ML20211B612
| ML20211B612 | |
| Person / Time | |
|---|---|
| Site: | South Texas  |
| Issue date: | 02/05/1987 |
| From: | David Brown, Mcintyre A HOUSTON LIGHTING & POWER CO. |
| To: | |
| Shared Package | |
| ML20211B603 | List: |
| References | |
| 87-S-0009, 87-S-9, ST-HL-AE-1914, NUDOCS 8702190459 | |
| Download: ML20211B612 (4) | |
Text
ATTACHMENT ST HL AE-191ll PAGE / OF e/
INTERIM FINAL X
PROBLEM INVESTIGATION REPORT Report No.: 87-S-0009
Subject:
Thermowell 0-Ring Leakage Problem Description Injur'y/ Damage Report:
During Hot Functional Testing (HFT), an area survey detected a leak at thermowell TE-461.
The leak was discovered while HFT was in progress with the reactor coolant system (RCS) at 450 and 1800 PSIG. TE-461 was inspected to determine the specific location of the leak. The seal weld joining the thermowell and the pipe boss had a pin hole. Apparently the o-ring forming the primary seal was not properly seated, thus allowing primary coolant to pass by the o-ring, up through the threaded connection and to the atmosphere.
Subsequent to this event the RCS was drained and the thermcwell removed. The o-ring was not properly seated in the o-ring groove.
Root Cause:
Due to the configuration of the thermowell/ boss o-ring seating surfaces it is difficult to assure proper seating.
Investigation during removal of the problem thermowells demonstrated the necessity for more attention to detail during installation to assure proper seating.
Corrective Action Taken:
All vertical thermowell installations were fcund to be acceptable. Thermowell installations at an angle other than vertical (refer to ) were unacceptable. These have been reinstalled in accordance with a detailed procedure requiring use of depth measurements to ensure o-ring seating.
Transferred to:
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Organization DelpartadTit & Title Date OCMd#;v 92.
2-s- n Approved by
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Title Date BEC Field Construction Manager ECI Construction Manager HL&P Deput Project Manager / Designee 02-65' O n_
Kee Date f
Ge=O">1 A9ltG' houkG &
F%e&1 Review (not required Title
/Date for interim reports) 1847A/0046A 8702190459 870205 PDR ADOCK 05000498 S
PDR S3/87-S-is.TXT
1 ATTACHMENT ST4fL AE /Y/
Corrective Action Taken PAGE 4L OF Page 2 of 2 The following paragraphs describe the sequence of events after the discovery of the leaking thermowell to boss weld.
After cooldown and draining of the Reactor Coolant System a detailed inspection of thermowell to boss wolds was conducted. TE474 was also found to be leaking.
Liquid penetrant inspections were made on all twenty thermowell seal welds. No additional leaking conditions were observed. Inspection following removal of TE-461 and TE-474 revealed that both o-rings were not properly seated. During installation the o-ring had been allowed to seat on the upper surface of the ring groove with the result that the thermowell would not seal as designed.
Since TE-461 and TE-474 were both installed about 120 from the vertical (refer to Attachment 2), the balance of the installations at the 120 locations were removed and inspected.
This included removal of TE-471, 462, 472, 464, 473, and 463. All o-rings were found to have been improperly seated. To establish a degree of confidence on the vertical o-ring installations TE-452 was removed. This o-ring was seated properly.
To determine acceptability of those o-rings installed at 45 from the vertical, TE-414 and TE-424 were removed. Both o-rings were found to have been improperly seated. The remaining thermowells at the 45 orientation, TE-444 and 434, were then removed with similar results.
Since only one vertical thermowell had been removed up to this point (TE-452), the decision was made to remove TE-451 to provide increased confidence that vertical o-rings were properly seated. TE-451 was removed and its installation was found to be acceptable.
During removal of the thermowells, breakaway torque values were recorded.
Several of the assemblies with improperly seated o-rings were found to have less than acceptable breakaway torque values. Properly seated o-ring assemblies were found to have acceptable breakaway values.
The seal weld used between the thermowell and the pipe boss serves as a secondary boundary. The o-ring is the primary boundary for primary coolant seal and the thermowell threads provide the strength. The seal weld requirements at time of installation required use of a single pass process. A review by the supplier (Westinghouse) recommended that a two pass procedure be used. The decision was made j
to apply the two pass procedure to all twenty thermowells of this configuration.
l When applying the second pass on TE-443 the welder reported experiencing a weld l
" bubble." This indicated a possibility of a leaking o-ring in a vertical assembly.
l This unit was disassembled. The cavity was dry and the o-ring had been properly seated.
NCR's SN-03258, SN-03259, and SN-03262 were written to document nonconforming conditions and corrective actions required.
All corrective actions have been completed and NCR's closed.
Plant thermowells, other than those shown on attachment 2, are of an all weld design not susceptible to this problem.
Work on Unit 2 thermowell installation has been placed on hold.
The rmowells
{
will not be installed until after the system has been turned over to Startup for final cleaning.
Specific installation instructions will be included in the work i
package to preclude reoccurrence of this problem on Unit 2 and future replacements by NPOD.
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l S3/87-S-A.TXT
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