ML20101J587

Rev 0 to Technical Rept NE-876, North Anna Unit 2,Cycle 8 Core Performance Rept

ML20101J587
Person / Time
Site:	North Anna
Issue date:	05/31/1992
From:	Dziadosz D, Thanh Nguyen, Psuik T VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.)
To:
Shared Package
ML20101J555	List: ML20101J552 ML20101J587
References
NE-876, NE-876-R, NE-876-R00, NUDOCS 9207010148
Download: ML20101J587 (53)

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TECilNICAL REPORT NE-876 - Rev. 0 NORTti ANNA UNIT 2, CYCLE 8 CORE PERFORMANCE REPORT r

NUCLEAR ANALYSIS AND FUEL POWER ENGINEERING SERVICES VIRGINI A POWER MAY, 1992 4/ ICV Q PREPARED BY [/ [

0 4:y,v T. T. Nguyen date' REVIEWED BY:

5/tc/ 94 T. S. Psuik Date REVIEVED BY: I/1 hos f 41-f t.

T. /.. Brookmire Date REVIEWED BY: j b 7 '7 1

5' 274 z.

A. P. Main Date APPROVEDBY:m[

P ' 0 * > / #-27-72

0. Dzietlosz 6' Date OA Category: Nuclear Safety Related Keywords: N2C8, Core Performance

s s-TABLE OF CONTENTS PAGE 1

Table of Contents List of Tables 2

3 List of Figures.

Section 1 Introduction and Sunmary.

5 Section 2 Burnup.

12 Section 3 Reactivity Depletion.

. 23 Section 4 Power Dist:ibution.

25 Section 5 Primary Coolnnt Activity.

45 Section 6 Conclusions 50 Saction 7 References.

51 e

l l

l NE-876 N2C8 Core Performance Report Page 1 of 51

IS LIST OF TABLES TABLE TITLE PAGE 3

r

-4.1 Summary of Flux Maps for Routine Operation

. 29 s

'\\

1 NE-876 -N2C8 Core Performance Report Page 2 of 51

s LIST OF FIGURES FIGURE TITLE PAGE 1.1 Core Loading Map....

8 1.2 Burnable Poison and Source Assembly Locations.

9 1.3 Movable Detector Locations.

10 1.4 Control Rod Locations.

11 2.1 Core Burnup llistory 14 1

2.2 Monthly Average Load Factors 15 2.3 Assemblywise Accumulated Burnup: Measured and Predicted 16 2.4 Assemblywise Accumulated Burnup:

Comparison of Measured and Predicted 17 2.5A Sub-Batch Burnup Sharing 18 2.5B Sub-Batch Burnup Sharing 19 2.5C Sub-Batch Burnup Sharing 20 2.5D Sub-Batch Burnup Sharing 21 2.5E Sub-B tch Burnup Sharing 22 3.1 Critical Boron Concentrar. ion versus Burnup - IlFP-ARD 24 4.1 Assemblywise Power Distribution - N2-8-07 30 4.2 Assemblywise Power Distribution - N2-8-14 31 4.3 Assemblywise Power Distribution - N2-8-25 32 4.4 lbt Channel Factot Normalized Operating Envelope 33 o

4.5 lient Flux Hot Channel Factor, F (Z) - N2-8-07 34 4.6 Heat Flux Hot Channel Factor, F (Z) - N2-8-14 35 q

4.)

Hea: Flux Hot Channel Factor, F (Z) - N2-8-25 36 q

4.8 Maximum Heat Flux ilot Channel Factor, F ( Z )*P, vs

?.

n Ax ia 1 Posit ic;n 37 NE-876 'N2C8 Core Performance Report Page 3 of 51

l -.

LIST OF FIGURES CONT'D

. FIGURE TIT 1.E PAGE 4.9 - Maximum lleat Flux flot Channel Factor, F (Z), vs. Burnup

. 38 q

4.10- Maximum Enthalpy Rise !!ot Channel-Factor, F-delta-Il vs.

Burnup.

.-39

.. 40

[

4.11 Target Delta Flux versus Burnup

'4.12 Core Average Axial Power Distribution - N2-8.

