ML18344A265
| ML18344A265 | |
| Person / Time | |
|---|---|
| Site: | Palisades  |
| Issue date: | 08/16/1993 |
| From: | Consumers Power Co |
| To: | Office of Nuclear Reactor Regulation |
| References | |
| Download: ML18344A265 (38) | |
Text
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Consumers Power Company Palisades Plant Docket 50-255 RESPONSES TO RAI NON-PROPRIETARY COPY August 16, 1993 13 Pages
RESPONSES TO NRC'S REQUEST FOR ADDITIONAL INFORMATION ON PALISADES PLANT REGARDING RECENT FUEL FAILURE EVENT MECHANICAL DESIGN INFORMATION REQUEST:
- 1. Details of the inspections performed to date, including results and conclusions.
Plans for other inspections.
RESPONSE
Details of the inspections performed to date are includ~d as.
Results and some conclusions are included.
More conclusions are further summarized in other documents including the root cause analysis report and the summary of how the failed rod was removed from the bundle {Note: fuel assembly and bundle are used interchangeably throughout this letter). The last page of contains the planned inspections that have not yet been performed.
The summary of how the failed rod was removed from the bundle is contained in Attachment 8.
The root cause analysis report was discussed in summary with the NRC Staff during the August 3, 1993 telephone conference call. The root cause analysis report will be updated and forwarded to the NRC Staff following completion of the planned inspections.
INFORMATION REQUEST:
- 2.
Justification for the selection of the L-bundles as replacement bundles. Detailed L-bundle design and core configuration and its impact on core physics and thermal hydraulic performance.
RESPONSE
The selection criteria that were used for the I-Hafnium replacement fuel assemblies are listed in Attachment 2.
All J, K, and L fuel assemblies were evaluated against these criteria. Some fuel assemblies from each of the J, K, and L batches met all the required criteria. Batch L fuel assemblies were chosen over the J and K assemblies for the reasons detailed in the selection criteria.
1 The modified L-assembly rod loading pattern is shown on the attached fuel assembly rod load map, Attachment 3.
These modifications have been performed to the replacement L-assemblies to assure that they will not experience fuel rod failures under conditions that caused the Cycle 10 fuel as~embly, I-024, rod failure.
I 2
- a. Eight (8) stainless steel rods are replacing fueled rods in all 16 L assemblies in the assembly corner that will be located at the core shroud corner during Cycle 11 operation. Thus, fuel rod failures similar to that which occurred in assembly 1-024 during Cycle 10 will be avoided.
Also, should spacer fretting occur similar to what happened to I-024, the corner stainless steel rod will remain in place captured by spacers 1 through 5 and 10.
- b.
For the other three assembly corners, one stainless steel rod of 0.437 inch outer diameter (OD) has replaced the one 0.417 inch fueled rod in each corner. This modification was deemed prudent to address fuel rod fretting that was seen during inspections of four and five times burned fuel assemblies.
An additional stainless rod in each of the three corners, for a total of two stainless steel rods in each corner, was used to replace fueled rods to provide additional conservatism.
The two stainless steel rods in these corners act to place a symmetric force on the adjacent fuel rods through the lantern spring. The stainless steel rods with their larger OD tighten up all four rods in each corner since they share a common lantern spring. This is supported by data taken on withdrawal forces of surrounding rods both before and after the stainless steel rod replacements in these corners (see item c below). Additionally, the stainless steel rod will not fret during operation like a zircaloy cladded fuel rod.
Therefore, any subsequent loosening of the four rods in any one corner due to an initial fretting of the very corner rod will be avoided.
The greater stiffness of the stainless steel rods and also their projected minimal wear will reduce any vibration response amplitude and thereby reduce any mechanically or hydraulically transmitted motion to adjacent fuel rods.
- c.
The effect on the fueled rods that share a lantern spring with two larger diameter stainless steel rods has been evaluated by utilizing design documents, testing on archive spacers, as well as withdrawal force data collected during the recent L-bundle reconstitution effort. The primary effect is to displace the lantern spring towards the neighboring fueled rods providing an estimated spring force increase of 0.6 lbs.
The resulting nominal increase in adjacent cell spring forces will improve the fretting resistance of the adjacent fuel rods.
The resulting increased spring forces are well below the maximum beginning of life design spring force.
The Cycle 11 core configuration utilizing the 16 L-bundle replacements is shown in Attachment 4.
Several fuel assembly position changes have been made to the original planned Cycle 11 core design to keep peaking factors and reactor vessel fast neutron fluence within limits. Core physics and thermal hydraulic performance of the resulting core configuration are being addressed in the reload methodology by Consumers Power and Siemens Power Corporation and will be covered with the Cycle 11 core design 10CFRS0.59 evaluation.
3 INFORMATION REQUEST:
- 3. Grid spacer spring retention forces as a function of core residence or burnup.
RESPONSE
Rod withdrawal force data was obtained on three I, one H, one J, one K, and two L assemblies. All these fuel assemblies have the bi-metallic lantern spring spacer design.
I-024 contained the failed rod at EOC 10.
I-021 was chosen as it was observed to have a loose corner rod in a visual exam and I-021 was in a symmetric location (X19) to I-024 (819).
I-048 was chosen as it was in the 819 location in Cycle 9.
The H-031 assembly was in 819 during Cycle 8.
The J-021 and K-031 assemblies were examined as potential replacements for the I assemblies.
L-024 was located between I-024 and I-048 during Cycle
- 10.
L-054 was the highest burnup L assembly.
Rod withdrawal forces were measured with a load cell as selected rods were removed from the assemblies for visual and eddy current examination.
Forces per celJ were determined from the rod withdrawal force versus distance data traces. The cell force was derived from the difference in withdrawal force as the rod was removed from each spacer.
Rod drag forces for each spacer cell are provided by assembly in the attached Tables 1 through 8, in Attachment 5. They are also averaged by spacer, by rod, by assembly, and by peripheral versus internal rod locations.
Individual cell forces are highest at spacer 10 which is located in the top plenum region.
The lowest forces are seen toward the top of the active fuel length. Multiple zero force values on a rod are usually associated with locations of fretting wear.
On some rods zero force values are present without fretting.
Isolated zero force values may be caused by variability in the measurements.
Zero forces would not be expected to cause flow vibration wear except when they represent clearances or if they are at the rod ends where unbalanced vibration forces are expected.
The tables also show the location of fretting marks determined from the eddy current examinations.
Locations are shaded in gray for minor wear and in black for severe wear.
Severe wear was defined as a distinct eddy current indication at both axial sets of spacer dimples within the cell. Minor wear was defined as a single eddy current observation within a spacer.
The fretting observations were generally located within the assembly and not at the ends. This would tend to indicate that the fretting wear was not caused by fuel assembly induced pressure drop differences or crossflows.
The data from Tables 1 through 8 are plotted versus burnup (Figures 1 and 2) and versus* number of cycles of operation (Figure 3), Attachment
- 5.
The plotted data does not include forces from spacers with severe eddy current ind-ications. These data were not included in the plot
average because of the likely reduction in force due to wear of the spacer dimples.
