ML20084S188
| ML20084S188 | |
| Person / Time | |
|---|---|
| Site: | Quad Cities  |
| Issue date: | 04/30/1974 |
| From: | COMMONWEALTH EDISON CO. |
| To: | |
| References | |
| NUDOCS 8306160207 | |
| Download: ML20084S188 (14) | |
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QUAD CITIES 1 NUCLEAR GENERATING PLANT l
CONTROL ROD BLADE INSPECTION AND EVALUATION I
l April 1974 l
l REMTRN TO DIRECTORATE OF REGULATORY OPERAHONg i
h haranER JS8Sjh
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1.
SUMMARY
AND CONCLUSIONS This report describes tne oddy current examination conducted to identify inverted B.C tubes in the Quad Cities I control rod blades. Evaluation of the effect on safety margins by the inverted absorber tubes remaining in the reactor is also gNen. Two control rod b!ades were rejectab!e under the acceptance enteria established. These blades were replaced during the Apnl 1974 refueiing outage. The remainder of the inverted absorber tubes (57 in number) have negligible effect on safety margins even if maximum stumping of B.C is assumed.
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INTRODUCTION During July 1973 a manufactunng deviation (inverted B.C tubes) in the control rod blades became known. This was reported as an abnormal occurrence to the USAEC, initially by the Millstone Point Company. A description of the inversions, the predicted population of the inversions, and evaluation of effects on safety margin were first introduced on the Millstone docket (Reference 1) and later on a generic basis (Reference 2).
l At the Quad Cities 1 site, an eddy current examination was used to detect inverted absorber tubes. During April 1974 of the refueling plant outage all control rod blades were successfully examined.
The purpose of this report is to:(1) report the results of the examination and (2) evaluate the effects on safety margi.is by control rod blades with an acceptable number of inverted absorber tubes left in the core.
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3.
SUMMARY
OF EXAMINATION During the period of April 15,1974 through April 26,1974, an eddy current examination was performed on 177 control l
rod blades located in the Quad Cities 1 reactor pressure vessel. The objective was to locate the steel wool inserted in the bottom end of the absorber tube prior to installation of the end plug. The detection of the steel wool is an indication that the absorber tube had been installed in the blade in an upside down position. This examination was conducted by personnel i
certified in eddy current techniques in accordance with the recommended practices of the American Society for Nondestruc-tive Testing. Specifics of the equipment and the detailed procedures are contained in Reference 3.
Preparatory to the examination, bundles diagonally opposite each other in a cell were removed from the core one at a time, so that the tops of the control blades were accessible for examination. The probe of a precalibrated eddy current tester was lowered into the reactor vessel onto one of the wings of a blade. During the scanning, any change in the conductive or magnetic properties of the wing results in change in the intemal eddy current" induced in it by the coil, and these changes in i
tum affect the impedance of the coil. This impedance variation is electronically processed and displayed on an oscilloscope or strip chart. A trained inspector then compares this trace with a trace developed from known conditions in a control rod blade standard.
As each control rod blade was examined, results were recorded on data sheets identifying the core position and the wing in the control rod blade.
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4.
RESULTS AND EVALUATION 4.1 PESULTS At conclusion of the examination,it was determined that of the 177 blades examined,91 absorber tubes in 32 control rod blades were inverted. One wing was completely inverted in one blade. The summary shown in Table 1 identifies the inverted absorber tubes by core position, control rod blade number, and location in each wing of the blade.
4.2 EVALUATION 4.2.1 Acceptance Criteria An extensive critical experiment program was conducted at the KWU facility at Grossweigheim, Germany, for the purpose of obtaining data for use in establishing acceptance criteria for inverted control rod blades (see Reference 2). Based on these results the following acceptance criteria were derived. Any control rod blade is acceptable if it meets these requirements:
1.
No wing sliould contain more than four inverted tubes.
2.
Each inverted tube location is assigned a value for a change in control rod blade strength in accordance with Figure 1. The sum of the values for each inverted tube in a control rod blade (all four wings) must not exceed 4%.
The basis for the above criteria is that the permitted deviation shall not cause a decrease in shutdown margin of more thanapproximately0.0025 Akg,if the B.C in tubes should settle the maximum amount of 16 inches in a particular area of the core.
