ML20195B193

Calculation B13-01739-23, Upper Lateral Stabilizer Mod Stress Analysis

ML20195B193
Person / Time
Site:	Nine Mile Point
Issue date:	05/13/1999
From:	Cole K, Mahadevan A, Ranganath S GENERAL ELECTRIC CO.
To:
Shared Package
ML20195B191	List: ML20195B186 ML20195B193
References
B13-01739-23, B13-1739-23, NUDOCS 9905280178
Download: ML20195B193 (17)

Text

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GE NuclearEn:rgy Engineering Calculation Sheet Sheet 1 of 8 Subject Orgenner/ Preparer:

Date Nine Mde Point 1 : Shroud Repeer - Upper Laters! Stabdeer Modifcatwi Stress Analysis A. Mahadevan S/6/99 Number Vertfier:

Date B13-01739 -23 som Renneneth -

5/6/99 Legible e Reproducible e ReviewerNenfer e identifebie Structure, System, Component e Purpose e Method o Units Nine Mile Point 1 Shroud Repair Upper Lateral Stabilizer Modification Stress Analysis

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9905200178 990521 PDR ADOCK 05000220 P

PDR NEO-087 (Rev 6/2G96) i,.

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GE Nucteer En:rgy Engineering C:Iculation Shtet sneet 2 e s amou oegenwiPrope=r Dew P+ne use Pome 1 Snreud Reper Upper tmoref Samptaer Modtcesca Stress W A Meneseven 5599 her w

Dew S13-01739 23 seen newnem 5599 w

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r Nine Mile Point 1 Shroud Repair i

Upper Lateral Stabilizer Modification Stress Analysis j

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Prepared By:

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A. Mahadevan. Pnncipal Engineer, Rescerx Modi 5 cations 9

I3ji9 Reviewed By: M *-W~^

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S. Ranganath.M/ nager,Mardware Design

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Approved By:

S/LS/Gd K.'ColeIPro'3ects Manager, Reactor Moliific'ations

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Ato.oer m swes)

GE Nuclear En:rgy Engineering Calculation Sheet Sheet 3 of B

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rer.

Dois sue,ect Nine Mde Posnt 1 Shroud Reper. Upper Latere! Stabdeer Modrficolson Stress Analyes A. Mehedeven 5 @ 99 VerWor:

Doio Nummer Sam Renooneth 5599 813 01739 23 Legstne e Repmm e ReviewerNonfier e toontifetWe Structure. System. Component e Purpose e Method e Units Table of Contents

1. INTRODUCTION AND SCOPE

-4

2. MODIFICATION DESIGN 4

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3. STRESS ANALYSIS 4

3.1 APPUCABLIb) ADS...

.. 5 3.2 A. ALNSIS :

....................................................................5 N

4. CONCLUSION 6

5. REFERENCES 7

ATTACHMENT Detailed Analysis Spreadsheen :

Up-Stabil-Modsf-Normal.xis Up-Stabil-Modsf-Thu.xis i

NEO.067 (Rev IV21W96)

u GE NuclearEn:rgy Engineen'ng Calculation Sheet Sheet 4 of 8 C_

n Deee Suspect None Md. PowW i ' Shroud R.p.ir Upp.r Lateral St.tme.r Modme. tion Stress An. lysis A Meh.o.v.n 5 @ 99 VerWeer:

Date Nummer Som Ranoeneth

&#S/99 B13 01730 -23 L.on. a.producien.. n.wi ru.re.r. i nim.in. - siructur., sysar., c+. at. Purpose e M.tnod e Unsts

1. Introduction and Scope During the RF-015 outage the torques in the shroud repair tie rod nuts were verified to assure that there is no unacceptable preload loss or ' looseness'. Upon such veri 6 cation, during the reinstallation of the upper lateral stabilizer, it was noticed that a single cap screw (1 of 4) attaching the upper contact wedge to the upper stabilizer spring had failed. Failure location was at the plane where upper wedge bracket and the spring interface. The root cause was identified to be intergranular stress corrosion cracking (IGSCC) in the X-750 bolt material in conjunction with large, sustained differential thermal expansion stress due to

' dissimilar materials at the inte6 ace (upper spring and belt: X-750; wedge bracket: Stainless Steel).

