05000250/LER-2007-004, Regarding Reactor Coolant System Boundary Leakage
| ML073180498 | |
| Person / Time | |
|---|---|
| Site: | Turkey Point  |
| Issue date: | 11/07/2007 |
| From: | Jefferson W Florida Power & Light Co |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| L-2007-173 LER 07-004-00 | |
| Download: ML073180498 (8) | |
| Event date: | |
|---|---|
| Report date: | |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications 10 CFR 50.73(a)(2)(i) 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability 10 CFR 50.73(a)(2)(ii)(A), Seriously Degraded 10 CFR 50.73(a)(2)(viii)(A) 10 CFR 50.73(a)(2)(ii)(B), Unanalyzed Condition 10 CFR 50.73(a)(2)(viii)(B) 10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(ix)(A) 10 CFR 50.73(a)(2)(iv)(A), System Actuation 10 CFR 50.73(a)(2)(x) 10 CFR 50.73(a)(2)(v)(A), Loss of Safety Function - Shutdown the Reactor 10 CFR 50.73(a)(2)(v)(B), Loss of Safety Function - Remove Residual Heat 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(v), Loss of Safety Function |
| 2502007004R00 - NRC Website | |
text
FPL November 7, 2007 10 CFR § 50.73 L-2007-173 U. S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, D. C. 20555-0001 Re:
Turkey Point Unit 3 Docket No. 50-250 Reportable Event: 2007-004-00 Date of Event: September 8, 2007 Reactor Coolant System Boundary Leakage The attached Licensee Event Report 05000250/2007-004-00 is being submitted pursuant to the requirements of 10 CFR 50.73(a)(2)(i)(B) to provide notification of the subject event.
If there are any questions, please call Mr. Paul Infanger at (305) 246-6632.
William Jefferson, Jr.
Vice President Turkey Point Nuclear Plant Attachment cc:
Regional Administrator, USNRC, Region II Senior Resident Inspector, USNRC, Turkey Point Nuclear Plant an FPL Group company
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB:. NO. 3150-0104 EXPIRES: 08/31/2010 (9-2007)
Estimated burden per response to comply with this mandatory collection request:
80 hours9.259259e-4 days <br />0.0222 hours <br />1.322751e-4 weeks <br />3.044e-5 months <br />. Reported lessons learned are incorporated into the licensing process and fed back to industry. Send comments regarding burden estimate to the Records and FOIA/Privacy Service Branch (T-5 F52), U.S. Nuclear Regulatory Commission, LICENSEE EVENT REPORT (LER)
Washington, DC 20555-0001, or by internet e-mail to infocollects@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used efor required number of to impose an information collection does not display.a currently valid OMB control (See reverse fnumber, the NRC may not conduct or sponsor, and a person is not required to digits/characters for each block) respond to, the information collection.
- 3. PAGE Turkey Point Unit 3 05000250 1 OF 7
- 4. TITLE Reactor Coolant System Boundary Leakage
- 5. EVENT DATE
- 6. LER NUMBER
- 7. REPORT DATE
- 8. OTHER FACILITIES INVOLVED MONTH DAY YEAR YEAR SEQUENTIAL REV MONTH DAY YEAR 05000 NUMBER NO.
