ML20116C395
| ML20116C395 | |
| Person / Time | |
|---|---|
| Site: | Monticello  |
| Issue date: | 07/19/1996 |
| From: | Hill W NORTHERN STATES POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| TAC-M94307, NUDOCS 9607310137 | |
| Download: ML20116C395 (6) | |
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Northem States Power Cortpany Monticello Nuclear Generating Plant 2807 West Hwy 75 Monticello, Minnesota 55362-9637 July 19,1996 US Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 MONTICELLO NUCLEAR GENERATING PLANT Docket No. 50-263 License No. DPR-22 i
Supplemental Information Conceming Notification of a Deviation from Ucensing Basis information Concemino Tomado Effects on the Reactor Buildino Superstructure frac M94307)
By letter dated February 12,1996, Monticello provided notification to the NRC of an identified condition which is a deviation in implementation of the Monticello licensing basis. Evaluations performed as part of the Monticello design basis reconstitution project determined that information, incorporated into the plant licensing basis by reference, is inconsistent with design basis information for the Monticello plant. The Monticello staff identified that information incorporated by reference into the Updated Safety Analysis Report (USAR), conceming the capability of the Reactor Building superstructure to withstand tomado wind loading conditions, is inconsistent with the design basis for the structure as well as specific licensing basis statements contained in the USAR.
Our February 12,1996 letter established the following commitment:
Modifications are to be implemented to resolve the deviation from the current licensing basis. The modification design will ensure that a failure of the structural members of the Reactor Building superstructure will not occur when exposed to tomado winds.
Attachment A to our February 12,1996 letter provided a description of the corrective actions to I
resolve the identified deviation. Detailed design work has been completed for the modification I
to implement the proposed corrective actions. The detailed design has identified that changes to our original corrective actions are required. Attachment A to this letter restates the corrective actions identified in our February 12,1996 letter and provides a description of the p
revised corrective actions being implemented. These corrective actions are to be implemented by a plant modification performed in accordance with the provisions established by 10CFR50.59.
310012 7/18/96 NSP H:\\ DATA \\NRCCORR\\RX-BLDG. DOC 9607310137 960719 PDR ADOCK 05000263 P
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4 USNRC NORTHERN STATES POWER COMPANY July 19,1996 Page 2 This letter contains no new NRC coinmitments, nor does it modify any prior commitments.
However, revised information concerning implementation of our previous commitment is provided. Please contact Mary Engen, Sr Licensing Engineer, at (612) 295-1291 if you require furtherinformation.
/E711 l1 William J Hill Plant Manager Monticello Nuclear Generating Plant c:
Regional Administrator-I!!, NRC NRR Project Manager, NRC Sr Resident inspector, NRC State of Minnesota, Attn: Kris Sanda Attachments: A - Corrective Actions for the Monticello Reactor Building Superstructure
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f Attachment A l
Corrective Actions.for the Monticello Reactor Buildina Suoerstructure j
Evaluations performed as part of the Monticello design basis reconstitution project have i
determined that licensing basis information, incorporated by reference, conceming the
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protection of stored spent fuel from the possible effects of missiles resulting from tomado j
phenomenon, is inconsistent with design basis information for the Monticello Reactor Building l
superstructure. Information contained in General Electric Company Topical Report APED-5696, which has been incorporated into the Monticello licensing basis via reference, states the reactor building superstructure is designed to withstand tomado winds. This statement is inconsistent with the design basis and specific licensing basis information contained in the i
license application, FSAR, and USAR; in that the Reactor Building superstructure was designed to withstand winds of approximately 189 mph. This capability is above the current design basis for wind loading (which is applicable to the superstructure), but does not satisfy j
the tomado loading resulting from a tomado with 300 mph tangential wind speeds (which is beyond the design basis of the superstructure).
l Corrective Actions as Described in NSP Letter Dated February 12.1996 As stated in Attachment A to Monticello's February 12,1996, the following corrective actions 4
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The north and south walls of the Reacar Building superstructure are to be strengthened by forming a composite beam from the existing verticall-beam i
columns of the superstructure wall and the existing vertical 1-beam columns l
which provide structural support to the Reactor Building overhead crane.
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The east and west walls of the Reactor Building superstructure are to be modified to provide pressure relief during postulated wind conditions in excess of approximately 160 mph. Pressure relief is to be provided by establishing l.
seams in the inner layer of steel siding along the wind girt such that the inner layer will tear when exposed to the excessive forces of high winds, thus relieving the windward pressure force such that structural integrity is maintained.
Revised Corrective Actions 3
Detailed design work has been completed for the modification to implement the proposed corrective actions. The detailed design has identified that changes to our original corrective actions are required.
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To preclude the Reactor Building steel superstructure from failing due to a 300 mph tomado wind loading, the following corrective actions are to be implemented.
- 1. The steel superstructure will be reinforced to allow the structure to withstand the postulated wind loading. To reinforce the superstructure, structural members and
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connections will be modified to strengthen their load carrying capacity. Various
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AttachmentA i
July 19,1996 i
Page 2 l
existing truss and column members will be strengthened by the addition of plates or structural shapes. Base plates for the columns will be strengthened by adding.
stiffening plates. Various connections in the trusses and the wind bracing will be strengthened by added weld and/or additional reinforcing plates.
- 2. A number of fasteners for the inner siding will be removed to assure wind forces on the superstructure during postulated tomado winds remains consistent with the j
analysis of the structure for the 300 mph tomado wind condition.
It is estimated that the modification will be implemented within five (5) months of initiating j
construction activities, with construction being initiated in July of 1996.