4.2.2 Safety Analysis Based on the above enteria, two control rod blades listed in Table 1 are rejectable. These are blades in core positions 22-11 and 30-59. Two of these were replaced during the April 1974 refueling outage. From Table 1 it can be seen that a total of 91 absorber tubes are inverted. If the number of inverted absorber tubes in the two replaced rods are subtracted, a net of 57 inverted absorber tubes remain in the reactor. In terms of fraction of total B C tubes, this is:
57
= 0.0038 4x21x177 Shutdown Margin Based on the inverted tubes found in Quad Cities Unit 1, the shutdown margin could be reduced 0.04% AK for certain fully withdrawn control rods due to B C in inverted tubes slumping to the maximum during the next operating cycle. The rod position most affected by surrounding blades with inverted tubes is position 10-15. This is adjacent to a number of rods which contain inverted tubes. Other inverted tubes will be sufficiently separated from the rod of concern (10-15) so as to have no effect on shutdown margin with the rod fully withdrawn.
Compliance with the technical specifications shutdown margin requirements can be confirmed by demonstrating at startup that the reactor can be shut down by R +0.25% AK with the strongest rod withdrawn where R includes 0.04% AK for B.C slumping. During the operating cycle the shutdown margin will not decrease. in summary, the technical specifications shutdown margin requirement of 0.25% AK can be met with extra shutdown margin with maximum theoretical settling of B.C assumed in the remaining few inverted absorber tubes.
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Table 1 QUAD CITIES 1 CONTROL ROD BLADE EXAMINATION Total inverted B.C Tubes Blade Core Wing and Location of Inverted B.C Tubes
- per CRD ltem Serial No.
Position A
B C
D Blade 1
CX56R 54-23 10 13 2
2 CX63R 54-27 15 1
3 CX28R 54-47 19 1
4 CX21R 50-27 2,11 2
5 CX80R 50-23 8, 9 2
6 CX74R 50-15 8, 9 2
7 CX12R 46-23 13 1
8 CY21R 46-43 3
1 9
CY12R 38-11 12,13,14 3
10 CX94R 38-51 7
1 11 CX101R 34-51 16,17,18 3
12 CX62R 34-35 11,12,13 3
13 CX127R 30-07 5
8,13 4,7,12 6
14 CX30R 30-15 13 1
15 CX53R 30-59 9 thru 21 13 16 CX84R 26-31 17,18 2
17 CX33R 26-15 10,11,12 20,21 5
18 CX34R 26-11 18 1
19 CX31R 22-03 3, 4 10 3
20 CX75R 22-11 1 thru 21 21 21 CY2R 22-43 10,19 2
22 CX60R 18-15 10 1
23 CX78R 14-15 12,15 2
24 CX23R 10-23 1
1 25 CX24R 10-19 1
1 26 CX41R 10-11 13 1
2-'
CX72R 06-15 6
1 28 CX32R 06-19 2
1 29 CX79R 06-35 5
1 30 CX122R 06-39 4
1 31 CX118R C2-31 13,14,20,21 4
32 CX144R 02-23 3
1 Total 91
- Numbers represent location of 8,C tubes in the wing per Fa;n.re 1.
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Other Accidents and Transients Effect of the potential settling of B.C in the remaining inverted absorber tubes was reviewed for the rod drop accident and the pressurization transients.
in the case of the rod drop accident, it was concluded in Reference 1 that if all of the absorber tubes in all four wings were inverted in one control rod blade and if the B.C has sett:9d the maximum of 16 inches, the worth in the adjacent blade would be increased by an estimated 0.002 AK. Since the max: mum number invc ed absorber tubes remaining in any given blade is six (position 30-07), or approximately 7% of total of 84, the effect on an adjacent rod is negligible.
For the pressurization transient (turbine trip with t ypass failure),it was shown in Reference 2 that if 5% of the absorber tubes were inverted and the B C slumped the maximum amount, a loss in pressure margin (dfterence between peak l
transient pressure and setting of the first safety valve) of about one psi would result. With 0.38% of'he absorber tubes in the core inverted, the loss in pressure margin is negligius. -
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REFERENCES 1.
" Reactor Control Blade Evaluation," July 23,1973 (MNPS-1 Special Report).
2.
J. A. Hinds (GE) letter to D. J. Skovholt (USAEC), dated October 8,1973.
3.
D. L Richardson and T. D. Smith, " Procedure for Locating Carbon Steel Heat Sinks in Control Rod Tubes During Plant Outages," November 1973 (NEDO-20211).
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