05000289/LER-2003-003
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| Initial Reporting | |
| ENS 40296 | 10 CFR 50.72(b)(3)(ii)(A), Seriously Degraded |
| 2892003003R00 - NRC Website | |
1. REPORTING REQUIREMENT (S):
This LER is being submitted pursuant to 10CFR50.73(a)(2)(i)(B), and 1 OCFR50.73(a)(2)(ii)(A).
On November 4, 2003, at approximately 10AM, during the TMI Unit One 15th (T1R15) refueling outage, an inspection of the Pressurizer Heater Bundle (PHB) *[AB/PZR HTR] was completed. This inspection identified a primary system leak at the lower Pressurizer Heater Bundle Diaphragm Plate. Technical Specifications (TS) Limiting Condition for Operation (LCO) 3.1.6.4 does not allow any leakage through a nonisolable RCS strength boundary. Therefore, the discovery of leakage from the PHB Diaphragm Plate was considered a degradation of a principal safety barrier. Three Mile Island Unit One was already in Refueling Shutdown and the conditions of the LCO were already satisfied.
The initial Indication of a potential Reactor Coolant System (RCS) leak was boric acid residue located between the PHB Diaphragm Plate and the PHB Cover Plate. Following disassembly of the PHB Cover Plate and performance of NDE, it was determined that the teak pathway was through the edge of the PHB Diaphragm Plate.
On November 4, 2003 at 15:50 EST, TMI Unit One made prompt notification of the event to the Nuclear Regulatory Commission (NRC) via the emergency notification system (EN# 40296).
2. DESCRIPTION OF STRUCTURE (S), SYSTEM (5) AND COMPONENT (S):
The pressurizer heaters are used to control the pressure of the RCS. There are three heater bundles. Each heater bundle has 39 heater elements. The heaters are attached by welding to the PHB Diaphragm Plate. The two-inch thick PHB Diaphragm Plate was manufactured from Alloy 600, and was designed to be inserted into the pressurizer opening. The pressurizer opening is a stainless steel 308L clad surface over the ASME SA 508 Class 1 base metal forging. Once inserted into the pressurizer opening, the PHB Diaphragm Plate projects 0.25 inches above the pressurizer shell surface. The PHB Diaphragm Plate was attached to the pressurizer shell with an alloy 82 weld. The PHB Cover Plate (SA 533 Grade B) is installed over the PHB Diaphragm Plate and is connected to the pressurizer with sixteen 2-inch studs (SA 320 L 43). The PHB Cover Plate had a seating diameter of 20.50 inches, which rests on the 20.73-inch diameter diaphragm plate. The PHB Cover Plate and studs are designed to take the full pressure loads; and the weld of the PHB Diaphragm Plate to the pressurizer is designed as a seal weld.
3. INITIAL PLANT CONDITIONS:
On November 4, 2003, TMI Unit 1 was in Refueling Shutdown (T1R15). There were no structures, systems, or components that were inoperable at the time of discovery that contributed to this condition.
4. EVENT DESCRIPTION:
During the TMI Unit One T1R15 Refueling Outage, boric acid residue was found at the lower pressurizer heater bundle. After removal of the insulation and PHB Cover Plate, boric acid residue was removed to allow for a detailed inspection of the area. The inspection was performed using both Visual (VT-2) and Dye Penetrant (PT) techniques.
The inspection found six indications. Three were located at approximately thel8 o'clock position" and the other three at the '2 o'clock position." Four of the indications were surface indications not associated with a throughwall leak. Progressive grinding and PT inspection identified two throughwall cracks, indicating a leak path through the PHB Diaphragm Plate.
All 360 degrees of the PHB Diaphragm Plate seal weld area were inspected with no other findings. The individual heater bundle area was observed during PHB Cover Plate removal and no evidence of leakage was found. The two other pressurizer heater bundles were visually inspected without removal of the PHB Cover Plate with no boric acid deposits found.