Similar. dissimilar metal connections using X-750 bolts exist at a total of 24 locations [(4 tie rod / upper stabilizers) x (4 bolts at the upper end + 2 bolts at the lower end, of each stabilizer spring)). The existing assembly is detailed in References 1 thru 3. A mod:Scation to the assembly is designed so as to eliminate this problem at all 24 locations, as described in section 2. This calculation documents the stress analysis performed to qualify the modification design. The analysis is performed using the acceptance criteria required in the original design specifications (Reference 1) for the Nine Mile Point I shroud repair hardware.

2. Modification Design The details of the modification are contained in Reference 2. The modification hardware details are shown in Reference 3d. The modification involves the addition of 316L SS clamps (nut plate) secured to the wedge block at the bottom with one bolt (XM-19) and to the wedge block at the top with two bolts (XM-19). The detailed stress analysis shown in the attached spreadsheets considered both XM-19 and X-750 bolts. Both materials were shown to be acceptable both from the viewpoint of meeting Reference 1 Design Specification allowable stresses and demonstrating acceptable IGSCC margin. The bolt material is chosen as XM-19 based on material availability. The analysis envelopes the effects on both the upper and lower contact locations of the upper spring.

3. Stress Analysis The modification involves the addition of the clamp, and the bohs. Stress analysis is performed to qualify the modification for the applicable loads, consistent with the original shroud design speci6 cation (Reference 1). Both tensile and shear stresses from the thennal and other operationalloads were evaluated. Also beanng stresses at the wedges are veri 6ed. Analysis was performed for normal operation and the thermal upset condition.

NEo 087 (R.v 6f2858)

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GENuclearEn rgy Engineering Calculation Sheet Sheet 5 of 8 orpensermeperor.

one s,npai Nww Mme Poird 1 Shroud Receir. 'M Lateret Stahihzer Modrncehon Strees Analvois A Mehedsven 5 @ 99 vermer-Dmie w

som mensenseh sees 813 01739-23 tegene. n.proaucane. Revieweriventwr. ioenunstne. structure, synism c...,.

i.: o Purpose e Method e unas 3.1 Applicable Loads The applicable, significant loads are as follows:

1. Differential thermal expansion loads (tensile and shear) due to the clamping / bolting of dissimilar materials.

2. Mechanical installation torque (Installation torque of 20 ft.lbs. is assumed per bolt).

j

3. Loads in the bolts due to potential friction between the RPV ID and the contact wedges The nonnal, steady state condition as well as upset thermal transient condition were evaluated.

During dynamic events the modification hardware will not be loaded in any significant tensile or shear loads. Therefore, the load combinations containing dynamic loads do not govern and hence not evaluated.

However, for purpose of checking the bearing at the contact wedges, seismic load is also included along with the upset thermal condition loads.

3.2 Analysis The critical components evaluated are the bohs and the nut plate and the stabilizer spring plate and the upper contact at the bok hole location. The analysis envelopes the effects at the upper and lower ends of the stabilizer spring. The stress results are summarized below.

Bolts The detailed analysis for the bolts is documented in spread sheets Up-Stabil-Modif-Nonnal.xis and Up-Stabil-Modif-7hU.xis for the normal and upset operating conditions respectively. As mentioned in section 2, analysis was performed based on X-750 as well as XM-19 materials for the bolt. Even though both materials were found acceptable, XM-19 was chosen as the favorable material based on material availability. These spread sheets are included as part of this calculation. In the evaluatic t, it is assumed that the currently remaining boks (X-750) are ineffective.

The tensile stress in the boh was determined to be 13.7 ksi (larger of normal and thermal upset conditions), which is well within the allowable stress of 35.4 ksi (= 0.9 Sy at the upset temperature) for the XM-19 material bolt. The allowable stresses for the XM-19 material were based on annealing at 2025-2075* F. Bolt head shear stress was also evaluated for the upper contact bolts and shown to be acceptable. Fatigue was also assessed. The allowable fatigue cycles were conservatively determined to be NEo 087 (Rev 6/2846)

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r-GE Nucl=r En:rgy Engineering Calculation Sheet Sheet 6 of 8 C.

._ _ ANeper.r; Det.

