ML20090M518

Forwards Control Rod Blade Insp & Evaluation Performed Under Supervision of GE

ML20090M518
Person / Time
Site:	Monticello
Issue date:	05/10/1974
From:	Mayer L NORTHERN STATES POWER CO.
To:	Skovholt D US ATOMIC ENERGY COMMISSION (AEC)
References
NUDOCS 9105030433
Download: ML20090M518 (12)

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Ltr te our 4-1-74 Itr, trane the followingt RPPORT: Control Rod Blade Inspectioa and Evalua ti on

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ACKNOWLEDGED DO NOT REMOVE PLANT NAME: Monticello

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Dear Px. Skovholt:

H3NTICEllA NUCIIAR GENERATING PIANT Docket No. 50-263 License No. DPR-22 Inverted Poison Tubes in control Blades Your April 1,1974 letter asked that we meet certain conditions until inspection of all control blades in the Manticello reactor is completed.

The reactor was shut down on March 14, 1974 for the annual refueling outage. A 1007, eddy current inspection of all control blades in the core was included in the outage activities. The test was performed under the direction of General Electric; their report of the test results is attached.

Two corrections should be made to page 8 of the report.

The " core position" in item 3 should be 10-19 rather than 10-17.

Also, the thirs sentence on the page should be changed to read, "Since the maximum number of inverted absorber tubes remaining in any given blade is three (position 42-15), or approximately 41, of the total of 84, the ef fect on an adjacent rod is negligible."

As a result of the test, six control blades were replaced; in addition, certain blades which remain in the core were identified as having one or more inverted poi son tubes. We have no plans for prematurely re-placing the letter blades.

Calculations show that should the poison powder reach its maximum compaction, the effect on the shutdown margin of the core would be 0.047. A k.

Until these blades are replaced or relocated to reduce this effect, we will include an additional 0.047.ok b t.&

Noh, HERN OTATE'3 POWER Ct.,,dPANY Mr. D J Skovholt May 10, 1974 Page 2 l

for potential compaction in demonstrating the R + 0.257.ok shutdown margin required by Technical Speci fication 4.3. A.1.

The value of R for Cycle 3, prior to including the 0.047.ok allowance for potential compaction, was calculated to be 0 as reported in our November 19, 1973 Second Reload Submittal.

k'e conclude that the inverted absorber tubes remaining in the reactor should have no measurable effect in either steady state or transient operations.

Yours very truly, O.1 y

L 0 Mayer, PE Director of Nuclear Support Services tom /Muv/1h j

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J G Keppler G Charnoff J

Minnesota Pollution Control Agency Attn. E A Pryzina attachment i

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SUMMAFlY AND CONCLUSIONS This report desenbes the eddy current ex amination conducted to identify inverted B.C tubes 6n the Monticello control rod blades Evaluation of the effect on safety margins by the inverted absorber tubes remaining in the reactor is also given Six control rod blades were rejectable under the acceptance entena estabhshed These blades were replaced during the March 1974 refuehng outage. The remainder of the inverted absorber tubes (t 9 in number) have negt gible effect on saf ety margins even if maumum stumping of 0.C is assumed.

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INTRODUCTION Dunng July 1973 a manufactunng deviation (inverted D.C tubes) in the control rod blades became known. This was r: ported as an abnormal occurrence to the USAEC,6nitially by the Millstone Point Company. A desenption of the inversions, the predicted population of me inversions, and evaluation of effects on safety margin were first introduced on the Millstone docket (Reference 1) and later on a genenc basis (Reference 2).

At the Mont# cello site, an eddy current excmination was used to detect inverted absorber tubes. Dunng March 1974 of the refuebng plant outage all control rod blados 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 sn acceptable number of inverted absorber tubes left in the core.

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3.

SUMMARY

OF EXAMINATION Dunng the penod of March 29,1974 through Apol 1,1974 an oddy current examination was pertormed on 121 controf tcd blades located in the Monticello reactor pressure vessel The objective was to locate the steel wool snserted in the bottom end of the absorber tube pnot to installation of the end plug The detection of the sleel wool 65 an iridication that the absorber tube had been installed in the blade in an upside down posttion This examination was conducted by personnel certified in oddy current techniques in accordance wfth the recommended practices of the Amencan Society for Nondestructive Testing 3

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

Preparatory to the c: amination, two fuel bundles diagonally opposRe 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. Dunng the scanning, any change in the conductive or

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magnetic properties of the wing resutts in change in the intemal" eddy current" induced in it by the coil, and these changes in turn attect the impedance of the coil. This impedance variation is electronically processed and displayed on an oscilloscope or stnp chart. A trained inspector then compares this trace with a trace developed from known conditiont in a control rod blade standard.

