ML20085G208
| ML20085G208 | |
| Person / Time | |
|---|---|
| Site: | Saxton File:GPU Nuclear icon.png |
| Issue date: | 04/01/1969 |
| From: | Neidig R SAXTON NUCLEAR EXPERIMENTAL CORP. |
| To: | |
| Shared Package | |
| ML20083L048 | List:
|
| References | |
| FOIA-91-17 NUDOCS 9110240187 | |
| Download: ML20085G208 (1) | |
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- 1. In cupport of C.mcc Rercaest Ib. 32, Applicant here'ay cub:.its a repo.-t entitled, "SU:;2.RY FZ202T 0:: BUC::LI:!3 Ol' SA::TUi: CORE II FJEL ASS.~:BLI53 /J.") PET:TIO:! 0F EUCl:LI::3 L CO?.E III".
SAXTO : .!FJC2:AR D:P3.;I31:TAL CORPORAT10:1 gj /s/ R. E. lieldic Prc;1 dent (S E A L)
Attest:
/s/ lt . B. Heist Secrctcr/ .
1st da: of April, 1969.
Saorn cr.d subscribed to before me this l
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! (S E A L) l .
/s/ Charles J. Ausel
!!otary Pe.blic Muhlenberg Township, Berks County '
My Comiasion Expires October IL,1970 1
1 9110240187 910424 PDR FOIA -
DEKOK91-17 PDR 5
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di v l SLMfARY REPOnf ON EUCKLING 0F SAXTON CORE II FI'FL ASSEMELIES AND PREVENTION OT . BUCKLING IN CORE III I
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SUMMARY
Buckling of the cans on some Saxton Core II assemblies was observed after Core II operation. The extent of the buckling of the Core II cans and the design modifications used to prevent buckling in Core III are herein reported.
The buckling of the central plutonium assemblies was caused by frictional Jorces between the grids and fuel rods arising from differential thermal expansion between the stainless steel acaembly can and Zircaloy fuel cladding, The buckling of the peripheral uranium dioxide fueled assemblies was '
caused by thermal gradients across the assembly and was limited to t.ose assemblies with the Core al type grid design.
The major modifications to the loose lattice assemblies to prevent buckling during Core III , operation consist of: (a) reduction of the grid friction loads through resetting of grid springs; and, (b) stiffening of the can structure through the use of full length clips between can halves and replacement of six Zircaloy water tubes by stainless steel water tubes with angle braces welded between the tubes and cans.
To prevent buckling in the load follow assemblies during Core III operation, the assemblies have been modified by reducing the grid friction loads through resetting of grid springs and stiffening of the can structure through: (a) replacement of fuel rods in two corner locations by square stainless steel bars with angle braces welded between the bars and cans; and, (b) angles welded to tne inside of the can between fuel rods on the long sides of the can.
None of the buckled assemblics will be reused in Core III.
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1.0 SUgyARY,0{, BUCKLING 11 Sers_II.plutenium gssemb!!eg Bnckling was observed in eight of the nine central plutonium as s emblie s . The buckling appeared to be of a random nature with no apparene pattern or consistency. However, the four corner assemblies of the square pattern formed by the central ninc assemblies appeared, in ganeral, to have the worst buckling.
The maximum lateral deficction of the buckles were estimated by visual inspection to be 0.06 to 0.08 inches.
The conter span between the second and third grids of the plutonium assemblies experienced the worst buckling with the greatest f requency, the frequency and severity of buckling decreasing towards the end of the assembly. The direction of the buckling (toward or away from the fuel rods) appeared to be completely random and the severity of buckles independent of direction.
Rub matks, which wire observed on several assemblies, could be l
attributed to hcndling or contact with spacer bars in the spent l fuel cask during shipment. However, in at least one case it is definitely concluded, based on the appearance of the marks, that 1 the rubbing occurred in the core and resulted from interference ' t#'Qr with a control rod assembly. 5>
g4F The single plutonium assembly which did not -exhibit buckling i contained eighteen stainless steel clad rods. The effect of stainless steel clad fuel rods would be to reduce the friction load exerted by the Zircaloy rods and increase the effective strength of the c:n (the stainless rods being put into compression as well as the can during dif ferential thermal expansion),
t l The one plutonium assembly which contained eight stainless steel clad rods was found to have only minor buckling. By analysis, this number of stainless rods is insufficient to prevent buckling of the assembly but the reinforcing effect evidently did reduce the extent of buckling in this assembly.
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1.2 gore,II,UO ,2 Assemblies Buckling was observed on three of the seven Core II design 002 fuel-asserblies. In this case, however, tae buckling was minor in nature and generally was restricted to the upper spans on the sides of the assemblies facing the center of the core. No buckling was observed on any of the Core I desiEn assemblies used in Core II.
A detailed summary of the buckling is given in Table 1 and Figures 1 and 2 show some typical buckles. Figure 3 shows a core cross section indicating buckled assemblies.
