ML20090L590
| ML20090L590 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 02/06/1976 |
| From: | Mayer L NORTHERN STATES POWER CO. |
| To: | Stello V Office of Nuclear Reactor Regulation |
| References | |
| 1310, NUDOCS 9102120444 | |
| Download: ML20090L590 (9) | |
Text
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Washington, DC 20555
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Dear Mr Stellos co p-a MONTICELID NUCIIAR GENERATING PIANT Docket No. 50-263 License No. DPR-22 Documentation of Basis for Continued Operation Subsequent to your April,1975 request for additional information concerning the design and performance of Mark I containment systems, utility owners of plants with these systems corrnissioned General Elcetric to undertake a Mark I containment evaluation program. The Nuclear Regulatory Cournission has been informed of progress of this program from time to time through the series of reports and letters sub-mitted by General Electric. In a January 28, 1976 meeting in the NRC Bethesda, Maryland office with representatives of the General Electric Company (GE) and representatives of the cuner utilities you were presented with the latest information developed by the Mark I containment evaluation program. At the conclusion of this meeting, you advised us of your finding of no requirement for immediate shutdown of any Mark I plant then operating, and you requested that each licensee provide a letter docamenting their basis for continued operation. This letter is the Northern States Power Company (NSP) response to your request concerning the lionticello Nuclear Generating Plant, e
Undfr the Mark I containment evaluation program conducted by GE the load basis was developed from model test data, and torus structural capability was analyzed with respect to these loads. We conclusion of this evaluation was that the Monticello containment would perform without loss of function in the course of the DEA-1OCA event. NSP agrees with this conclusion, ne basis for our concurrence with this conclusion involves tnany f acets. Por pur-poses of explanation the discussion is organized as rollows:
(a) Torus support system structural response to the defined loads.
(b)
Conservatisms in the presently defined loads.
(c) Probability of the event associated with origin of significant loads.
9102120444 760206 jMQ PDR ADOCK 05000263 P
NORTHEi. 4 CTATED POWER COMPANY Mr Victor Stello Page 2 February 6, 1976 I.
Torus Support System Structural Evaluation The results of the structural evaluation by GE are in the form of ratios of applied load to lower bound ultimate capability of structural elements of the torus support system. These elements included the torus ring, shell, upper column connections, columns, lower column connection end column base plate anchor bolting.
The ring, shell and upper column connection nssemblies were found to have large margins with respect to the analyzed loads, both upward and downward, and are not an area of concern.
For down loads, the load capacity ratio of structural elements of the Monticello torus support system having a ratio above 0.9 were as follows:
Component Ratio Columns 0.92 Lower pin connection elevis lugs 1.00 Fcr the columns there may be a potential for limited and local yielding near the top connection, as a result of combined bending and axiel forces. However, this would be expected to be minimal because of the short duration pulse-character of the load. Yield across the total column cross-section would not be expected because the axial load is not sufficient to produce this result.
The clevis lugs of the lower column pin connection could potentially be subject to moderate local yield in bearing, but again this would be minimal because of the pulse-character of the down load.
For upward loads, the column base anchor bolts would be the only element of 4
potential concern; the CE-Bechtel evaluation indicated that maximum elongation of these embedments would be 0.13 inches, or about one third of the elongation associated with ultimate pullout capability for a 1-1/2 inch embedded bent anchor as reported by Bechtel.
This conclusion with respect to applied load can be con-sidered ccnservative because of the pulse nature of the uplift load component attributabic to vent header response reactions.
As a parallel effort to the GE-Bechtel evaluations, and for further assurance of these conclusions, NSP retained NUTECH as a consultant to perform an independent structural evaluation for the same applied loads.
The results of the NUTECH evaluation are in concurrence with those of GE-Bechtel.
Notwithstanding that torus uplift is expected to be 0.13 inches or less, NSP has performed a field survey of all ECCS piping connected to the torus, and an analysis of the capability of this piping to withstand such uplift has been completed.
