ML19339A057

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Forwards Evaluation of Cause of Discrepancy as Suppl to 740918 Unusual Event Rept.Incorrect Burnups Used for Shuffled Fuel in Calculation of Control Rod Worths
ML19339A057
Person / Time
Site: Yankee Rowe
Issue date: 12/05/1974
From: Heider L
YANKEE ATOMIC ELECTRIC CO.
To:
US ATOMIC ENERGY COMMISSION (AEC)
References
WYR-74-47, NUDOCS 8010310694
Download: ML19339A057 (11)
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Telephone 6l7 366-90ll 1

rwx 710-390-0739 YANKEE ATdMIC ELECTRIC COMPANY m.a.,.,

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United States Atomic Energy Commission II ',

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Attention: Directorate of Licensing

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References:

(a) License l'o. DPR-3 (Docket No. 50-59) d '.

(b)

YAEC letter to USAEC dated 9/18/74

Dear Sir:

Yankee Atomic Electric Company submits the attached supplement to Eeference 1.

This supplement is an evaluation of the cause of the discrepancy as promised in the original Unusual Occurrence Report.

As srate 3 in the original report, the plant operation below 400 ppm all-rods-cut critical boron will :ce addressed in a separate submittal.

The present all-rods-out critical bcron concentration is 950 ppm.

1:e trust you will find this information satisfactory; however, should you desire additional i1 formation, feel free to contact us.

Very truly yours, YA!!KEE ATOMIC ELECTRIC COMPAIN Il ity

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L. H. Heider 87 DEC9 19 M l'* J-xanager of Operations REGULAiC W. h C' dm

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' Chronology p.:pvd vf.ut tmd l9 '_. P7

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8/13 (Tues)-8/17 (Sat)-Measurements of control rod worth in Core XI.

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a 8/14 0.'ed)-S/19 (Man)-Preliminary reduction of data from worth measurements 8/19 (Mon)-Notification of AEC by phone call to F. Burger.

8/19 (Mon)-Internal non-conformance report filled out; by plant and reviewed by PORC, recommending that " power operation (bel based on a satis-factory safety evaluation being performed by NSD [ Yankee Nuclear Services Division) justifying operation with the measured rod worths."

8/19 (Mon)-NSAR Committee approves operation at 100 percent power based on NSD safety analysis review.

8/19 (Mon)-Investigation of discrepancy begun.

9/19 (Wed)-Unusual oc,:nrrence report filed with AEC.

2.

Core Description The configuration of Core XI is described in the Yankee Rowe FHSR, as nodi-fic.d by Amendacr.: No.

9.

A brief description is included here to facilitete.under-standing the explanation which follows:

The core consists of,:

e 36 fresh, 4 w/o zire clad assemblies; 36 once bu*ned, zirc-clad assemblies of 6 w/o initial enrichment; e

o 4 twice-burned, stainless steel clad assemblies of 4.94 w/o initial enrichment.

Each zire asse bly is surrounded by an integral perforated stainless steel shroud for structural support. As shown in Figure 1, the core was-loaded in an annular fashion, with the turned fuel preferentially toward the center.

The same kind of loading pattern had been used in previous Yankee cores.

Figure 1 also displays

  • he placement of the various contro] rod groups relative to the fuel loading pattern.

3.

Summary of Discrepancy Table 1 compares the measured control rod worths with the original calcu-lated values.

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4 ASSD1BLIES Stainless Steel Clad, Twice Recycled 36 ASSDiBLIES Zircaloy Clad, Once Recycled 36 ASSDIBLIES Zircaloy Clad, Fresh Figure 1 YA';KEE EUCLEAR YANKEE CORE XI FUEL LOADING AND CONTROL PO'4ER STAT 10S ROD BA.X LOCATIOSS l

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- Table 1

~ Control Rod Bank Worths 4

Measured vs. Original Calculated Values j

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i Rod Bank Temperature Calculated Measured North i

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Designation Condition Worth (%Ap)

(% Ap)

A Cold 0.66 0.52 1

B Cold 2.04 1.77 I

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4.

Cause of Discrepancy n

It has been de'termined that incorrect burnups were used for the shuffled fuel in the calculation of control rod worths.

Because of a numerical error, these burnups were approximately.eight (8) percent lower than the true values, as shown in Table 2.

The relative locations of the shuffled fuel and of the control rod banks are shown in Figure 1.

The increased burnup of the shuffled fuel in the center of the core represents a decrease in the reactivity of this region, which causes the decrease in the worth of control rods located here.

5.

