ML20077F095
| ML20077F095 | |
| Person / Time | |
|---|---|
| Site: | McGuire, Mcguire  |
| Issue date: | 06/03/1991 |
| From: | Tucker H DUKE POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9106110413 | |
| Download: ML20077F095 (12) | |
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%.r DUh2 POWER June 3, 1991 U.S. Nuclear Regulatory Commission ATTN:
Document Control Desk Washington, D.C. 20555
Subject:
McGuite Nuclear Station Docket Nos. 50-369, -370 Operational Problems Involving the Moveablo Incore Detector System (MIDS) i Gentlemen:
The purpose of this letter is to familiarize the NRC with details regarding recent problems that Duke Power Company has been experiencing with the Moveable Incore Detector System (MIDS) for McGuire Unit.1, as well as our plans for resolving these problems both in the short and long-term.
Over the last two operating cycles, we have experienced difficult.y accessing the minimum number of thimbles required for system operability, and t.his has led to our requesting and the NRC granting relief from the existing technical specification operability requirements for the system.
This letter presents a summary of the MIDS design, discusses our experience and history of problems with t.he system, compares our l
experience with that of t.he industry, and establishes our action plan l
for the next refueling outage for improving MIDS reliability. This action plan includes contingency plans in the event. that actions taken during the refueling outage prove unsuccessful at improving the reliability of the system. A discussion of our long-term plans for t.his system is als= included.
MIDS Design The Moveable Incore Detector System is an electro-mechanical system used to drive miniature fission chambers through retractable flux thimbles which penetrate up through the fuel assemblies in various locations l
throughout t.he core.
The thimbles are insurted into the reactor core through condult s 8
extending from t.he bottom of the reactor vessel through the concretc l
shield area and then up to a thimble seal table.
Since the thimbles are closed at the leading (reactor) end, they are dry inside. The thimbles flO6110413 910603 fDR ADoCN 05000369 g60l PDn
i Document. Contiol Desk page 2 June 3, 1991 thus serve as a pressure barrier between the reactor coolant system pressure and the at.mosphere.
Mechanical seals between the retractable thimbles and the conduits are provided at the seal table. The conduits are essentially extensions of the reactor vessel, with the thimbles allowing the insertion of t.he fission chambers.
In all, there are six fisston chambers and fifty-eight flux thimbles.
The miniature fission chambers employ uranium oxide enriched to more than 90% in U-235 and can be remotely positioned in the guide thimbles to provide flux mapping of the core.
Approximate chamber dimensions are 0.188 inch in diameter and 2.1 inches in length.
The stainless steel detector shell is welded to the leading end of a helical wrap drive cable and t.o e stainless steel sheathed mineral insulated coaxial cable.
Each detector is designed to have a minimum thermal neut.ron sensitivity of 1.0E-17 amps /nv and a maximum gamma sensitivity of 3.0E-14 amps /R/hr.
The six fission chambers are designated as Detect ors A, B, C, D, E, and F.
The drive systems used to drive t.he fission chambers into the core consists of six drive units, twelve limit switch assemblies, six 5 pat.h rotary t.ransfer devices, six 10-path rotary transfer devices, and fi f t y-eight. isolation valves. The drive units, 5 path rotary transfer devices, and limit. switch assemblics are mounted permanently on a pl a t form. The 10-path rotary t.ransfer devices and the isolation valves are mounted directly above the seal tabic, on a moveable support assembly which can be moved aside during refueling. These components are t. icd together with interconnect.ing tubing, and are therefore extensions of the thimbles.
The normal pat.h of a detector during a flux map is f rom it.s storage wheel (located inside t.he drive unit), through a drive wheel / encoder assembly (also located inside the drive unit), through two limit swit.ch assemblies, a 5 path rotary transfer device, a 10-pat.h rotary transfer device (which can choose any one of up to ten different thimbles), an isolation valve, and into t.he thimble proper. The positions of the detectors are recorded by the encoders and read out. on nixie tube displays located on t.he console in the control room. All six detectors can be run at the same time, each detector accessing a thimble chosen by its respective 10 path rotary t.ransfer device. Thus, in t.en passes, all fifty-eight thimbles can be accessed.
( A t.ypical flux map requires twelve passes since all detectors must run t.hrough a common path twice for calibration purposes.)