There were no significant operational issues noted over the last operating cycle associated with the pressurizer heaters.
This event was reported as a non-emergency f8-hour] in accordance with 10 CFR 50.72(b)(3)(ii)(A).
5. ASSESSMENT OF SAFETY CONSEQUENCES:
The pressurizer is an integral part of the RCS pressure boundary. The pressurizer serves the following functions: (a) establishes and maintains RCS pressure; (b) provides a surge volume for limiting pressure excursions; and, (c) provides a water reserve to accommodate reactor coolant temperature/density changes during operation. The primary safety function provided by the pressurizer is to maintain RCS pressure boundary integrity.
The cracking of the Alloy 600 PHB Diaphragm Plate at Three Mile Island Unit One has been attributed to primary water stress corrosion cracking (PWSCC). It was not considered an immediate significant threat to the structural integrity of the RCS boundary or pressurizer due to the low leakage rate and slow crack growth. Industry experience with PWSCC of Alloy 600 components, and pressurizer heater bundle leakage, has shown that leakage rates are low and crack growth rates are relatively stow.
There were no releases from the plant due to the subject leak. The leakage from the heater bundle had no impact on the health and safety of the public.
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 TMI Unit 1 Condition Reporting program.
PWSCC was determined to be the most likely cause of the PHB Diaphragm Plate leakage, resulting in cracking of the Alloy 600 material. PWSCC is a degradation mechanism that requires three conditions for initiation, all of which were present at the heater bundle: (1) a susceptible material, (2) a high residual state of stress, and (3) an aggressive environment. The PHB Diaphragm Plate was manufactured from Alloy 600, an alloy that is known to be susceptible to PWSCC. PWSCC-induced failures in industry have occurred in a number of Alloy 600 components, including pressurizer heaters, pressurizer nozzles, and steam generator tubes. Welding induces high residual stresses, and the diaphragm plate-to-pressurizer seal weld is a location where these residual stresses were present. The environment within the pressurizer is sufficiently aggressive to induce PWSCC: the pressurizer normally operates at a temperature of approximately 650 degrees Fahrenheit, which is above the As described above, industry data demonstrates that PWSCC cracks typically become evident through small leaks prior to significant degradation of the pressure boundary.
7. CORRECTIVE ACTIONS:
The defects in the PHB Diaphragm Plate were at two locations. The seal weld metal and PHB Diaphragm Plate were progressively ground and PT examined until no further evidence of cracking was found. An initial weld repair consisted of redepositing a seal weld over the areas of the PHB Diaphragm Plate that were removed by the aforementioned grinding. On November 23, 2003 during post maintenance testing (PMT) of the repaired seal weld at normal operating pressure and temperature, leakage was observed from the repaired seal weld area. The plant was returned to Cold Shutdown conditions to perform additional repair(s).
Subsequently, the lower Pressurizer Heater Bundle Assembly including the PHB Cover Plate was replaced with a new heater bundle assembly. This new PHB Diaphragm Plate is constructed of Type 304 austenitic stainless steel, which is resistant to PWSCC. The new PHB Diaphragm Plate was seal welded to the pressurizer using Alloy 52 weld material which is known to be highly resistant to PWSCC.
Successful post maintenance testing of the new N-IB Assembly at normal operating pressure and temperature has demonstrated the acceptability of the installation.
8. PREVIOUS SIMILAR EVENTS:
A similar previous condition was reported in LER 50-28912001-002-00 where the Reactor Vessel Upper Head Nozzle Penetrations were found to have evidence of leakage (boric acid residue). Similarly, these conditions were attributed to PWSCC and the nozzles were repaired using more corrosion resistant materials. The Reactor Vessel Upper Head was replaced this past outage, T1R15, using PWSCC resistant materials.
- Energy Industry Identification System (EllS), System Identification (SI) and Component Function Identification (CR) Codes are included in brackets, [SI/CFI) where applicable, as required by 10 CFR 50.73 (b)(2)(ii)(F).