05000271/LER-1988-001, :on 880211,loss of Flow to Svc Water Effluent Radiation Monitor Occurred.Caused by Design Deficiency. Alternate Path Made Available to Monitor & Periodic Surveillance Initiated to Verify Flow
| ML20151A544 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 04/01/1988 |
| From: | Pelletier J VERMONT YANKEE NUCLEAR POWER CORP. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| LER-88-001, LER-88-1, VYV-88-046, VYV-88-46, NUDOCS 8804070068 | |
| Download: ML20151A544 (6) | |
| Event date: | |
|---|---|
| Report date: | |
| 2711988001R00 - NRC Website | |
text
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On 9-3-87, with the nt maintenance outage, plant personr.el discovered that the Service Water (SW)(EIIS=BI)
Effluent Radiation Monitor was not receiving flow.
As required by plant Technical Specifications when this nonitor is not operable, daily M r.o samples of the SW effluent stream were imediately initiated.
Investigaticn of this event has found the root c4use for loss of flow to the monitor to be a procedure deficiency when making system lineup changes that affect I
the monitor sample source.
The intermadiate cause was a malfunctioning flow switch which should have caused loss of flow annunciation.
On 3-2-88, while evaluating the above event, it was recognized that releases in excess of 10 CFR 20 limits are possible in some plant operating modes. The SW Effluent Radiation Monitor alarm setpoint is currently set at three times normal background per the Vermont Yankee Offsite Dose Calculation Manual. This setpoint assumes mixing with other clean plant effluent streams during open cycle plant opera-tions and exceeds 10 CFR 20 limits w'ien this nixing does not occur in certain operating modes.
The root cause of inadequate monitoring during certain nodes is due to radiation monitoring system design and calibration assuming open cycle plant operating conditions.
An evaluation of existing equionent and procedures will be performed to develop a revised nethod of monitoring effluent streams in all plant operating nodes,
'j A Potential Reportable Occurrence was initiated on 9-3-87 to address the loss of j
monitor flow event.
The event was determined to be Not Recortable at that time i
although additional investigation of the problem was initiated. After gaining further information, it i,ts determined on 2-11-88 that this event should be re-evaluated per this LER.
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DESCRIPTION OF EVENT
On 9-3-87, with the plant shutdown for refueling, technicians were performing a routine calibration of the Service Water (SW) (EIIS=BI) Effluent Radiation Monitor flow switch (EIIS=FS)(Worcester Valve Company Model +F16602X). During the calibra-tion, the flow switch was found to be inoperable due to silt buildup in the switch.
In addition, it was found that ti.ere was no flow through the SW Effluent Radiation Monitor corresponding to the plugged flow switch.
Grab samoles of SW i
effluent were then initiated immediately and taken every 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> as required by l
Technical Specifications until flow was restored to the Radiation Monitor on 9-26-87 After reviewing system evolutions that occurred during the plant shutdown, it was determined that the SW Effluent Radiation Monitor was not receiving flow for
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twelve days (8-22-87 to 9-3-87).
This was due to a valve lineup change that occurred on 8-22-87 which greatly reduced SW aystem flow.
On 3-2-88, while evaluating the current alarm setpoint of the Service Water (SW)
Effluent Radiation Monitor, it was discovered that effluent releases in excess of 10 CFR20 limits are possible during certain plant operating modes.
The alarm setpoint is currently set at three times normal background per the Vermont Yankee Offsite Dose
alculation Manual.
This setpoint assums mixing with other clean plant effluent streams during open cycle plant operation and exceeds 10 CFR 20 limits when this mixing does not occur in certain plant operating modes.
CAUSE OF EVENT
On 8-22-87, the Service Water system flow path that supplies all Turbine Building Cooling equipment was isolated to allow for valve maintenance.
Since the Turbine Building is normally a major load on the Service Water system, this isolation greatly reduced total required Service Water flow.
1 As a result, the SW system discharge line was no lorx;cr completely filled.
l Therefore, the Radiation Monitor suction line, which enters from the side of the SW discharge line, became uncovetad. When this occurred, sample suction flow ceased 1
which in turn caused the breaker for the sample pumo supplyino the monitor to trip.
1 Normally, a Radiation Monitor downscale alarm occurs in the Control Room upon failure of its corresponding radiation detector or upon loss of flow throuch the detector. However, the flow switch which initiates the loss of flow signal was found to be clogged with silt and did not function.
This malfunctioning flow switch has been determined to be the intermediate cause of this event.
The evaluation of this event has identified the root cause to be a procedural deficiency in not recognizing the loss of flow to the Radiation Monitor when the i
Service Water system valve lineup was changed.
The "High" alarm setpoint for the SW Effluent Radiation Monitor was set at a level of 3 times normal detector background levels per procedure and the Vermnt Yankee Offsite Oose Calculation Hanual (ODCM) guidelines.
Since current detector background levels are approximately equal to 10 CFR20 release limits, the current i
alarm setooint is at accroximtelv 3 times the release limit.
