ML19345A298
| ML19345A298 | |
| Person / Time | |
|---|---|
| Site: | Yankee Rowe |
| Issue date: | 03/30/1973 |
| From: | Burger A US ATOMIC ENERGY COMMISSION (AEC) |
| To: | |
| Shared Package | |
| ML19345A297 | List: |
| References | |
| NUDOCS 8011180632 | |
| Download: ML19345A298 (6) | |
Text
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UNITED STATES
!Y-ATOMIC ENERGY COMMISSION b D d.*
I w Asm uoros, o.c. : 2os4s (A
%n e Farch 30,1973 Files (Docket No. 50-29)
DEU:
g, Chief, ORB #1, L-SAFEfl EEVIEi 0F PEOPOSED CHANGE NO.106 REQUESTING AUTEORIZATION FOR OPE:iATE.G TEE YAMKEE REACTOR Wl'IH A NEi LOWER SFEDUD IUEE ASSMIX Ey letter dated February 5,1973, and supplemental informatica stimitted by letter dated March 16, 1973, Yankee requested authorization for the installation of a new lower shroud tube asse21y which is a part of the guidance system for the centrol rods.
The criginal shroud tube assed ly, as described in the S1fety Analysis-Repcrt, includes 32 shroud tubes. Each tube has an cpening at the lcwer end to allcw coolant flow through the tube. The guide biccks at the upper end of the tube serves for the alignment of the cmciform centrol rod within slotted cpenings. One shroud tube each is used in this mnner fcr the 24 control reds. The mmaining eight shroud tubes accenrodate the extension m2ers of the eight fixed cruciform shim reds.
These cenpenents occupy the water slots within the ccre for reducing flux peaking. At their 1cwer ends the shroud tubes am bolted to a welded tie bar.asse-bly for spacing and rigidity. At their upper ends each shroud tube flange is directly bolted to the lcwer plate of the core suppcrt plate. A crirp locking device is provided for each standard stainless steel bolt used in the shroud tube assembly. These devices are cri: ped in gr: oves inside the shroud tube flanges, and the ti bar asse217 m2ers and into indents in the bolt heads for preventing bolts from working loose and drcpping cut in the event the bolt cla@ing preload is lost. Ecwever, the original shroud tube assedly does not include other significant features for controlling lateral displacemnt of asse2Ly cenpenents by-vibratory forces that could occur if the bolt cla: ping prelcad were lost.
- The Yankee reactor has been cperated safely with the original shroud j
tube assed ly which performed reliably the intended ftmetien of guidirg the control rode sver a period of 12 years. After resurption of cpem-tion with Core X in May 1972 isolated difficulties were experienced with two centrol rods. These msulted in raquirements for cperating the reactor at a lower pcwer level (83% of full pcwer) with more restric-tive contml rod insertion limits and with additional control rod surveil-lance until the reacter was shut dcwn in October 20, 1972, for replace-mnt of the centrol rods.
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Files-March 30, 1973
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a Routine inspecticns of a' shim rod and control rod No. 19 during this
' scheduled maintenance outage -led to the observation of foreign objects
-on the core support plate. - Subsequent examination of the reactor inter-t mis by the use of an undeIvater TV camera identified the foreign objects to be shmud tube tie bolts. These examinations also revealed that'
- two centml rod shrcud tubes' were displaced.. Independent detailed inspec-tien programs were carried out by Yankee and Westinghcuse. The results of their inspections were sumarized initially in Yankee's letter dated
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Deceder 14, 1972.. Altogethe ; seven tie celts were found drcpped cut and five tie bolts had low preload. The locking devices associated with these. bolts were found either dropped cut or still in place. In additien, one bolt was also found dropped cut and another ene partially withdrawn at the bottcm connecticns of one' shroud tube to the tie bar assedly.
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Because of the bolting failures identified in the shroud tube connecticns and in view of the igortance of the shroud tube assembly integrity for l
the continued functicning of the centml mds, Yankee proposes to replace the original shmud tube assembly described above in this safety evaluation.
The new shroud tube assembly, designed and fabricated by Westinghouse, incorpcrates immved design features and supericr bolting systems and meets current fabrication stancards and quality assurance requirements.
ucwever, the basic functior'al design of the shroud tube assembly as part of the contml md guide s;, tem remains the same. The new asse2 1y is also made of stainless steel and includes 24 shroud tubes cc@ared with 32 shroud tubes in the original asse d ly. The eight shrouds that acccm-modated the statierer/ shim md extensions are no lenger required and have therefore not been incorporated in the new assembly. New similar i
f'.xed shim rods without extensicn meders.will be installed that will be positioned and suppcrted by the new asse2Ly tcp plate. The new shrcud tubes are similar to the existing ones, except that they have tumed diameters at the top that engage in ecunterberes in the new tcp plate.