Spring normal forces may be estimated by the relationship; D = µN, where, for the bi-metallic spacer with diagonal spring; D = rod withdrawal drag N = total normal force on rod = {l + v'2) F = 2. 414 F F = spring force
µ = coefficient of friction Tests of new and oxidized r9~~ in bi-metallic spac~r~_have shown friction coefficients of u;~::::::tt:n for new clad and [fII!IIJ for oxidized clad.
The nominal relatiofrshlp between the spring..... __ tci"rce and the rod withdrawal force in the irradiated assembly is therefore:
~nd µ 2. 414 F = Cii!t!i!i[tj::J ( 2. 414) F = Ct!ti.!Ii!i::J F F = [jjjj!!ij[i!i!i!i!iill] D
- 4 To compare performance of the spacers with design predictions and with other Siemans Power Corporation (SPC) experience, the data are normalized and plotted in Figure 4, Attachment 5 as percent relaxation.
The percent relaxation was determined from t~~_.fo 11 owing ca lcul at ion. A nominal beginning of life drag force of OI:It:IH pounds per cell was calculated from the average spring force of ************
Fmax~Fmi n = [:::::I:::::::::::::::1::::I:::] = [i:::::::::::m:::::::] lbs.
and the new clad coefficient of friction of Ulii!lilili::J.
The calculated dr..~g_..force is confirmed by reported average iri"s.e"ft ion drag forces of
[JI]})) lbs per ce 11.
Ce 11 rel ax at ion percentages are determined as ori"e****foinus the ratio of the measured to beginning of life (BOL) value.
The nominal drag force at 30 to 40 GWd/MTU is expected to relax to about 86% of the beginning of l if e v aJ.1,1.~_ 1 _ or about LiititilllD lbs, based on oxidized friction. A minimum of Ullltfl lbs drag ffrfce might be expected based on the minimum BOL sprlri*9*******force.
These projections are consistent with the Palisades H, I, J, K, and L data.
The variability in initial spring force of+/- 30% is already noted.
There is an additional potential variability in the 25% difference between oxidized and unoxidized friction.
The two sigma variability of the friction coefficient data is+/- 20%.
A total variability of 75%
about the nominal spring force projection may therefore be expected due to spring tolerances and friction coefficient uncertainty.
Variations of this magnitude are seen in the individual cell measurements.
The Palisades average force data show slightly less relaxation than observed in other SPC fuel.
The normal relaxation combined with the severe wear at particular rod and core locations, suggests that a fretting mechanism other than normal rod flow vibration is active in the fretted fuel assemblies.
THERMAL HYDRAULIC DESIGN INFORMATION REQUEST:
I. Localized axial flow and cross flow conditions as a result of replacement assemblies and the knowledge gained through root cause analyses.
RESPONSE
5 Fourteen oversized (0.437-inch O.D. versus 0.417-inch O.D. for a fuel rod) stainless steel rods will replace fuel rods in the sixteen replacement L assemblies.
The primary hydraulic impact of these rods will be to reduce the total flow in these L assemblies by reducing the flow area.
Because the resulting reduction in the total flow area of the core is insignificant, the flow through all the subchannels in the core without stainless steel rods on their boundaries will be essentially unaffected.
The flow in subchannels which contain one or more of the stainless steel rods will be less than that of the "intact" flow subchannel.
The two rod sizes will cause a change in the internal flow distribution within each of the L assemblies but will not have a significant effect on the inter-assembly cross flows between the L assemblies and the rest of the core.
The flow distribution within the L assemblies will develop in the region just downstream of the entrance to the assembly and will be consistent with the two different rod sizes and flow subchannels. There will be no significant intra-assembly cross flows within the L assemblies as a result of the two different rod sizes and this distribution will remain nearly unchanged axi~lly in the L assemblies.
With regard to DNB margins, the flow area affected by the replacement of fuel rods by stainless steel rods represents a very small fraction of the full flow area of the core and the impact will be almost undetectable.
Small though it may be, the core-wide hydraulic impact of the stainless steel rods will be positive. The reduction in flow area at the periphery of the core will force a minute increase in flow in the interior where the limiting assemblies reside. This increase in flow will result in a minute increase in the core pressure drop and a minute increase in the calculated MDNBR.
The DNB margin of the L assemblies themselves is not a concern because the power in the fuel rods in these assemblies is such that the L assemblies cannot become DNB limiting assemblies and the subchannel flows of the "intact"
- subchannels is slightly greater than those with the stainless steel rods.
Standard design fuel assemblies (Bi-metallic spacers) have a lower pressure drop for a given flow rate due to their smaller, less restrictive spacer grid design than the pressure drop expected for an assembly with an High Thermal Performance (HTP) design spacer.
A concern has been raised that with an HTP assembly installed adjacent
6 to a standard assembly, flow will tend to be channelled away from the HTP assembly and flow crosswise to the standard assembly.
This assembly crossflow could result in increased rod fretting, and axial loading beyond the design capability of the assembly.
The following paragraphs provide a qualitative discussion of the impact on localized flow conditions with an HTP design fuel assembly installed adjacent to a standard design fuel assembly in location B-19. SPC has determined that the flow resistance is 5% higher at each spacer grid in the HTP assembly design. Other flow losses along the spans between the spacers and through the upper and lower tie plates are the same as the standard design. Consequently, there will only be a localized flow differential in areas immediately adjacent to spacer grids.
The B-19 core location is in a core shroud corner with the other two sides facing a control rod gap.
Assuming that there are two HTP assemblies adjacent to a standard assembly in core location B-19, it is ~ossible that the flow will redistribute in the vicinity of the spacers. However, with the wide water gap associated with the control rod gap, the flow will preferentially redistribute into the gap (not the standard assembly) and there will be reduced crossflow into the standard assembly.
Crossflow induced fretting damage to fuel rods related to the design of the fuel assembly is usually worse at the inlet or the outlet of the fuel assembly.
The wear observed on fuel assemblies at Palisades is not close to either the inlet or the outlet. It is therefore concluded that the fuel rod fretting caused by cross flows related to fuel design is not indicated and, consequently, it is not conside~ed to be a root cause.
The root cause for the wear phenomenon appears to be a local condition outside of fuel design considerations.
MISSING FUEL RECOVERY NOTE:
The responses which follow do not address the missing fuel.
The efforts to locate and evaluate the impact of missing fuel are to be addressed at a later date.
INFORMATION REQUEST:
- 1. Accountability of the missing debris (pellets, cladding, spacer strip and spacer spring).
RESPONSE
A material accountability of assembly I-024 failed rod S-15 is attached. This accountability covers both present and missing material. The calculated weight of the material present shown on the attached accountability table agrees well with actual measurements made during the retrieval process. Material missing from assembly I-024 rod S-15 is summarized as:
Missing Cladding Missing Pellets Missing Insulator Disk 56 982 2
grams grams grams
In addition to the missing fuel rod material, spacer material is missing from the damaged corner of assembly I-024.
Most of the missing material are portions of the side plates in the area of the failed rod.