. 41 4.13 Core Average Axial Power Distribution - N2-8-14

, 42 4.14 Core Average Axial Power Distribution - N2-8-25 43 4.15 Core Average Axial Peaking Factor vs. Burnup.

. 44 5.1. Dose Equivalent 1-131 vs. Time.

. 48 5.2 I-131/I-133 Activity Ratio vs. Time

. 49

\\

/

NE-876 N2C8 Core Performance Report Page 4 of 51

+'

Section 1 INTRODUCTION AND

SUMMARY

On February 26, 1992, North Anna Unit 2 completed Cycle 8.

Since the Initial criticality of Cycle 8 on November 1,

1990, the reactor core 8

produced approximately i.0862 x 10 MBTU (18,239 Megawatt days per met ric ton of contained uranium).

The purpose of this report is to present an analysis of the core pe r f o rtna n c e for routine operation during Cycle 8.

F The physics tests that were performed during the startup of this cycle were covered in the North Anna 1:n i t 2,

Cycle 8 Startup P;iy s i cs Test Report

• and, therefore, will not be included here Nort h Anna Unit 2 was in coastdown f rom January 14, 1992, at which t ime

~

the burnup was approximately 16,829 MWD /MTU. The coastdown accounted for an additional core burnup of roughly 1,410 MWD /MTU from the end of full power reactivity.

The Cycle 8 core consisted of 14 sul-batches of f uel: four once-burned batches from Cycle 7 (batches 9A, 9B, N1/10A, and N1/10B); seven twice-burned batches, one from North Anna 1 Cycles 3 and 4 (batch N1/5),

one f rom North Anna 2 Cycle 5 and North Anna 1 Cycle 4 (batch N1/6), one f rom Nort h Anna 1 Cycles 5 and 6 (batch N1/ 7), one f rom North Anna 2 Cycles 2 and 3 (batch 4), one from North Anna 2 Cycles 3 and 4 (batch 5A), and two from North Anna 2 Cycles 6 and 7 (batches 8A and BB); one NE-876 N2C8 Core Performance Report Page 5 of 51

L

thrice-burned' batch f rom North Anna 2 Cycles 5,6, and 7 (batch 7A); and' two fresh batches (batches 10A and 10B).

The North Anna 2 Cycle'8 core loading map specifying the fuel batch identification, and fuel assembly locations is shown in Figure 1.1.

The burnable poison locations and source assembly locations is shown in Figure 1.2.

Movable detector locations are shown in Figure 1.3.

Control ro'i locations are shown in Figure 1a.

Routine core follow involves the analysis of four principal performance indicators.

These are burnup distribution, reactivity depletion,-power distribution, and -primary coolant activity.

The core burnup distribution is followed to verify both burnnp symmetry and proper batch - burnup sharing, thereby ensuring that the fuel held over for the next cycle will be compatible with the new fuel that is inserted.

Reactivity depletion Js monitored to detect the existence of any abnormal reactivity behavior, to determine if the core is depleting as designed, and to indicate at what burnup level refueling will be -required.

Core power distribution' follow includes the monitoring of nuc1c r hot channel factors _to verify that they are within the Technical Specifications:

1imits, thereby ensuring that adequate margins for linear power density and critical-heat flux thermal limits are maintained.

Lastl;, as part of normal core follow, the primary coolant activity is monitored to verify that the dose equivalent iodine-131 concentration is within the limits 2

specified by the North Anna Unic 2 Technical Specifications 4

radiciodine analysis based on the iodine-131 concentration in the coolant is performed to assess the integrity of the fuel.

NE-876 N2C8_ Core Performance Report Page 6 of 51

Each of the four performance indicators is discussed in detail for the North Anna Unit 2, Cycle 8 core in the body of this report. The results are summarized below:

1. Burnup - The burnup tilt (deviation from quadrant symmetry) no greater than 10.30' with the burnup accumulation in on the core was each batch deviating f rom design prediction by no recre than 1.69*..

2. Reactivity Depletion - The critical boron concentration.

used to monitor reactivity depletion, was consistently within 10.39' AK/K of the desigii prediction which is within che ilt AK/K margin allowed by Section 4.1.1.1.2 of the Technical Specifications 3.

