05000237/LER-2015-001
| Dresden Nuclear Power Station, Unit 2 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 2372015001R01 - NRC Website | |
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PLANT AND SYSTEM IDENTIFICATION
Dresden Nuclear Power Station (DNPS), Unit 2, is a General Electric Company Boiling Water Reactor with a licensed maximum power level of 2957 megawatts thermal. The Energy Industry Identification System codes used in the text are identified as [XX].
A. Plant Conditions Prior to Event:
Unit: 02 Event Date: 01/13/15 and 02/16/15 Reactor Mode: 1 Mode Name: Power Operation Event Time: 1903 and 1227 CST Power Level: 100 percent
B. Description of Event:
On January 13, 2015, operators inserted a manual scram on Unit 2 due to increasing reactor water level. It was determined that the Feed Water Level Control [JB] (FWLC) system had failed. All control rods inserted, all system responded as expected. The FWLC system processor cards were replace and the unit was returned to operation on January 16, 2015 On February 6, 2015, during maintenance activities to continue troubleshooting additional FWLC alarms, the reactor water level increased as a result of a momentary interruption in the FWLC power supply. Feedwater regulating valves were manually operated to control reactor water level, however the transient resulted in an automatic scram due to low reactor vessel water level. During the transient, High Pressure Coolant Injection [BJ] (HPCI) was initiated, but proceduralized operator response actions prevented injection. The Unit 2 and 2/3 Emergency Diesel generators [EK] started but did not load on to their respective busses as offsite power was not lost.
During both events, main feed water [SJ] flow was maintained in accordance with station operating procedures.
These events are being reported under 10 CFR 50.73(a)(2)(iv)(A) "Any event or condition that resulted in manual or automatic actuation of any of the systems listed in paragraph (a)(2)(iv)(B)... "
C. Cause of Event:
The cause of these events was a spurious power interruption on a historically improperly landed power supply ribbon cable connection for the digital FWLC system, which caused a momentary 5Vdc power supply loss to the FWLC Multi-Function Processors (MFP). The loss of power resulted in a runback of the 2A Reactor Recirculation [AD] pump and a reactor level transient. The runback signal from FWLC was caused by mis-positioned DIP switches on another FWLC digital circuit board.
A second root cause for the scram on 02/06/15 was less than rigorous organizational challenge of troubleshooting activities, and less than rigorous troubleshooting that allowed the conditions to remain undetected following the scram on 01/13/15. The troubleshooting was focused upon a pre-conceived assumption of the issue and did not fully validate all potential causes nor challenge their disposition.
D. Safety Analysis:
Following the actuations, all other systems responded as expected, therefore, this event is of low safety significance.
E. Corrective Actions:
1. The degraded five-ribbon cable was replaced per WO 01805706-03, with five single-ribbon cables per the vendor's recommendations.
2. Develop and present a Case Study from this RCR that specifically highlights the level of challenge needed to ensure that a troubleshooting problem is thoroughly understood to the Senior Management Team, Engineering, and Maintenance.
3. Improve the formality and rigorous use of the troubleshooting process by performing the following:
- Brief lessons learned from RCR 2437067-02 at an Engineering All-Hands Meeting.
- Brief the troubleshooting process with the Engineering management team.
- Perform training on the Troubleshooting process emphasizing the correct mindset, being open-minded about other potential causes, and comprehensive. Train the engineering population on the troubleshooting issues associated with this root cause.
- Increase oversight of the troubleshooting teams for the next six months with senior manager observations of troubleshooting activities and troubleshooting briefs.
4. Revise Unit 2 and Unit 3 Preventive maintenance Model WOs to document as-found/as-left positions of DIP switches and other positionable components if circuit cards are removed.
5. Correct Unit 2 DIP Switch Positions.
6. Perform a review of 100% of open Complex Troubleshooting Support/Refute Matrices and review 100% of the matrices closed within last year.
7. Station Manager/Site VP to review root cause with site senior management and emphasize the challenges missed.
8. Review existing event response procedures checklist for team initiation and review of roles and responsibilities.
F. Previous Occurrences:
No previous occurrences of this event were determined through the investigation.
G. Component Failure Data:
Manufacturer Model S/N Type Bailey YIMMFP12 PTJED11391 and PTJED11487 Module Assembly