05000289/LER-2012-003
| Three Mile Island, Unit 1 | |
| Event date: | 08-22-2012 |
|---|---|
| Report date: | 10-22-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown |
| Initial Reporting | |
| ENS 48220 | 10 CFR 50.72(b)(2)(i), Tech Spec Required Shutdown |
| 2892012003R00 - NRC Website | |
There were no structures, systems, or components out of service that contributed to this event.
Background
TMI-1 was designed by Babcock and Wilcox (B&W), currently Areva. The pressurizer (PZR) was built with heater bundles that could be removed and replaced in the event that a pressurizer heater element would need to be replaced. Pressurizer heaters provide the function of maintaining RCS pressure. This function is accomplished during steady state conditions as well as transient or post trip recovery.
A diaphragm sits in a recessed channel in the stainless steel cladding on the pressurizer shell. The diaphragm is seal welded to the shell and is retained by a large bolted cover. The cover and associated bolting retain the majority of forces developed by the RCS.
On July 6, 2012 the Reactor Coolant System (RCS) leak rate step changed by an approximate factor of four. On August 12, 2012 the RCS leak rate changed again, this time by a factor of ten, placing the leak rate at approximately 0.25 gallons per minute (gpm). Significant efforts were undertaken to determine the source of the change in the leak rate. TMI technical specifications allow an unidentified leak rate of 1.0 gpm; however, no leakage is permitted through a primary pressure (strength) boundary.
Numerous walk down inspections were performed in the reactor building outside the D-rings but no source of this size leak was located. Camera and robotic inspections were performed inside the D-Rings and during the night shift on August 21/22, 2012 the leak was determined to be coming from the upper pressurizer heater bundle.
TMI-1 reported the event via EN# 48220 on August 22, 2012 at 03:53 under 10 CFR 50.72(b)(2)(i) "the initiation of any nuclear plant shutdown required by the plant's Technical Specifications." The unit commenced a shut down in the early morning hours of August 22, 2012 and was down for approximately two weeks. During the shutdown the upper heater bundle was replaced with the stainless steel 12- element high watt density Watlow bundle which the station had purchased in 2004. This bundle is stainless steel and is not susceptible to PWSCC.
The source of the leak identified on August 22, 2012 was a crack in the diaphragm on the heater bundle.
The leak location was identified prior to the removal of the heater bundle. The relaxation of the stresses on the diaphragm allowed the crack to tighten sufficiently to prevent detection after shutdown. Very tight cracks can typically only be observed during destructive examination. Non destructive examination (NDE) such as the use of ultrasonic transducers (UT) and Dye Penetrant Testing (PT) have been shown to be unreliable in the detection of PWSCC through experience at TMI-1.
The diaphragm was not destructively examined to positively confirm PWSCC as the cause. However, the high level of susceptibility of Alloy 600 at the elevated pressurizer temperature coupled with the similar failure in the lower bundle in 2004 clearly indicate this defect was caused by PWSCC. The behavior of the defect was also noted to be similar to other verified PWSCC cracks. The amount of uncertainty is small and is considered acceptable by plant management to eliminate the added radiation dose destructive examination would have entailed. The diaphragm and its seal weld are primarily there to prevent water leakage past the pressure retaining structure.
TMI station replaced the upper bundle with the 12-element high watt density Watlow bundle that was purchased in 2004. The stainless steel diaphragm is not susceptible to PWSCC. Once the upper pressurizer bundle was replaced the station returned to power and RCS leak rate calculations were lower than prior to the event.
B. CAUSE OF EVENT
Problem Statement (Root Cause):
The pressurizer heater bundle diaphragm was constructed of Alloy 600. This material is susceptible to PWSCC.
Statement of Cause (Root Cause):
The use of Alloy 600 materials in high temperature locations was a design weakness in the construction of the TMI station and the industry.
Basis for Cause Statement (Root Cause):
Alloy 600 is susceptible to cracking at the pressures and temperatures experienced at the pressurizer heater bundle location. A similar leak in 2003 resulted in a similar root cause performed in 2004. The action to resolve the 2004 root cause was to mitigate the Alloy 600, i.e., replace the heater bundles. The root cause in 2004 took appropriate actions. Efforts to obtain a heater bundle that was reliable was a continuing effort. As a result, the bundle was scheduled for replacement but had not been replaced at the time of the event.
Extent of Condition (Root Cause):
The extent of condition is limited to Alloy 600 susceptible materials. The pressurizer is considered the highest risk because the temperature is the highest. Corrective Actions to Prevent Recurrence (CAPRs) 1403278-15 and 1403278-16 will resolve the material issue for the pressurizer by replacing the middle bundle, which is the last external high temperature Alloy 600 location. Other alloy 600 locations within the TMI plant are being addressed under the Alloy 600 program.
Extent of Cause Cause Being Addressed Extent of Cause Root Cause: New materials and designs that have not been The use of Alloy 600 materials in high proven through years of service are subject to temperature locations was a design failures that may not have been predicted nor weakness in the construction of the TMI expected. Appropriate NDE and monitoring station. techniques in accordance with the codes and standards of construction are barriers to Cause: degrading conditions causing material failures.