An incore flux map is obtained by measuring the signal response from the moveable detectors as they traverse t.he selected instrument thimbles from top to bottom. The detector signal is proportional to the neutron density incident t.o the detector as it traverses the length of the fuel column. The signal response can then be used to evaluate t.he reactor core relat ive power distribution. Thus, the moveable detectors measure
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Document Control Desk Page 3 June 3, 1991 local neutron densit.y or flux.
Figure 1 111ust rates the McGuire Moveable incore Detector Syst em.
Discussion of present Problem McGuire has recently been experiencing a problem wit h sticking t.htmble tubes.
This problem is especially severe on Unit I and occurs in the area where the t himbles penetrate t he mirror insulation at the bottom of the reactor vessol.
It is presently bolloved that Neolube from the det ectors and carbon from the CO., purge gas has built up over t he years, coating t he inside of tho thimblOs.
IL la postulated that. these materials 1(quify or vaporize in the area of the reactor coru, migrato down t.o t he bottom of t.ho reactor vessel, resolidify due to the temperature gradient in the area of the mirror insulation, and whsequently block access to the thimble tubes.
Most of the thimbio blaciongo has occurred over a span of several inches.
The thimble blockage prohtem became severo on Unit. I following the end-of-cycle 5 refueling outage, when the tubes were eddy current tested using an oversizo probe.
I t. is speculated that. t ho probe dislodged material from the inner walls of the tubes and pushed it. ahead into t he core region, thereby creating the blockage.
Technical specif f cat.lons require that a minimum of 75% of the thimble tubes be available for the MIDS to be considered operabic. As a result of the t.htmblo st.icking problem ' Duke power Company proposed a temporary change to the technical specifications for Unit 1 Cycle 6, and again for Cycle 7, to reduce the minimum requirement from 75% to 5d%. The NRC subsequently approved both of these tempocary changes.
For Cycle 6, it was not. necessary to actually ut.111ze the reduced requirement, since enough thimbles were able t.o be accessed t.o meet. the 75% requirement.
For Cycle 7. however, the t.htmbic sticking problem has becomo so savore that the 75% requirement could not always be met.
This temporary chango will expire at tho end of Cycle 7.
Figures 2 and 3 depict the thimble sticking historica for Units 1 and 2, respectively. Note the marked increase in the number of st.-icking thimbles following the Unit I end-of-cycle 5 refueling outage, when the eddy current testing was conducted using the oversize probe.
For Unit 2
it. can be seen that the thimble sticking problem was significant ly reduced following the acetore cleaning performed during the end-of-cycle 3 refueling outage. The s-cess in alleviat ing Unit 2's problem is speculated to be due large1/ to the fact that the Unit 2 thimblen were not oddy current tested with the oversize probe, hence no large scale dislodging of material by the probe occurred as is postulated to have occurred for Unit 1.
Also, the inside surfaces of the Unit 2 thimbles were not coated with Neolube as was t he case f or Unit 1.
Only small amounts of Neolube are present inside the Unit 2 thimbios (from the t
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I Document Control Desk page 4 June 3, 1991 helfcal detector cables).
Based upon the observed thimble sticking history, it is difficult to predict the number or location of sticking thimbles for any given flux map.
A few thimbles far each unit have been observed to stick repeatedly for each flux map over several fuel cycles. Horo often, thimbles will encounter sticking problems for a few flux maps only.
In addition to the thimble sticking problem, other problems exist which impact the reliability of the MIDS.
Drive system components are experiencing age-related degradation.
Slip clutches, encoders, and relays frequently need to be replaced.
The drive motors, limit switches, and rotary transfer devices are also exhibiting signs of wear.
A comprehensive preventive maintenance program exists for the MIDS and the system is serviced every refueling outage.
Industry Experience uith Moveable incore Detector Systems In an aticmpt to determine if the thimble sticking problems experienced at McGuire were common throughout the industry, maintenance personnel responsible for the MIDS conducted a survey of fourteen plants which utilize comparabic systems. The results of this survey are included as Tablo 1.
The survey data reflects a wide variety of years in operation, thimbic sir.es, purge systems, and cleaning and maintenance practices.
In general, plants that utilize small-diametor thimbles and CO, purge systems sceu to be experiencing more severe 8 ticking problems.
A correlation between thimble sticking and the use of Neolube was inconclusive.
