ML17252B050

Control Rod Blade Inspection and Evaluation

ML17252B050
Person / Time
Site:	Dresden
Issue date:	04/30/1974
From:	Commonwealth Edison Co
To:	US Atomic Energy Commission (AEC)
References
Download: ML17252B050 (10)

Text

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'<' ' , , I r DRESDEN 3 NUCLEAR GENERATING PLANT CONTROL ROD BLADE INSPECTION AND EVALUATION April 1974

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1.

SUMMARY

AND CONCLUSIONS This report describes the eddy current examination conducted to Identify inverted B4 C tubes ifl the Dresden 3 control rod blades. Evaluation of the effect on safety margins by the inverted absorber tubes remaining in ~e reactor is also given.

Seven control roo blades were rejectable under the acceptance criteria established. These bli;ides were replaced during the April 1974 refueling outage. The remainder of the inverted absorber tubes (a2 in number) have negligible effect on safety margins even if maximum slumping of s.c is assumed.

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2. INTRODUCTION During July 1973 a manufacluringC!eviation (inverted B4 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) .

.At the Dresden 3 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 margins by control rod blades with an acceptable number of. inverted absorber tubes left in the core.

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3.

SUMMARY

OF EXAMINAT,ON

. D_uring the period of Apri! 4, 1~74 through April 10, 1974, an edgy Gurrent examination.was performed on 177 control .

rod blades located in the Dresden 3 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 certified in eddy current techniques in accordance with the reeommended practices of the American Society for Nondestructive Testing.

Specifics of the equipment and the detailed procedures are contained in Refe.rence 3.

Preparatory to the examination, two fuel bundles diagonally opposite each other in a cell were removed from the core 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 internal "eddy currenf' induced in it by the coil, and these changes in 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 RESULTS n

At conclusion of the examination, it was determined that of the 1 blades examined, 187 absorber tubes in 25 control rod blades were inverted. One 'wing was completely inverted in four blades> The summary shown in Table 1 identifies the inverted absorber tubes by core position, control rod blade number, and location in each wirig of the blade ..

Table 1 I;. r.; !* ."!. r DRESDEN 13'C'C)NTROl.'.l ROD" BlADE 'EXAMINATION Total.

.,. .. <Inverted * **

B4 C Tubes Blade Core* Wlng'"and Location of Inverted B4 C Tubes

• per CRD Item Serial No. Position A B c D* 'Blade* - :

1 DD-1n 02-35 16,17 ..

- ** 1 '2 ..

2 CY-87R 06-35 6 . r .,

3 DD-19 10-35 15, 18, 19 3 4 DD-122 10-47 13,16 2*

5 DD-121 10-51 6 1 6 DD-9 14-39 14,15,16, 13, 14, 15, 16, 4,5,8,9, 14,15, 16, 34 17, 18, 19,20,21 17, 18, 19,20,21 10,11, 12, 13, 14 17, 18, 19,20;21 '

7 DD-49 14-47 3,4 2 8 DD-10 18-39 All *21 9 DD-64' 18"47 '5 1;

• 10 'DD-45 3 . :.-*.

18-51 1 11 DD-117 26-27 6;7,8 *3 12 DD-40 26-43 1,2,3,4,5,6,7 - 7 13 . DD-94 26-51 '

20,21' *2 14 DD-17 30~39 . 8, 12, 13, 14! 4,5,6,7,8;9,' 4 22 . '*

15, 16, 17, 18 10, 1'1~12, 13, 14, 15, 16 '

15 DD-1 30-47 ' *: 4,5 2 16 DD-50 34-47 All 1,2,3,11~12 26 17 DD-111 34-55 - 4

. CY-163R i *.*

18 38-43 9 1 19 DD-155 38-51 18,19,20 All 24 20 DD-7. 42-55 - 3,4 - 2 21 DD-141 46-31 7,8,9,10 4 22 CY-153R 46-43 16 23 CY-149R 46-51 - 6' 24 . DD-125 54-23 14,15 2

,,) 25 DD-154 54-43 Air** - 21' Total 18:7 i1 4.2 EVALUATION . '

4.2.1 Acceptance Criteria An extensive critical experiment program was conducted at the KWU facility at Grosswelzheim, Germany, for the purpose of obtaining data for use in establishing acceptance criteria for inverted control rod blades (see Reference 2). Based 4-1

on these results the following acceptan~ critE1ria were:deriv~d. Any control rod blade is acceptable if it meets these requirements:

1. No wing should contain more than four inverted tubes.

2, Ea¢h 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 eacti inverted tube in .a control rod blade (all four wings) must not exceed 4%. . ' .

