05000529/LER-2006-002
| Palo Verde Nuclear Generating Station | |
| Event date: | 07-16-2006 |
|---|---|
| Report date: | 09-14-2006 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability |
| 5292006002R00 - NRC Website | |
Note: All times listed in this event report are approximate and Mountain Standard Time (MST) unless otherwise indicated.
1. REPORTING REQUIREMENT(S):
This LER (50-529/2006-002-00) is being submitted pursuant to 10 CFR 50.73(a)(2)(vii), to report an event where a single condition caused two independent trains to become inoperable in a single system designed to remove residual heat and mitigate the consequences of an accident. Specifically, on July 16, 2006 a watertight fire door (EIIS:
NM, DR) functioning as the train separation barrier between Auxiliary Feedwater (AF) (EIIS: BA) pump rooms 'A' and 'B' was left open and unattended for approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and 20 minutes. As a result, AF trains 'A' and 'B' were rendered inoperable.
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The AF system consists of one essential steam turbine driven pump (train 'A') (EIIS: BA, P, TRB), one essential motor driven AF pump (train 'B') (EIIS: BA, P, MO), and one non-essential motor driven AF pump (train 'N') configured into three trains. The essential steam turbine-driven and motor-driven AF pumps are located on the 80 ft level in the Main Steam Support Structure (EIIS: NM) in separate rooms designed to seismic category I requirements.
- Each essential pump provides 100 percent of AF flow capacity to the steam generators (EIIS: AB, SG) as assumed in the accident analysis. The AF system mitigates the consequences of any event with a loss of normal feedwater (EIIS:
SJ). The design basis of the essential AF trains is to supply water to the steam generator to remove decay heat and other residual heat, by delivering at least the minimum required flow rate to the steam generators at pressures corresponding to 1270 psia at the entrance to the steam generators. A watertight fire door (C-A-06) functions as the train separation barrier between train 'A' and 'B' AF pump rooms.
3. INITIAL PLANT CONDITIONS:
On July 16, 2006, at 14:52, Palo Verde Unit 2 was in Mode 1 (power operations), operating at approximately 100 percent power. There were no major structures, systems, or components inoperable at the start of the event that contributed to the event.
4. EVENT DESCRIPTION:
On July 16, 2006 at 10:28, a Fire Department Emergency Services Officer (ESO) (utility, non-licensed) entered the 80' Main Steam Support Structure (MSSS) through security door C-A-01 to perform assigned tasks in the train 'A' AF room. The ESO then entered C-A-06 to conduct inspections in the train 'B' AF room. The ESO completed the tasks and exited the area at 10:32. Later the same day at 14:52 an Area Operator (AO) (utility, non-licensed) entered door C-A-01 and found door C-A-06 open. The AO secured door C-A-06, exited the area, and notified the Unit 2 Control Room Supervisor (CRS) (utility, licensed) of the open door. Operations personnel declared both train 'A' and 'B' AF pumps inoperable for the duration door C-A-06 was left open and unattended.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The condition of two Auxiliary Feedwater (AF) trains inoperable in Mode 1, 2, or 3 is addressed by Limiting Condition for Operation (LCO) 3.7.5 Condition C requiring the unit to be in Mode 3 within 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> and in Mode 4 within 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br />. Since the duration of this event was 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> and 20 minutes, no condition prohibited by Technical Specifications existed.
In this case, there was no flooding, fire, or seismic event; therefore, there was no actual consequence to safety related functions.
Total Risk Impact of having the door between the two Class lE AF pump rooms open is the sum of Incremental Conditional Core Damage Probability (ICCDP) (fire), ICCDP (seismic), and ICCDP (flood).
The fire delta core damage frequency (CDF) was conservatively assessed to be the sum of risk due to a fire in train 'A' AF room with the train 'B' AF out of service and a fire in train 'B' AF room with train 'A' AF out of service (00S), minus the baseline fire risk.
(3.5E-8/yr). The ICCDP is the delta CDF times the event duration (4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />, 20 minutes) which was rounded to 5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />.
ICCDP (fire) = 3.5E-8/yr * 5hr/(8760hr/yr) = 2E-11 The open barrier does not provide any seismic mitigation function, so the seismic delta CDF is null, ICCDP (seismic) is equal to 0.
The flood delta CDF was conservatively assessed as a miscellaneous reactor trip event with the flood analysis pipe break frequency for the train 'A' AF and train 'B' AF pump rooms, assuming train 'A" AF and train 'B' AF are out of service (3.56E-6/yr.) ICCDP(flood) = 3.56E-6/yr * 5hr/(8760hr/yr) = 2E- 9 1CCDP total =2E-11 + 0 + 2E-9 = 2E-9 This indicates the risk impact of the condition described above with barrier door C-A-06 between train 'A' AF and train 'B' AF pump rooms being left open and unattended, is not significant.
6. CAUSE OF THE EVENT:
The direct cause of the event was the ESO failed to follow procedural requirements for closing and securing door C-A-06. The root cause was the ESO inspecting the area inside door C-A-06 did not maintain sufficient attention to ensure the door was closed.
7. CORRECTIVE ACTIONS:
The company implemented its' disciplinary policy for this event.
Signs for Units 1, 2, and 3 were posted at the entry to the A train AF room (C-A-01) alerting employees to secure the B train AF room door for each entry and exit.
A site wide communication was distributed and the Vice President of Nuclear Operation directed a plant stand down be held to discuss the importance of door C-A-06 being closed after entry or exit.
A local door alarm will be added to C-A-06 to alert personnel when the door is open.
Until the door alarm is installed, a compensatory measure will be established by posting a Security Officer at the entry point to the AF rooms to ensure plant personnel entering the AF rooms secure C-A-06.
8. PREVIOUS SIMILAR EVENTS:
In the past three years, two similar events were reported (LER 50-529/2005-003-00, and 50-530/2006-001-00). The corrective actions associated with these events did not effectively prevent recurrence. Note that the corrective action discussed in LER 50 530/2006-001-00 to install a local door alarm on C-A-06 had not yet been completed at the time of this event; however, the interim corrective actions were ineffective.