05000387/LER-2014-001
| Susquehanna Steam Electric Station - Unit 1 | |
| Event date: | 02-06-2014 |
|---|---|
| Report date: | 04-01-2014 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 3872014001R00 - NRC Website | |
CONDITIONS PRIOR TO THE EVENT
Unit 1 - Mode 1, 100 percent Rated Thermal Power Unit 2 - Mode 1, 100 percent Rated Thermal Power There were no systems, structures, or components that were inoperable at the start of the event and contributed to the event.
EVENT DESCRIPTION
On February 6, 2014, Susquehanna Steam Electric Station (SSES) discovered a previously unrecognized failure to enter Technical Specification (TS) LCO 3.4.10 when on 27 occasions, the Reactor Pressure Vessel (RPV) (EIIS:
RPV) pressure dropped below 0 psig during past reactor startups and shutdowns. At the time of discovery, both units were operating in Mode 1 at 100 percent thermal power. All systems were performing as designed.
This LER is being submitted to the NRC in accordance with 10 CFR 50.73(a)(2)(i)(B) for a condition prohibited by TS 3.4.10 since the condition existed for a time longer than permitted by the TS.
CAUSE OF THE EVENT
The apparent cause of the failure to enter the LCO was the condition was procedurally allowed and aligned with training provided to the licensed operators. Since the P-T limit curves do not include values for pressure and temperature below 0 psig, operating with the RPV being below 0 psig was not recognized as a condition prohibited by TS until the receipt and evaluation of I NPO OE 309129.
ANALYSIS / SAFETY SIGNIFICANCE
There were no actual or potential consequences to the health and safety of the public as a result of this event. The 27 instances did not challenge any design or safety limit. Nuclear safety was not significantly compromised because the negative (vacuum) internal pressures do not adversely impact the structural integrity of the vessels. Further, operation at a vacuum does not reduce any design margin to non-ductile failures.
SSES operated in an analyzed condition and within established margins for brittle fracture of the RPV. There are no safety significant issues created by this condition. During this condition the RPV steam dome operates with a vacuum of approximately -14 to -6 psig during startup with the RPV metal temperatures at approximately 160 degrees Fahrenheit (°F) to 170°F. Operation below 0 psig on the pressure/temperature (P-T) limit curves has been evaluated and determined to be acceptable. When the RPV experiences a vacuum, there will be a uniform compressive membrane loading across the vessel wall caused by the ambient external pressure being larger than the internal pressure. Consequently, the driving force acting on the tip of a postulated flaw in any location will be reduced from that calculated for the 0 psig point on the P-T limit curves. In other words, the applied stress intensity factor at the postulated crack tip, when the RPV experiences a vacuum, is less than the applied stress intensity factor when the RPV experiences a positive internal pressure. Thus, the RPV metal temperature required for an internal pressure of 0 psig on the P-T limit curves is bounding for RPV operation with a vacuum.
CORRECTIVE ACTIONS
Completed Actions:
- The Unit 2 operating procedure for reactor startup and heatup has been revised to proceduralize plant start-up with the MSIVs closed and then opening them between 10 and 40 psig RPV pressure. Startup in this manner maintains RPV pressure above 0 psig at all times and minimizes level transients caused by pressure changes when opening the MSIVs.
- Operators were trained on the revised reactor startup and heatup procedure.
Planned Actions:
- Prior to startup from the 2014 Unit 1 refueling outage, the operating procedure for reactor startup and heatup will be revised to be consistent with the changes made to the corresponding Unit 2 procedure.
- Prepare a change to the P-T limit curves.
PREVIOUS SIMILAR EVENTS
There were no previous similar events identified at SSES in the past three years relative to RPV P-T limit curve limitations.