05000456/LER-2006-001

LER-2006-001, Unit 1 Reactor Coolant System Pressure Boundary Leakage Due To Inter-Granular Stress Corrosion Cracking of a Pressurizer Heater Sleeve

Docket Number
Event date:	04-25-2006
Report date:	07-18-2006
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded
Initial Reporting
4562006001R01 - NRC Website

A. Plant Operating Conditions Before The Event:

Event Date: April 25, 2006 Event Time: 0030 Unit: 1 MODE: 6 Reactor Power: 0.0 percent Unit 1 Reactor Coolant System (RCS) [AB] Temperature: 92 degrees F, Pressure: N/A

B. Description of Event:

There were no additional structures, systems or components inoperable at the beginning of the event that contributed to the severity of the event.

On April 19, 2006, at 0300, while removing insulation from the surge line of the Unit 1 pressurizer, an insulation contractor discovered boric acid on the insulation.

On April 25, 2006, at 0030, following an extensive investigation, Exelon Generation Company, LLC (EGC) identified rouging on the insulation penetration for pressurizer [AB] heater number 52. Rouging is the development of surface deposits or staining, and is an indication of possible high temperature steam impingement on stainless steel. The leakage was identified as originating from the number 52 heater near the upper weld between the pressurizer heater sleeve and heater coupling. Technical Requirements Manual Limiting Condition for Operation (TLCO) 3.4.f Condition A was entered for one or more ASME components not in conformance due to pressure boundary leakage.

Analysis of the boric acid found on the pressurizer and the pressurizer insulation determined a leak existed during the past operating cycle which is not in compliance with Technical Specification (TS) 3.4.13, "RCS Operational Leakage," that states there will be no pressure boundary leakage in Modes 1, 2, 3 and 4. This leak was identified through refueling outage inspection activities, not during plant operation.

The heater coupling and a portion of the sleeve was cut out of the system and the remaining portion of the heater sleeve was plugged and welded using an engineered ASME Section III repair detail. A complete visual inspection of all 78 pressurizer heaters was performed to determine the initial extent of condition. Heater number 52 was identified as the only source of boric acid leakage from the pressurizer.

On April 28, 2006, at 0143, following the repair of the pressurizer, TLCO 3.4.f Condition A was exited.

. Cause of Event The coupling and portion of the heater sleeve removed were shipped to a testing facility to determine the cause of the observed leak.

The failure analysis of the removed heater sleeve identified a through-wall crack located in the heat affected zone of the heater sleeve near the upper coupling weld. The failure was caused by circumferentially oriented intergranular stress corrosion cracking (IGSCC). The crack propagated through the heater sleeve heat affected zone appearing to be heavily sensitized during fabrication welding. Typically, the material used for the sleeve, Type 316 stainless steel, is not susceptible to IGSCC in a pressurized water reactor (PWR) environment. However, sensitized 316 stainless steel can be susceptible to IGSCC in stagnant PWR environments containing oxygen. With the heater element inserted into the sleeve, a long, cylindrical, crevice, approximately 0.015" wide by 12" long, is created between the heater element and the sleeve.

Analysis of the boric acid found on the pressurizer and the pressurizer insulation determined a leak existed during the past operating cycle which is not in compliance with TS 3.4.13, "RCS Operational Leakage," that states there will be no pressure boundary leakage in Modes 1, 2, 3 and 4. This leak was identified through refueling outage inspection activities, not during plant operation.

The root cause of the observed boric acid leakage was IGSCC of the number 52 pressurizer heater sleeve though a locally sensitized section of the Type 316 stainless steel base material. The heater sleeve was sensitized through minor cold working of the inner diameter bore and by the heat generated by multiple weld passes on the coupling to heater sleeve weld.

D. Safety Consequences:

There were no safety consequences impacting plant or public safety as a result of this event.

All 78 PZR heater sleeves and couplings were visually inspected in the refueling outage. Heater 52 was confirmed to be the only leakage source. Based on the amount of boric acid present, the leak size was determined to be extremely small and the associated leak rate would be too small to be captured by normal surveillance methods. If leakage had increased, the normal charging system would be used to compensate for the leakage. The leak was identified through refueling outage inspection activities, not during plant operation. Had the pressure boundary leakage condition been identified in Modes 1, 2, 3 or 4, TS 3.4.13, "RCS Operational Leakage," would have required a Unit 1 shutdown to identify the leakage source and to perform the necessary repairs prior to resuming operation.

Operating experience for this design of stainless steel heater sleeve indicates that the heater at location 52 of Braidwood Unit 1 represents the only occurrence of IGSCC-induced circumferential cracking in the industry. With no other occurrences of leakage occurring in this large population of stainless steel heater sleeves, this data suggests an extremely low probability of occurrence of cracking in another heater sleeve.

A postulated severance or ejection of a heater sleeve due to circumferential cracking was evaluatAd. Ejection of A heater sleeve was found to be equivalent to a small break loss of coolant accident (SBLOCA). The consequences of such an event are bounded by the results of existing SBLOCA emergency core cooling system performance analysis.

Therefore, the impact on normal plant operation was insignificant.

This event did not result in a safety system functional failure.

E. Corrective Actions:

The corrective actions include:

• Cut out the failed heater sleeve portion, plug, and seal weld the sleeve to prevent further leakage. This action has been completed.

• Visually inspect all of the pressurizer heater sleeves for signs of boric acid leakage every refueling outage on Unit 1 and Unit 2, beginning with the next Unit 2 refueling outage, as the first inspection has already been completed on Unit 1. Inspections will be performed at every refueling outage, pending more definitive industry guidance. Inspections are to be bare metal inspections conducted by opening the convection shields or by removal of the mirror insulation.

The corrective action to prevent recurrence will be the implementation of long-term recommendations provided by the EGC Asset Management Group to prevent future leakage, and implementation of actions required to comply with industry guidance to accept visual examinations for evidence of leakage.

Previous Occurrences:

There have been no previous similar events at Braidwood Station.

Component Failure Data:

Manufacturer � Nomenclature � Model � Mfg. Part Number Westinghouse � Pressurizer � 1100J48 � N/A NRC FOR 66A (1-2001)