05000323/LER-2011-001
| Diablo Canyon Power Plant | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 46701 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation, 10 CFR 50.72(b)(3)(iv)(A), System Actuation |
| 3232011001R00 - NRC Website | |
I. Plant Conditions
At the time of the event, Diablo Canyon Power Plant (DCPP) Unit 2 was in Mode 1 (Power Operation) at 98 percent reactor power with normal operating reactor coolant temperature and pressure.
II. Description of Problem
A. Background
During the month of March 2011, plant operators identified that a relief valve, XS-2-RV-95, on a feedwater heater ISJI had developed a leak and would require monitoring and repair. The relief valve leak worsened and began spraying hot feedwater on plant components in the general area of the relief valve.
B. Event Description
On March 23, 2011, the Unit 2 Turbine Watch identified a developing steam leak on XS-2-RV-95.
On March 26, 2011 at 0930, a plant operator identified that the leak on XS-2-RV-95 had increased, prompting a decision to isolate the associated feedwater heater 2-1A. During the initial attempt to isolate XS-2-RV-95, the leak increased significantly and wetted the adjacent Main Feedwater (MFW) Pump 2-1 control panel, causing several MFW Pump 2-1 alarms. The MFW Pump 2-1 control panels were covered and plant operators subsequently reduced the magnitude of the leak by isolating feedwater heater 2-1A.
On March 26, 2011 at 14:49, plant operators manually tripped Unit 2 reactor from 98% power following an automatic trip of MFW Pump 2-1.
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event None D. Other Systems or Secondary Functions Effected None
E. Method of Discovery
A plant operator observed leakage from relief valve XS-2-RV-95 during normal rounds. They also observed that the leakage was spraying the MFW Pump 2-1 and wetting the associated control panel.
F. Operator Actions
Plant operators initiated a manual reactor trip in accordance with Abnormal Operating Procedure OP AP-15, "Loss of Feedwater Flow," following the automatic trip of MFW Pump 2-1.
G. Safety System Responses
An automatic actuation of the auxiliary feedwater system, steam generator blowdown isolation, and main feedwater isolation occurred during the plant shutdown. These actuations were expected and all components functioned as designed.
III. Cause of the Problem
A. Immediate Cause
The local MFW Pump 2-1 electronic components were subjected to hot feedwater spray from a leaking relief valve. As a result, when MFW Pump 2-1 was subjected to the hot feedwater spray from the XS-2-RV-95 failed inlet flange gasket, a grounded circuit for low-low lube oil reservoir level [Si] caused the MFW Pump 2-1 automatic trip.
B. Root Cause
The preliminary root cause identified that Maintenance Procedure MP M-54.1, "Bolt Fabrication and Tensioning," allowed the fasteners for bolted connections using sheet type gasket material to be tensioned to "snug tight" versus providing specific bolt torque requirements. As a result, flange gasket preload has not been consistently applied when sheet type gasket material is used. The failed gasket for the relief valve was installed using this maintenance procedure.
IV. Assessment of Safety Consequences
The DCPP Unit 2 shutdown was normal and as expected following the automatic feedwater pump trip and subsequent manual reactor trip. The Solid State Protection System functioned as designed upon receipt of the manual reactor trip signal. The reactor trip breakers opened and all control rods and shutdown rods inserted as designed. The automatic auxiliary feedwater system actuation was expected with all system components fuctioning as designed. This Condition I event was within the bounds of the Updated Final Safety Analysis Report and the post trip transient response was as expected. As such, this event is not considered risk significant and it did not adversely affect the health and safety of the public.
V. Corrective Actions
A. Immediate Corrective Actions
Plant operators initiated a manual reactor trip in accordance with OP AP-15 following the automatic trip of MFW pump 2-1.
B. Corrective Actions to Prevent Recurrence (CAPR) Using the change management process, Maintenance Procedure MP M-54.1, "Bolt Fabrication and Tensioning," was revised to specify torque requirements for tensioning fasteners that are greater than one half inch on bolted connections, regardless of classification and use, with the exception of soft gasket material (e.g. rubber, Teflon, and Neoprene).
05000 323
VI. Additional Information
A. Failed Components
XS-2-RV-95 inlet gasket
B. Previous Similar Events
The XS-2-RV-95 inlet gasket had previously failed during a plant shutdown for refueling on October 26, 2004. This previous gasket failure did not produce a MFW pump trip or unit shutdown.
C. Industry Reports
INPO Operating Experience Report OE 33263, Main Feedwater Pump Amphenol Connection Water Intrusion Results in Manual Plant Trip (DCPP2).