05000275/LER-2002-002
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| 2752002002R00 - NRC Website | |
A. Background
Technical Specification (TS) 5.5.9, "Steam Generator (SG) Tube Surveillance Program," requires that the results of each SG tube inspection be classified as Category C-3 if more than one percent of the total tubes inspected are defective. Defective tubes must be repaired or removed from service by plugging.
TS 5.6.10, "Steam Generator (SG) Tube Inspection Report," paragraph a, requires the number of tubes plugged in each SG [AB][TBG] to be reported within 15 days following the completion of each inservice inspection. TS 5.6.10, paragraph c, requires the results of SG tube inspections, which fall into Category C-3, to be reported in a special report to the Commission within 30 days and prior to resumption of plant operation. TS 5.6.10, paragraph d, requires NRC notification prior to returning the SG to service if circumferential crack-like indications are detected at tube support plate (TSP) intersections, or if indications are identified at TSP intersections that are attributable to primary water stress corrosion cracking (PWSCC).
If the results of the SG tube inspections are classified as Category C-3, then NRC notification is required in accordance with 10 CFR 50.72(b)(3)(ii), and submittal of a special report is required in accordance with TS 5.6.10.c.
B. Event Description
On May 19, 2002, final analysis of eddy current testing on SG 1-2 indicated that greater than one percent of the active tubes inspected were defective, therefore classifying SG 1-2 as Category C-3, per TS 5.5.9.
Defects in 36 active tubes in SG 1-2 were removed from service by tube plugging in 1R11. Two additional non-defective tubes were plugged as a preventive measure.
On May 19, 2002, at 1759 PDT, a nonemergency report was made in accordance with TS Table 5.5.9-2 and 10 CFR 50.72(b)(3)(ii)(B).
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION DOCKET NUMBER (2) � LER NUMBER (6) 51❑ 0 0 2 FACILITY NAME (1) Diablo Canyon Unit 1 0 7 15 r2002 2
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On May 20, 2002, PG&E discussed the preliminary results of the inspection with the NRC during a phone conference. During this phone conference, PG&E also notified the NRC that PWSCC and circumferential indications were detected at TSP intersections, satisfying TS 5.6.10 paragraph d.
In SGs 1-1, 1-3, and 1-4, the number of defective tubes plugged in 1R11 was 22, 9, and 10, respectively, thus classifying these SG inspection results as Category C-2, per TS 5.5.9.
C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event None.
D. Other Systems or Secondary Functions Affected
None.
E. Method of Discovery
The defective tubes were found during routine scheduled eddy current testing of Unit 1 SG tubing performed during 1 R11.
F. Operator Actions
None.
G. Safety System Responses
None.
Cause of the Problem
A. The number of defective tubes in SG 1-2 identified during 1R1 exceeded one percent of the total tubes inspected, placing the SG in Category C-3, per TS 5.5.9.
B. Root Cause
The majority of the tube defects are attributed to outside diameter stress corrosion cracking (ODSCC) and PWSCC at hot leg TSP intersections.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION DOCKET NUMBER (2) PAGE (3) FACILITY NAME (1) Diablo Canyon Unit 1 0 5 { 0 0 1 0 2 7 5 2002 LER NUMBER (6) - 1 0 0 2 0 1 0 C. � Contributory Cause None.
IV. Assessment of Safety Consequences
The licensing-basis, large-break, loss-of-coolant accident analysis assumes a tube-plugging limit of 15 percent per SG. Including the tubes plugged during 1R11, the following table presents the number of tubes (out of a total of 3,388 tubes for each SG) that are currently plugged in each of the four Unit 1 SGs.
SG
Active Defective Tubes Plugged in 1R11 Total Tubes Plugged to Date Total Percentage of Tubes Plugged 1-1 22 154 4.5 1-2 36 230 7.0 1-3 9 49 1.5 1-4 10 75 2.2 The plugging percentage for each Unit 1 SG remains within the current allowable limit of 15 percent.
There were no actual safety consequences involved in this event since all defective tubes met the criteria of Regulatory Guide 1.121, "Bases for Plugging Degraded PWR Steam Generator Tubes," for structural integrity at the end of Cycle 11.
Also, the condition is not considered a Safety System Functional Failure.
Therefore, the event is not considered risk significant and it did not adversely affect the health and safety of the public.
V. � Corrective Actions
A. Immediate Corrective Actions
All Unit 1 SG tubes classified as defective during 1R11 have been plugged in accordance with Diablo Canyon Power Plant (DCPP) TS 5.5.9.
B. Corrective Actions to Prevent Recurrence
PG&E has initiated several programs to minimize SG tube degradation, license alternate repair criteria, and license tube repair techniques.
) FACILITY NAME ( LICENSEE EVENT REPORT (LER) TEXT CONTINUATION DOCKET NUMBER (2) LER NUMBER (6) Secondary side initiatives to minimize tube degradation:
1. Electric Power Research Institute secondary chemistry recommendations were implemented to minimize ODSCC at TSPs (e.g., hydrazine levels were increased in 1992, the secondary side pH treatment was converted from ammonia to ethanol amine in 1993/1994, and a molar ratio control program was implemented in 1993).
2. Tube sheets are sludge lanced during each refueling outage to minimize ODSCC at the tube sheet.
3. DCPP has an upgraded plant makeup water system to minimize SG contaminate levels.
4. SG blowdown is maintained at approximately one percent of the main steaming rate to minimize SG contaminate levels.
5. A boric acid addition program is in effect, including boric acid soaks at startup to mitigate denting and ODSCC at TSPs.
6. DCPP has condensate polishers and emergency (plant curtailment) procedures to protect against seawater condenser tube leaks.
Primary side initiatives to minimize tube degradation:
1. Rows 1 and 2 U-bends have been heat treated in 1987/1988 to prevent PWSCC.
2. The tubes in the hot leg tube sheet region were shot peened in 1992/1993 to minimize PWSCC.
3. Reactor coolant system (RCS) contaminants are maintained at low levels in accordance with EPRI guidelines.
4. Lithium and boron concentrations are coordinated to minimize pH swings in the RCS.
5. Zinc addition to the RCS was implemented in Units 1 and 2 starting in Cycle 9 to inhibit PWSCC in SG tubes.
LICENSEE EVENT REPORT (LER) TEXT CONTINUATION 5 7 i 5 LER NUMBER M) FACILITY NAME t1) Diablo Canyon Unit 1 0 DOCKET NUMBER (2) 0 1 0 0 2002 PAGE 13)
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0 0 2 Alternate repair criteria (ARC):
1. Voltage-based ARC for axial ODSCC at TSPs was implemented starting in Unit 2 eighth refueling outage and Unit 1 ninth refueling outage.
2. W* ARC for axial PWSCC contained in the WEXTEX tube sheet was implemented starting in Unit 1 ninth refueling outage and Unit 2 ninth refueling outage.
3. PWSCC ARC for axial PWSCC at dented TSPs was implemented starting in 1R11.
Tube repair techniques:
PG&E is reviewing methods of tube repair and sleeving techniques for application at DCPP.
VI. � Additional Information
A. Failed Components
Component: � SG tubes (Series 51 SG) Manufacturer: � Westinghouse
B. Previous Similar Events
inspected in SG 1-1 and 1-2, during the Unit 1 eighth refueling outage, were defective.
inspected in SG 1-2, during the Unit 1 tenth refueling outage, were defective.