05000 FACILITY NAME DOCKET NUMBER 9
08 2007 2007 - 004 - 00 11 8
2007 05000
- 9. OPERATING MODE
- 11. THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF 10 CFR§: (Check all that apply)
El 20.2201(b)
El 20.2203(a)(3)(i)
El 50.73(a)(2)(i)(C)
El 50.73(a)(2)(vii) 6 El 20.2201(d)
El 20.2203(a)(3)(ii)
El 50.73(a)(2)(ii)(A)
El 50.73(a)(2)(viii)(A)
El 20.2203(a)(1)
El 20.2203(a)(4)
[1 50.73(a)(2)(ii)(B)
El 50.73(a)(2)(viii)(B)
El 20.2203(a)(2)(i)
E3 50.36(c)(1)(i)(A)
El 50.73(a)(2)(iii)
El 50.73(a)(2)(ix)(A)
- 10. POWER LEVEL El 20.2203(a)(2)(ii)
El 50.36(c)(1)(ii)(A)
El 50.73(a)(2)(iv)(A)
El 50.73(a)(2)(x)
[E 20.2203(a)(2)(iii)
El 50.36(c)(2)
El 50.73(a)(2)(v)(A)
El 73.71(a)(4)
El 20.2203(a)(2)(iv)
El 50.46(a)(3)(ii)
El 50.73(a)(2)(v)(B)
El 73.71(a)(5) 0 El 20.2203(a)(2)(v)
El 50.73(a)(2)(i)(A)
El 50.73(a)(2)(v)(C)
El OTHER El 20.2203(a)(2)(vi) 0 50.73(a)(2)(i)(B)
El 50.73(a)(2)(v)(D)
Specify in Abstract below or in RTD history megger readings were reviewed for adverse trends for the related population on Unit 4. No adverse trends were found indicating that no moisture was likely present in the RTD enclosures. As such, no inspections were required for the Unit 4 RTDs.
CORRECTIVE ACTIONS
The Thermowell Project Team has completed the following Actions to Date to investigate TE-3-420A/B:
" Pressurized the RTD enclosure to. 100 psig and inspected for leaks. The enclosure was also vacuum tested. No leaks were identified.
- Performed visual inspection (VT) of thermowell boss and surrounding area, and no indications or through-wall cracks were observed.
" Performed dye penetrant tests (PTs) of the following welds: (a) pipe boss-to-RCS full penetration weld, (b) thermowell-to-pipe boss seal weld, and.(c) thermowell-to-adapter weld. No indications or cracks were found.
- Removed thermowell and shipped to independent laboratory for forensic analysis and testing.
- Thermowell was pressure tested with helium at 2400 psig for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br />, and no leaks were found.
- Performed metallurgical analysis on thermowell, and no flaws or defects were identified in the microstructure that would have caused a leak.
Reviewed historical megger test data for the same type RTD enclosures on Unit 3, and the megger checks were satisfactory (with no decreasing trends) for the other RCS Cold Leg RTDs (TE-3-41OA/B and TE-3-430A/B) and RCS Hot Leg RTDs (TE-3-413A/B, TE-3-423A/B and TE-3-433A/B). Good megger readings indicate that moisture is not likely present in any of the remaining Unit 3 RTDs of this type.
" Reviewed past megger test results for the same type RTDs on Unit 4, and all readings were in the megohm range with no adverse trends identified on any of the RCS Cold or Hot Leg RTDs (TE 410A/B, TE-4-420A/B, TE-4-430A/B, TE-4-413A/B, TE-4-423A/B and TE-4-433A/B). Good megger readings indicated that moisture is not likely present in any of the Unit 4 RTDs of this type.
" Performed history search using Operating Experience (OE), Condition Report, and work order databases, and no failures of this type of thermowell were found.
" Replaced the thermowell and temperature detector (including enclosure) with new components. Leakage inspections performed during overpressure tests subsequent to the Unit 3's return to service identified no leaks from new thermowells/detectors or in the surrounding area.
Second Event: TE-3-422B2 On 8/28/07, just days prior to the recent Unit 3 PT3-23 outage, a scheduled Preventive Maintenance for time response testing of the protection channel RTDs was performed. During the PM, a low (but still within acceptance criteria) megger reading was identified on RTD element TE-3-422B2. The thermowell/RTD assembly for TE-3-422B2 is a different type and different design than the one used for TE-3-420A/B. The assembly uses a fast-response dual-element RTD in conjunction with a tapered-wall thermowell design.
There are twelve (12) of these dual-element RTDs in the RCS loops that provide temperature inputs to the Reactor Protection System Tavg and Delta T channels. There are three (3) Hot Leg RTDs and one (1) Cold Leg RTD per loop for each of the A, B and C Loops. On 9/13/07, the Hot Leg "B" RTD housing, TE 422B2, was inspected per the troubleshooting work order and a small amount of water (-5 to 10 ml) was found in the head assembly. A small sample was collected and taken to Chemistry for analysis. Results of the chemical analysis concluded that the water sample contained constituents of RCS fluid. The action plan to address leakage at the second location on thermowell TE-3-422B2 was very similar to the previously established plan for TE-3-420. Several additional inspections were recommended by the team, and those additional actions taken to this point are summarized below.