At several plants, thn thimble sticking problem was severo enough to warrant complete thimble replacement.
Previous Corrective Actions Taken and Future plans for the System Maintenance History and Plans for Next Refueling Outage As mentioned previously, the thimble sticking problem became severe on Unit I beginning with Cycle 6. following the eddy current testing using the oversize probe during the end-of-cycle 5 refueling outage.
During the end-of-cycle 6 refueling outage, extensive cleaning and repairs were perf ormed on the system.
The thimble tubes were flushed with acetone and subsequently flushed twice with water.
Extensive preventive maintenance was conducted on the drive systers, the tubing the drives pass through before entering the thimble tubes, and the fittings on the ends of the thimble tubes.
In addition Unit I underwent a mini-outage during an approximate four-week period in October and November of 1990.
During this outage.
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Document Control Desk page 5 June 3, 1991 modifications to lessen bending in the t.htmble tubing were implemented and tubes were again soaked in acetone and subsequently watcr flushed.
While the acetone / water flush appears to alleviate the thimble st.f cking problem in t.ho short t.crm (i.e., for part of a fuel cycle), it is not a complete solution, as it only softens the thimble blockage and does not remove it.
McGuire Unit I continues to experience some of its most severe thimble sticking problems during Cycle 7 operat ion. While it was not necessary to utilize the technical specificat. ion relief granted by the NHC for Cycic 6, it has been necessary to utilize the rollef granted for Cycle 7.
During May of 1991, Unit I experienced a forced outage as a result of reactor coolant pump problems.
During this outage, the most severely blocked thimble tubes (nineteen in all) were filled with isopropyl alcohol and allowed to soak for several days. The tubes were then flushed. All nineteen thimbles were subsequently accessed a.11 the way to the top-of-core limits.
It is unknown at. present how long they will remain clear, since previous cleaning efforts have not resulted in a permanent solution to the thimble sticking problem; however, during a flux map t aken following this cleaning, fif ty-four thimbles were successfully accessed.
In addition, the C0 purge system has been 7
turned off for both units.
Plant modifications have been approved to delete the purge systems.
During the end-of-cycle 7 refueling outage, the thimbles will again be filjed with cleaning fluid (either acetone or isopropyl alcohol),
capped, and allowed to soak for approximately 35 days to soften the blockage. The thimbles will then be wire brushed, flushed, and air dried.
It is hoped that the combination of extended soaking and wire brushing will permanently remove the thimble deposits.
In addition to the above thimbic cleaning efforts, an assessment of the performance of the incore instrumentation room ventilation system is being conducted.
The possibility of localized hot spots in the vicinity of the MIDS equipment (which can result in an increased potential for component failure and the subsequent need for increased maintenance) is being examined and the feasibility of redirect.ing ductwork to enhance cooling capability is being assessed.
As noted previously, in addition to the thimble sticking problem, other problems exist which can impact the reliability of the MIDS.
Based upon manufacturer recommendations as well as previous operating experience, a preventive maintenance program has been established for the system.
Most mechanical and electrical components are serviced every outcge.
The detectors are replaced as needed and the chart drives are serviced monthly.
Document Control Desk l
page 6 June 3, 1991 Contingency Plans for Cycle 8 It is hoped t hat t he extensive cleaning and wire brushing planned for the end-of-cycle 7 refueling outage will permanently resolve the problem with sticking t himbles.
In the event that t he problem persists, however, future technical specificatton relief will most likely be necessary.
Duke power Company is proposing to take contingency measures to prevent a forced shutdown in the event that 75% of the thimbles cannot be accessed for the flux maps required for Cycle 8.
We are presently examining the possibility of obt aining the required technical support for a Cycle 8 1icense amendment, should one be required. The Cycle 8 core will consist. of both Il&W and Westinghouse fuel. Therefore, an analysis similar to those performed in support of the Cycle 6 and 7 license amendments will be more time consuming as a result of the need to develop core physics models for the mixed core.
The analysis would use the same methods that were employed in support of the Cycle 6 and 7 license amendments and will consider the mixed core design.
If it is performed, we would like t.o provide this analysis to the NRC for review prior to the beginning of Cycle 8.
If an amendment becomes necessary for Cycle 8, then the remainder of the license amendment. package can be submitted at the appropriate t ime.
It is hoped that this approach will reduce the amount of time required for the NRC to review the license amendment submittal if thimble sticking problems continue to be experienced early in Cycle 8.
