05000530/LER-2004-001
| Palo Verde Nuclear Generating Station Unit 3 | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 40556 | 10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded |
| 5302004001R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
This LER is being submitted pursuant to 10CFR50.73(a)(2)(ii)(A).
Specifically, on February 29, 2004, engineering personnel, in accordance with the PVNGS boric acid walkdown procedure discovered boric acid residue on a reactor coolant system (RCS) (EIIS:AB) pressurizer heater sleeve (EIIS:EHTR). Technical Specifications (TS) Limiting Condition for Operation (LCO) 3.4.14 permits no RCS pressure boundary leakage. Therefore, the discovery of leakage (boric acid residue) from the heater sleeve was considered to be a serious degradation of a principal safety barrier.
On February 29, 2004 at 0944 Mountain Standard Time (MST), APS made prompt notification of the event to the Nuclear Regulatory Commission (NRC) via the emergency notification system (ENS# 40556).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The degraded pressurizer heater sleeve (EIIS: EHTR) (3MRCEA03) is the location for pressurizer heater "A03" and was fabricated from alloy 600.
3. INITIAL PLANT CONDITIONS:
On February 29, 2004, Unit 3 was in Mode 3, Hot Standby. The Unit had been shutdown and was being maintained in Mode 3 while trouble shooting a turbine- generator excitation problem. At the time of discovery, the RCS was at approximately 565 degrees F and 2245 psia. There were no structures, systems, or components that were inoperable at the time of discovery that contributed to this condition.
4. EVENT DESCRIPTION:
On February 29, 2004, APS engineering personnel were performing pre-planned boric acid walkdowns of RCS components in accordance with the Boric Acid Corrosion Prevention Program procedure (70T1-9CZ01). This procedure incorporated NRC Bulletins 2002-01 and 2002-02, which address RCS pressure boundary integrity.
The purpose of the boric acid walkdown is to identify boric acid leakage for corrective maintenance during the outage. The walkdowns are part of APS' systematic measures to ensure that boric acid corrosion does not lead to degradation of the reactor coolant pressure boundary and that there is an extremely low probability of abnormal leakage, rapidly propagating failure, or gross rupture.
During the walkdown of the RCS, a detectable amount of boric acid residue was observed around the heater sleeve penetration for pressurizer heater A03.
The control room personnel entered LCO 3.4.14 Condition B at 0521 MST and took actions to place the Unit in Mode 5 within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />. The TS Condition B was exited on March 1, 2004 at 0608 MST when the plant entered Mode 5, Cold Shutdown, and the LCO was no longer applicable.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The cracking of Alloy 600 materials at Palo Verde has been attributed to primary water stress corrosion cracking (PWSCC), and has not been considered an immediate INRC FORM 366A U.S. NUCLEAR REGULATORY COMMISSION (7-2001) significant threat to the structural integrity of the RCS boundary due to the orientation and location of the cracks and the low leakage rates.
The bases for this conclusion is that when PWSCC has occurred at Palo Verde, the cracks have resulted in detectable (albeit insignificant) leakage. Past examinations of hot leg piping instrument nozzles and pressurizer heater sleeves have confirmed this conclusion (references: LER 50-530/2003-002-00, LER 50-53012001-003-00, and LER 50- 528/2001-001-00). At Palo Verde, the boric acid leakage has been identified during visual examinations prior to any significant degradation of the reactor coolant boundary.
There were no failures that rendered a train of a safety system inoperable and no failures of components with multiple functions were involved.
The condition would not have prevented the fulfillment of the safety function, and the condition did not result in a safety system functional failure as defined by I OCFR50.73(a)(2)(v).
6. CAUSE OF THE EVENT:
An investigation of this event was conducted in accordance with the PVNGS Condition Reporting program. PWSCC was determined to be the mechanistic cause of the heater sleeve leakage, resulting from cracking of the alloy 600 material. This type of cracking is known to be affected by high temperatures and time of service.
Industry and Palo Verde specific data demonstrates that PWSCC cracks will become evident through small leaks prior to significant degradation of the pressure boundary.
No unusual characteristics of the work location (e.g., noise, heat, poor lighting) directly contributed to this event. No personnel or procedural errors contributed to this event.
7. � CORRECTIVE ACTIONS:
The degraded heater sleeve was repaired using an NRC-approved mechanical nozzle seal assembly (MNSA.) The MNSA is a mechanical device consisting of a split gasket/flange assembly that is placed around the leaking penetration. The gasket is made of Grafoil packing, a graphite compound that is compressed within the assembly to prevent RCS leakage past the penetration. The assembly is bolted into holes drilled and threaded on the outer surface of the pressurizer. Another assembly is bolted to the flanges, which serves as the structural attachment of the sleeve to the wall. This assembly serves to carry the loads in lieu of the "J" welds on the Alloy 600 penetrations. Post installation testing of the MNSA at normal operating pressure and temperature has demonstrated the acceptability of the installation.
The scheduled outage duration of this fall's Unit 3 refueling outage is being increased by 20 days. The scheduled outage start date is October 2, 2004. This increased duration will allow enough time to replace the pressurizer Alloy 600 heater sleeves with Alloy 690 material. The replacement schedule is to replace all alloy 600 heater sleeves in this current refueling outage but no later than the following refueling outage. Also, all installed MNSA's will be removed from the pressurizer.
Any additional corrective actions taken as a result of this event will be implemented in accordance with the PVNGS corrective action program. If information is subsequently developed that would significantly affect a reader's understanding or perception of this event, a supplement to this LER will be submitted.
8. PREVIOUS SIMILAR EVENTS:
Similar previous conditions were reported in LER 50-530/2003-002-00, LER 50-530/2001- 003-00, and LER 50-528/2001-001-00 in which different hot leg instrument nozzles and/or heater sleeves were found to have evidence of leakage (boric acid residue). Similarly, these conditions have been attributed to PWSCC and the nozzles/sleeves were repaired using an NRC-approved mechanical nozzle seal assembly (MNSA), and/or a permanent repair design.