TE-3-422132 Actions to Date:
- Pressurized the RTD enclosure to 100 psig and inspected for leaks. The enclosure was also vacuum tested. No leaks were identified.
- Performed visual inspection (VT) of thermowell boss and surrounding area, and no indications or through-wall cracks were observed.
" Performed PT of thermowell-to-pipe boss weld area. There were no indications and no evidence of degradation or past leakage around the pipe boss location.
- Removed thermowell and shipped to an independent laboratory for forensic analysis and testing.
- Pressure tested the thermowell with helium to 3200 psig for 2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> and no leaks were found.
- PT inspected thermowell using fluorescent penetrant with an extended hold time of 1 hour1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br />, and no cracks or other indications were found.
- Performed a review of historical megger data for twelve (12) RCS temperature detectors having similar thermowell designs on Unit 3. Those temperature detectors with negative data trends were opened and internally inspected for moisture or evidence of past moisture. Those Temperature detectors lacking recent (i.e., 2007) hot megger readings were also inspected. As such, the eight (8) "A" and "C" Loop RTD housings were opened and inspected, and no moisture was found in any of these. For the "B" Loop, two (2) RTDs had satisfactory historical megger readings and two (2) RTDs had unsatisfactory historical readings, one being the subject RTD (TE-3-422B2 that has been replaced. The other.
remaining "B" Loop RTD with unsatisfactory readings (TE-3-422C/D) was opened and inspected, and no moisture (or sign of past moisture) was found.
- Reviewed past history of megger test results for all twelve (12) of the same type RTDs on Unit 4, and no adverse trends were identified on any of the three (3) RCS Cold Leg or nine (9) Hot Leg RTDs for the reactor protection system. Good megger readings(and no adverse trends) indicated that moisture is not likely present in any of the Unit 4 RTDs of this type. No further action is required for this population on Unit 4
- Replaced the thermowell and temperature detector (including enclosure) with new components. Leakage inspections performed during overpressure tests subsequent to the Unit 3's return to service identified no leaks from new thermowells/detectors or in the surrounding area.
CAUSE OF THE EVENT
The root cause analysis is still in progress.
Based on the results of the extensive NDE and destructive examinations performed and the robust design, it can be concluded that it is unlikely that a failure of the thermowell pressure boundary was the source of the water in the RTD housing. Nonetheless, the thermowells were replaced and the RCS system pressure boundary restored per the original design. As such, the focus of the continuing investigation is associated with the breach of the environmental qualification boundary of the RTD enclosures and potential sources of water intrusion. Although the field performed leak tests of the enclosures did not identify any leak path, further inspections and testing are planned on the RTDs which were replaced.
ANALYSIS OF THE EVENT
The thermowells and associated RTDs are safety related. The RCS TE-422B2 RTD provides input to the Reactor Protection System for RCS temperature monitoring. The RCS TE-420 A/B RTDs provide input to the Subcooled Margin Monitoring System and provide input to the Qualified Safety Parameter Display System. The thermowells are considered part of the Reactor Coolant System Pressure Boundary. In both cases, the instrumentation design provides redundancy so that failure of one Temperature Element does not result in loss of the required safety function. As a result of the extensive tests performed-on the thermowells, there is no reason to suspect RCS pressure boundary leakage from the thermowells/RTDs.
However, the root cause of this event is ongoing and any resulting additional measures determined to be required will be identified and implemented.
REPORTABILITY
Turkey Point Unit 3 Technical Specifications (TS) Limiting Condition for Operation (LCO) 3.4.6.2.a permits no RCS pressure boundary leakage in modes 1 thru 4. As a result of the chemical analysis indicating the water found had RCS constituents and indication of fluid in the thermowell, this condition was reported to the NRC pursuant to the requirements of 10 CFR 50.72(b)(3)(ii)(A) as a degraded condition.