This modification will assure that the Reactor Budding superstructure will be adequate for the following loading conditions:
Class 11 loads - within code allowables: Dead Load + Uve Load + wind load.
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Class ll loads + Seismic - within USAR allowables for Class I structures: Dead Load +
l Live Load + Seismic.
Tomado loads - within USAR allowables for Class I structures: Dead Load + Live Load
+ Tomado wind loads.
This modification will add approximately 88,000 lbm of reinforcing steel to the Reactor Building steel superstructure. From Appendix A of the USAR, the weight used for the existing Reactor l
Building superstructure seismic analysis is 876,000 lbm. The 88,000 lbm of reinforcing steel being added represents approximately a 10% increase in the weight. The total weight of the structure used in the seismic analysis of the Reactor Building was 144,124,000 lbm. Thus the weight of the Reactor Building (including the additional reinforcing steel being added) represents less than 0.7% of the total Reactor Building weight. A 10% increase in the Reactor I
Building superstructure weight, which is less than 0.7% of the total weight of the Reactor Building, is considered negligible. Therefore the additional mass does not have any significant effect on the seismic response of the Reactor Building below the refuel floor and therefore does not affect any previous seismic analysis of this portion of the Reactor Building or any equipment in the Reactor Building.
Since the superstructure has significant reinforcing added due to this modification it has been reanalyzed in its modified configuration for seismic loading. It has been verified that the superstructure is adequate for a seismic event and that the rest of the Reactor Building is adequate for any loads that would be superimposed on it by the superstructure due to a seismic event.
The metal siding consists of an exterior aluminum siding which is screwed to subgirts. The subgirts are clipped to the inner panel at the inner panel ribs. The inner steel panels are attached to the wind girts by screws. The wind girts are horizontal members that span the
Attachment A July 19,1996 Page 3 columns of the superstructure. Insulation is attached to the subgirts and sandwiched between the inner and exterior panels. it has been determined that the exterior siding will blow away at wind speeds less than 300 mph, which is acceptable. The interior panels on the windward side remain attached to the superstructure for wind speed of 300 mph. However, the ends of the panels have been shown to fail at a wind speed of approximately 200 mph which would allow for partial venting of the windward side. Upon windward side vanting, the panels on the leeward side will fail. Prior to venting on the windward side, at approximately 189 mph, the panels.on the other two sides will fail due to the vacuum load from air flowing around the superstructure.
The analysis of the reactor building superstructure recognized the venting action of the windward inner panels and the corresponding load reduction on the structure. The ends of the windward inner panels, where the panel pieces are butted up against each other, shear out at the location of the wind girt fastening screws at a wind speed of approximately 200 mph, which corresponds to a pressure of 160 psf. This 160 psf loading was analytically determined based on installation drawings showing the panels having two screws at the buttjoint. To verify the loading at which the inner panels sheared away from the screws, panels were fabricated and tested to determine the corresponding pressure at which the ends of the panels would shear out and provide partial venting of the windward loading. Results from the tests on the inner panels confirmed the analyzed behavior of the windward inner panels. Field investigation of j
the installed configuration showed that there are two to four screws at the butt joint. The fasteners at the panel ends will be modified by removal of the excess screws to satisfy the analyzed configuration for the reactor building superstructure of two screws per panel at the butt joint. It should be noted that standard industry practice is two screws per panel and the existing panels have two screws per panel at the intermediate wind girt, it is only the buttjoints at the ends of the panel that have more than two screws.
Installation Considerations The screw holes that are created from the removal of the panel screws will be sealed with caulking to prevent potentialin leakage of outside air and any adverse effect on secondary containment integrity. The total area of openings in the secondary containment created during this modification will be limited by the installation procedures to ensure no adverse effect on i
secondary containment integrity.
When construction activities are taking place over the spent fuel pool during this modification the fuel pool will be covered to prevent material from being dropped into the pool. Any cover used over the spent fuel pool will be an engineered design to preclude failure of the cover (the cover will be designed to prevent collapse due to a seismic event) and thus prevent foreign material from entering the spent fuel pool. A platform bridging the overhead crane rails will be erected to support installation of the truss reinforcing members. This platform will be an engineered design that will prevent any structural member that is being installed from accidentally falling into the spent fuel pool.
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Attachment A i
July 19,1996 Page 4 Hoisting of new structural reinforcing members will not take place over the spent fuel pool thus precluding the possibility of them falling into the spent fuel pool. There are no pieces of material that will be lifted that weigh greater than 1500 pounds, thus there are no heavy load concems.
l Local High Efficiency Particulate (HEPA) filters will be used to minimize the airbome particulate created from welding or grinding activities.
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4 Welding and vibration in the vicinity of the detector for Spent Fuel Pool Radiation Monitors have the potential for causing a spurlous high radiation signal. This would initiate a Reactor Building ventilation system shutdown and a Standby Gas Treatment system initiation. To prevent this from occurring due to a spurious signal the trips will be bypassed. The trips will be 4
bypassed when ever it is prudent to prevent a spurious signal due to welding, mechanical vibration (caused by chipping out concrete for base plate repair) or if the detectors need to be temporarily moved due to interference concems. Technical Specifications (3.2.E and Table i
3.2.4) require that the instrument channels be operable whenever secondary containment is required; however, one of the two monitors may be bypassed for maintenance and/or testing.
Only one monitor will be bypassed at a time. When one detector is bypassed construction l
activities in the vicinity of the other detector will be stopped. Bypassing of the Spent Fuel Pool Radiation Monitor will be controlled administratively via the installation procedure.
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