ML20090L803
| ML20090L803 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 07/02/1976 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Stello V Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9102120682 | |
| Download: ML20090L803 (4) | |
Text
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NSF NORTHERN 5TATEs POWER COMPANY Q[,i usNNuaeous. McNwa eotA esmot
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Dear Mr Stello:
MONTICELLO NUCLEAR GDIERATING PLANT Docket No. 50-263 License No. DPR-22 Supplementary Information to 1/26/76 License Amendment Request We understand from verbal contacts with your staff that questions have arisen con-cerning our January 26, 1976 License Amendment Request which superseded the initial License Amendment Request dated June 9, 1975.
nis request proposes that the con-trol rod drive (CRD) exercise surveillance required by Technical Specifications be changed fran weekly to monthly.
General Electric has stated their position as follows:
"llistorically, CRD exercising was first proposed at Dresden 1 and was instituted on a daily basis. We initial intent was to prevent crevice corrosion in the drive, especially in the metallic surfaces in contact with the seals.
It was recognized, however, that this operation was also a valuable check on drive condition and hydraulic system continuity.
W e original exercising requirements were specified in instruction books, not in technical specifications.
Early in the 1960's, an analysis was made on the scram mode reliability based on the system readiness logic, i.e., instrumented accumulator energy storage plus tested scram valves plus low friction drive line and hydraulic lines intact (as demonstrated by the periodic exercising test).
We analysis indicated that with these conditions satisfied, a satisfactory scram would occur.
Wis emphasized the safety aspects of this test, because it verified a moving drive line which is probably why the exercisirs requirement moved from the Instruction Books into the Technical Specifications, 11owever, this emphasis may have caused us to lose sight of the corrosion prevention aspect. With the introduc-tion of nitrided parts, which are used at Monticello, the prevention of corrosion became significant.
Experience indicates that crevice corrosion exists on drives which re-main in a given position for long periods of tixne. A ntunber of index tubes have had to be replaced because these tubes had pitting in the seal contact area corresponding to the full-in drive position. Rese 9102120602 760702 PDR ADOCK 05000263 PDR p
NORTH IN OTATED POWtER COMI
.N Y Hr Victor Stello
-2 July 2.1976 index tubes were from drives that had been in the full-in position for most af a fuel cycle and were not exercised.
W ie experience indicates the need for exercising all drives, even those in the full-in position.
Were is no corrosion data available at this time to provide a basis to lengthen the weekly surveillance frequency. GE recousnends that a lead plant be selected to develop the data base necessary to sup-port a lengthening of the thne interval.
his lead plant should give preference to those CRD's exercised least when selecting the sample of drives for the normal preventive maintenance program to provide the data base to support a longer surveillance interval."
A number of points raised in this generic position should be discussed individually as they apply to Monticello.
Were has been no pitting observed at Monticello that can be attributed to crevice corrosion. Should seal deterioration occur, the result may be higher stall flows and slower scram times, llovever, experience has shown that the CRD's performance in the " notch in" and " notch out" modes of operation would be noticeably af fected before a significant effect on scram times would be obse rved.
Since there are Technical Specification limits and surveillance requirements on scram times, the safety aspects of seal performance are assured.
Should lack of control rod exercising result in more frequent CRD maintenance, the choice between exercise frequency and maintenance becomes an economic decision rather than a safety decision, and therefore should not be a Technical Specification consideration. We a re ve ry concerned with seal performane from both the operational safety and economic view-point.
Our major reason for initially pursuing the change in exercise frequency was to avoid fuel failures in the initial core fuel and loss
' plant capacity, llaving since re-placed all initial core fuel with fuel of an improved design, we now are attempting to minimize fuel duty cn the improved replacement fuel as much as possible.
Based upon this desire, we believe it prudent to continue to pursue the Technical Speci-fication change. We believe that the safety aspects of a periodic operability demon-stration are addressed in our January 1976 submittal, which conservatively quantifies a reasonable exercise frequency as being once per month.
As discussed in the detailed safety evaluation that accompanied the amendment request, the Monticello control rod drives are of the same General Electric design as those at other plants where cracks have been discovered in the collet retainer tubes (CRT).
It should be emphasized that (1) no cracks have been observed at Monticello; (2) ob-served cracks at other plants have not resulted in a single inoperable drive; and (3) in simulated environmental tests, control rods with cracked CRT's have undergone more hot scrams (by a factor of 9.7) than anticipated in the life of a reactor with-out the failure to operate.
The fact r(nains that if a control rod drive became inoperable due to a complete severance of CRT, it would be discovered with the peri-odic surveillance exercising.
Because of the standard technical specification re-quirement for weekly control rod exercising of partially or fully withdrawn operable control rods, it has become cocunon practice to state that in the remote event of a l
completely severed CRT, the inoperable rod would be discovered by the weekly exercise su rveillanc e.
From the General Electric position stated above, it is clear that such reference to weekly exercising is a statement of current practice and not a I
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NORTH iN OTATED POWER COMF..NY
' Mr Victor Stello
-3 July 2, 1976 reconsnenda t ion.
With all the above considerations in support of extending the surveillance interval, t r. addition to the statistical analysis in our previous safety evaluation, and lack-ing any concrete information to the contrary, we continue to request action on our Janua ry 26, 1976 License Amendment Request.
Our objective remains to update the l
surveillance program in support of a more optistan frequency which decreases fuel duty.
Based on discussions with your staff, certain supplementary remarks are also approp.
]
riste regarding the potentist for a cracked CRT which is also discussed at length in l
our amendment request.
Our analyses for the proper surveillance frequency treats the failure of the control rod to insert as a random independent failure.
The September 24, 1975 NRC staff safety evaluation on the cracked CRT issue states:
" Distribution of failures of similar specimens generally follow a log nomal patte rn, with one to two orders of magnitude in time or cycles between failures of the first and failures of the last specimen. As no collet housing has yet failed, we are confident that there would be very few, if any, failures during the next time period correspond-j l
ing to the total service life to date."
j Even though identical components may bc. subject to the same failure, we maintain l
that they should properly be treated as random failures if the time interval between failures is expected to substantially exceed the surveillance interval. We concur i
l with the above staff position which clearly indicates that the expccted tfane interval between failures grossly exceeds the proposed surveillance interval.
The staff position conceming improved system reliability in light of common mode failure is sunanarized j
on Page 59 of WASil-1270 as follows:
"It appears, therefore., that only limited improve-j ment can be obtained by increasing the frequency of testing." Frcun this we deduce that surveillance intervals are to ba detemined on the basis of random failures as we have done, and that cocunon mode failures should be treated as another subject.
l We believe that this additional infomation should allow the staff to resume their review of the January 26, 1976 License Amendment Request.
If you require other clari-fying information to process this request in a timely manner, please advise us.
Should l
you be unable to issue authorization for this change, we shall appreciate receiving l
written notification with the technical basis for its unacceptability.
l
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Yours very tmly, V
f L 0 Mayer, PE Manager, Nuclear Support Services i
LOM/MllV/ deb ces J G Keppler j
G Charnof f i
MPCA i
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furnishing supplementary infortnation to 1/26/76 License andt. request regarding the Control Rod Drive exercise surveillance.
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