05000388/LER-2020-001-01, Manual Scram Due to Rising Main Condenser Backpressure Caused by Failure of an Offgas Recombiner Inlet Valve
| ML20310A258 | |
| Person / Time | |
|---|---|
| Site: | Susquehanna  |
| Issue date: | 11/05/2020 |
| From: | Cimorelli K Susquehanna |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| PLA-7887 LER 2020-001-01 | |
| Download: ML20310A258 (5) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3882020001R01 - NRC Website | |
text
Kevin Cimorelli Site Vice President Attn: Document Control Desk U.S. Nuclear Regulatory Commission Washington, DC 20555-0001 Susquehanna Nuclear, LLC 769 Salem Boulevard Berwick, P A 18603 Tel. 570.542.3795 Fax 570.542.1504 Kevin.Cimorelli@TalenEnergy.com SUSQUEHANNA STEAM ELECTRIC STATION LICENSEE EVENT REPORT 50-388/2020-001-01 UNIT 2 LICENSE NO. NPF-22 PLA-7887 TALEN~
ENERGY 10 CFR 50.73 Docket No. 50-388 Attached is Licensee Event Report (LER) 50-388/2020-001-01. The LER supplement reports an event involving a manual scram due to rising main condenser backpressure. The condition is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in a manual actuation of the Reactor Protection System (including a reactor scram).
There were no actual consequences to the health and safety of the public as a result of this event.
Attachment: LER 50-388/2020-001-01 Copy:
NRC Region I Mr. C. Highley, NRC Sr. Resident Inspector Ms. S. Goetz, NRC Project Manager Mr. M. Shields, PA DEP/BRP November 5, 2020
Abstract
On February 14, 2020 at 00:25, Susquehanna Steam Electric Station (SSES) Unit 2 reactor was manually scrammed due to rising Main Condenser backpressure caused by a loss of the Unit 2 Offgas Recombiner.
This event was reported by Notification EN 54525 in accordance with 10 CFR 50. 72(b)(2)(iv)(B) and (b)(3)(iv)(A). This event is also reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in a manual actuation of the Reactor Protection System (RPS) (including reactor scram), as well as associated isolation and actuation of other systems listed in 10 CFR 50. 73(a)(2)(iv)(B), including the Reactor Core Isolation Cooling (RCIC) system.
The most probable cause was an inadvertent closure (drifting closed) of the Unit 2 Offgas Recombiner System Inlet Control Valve. Stroking, disassembly, reassembly, and air operated valve diagnostics alleviated the cause of the inadvertent closure.
There were no actual safety consequences associated with the condition.
U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 08/31/2023
- 3. LERNUMBER Susquehanna Steam Electric Station Unit 2 05000-388
NARRATIVE
CONDITIONS PRIOR TO EVENT
Unit 1 - Mode 1, approximately 86 percent Rated Thermal Power Unit 2 - Mode 1, approximately 98 percent Rated Thermal Power YEAR 2020 SEQUENTIAL NUMBER 001 REV NO.
01 Loss of the Unit 2 Offgas Recombiner caused rising Main Condenser backpressure, which resulted in the need for a manual scram.
EVENT DESCRIPTION
On February 14, 2020 at 00:25, Susquehanna Steam Electric Station Unit 2 reactor was manually scrammed due to rising Main Condenser [EllS System Code/Component Code: SG/COND] backpressure caused by a loss of the Unit 2 Offgas Recombiner [EllS System/Component Code: WF/RCB]. Following is a timeline of the events associated with the scram:
February 14, 2020 at approximately 00:12-Unit 2 Offgas Recombiner OC145 Panel Trouble and 2C198 Hydrogen Water Chemistry Panel Trouble alarms were received along with indication of rising Main Condenser backpressure. Initial Main Condenser backpressure was 2.6 inches HgA (Mercury Absolute) and was rising at approximately 0.3 inches HgA/minute.
February 14, 2020 at approximately 00:15-A Recirculation Limiter 2 run back was inserted to lower reactor power. Main Condenser backpressure continued to rise following the reduction in reactor power.