The results of this preliminary piping analysis are that the limiting case line can
i NORTHEi J OTATEC POWER COMPANY
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Mr Victor Stello
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Page 3 l
February 6, 1976 withstand uplift displacement in excess of twice the calculated torus displacement
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(Section III, Class 2 allowable stress of 2.4 S, considering weight, seismic and h
pressure).
Several other general aspects of conservatism are worthy of consideration on a qualitative basis:
i (a) the ultimate capacity of structural elements as determined by analytical methods is generally fouad to be conservative in comparison to actual failure tests; (b) the mechanical properties of the materials used in the structural elements of the torus support system generally expected to range from several percent to twenty percent greater than the minimum specification for the material designation; (e) dynamic yield properties for these materials are generally accepted to be abov-ten percent above static yield strengths for the rapid strain rates expected to occur in these elements during I4CA.
II.
Load Jonsiderations one-twelf th scale tests of a Mark I torus were run to obtain upwarded and down-ward loads on the torus due to postulated LOCA events. Recognizing the uncer-tainties fnherent in numerical scaling techniques, the methods used in the determination of upward and downward pressure loads exceed a most probable load analysis approach by incorporating many conservatisms in data interpretation and analytical technique.
1.
Upward Load Conservatisms The upward pressure load is sensitive to the pressure history of the drywell following a postulated LOCA because the driving force for the pool swell and the reaulting torus air space compression are increased with a greater drywell pressurization rate. The upward pressure load on the torus has been defined for the Short Term Program by application of the calculated FSAR drywell pressurization rate.
Specifically, the 1/12th scale tests were run and enalyzed to obtain loads based on the FSAR pressurization history. Ilowever, since this pressure history has been used to assure the adequacy of the drywell design pressure, it is biased towards high values for that purpose.
The FSAR pressure history assumes an in-stantaneous break (mass fluxes evaluated using the Moody Critical flow model assuming slip), no steam condensation in the drywell, and a homogeneous air-steam-liquid flow mixture in the vent.
This results in a high pressurization rate and increases the upward load definition.
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NORTHEM OTATED POWER COMPANY 4
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1 Mr Victor Stello l
Page 4
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February 6, 1976 l
5 As an example of the conservatism for the upward pressure load produced by the application of the FSAR pressurization rate, consider the reduction in mase flux which occurs with the application of the homogeneous rather than the slip formulation of the Moody Critical flow mods 1.
Even for the 20 Btu /lb mass subcooled liquid in the recirculation system, the gomogeneous model shows a reduction in the 9
mass flux from 8100 to 7100 lb m/sec. f t
- Using the sensitivity curves, this flow l
reduction produces a reduction in the upward pressure load of 2 percent. The other l
conservatisms in the FSAR pressure history will add to this margin.
Another conservatism for the upward load used in the Short Term Program is the 8
assumption of a 1007, air flow in the vent system.
This conservatism confilets 1
directly with the homogeneous air-steam-liquid vent flow assumptions used to define the FSAR pressurizati'on rate. More consistent assumptions aimed at determining i
the most probable load basis are possibic. One alternative is te apply the FSAR hotogeneous air-steam-liquid vent flow assumption for both the pressure history and the non-condensible flow rate into the bubble. The other alternative is to assume 1007, air vent flow for both the pressure history ar.d the flow rate into the-bubble.
If, for example, the former is evaluated, the non-condensible bubble flow j
rate is reduced by a factor of three and the sensitivity analysis for (Apoog/Avent) shows that the maximum upward load will be reduced by a factor of two.
Another contribution to the total upward load on the torus structure is the impact.
3 1
load on the vent header.
'Ihe impact pressure on the vent header for the Short Tenn Program was determined by applying the impact velocity measured in the 1/12th Scale tests and the results of the PSTF impact data. However, the PSTF data was r
obtained for the impact of a slug having a thickness greater than the diameter of
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the target.
In contrast, the 1/12th Scale slug thickness is thinner than the vent header. The reduced slug thickness in the torus allows the liquid to be quickly decelerated under the header immediately following impact.
This deceleration, which was observed in the 1/12th Scale tests, would be expected to yield a lower f
impact load. 'Indeed, the impact pressure history measured in *.he 1/12th Scale test l
by a strain gage on the vent header was a factor of three less in magnitude and j
three times longer in duration.