Sungsor Nan. Mil. Posnt 1

• St.roud Reper 'M Leler.1 Statiiteer Modshcotion Strees Anoty as A Meh.d.ven SW99 v.rvier-omne sun.or eisome-23 s n.w w ee L.9es.. n.prooucatise. a. viewer /verw r. io.ntifisi. structure sysiem. component. Purpose. Metnoa. units 7 E5 cycles. The actual number of thermal cycles (startup and shutdown) is conservatively assumed as 200 which is much smaller than the allowable of 7 E5 cycles. Thus fatigue is not a concern.

Nut Plate The bolts are threaded into the nut plate. The only significant loading on the nut plate is due to the tensile load in the bolt. The thread shear in the nut plate (SS 316L) due to the bolt load was determined and was found to be less than 3.5 ksi which is well within the allowable of 11.3 ksi (= 0.6 Sy at the thermal upset temperature) for 316L plate material.

Stabilizer Spring and Wedge Bearing The stabilizer spring (X-750) was evaluated for the potential stresses due to the loads at the bolt hole location at the wedge end. The stresses were insignificantly small.

/.5 ksi Pm + Pb Stress

=

79.9 ksi Allowable Stress (1.5 Sm)

=

Bearing stress at the wedge contact (lower wedge was evaluated as the governing location) was assessed indirectly by calculating the required bearing area to resist the load. The wedge material was used as the basis (316L assumed) for the allowable bearing stress of 1.0 Sy. The required bearing area was determined to be 1.0 sq.in.. The actual bearing area available is substantially larger than 1.0 sq. in. Thus bearing is acceptable.

Upper Contact The upper contact ligament near the bolt hole location was evaluated for shear due to the bolt load and shown to be acceptable.

4. Conclusion The results of the structural analysis are summarized in Table 1. Bounding stresses are reported considering both the upper contact and the upper wedge.

Based on the calculations detailed above, the modification design meets the allowable requirements of the original shroud repair design specifications and is thus qualified for the remaining design life of the shroud repair.

NEo 087 (Rev Sf26/96)

r GE Nuclear En:rgy Engineering Calculation Sheet Sheet 7 of 8 Eutgect Organger/ Preparer; Date l

Nine Wie Por:it 1 Shroud Repair. Upper Lateral Stabilmer Modification Stress AnaWis A Mahadevan SW99 l

Number Verifeer:

Date l

B1341739 23 Sam Ranganath 5499 Legible e Reproducible e ReviewerNonfer e identifiab6e. Structure, System. Component e Purpose e Method e Unsts

5. References

1. Design Specification, Nine Mile Point 1 Shroud Repair,22A5583, Revision 2.

2. Field Deviation Disposition Request, FDDR No. EA-10040

3. Drawings:

a)

Upper Spring Assembly,112D6574 Revision 2 b)

Detail - Spring, Upper,112D6563, Revision 2 l

c)

Detail - Upper Contact,112D6577, Revision 1 d)

Upper Spring Clamp Details,237C7252, Revision 0 e)

Reactor Vessel,104R859, Resision 9 f)

Shroud, 706E231, Revision 4 g)

CEI Vessel General Arrangement,231-560-1, Revision 1 l

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NEo-087 (Rev 6/26/96)

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GE Nuclear En:rgy Engineering Calculation Sheet Sheet 8 of 8 6uesom C.- -_ _ w Done None Mme Posnt 1 F.hroud Regeer. Useer t.ateral StatNkrer Modthcstson strees ArwWe A Meneseven 5499 vertier.

Doie wmeer 813 01739 23 som stensensen Sees t gen. nepromuomie e novis.ortverwier. isenirnesiae struciare. system componeni. Purpose e Methoc e Unsts Table 1. Summary of Stresses L

Calculated

. AEowable

,"Amacarientie= W

;,N Ksi " ~Sm Ksi Connponente llMatesia'IP #NConditiesys $jjhqgightgyetN v, c.r:p Qsgggge Bolt XM-19 Level A Stress Intensity, P+Q 12.0 0.9 Sy =

(Normal) 29.8 Bolt li-ad Shear 4.5 0.6 Sm =

Stressfhw contact) 17.8 Shear Stre',s (due to 2.2 0.6 Sm =

friction force) 17.8 Sustamed Stress for 7.5 33.5 IGSCC Level B Stress Intensity, P+Q 13.7 0.9 Sy =

(Thermal Upset) 35.4 Bolt Head Shear 5.1 0.6 Sm =

Stress (Upper contact) 18.8 f

Shear Stress (due to 3.0 0.6 Sm =

friction force) 18.8

.1 r.wn:;- m ro~

3.zmynygemn wwn ~~m ;