As each control rod blade was examined, results were recorded on data sheets identifying the core poskion and the wing in the control rod blade.

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RESULTS AND EVALUATION 4.1 RESULTS i

At conclusion of the er amination, it was determined that of the 121 blades examined,129 absorber tubes in 21 control rod blades were inverted. One wing was completely inverted in five blades 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 blado.

l 4.2 EVALUATION 4.2.1 Acceptance Criteria An extensive entecal expenment program was conducted at the KWU facility at Grossweigheim, Germany, for the purpost, of obtaining data for use in establishing acceptance criterla for inverted control rod blades (see Reference 2). Based on these results the following acceptance entena were derived. Any control rod blade is acceptable if it meets these requirements:

1.

No wing should 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 enteria is that the permitted deviation shall not cause a decrease in shutdown margin of more thanapproximately0.0025 Akgr,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 Analysta Dased on the above enterla, six control rod blades listed in Table 1 are rejectable. These are blades in core positions 0615,10-23,1819,2211,26 51, and 34-39. All six of these were replaced during the March 1P74 refueing outage. From Table 1 it can be seen that a total of 129 absorber tubes are inverted. If the number of inverted absorber tubes in the six replaced rods are subtracted, a net of 19 inverted absorber tubes remain in the f eactor, in terms of fraction of total B.C tubes, this is:

19 4x21x121 Shutdown Margin Dased on values given in Figures 1 and 2, 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 aff ected by surrounding blades with inverted tubes is position 38 19. This is adjacent to positions 34 19,42 15, and 42 23 all of which contain inverted tubes. For conservatism Rod 3015 was also included in the evaluation. Other inverted tubes will be sufficiently separated f rom the rod of concem (38 19) 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 spa 3fications shutdown margin requirement of 0.25% AK can ba met with extra shutdown margin with maximum theoretical settling of B.O assumed in the remaining few inverted absorber tubes.

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Table 1 MONTICELLO CONTROL ROD SLADE EXAMINATION

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B.C Tubee Blade Core Wing and Locetion of inverted B.C Tuboe' per CRD item Seriet No.

Position A

B C

D Blade l

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All 1

MT49 06 15 1

6 2

MT113R 06 35 1

3 MT51R 10 17 12 21 All 4

MT59R 10 23 All 21 5

MT16R 18-19 1

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MT7R 18 31 1

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MT104R 18-43 1

3 1

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8 MT53R 22 07 All 21 9

MT12R 22 11 1

21 10 MT18R 22 31 1

21 11 MT112R 22 39 1

1 12 MT74R 22 51 17 1

13 MT93R 26 11 1,8,16,17 & 21 5

14 MT118R 2651 2

15 MT65R 30 15 12 2

2 4,19 16 MT100R 30-39 1

17 MT92R 34 19 1

18 MT96R 34 39 All 21 10,20 3'

19 MT14R 42 15 1

1 20 MT40R 42 23 5

1 4

21 MT99R 42-43 Total 129

'NumWs recrnent locaten of B.C tutwo m Ow wng per Fepure 1.

Other Accidents and Transiente Eff ect of the potential settlar,9 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 settled the maximum of 16 inches, the worth in the adjacent blade would be increased by an eemated 0.002 AK. Since the maximum number inverted absorber tubes remaining in any given fl blade is)wd'(position 3F39). or approximatelM of total of 84, the effect on an adjacent rod is negligible.

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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 B.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 0.19% of the absorber tubes in the core inverted, the loss in pressure margin is negligible.

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REFERENCES 1.

Reactor Control Blade Evaluation, Juty 23,1973 (MNPS.1 Special Report) 2.

J. A. Hinds (GE) letter to D. J. Skovhort (USAEC), dated October 8,1973 3.

D. L. Richardson and T. D. Smith,

• Procedure for Locating Carbon Steel Heat Sinks in Control Rod Tut >es Dunng Plant Outages," November 1973 (NEDO-20211) h 11./.12

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