The more conservative vent header impact pressure was used in the analysis as an added conservation. The 1/12th Scale test results l
will be substantiated by future 1/6th Scale testing.
i i
2.
Downward Load Conservatisms Similar conservatisms have been used in defining the downward pressure load on the l
torus. The calculated FSAR pressure rate was also used to establish the downward I
pressure load on the torus.
If the finite opening tine of the break, reduced mass l
flux at the break, and steam condensation in the drywell were accounted for, the drywell pressure at vent clearing would be less and the downward pressure load I
would be reduced. The reduction in the downward pressure load for using a mass flux of 7100 instead of 8100 lb m/sec. ft2 is 6 percent.
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Mr Victor Stello Page 5 February 6,1976 i
The data used from the 1/12th Scale tests to define the downward pressure loads was also analyzed in a conservative manner.
There was some variation in the maximum downward pressure loads measured for the medium orifice runs considered as a group and for the large orifice runs considered as a group.
Instead of averaging the loads measured for the medium and large orifice runs, the greatest magnitude down-ward pressure loads were identified for both orifice sizes. The Reference Plant downward pressure load was then determined by interpolating between the maximum i
of the maximum downward pressure loads.
The analysis of the 1/12th Scale test results also did not take credit for any reduction in the downward force due to three dimensional effects and pressure attenuation.
The submerged pressure transducers are located at the mid-width of the test section and will sense most directly the pressure of the bubbic formed at the downcomers and the water jet forces.
Both the bubble pressure and the water jet force will attenuate as one moves circumferential1y away from directly below the d ownc omers. However, since the pressures measured by the transducers were assumed to act uniformly over the width of the test section, a higher than actual reaction force was calculated.
In the typical torus, the downcomers are not spaced uniformly leaving a large section below the vent pipe where the influence of the downcomers is decreased.
The downward pressure load produced by the bubble pressure at the downcomers and the water jet forces will be reduced in this section because of the increased distance to the nearest downcomers. However, the pressure loads should not be significantly increased where the downcomers are closely spaced because the measured pressure load of 16.33 paid approaches the driving pressure - the drywell pressure is 17.0 psid at the time of vent clearing. Therefore, due to three dimensional effects and variable downcomer spacing the maximum downward pressure cited for the reference plant of 16.33 psid is conservative.
III.
Probability of the Event Associated with Origin of Defined Loads The defined loads used in the structural evaluation were based on containment pressurization rates resulting from the DBA-LOCA event; that is, the worst case break of a 28 inch recirculation line at the reactor vessel nozzle.
This piping would require a major f ailure such as complete severence and separation or a large axial break to produce the containment loading used in the evaluation.
We believe that the probability of such f ailures for this line size would be on the same order as reactor vessel f ailure estimates which are associated with an ex-tremely low probability.
In the case of Monticello, six of the thirty-nine welds on the 28 inch piping have been the subject of ISI examination since initial service.
The nozzle to safe end end safe end to pipe welds on both recirculation loops ware included in these ISI examinatioas; we believe that these inspections in the location of the " worst case" break, significantly reduce the probability of that event.
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1 NORTHE.. 4 GTATES POWER COMPANY
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Mr Victor Stello j
Page 6 j
February 6,1976 2
Considering 22 inch recirculation line breaks, and adjusting containment loadings using CE senstivity curves to this break size for Monticello, we conclude that relative to DSA-LOCA loads the upward loadings would be reduced by 18% and the downward loadings would be reduced by 25%. We believe this illustrates that breaks on the 22 inch line sizes are associated with sufficiently reduced dynamic loadings j
on the torus structure to be below the potential range of immediate concern.
3 Three of the nineteen welds on 22 inch piping have received ISI inspection since
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initial service.
Stepping to the next larger line size, 18 inch lines, the torus dynamic loadings 4
would be further reduced from the DBA-LOCA event by 55% for upward loads and 677.
j for downward loads, t
IV.