Some small amount of internal strip material is also missing.
One lantern spring from spacer 9 is also missing.
The most significant non-fuel debris that has not been accounted for is the missing half cladding. This 30 inches of half cladding may have been worn away to a large extent during Cycle 10 operation.
Additionally, this half cladding section may be in several pieces as opposed to one piece. A search for missing material has been performed in the core location where assembly I-024 resided, on the bottom of.the reactor vessel under the core support plate, on top of the core, and in the reactor cavity tilt pit. The missing half cladding was not seen.
INFORMATION REQUEST:
- 2. Disposition of the located debris.
RESPONSE
7 The fuel rod fragmented pieces have been placed into two long and one short storage baskets.
The two long storage baskets are located within a storage can stored in the spent fuel pool.
These two long storage baskets each contain some amount of fuel and can remain in the spent fuel pool for long term storage similar to spent fuel assembl1es.
The short storage basket contains a small piece of cladding and no fuel.
The radiation readings are relatively low enough to disposition this small cladding piece similar to other radioactive waste encountered during the refueling process. (For example, the filters used for vacuuming and cleaning the refueling cavity are typically near or above the radiation level of the small cladding piece.) The two small pieces of spacer side plate material that were located on the core shroud and the core plate have been retrieved using the normal vacuuming equipment.
The filters used for this vacuuming process are disposed of as radioactive waste.
It is possible that other missing spacer pieces and small cladding pieces were vacuumed up during the several tilt pit, reactor cavity floor, or core region vacuuming that took place during the refueling outage.
INFORMATION REQUEST:
- 3. Potential impact of any remaining debris on startup and operation.
RESPONSE
Some spacer and cladding material have not been found.
If this material still resides within the primary coolant system, then the potential exists for fuel rod fretting after the primary coolant pumps are returned to service.
Some of the impact will be mitigated by 136 of the 204 total core assemblies being debris resistant by design.
SPECIFIC QUESTIONS INFORMATION REQUEST:
8
- 1. The proposed bundle reconstitution method does not appear to be consistent with the Siemens Report ANF-90-082, Revision 1, wherein the replacement rods should have zircaloy cladding and the same dimensions as the fuel rods.
The staff is interested in the effect of the oversized rods on the potential fretting of the adjacent rods and the relaxation of spacer springs.
RESPONSE
Siemens prepared a generic report, ANF-90-082, entitled "Application of ANF Design Methodology, for Fuel Assembly Reconstitution," which summarized the application of NRC approved design methods to justify insertion of irradiated fuel assemblies, which have been "reconstituted" by replacing fuel rods with inert rods, into a reactor. This document is generic in that it describes and justifies the criteria required to qualify such a reconstituted assembly for continued irradiation. The reconstitution performed on the L assemblies for Cycle 11 is not completely consistent with that described in the generic report.
Specifically, the generic report identifies replacement rods as being inert rods, filled with zircaloy or stainless steel, or water rods.
These two types of rods were specified in the generic report as Siemens typically uses this type of replacement rod in the repair of fuel assemblies with failed fuel rods.
Fuel assemblies which have been reconstituted in accordance with the generic report can be used in any core location. The analytical methodology described in the generic report called for an evaluation of the impact of the reconstitution in the following areas:
Neutronics Power Distributions - local peaking, axial shapes, etc.
'Kinetic Parameters Control Rod Reactivity Core Monitoring Coefficients
- Thermal Hydraulics Hydraulic Compatibility Thermal Performance {DNBR)
Fuel Centerline Temperatures Rod Bow LOCA Rod Ejection Accident Mechanical Design Differential Rod Growth and Assembly Growth Assembly Liftoff Linear Heat Generation Limits
9 The Palisades Reload L assemblies are being reconstituted using fourteen solid stainless steel rods that are approximately twenty mils larger in diameter than a standard fuel rod (0.437-inches O.D. versus 0.417-inches 0.0.). The inconsistency with this reconstitution and
-l~d:!~::H~
the generic report identifies the evaluations required to support the use of a reconstituted fuel assembly (see above}.
The safety implications of the reconstitution of the Reload L assemblies have been evaluated in a safety evaluation which is specific for proposed operation of Cycle 11 and a revised Safety Analysis Report (SAR} was prepared.
The SAR includes explicit modeling of the fourteen replacement stainless steel rods. inserted into the sixteen L assemblies.
The principal impacts of the insertion of the stainless steel rods in the L assemblies are the changes which occur in the local power and flow distributions. These are discussed in Thermal Hydraulic Design, item 1, above.
The reconstituted L assemblies were also explicitly treated in the generation of the core monitoring coefficients. The impact_that these reconstituted Reload L assemblies will have on the monitoring of assembly power and power peaking is therefore accounted for by these coefficients.
The use of the [l!]!]!i!i!:::::::::::::::t::II!!jl]l::::::::::::I::Il!l!Il::::::::::J:::::I::i::I!I!ll for Pal i sades does not degrade the safety**m~irglri"s****af".. Pallsides***because of the planned location the reconstituted assembly in the core.
The inert rods are in the corners of very low (z20%) power assemblies located next to the core baffle.
DNB is not a concern in these very low power locations.
Flow maldistribution is also not a concern because the inert rods are located far from DNB limiting fuel assemblies.
The mechanical impact of inserting larger and stiffer stainless steel rods in the*fuel assembly has also been addressed for Cycle 11 operation. The stainless steel rods are about 0.2 lbs heavier than the fuel rods they replace due to their larger diameter.
This increased weight has a positive effect on assembly holddown.
Other mechanical impacts are addressed within various responses to other information requests herein.
As is noted in these responses, the presence of oversized stainless steel rods tends to make the L assemblies more rigid and less prone to vibration.
The potential for fretting of neighboring fuel rods by the stainless steel rod should thus be lower than in an L assembly without stainless steel rods.
CPCo has experience with the use of these particular rods, as they are being taken from a selection in the "H" reload neutron shield assemblies used during Cycle 8.
The use of the "H" assemblies in Cycle 8 was addressed as part of the SPC reload methodology similar to the analytical methodology described in the generic report.
In addition, stainless steel partial shield assemblies used in Cycle 10 (which will receive their second burnup in Cycle 11} have been addressed under the SPC reload methodology in a similar manner.
However, it is noted that the outside diameter of the stainless steel rods used in the "N" partial shield assemblies is the same as the fuel rod.
As a point of further clarification, the following discussion is presented to address the general subject of Generic Letter 90-02 and its supplement.
10 CFR 50.59 allows licensees to make changes to the plant without prior NRC approval if the changes: a) do not involve an unreviewed safety question and, b) do not involve a Technical Specification (TS) change.
10 Since the Palisades TS do not specify the number of fuel rods in a fuel assembly, replacement of fuel rods with non-fuel rods does not conflict with the Palisades TS.
Therefore, such replacements may be completed without prior NRC approval provided that the replacements do not involve an unreviewed safety question.
The NRC has issued a Generic Letter, GL 90-02, which discusses replacement of fuel rods with inert rods or with open water channels.