As a result of the on-going root cause analysis, it is conservatively being considered reportable under 50.73(a)(2)(i)(B) for operation in a condition prohibited by technical specifications.
APPLICABILITY: MODES 1-4 ANALYSIS OF SAFETY SIGNIFICANCE TE-3-420A/B is a dual element RTD installed in Loop B Cold Leg. Different channels of QSPDS (Channel A and Channel B) utilize each element. Identical detectors are installed in Loops A and C. For a channel of Qualified Safety Parameter Display System (QSPDS) [IP] cold leg temperature to be considered OPERABLE, it must have at least two of its three detectors operable. This removed operating margin which would not allow forfailure of any additional RTDs. The MINIMUM CHANNELS OPERABLE is one (with two of the three detectors.) To call a channel operable, it must have at least two of its three
YERISEQUENTIALINME REVo NUMBERPage 6 of 7 2007 004 00 detectors operable. Therefore, even with TE-3-420A/B inoperable, the technical specifications would be met and there would be no safety concern.
TE-3-422B2 is one of three Hot Leg RTDs located in reactor vessel output loop "B". The three RTD outputs are averaged and provide inputs to control circuits, reactor delta T, and reactor Tav. Tave is utilized in the overpower set point calculations and over-temperature set point calculations of Eagle 21 (Reactor Protection System) [JC]. Reactor delta T is compared against these two calculated set points and provides output trips when the set points are exceeded.
Three hot leg RTDs are not required for channel operation. If an RTD does not meet tested acceptance criteria, it is removed from scan. The channel is still operable but a trouble alarm is generated to alert the operators of the RTD failure.
TE-3-422B2 remained operable. Insulation resistance of this RTD had decreased from 80 MQ to 10 MO.
The 10 MQ reading met procedural acceptance criteria. Continued operation with this degrading trend would not have affected the Eagle 21 safety margin. An RTD that for some reason drifts out of the required accuracy limits set into Eagle 21 algorithms is automatically removed from scan and is no longer included in the RTD average. For this reason the difference in Thot average would be minimal.
No adverse safety issues were associated with the 10 MQ readings of TE-3-422B2.
ADDITIONAL INFORMATION / SIMILAR EVENTS Engineering reviewed the event history, OE, CR and work history databases for failures of fast-response RTD thermowells. Two (2) failures of the fast-response thermowells were identified in the nuclear industry, one at Palo Verde and one at the Robinson Plant.
The Palo Verde failure occurred in the late 1980's when a thermowell broke off in the flow stream during startup testing due to excess flow vibration. The thermowell was a very different design and much less robust than the PTN design. They replaced the thermowells with a more robust design and have not indicated any problems since.
The Robinson failure was caused by "field machining" performed on the thermowell at the site to accommodate fit-up issues into RCS piping. Their thermowell design was later updated to a more robust design that is "slightly different" (except at the tip) than the PTN design. Again, Turkey Point's design is more robust than the Robinson design.
A similar history/Operating Experience search for the normal-response RTD thermowell revealed no failures in the industry.
EIIS Codes are shown in the format [EIIS SYSTEM: IEEE system identifier, component function identifier, second component function identifier (if appropriate)].
FAILED COMPONENTS IDENTIFIED:
The fluid found inside the EQ enclosure for TE-3-422B2 and TE-3-420A/B did not cause a failure or loss of function of the associated channels of QSPDS or RPS. Although the fluid had some constituents of RCS, no cracks or flaws were found in either of the thermowells. No further tests will be performed on the thermowells for RCS pressure boundary leakage paths since no defects were discovered in the tests performed to date. However, based on the RTD TE-3-420A/B being declared inoperable, this would be considered a failure of the RTD. The operability of RTD TE-3-422B2 was never in question, so it is not considered a failure of the RTD. Based on the current information from the ongoing investigation, the source of leakage was not RCS pressure boundary leakage, but the most probable leakage path was through threads, seals or mechanical joints in the RTD enclosures.