05000416/LER-2013-005
| Grand Gulf Nuclear Station, Unit 1 | |
| Event date: | 12-12-2013 |
|---|---|
| Report date: | 10-06-2014 |
| 4162013005R01 - NRC Website | |
A. REPORTABLE OCCURRENCE
This Licensee Event Report (LER) is being submitted pursuant to Title 10 Code of Federal Regulations (10 CFR) 50.73(a)(2)(i)(B) for an operation or condition which was prohibited by Technical Specifications by not entering Limiting Condition of Operation (LCO) 3.4.11, RCS Pressure and Temperature (PIT) Limits during six Reactor startups with Reactor Pressure Vessel (EllS:RPV) steam pressure below zero (0) pounds per square inch gage (psig).
B. INITIAL CONDITIONS
At the time of discovery of the issue, the reactor was in operational mode one with reactor power at 100 percent and normal operating pressure. There were no additional inoperable structures, systems, or components at the time of discovery that contributed to this event. This event is considered a discovery of an existing but previously unrecognized condition.
C. DESCRIPTION OF OCCURRENCE
On December 12, 2013, Grand Gulf Nuclear Station discovered a previously unrecognized failure to enter LCO 3.4.11 when Reactor Pressure Vessel (RPV) pressure dropped below zero psig during six reactor startups. The plant was operating in Mode 1 at 100 percent thermal power during discovery. All systems performed per design.
D. APPARENT CAUSE
The cause of the failure to enter the LCO was the condition was procedurally allowed and aligned with training Operations personnel had received. Therefore, RPV being below 0 psig was not recognized to be a condition that required entry into the LCO.
Integrated Operating Instruction (101) 03-1-01-1 was revised in 1994 to allow startup with a vacuum in the reactor vessel because opening Main Steam Isolation Valves (MSIVs) with pressure in the reactor vessel can cause a rapid drop in level.
E. CORRECTIVE ACTIONS
A corrective action has been issued to revise the 101 03-1-01-1 to have instruction to prevent drawing a vacuum on the RPV. Interim corrective action of MSIVs remaining closed during start-up until a slight pressure (-5-10 psig) is observed in the reactor was issued to ensure negative pressure is not applied to the RPV until 101 03-1-01-1 is revised. A corrective action has also been issued for training to update all applicable training materials related to changes to the 101 03-1-01-1.
F. SAFETY ASSESSMENT
The six events posed no threat to public health and safety as the RPV performed as designed. All safety systems performed as designed.
The six events did not challenge any design or safety limit. Nuclear safety was not significantly compromised because the negative (vacuum) internal pressures identified do not cause any concerns with applicable material stresses or analysis for the GGNS RPV. GGNS remained in an analyzed condition and within established margins in regards to brittle fracture of the RPV ferritic materials. There are no known or understood safety significance issues created by allowing the RPV to have a relatively small vacuum of approximately -9.9 to -6 psig during startup with the RPV metal temperatures being at approximately 160 degrees Fahrenheit (°F) to 170°F. There is reasonable assurance that operating below 0 psig on the pressure/temperature (PIT) curves is acceptable. The PTLR is set for the limiting components which are nozzle welds for the RPV inlet. These locations get compressive and tensile stresses during startup and shutdown, but the vacuum in the steam region had an immeasurable effect on these limiting locations. The locations were still under pressure from the static fluid head (just less pressure due to being at vacuum). Therefore, margin remained.
During the six events, no Technical Specification defined Safety Limits were challenged.
Radiological Safety was not affected since there was no radiological release to the public during the events.
There was no impact to the safety of the public, industrial safety or radiological safety as a result of these events.
G. ADDITIONAL INFORMATION
There have been no indications of RPV brittle fracture in the past 3 years due to the cause documented in this LER. There have been no identified failures to enter LCO 3.4.11 due to inadequate procedure or training documented in an LER in the past 3 years.