GL 90-02 provides guidance to licensees whose existing TS do not allow such replacement. This GL suggests changes that may be proposed to TS which would amend an explicit fuel assembly description, like that in the former Standard Technical Specifications (STS), to allow such replacement of fuel rods if the associated analyses have been completed using NRC approved methodology.
GL 90-02 is similar, in intent, to GL 88-16 which suggests proposing TS changes which relocate specific core operating limits from the TS to a Core Operating Limits Report and add the TS requirement that these limits be calculated in accordance with NRC approved methodology.
In contrast to the Palisades TS, the former STS contained an explicit description of a fuel assembly in section 5.3.
(See the former CE STS, NUREG 0212)
Since this description stated the number of fuel rods in a fuel assembly, fuel rods could not be replaced with non-fuel rods without an amendment to the TS and the associated NRC review.
GL 90-02 provides relaxed TS wording which would allow replacement of fuel rods with certain non-fuel rods or with open water channels without prior NRC approval 11 if justified by cycle-specific reload analyses using NRC-approved methodology."
The NRC issued Supplement 1 to GL 90~02 on July 31, 1992.
The stated purpose of that supplement is to "clarify the limitations on the application of currently NRC-approved analytical methods and to withdraw and replace the model technical specifications (TS) which were recommended by GL 90-02...
11 The supplement states, in part, "licensees are not required by this supplement, or by GL 90-02, to change their TS.
11 Since Palisades has not chosen to submit a TS change request based on GL 90-02 or the supplement, those sections of the supplement discussing TS changes which emulate the model TS of GL 90-02 and its supplement, and the interpretations of limits contained therein, do not apply to Palisades.
11 For the second part of this information request, reference Mechanical Design responses 2 and 3 above which covers the effect of the oversized rods on the potential fretting of the adjacent rods and the relaxation of spacer springs.
INFORMATION REQUEST:
- 2. Please provide the available spring retention data to the NRC or provide a schedule for that information.
RESPONSE
Data has been provided within this information package and was presented in the Mechanical Design response 3 above.
INFORMATION REQUEST:
- 3. The plan to install stainless steel rods in the L-bundles is not consistent with the inspection data from the I-21, I-24, and H-31 bundles.
In light of the above, your current plan to replace two fuel rods at each corner that is not facing the core shroud with stainless steel rods does not appear to be conservative.
Please provide justification for the stainless steel rod placement design.
RESPONSE
Consumers Power and Siemens have reviewed the data to the stainless steel rod replacement modifications and have concluded they are consistent. Reference response 2 above to the Mechanical Design section for detailed discussion on stainless steel rod placement design.
- The inspection data from I-21, I-24, I-48, and H-31 bundles shows up to 0.006, 0.012, 0.006, and 0.008 inches of fretting, respectively, for assembly corner rods not located at the core shroud corner.
The selected stainless steel rod placement design will eliminate rod fretting at each corner as a maximum potential result and will reduce fretting to less than that seen by I-21, I-24, I-48, and H-31 as a minimum potential result. Therefore, the use of 2 stainless steel rods at each corner that is not facing the shroud is justified by the potential results being within this acceptable minimum to maximum range.
INFORMATION REQUEST:
- 4. How will the use of stainless steel rods stop the fretting?
RESPONSE
Use of stainless steel rods will not stop the fretting of grid spacers as observed on I-24 assembly.
The stainless steel rod placement design is a remedial corrective action to cover use of the L-bundles in the core shroud corner locations for one cycle during Cycle 11.
For Cycle 11, any fretting of the spacers at the core shroud corners will potentially only affect adjacent stainless steel rods.
The stainless steel rod placement design has a stainless steel rod barrier
from guide bar to guide bar for the assembly corner that faces the core shroud corner.
INFORMATION REQUEST:
- 5. What is the impact of the increased core flow experienced after the steam generator replacement as related to flow-induced vibration and fretting?
RESPONSE
The impact of the increased core flow after the steam generator replacement on the flow-induced vibration and spacer fretting was evaluated.
With the increased core flow, the projected end of life restraint is greater than the projected vibration force.
Siemens' N.
Harbinson, letter to G. Goralski (CPCo) dated August 1, 1993 is attached, Attachment 7 which provides detailed discussion on the restraint force design and flow-induced vibration.
INFORMATION REQUEST:
12
- 6. Provide justification for rotating the bowed assemblies so that the bowed area is facing the control blade instead of the core shroud.
What assurance do you have that the bowed assembly will not impede the movement of the control blade? Provide the clearances between the assembly and the core shroud and between the assembly and the control blade as well as the estimated amount of assembly bow.
RESPONSE:.
Sixteen fuel assemblies from Reload Batch L have been selected for insertion into peripheral corner positions for Cycle 11.
All sixteen assemblies will be oriented with their burnup gradient away from the shroud in order to minimize the possibility of contact between these assemblies and the shroud.
Eight of these assemblies will be placed next to control blades and their burnup gradients will be towards the control blades. Since burnup gradient correlates positively to fluence gradients which correlate positively to fuel assembly bow, burnup gradients have been evaluated to determine acceptable assembly bow.
Expected burnup gradients at the end of Cycle 11 have been calculated for the L assemblies which will be used on the periphery.
The maximum projected gradient calculated was approximately 6,400 MWD/MTU.
This maximum was for an assembly which was not next to a control blade.
The maximum projected burnup gradient towards a control blade is about 4,600.MWD/MTU.
During operation of Palisades, it has been quite common to have burnup gradients in excess of 9,500 MWD/MTU not oriented towards a control blade and 6,000 MWD/MTU towards a control blade.
In some cases, the burnup gradients have exceeded 7,000 MWD/MTU towards a control blade.
Palisades has *not* experienced any difficulties related to the operation of the control blades or to contact between the fuel assemblies and the control blades as a result of such gradients.
Given the projected burnup gradients for the L assemblies, past operating history for Siemens Power Corporation fuel at Palisades bounds Cycle 11.
The assembly bow for the L assemblies in Cycle 11, based on the conservative nature of the burnup.gradients in the L assemblies, is also expected to be bounded by past operating history for Siemens fuel at Palisades.
Clearances between fuel assembly, control blade and shroud; Fuel Assembly Guide Bar to Shroud = 0.1365 inches Fuel Assembly Guide Bar to Control blade = 0.0925 inches
- Gap between assemblies is 0.365 inches with a nominal 0.180 inch wide control blade that inserts between the assemblies.
The maximum envelope for the width of the control blade over its entire length is 0.240 inches.
INFORMATION REQUEST:
13
- 7. Provide justifications for not conducting a metallurgical analysis on the failed fuel rod to determine or confirm the failure mechanism as well as the condition of the fuel pellets.
RESPONSE
The failed fuel rod fragments, assembly I-024, and the core location B-19 examinations all provide a consistent picture of how the bundle was damaged and how the rod was failed. A summary of the failure and damage mechanisms is attached. Additional dose and dollar costs to support the suggested metallurgical analysis can not be justified given the shroud/assembly interface problem that has been determined to exist. If the rod had just failed with no physical damage imposed from the shroud/assembly interface, such a metallurgical analysis would be warranted.