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TABLE 1 j
Core Buckling Location Observed Serial No, M SS Clad UO3 Rods ID No 503-1-7 3F No 503-1-19 SS Clad UO2 Rods SD No 503-1-10 SS Clad UO2 Rods 1C Yes Minor 503-10-6 SS Clad UO2 Rods 2B No 503-10-2 SS Clad UO2 R ds 2T Yes Minor 503-10-3 SS Clad UOy Rods SS Clad UO2 Rods -4B No 503-10-4 SE No 503-10-5 SS Clad UO2 Rods 2C Yes 503-12-2 Zr Clad Pu02-UO2Rods 2D Yes 503-12-5 Zr Clad Pu02-UO2 Rods 2E Yes 503-12-4 Zr Clad Pu02-UO2 Rods 3C No 503-12-3 Zr/55 Clad Pu0 2-UO 2 Rods 3E Yes 503-12-6 Zr Clad Pu02-UO2 Rods Zr Clad Puo 2~UO2 Rods 4C Yes 503-12-7 4D Yes 503-12-1 Zr/SS Clad Pu02-UO2 Rods Zr Clad Pu0 2-UO 2 Rods 4E Yes 503-12-8 1E No SS Clad UO2 Rods 503-7-1 SS Clad.UO2 Rods 3B No 503-2-3 4F No 503-11-1 SS Clad UO2 Rods SS Clad UO2 Rods SC Yes Minor 503-11-2 Zr Clad Pu02-UO2 Rods 3D -Yes 503-13-1 l
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FIGURE 3: Saxton Core Cross-Section Showing Buckled Fuel Assembly Locatiens "B" l
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2.0 g6pjj,g{, ppg [L}yg The following possible loading methods were examined a; buckling modes g for the assemblies:
A. Externally applied loads on the assembly due to:
- 1. Shipping ar.d handling
- 2. Interference with reactor internals B. Loading generated internally to the assembly throught
- 1. Frictional effects during differential expansion
- 2. Temperature differentials across the assemblies 2.1 Externa}1y,A2pligd, Loads, Evidence of buckling due to these types of loading would have been '
the collapse of the fuel assembly.end spans between the nozzles and end grids. The required end loading would also have hai to have been of such magnitude that the top nozzlo hold down springs would have collapspd. Examination of the assemblies showed no evidence of either of these conditions.
In addition,, analysis showed that the conditions in the reactor during handling operations, which would be necessary to produce this magnitude of leading, could not be realistically predicted.
It was concluded, therefore, that the buckling did not result from externally applied loads.
2.2 Intg593y_gggg5333g_Leggs 2.2.1 ((u3931gg,331gg)}}gg (Zircaloy Cladding) Calculations show that the buckling of these assemblies occurred due to differential expansion on initial heatup. The buckling, however, would be of an ciastic nature at that point, disappearing on subsequent cool down, except for some small amount of permanen: set resulting from
relaxation due to irradiation. The large buckles developed by a ratchetting mechanism through a number of full temperature cycles of the core, the buckles growing by the additional permanent sec occurring with each cycle. A full temperature cycle is from cold shutdown, through hot operating temperature back to cold shutdown conditions. Examination of the reactor's thermal history, shows that only five such cycles had occurred prior to the mid-life detailed observation of three fuel assemblies. Estimations of the permanent set indicate that only small deformations would have been present at this stage; this is probably the reason that the buckling was not observed. Subsequent to this observation, eighteen additional cycles occurred which account for the large buckle,s observed at the end of life. 2.2.2 pg2_{yglgp_3ssgeblies (Stainless Steel Cladding) Although the UO2 assemblies which exhibited buckling were of the Core II design with six point contact grid support, the buckling in the assemblies is not attributed to , ! frictional loading. In these assemblies, both the assembly can and fuel cladding are stainless steel. Therefore, any frictional loading in the can caused by differential expansion between the rods and can at operating temperatures would be small and would result from tensile stresses in the can. In addition, buckling was only observed on the hot side of the cans and not randomly distributed around all sides as would be expected with axial friction loads. It appears, instead, that the observed buckling in these l assemblies resulted from a combination of thermal gradients across the assemblies and the resistance to bowing exerted by the rod bundles in the grids used in the Core II design. l 1
The six point contact support used in the Core II grids provide an effective bailt-in condition for the rods at each grid location and thus resist bowing of the assemblies through resttaining moments on the rods. If thermal gradients sufficient to produce bowing in an unrestrained condition were present in the Core II assemblies, the restraint offered by the grids could result in compressive buckling stresses on the hot face of the assemblies. This would not be the case with Core I design assemblies where the grids provide a four point support for the rods and little restraining moment. Examination of the core temperature distributions based on power dibtributions during Core II operation showed four assembly positions where thermal gradients would be sufficient a to cause buckling in Core II design as)emblies and one location which was marginal. Of the four locations, one was occupied by a Core I assembly which exhibited no buckling. Two of the remaining locations were occupied by Core II assemblies which did exhibit buckling. The Core II assembly occupying the fourth location showed no buckling. In this case, the actual average temperature of w coolant flowing through the assembly is in question. From , instrumenta* ion in the 3 x 3 test assembly (503-4-29) which was suspended in the 9 x 9 assembly at this locatica, coolant temperatures approximately 00'F below expected were indicated. Because of channeling effects through the 3 x 3 assembly, the indicated tempetsture would be basically the discharge temperature from the 3 x 3 assembly and would reflect the effect of deleted fuel rods in the assembly. However, the low temperatures in the 3 x 3 would also tend to reduce coolant temperatures in the 9 x 9 assembly and thus reduce temperature gradients across the assembly since these are directly related to coolant temperature. Although the temperature effects cannot be accurately predicted, it would appear that buckling did not occur because of reduced coolant temperatures. 4 -
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! , e i l l l The last of the three Core II assemblies which exhibited buckling was in the marginal location where the thermal l gradients were not sufficiently high to predict buckling. The buckling in this case, however, was very minor and localized and could possibly have resulted from a local ! weakness in t..e can (a thin ligament or out of flat condition). l l t l ( l l L 1 l l
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. e 30 0991f!S6I!95.9f_S93!_!!!.6!!F0! LIE!.19.fBEYSEI_!ESELIE9 The center fuel assemblies in Core III were of Core II design and contained Zircaloy water tubes and/or Zircaloy clad fuel. Therefore, they would experience high friction loads due to differential expansion between the stainless steel assembly can and the Zirealoy rods and would be expected to buckle. To prevent buckling during Core III operation, the Core II assembly design required modification.
