber Na N/A | |
| Event date: | 01-29-2007 |
|---|---|
| Report date: | 03-30-2007 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| 3542007001R00 - NRC Website | |
- Energy Industry Identification System {EIIS} codes and component function identifier codes appear as {SS/CCC}
IDENTIFICATION OF OCCURRENCE
Event Date/Time: January 29, 2007 - 23:12 Discovery Date/Time: January 29, 2007 - 23:12
CONDITIONS PRIOR TO OCCURRENCE
Hope Creek was in Operational Condition 1 with reactor power at approximately 21% during startup following a planned maintenance outage. No structures, systems, or components were inoperable that contributed to the event.
DESCRIPTION OF OCCURRENCE
Plant Startup was in progress with reactor power at 21% and reactor level in a stable oscillating pattern.
Level ranged from 36 to 33.5 inches repeating every five minutes in a sinusoidal pattern. The "C" RFP was in service, manually maintaining a differential pressure of— 180 psid across the Start Up Level Control (SULC) valve. The SULC valve was in automatic, single element control maintaining reactor vessel level.
The "C" RFP total flow was operating in manual at between 4200 and 4800 gpm, with the minimum flow valve maintaining indicated pump total flow at greater than 5,000 gpm, the minimum flow automatic initiation set point. Indicated "C" RFP minimum flow ranged from zero to 800 gpm, as the system oscillated in response to injection demand.
Within three minutes of synchronizing the Main generator to the grid, reactor level began to lower out of the control band. The Licensed Plant Operator (PO) adjusted the 'C' RFP speed to control discharge pressure. Several speed adjustments were made by the PO in an attempt to maintain Reactor water level in the control band. During this level excursion, the PO returned the RFP speed signal to the previous setting in order to stabilize level. As Reactor Level continued to lower, the PO raised the speed signal to above the previous injection setting. The expected rise in discharge pressure was not observed.
At this point the PO suspected an equipment failure associated with the 'C' RFP Minimum Flow valve, however, the Control Room indications showed the Minimum Flow at 0 gpm. Reactor Level continued to lower from 35" to 30" within 15 seconds and the Low Level Alarm was received at Level 4 (30"). The PO, having indications/response not as expected, attempted manual operation of the SULC valve. This had no effect due to a low differential pressure across the SULC valve, which provided little throttling. An attempt was made to place the 'A' RFP in service as Reactor Level continued to lower. Reactor Level lowered from 30" to 12.5" within one minute. As Reactor Water Level approached the Low Level setpoint, the Reactor Operator placed the Mode Switch in Shutdown, however the Reactor automatically scrammed on Low Water Level at 12.5" (Level 3) approximately 2 seconds prior to the insertion of the manual scram.
The leaking instrument tap weld provided input to instruments that supply "C" RFP minimum flow system flow indication to the CRIDS system, and indication and control feedback to the Foxboro Digital Feed Control System.
The failure reduced indicated flow to the "C" RFP minimum flow control system logic, which in turn increased valve opening demand, and resulted in significant excess flow through the "C" RFP minimum flow line.
Concurrent with this, as actual flow increased and significantly degraded pump performance, control room indication for minimum flow reduced to zero on both the Control Room Integrated Display System (CRIDS) and the Foxboro Digital Feed Control System (DFCS), and remained so throughout the bulk of the event.
Manual feed water control manipulation in response to the equipment failure was not effective in preventing the level from reaching the scram setpoint. Although manual action was taken to control RFPT speed, the SULC valve was left in the automatic level control mode. This configuration amplified the feed water system transient response, due to the SULC valve response not being in step with the operators' actions.
The effect of the line crack was also pressure dependent. At low pressures, the crack had little or no effect on system performance, but as pressure was raised above 650psig, the crack caused at first unstable indication, then as pressure approached normal operating pressure, indication became more and more erroneous.
PREVIOUS OCCURRENCES
A review of previous reportable events at Hope Creek was performed to determine if a similar event had occurred. No similar events were identified.
The risk presented by this condition is minimal. The lowest recorded reactor level for the transient (-16") did not challenge ECCS set points.
A review of this event determined that a Safety System Functional Failure (SSFF) has not occurred as defined in Nuclear Energy Institute (NEI) 99-02.
CORRECTIVE ACTIONS
The weld repair at flow nozzle H1AE-1AEFE-1770C was completed.
Penetrant Testing on each of the instrument tubing welds for flow indication associated with 'A', 'B' & 'C' RFP's will be performed during an outage.
Standardized methodology for responding to challenges from feedwater in low-power conditions will be developed.
COMMITMENTS
This LER contains no commitments.