ATTACHMENT 1 Consumers Power Company Palisades Plant Docket 50-255 FAILED FUEL ROD INSPECTION RESULTS CONTAINS NON-PROPRIETARY INFORMATION August 16, 1993 10 Pages
1 1-024 FAILED ROD EVENT INSPECTION RESULTS
SUMMARY
A.
EXECUTIVE
SUMMARY
INSPECTION TYPE NUMBER OF NUMBER INSPECTIONS STILL PERFORMED PLANNED VISUALS OF FRAGMENTED FUEL ROD 7
0 PIECES REFUELING EQUIPMENT INSPECTIONS 2
0 RE-REVIEW OF PAST ASSEMBLY 5
0 VISUALS VARIOUS VISUALS ON FAILED 3
0 ASSEMBLY I-024 OTHER ASSEMBLY VISUAL 28 0
EXAMINATIONS FOR ROOT CAUSE ASSEMBLY VISUALS ON REPLACEMENT 16 0
BUNDLES INDIVIDUAL FUEL ROD EXAMINATIONS 10 0
(WITHDRAWAL FORCE, EDDY CURRENT ASSEMBLIES TESTING, AND VISUALS) 125 TOTAL RODS CORE SHROUD LOCATlON VISUALS 9
1 ASSEMBLY LENGTH MEASUREMENTS 3
0 UPPER TIE PLATE HOLE GAUGING 1
0
B.
CHRONOLOGICAL
SUMMARY
OF COMPLETED INSPECTIONS
- 1.
7-1-93 0930 INSPECTION OF UNKNOWN ITEM Unknown item inspected in south tilt pit using camera and known OD (outside diameter) plug to determine its OD.
RESULTS:
Item appears to be a piece of fuel rod, about 5 feet long.
- 2.
7-1-93 2100 TILT PIT AREA INSPECTION Accessible areas of the reactor cavity tilt pit were inspected using a camera to determine if other pieces existed.
RESULTS:
Four pieces found totaling about 11 feet; one had a lower end cap. Total length of one of the pieces could not be positively estimated, since it was not entirely visible.
- 3.
7-2-93 A-SHIFT INSPECTION OF LOWER END CAP Camera inspection of lower end cap was performed in an attempt to identify its serial number.
RESULTS:
Serial number was read and later that morning was identified to belong to assembly I-024 corner rod S-15.
- 4.
7-4-93 A-SHIFT DETAILED EXAM OF ROD PIECES BY SIEMENS 2
Rod pieces were moved to the south end of the tilt pit and a detailed examination was performed.
Only 3 pieces were located at this time.
Total length is still approximately 11 feet.
Fourth piece that was seen earlier was not found.
At this time it was postulated that a hose and shadows had been mistaken for a piece of fuel rod.
RESULTS:
The three pieces were characterized in detail.
Summary results are:
- a. One piece has a severe longitudinal split with an estimated 5 feet of fuel missing.
- b. Severe wear exists at spacer locations.
- c. Upper end cap not located.
- d. One 18 inch piece appears to contain fuel.
- e. No obvious fuel pellets seen on the floor.
- 5.
7-4-93 C-SHIFT TO 7-5-93 A-SHIFT FRAGMENTED ROD RECOVERY Siemens retrieved the rod pieces and placed them in storage baskets.
Visuals were made of the process.
RESULTS:
One end of the 4-1/2 foot piece verified visually to contain fuel.
The larger 5 foot piece appeared to be more than just split. Likely cladding material is missing from this piece.
The 4-1/2 foot piece fit well to the 18 inch piece.
- 6.
7-5-93 A-SHIFT REFUELING MACHINE HOIST BOX INSPECTION A camera inspection of the refueling machine hpist box was performed to check interior for damage or protrusions.
RESULTS:
Completed the camera inspection with no damage found.
- 7.
7-6-93 B-SHIFT TILT MACHINE CARRIER INSPECTION A camera inspection of the tilt machine carrier was performed to check interior for damage or protrusions.
RESULTS:
Completed the camera inspection with no damage found.
- 8.
7-6-93 1800 REVIEW OF EARLIER VISUAL EXAM OF ASSEMBLY 1-021 The video tape of assembly I-021 visual exam performed earlier in the refueling outage was reviewed in light of the damage to assembly l-024.
3 RESULTS:
The corner rod of assembly I-021 that was located next to the core shroud was observed to be loose at spacer number 7.
This is an equivalent position to the assembly I-024 failed rod.
- 9.
7-8-93 REVIEW OF CYCLE 8 VISUAL EXAMS OF I-021 AND I-024 ASSEMBLIES The video tape of I-021 and I-024 assemblies that was taken during cycle 8 to support the I-Hafnium installation project was reviewed.
RESULTS:
No problems were observed.
- 10. 7-12-93 REVIEW OF CYCLE 9 VISUAL EXAMS OF 1-021 AND 1-028 ASSEMBLIES The video tape of the last refueling outage visual exams of assembly I-021 and its E-W axis mirror assembly I-028 was received from Siemens and reviewed.
RESULTS:
No problems were observed.
~
- 11. 7-12-93 ASSEMBLY I-024 IN CORE INSPECTION The view of assembly I-024 from the top as it sat in the core after the UGS removal was taped.
4 RESULTS:
No observed damage or markings to note on the upper tie plate.
A piece of spacer side plate of about 1/4 inch width was located on the shroud.
{This piece was later vacuumed.)
- 12. 7-13-93 ASSEMBLY I-024 IN CORE INSPECTION PRIOR TO REMOVAL The view of assembly I-024 as it sat in the core prior to removal and the upper end piece that later fell from the bundle, as it was moved to the tilt machine, was taped.
RESULTS:
The observed damaged was documented in detail.
Summary results are:
- a. Severe spacer damage exists.
- c. The assembly was judged to be acceptable for transport to the spent fuel pool using the normal refueling equipment.
- b. The dropped piece was the upper end of the failed rod and accounts for the remaining length of the failed rod.
- 13. 7-13-93 DETAILED VISUAL EXAM OF ASSEMBLY 1-024 IN THE SPENT FUEL POOL A detailed visual of all four sides of assembly I-024 was performed.
RESULTS:
The observed damage was documented in detail.
Summary results are:
- a. Spacers 1-5 appear recently torn.
- b. Spacers 6-9 have corner material missing.
These spacers show a failure mechanism apparently different than the first five spacers, (as determined by oxide, rub marks, and failure locations).
- c. Rods adjacent to the failed position show fretting within span 9, suggesting the failed rod was severed partially or completely in this span during operation.
- d. A fuel plume was observed starting near the top of span 9.
- 14. 7-13-93 VISUAL EXAM AND RETRIEVAL OF THE ROD PIECE ON THE CORE The rod piece that fell from assembly I-024 during its transit to the tilt machine was retrieved while being taped.
It was placed in the storage basket along with the remaining fragmented rod pieces.
RESULTS:
This piece was about 6 inches long and seemingly fits well with one end of the 5 foot long piece.
