05000530/LER-2003-002
| Palo Verde Nuclear Generating Station Unit 3 | |
| Event date: | 03-29-2003 |
|---|---|
| Report date: | 05-27-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(ii) 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 39714 | 10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded |
| 5302003002R00 - NRC Website | |
1. REPORTING REQUIREMENT(S):
This LER is being submitted pursuant to 10CFR50.73(a)(2)(ii).
Specifically, on March 29, 2003, engineering personnel, in accordance with the PVNGS boric acid walkdown procedure discovered boric acid residue on a reactor coolant system (RCS) (EIIS:AB) hot leg instrument nozzle (EIIS:NZL) and a 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 nozzle and heater sleeve was considered to be a serious degradation of a principal safety barrier. Upon notification of the discovery, Operations personnel entered LCO 3.4.14 Condition B at 1150 Mountain Standard Time (MST). The TS Condition B was exited on March 30, 2003 at 0845 MST when the plant entered Mode 5, Cold Shutdown, and the LCO was no longer applicable.
On March 29, 2003 at 11:40 MST, APS made prompt notification of the event to the Nuclear Regulatory Commission (NRC) via the emergency notification system (ENS# 39714).
2. DESCRIPTION OF STRUCTURE(S), SYSTEM(S) AND COMPONENT(S):
The degraded hot leg instrument nozzle (EIIS: TW) (2JRCCTVV0112HC) penetrates the RCS Loop 1 hot leg piping and was fabricated from NiCrFe Alloy 600 (Inconel 600).
The degraded pressurizer heater sleeve (EIIS: EHTR) (3MRCEA01) is the location for pressurizer back-up heater "A01" and was fabricated from Inconel 600. Heater sleeve "A15" (EIIS: EHTR) (3MRCEA15) was initially suspected to be leaking, but a subsequent VT-2 examination revealed no indication of leakage.
3. INITIAL PLANT CONDITIONS:
On March 29, 2003, Unit 3 was in Mode 4, Hot Shutdown. The RCS was being cooled down in preparation for Unit 3's tenth refueling outage. There were no structures, systems, or components that were inoperable at the time of discovery that contributed to this condition.
4. EVENT DESCRIPTION:
On March 29, 2003, 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 walkdowns is to identify boric acid leakage for corrective maintenance during the refueling 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 an instrument nozzle that penetrates the Loop 1 hot leg. The boric acid had accumulated on the exterior of the hot leg piping around the outer perimeter of the instrument nozzle. In addition, a detectable amount of boric acid residue was observed around the penetration for pressurizer back-up heater A01.
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The cracking of Alloy 600 hot leg piping instrument nozzles at Palo Verde has been attributed to primary water stress corrosion cracking (PWSCC), and has not considered an immediate significant threat to the structural integrity of the RCS boundary or instrument nozzles, due to the orientation 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 been axial in orientation and have resulted in detectable (albeit insignificant) leakage. Past examinations of hot leg piping instrument nozzles have confirmed this conclusion (reference LERs: 50-530/2001-003-00, 50-528/2001-001-00 and 50-528/1999-006-00). At Palo Verde, the boric acid leakage has been identified during visual examinations prior to any significant degradation of the reactor coolant boundary. Eddy Current examination at heater sleeve A01 confirmed the presence of axial orientated PWSCC.
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 10CFR50.73(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 instrument nozzle and heater sleeve leakage, resulting from axial cracking of the Inconel 600 material. This type of cracking is known to be affected by high temperatures and time of service.
Alloy 600 nozzles show significant variability with respect to PWSCC. Forgings and hot worked bar stock may be more susceptible to this form of degradation than cold drawn and annealed pipe material. Nozzles with a wide variety of yield strength levels, from near the specification minimum to very high yield strength, have cracked. Nozzles fabricated from Alloy 600 with a variety of microstructures, including some that steam generator experience indicated should have been resistant to PWSCC, have cracked, as have nozzles fabricated from material with high and low final mill-anneal temperatures.
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 Alloy 600 instrument nozzle that penetrates the RCS Loop 1 hot leg piping was repaired with a more corrosion resistant Alloy 690 nozzle.
The degraded heater sleeve was repaired using a 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.
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 LER50-530/2001-003-00, LER 50- 528/2001-001-00 and LER 50-528/1999-006-00, in which different hot leg instrument nozzles were found to have evidence of leakage (boric acid residue). Similarly, these conditions have been attributed to PWSCC and the nozzles were repaired using Alloy 690 instrument nozzles. The corrective actions to prevent recurrence from the previous conditions are the same as for the conditions reported in this LER.