February 14, 2020 at approximately 00:25-A manual scram was inserted by placing the Reactor Mode Switch to Shutdown when Main Condenser backpressure rose to 6 inches HgA. All control rods inserted.
Reactor water level lowered to -30 inches causing a Level 3 (+13 inches) isolation and a partial (Division
- 2) Level 2 (-38 inches) isolation. The Reactor Core Isolation Cooling (RCIC) system [EllS System Code:
BN] actuated as expected for given plant conditions. Operators subsequently maintained reactor water level at the normal operating band using the Reactor Feed Water system [EllS System Code: SJ]. No steam relief valves [EllS System/Component Code: SB/RV] opened. The Reactor Recirculation Pumps [EllS System Code/Component Code: AD/P] remained in service.
This event was reported by Notification EN 54525 in accordance with 10 CFR 50.72(b)(2)(iv)(B) and (b)(3)(iv)(A). This event is also reportable in accordance with 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in a manual actuation of the Reactor Protection System (RPS) (including reactor scram), as well as associated isolation and actuation of other systems listed in 10 CFR 50. 73(a)(2)(iv)(B), including RCIC.
CAUSE OF EVENT
The most probable cause was an inadvertent closure (drifting closed) of the Unit 2 Offgas Recombiner System Inlet Control Valve (HV26912) [EllS System/Component: WF/FCV]. Indications, system responses, and troubleshooting narrowed the failure to HV26912 inadvertently drifting closed, resulting in increased Main Condenser backpressure and the stalling of the Steam Jet Air Ejector [EllS System/Component Code: SH/EJR].
SEQUENTIAL NUMBER 001 REV NO.
01 Contributing to this event were a degraded antenna cable connector that resulted in poor radio communication and lack of clear procedural direction to place the standby offgas recombiner in service for the condition that existed. These contributors resulted in delay in placing the standby offgas recombiner in service.
ANALYSIS/SAFETY SIGNIFICANCE
The actual consequences from the loss of the Unit 2 Offgas and Condenser Air Removal Systems were rising main condenser backpressure, a forced down power, and subsequent manual scram.
The potential consequences included the following:
Without prompt operator action, an automatic scram would have initiated, potentially with complications.
Had the Common Offgas Recombiner not been available, the scram would have been complicated by the continued loss of condenser vacuum until the Mechanical Vacuum Pump [EllS System/Component Code: SHIP] was placed in service, and steam would have been directed to the Suppression Pool.
Based on the results of a risk significance evaluation, the event was classified as "very low" safety significance. During the event, very few risk significant components were out of service. The manual scram was uncomplicated, did not require Emergency Core Cooling System (ECCS) initiation, and Main Steam Isolation Valves (MSIVs) remained open (i.e., the main condenser remained available as the primary heat sink).
The condition described herein did not result in a safety system functional failure. Accordingly, this event will not be counted as a safety system functional failure in the Reactor Oversight Process Performance Indicators. There were no actual consequences to the health and safety of the public as a result of this event.
CORRECTIVE ACTIONS
Key corrective actions include:
- 1. HV26912 stroking, disassembly, reassembly, and air operated valve diagnostics alleviated the cause of the inadvertent closure.
- 2. The degraded antenna cable connector was repaired and signal strength checks were added to the annual radio health check preventive maintenance.
- 3. Established a tracking system to ensure Operations hand held radios are periodically maintained, including re-tuning.
- 4. Off normal vacuum procedures were revised to include abnormal offgas recombiner operation with rapidly degrading vacuum.
- 5. Replacement and testing of SV26912 during the next system outage window is planned and Susquehanna will revise the cause analysis if conflicting information is found.
COMPONENT FAILURE INFORMATION
Component failure information is as follows:
Manufacturer: Valtek, Inc.
Model No.: Mark I Valve Function: Flow Control Valve Valve Type: Globe Size: 10 inches Actuator Type: Air-Operated
PREVIOUS OCCURRENCES
YEAR 2020 SEQUENTIAL NUMBER 001 REV NO.
01 LER 50-388/2015-003, "Unit 2 Automatic Reactor Scram Caused by Main Turbine Trip Due to Loss of Main Condenser Vacuum," dated June 8, 2015 Page..!_ of..!_