5
- 15. 7-14-93 VISUAL EXAM OF ASSEMBLY 1-021 A visual exam of assembly I-021 was performed with particular emphasis on
~amera focus of the corner rods.
RESULTS:
- 16. 7-15-93 ASSEMBLY 1-024 INDIVIDUAL ROD EXAMS Individual rods from assembly I-024 were examined for withdrawal force, eddy current testing, and visual examinations.
The 3 rods adjacent to failed rod S-15 were not examined due to spacer cell damage.
RESULTS:
- 17. 7-16-93 ASSEMBLY I-021 INDIVIDUAL ROD EXAMS Individual rods from assembly I-021 were examined for withdrawal force, eddy current testing, and visual examinations.
RESULTS:
Corner rods on sides adjacent to the shroud during Cycle 10 had several indications of up to
[!:::r:::r:rt:::::tr::titi:~:~t:J from fretting at the spacer cell locations.
The****\\ifs"U*ar******1:i"n-*******rod S-1 confirmed rubbing wear at spans 5 and 6 from the shroud corner during Cycle 10 operation.
Note:
At this time, the decision was made not to re-use the I-Hafnium assemblies in Cycle 11.
Inspections for root cause continued with emphasis on ability to justify use of replacement bundles.
- 18. 7-17-93 ASSEMBLY I-048 EXAMINATION A visual of assembly I-048 was performed as well as examinations of selected individual rods for withdrawal force, eddy current testing, and visual examinations. This bundle was in core location B-19 during the previous Cycle 9 (the assembly I-024 location in cycle 10). Also, rod selection pattern for individual rod examinations was modified to better check status of interior rods.
RESULTS:
The overall assembly visual exam gave satisfactory results.
Corner rods on sides adjacent to the shroud during Cycle 9 or 10 had several indications of up to [iltt:::::i::ri:ttt:tt:n from fretting at the spacer ce 11 l ocat i ons**:*************1hfifrl()"f rods looked good.
* --i
6
- 19. 7-18-93 ASSEMBlIES J-021 AND K-031 EXAMINATIONS Potential replacement assemblies J-021 and K-031 were examined.
Assembly visual exams were performed as well as examination of individual rods.
RESULTS:
The overall assembly visuals looked good.
Corner rods and interior rods looked good.
Only one minor indication was seen on each assembly.
- 20. 7-18-93 ASSEMBLY H-031 EXAMINATION A visual exam of assembly H-031 and of selected individual rods was performed. This bundle had resided in core location B-19 during Cycle 8.
Cycle 8 was its fourth cycle.
EOC 8 burnup was 37,625 MWD/MTU.
- Also, assembly H-031 never saw the increased PCS flow that resulted from the steam generator replacement that occurred at after Cycle 8.
RESULTS:
Assembly visual examination indicated one side may have been rubbing against the core shroud during Cycle 8, however, this is not conclusive. Corner rods on a side adjacent to the
~.h:r.9.l!cf during Cycle 8 had several indications of up to an 0!1]i::::rn::::~:
liilllllililililI:J from fretting at the spacer cell locations.
Add.iTfonally, a corner rod that was not on the core shroud had sever a 1 i nd i cat i 0 n s. 0 f up to an n::::::::::::::::i::::::::::::::::::::::::::::=:::::=:::::::n from fret ti ng at the spacer cell locations.
- 21. 7-19-93 ASSEMBLY L-024 EXAMINATION An assembly visual examination of L-024 and of selected individual rods was performed. This bundle resided on the core shroud between assemblies 1-048 and 1-024 during Cycle 10.
RESULTS:
Assembly visual and individual rod exam results were good.
- 22. 7-19-93 ASSEMBLY L-054 EXAMINATION An assembly visual examination of L-054 and of selected individual rods was performed. This bundle had the highest 3 cycle core residence burnup of all standard bundles; 37,709 MWD/MTU.
Also, assembly L-054 had never resided on a core shroud location.
RESULTS:
Assembly visual and individual rod exam results were good.
Two minor indications were seen on one rod.
7
- 23. 7-20 l 21-93 ASSEMBLIES L-059 AND L-060 EXAMINATIONS Lead High Thermal Performance (HTP) spacer design assemblies L-059 and L-060 were examined to check performance of spacer design that affects the majority of Cycle 11 core design.
Assembly visuals and individual rod examinations were performed.
RESULTS:
Assembly visuals and individual rod exam results were good.
Some minor indications were seen on a few rods.
These minor indications were associated with original fuel rod loading during assembly fabrication (i.e., burrs) and are not associated with fretting wear during operation.
- 24. 7-21-93 ASSEMBLIES I-025, I-027, I-028, AND I-052 EXAMINATIONS Assembly visuals on these 4 I-Hafnium assemblies were performed to check for any indications of rubbing or damage.
RESULTS:
[!J!Illt!:!l!l!:!l!l!l!]!llI::::!l!l!l!]ltlllllllll:jj::]l!:::::'J were seen in assemblies I-025 and I-027.
- 25. 7-26-93 ASSEMBLIES M-048 AND M-049 EXAMINATIONS Visual examinations of assemblies of M-048 and M-049 were performed to check for any indications of damage.
These two assemblies plus the previously examined assembly 1-025 were adjacent to assembly 1-024 during Cycle 9.
The Cycle 9 core loading indicated some difficulty about this area dealing with the initial seating of assemblies 1-024 and M-049.
RESULTS:
Assembly visual exams showed no handling damage.
- 26. 7-22-93 CORE LOCATION B-19 VISUAL Core location 8-19 where* assembly 1-024 resided during Cycle 10 was visually examined.
RESULTS:
Rub marks from assembly I-024 were evident. Guide bar rub marks were heavier on the west side and spacer rub marks heavier on the south side.
Rub marks from the S-15 corner rod were evident from assembly spans 5 through 9.
- 27. 7-23-93 CORE LOCATION X-19 VISUAL Core location X-19 where assembly I-021 resided during Cycle 10 was visually examined.
RESULTS:
Rub marks from assembly I-021 were evident.
Rub marks from the S-1 corner rod were evident from assembly spans 5 and 6.
8
- 28. 7-27-93 ASSEMBLIES I-056 1 J-056 1 AND K-056 EXAMINATIONS These assemblies resided in core location 8-19 during Cycles 5, 6, and 7, respectively.
Minor wear due to interfacing with the shroud likely would not be seen due to two subsequent cycle burns for each of the assemblies.
However, any significant wear or damage that could be seen may help determine whether shroud/assembly interfacing issue is an old problem.
RESULTS:
No conclusive evidence of shroud rubbing was seen.
Assembly I-056 did have a damaged spacer grid, however, the damage appeared unrelated to the assembly I-024 failed rod.
- 29. 7-28-93 ASSEMBLIES SAN-08 1 I-021 1 AND L-052 LENGTH MEASUREMENTS Length measurements of these bundles were made to verify growth within design expectations.
RESULTS:
Length measurements were within design expectations and, more importantly, were not indicating the growth necessary to interfere with UGS bottom plate.