There were two possible approaches to assembly modification to prevent the buckling:
- 1. Increase the stiffness of the can sufficiently to withstand imposed loads.
- 2. Decrease the friction load by reducing the normal force applied to the fuel rods by the grid springs.
A combination of both approaches was-used. The sp ' contact force was reduced to the minimum which would not risk fretting of the fuel rods. However, this minimum contact force would still be sufficient to cause
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buckling. Therefore, the can was also strengthened. 3.1 Loose, Lattice _ Assemblies The fuel assembly has been strengthened by replacing six Zircaloy g water tubes by six stainless steel ones and spot welding 0.028 inch L thick angles between the can and the tubes. The one inch long clips previously used to fasten the two halves of the enclosure i have been replaced by full length clips in the spans between grids. A cross-section of a repaired loose lattice assembly is shown in l I Figure.4. The grid springs have been reset to give a nominal 6.5 lbs contact l force. compared to the 15.5 lbs-force previously used. 'The combined effect of the changes produces a safety factor of 1.5 between friction forces generated'by the grido and the buckling strength of the cans. 5
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FIGURE 4: Cross-Section of Repaired Loose Lattice Assembly 12 -
3.2 Load,Pollow Assemblies , A slightly different method has been used for the load follow assemblies to obtain the required strength. A solid square
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stainless steel bar, with two angles's' pot velded between the bar and ttie can, is used in place of two stainless steel clad fuel rods in opposite corners of the asse. ably. A full length angle 0.05 inch thick has been spot velded to the inside of the enclosure skin at the center of each long span between grids. Full length angle clips are also used betweer the ends of enclosure halves as was done with the loose lattice assemblies. A cross-section of a repaired load follow assembly is shown in Figure S. The MAPI assembly to be used in the periphery of the core will be similarly treated. The same buckling strength f actor has been achieved for these assemblies as in the loose lattice.
- 3. 3 Peripheral,UO .2 Assemblies Because of the large thermal gradici.ts across these assemblies in Core III, on,1y Core I type will be used.
An examination of the thermal gradients across the assemblies in the whole core shows that, although no buckling will occur, the worst gradient will produce s bow approximately 0.015 inch over the length of the assembly. This will always be of an elastic nature and will cause no interference problems. 3.4 Thermal,Hydrgulic, Considerations, The taodifications used for both the loose lattice and load follow assemblies have not compromised the thermal-hydraulic performance in Core Ill operation. For both types of assemblies the minimum DNB ratio will not be below the current. limit (1.30) specified in the operating license for the Saxton reactor. l l
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- . t With the modifications discussed herein buckling is not anticipated in either the new or irradiated assemblies used in Core III.
! Assuming buckling did occur, however, up to 0.060 inch'inward deflection of te.e can surfaces can be tolerated without danger i of exceeding the minimum allowable DNBR.- Deflections of this magnitude in the outward direction could also be tolerated without problems, i.e., binding of control rods would not occur 4 l The fuel assemblies will be inspected at mid-life for any evidence of cu.. buckling before reinsertion for continued reactor operati u,. , In addition, the control rod scram times are normally checked every i six months as a check against gross buckling. 3.5 yy3} gay _6spe33s The modifications proposed will have no effect on the operation of l the load follow assemblies. However, the addition of stainless ! water tubes in the loose lattice assemblies will reduce the power output of these assemblies by approximately 3%. This may be l alleviated slightly_by increasing the nominal power output from-26 MWT to approximately 26.3 inCE without seriously affecting any I thermal hydraulic or nuclear margins. i 3.6 yealth,and_ Safety l l The modifications to Core III assemblies of Core II design will prevent buckling and present no hazard to the health and safety of the public. The assemblies will be tiven a detailed examination for buckling at mid core life. 15 - e ~ - - , - - , . ._ -
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