ML20091B970
| ML20091B970 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 03/27/1992 |
| From: | Clement C, Scace S NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| MP-92-328, NUDOCS 9204020201 | |
| Download: ML20091B970 (3) | |
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HARTFORD CONNECTICUT 051 1 4 270 ratnaast New tregy C,,waN (2031 % 5 4 000 Docket No. 50-423 Re: 10CFR50.
itarch 27, 1992 MP-92-328 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555 Gentlemen:
Millstone Nuclear Power Station, Linit No. 3 Re.[upling Water Storage. Tank Level.lwitclLFaihtte This Special Report is being submitted to disseminate information to the industry regarding the presence of microbiological induced corrosion (hilC) in staenant instrument sensing lines.
finnunary During the performance of Operations surveillance procedure SP 3451B03, on March 1,199L at 1000 hours0.0116 days <br />0.278 hours <br />0.00165 weeks <br />3.805e-4 months <br /> while in Mode 0 at OE power, atmospheric pressure and 88 degrees Fahrenheit, RWST Level Switch 30SS*LS54A was found to have failed in the low-low level condition. With this level switch indicatine a low-iow level condition, it ivould block the "'A" Residual Heat Remcval (RHS) Punip from starting automatically upon receipt of_ a Safety injection Signal (SIS).
c The root cause of the ASCO level switch failure is microbiolocical induced corrosion
'(MIC) of the viton diaphragm. Cerrosion products from the zinc-plated carbon steel backing plate and from the breakdown of the viton blocked the instrument sensing port.
- As short term corrective action, the failed level switch was replaced with a spare of the same m_odel and the failed switch was destructively disassembled and examined to determine the failure mechanism. The long term corrective action plan included chemical sampling the instrument sensing lines when plant conditions permitted.
- Investicatinn The failed switch was destructively disassembled to determine the cause of the failure. [O
~ This examination revealed the breakdown of the viton sensine diaphragm and its zine plated carbon steel pressure (backing) plate. The sensing port was completely blocked 40 g6g)$
by corrosion products. The sensing lines and switch heusing are made of stainless steel and showed no evidence of corrosion or degrcdation. The corrosion by-products are from the breakdown of the zinc-plated carbon steel pressure (backing) plate. The viton
- diaphragm was clearly degraded and deformed. The diaphragm had become permeable j
and leaked by, as evidenced by corrosion of the pressure (backing) plate.
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- Chemical sampling of ~the fluid in the sensing lines to all of the level switches was performed on January 28 & 30, 1992 and March 6,1992. The samples obtained on
. January 28 & 30,1992 revealed a depressed level of pH (2.0) in the process fluid of the sensing lines to the level switches This was sienificantly more acidic than the RWST pH of 4.6-4.S. These results indicated the presence of microbiological activity in the sensmg
- lines, RWSTlevel switches 30SS'LS54B and 30SS*LS56B were destructively examined on
- February 17,1992 by Buckman Laboratories. Samples of the bacteria were obtained from the switches and the sensing lines. The chemistry report from Buckman Laboratories indicates the growth of an anaerobic microflora, not typically associated with MlC.
This is involved either as an inducing mechanism or an influencing mechanism for the failure of the RWSTlevel switches. A supplemental _ chemistry report may be provided by Buckman Laboratories if the samples will grow over a longer period of time. This supplemental report will provide additional chemical _information in determining the exact form of bacteria (MIC) present.
The chemical samples obtained on March 6.1992 revealed an increase in the pH measurement (4.6-5,84). This indicates degradation of the viton diaphragms in the level switchesc The samples were rust colored which indicates that the backing plates are corrodinc due to contact with the corrosive borie acid fluid matrix -The resultant iron Hidroxide (3FeOH2); formed as the water -dis'ssociates causes a caustic imbalance, which causes the pH to increase.
The gas created by MlC apparently permeates the viton diaphragm which protects the zine plated carbon steel pressure (backing) plate. This exposes the pressuce plate to corrosive decradation causine failure of the switch due to cloccinc of the orifice from the deformationlof the backing ipressure) plate.
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- EntluMkm The corrosion products created by the MIC clogged the level switch orifices, blocking pressure sigtnis from reaching the diaphragm.- There have been three (3) level switch failures in six (6) years. All failures occurred in 30SS*LS54A, and C which are interlocked to the "A" RMS pump.. These switches (LS54 series) have 1/8" orifices inteeral with the switch and all three failures appear to have been caused by clogging of the orifice. The other level switches,30SS*LS56A, B, C & D do not have an orifice, and are not subject to this failure mechanism.
-Surveillance testing of the RHS and OSS pumps verifies operability of the level switches on a quarterly basisc Two switches on separate trains have not failed within the same quarter. Since it is felt that the life expectancy of the switches would depend upon the amount of contaminants in the switch land since the amount of contaminants could vary sicnificantly, it is unlikely that both trains would be affected at the same time: A review of t.he equipment history indicates that there have been three (3) failures of the level switches interlocked to the RHS pumps, (30SS* LS54A, & C), over a six (6) year period.
All three failures only affected the _"A" RHS pump. These switch failures occurred to
-30SS*LS54A and C, which affect the "A" RHS pump only. A check of all switeces at the time of the third failure found one switch interlocked to the _"B" RHS pump to be sluggish, which indicated partial blockage of the switch orifice. However, the switch was still operable. There have not been any failures of the level switches interlocked to the OSS pumps (30SS*LS56A; B, C & D). The switches interlocked to the RHS pumps
-(30SS*LS54A, B, C & D) will annunciate upon a LO-LO RWST Level (failed position of a switch) when a Safety Injection Signal (SIS) is initiated. The level switches -
ir erlocked to the OSS pumps will annunciare upon a LO-LO RWST Level (failed
- ion of a switch), regardless of a Safety injecticn Signal (SIS) initiation.
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hiilistone 3 Engineering will implement a design chance to allow for the immediate -
detection of a failed level switch interlocked to an RHS pump start.13ased on the recommendations of 13uckman Laboratories, a chemistry sample and fiufh program has i
been established with a monthly frequency. Should these samples indicate accelerated hilC related anaerobic microflora crowth, the sample and flush frequercy will be increased. Also, Millstone Unit 3 Engineerine will consider a preventative maintenance program to replace the switches every two (2) years at that time.
Since the shortest experienced time to faliure of the switches is five (5) years, the two (2) year replacement schedule is considered conservative. MP3 Encineerine and Chemistry are investicatine the possibility of performine a flush of the sen' sing line's with a chemdal cleaning agent'.
Also, MP3 Engineering, Chemistry id 1&C are investigating the possibility of replacing the viton diaphragm with a different material.
The Licensee contact for this Special Report is Alark A. Smith, who may be contac ed at (203) 444-5487.-
- Very truly yours, NORTHEAST NUCLEAR ENERGY COMPANY FOR: Stephen E. Scace Director. Millstone Station
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Carl H. Clement hiillstone Unit 3 Director SES/ MAS:bjo ec-T. J. Martin, Recion I Administrator W. J. Raymond,' Senior Resident inspector, Millstone Units Nos.1, 2, and 3 V. L. Rooney, NRC Project Manager, Millstone Unit Nos.1 and 3.
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