The susceptibility to cracking due to This event resulted in a leak in the diaphragm plate of a pressurizer heater bundle. The leak was not a threat to the safety of the reactor; however, the leak caused a mid-cycle shutdown that had radiological dose implications.
Actual Safety Consequences:
The actual safety consequence was minimal. The leak was approximately 0.25 gpm and the unit was shutdown in advance of significant material wastage or long term boric acid leakage. The pressurizer heater bundle diaphragm cover bolts were inspected when the heater bundle was replaced and found to have exhibited no loss of material. The diaphragm bolts were potentially exposed to boric acid from July 6, 2012 to August 23, 2012. Over this 48 day period the leak rate weighted average was 0.175 gpm.
This average leak rate had no effect on the bolts, and very little boric acid buildup collected around the bolts.
Safety Consequences if Occurred with a Design Basis Accident:
A design basis accident requires maximum injection flow from High Pressure Injection (HPI), Low Pressure Injection (LPI), and Core Flood. This large volume of makeup flow will pass through the core and exit the RCS via the break location (cold leg double ended guillotine shear). A small leak in the pressurizer diaphragm would not contribute to this event. The pressurizer would be empty in this event resulting in no contribution to the total accident response. Also, the RCS depressurizes during a design basis accident which would eliminate the diaphragm leak path as well.
System Functional Failure:
This event affected the pressurizer system function to maintain the integrity of the reactor coolant system pressure boundary. The small amount of leakage from the pressurizer heater bundle diaphragm was classified as a pressurizer system functional failure. The reactor coolant system leakage made a step change on July 6, 2012, and remained elevated until the plant was shutdown and depressurized (August 23, 2012). The system was fully capable of maintaining RCS pressure during the 48 day period of leakage. The risk of a leak that could affect reactor safety was small because the leakage potential for a cracked diaphragm is controlled by the strength of the cover which contains the majority of the forces developed by RCS pressure, not the diaphragm. The diaphragm and its seal weld are there to prevent water leakage past the pressure retaining structure, the cover, although the interior part of the diaphragm As part of the inspection during the maintenance outage the remaining alloy 600 middle bundle was inspected. There was no evidence of leakage or observable degradation.
D. CORRECTIVE ACTIONS
- Perform replacement of the middle pressurizer heater bundle.
E. PREVIOUS OCCURRENCES
Similar industry occurrences were reported over the last 30 years for PWSCC. Pressurizer heaters and nozzles were reported leaking after boric acid residues were found. The table below identifies TMI occurrences. The base materials were Alloy 600 and the failure mechanism was PWSCC. The TMI pressurizer design conditions (2500 psi and 670 degrees F) and normal operating conditions (2155 psig and 648 degrees F) meet the thresholds at which PWSCC has been shown to occur.
Previous Events Previous Event Review TMI IR 184753, The pressurizer lower heater bundle was identified as leaking during Outage Pressurizer Heater T1 R15.. Weld repairs were made to the diaphragm to pressurizer seal weld and Bundle Diaphragm to the diaphragm plate itself. The root cause was determined to be inadequate Plate Leakage, design of the diaphragm plate (material selection) and weld materials. The November 4, 2003 materials were Alloy 600 and 82/182 which is known to be susceptible to PWSCC. The other two bundles were determined to be susceptible to PWSCC.
The lower bundle was repaired during one outage prior to the scheduled replacement (2005 refueling outage). The OE identified numerous instances of PWSCC cracking specific to pressurizer heater locations. CAPR 184753-25 was issued to revise engineering design standards addressing Alloy 600 and 82/182 material and their susceptibility to PWSCC. The lower bundle was subsequently replaced during T1 R15 when the bundle was identified leaking during start-up.
Corrective actions were issued to inspect the bundles during T1 R16 with insulation removed. The risk to the plant from this failure was considered low because the damage mechanism has a slow growth rate and RCS leakage is closely monitored. This IR has direct applicability to TMI and the decision to replace rather than repair the bundle during the 2012 outage was a direct lesson learned from the 2003 leak.
TMI IR 187903, Leak on the Lower Heater This IR documents that the seal weld, repaired as described under IR 184753, was the most probable root cause for this leak that was identified during start-up Bundle of Pressurizer, following the 2005 refueling outage. This IR was used in proceeding directly to November 23, 2003 replacement of the bundle in 2012.
INPO Level 3 Event This OE discusses significant damage and leakage to stainless steel heater Report 12-10, "Primary elements at a foreign plant. The initial leakage was due to stress corrosion Coolant Leak Caused cracking in a pressurizer heater sheath that allowed water ingress into the heater by Swelling and element. This water ingress resulted in swelling of the magnesium oxide Mechanical Failure of insulation used for the heaters which then swelled the heater element sheath Pressurizer Heaters" outside the pressurizer and resulted in a significant leak. This OE supports concerns over failed heater elements that goes beyond heater capacity alone and emphasizes the PWR fleet vulnerability to pressurizer heater performance.
Reference IR 1321322 for Exelon review of this OE. This event is not applicable to this root cause due to the mechanism and the material, although it is applicable to TMI.
- Energy Industry Identification System (EMS), System Identification (SI) and Component Function Identification (CFI) Codes are included in brackets, [SI/CFI] where applicable, as required by 10 CFR 50.73 (b)(2)(ii)(F).