Nut Plate Type 316L Level B Thread Shear Stress

< 3.5 0.6 Sy =

(Thennal Upset) 11.3

+ y,,

-ay;pya4p.;;y av., ng._., v>m,wis.e Upper Type 316L Level B Ligament Shear Stress 3.7 0.6 Sm =

Contact (Thermal Upset) due to bolt load 10.0

.zas.x mm a.m

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+ gvs.

c..w, Upper X-750 Level B Stress due to wedge 1.5 1.5 Sm =

Spring (Thermal Upset) friction force 79.9 t

NEo 067 (Rev 6/26/96) l

ENCLOSURE 4 l

SUMMARY

OF THE NIAGARA MOHAWK 10CFR50.59 SAFETY EVALUATION

.e A.

DESCRIPTION:

d.1 Background & Purpose The core shroud tie rod assemblies were designed and installed during RFO13 in 1995, to replace shroud welds H1 through H7 (UFSAR Figure XVI-12a). Each tie rod assembly includes an upper spring which is attached to an upper spring bracket by (4) 3/8" cap screws.

One of the cap screws was found to be broken during a visual inspection of the 166* location tie rod following re-installation of the upper spring assembly. 'Ihe apparent cause was identified to be intergranular stress corrosion cracking (IGSCC) in the alloy X-750 cap screw material in conjunction with large, sustained differential thermal expansion stress due to dissimilar materials at the interface; the upper spring and cap screws material is alloy X-750 and the upper spring bracket material is 300 series stainless steel. Additionally, sustained stresses caused by the original cap screw installation torque and by the potential for binding between the vessel wall and the upper spring contact points, cannot be ruled out as potential contributors to the high sustained stress condition that eventually. led to the IGSCC failure.

Results of the metallurgical evaluation confirmed IGSCC as the failure mechanism. Other tic rod dissimilar metal connections affected by this condition exist at a total of 24 locations [(4 tie I.

rod upper springs) x (4 cap screws at the upper end + 2 cap screws at the lower end of each upper spring)]. A modification to the upper spring assembly is designed so as to eliminate this problem at all 24 locations as described in Part A.2 below.

The original design of the tie rod assemblies and subsequent modifications to the tie rods made during RFO14, were performed as an alternative to American Society of Mechanical Engineers (ASME)Section XI as permitted by 10CFR50.55a(a)(3). Consequently, Nuclear Regulatory Commission (NRC) approval of the original design and subsequent modifications made during.

RFO14, were submitted to and approved by the NRC. The proposed upper spring

modification is a change to the installed tie rod assemblies as described in NMPC's previous

. NRC submittals and the NRC Safety Evaluation (SE) of the original tie rod design. The NRC y

~SE, dated March 31,1995, for the original tie rod design is mentioned in UFSAR Section t

i XVI.A.5.0. ' Therefore, the proposed upper spring modification affects the UFSAR by

reference to the NRC SE. BWRVIP-04, Section 3.1, Item 2 requires the licensee to prepare a SE in accordance with 10CFR50.59 to evaluate the changes made to the plant as described in 4

the UFSAR prior to submitting the repair to the NRC for review under 10CFR50.55a(a)(3)(i).

4 This_SE documents the NMPC review of the upper spring modification, in accordance with the

- provisions of 10CFR50.59, prior to NRC submittal.

L A.2 The upper spring modification involves the addition of a clamp fabricated from 300 series stainless steel material, secured to each existing upper spring bracket by two XM-19 bolts, installed perpendicular to the plane of maximum thermal expansion. Currently (4) 3/8" cap screws function to secure the upper spring bracket to the upper spring. The two XM-19 bolts replace the function of the (4) existing 3/8" cap screws. The XM-19 threaded bolts are staked i

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.g in place by Type 316/316L locking pins. The clamp functions to prevent the existing 3/8" cap nerews from becoming loose parts.

The stainless steel upper contacts, on the lower end of the upper springs, will also be modified by a similar method. A clamp fabricated from 300 series stainless steel material secured to each upper contact and to the upper spring by one XM-19 bolt installed perpendicular to the plane of maximum thermal expansion will be added. Currently, (2) 3/8" cap screws function to secure the upper contact to the upper spring. The XM-19 threaded bolt wi'11 replace the function of the (2) existing 3/8" cap screws. The portion of the existing 3/8" cap screw not locked in to the upper spring will be removed prior to re-installation of the upper spring. This assures that the part of the screw that could potentially be cracked is removed so that there is no possibility of a loose part. The function of the new clamp is to secure the upper contact to the upper spring. The XM-19 threaded bolt is staked in place by Type 316/316L locking pins.

