1.1 Problem Statement
“On August 23, 2011 at 1351 North Anna Power Station experienced a magnitude 5.8 earthquake followed by a dual unit trip and a loss of offsite power. The purpose of this Root Cause Evaluation is to:
0 Identify the cause for the reactor trips on NAPS Unit 1 and Unit 2.”
1.2 Root Causes
1.2.1 “The Direct Cause for the both the Unit 1 and Unit 2 reactor trip was the initiation of the rate Nuclear Instrument (NIs) Power (PWR) Range Hi Flux Rate Reactor Trip. Both Unit 1 and Unit 2 met the required coincidence of 2 out of 4 Power Range Nuclear Instruments (PRNI) with greater than a 5 % change in 2.25 seconds.
(RC1) The Root Cause of this event was a synergistic combination of seismically induced conditions which include core barrel movement, detector movement, and small reactivity effects from core movement and thickening of the thermal-boundary layer along the fuel rods. The additive effects of the combined conditions resulted in momentary under moderated core conditions as evidenced by the oscillatory but overall decreasing flux profiles from both Unit 1 and Unit 2.”
1.3 Contributing Cause(s):
1.3.1 “No Contributing Causes are applicable to this event. All potential candidates for Contributing Causes were investigated in detail and determined to not be credible.”
1.4 Corrective Action
“As part of this investigation, a detailed review was performed to determine the validity of the Dranetz Sequence of Event Recorder data and PCS alarms for Unit 1 and Unit 2. The results of this review determined that all data points were valid, equipment operated as expected, with the exception of 5 points that are being tracked by NAPS Corrective Action Program. Notable Equipment related Narrative Logs and Condition Reports submitted immediately following the Seismic Event are referenced in this report and resolution of concerns is tracked through the Corrective Action Process.”
“On August 23, 2011 at 13:51 hours, a magnitude 5.8 earthquake occurred approximately 11 miles WSW of North Anna Power Station. The earthquake caused a series of reactor trip signals to both Unit 1 and Unit 2 reactors, as well as a total loss of offsite power to the station. The “First Out” reactor trip signals for both Units were “High Flux Rate Reactor Trip”. The “First Out” turbine trip signal for Unit 1 was “Main Transformer Lock Out”.
The “First Out” for Unit 2 was “Reactor Trip/Turbine Trip”.
To facilitate the investigation the following investigative techniques were used;
1. Fault Diagram – “Why” chart (Attachment 2) …
2. Event Timeline (Attachments 4, 5 and 6) …
3. Analysis Of Dranetz Event Data vs Plant Computer System (Attachment 10)”
Specific Indicated Nuclear Power Transient Cause Discussion
“Operating experience and testing associated with Japanese nuclear reactors indicates core reactivity changes can occur as a result of small changes in overall fuel geometry caused by earthquakes. This phenomenon would cause a down power created by seismic vibrations disrupting the laminar sub-layer along the cladding wall resulting in rapid and transitory bubble bursts that would add negative reactivity due to the void defect. The RCE Team noted that the down power at the start of the event coincides with the start of seismic activity. Once the Reactor Trip Breakers opened there were several more oscillations which coincided with the highest seismic peaks noted during the earthquake. Figure 4 and 5 show the correlation between the oscillations and seismic activity on both Units. Also plotted on the graphs is the time that reactor trip breakers opened.