- 30. 7-29-93 CORE LOCATION B-5 VISUAL Core location 8-5 was where assembly I-025 resided during Cycle 10.
This core location visual exam was chosen to see what a potentially "good" corner shroud location looked like, since the assembly I-025 visual exam did not indicate any lnterference rubbing.
RESULTS:
Rub marks were evident from the I-025 assembly guide bar span 9 on the north side. Spacer 9 may have also been rubbing.
- 31. 7-30-93 REPLACEMENT ASSEMBLY VISUAL EXAMINATIONS Assembly visuals were performed on 16 assemblies selected as candidate replacements for the I-hafnium assemblies.
RESULTS:
All assemblies were found to be acceptable.
- 32. 8-6 and 7-93 11SHARD 11 VISUAL EXAM AND RETRIEVAL FROM TILT PIT The fourth fuel rod fragment was located. A visual examination was performed on this fragment and it was then placed into a small storage basket.
RESULTS:
This fuel rod fragment is approximately 12 inches long.
The piece was only a portion of the total rod circumference, about 90 degrees.
The piece is believed to fit with the five foot portion that was retrieved earlier.
- 33. 8-9-93 ASSEMBLY 1-024 UPPER TIE PLATE HOLE GAUGING The upper tie plate was gauged to eliminate large hole size as being a root cause.
RESULTS:
The upper tie plate holes accepted the 0.999 11 pin gauge and the 4-pin true position gauge.
The larger 1.001" pin gauge did not go into any holes.
- 34. 8-11-93 ASSEMBLY C-133 ASSEMBLY VISUAL EXAMINATION Assembly C-133 was in core location B-19 during Cycle 1 when the core barrel vibration problem existed. This assembly was only a one cycle burnup, therefore, any signs of minor interference with the shroud may still be visible.
9 RESULTS:
The visual examination did not provide any conclusive evidence that any rubbing against the shroud had occurred.
- 35.
VISUAL EXAMINATION ON ALL REMAINING I-HAFNIUM ASSEMBLIES AFTER REMOVAL FROM CORE Due to the limited room in the. spent fuel pool, visual examinations on all I-Hafnium assemblies were not able to be performed earlier as would have been preferred.
Remedial corrective action modifications to replacement bundles had to assume that the assembly 1-024 problem was limiting. The remaining 16 I-Hafnium replacement assemblies were also modified in the same manner as the 1-024 replacement assembly which is perhaps conservative, if the shroud/assembly interface problem is only limited to a few core locations.
The visuals on all remaining I-Hafnium assemblies is required to assure that the remedial corrective action modifications remain bounding.
These visuals will also be utilized as input for the root cause evaluation.
- 36.
VISUAL EXAMINATIONS ON OTHER I-HAFNIUM CORE LOCATIONS Visual examinations will be performed on core locations which have suspected shroud/assembly interference based on the visual examinations of the remaining I-Hafnium assemblies.
RESULTS:
The remaining I-Hafnium visual examinations that were completed {Item 35 above) did not require expansion of the planned core location visual examinations.
C.
INSPECTIONS PLANNED
- 1.
VISUAL EXAMINATION OF CORE LOCATIONS A-16, E-2, E-22, H-23,.X-5, Z-8, Z-16 Originally, only visual examination of core locations B-5 and X-5 were planned in addition to any locations that I-hafnium assembly visual examinations indicated interference with the shroud was likely.
Locatio~s 8-5 and x~5 were chosen as 11good 11 locations, since assembly visuals did not indicate any signs of interference. After location B-5 showed indications of some minor rubbing from the assembly, the scope of
10 the core shroud inspections was expanded to better characterize pattern, if any.
RESULTS:
This activity is still ongoing.
The visual examinations of core locations A-16, E-2, E-22, H-23, Z-8, and Z-16 have been completed.
The video tape of these visual examinations are currently being reviewed.
Initial review indicated no significant observations and no surprises were seen.
The visual exam of core location X-5 is still planned.
ATTACHMENT 2 Consumers Power Company Palisades Plant Docket 50-255 SELECTION CRITERIA FOR I-HAFNIUM REPLACEMENTS August 16, 1993 1 Page
- 1.
Selection Criteria For I-Hafnium Replacements Burnup < 37,500 MWD/MTU at EOC 11 for J and K assemblies.
Reason:
Criteria is somewhat arbitrary and less important if L bundles are used, however, the objective is to stay at
< 1-024, beginning of cycle 10 burnup when the 1-024 assembly was not failed.
The design limit of 46,000 MWD/MTU for the L bundles is still applicable due to different spacer grid stamping.
- 2.
Previously not on one of the eight octant symmetric corner shroud positions that assembly 1-024 was in when it failed.
Reason:
Conservative perhaps to apply to all eight octant positions.
It is advantageous to avoid previous X-19 and B-19 residence bundles in the event some damage may have already occurred.
- 3.
Consider bundle bowing to avoid largely bowed assemblies.
Reason:
The bundle to shroud interference already exists at B~l9 and X-19 locations.-
Bundle bow towards the shroud may add to the problem.
Bundle bow away from the shroud must be considered for potential impact on an the adjacent control rod.
- 4.
L assemblies may have some advantage over J & K assemblies.
Reason:
Spacer plate material was stamped in preferential directional to minimize cell opening due to growth.
Also using L will not require ultrasonic examination whereas J and K assemblies will.
- 5.
Fluence rate values should not exceed Cycle 9 values.
Reason:
Cycle 9 fluence rates were used for consistency with the Pressurized Thermal Shock data currently being reviewed by the NRC.
ATTACHMENT 3 Consumers Power Company Pali sades Pl ant Docket 50-255 L-ASSEMBLY ROD LOAD MAP August 16, 1993 1 Page
SIDED 2
3 4
5 8
1 8
9 10 11 12 13 14 15 FUEL ASSEMBLY ROD LOAD MAP CYCLE 11 L-ASSEMBLY DESIGN PALISADES SIOEA A B C D E F G H K L M N P R S
- ~
&.a SIOEC SHROUD GB
- GUICE BAR IT* INSTRUMENT TUBE
- STAINLESS STEEL ROD SIOEB EXAMPLE PATTERN SHOWN FOR THE LOADING OF 14 STAINLESS STEEL RODS INTO AN L ASSEMBLY BASED ON ASSEMBLY TOP VIEW.
SYMMETRIC PATTERN USED FOR ALL 16 L ASSEMBLIES WITH THE 8 STAINLESS STEEL ROD CORNER ALWAYS POSITIONED TO BE AT THE CORE SHROUD CORNER.