Power Distribution - Incore flux maps taken each month s

indicated that the assemblywise radial power distributions deviated from the design predict ions by a muimum average dif ference of 2.5*,.

All hot channel facters inet their respective Technical Specifications limits.

4.

primary Coolant /,c t. i v i t y - The average dose equivalent iodine-131 activity level in the primary coolant during Cycle 8 was approximately 0.0242 pCi/gm.

This corresponds to less than 31 of the operating limit for the concentration of radiciodine in the primary coolant. Radiciodine analys is indicated several fuel rod defects, which prompted ultrasonic testing (UT) during the Cycle 8 to Cycle 9 refueling outage During UT testing, it. was confirmed that night f ue l rcxis in five fuel assemblies were defective NE-SM N2C6 Core Performance Report page 7 of 51

Figure 1.1 NORTH ANNA UNIT 2 - CYCLE 8 CORE 1.0ADING MAP

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NE-876 N2C8 Core Performance Kaport Page 8 of 51

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Page 9 of 51

Figure 1.3-NORTH ANNA UNIT 2 - CYCLE 8 h.%MOVAlli.E - DETECTOR LOCATIONS N '%,

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NE-876 N2C8 Core Performance tieport Page 10 of 51

4 Figure 1.4 NORTil ANNA UNIT 2 - CYCLE 8 CONTROL ROD LOCATIONS R

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SP tSpare Rod Iocations) 8 NE-576 N2C8 Core l'erformance Report Pap,e 11 of 51

Section 2 4

1 BURNUP t

The_ burnup history for -the North Anna Unit 2,

Cycle 8 core is graphically depicted in Figure 2.1.

The Nor th Anna 2,

Cycle 8 core achieved'a burnup of 18,239 tlWD/MTU.

.As shown in Figure 2.2, the average load factor for Cycle 8 was 95.2% when referenced to rated thermal power J

= (2893 MW(t)). Unit 2 performed-a power coastdown starting on January 14, 1992 until shutdown for refueling on February 26, 1992.

Radial - (X-Y) burnup distribution maps show how the core burnup is shared'among the various fuel assembl!es, and thereby allow a detailed burnup distribution analysis.

The NEWTOTE' computer code is used to calculate these assemblywise burnups.

Figure 2.3 is a rcdial burnup

-distritiution map in which the assemblywise burnup accumulation of the core at the end of Cyc1c-8 operation is given.

For comparison purposes, the design values are also given. - t'igure 2.4 is a radial-burnup distribution map in which the percentage difference comparison of measured and

_ predicted assemblywise burnup accumulation at the end of Cycle 8 operation is also given. As can be seen from this figure, the accumulated assembly burnups were generally within 12.98% of the predicted values.

In

- addition, deviation from quadrant symmetry in the core throughout the cycle was'no greater than 10.30%.

The burnup sharing on a batch basis is monitored to verify that the e

core is operating as designed and ;o enable accurate end-of-cycle batch NE-876 N2C8 Core Performance Report Page 12 of 51 l

~

burnup predictions to be made for use in reload fuel design studies.

Batch definitions are given in Figure 1.1.

As seen in Figures 2.5A, 2.5B, 2.5C, 2.5D, and 2.5E the batch burnup sharing for North Anna 2, Cycle 8 followed design predict ions closely with no batch deviating from prediction by more than 1.69%.

Syemetric burnup in conjunction with agreement between actual and predicted assemblywise burnups and batch burnup sharing indicate that the Cycle 8 core did deplete as designed.

i i

SE-876 N2C8 Core Performance Report Page 13 of 51

Figure 2.1 NORTH ANNA UNIT 2 - CYCLE 8 CORE BURNUP HISTORY non.- -

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WAXlWUW DESICS SURNVP -

19800WWD/VIU NE-876 N2C8 Core Perrormance Report Page 14 of 51

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MONTH NE-876 N2Cd Core Perfornance Report Page 15 of 51