Figures 1, 2, 3, and 4 of Enclosure 3 provide an illustration of the tie rod assemblies including the modification of the upper spring.

The upper springs will be removed from the reactor water and the modification will be performed dry in air on Reactor Building Elevation El. 340. An as low or reasonably achievable (ALARA) review of the activities has been performed and the appropriate

~

radiological controls have been established in the work documents. Since it is expected that there will be no fuel in the core when the upper spring is removed, this SE does not evaluate the temporary change to the UFSAR associated with removing the tie rod upper springs concurrent with having fuel in the core. A separate SE was approved addressing the safety significance associated with removing the upper springs with fuel in the vessel, should that plant condition become necessary. There are no other nuclear safety issues associated with the activities required to perform the proposed modification, hence none are evaluated herein.

A.3 S=It=hility of the Renair Desien to Address Appan ent Cause of Cap Screw Failure The proposed repair design addresses the three potential contributors to the high sustained stress condition that eventually failed the cap screw by IGSCC. The threeLpotential stress contributors are: 1) tensile stress due to thermal expansion of the dissimilar materials, 2) tensile stress due to the cap screw installation torque, and 3) shear stress due to the potential for binding on the RPV wall. These three potential stress contributors are addressed by the y,

proposed design as follows:

1 1)

Thermal tensile stress: The XM-19 bolt material's thermal coefficient of expansion is j

similar to that of the 316L stainless steel parts that it is adjoined to, which minimizes the thermal expansion tensile stress. The XM-19 bolt is installed perpendicular to the g.;

plan of maximum thermal expansion which minimizes the thermal expansion stress.

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- 2)

Installation torque stress: A larger bolt size combined with a maximum installation torque of 20 +/- 3 ft-lbs reduces the nominal installation torque stress by about a factor of six compared to the estimated nominal installation torque stress computed for the failed 3/8" cap screw.

3)

Binding shear stress: The potential for binding on the vessel wall could not be totally ruled out as a potential contributor to the sustained stress. As a preventive measure, the corners and leading top and bottom edges of the upper wedge and upper contact which contact the vessel wall will be rounded to promote sliding on the vessel wall.

A more detailed discussion of the suitability of the repair design is included in the analysis section below.

B.

ANALYSIS:

The applicable criteria and conformance for this analysis are as follows: The criteria are the

.same criteria as those used for the original tie rod repair SE, as applicable. The conformance sections discussed in this SE specifically address the proposed modification to the upper springs.

B.1 Renair DecIgn Life (Criteria):

The design life of all repair hardware will be the same as that for the shroud modification hardware originally installed in 1995.

B.1 Rennte Declen Life (Canformance):

Assuring an adequate design life for this hardware is mainly a material selection, design stress and process control effort. The selection of low carbon stainless steel for the plate and XM-19 for the bolts assures the best available materials for the nuclear reactor environment. These materials, when exposed to low sustained operating stresses, have been successfully used in the boiling water reactor environment and are resistant to IGSCC. Solution annealing and sensitization testing are imposed to guard against IGSCC. The same process chemical controls are imposed for the fabrication on site as were imposed in the shop in order to assure that contamination by heavy metal and chlorine or sulfur compounds will not occur. The design, configuration and materials have been selected to ensure that the sustained stresses due to the installation torque and due to differences in thermal expansion of the material are minimized to avoid the potential for stress corrosion cracking. Also, there is nothing in the new hardware or installation such as creep, fatigue or radiation degradation that reduces the design life of the shroud modification hardware.

B.2 Unner Snrine Functlanni Reauiremenic - UFSAR Section IV-B.7.1.9.2 (Criteria):

b The functional requirement of the upper spring described in the UFSAR is to provide lateral support to the shroud.

3

.y B.2 '

U5= "- ^- F-f H hayL- ---rf = - UFS AR erS= IV.B 7.1.9.2 (Confannanceh The new plate and bolts replace the function of the existing 3/8" cap screws which was to attach the upper wedge and upper contact to each upper spring assembly. This change does not impact the design function of the upper spring to provide lateral support of the shroud.