Conclusions 1
In light of these considerations we believe that the probabikity of a DBA-LOCA is extremely low, and that the only LOCA break that could produce loadings event 4
as used in the evaluation are associated with those approaching the " worse case" i
event related to 28 inch lines close to their reactor connection. Second, we j
believe that the structural evaluation illustrates with reasonable conservatism that the containment system can perform its function under the load basis used in i
the evaluation.
Third, we believe that the derivation of loads developed for this evaluation includes significant conservatisms. On this basis we conclude that 1
the Monticello Nuclear Generating Plant can continue operation for an interim period without undue risk to the health and safety of the public.
1 Notwithstanding these conclusions for interim operation, NSP recognizes that the margins of structural capability for the torus support system must be improved to
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a range which approaches the margins required for normal design practice. With this objective, NSP has undertaken a course of action to develop detailed engineer-ing designs and determine feasibility of installation of these designs for j
modifications to improve the structural elements of the torus support system that arc subject to high loedings.
The modifications being considered involve rein-j forcement of torus support columns, reinforcement of lower column to base connection, and improvement of column base anchoring. NSP is prepared to discuss this action plan at your convenience, i
Yours very truly, L 0 Fbyer, E I
Manager of Nuclear Support Services LOM/LLT/ak l
cc: J G Keppler i
G Charnoff NTCA J
Attn: J W Ferman 4
1 UNITED STATES NUCLEAR REGULATORY COMHISSION I
NORTHERN STATES POWER COMPANY i
MONTICELID NUCLEAR GENERATING PIANT 1
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Docket No. 50-263
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l License No. DPR-22 i
LETTER DATED FEBRUARY 6,1976 1
RESPONDING TO NRC REQUESTS
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FOR INFORMATION ON CONTAINMENT DESIGN Northern States Power Company, a Minnesota corporation, by this letter dated February 6, 1976 hereby submits information in response to NRC j
requests for information made during a meeting with the Mark I Containment Owners' Group on January 28, 1976.
This request contains no restricted or other defense information.
NORTHERN STATES PORER COMPANY 1
By Ojd/n/
4 L'J'Wa ch t e r '
Vice President, Power Production
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& System Operation On this 6th day of February,197 6, before me a notary public in and for said County, personally appeared L J Wachter, Vice President, Power Production and System Operation, and being first duly sworn acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof and that to the best of his knowledge, information and belief, the statements made in it are true and that ic is not interposed for delay.
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I DENISE E. BRANAU U
NOTARY PUBLIC - MINNESOTA HENN; PIN COUNTY j
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3 UNITED STATES NUCLEAR RECUIATORY COMMISSION NORTHERN STATES POWER COMPAT. l HONTICELID NUC12AR GENERATING PIANT j
Docket No. 50-263 License No. DPR-22 1
LETTER DATED FEBRUARY 6,1976 RESPONDING TO NRC REQUESTS 4
FOR INFORMATION ON CONTAINMENT DESIGN l
Northern States Power Company, a Minnesota corporation, by this letter dated February 6,1976 hereby submits information in response to NRC requests for information made durin8 a meeting with the Mark I Containment Owners ' Group on January 28, 1976.
'Ihis request contains no restricted or other defense information.
NORTHERN STATES POWER COMPANY By
/s/ L.T Wachter L J Wachter Vice President, Power Production
& System Operation On this 6th day of February,197 6, before me a notary public in and for said County, personally appeared L J Wachter, Vice President, Power Production and System Operation, and being first duly sworn acknowledged that he is authorized to execute this document on behalf of Northern States Power Company, that he knows the contents thereof and that to the best of his knowledge, information and belief, the statements made in it are true and that it is not interposed for delay.
/s/ Denise E Branau Denise E Branau Notary Public, Hennepin County, Minnesota My Commirsion Expires October 10, 1981 i
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V. Stello None 40 SENT LOCAL PDR _XXT CLASS UNCLASS PROPINFO INPUT NO CYS W-'"n DOCKET NO:
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DESCRIPTION:
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1 Letter re, our letter of 4-75.. notarized 2-6-76..
l Letter furnishing additional info, concerning the l
Documentation of Basis for Continued Operation..
AC' KNOWLEDGED (40 Cys. Received)
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