Figure 2.3 NORTH ANNA UNIT 2 - CYCLE 8 ASSEMBLWISE ACCUMULATED BURNUP MEASURED AND PREDICTED (GWD/MTU)

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NE-876 N2C8 Core Performance Report Page 16 of 51 i

i

4' 4

Figure 2.4 NORTH ANNA UNIT 2 - CYCLE 8 ASSEMBLWISE ACCUMULATED BURNUP COMPARISON OF MEASURED AND PREDICTED (GWD/MTU)

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37,921 45,097 7,176 BURNUP TILT N1/7 1

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---

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2 23,869 42,678 18,809 SW =

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21,890 21,890 CYCLE AVERAGE ACCUMULATED BURNUP = 18,239 I

NE-876 N2CS Core Pelformanct R e po r t.

Page 17 of 51

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e-Section 3 i

REACTIVITY !)El>LETION The primary coolant cr it ical borcr cot.cerit rat ion is monitored f or the purposes of f ollowir g core reactivity and to ider.t i f y any anomalous reactisity be ha,' i o r. The FULLOW' computer code was used to no. cn a l i ze

" actual" critical boron concentration measurements t o ries ign cond i t iote, taking Jnto canalderation control rod position, xenon concentration, noderator temperature, and power level.

The normalized critical boron conc"nt rat ion versun burnup curvr for the North Anna ? Cycle 8 core is shown in rigure 3.1.

It :an he seen that the measured data typically compared to within 58 ppm of the design prediction. This correspondr, to 10.39*,

AK/K which is within the 1 1 *. AK/K criterion for reactivity anomalies set f or th in Soc'. ion 4.1.1.1.2 of the Te chn ica l Spec i f icat i_ons.

In conclusion, the trent indicated by the cr it ical horon coacentration verifies that t.he Cycle 8 core deplettd as expected wit hout any r eact iv it y anomalics, i

NE-876 N2C6 Core Performance Report

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NE-A76 N2CS Core Performance keport Page 24 of 51

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Section 4 i.

POWER DISTRll3UT10N Analysis - of core power distribution data on a routine basis is necessary to verify that the hot channel factors are within the Technical j

Specifications limits and to ent.ure that the reactor is operating without 1

any= abnormal conditioon which could cause an

' uneven" burnup i

distribution. Three dimensional core power distribut ions are determined S

Irom movable dutcetor flux map measurements using the INCORE computer program. A summary of all full core flux maps taken since the completion of startup physich testing for North Anna 2, Cycle 8 is given in Table

-4.1.

Power distribution maps wern generally taken at mont hly intervals with addioonal.uaps taken as needed, l

i I

Radial (X-Y) core power distribut ion f or a representative series of l

incore' flux.aps are given in f igures 4.1, 4.2, and 4.3.

Figure 4.1 show-a power distribution map that was taken early in cycle life.

Figure 4,2 shows a powca distribution map that was taken near mid-cycle burnup.

Figure 4.3 shows a map that was taken near the end of

.cle 8.

The measured relat ive assembly powers were generally within 7.0*.

and the maximum average percent dif ference was equal.to 2,5*,.

In addition, as

' indicated - by the INCORE tilt factors, the power distributions were essentially symmetric for each case.

hn important aspect of core pbwer distribution follow is the monitoring of nuclear hot' chennel factors.

Verification that these factors are NE-876 N2C8 Core Perfo mance Report Page 25 of 51

within Technical Specifications limits ensures that linear power density and critical heat flux limit:: wall not be violated, thereby providing adequate thermal margin and maintaining fuel cladding integrity.

North Anna Unit 2 Technical Specification 3.2.2 limited the axially dependent heat flux hot channel factor, F (Z), to 2.19 x K(Z), where K(Z) is the g

hot channel factor normalized operating envelope, and 2.19 is the Fq limit at. rated thermal power, both as specified in the Core Operations Limit Report (C0hR)*

Figure 4.4 is a plot of the K(Z) curve associated with the 2.19 F (Z) limit.

q The axially dependent heat flux hot channel factors, F (Z), for a 9

representative set of flux maps are given in Figures 4.5, 4.6, and 4.7.

