05000482/LER-2004-002
| Wolf Creek Generating Station | |
| Event date: | 02-13-2004 |
|---|---|
| Report date: | 04-09-2004 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 4822004002R00 - NRC Website | |
Background:
Steam Generator (SG) [EIIS Code: SG] level is normally controlled by automatic adjustments in the Main Feedwater System [EIIS Code: SJ] main feedwater regulating valve (MFRV) [EIIS Code: FCV] position and main feedwater pump [EIIS Code: P] speed. The MFRV stem/plug assembly has a taper junction connection where the valve stem connects with the plug. This junction consists of a hole in the top of the plug that the valve stem fits into. The top half of this hole is tapered and the bottom of the hole has threads that the stem is screwed into. When the valve stem is screwed (and torqued) into the hole the tapered area creates a tight interference fit between the stem and plug. The stem/plug assembly is then pinned with a solid pin to prevent the plug from rotating and unscrewing from the stem. If the stem/plug joint is loose, more of the load will be borne by the pin than intended by design.
Plant Conditions Prior to the Event:
MODE —1 Power —100 percent Normal Operating Temperature and Pressure
Event Description:
On February 13, 2004, Wolf Creek Generating Station (WCGS) was operating at 100 percent steady state power. At 8:04 AM CST the valve plug in the "D" MFRV separated from the valve stem and failed to the closed position, isolating the Main Feedwater supply to SG "D". This caused a rapid lowering of water level in SG "D" that initiated a reactor trip on Lo-Lo SG level.
At the time of the trip, all control rods fully inserted and all safety equipment performed as designed. Main feedwater was isolated and motor-driven and turbine-driven auxiliary feedwater pumps started as expected. There were no significant maintenance or operating evolutions in progress at the time of the trip, nor were there any major pieces of equipment out of service.
Basis for Reportability:
The reactor trip and subsequent actuation of Engineered Safety Features (ESF) described in this event is reportable per 10 CFR 50.73(a)(2)(iv)(A), which requires reporting of "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B) of this section.
Paragraph (B)(1) of 10 CFR 50.73(a)(2)(iv) includes "Reactor protection system (RPS) including: reactor scram or reactor trip?
Root Cause:
The cause for the reactor trip was separation of the MFRV plug from the stem, which caused the valve to fail in the closed position and isolate main feedwater to SG "0".
The root cause for the separation of the plug from the stem was insufficient problem assessment and resolution to resolve loose stem conditions identified in the "A", "B" and "D" MFRVs in October 2003 during WCNOC's Refueling Outage 13. At that time WCGS engineering staff incorrectly reasoned that installation of new solid pins would provide a sufficient margin of strength to allow the continued use of the loose stem/plug assemblies. This decision was not documented and reviewed as required by plant procedures.
Corrective Actions:
To address the hardware issues, the stem/plug assemblies for all 4 MFRVs were replaced with new stem/plug assemblies with tight stem to plug connections.
Actions to address the failure to adequately assess and resolve the loose stem-to-plug fit condition previously identified are being developed within the WCNOC corrective action program.
Safety Significance:
The safety significance of this event is low. This event is bounded by the current licensing basis analysis as reported in WCGS Updated Safety Analysis Report (USAR) section 15.2.7 "Loss of Normal Feedwater Flow." The event reduced normal feedwater flow to the "D" SG, resulting in the reduction of level in the secondary side of the SG. The reduction in SG level provided the input (Lo-Lo SG Level Signal) to trip the reactor, isolate all normal feedwater, and start the motor and turbine driven auxiliary feedwater pumps.
There were no adverse effects on the reactor core, the reactor coolant system, or the main steam system, due to the auxiliary feedwater system's capacity to supply the necessary heat sink.
All safety related equipment performed as designed and there were no adverse effects on the health and safety of the public.
Operating Experience/Previous Events:
In June, 1996, WCGS experienced a reactor trip because of a stem/plug assembly separation in MFRV "C" caused by failure of a roll pin due to spare stem/plug assemblies installed in March 1996 (Refuel 8) not being upgraded from roll pins to solid pins during a 1988 pin upgrade.