05000389/LER-2009-004
| Docket Number Sequential Revmonth Day Year Year Month Day Yearnumber No. | |
| Event date: | |
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| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| 3892009004R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6)
Description of the Event
On September 20, 2009, St. Lucie Unit 2 was performing a plant startup following a 2B2 reactor coolant pump (RCP) [EIIS: AB] maintenance outage. St. Lucie Unit 2 was entering Mode 2 when a Control Room Operator observed that the primary safety valve (PSV) [EIIS: AB] V1202 had indications of seat leakage based on increasing tailpipe temperatures.� Quench tank [EIIS: AB] levels did not increase, indicating that the leakage was only non-condensable gas from the pressurizer steam space. Control Room operators stopped the leakage by reducing reactor coolant system (RCS) [EIIS: AB] pressure by 200 psi and slowly increasing the pressure over the next 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> to minimize thermal loading on the valve. Full RCS pressure of 2250 psia was reached at approximately 0900 on 9/21/Q9 with all pressurizer safety valve temperatures stable; reactor start-up commenced at 1227 on 9/21/09.
At 1333 on 9/21/09, the safety valve V1202 tailpipe temperature abruptly increased to approximately 224 °F coincident with quench tank pressure increasing from 4.0 psig to 4.4 psig. Control Room Operators stopped the leakage by lowering RCS pressure to 2050 psia and the Unit Supervisor ordered a manual reactor trip. All safe shutdown equipment operated as designed with no adverse impact on the health and safety of the public. The reactor did not yet achieved criticality.
Cause of the Event
An evaluation determined the causes for this event can be attributed to PSV seat leakage resulting from the valve not achieving thermal stability early enough to resist the increasing pressure. Contributing causes included gas management during the startup process, missing PSV inlet flange insulation, and procedure deficiencies.
Analysis of the Event
This event is reportable under 10 CFR 50.73(a)(2)(iv)(A), as any event or condition that resulted in a manual or automatic reactor trip.
Analysis of Safety Significance The PSVs are installed to provide over pressure protection for the RCS. The PSVs provide an active safety function which provides RCS overpressure protection during increasing pressure transients in accordance with ASME Boiler and Pressure Vessel Code Section III. The PSVs also provide a passive safety function to maintain the RCS pressure boundary.
Actions taken by the Operators to manually trip the Unit precluded any safety concerns for the PSVs to seat during startup; safe shutdown was achieved without impacting the health and safety of the public.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6)
Corrective Actions
The following corrective and supporting actions were entered into the site corrective action program.
1. Replaced PSV valve V1202 with an identical model valve.
2. Added insulation collars on all three PSV inlet pipe flanges.
3. Upgrade PSV removal and installation maintenance procedure 0-MMP-01.09 4. Revise Operation procedure 2-GOP-502 to mandate slow RCS pressurization rate and annotate these changes to sustain actions for sustainability (PSV thermal stability issue) Similar Events Review of operating history for St. Lucie identified five events of seat leakage from the Pressurizer Code Safety Relief Valves. The causes varied; this is a repeat event.
Failed Components
Crosby Valve and Gage Company pressure relief valves; The valves are stainless steel forged body model DS-C-84217 size 3K6 (3" inlet and 6" outlet) with a 3" 2500 # large tongue inlet flange, and a 6" 300# raised face exit flange.