ML19326B439
| ML19326B439 | |
| Person / Time | |
|---|---|
| Site: | Arkansas Nuclear  |
| Issue date: | 02/13/1974 |
| From: | ARKANSAS POWER & LIGHT CO. |
| To: | |
| Shared Package | |
| ML19326B431 | List: |
| References | |
| NUDOCS 8004150793 | |
| Download: ML19326B439 (4) | |
Text
- '
. ~.
ARKANSAS POWER & IlGHT COMPANY
' ARKANSAS NUCLEAR ONE-UNIT 1 FUEL ASSEMBLY - ORIFICE ROD ASSEMBLY BINDING I.
BACKGROUND On December 28, 1973, inspection of the forty (40) fuel assemblies containing orifice rod assemblies revealed that the orifice rod assemblies were binding in the guide tubes of seven (7) of the fuel assemblies.
All c
of the fuel assemblies involved belong to Batch 3 of the initial core loading of Arkansas Nuclear One-Unit 1 (ANO-1).
On January 12, 1974, a further inspection was performed by AP&L and B&W representatives, including a Quality Control Inspector from B&W's Commercial Nuclear Fuel Plant (G FP).
This inspection confirmed that there was unacceptable drag during inspection and removal of the orifice rod assent:11es; however, inspection of the upper nut and guide tube weld areas did not reveal a cause for the rod drag. nor was any out-of-specification condition apparent.
It was then agreed that the subject fuel assemblies and -orifice rod assemblies would be shipped back to B&W's CNFP for more detailed examination.
The components were shipped back to B&W on January 17, 1974.
II.
EXAMINATION OF THE COMPWENTS
.The fuel assemblies were subjected to a detailed visual examination, followed by control rod drag tests, envelop inspection and fuel rod spacing measurements.
Members of the CNFP Manufacturing and Quality Control organizations drafted. a procedure for systematic fuel bundle disassembly, in order to gain access for addi-tional measurements which could not be made with the fuel bundle in the " assembled" condition.
This proce-dure was reviewed and approved by' appropriate members of B&W Fuel Engineering, Quality Assurance and Project Management and representatives of Arkansas Power &
Light Company prior to implementation.
Under this procedure, parallelism of the ' upper end fitting is checked first, followed by insertion of the ori#1ce rod so7'/3 soon
m ID) assembly with selective removal of particular orifice plugs.
In addition, the ease of insertion of individua-orifice plugs into the guide tubes is checked.
After this, the holddown spider and spring are removed and detailed measurements are made of the orientation and location of the upper guide tube nuts.
Finally, the guide tube nuts and guide tubes are removed with addi-tional measurements being made during and after this procedure.
III.
RESULTS OF EXAMINATION Examination of the fuel assemblies and orifice rod assemblies did not reveal any out-of-specification conditions.
The control rod drag tests showed a maxi-mum drag of 2 lbs. on any single assembly vs. a speci-fication requirement of 7 lbs. maximum.
These tests confirmed the results of earlier drag tests as recorded in the fuel assembly Quality Control records.
The holddown spring and spider and two guide tubes were removed from fuel assembly No. 1C42 in accordance with the previously mentioned procedure.
As a result of measurements made during this disassembly process, it O
was found that some of the upper guide tube nuts were cocked.
IV.
ANALYSIS AND CCNCLUSIONS During welding of the upper end fitting grillage plate to its side supports (plenum assembly) minor d.is tortion (bowing) of the grillage upper surface occurs.
When the guide tube nuts are torqued on the guide tube and welded to the grillage, they ass ume the slope of the grillage at that location.
If the surface of the grill-age is not perfectly flat, the nuts are tilted or cocked slightly depending on the local slope.
Because of the close fit (.014 minimum diametral gap) between the orifice rod and the guide tube nut I.D.,
l the orifice rod assembly design is extremely sensitive to local misalignments and adverse positional tolerance s tackups.
A slight nat tilt (N30 minutes) at any of 1
the.16 guide tube locations could be sufficient to pre-
]
vent insertion of the orifice rod assembly.
This ccmbi-nation of grillage bow ar.d tolerance accumulation reduced the effective free path for insertion of the orifice rods enough so as to cause binding during orifice p
rod assembly insertion or removal.
t 1
.%d. _.. _.
In contrast to the orifice rod assembly, the control rod assembly, burnable poison rod assembly and axial power shaping rod assembly have a relatively large diametral gap (.054 minimum) between the clad O.D.
and the nut I.D.
Every ANO-1 first core fuel assembly has been inspected with a functional gauge that simulates the maximum tolerance stackup possible for the control, axial power shaping and burnable poison rod assemblies; and drag loads were within acceptable limits.
Thus, the inser-tion problem does not exist on the control components.
V.
CORRECTIVE ACTION A.
The orifice rod diameter will be reduced from 0.480 inches do 0.440 inches to eliminate the binding problem.
1.
All first core fuel assemblies have been inspected with a fractional gauge that accurately simulates the maximum tolerance stackup possible for the 0.440 inch diameter control rod, axial power shaping rod and burnabla poison rod assembly.
Reduction of O'
the orifice plug diameter to 0.440 inches will insure free insertion of the orifice rod assembly into any fuel assembly.
2.
The incorporation of 0.440 inch diameter orifice plugs into the design has been reviewed by B&W Fuel Engineering and it has been concluded that this change will not result in lowering of the thermal hydraulic design limits.
B.
The fuel assembly upper end fitting design will be reviewed and appropriate controls and changes will be incorporated to prevent this problem in the future.
C.
The two guide tubes removed from 1C42 were replaced with new tubes and guide tube nuts; the holdown spring and spider were replaced, and the fuel assenbly was inspected and recerti-fled to original specifications.
VI.
SAFETY IMPLICATIONS B&W Fuel Engineering has reviewed the corrective actions g
noted above and has concluded that there are no safety k -
,a
\\
inplications that require modifications to any of the fuel assemblies.
Safety related concerns have been evaluated based on the maximum distortion possible in the design and found to be acceptable.
In this review, the following areas were covered:
A.
Effect on Control Rod Drop Times The simulated worst tolerance control rod gauge check conducted on each assembly gave drag loads within acceptable limits.
Thus, free insertion of a control rod without excessive drag is assured for any fuel assembly and the control rod drop times are not affected.
C B.
Effect of Shipping and Handling Loads Shipping and handling loads do not result in any permanent deformation of the fuel assembly structure.
C.
Effect of Normal Operation T'.se effects of temperature and irradiation on the fuel assembly during normal operation will not amplify the as-built distortion.
The fuel assembly holddown load will not produce any permanent deformation of the upper end fitting and is in the direction to reduce the as-built dis tortion.
D.
Faulted Conditions The axial loads exerted on the fuel assembly during a combined seismic and LOCA condition will not result in additional distortions of a magnitude to prevent control rod insertion or affect the fuel assembly structural integrity.
t V
l L -
.--