B.3 Sinns Analysis (Crliariah The stress analysis shall conform to the same requirements, including material properties, as specified in the Design and Code Specification for the original tie rod repair design.

B.3 -

Stress Analysis (Confonnanceh The stress analysis used material properties, material allowable stresses, applicable design load combinations and nomenclature for stress intensities consistent with those used in the original tie rod stress analysis. Therefore, the modified upper spring assembly satisfies the design I

specifications for the original shroud modification hardware as specified in GENE 25A5583.

Details of the analysis are contained in the GE Nuclear Energy Stress Analysis. Table 1 below provides a summary of the resulting stresses. The stresses as shown in the Table are well

{

below the allowable stresses for each loading condition. Furthermore, the stress analysis demonstrates that the high sustained stress condition due to thermal expansion of dissimilar

. materials and the installation torque are significantly less than that of the original bolted joint design.

Table 1. Sununary of Stresses cu p 9assesif 3Cesense d Ph%

Ma seanss? ' AEowabis Snes

' MM99M@7 e W, WW y '

"D"

. N

, W i ?@3 4 OFMgKggl1W % k

%iW!ggl5 06 '

g Bok XM-19 level A(Normal)

Stress hdensity, 12.0 0.9 Sy=29.8 P+Q r.

1 Bok Heed Shear 4.5 0.6 Sm=17.8 Stress (Upper Condact)

Shear Stress (due 2.2 0.6 Sm= 17.8 to friction force)

Sustamed Stress 7.5 33.5 for 1GSCC level B (Thermal Stress Intensity, 13.7 0.9 Sy=35.4 Upset)

P+Q Bolt Head Shear 5.1 0.6 Sm=18.8 Stress (Upper Contact) 4 x

r g

Cassponent:

.Matersal 3.

Condition Classification Calculated Stress -

'Allonie Stress 7

Siwar Strets (due 3.0 0.6 Sm = 18.8 i

to friction force Nut Plate Type 316L Level B (Thermal Thread Shear

< 3.5 0.6 Sy= ll 3 Upset)

Stress Upper Contact Type 316L Level B (Thermal Ligament Simar 3.7 0.6 Sm= 10.0 Upset)

Stress due to bolt load Upper Spring X 750 Level B (Ihermal Stress due to 1.5 1.5 Sm=79.9 Upset) wedge friction force B.4 Radiation Effects on Rennte Decion (Criteria):

The design of the repair shall account for the affects of irradiation relaxation utilizing end-of-life fluence on the materials.

B.4 Radiation Effects on Repair Design (Conforrnance):

The XM-19 bolts are prevented from backing out of their threaded holes by interference fit pins that stake the bolts. Therefore, if end oflife irradiation relaxation of the bolt fabrication installation torque were postulated to occur, it would have no effect on the capability of the new bolt to maintain the bolted joint intact for the design life of the repair.

B.5 Thermal Cycles (Crheria):

The repair hardware shall consider the effects of thermal cycles for the remaining life of the plant. Analysis shall use original plant RPV thermal cycle diagrams. The design shall assume a number of thermal cycles equal to or greater than the number assumed in the original RPV design. Alternatively, thermal cycles defined by actual plant operating data may be employed if technically justified. Using this thermal cycle information, repair components and the repaired shroud shall be evaluated for fatigue loading along with any other design vibratory

loads, i

B.5 Therrnal Cycles (Conformance):

An analysis has evaluated the modified upper spring assemblies for their capability to withstand loading conditions due to thermal differential vertical displacements between the RPV and the upper spring. The analysis considered the loads applied to the new hardware during reactor heat-ups and cool downs due to the radial upper spring force combined with friction on the RPV wall between the RPV wall and the upper wedge / contact. The analysis 5

p

. t i4-concluded that the allowable number of thermal fatigue cycles (for plant heat-ups and cool downs) was much larger than the actual number of thermal cycles assumed in the analysis.

Thus, fatigue is not a concern for the repair design.