ATTACHMENT 4 Consumers Power Company Palisades Plant Docket 50-255 REVISED CYCLE 11 CORE PLAN August 16, 1993 1 Page
1 2
3 4
5 6
7 8
9 A
Revised CYCLE II CORE PLAN PALISADES NUCLEAR PLANT N
B C D E F G R S T v w x TRUE\\
z 1
2 3
4 5
6 7
8 9
10 14
~m=~~~~~,f---r.":';~.2},,(----,~~~~~~~.-~~~~-e&.~~~ 15 19 20 21 22 23 A
B C D RST VWX Z
S Symmetry Lines 16 17 18 19 20 21 22 23 Batch 0: New Fuel !!'.ff Batch N: Once Burnt t? SAN: Once Burnt ' Batch M: Twice Burnt Im Batch L: Three Times Burnt SAN assemblies contain Stainless Steel pins Cycle 11 Core Plan uses !4 core rotational symmetry E
ATTACHMENT 5 Consumers Power Company Palisades Plant Docket 50-255 ROD WITHDRAWAL FORCES Siemens Power Corporation PROPRIETARY INFORMATION WITHHELD FROM PUBLIC DISCLOSURE August 16, 1993 12 Pages
ATTACHMENT 6 Consumers Power Company Palisades Plant Docket 50-255 ASSEMBLY I-024 ROD SIS MATERIAL ACCOUNTABILITY PROPRIETARY INFORMATION WITHHELD FROM PUBLIC DISCLOSURE August 16, 1993 I Page
ATTACHMENT 7 Consumers Power Company Palisades Plant Docket 50-255 GRID SPACER SPRING RETENTION FORCES PROPRIETARY INFORMATION WITHHELD FROM PUBLIC DISCLOSURE August 16, 1993 16 Pages
ATTACHMENT 8 Consumers Power Company Palisades Plant Docket 50-255 EVALUATION OF S-15 ROD REMOVAL FROM I-024 August 16, 1993 3 Pages
Evaluation of S-15 rod removal from I-024 The purpose of this document is to provide an evaluation which determines how the S-15 corner fuel rod was removed from assembly I-024 during the Cycle 10/11 refueling outage in June/July 1993.
During radiation surveys of the reactor cavity, a 900 R/hr particle was located in the reactor cavity two feet from the reactor flange.
Analysis of this particle later determined the particle to be fuel. A few days later, again during radiological surveys, a 7000 R/hr object was located in the tilt pit.
1 Video observation of the tilt pit area determined that the object was a piece of a fuel rod.
Further observation of the tilt pit area determined that there were four objects which appeared to be fuel rod fragments, one of which had an intact lower end cap whose serial number indicated that it was rod S-15 from assembly I-024.
During initial recovery of the fuel rod fragments, only three of the original four fragments could be located.
The fourth fragment was retrieved on August 6 and 7, 1993.
Fuel assembly I-024 was located in reactor vessel location 8-19 during Cycle 10.
In Cycle 11 it was to be in the same location, but rotated 180°.
Since 8-19 is in a core shroud corner location, the assembly cannot be picked, rotated and reinstalled at the location due to interference with the refueling machine camera and the shroud. Therefore it is necessary to pick the assembly, rotate the assembly with the mast, place the assembly in the tilt machine, rotate the mast back to its original orientation, pick the assembly from the tilt machine and lower it back into its core location.
It is believed that fuel rod S-15 failed in the core about July of 1992.
The failure was first detectable 87 days into the cycle; however, because it was a low power rod, the failure may have initiated earlier. It has also been concluded that the corner pieces of spacer grids six through nine were worn through and lost while the assembly was. in the core during Cycle 10.
The cladding in spans 5 and 6 was severely worn against the shroud corner during subsequent Cycle 10 operation. At or above this location was the likely primary failure location. About November 1992 a large axial split likely occurred resulting in subsequent loss of fuel.
The interface with the shroud created vibrations that caused wear at spacer locations for rod S-15.
The adjacent rods show evidence that a circumferential break existed in span 9 during Cycle 10 operation.
This gives us the following scenario at the beginning of fuel moves in the end of Cycle 10 refueling outage. Assembly I-024 was located in the 8-19 location with rod S-15 failed and the corresponding corners of spacer grids six through nine missing.
Rod S-15 was subseq~ently located in the corner of the core shroud with 4 of the top 5 spacer grids broken (spacer 10.was partially intact) in the corner locations.
The attached drawing provides an indication of how the pieces of rod S-15 fit into the assembly.
2 When the mast was lowered to grapple ass_embly I-024 the S-15 fuel rod may have been protruding out of the corner of the fuel assembly.
The normal maximum clearance between the fuel rod ~nd the shroud corner is 0.2045". This situation may have.resulted in the broken fuel rod being retained in the hoist box while the fuel assembly was being lifted from the reactor vessel.
The fuel bundle was transported to the tilt machine in the tilt pit. The mast was rotated and the bundle was lowered into the tilt machine.
The mast was then rotated. The operator then attempted to lift the bundle into the hoist box.
It is believed that the operator then got an Qverload indication on the refueling machine and lowered I-024 back. into the tilt machine.
The bridge was then moved about 0.4-inch to the west. The* 1-024 assembly was then successfully hoisted back into the hoist box~-
The greater clearances between the tilt machine and the assembly (possibly combined with looseness resulting from handling) resulted in the S-15 fuel rod getting caught on the outside of the hoist box at spari 9.
As the assembly was hoisted into the hoist box, rod S-15 was peeled out of the assembly.
It is believed that as the rod was peeled out of the assembly, it struck a hoist wheel block in the refueling machine mast which is approximately 4 feet above the top of the tilt machine at this point. This resulted in the rod breaking into the 4 to 5 foot sections observed. Spacers 1 through 5 have whit-appear to be freshly torn corners which is consistent with this method of removal.
The remaining upper portion of the rod (about 6 to 7 inches) including the upper end cap was retained in the assembly as it was returned to the core.
On July 13, 1993, when the 1-024 a~sembly was moved to the tilt pit ar~a, the
- upper portion of the rod was no longer in the assembly. _This upper portion was subsequently located on top of the core near th~ B-19 location and retrieved *. It is believed that the upper rod piece simply fell from the_
assembly as it was being lifted from core location 8-19 for transportation to -
the tilt pit and then to the spent fuel pool.
- ~.
PALISADES ASSEMBLY 1-024 140.702 1~.~2 _____________ _
Span 9 ;
1~-~7 ___________ _
Span 8 108.347 g~'!!7 ---------
7Z:~7 -------
15.!.:~7 -----,..--
"'!!:~7 ___ _
30.847 15.347 Lower Tie Plate Revised 8/13/Q3 e-r I~
UEC present, jagged lower end; possibly intact cllid spacer fret rI 3.5" :c Plenum spring present, captured "jl at mid-length; spring has slipped down from UEC; fully open clad both 1
- above and below spring ends; spring lower end stidls out.
Rest of section up to 2/3 of circumference of clad. gone and/or split open to top of ~d.
spacer sideplate fretting *,
ttvough--11 hole horimntal cut spacer sideplate fretting partial jagged end spacer fret; sideplate dimples &
- window imprint, right face
} spacer fret; sideplate dimples &
window imprint, right face shiny scratch spacer fret; sideplate dimples &
window imprint, right face even, shiny fracture surface
} even, shiny fracture swface spacer sideplate fret at fracture surface; spacer dimple & window imprint spacer fret, bottom and center lobes of spring, _,. mam on other side from fretted a..-
LEC ok Rod S15