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CAUSE CF EVENT (Cont. )
i NOTE: The Vermont Yankee Circulating Water (CW) system, which crovides Main Condenser cooling, is considered to be a "clean" stree.m P.nd operater in the follosting three modes:
a) Open Cycle - CW pumps "river to river" b) Closed Cycle - CW is in a clo ed cooling tower loop with SW effluent mixed in the loop. To maintain loop inventory, a volume equal to SW flow is discharged after mixing, c) Hybrid Cycle - CW is a mixture of the above two loops.
The Radiation Monitor alarm setpoint discussed previously is considered to be very conservative and well within 10 CFP. 20 discharge limit? In the "open" CW cycle in that the detector monitors Service Water effluent prior to its mixing with the plant Circulating Water effluent (a "clean" stream).
Typical flow rates for these two syrtems during open cycle operation are as follows:
Service Water = 13,400 gom Circulating water = 360,000 gom This mixing occurs in the plant discharge structure prior to being released off-site. Therefore, during open cycle operation, dilution in the discharge structure reduces effluents to well below 10 CFR 20 release limits.
However, during periods of hybrid or closed cycle operation and during plant shutdown, the above mentioned mixing does not occur.
During these modes, effluents may have radionuclide concentrations higher than 10 CFR 20 limits but lower than the corresponding alarm setpoint.
The root cause of the setpoint event has been determined to be that both the l
radiation monitoring system design and associated detector calibration guidelines assum open cycle plant operating conditions and discharge structure mixing.
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modes and the corresponding lack of mixing were not accounted for.
ANALYSIS OF EVENT
Although the Service Mter effluent stream was not beirg monitored for a twelve day period, there is good assurance that a radioactive '.10.'id effluent release in excess of 10 CFR 20 limits did not occur in tha'.
a) The Service Water System operates at a higher pre!!sure than the various equipment cooling systems it supplies. Thereforei leakage of any radioactive liquid into the Service Water system is highly unlikely, g....
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ANALYSIS OF EVENT (Cont.) l
//i b) During the period that the radiation monitor was made out of service, there were no substantial Aanges in the Service Water system configuration or Plant operating status.
Therefore, it can be conservatively assumed that grab samples taken after the discovery were reprewistive of the effluent stream during the twelve unmonitored days.,In addit'.os, grab samples taken of the effluent stream immediately after th0 discogry were well below release limits.
Uhe highest specific activity recorded during that period was 5.95E-8.
The 10 C9 20 Appendix B release limit is 3.T -6 or approxima-tely fifty timej tFp highest' measured activity during the sampling period.)
f The samples referencud above can be reasonably assumed t represent tyoica j,
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effluent release levels during plant outtges with the CW system secured. Thereforce ' e during the events of this report, %ere no's exceeded and that there were nu adver%go3d assurance that release limits were se safety in,plications to plant equipment, personnel, or to the public.
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CORRECTIVE ACTION
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l When plant valve lineup chano'<s occur as described above that greatly reubce SW system flow, an alternate flow path to the ra6'ation monitor is available to assure constant flow to tht imonitor.
Operating groceddres have been revised to assure ttat this alternate path is used wheh it is necessary to%aintain flow.
- - In addition-, a periodic surveillanc? ca9been initiated to verify flow from the radiation monitor, thereby assuring,de*ector flow regardles; of the SW system con-c figuration.
If loss of flow is observ b6, ihr$1 ate antion will be taken to restore flow.
Since tne olant is curlently operiting in t% open cycle and discharge structure mixing I's maintained, potential radioettive effluent releases are being adequately monitored and no immediate corrective action is necessary for current monitor setpoints.
Te, assure adequate plant effluent monitoring in the future, evalueklon of existing monitoring equipwnt and pracedures will be performed to devc2@ a method to more adequately monitor e# fluent in all plant operating modes.
In the' interim, at all times the plant is not operatir3 in'the opet cycle, grab samtkes of SW effluent will be taken daily or as determined necessary tu essure /0 CFR 2G/11mits are not exceeded.
ADDITIONAL INFORMATION
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No similar occurrence,s have been reported to the Commission in the last five years.
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A Potentia.' Reportable Occurrence (PRO) was initiated on 9-3-87 to address this event. The eve.it was determined to be Not Reportable at that time although addi-tional investigation of the problem was initiated. After gaining further infor-mation, it was determined on 2-11-88 that this event should be re-evaluated per this LER.
On 3-2-88, a second PRO related to the initial event was generated to further eva!uate current radiation monitor design and calibration methods and resulted in this revised LER.
j A Nuclear Plant Reliability Data System (NPRDS) inquiry showed no evidence of similar events at other plants, j
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l VERhiONT YANKEE N UCLEAR POWER CORPORATION P. O. BOX 157 -
GOVERNOR IIUNT ROAD VERNON, VERMONT 05354 r
April 1, 1988 VYV 88-046 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C.
20555
REFERENCE:
Operating License DPR-28 Docket No. 50-271 Reportable Occurrence No. LER 88-01
Dear Sirs:
As defined by 10CFR50.73, we are reporting the attached Reportable Occo:rence as LER 88-01, Rev 1
l Very truly yours, VERMONT YANKEE NUCLEAR POWER CORPORATION
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Samas P. Pelletier i
l Plant Manager cca Regicnal Administrator USNRC Office of Inspection and Enforcement Region I 475 Allendale Road King of Prussia, PA 19406 i
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