Long-Term Plans for the MIDS In an ef fort t o ensure future long-term reliability of the incore detector systems at McGuire, several options are being considered as potential solutions to the problems being experienced. These include replacement of all thimble tubes, replacement of the thimble tubes and other MIDS components, and replacement of the MIDS Ltself by a fixed incore detector system.
These cpt. ions vary as to their ef fectiveness as solutions t o the problems being experienced by the MIDS.
For example, replacement of the thimble tubes is an ef fective solution to t he thimble blockage problem, but would not alleviate other problems with the system such as the failure of electronic and mechanical components.
Replacing the MIDS with a fixed incore system is the most complete solut ion to the various problems being experienced by the syst em.
In addition to varying as t o their ef f ectiveness as solut ions, the options also vary in cost.
An economic analysis of the various proposed solut. ions has been ;)erformed and t his analysis indicates that the fixed incore system offers the greatest dollar-for-dollar payback over the long t e rm.
Document Control Desk Page 7 June 3, 1991 No decision has been reached yet as to which option will be selected, although replacement. of the MIDS with a fixed incore system is recognized as the most. techalcally complete and cost effective long-term solution. Duke Power Company is planning to establish a MIDS project team to pursue a permanent. solution to the problems aficcting t.he system.
No action is being requested of the NRC at this tino.
In the event that regulatory interface between Duke Power Company and the NRC is required in the future, we will cont.act you as appropriate.
Should you have any questions concerning this letter, please call L.J.
Rudy at (704) 373-3413.
Very truly yours,
/A-Ital B. Tucker IJR/s I
Attachment xc (W/ Attachment):
S.D. Ebneter Regional Administrator. Region 11 T.A. Reed, ONRR P.K. VanDoorn Senior Resident Inspector i
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Table 1 Results of Industry Survey of MIDS Experience Plant il Thimble 1D Age (:>rd Purne cleaning Comments 1
U1-15 CO 1st time Numerous probican with 2
U2-13 7 yrs st.icking thimbles, Water required complete replacement in 1987.
2
.201 5-6 CO,,
Every Shut down on ist. cycle cycle due to not meet.ing TS.
Alcohol.
Suspect bad mfg or Neolube.
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Apex Cleaned for 1st time about 2 yrs ago.
No problems since.
4 UI.201/.210 Ul-8 Orig None U1 orig had.201, U2.210 U2-3 CO, changed to.210.
U2 Pros.205 Quit orig had.210.
Ilad sticking on both units.
Changed to.205.
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.201 9
Orig Apex Apex cleans each outage CO.,
with demin water, then Quit sprays with Neolube.
No problems with st.lcking.
6
.210 14 Pres None Thimbles replaced in when lie,,
1985. Orig purged with replaced CC.,.
Clal.ms this along
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with Neolube caused sticking problems.
Cleaned old t.htmbles wit.h alcohol. Do not clean new t.htmbles.
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.201 1
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Demin Immediately had
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wire cleaned with alcohol brush then wire brushed.
Sti11 having aticking problems.
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None Demin Cleaned twice using wa t.e r/
demin water and wire wire brush.
Never used brush Neolube.
No problems.
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r Table 1 Results of Indust.ry Survey of MIDS Experience (continued) 1 Plant Il Thimble TD Age (yrs] Purne Cleaning Comments 9
.201 10 Orig Alcohol llaving problems getting CO every rid of alcohol / water Qulf outage mixed waste.
Did have problems with sticking thimbles.
10
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N Apex Usually 2 or 3 blocked 5 2
and 10 path transfers or bad detectors are reason for getting close to TS.
11
.237 20 None None Never cleaned, no problems.
12
.201 17 2 or 3 Domin Master Lee cleans their
.210-15/ unit yrs water thimbles with CO Alcohol roto-router, also Qu$t alcohol and domin water, 13 5
None Apex No problems.
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.201 Ul-14 Orig Apex Cleaned thimbles and U2-10 CO, stopped using CO, purge Quit about 5 yrs ago.'
Problems mostly mechanical now.
Notes:
1.
All sites use.188 diameter detectors.
2.
Sites marked
- have EANCO systems.
3.
All other sites have Teleflex systems.
4.
All sites are required by technical specifications to obtain 75% of the total thimbles per flux map.
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