The ba.sis for the above criteria is that the permitted deviation shall not cause a decrease in shutdown margin of more than approximately 0.0025 .:1kett. if the B4 C ii) 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 criteria, seven control rod blades. listed in Table 1 are rejectable. These.are blades in core

. positions*t4-39, 18-39, 26-43, 30-39, 3+:47, 38-51, and 54-43. All seven of these were replaced during the April 1974 refueling outage. From Table 1 it can be seen that a total of 187 absorber tubes are inverted. If the number of inverted absorber tubes in the seven replaced rods are subtracted, a net of 32 inverted absorber tubes remain in the rei;tctor. In terms of fraction of total B4 C tubes, this is:

32 0.0022 4x21x1n Shutdown Margin Based on the ex~mination of Dresden 3 control rods and replacement of seven of the rods, the effect of remaining inverted-B4 C tubes has been evaluated. The area most affected is control rod 14-51 which is adjaeent to five blades containing inverted tubes. Using Figures 1 and 2 of the Reference the resulting reduction in shutdown margin,would be less than 0.03% .:1k if the B4 C in these tubes should slump the f1laximum amount during the next cycle of operation.

Compliance Wit~ the technical specifications shUtdown rrlargin requirements can be CC>~firmed bydemonstrating at startup.that the reactor cari be shut down by R+0.25% ~K with the strongest rod withdrawn where R includes 0.03% .:1K for B4 C slumping. During th~ operatil"!g cycle the shutdoWJJ ~argin Win not decrease. In summary, the technical specifications shutdown margin requirement of 0.25% .:1K can be met with extra shutdown margin with maximum theoretical settling of B4 C assumed in the remaining few inverted absorber tubes.

Other Accidents and Transients Effect of the potential settling of B4 C in the remaining inverted absorber tubes was review0d for th~ 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 settled the maximum of 16 inches, the worth in 'the adjacent blade would be increased by an estimated 0.002 .:1K. Since the maximum number inverted absorber tubes rem~ini~g in any given blade is four (position 46-31), or approximately 4% of total of 84, the effect on an adjacent rod is negligible.

i For the pressurization transient (turbine trip with bypass failure), it was shown in Reference 2 that if 5% of the absorber.

tubes were inverted and the B4 C slumped the maximum amount, a loss in pressure margin (difference between peak transient pressure and setting of the first safety valve) of aboUt one psi would result. With 0.22%of the absorber tubes in the core inverted, the loss in pressure margin is negligible .

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2 3 4 5 6 .7 8 9 10 11 . 12 13 14 15 16 17 18 19 20 21 TIP POSITION OF EMPTY TUBE IN THE WING CENTER Figure 1. . Change in Control Blade Strength Per Empty Polson Tube

Q 0.28 0.26 BASIS FOR CONTROL ROD ACCEPTANCE CRITERIA 4%

0.24 0.22 REDUCTION IN WORTH OF THE END OF THE CONTROL BLADE 0.20

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NUMBER OF INVERTED BLADES ADJACENT TO.A STUCK ROD Figure 2. Loss of Shutdown Margin Versus Number oUnverted Blaqes with 12 ln.-15 in. B 4C Slump Placed Adjacent to a Stuck Control Rod-Equilibrium Core .With Ax/ally Distributed Gd

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 Rlchard$0n andT. D. Smith, "Procedure for Locating Carbon Steel Heat Sinks in Control Rod Tubes During Plant Outages," November 1973 (NED()-20211).

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