05000461/LER-2013-003
| Clinton Power Station, Unit 1 | |
| Event date: | 04-26-2013 |
|---|---|
| Report date: | 06-24-2013 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv), System Actuation |
| Initial Reporting | |
| ENS 48974 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 4612013003R00 - NRC Website | |
PLANT AND SYSTEM IDENTIFICATION
General Electric -- Boiling Water Reactor, 3473 Megawatts Thermal Rated Core Power Energy Industry Identification System (EllS) codes are identified in test as [XX].
EVENT IDENTIFICATION
Manual Reactor SCRAM Due To Main Electro-Hydraulic Control System Failure
A. Plant Operating Conditions Before the Event
Unit: 1 Event Date: 4/26/13 Event Time: 0855 hours0.0099 days <br />0.238 hours <br />0.00141 weeks <br />3.253275e-4 months <br /> CDT Mode: 1 Mode Name: Power Operation Reactor Power: 97 percent
B. DESCRIPTION OF EVENT
On 4/26/13, the plant was in Mode 1 (Power Operation) at 97 percent power. At 0855 hours0.0099 days <br />0.238 hours <br />0.00141 weeks <br />3.253275e-4 months <br />, operators in the Main Control Room received a Main Electro-Hydraulic Control (EHC) [TG] pressure alarm [ALM] due to lowering level in the EHC oil reservoir [RVR]. A non-licensed operator was dispatched to the EHC skid to investigate the alarm. The operator reported that the main EHC oil reservoir level was at minus 4.25 inches and rapidly lowering. An oil mist was reported on the 800-foot elevation of the Turbine Building. In response to these conditions, operators initiated a manual reactor SCRAM by placing the reactor mode switch [HS] in the shutdown position. Operators entered the Reactor Scram Off-Normal procedure and subsequently entered Emergency Operating Procedure (EOP) -1, "Reactor Pressure Vessel Control," due to an expected low reactor water Level 3 trip signal. All control rods fully inserted and the plant responded as expected to the SCRAM. Operators evaluated the need to enter Emergency Action Levels (EALs) and concluded that no entry thresholds were met. Operators controlled reactor pressure using Turbine bypass controls which were unaffected by the EHC failure.
At 1100 hours0.0127 days <br />0.306 hours <br />0.00182 weeks <br />4.1855e-4 months <br />, Operators exited EOP-1 in accordance with normal plant procedures as stable plant conditions were reached. Operators maintained the reactor in Mode 3, Hot Shutdown.
As expected during the event, when reactor water level lowered to the low reactor water Level 3 trip setpoint primary containment isolation valves [ISV] in Group 2 (Residual Heat Removal (RHR) [BO]), Group 3 (RHR), and Group 20 (miscellaneous systems) received signals to automatically shut; these valves are normally closed and operators verified that the valves remained in the closed position.
Following the reactor scram, a walkdown by Mechanical Maintenance personnel identified the cause of the EHC oil leak as a broken socket head cap screw that was used to attach a hydraulic shutoff valve [SHV] to Main Steam Turbine Control Valve Number 4 (1TGCV4). Maintenance personnel found the three remaining cap screws on the shutoff valve loose, with one of the three slightly bent.
This event was determined to be reportable under the provisions of 10 CFR 50.73(a)(2)(iv), as a condition that resulted in manual actuation of the reactor protection system (RPS). The station notified the NRC of this event via Emergency Notification 48974 at 1155 hours0.0134 days <br />0.321 hours <br />0.00191 weeks <br />4.394775e-4 months <br /> on 4/26/13. Issue Report 1506929 was initiated to investigate this event.
C. CAUSE OF EVENT
The cause analysis identified that the most probable cause for this event was inadequate work instructions in the 2008 work order that led to a failure to install appropriate lock washers on the shutoff valve connection to Main Steam Turbine Control Valve Number 4, resulting in the socket head cap screws becoming loose due to the dynamic service loading on the shutoff valve connection over the five years since installation, ultimately failing due to fatigue.
A contributing cause for this event is the mounting bolt installation kits for the shutoff valves came with the cap screws but did not have any lock washers for the shutoff valve connection. This may have led the installer to assume the kit was complete with all parts needed to install (i.e., human error trap).
D. SAFETY CONSEQUENCES
The actuation of the Reactor Protection System placed the plant in a safe and stable condition. There were no plant safety limits exceeded, and no other Engineered Safety Feature actuations, and risk significance was low. Safety-related systems functioned correctly in response to this event with critical plant parameters remaining within the bounds of plant design, Technical Specifications, Updated Safety Analysis Report, Offsite Dose Calculation Manual, and Core Operating Limits Report. The affected system (i.e., TG system) is not-safety related.
No loss of safety function occurred during this event.
E. CORRECTIVE ACTIONS
The hydraulic shutoff valve on Control Valve 1TGCV4 and its associated cap screws were replaced and torqued to the required 25 foot-pounds, and the EHC system was pressurized to verify it was free of leaks.
As an extent of condition review, the bolting torque for all hydraulic shutoff valves for turbine control valves, stop valves, and intermediate valves was verified to be at the proper torque value of 25 foot-pounds.
Work Orders are being created to install lock washers during the next refueling outage on the shutoff valve for 1TGCV4 and, as part of the extent of condition review, on the shutoff valve for turbine control valve number 2 (1TGCV2) which was also found without lock washers. The other turbine control valves (1TGCV1 and 1TGCV3) were found to have the required lock washers installed.
The model work order for replacing shutoff valves on control valves will be revised to add shutoff valve Original Equipment Manufacturer (OEM) installation requirements including job steps to validate that the proper lock washers have been installed.
A review of the model work orders used to install the hydraulic shutoff valves on the turbine control valves and combined intermediate valves was performed and found that the work orders included specific steps to install the lock washers. However, work orders will be created to validate lock washers are installed on all hydraulic shutoff valves on main steam valves.
The station will ensure the bill of materials in model work orders for installing shutoff valves on control valves specifies the appropriate lock washers for the application.
F. PREVIOUS OCCURRENCES
A review for previous occurrences did not identify similar events at Clinton Power Station.
G. COMPONENT FAILURE DATA
Component Description: Stainless steel shut-off valve for EHC System on main turbine steam valves Manufacturer: General Electric Company Manufacturer Part Number: 0843E704G0003