At the bottom ccnnections the. shroud tubes engage in counterbcres in the new-tie plate. These new features are significant for centrolling lateral displacerent of a shroud tube in the event the preload en its bolted connections is lost. The new lower tie plate-is a precisicn anchined integral plate of igroved'censtruction, whereas the original tie plate is a welded bar assembly. The top support plate will be fabricated in j
4 sections to allow clearance for the existing radial support arms of -
the lower core suppcrt plate. The new tcp plate has two alignment-sleeves in each quadrant. This new construction allows a cegact pre-assembly of the shroud tubes into e ' unit. Wereas the upper ends of the shroud tubes in the original asse...'y were directly bolted to the core 3
support plate, the new unitized shroud tube assedly will-be keyed to f
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Yarch 30,1973 1
the core support plate with the aligrzent sl: eves before being bolted to it. For ccnnecting the shroud assedly conpenents to each other and i
the entire assembly to the 1cwer core support plate, high strength bolts are used in the new assembly whereas standard stainless steel bolts i
were used in the origbal assembly. TM individual bolts are larger in size and the clanping preloads in the new assen:bly are increased fmm 7,400 lbs to 32,000 lbs in the upper shroud tube connections and frem -3,700 lbs to 36,000 lbs in the lower shroud. tubes connections.
The significant increases in the u rgins of the preloads in additicn j
to the inpmved 1ccking devices used en the assembly bolts provide l
a high degme of ccnfidence that the integ"ity of the bolted ccnnec-i tien in the new Ic/ der shroud tube assecbly will be mintained.
In the supplemntal informtion, Yankee has sumari::ed the scope and results of the detailed inspection prog"am follcwing initial indica-tiens of the bolting failures in the icwer shroud tube assembly.
Ccrple*,e visual inspections by the use of binoculars, unde-aater TV i
camras, and torcscopes included the lcwer intemals, significant l
areas of the upper internals, and areas inside the reactor vessel.
Nasurerents perferred by the use of specN'y designed. tools and gaging devices included torque checks en every other lcwer core support plate to core barrel connecting bolt. 'lhese masuremnts confirred that these inportant lead carrying bolted ccnnecticns remined tight. We ccnclude that this inspection progaam has identified the extent of the bolting failures in the 1:wer shroud tube assembly and it has ccnfi-ed that the other internal corpenents have not experienced significant structural degaadaticn. In addition, during this inspection prcgram an irpressicn was mde of the existing cladding defect inside the reacter 1
vessel. A check of reasurements of this 1 pression does not indicate
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reasureable changes from reasurenents made in 1965 and 1968 inspecticns.
i All dropped out bolts and locking devices were mtrieved from the core l
support plate and tM reactor vessel bottcm, except for one bolt.
Efforts will be continued to recover this missing bolt. A large foreign cbject, identified as part of en original low flux speciren holder was also retrieved frcm the underside of the lcwer core support plate.
Yankee has made an evaluation of the bolting failures in the lower shmud tube assembly. A sumTr/ report en their evaluation is contained in one of the supplements. Each of the significant potential rechanisms that could have centributed to the identified boltirg failures was examined. TMse include the effects of inprcper tightening of bolts and excessive fit up leads during the initial intemals assembly, pos-sible damge sustained during previous handling of t% internals i:
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Files Farch 30, 1973 package, steady and esaillating hyuraulic forces including the effects of 3-loop mther than tw mrral 4-locp cperation, colt clanping pre-load relaxation due to themal and radiation induced effects, and the reacticn leads between the shroud tuces and the icwer core support plate. Yariee concludes, based on the evidence frcm the inspection pmgram and exardnaticn of the potential failure moces, that flow induced vibraticns actirs en a shroud tube that has loose upper flange ccnnecting bolts cculd cause significat wear of the threads and locking devices, resulting in the eventual less of function of these connecting devices. However, the results of Yarlee's calculaticns of the hydraulic forces indicate that the driving forces due to flow induced vibraticns are too smll to cause the shroud tube belts to loosen. 'Iherefore, it is concluded that a precondition of locse bolts n:ust have existed.
Althct'sh the significant mchanism that could have resulted in the necessary precendition for the vibratory ction of the shrcud tubes have bean identified and examined, Yarkee cannot detemine the degree to which each of these mchanisms centributed to the preconditicn of loose bolts. Eased cn our review of the msults frcm the detailed inspecticn progrars, we agree with Yarkee's assessrent that the failures in the Icwer shroud tube asse d ly are local in nature, that the causes are mrgral, and that they occurred essentially for reascns su:rarized in Yarlee's failure evaluaticn. We are reinforced in this conclusion by the facts that the other bolted ccnnecticns in the shroud tubes were fcund to be tight upcn disassedly after 12 years of expcsure to flow induced vibratiens and that inspecticns confirmed that the other inter-als corscnents have not experienced signifi 'nt degradaticn du"ing this period of cperaticn.
