2CAN068104, Forwards Documentation of 810527 & 28 Telcons Re Fuel Assembly Spacer Grid Damage
| ML20004D665 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 06/04/1981 |
| From: | Trimble D ARKANSAS POWER & LIGHT CO. |
| To: | Clark R Office of Nuclear Reactor Regulation |
| References | |
| 2CAN068104, 2CAN68104, NUDOCS 8106090630 | |
| Download: ML20004D665 (6) | |
Text
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ARKANSAS POWER & LIGHT COMPANY POST OFFICE BOX 551 LITTLE ROCK, ARKANSAS 72203 (501) 371-4000 June 4, 1981 h.
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Director of Nuclear Reactor Regulation ATTENTION: Mr. Robert A. Clark, Chief 9
Operating Reactors Branch #3 Division of Licensing U. S. Nuclear Regulatory Commission Washington, D.C.
20555
SUBJECT:
Arkansas Nuclear One - Unit 2 Docket No. 50-368 License No. NPF-6 NRC Request for Information on Fuel Assembly Spacer Grid Damage
( File: 2-1510)
Gentlemen:
On May 27 and 28,1981 telephone conversations (between members of the NRC Core Perfonnance Branch, NRC's Bob Martin, representatives of Combustion Engineering, Inc. and Arkansas Power & Light Co.) were held to discuss the fuel assembly spacer grid damage observed during an inspection of discharged nuclear fuel following the first refueling of ANO-2.
Upon completion of the discussion, which included a description of the observations and subsequent analyses, the NRC personnel requested written documentation, of the information provided by AP&L/C-E during the call, be transmitted to NRC.
This information is included as Attach-ment 1 to this letter.
Further, NRC requested a coimitment that AP&L continue to investigate the cause of the grid damage and to take cor-rective action prior to the next refueling.
AP&L and C-E are actively pursuing a solution to the question of the cause of the grid damage.
This effort will continue until a solution is arrived at or all identified avenues of concern have been investigated.
Corrective action based 8106090(.oD 9
MEMBER M'OOLE SOUTH UTILITIES SYSTEM
]
c J$;ne 4,1981 Mr. Robert A. Clark
- on the results of these investigations will be implemented and findings will be reported to NRC prior to the next refueling of AN0-2.
Very truly yours, d) avid C. Trimble Manager, Licensing DCT:THC:JTE:1p Enclosure cc: ANO DCC
ATTACHMENT 1 ANO-2 FUEL ASSEMBLY SPACER GRID DAMAGE 1.0 OBSERVATIONS Arkansas Nuclear One - Unit 2 was shutdown to begin its first refueling outage on March 27, 1981.
The refueling core shuffle began on April 19, 1981 and was completed on May 10, 1981.
During this period several fuel examination were conducted including a visual inspection of 15 fuel assemblies using TV camera, periscope or both. The results of this examination were provided to NRC in a telephone conference call on May 12, 1981 and were documented in CEN-164(A)-P. Subsequent to completion of the refueling shuffle sequence and reassembly of the Reactor Vessel Internals and Closure Head Assembly, a series of TV examinations of the 60 discharged (Batch A) fuel assemblies was conducted.
The TV examinations revealed the presence of spacer grid damage to five fuel assemblies.
The damage to two assemblies ( AKA023 and AKA041) was extremely minor. However, the damage to three assemblies ( AKA042, AKA106 and AKA109) was significant. The damage to these thme assemblies is shown in sketches on figure 1.
Based on crud patterns and bright metal at the torn interfaces as viewed by undenvater periscope it has been concluded that the damage occurred during fuel handling following the operation of the first cycle.
It has also been concluded that the damage resulted from grid to grid interaction with adjacent fuel assemblies during fuel handling within the core.
2.0 REVIEW 0F OPERATING PRACTICES AP&L has examined its fuel handling procedures and refueling machine checkout / calibration procedums for indications of improper practi ces.
This review has indicated that the refueling machine was properly calibrated prior to starting fuel shuffle and should have provided an overload / underload trip prior to placing unacceptable loads on fuel assembly components.
The overload trips are set to trip when less than 100 lbs force is placed on a heavy assembly which has a part length control rod in it and could be as high as 220 lbs force on a light assembly containing no control conponent.
The underload trips are set to provide similar protection.
AP&L personnel ir.volved in the fuel handling operations (operators, and engineers) have been interviewed regarding their experience during fuel handling in the core.
These interviews have indicated that overload trips did occur at a frequency of about twice per shift (or approximately once per 10 fuel bundles).
Underloads were very rare and probably less than 5 total occurrences we're experienced. Since the overload l underload trips were believed adequate to prevent fuel damage, no special concern arose because of them. The practice used upon receipt of an overload / underload
trip was to relieve the extra load and clear the overload /under-load and then shake the hoist cable by hand while the assembly was being raised or lowered.
This practice causes some additional lateral motion of the fuel assembly and helps clear obstructions and points of contact.
It has been used successfully by AP&L on ANO-1 for several refuelings.
3.0 FUTURE EVALUATIONS / CORRECTIVE ACTION 0ur review of operating practices / procedures has not yet identified any problems which would have been expected to cause the observed grid damage. However, several areas for further examination have been identified.
These include (1) quantifying the additional force placed on a fuel bundle by momentum of the system after an overload trip occurs, (2) determining the effect of cable shaking during assembly movement and (3) determining more accurately the force required to cause grid damage in the configurations expected to be experienced during fuel handling.
These determinations and possibly others are planned and modifications to procedures, training and/or equipment will be made based upon the findings as requi red.
4.0 ACCEPTABILITY FOR CONTINUED OPERATION The observed damage on discharged assemblies raises questions as to the condition of the assemblies left in the core.
Two major areas of concern exist:
(1) If similar damage exists on fuel assemblies remaining in the core, are the fuel rods at the affected locations supported adequately to prevent failure during operation?
and (2) If loose pieces of spacer grid perimeter straps exist within the core can they cause damage to the fuel rods of assemblies in the core?
Tests conducted at C-E's Windsor f acility on 16 x 15 fuel bundles indicate that fuel rods unsupportei at one intermediate spacer grid elevation will not fail due t3 fretting or vibration. The lower grid which is made of inconel (a tougher material than zircalloy) is required, but these grids are not believed to be affected by the observed ?roblem.
The failure of individual fuel rods within the Cycle 2 core due to loose pieces of grid straps is believed to be possible although this failure mechanism has not been linked to previous fuel failures.
The type of failure which could be postulated is one in which fretting and subsequent wear of one or a few fuel pins could be caused by the lcose pieces.
This mechanism is thought to be unlikely because loose pieces are expected to become impinged and possibly lodged in the lower part of the vessel or lower grid plate of a fuel assembly.
Only a piece which managed to travel into the fuel region and happened
1 '
to become lodged at a location where it could flutter against one or a few fuel rods would be expected to cause damage.
This mechanism would not be expected to cause failure of a large number of fuel rods.
Any failures which might be caused by this means
-would be detected by the letdown monitors and/or periodic Reactor Coolant sampling and analysis. The Technical Specifications and safety analyses are based on a specified. amount of fuel pin leakage which far exceeds that which would be postulated due to this mechanism.
AP&L has concluded that the operation of ANO-2 with potentially degraded fuel assembly grid straps does not constitute a safety concern and is acceptable for Cycle 2 operation.
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FIGURE 1 h
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AKA106
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h/'f I$;9 frf,n f)d E'A j
AKA109
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NOTE: The bottom inconel grid is #0 and the top grid is #11.
P00R ORIGINAL