Supporting Evidence This explanation is supported by the following observations:

1.

Calculations using the corrected burnups in a slightly revised model shou nuch better agreecent with the measured control rod worths.

Table 3 shows the comparison of new and revised calculations withthe measured values.

2.

The increased burnup. lover reactivity) in the shuffled fuel manifests itsel# in a saall but observable discrepancy between measured and calcu-lated asseshly powers. Again, revised calculations with the corrected burnups.show cuch better agreement, as indicated in Table 4.

3.

The parcentage discrepancy in control rod worth is greatest for the bank locate?. in the midst of the shuffled fuel (Bank A) and least for those banks (B and C) located nearer the interface between fresh and shuffled fuel.

In addition, Banks B and C, which are about equally distant fren the interface, exhibit equal discrepancies.

Measures taken to insure that no other f actors contributed to.the discrepancy included the following:

N 1.

The reactor engineer verified that the core had actually been loaded as intended.

2.

The teasurements of control rod worth were repeated by Ucstinghouse Nacicar Services Division, using their own reactivity computer.

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' Table 2 Batch Burnup Comparison - Corrected vs. Uncorrected i

Uncorrected.

Percent Increase Burnup.

in Batch Average 1

i Fuel Batch (FED /MTU)

Burnup*

Fresh 0

0 Once Burned 11300 8

i Twice Burned 19600 10

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Control Rod-Bank Worths Measured vs. Revised Calculated Values Revised l-Rod Bank Temperature Calculated Measured Worth j

Designation.

Condition' Worth (% AD)

(% Ap) f A

Cold 0.53 0.52 f

B Cold 1.74 1.77 C

Cold 1.41-1.35 i

'A Hot 0.73 0.70 B

Hot 2.34 2.21 1

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The reactor engineer verified (by noting that a period resulted t

from incremental conrol rod motion) that each individual control L

rod was indeed latched.

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The model used to calculate control rod worths was checked f }

independently.

5.

Calculations were performed to show that control rod worth i

could not be affected by lateral movement of the stainless r

steel shroud surrounding the zire assemblies.

6.

Monte Carlo calculations of relative control rod worth were coni'eted by an outside party (NUS Corporation) to verify that contro.'. rod worth was not significantly affected by whether the fuel was clad with zire or stainless steel.

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6.

Safe v Implications I

A.

A review of the FHSR, including all amendments has established that the i

difference between measured and calculated control rod worths does not present any unreviewed safety questions except for a Steam Line Rupture at end-of-core life. For this accident, it has been determined tnat operation at full power will not violste the asrueptions used in the i

accident analysis as long as the full power, all-rods-out, critical I

boron concentration is greater than 400 ppm. Plant operation below

]1 400 ppm all-rods-out critical boron will be adaressed in a later sub-mittal.

The present all-rods-out critical boron concentration is about c'

JS8d ppm.

150 f

B.

The effect of the radial tilt-in core pcwer (discussed in Section 5)

' has been to increase the peak fuel rod power by less than 4 percent t

above the nominal predicted value.

Since the nominal predicted value was 10 percent below'the design value used in the accident analysis, (see Table 102-3 of Reference 1), this does not present any unreviewed safety question.

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Sumary i

.The discrepancy-in-control rod worths was due to the incorrect burnups This explanation is supported used for shuffled fuel in the original calculation.

'i by all available evidence. -The'only unreviewed safety quest on. which may occur d in

~ is related to a Steam Line Eupture at end-of-life, which will be addresse i

a later subnitral.

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4 P002 GEMAL AEC DISTRIBUTION FOR_P, ART SO DOCKET MATERI AL (TEMPORARY FORM)

CONTROL NO:

12376 FI LE :-._

FROM: Yankee Atomic Elec. Co.

DATE OF DOC DATE REC'D LTR TWX RPT OTHER Westborough, Mass. 01581 12-5-74 12-9-74 X

L.m %$w TO:

ORIG CC OTHER SENT AEC PDR XX E

DRL 3 signed 37 SENT LOCAL PDR CLASS UNCLASS PROPINFO INPUT NO CYS REC'D DOCKET NO:

40 50-29 xxx ENCLOSURES:

DESCRIPTION:

Ltr trans the following:

Supplemental Info re YAEC

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ltr to USAEC dated 9-18-74 re Unusual Occurrene re plant operation below 400 ppm all-rods-out critical boron......

(LO cys encl ree'd)

PLANT NAME: YAEC N -W'.

PP.mnun FOR ACTION /INFORMATION DHL 12-9-74 DUTLER (L)

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