We have reviewed the mchanical and therml and hydraulic evaluaticn of the new shroud tube assedly that are sunrnrized in one of the suppb-nents submitted by Yarlee with Proposed Change No.106. As indicated pmviously in this nfety evaluation, the basic functicnal design cf th'.s assed ly remains unchanged, except that eight shrcud tubes that accennedated the fixed shirs in the original assembly have been eliminated in the new shrcud tube assedly. lhe eight fixed shta mds have been replaced by units with shorter extensions that will resc on top of the tcp plate of the now assembly. The holes in these locations am being utilized for bolting the assedly tcp plate to the lower core suppcrt plate. Significant 1 Drovecents have been factored into the design of the new shrcud t tbe assembly based on the results of the inspections and the failure evaluaticn. As described before, these include ccunterberes in the assembly top plate and the lower tie plate for accurate mg:munt or the guide tubes and for centrolling lateral noticn of a guide tube in the event the clarping prelcad en its ccnnecting bolts is reduced;
Files 72rch 30,1973 superior bolts and bolt locking devices and incmased clarping preloads. In addition, the new tcp plate in the replacemnt assertly will provide further stiffening of the core support plate that will reduce the bending mcrents in the shroud tube flange and consequently lessen the fcrees acting en the ccnnecting bolts. The entire weight of the new shmud tube assembly is about 500 lbs less than the weight of the crigiral asserbly (8000 lbs). Yankee has calculated the stresses in the rest 151 ting section of a shroud tube (1: mediately under the upper flange of the tube) for the steady state, full ficw condition including centrol rod scram. The stresses en the bolts and assembly ccrpenents have also been calculated. Results of the calculations have been repcrted in one of the supplements to Prcpesed CPange "o.106 and they ccnfirm acceptable rargins in the stress values. YarAee has also evaluated tM effect of cbstructing ficw through the holes that are being vtilized for attaching the new assembly to the ccre support plate in places where the eight shrcud tubes were ellr:irated. Cal-culaticns s1cw that this msults in an insignificant increase in the nnxim:n aT in the 1cwer plate of the core support plate in tha order of 3 F, and no change in the raximza A T in the upper plate of the core su;; port plate. Rercval of the eight shroud tubes :culd also have an effect en the core inlet f1cw distribution. Experience from hydraulic redel resting for the Ccnrecticut Yarlee reacter has shcwn trat this effect will not be significant. We have ccncluded that Yarkee Pas censidered adequately the rechanical end thermal and hydraulic effects of the replacerent shroud tube asserbly and that the raisins are acceptable.
Yarkee has specified the guides, standards, and ccdes trat Pave been follcwed in the design and fabricaticn of the new shmud tube assembly, includirJ;; the quality assurance requirements in 10 CFR Part 50, Appendix 3, and Safety Guide 31. The application of these guides, standards, and ecdes to the r= placement shroud tube assembly are acceptable as specified in the supplerent to Propcsed CPange No.106 Yarkee has also outlined prog ams for precperaticral inspecticn and postcperaticral' surveillance of the replacerent shroud tube asserbly.
As indicated before, the guide tubes are part of the guidance system for the control rods. One of the causes of failure of a centrol rod to perfonn the intended functicn wculd be ancrnlies associated with the guide tube. Therefore, performance of the centrol rods within the specified limit is an drportant indication of the integ"ity of the guide tube asserbly. The pmg; ram of control rod surveillance cutlined in a supplerent of Pmpcsed Change No.106 will also provide surveillance
Files March 30, 1973 of the. integrity of the shmud tube assembly. This program requim s centrol rod drop testirg with the plant in the hot standby condition and all reacter coolant ptzps operating at increasing time intervals and centinuing every 12 weeks thereafter. We have previcusly reviewed this program in ccnnection with the installation of the new centrol rodsfor the Yankee reacter and it was authorized in Change No.104.
In addition, during each refueling cutage, Yankee will perform a pmgmm of inspection cn at least cne shroud tube in each quadrant of the asse'rbly. We have concluded that the precperaticnal and post-cperational inspection and surveillance propra are acceptable.
On the basis of our review of the information submitted by Yankee, we have ccncluded tnat operaticn of the reactor with the mplacement shroud tube assembly does not invulve significant incards censideraticns not described or inplicit in the Safety Analysis Report and there is reascnable assurance that the health and safety of the public will not be endangered. AcccMh. gly, Change No. 106 can be authcrized.
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7 A. Burger Operating Reactors Erench #1 Directorate cf Licensiy.