B.6 Potentini for Binding of Upper Spring Contacic on RPV Wall (Criterlah The potential for binding between the upper spring RPV wall contact points shall be addressed, i

B.6 Potential for Blnding of Unner Spring Cantacts on RPV Wall (Conrormance)'

An assessment was performed to determine whether binding on the vessel wall at the upper contact and wedge could have contributed to the cracking of the cap screw. Some of the considerations for the assessment are summarized below.

i The examination of the failed surface showed no evidence of plasticity that would be expected if the relative expansion of the shroud relative to the vessel were taken up by the cap screw.

i The stiffness of the upper :pring is relatively low (24 kips per inch) and any surface irregularities can be accommodated by compression of the upper spring. For example, if there is a radial interference of 0.1 inch on the vessel clad (a conservative assumption since the nominal thickness of the clad is likely to be 0.25 inch), the additional lateral force would be 2400 lb. Such a force can be accommodated by the lateral deflection of the spring. It is y

therefore expected that the upper wedge and contact will ride over any surface irregularities.

'If one assumes total binding to occur, the relative displacement that could occur between the wedge and the spring (i.e., shear displacement at the interface) is estimated to be 0.2 inch.

Assuming that the shear occurs over the 0.36 inch segment, the shear strain could be as high as 0.2/0.36 = 55%. This would lead to gross yielding. Even if the binding is partial, the

,^

associated strains would still be large. Such strains would have led to visible bending and plastic flow, something that is not seen in the failed cap screw.

The RPV cladding appeared to be gouged (metal deformation) or pushed up at the upper spring upper contact location of the 166* tie rod indicating that some sliding occurred.

Based on the above, it is reasonable to conclude that significant binding did not occur and could not have contributed to the observed failure. As a preventive measure, the upper contact and upper wedge leading edges are being rounded as part of the current repair. This will i

further reduce any possibility of binding.

h Available bearing area on the upper contact / wedge after rounding of the edges was evaluated

-and determined to be acceptable in the stress analysis.

6

'a B.7 I-Pads C-Marmitan (Criteria):

Repair hardware mechanical components shall be designed to minimize the potential for loose parts inside the vessel. The design repair shall use mechanical locking methods for threaded connections. All parts shall be captured and held in place by a method that will last for the design life of the repair.

B.7 I-Pads Cancidarmitan (Conformance):

The modified upper springs have been designed to minimize the potential for loose parts inside the vessel. The addition of the clamps are intended to prevent the existing 3/8" cap screws from becoming loose parts. The threaded bolts are captured and held in place by interference fit locking pins that stake the bolts. The locking pins are prevented from backing out of their installed holes by reducing the diameter of the hole edge after installation of the pins.

B.8 Crevices (Criteria):

('

The repair design shall be reviewed for crevices to assure that criteria for crevices to be immune to stress corrosion cracking acceleration are satisfied.

B.8 Crevices (Confor==nce):

The repair design has considered crevices and their impact on stress corrosion cracking by using materials which are highly resistant to IGSCC. The material's IGSCC resistance is verified by testing per the requirements of the fabrication specifications. Requirements for cold work control of machined threads are included in the fabrication specification.

B.9 Materials (Criteria):

i All materials used shall be in conformance with the original tie rod repair material specification previously approved by the NRC for the NMP1 shroud repair with the exception of the XM-19 bolts. XM-19 shall be in accordance with BWRVIP-02 material requirements or j

other NRC approved criteria.

B.9 - MaterInle (Conformance):

i Material requirements for the modified upper springs are essentially the same as the requirements of the original shroud repair hardware fabrication specification. The XM-19

, fabrication requirements are in conformance with the BWRVIP-02 requirements or other NRC j

U approved criteria. The XM-19 specified requirements for NMP1 are consistent with other XM-19 requirements used by GE Nuclear Energy for other tie rod designs that have been approved by the NRC. Either XM-19 or X-750 could have been used for the bolts. XM-19 was selected i

because of material availability and because it's coefficient of thermal expansion is similar to that of 300 series stainless steel.

i 7

E

ry W

s C.-

CONCLUSION:

ihe N PC SE has evaluated the modification to the tie rod upper spring assemblies that will restore them to their intended design. The modification includes the addition of bolted clamps which will perform the function of the failed 3/8" cap screws. This review demonstrates that the modification can be installed (1) without an increase in the probability or consequences of t

an accident or malfunction previously evaluated, (2) without creating the possibility of an -

accident or malfunction of a new or different kind from any previously evaluated, (3) and without reducing the margin of safety in the bases of a Technical Specification. Therefore, the tie rod upper spring modification does not involve an unreviewed safety question.

I P'

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