Throughout Cycle 8, thn measurou values of F (Z) were within the Technical 9

Specifications limit.

A summary of the maximum values of axially-dependent heat tlux hot channel factors measured during Cycle 8 is given in Figure 4.8.

This figure indicates that the minimum margin to the F limit in the axial region covered by the Technical Spec!fication 9

4.2.2.2 is 11.4%.

(Technical Specificat. ion 4.2.2.2.g states that Fq surveillence is not applicable in the lower core region from 0% to 15%

inclusive, and the upper core region from 85% to 100% inclusive.)

Figu re - 4.9 shows the maximum - values for the heat flux hot channel factor measured during Cycle 6.

As can be seen from the figure, there was an approxim.Ite 16.00'. margin f rom the maximum F (Z) to the 2.19 limit q

at - the beginning c,f thn cycle, which was the minimum margin seen for the cycle.

NE-876 N2C6 Core Performance Report page 26 of 51

The value of the enthalpy rfSe hot channel factor. F delta II, which is the ratio of the integral of the power along the rod with the hinhest inte3; rated power to that of the average rod, is routinely followed.

The Technical Specifications limit for this parameter is set stah that the departure from nucleate Soiling ratio (DN!!K) limit will not be violated.

Additionally, the F-delta-Il limit ensures that the value of this paramet er i

used in the LOCA-ECCS analysiv in not exceeded during normal operation.

North Anna Technical Fpecification 3.2.3 limited the enthalpy rise hot channel factor to 1.49(140.3(1-P)) for Cycle 8,

where 1.49 is the F-delta-ll at rated thermal power and 0.3 is the power f actor rnult iplier, both as specified in the C01.R.

A summary of the ma x i mt.m values for the enthalpy rise hot channel factor monaured during Cycle 8 is given in i

Figurr. /.10.

A, can be seen from this figure, the mini um margin to the limit was approximatelv 2. 2'.

The target delta flux

• is the delta flux which would occur at condit ions of f ull ; ower, all rods out, and equilthrium <enon.

The delta f?ax is measured with the core at or near these condit ions ar.] the t arget delta flux is established at this incasured paint. Since the target delta flux varies as a f unct ion of burnup, the target value is upda t ed rnonth ly.

I!y maintaining the value of delta flux r elatively constant, adverse axial power shapes due to xenon redistribution are avoided.

The plot of the target delta flux versus burnup, given in Figure 4.11, shows the value of this parameter to have been approximately -1.4*. at the Pt-Ph De l t a F lux = ----- X 100 where Pt power in top of rore ( %( t ) )

=

2893 Pb = power in bottom of core ( %( L ')

NE-876 N2C8 Core Performance keport Page 27 of 51

i ber. inning of Cycle 8.

Delta flux values decreased steadily to -5.3% near a cycle burnop of 14,300 MWD /HTU, where it then gradually increased to j

-3.6% before the coastdown. - At the end of Cycle 8, the target. delta flux lucrea*.ed to 44.2% due to ti.e coastdown. This axial power shif t can also be observed in the correspondf r; core average axial power distribution for a representative series of maps given in Figures 4.12 through 4.14.

[

In Map N2 8-07 (Figure 4.12), taken at 1505 MWD /MTU, the axial power dist ribution had a shape peaked towerd the middle af the core with a peaking ' factor of 1.208.

In Map N2 8-14 (Figure 4.13),

t.a k e n at r

approximately 9129 MW9/MTU, the axial power distribution peaked slightly toward the bottom of the core with an axial peaking factor of 1.157 Finally, in Map N2-8 25 (Figure 4.14), taken at 16,640 MWD /MTU, the axial peaking factor was 1.157, with the axial power distribution shifted alightly back toward the top.

The history of F-Z during the cycle can be seen more cicarly in a plot of F-Z versus burnup given in Figure 4.15.

In conclusion, t he North Anna 2 Cycle 8 core performed satisf actorily

. with power cistribution analyses verifying that. desigt, predictions wuri

+

f accurate and that the values of the F (Z) and F-delta-ll hot chnnnel q

factors were within the limits of the Technical Specifications.

t i

r 1

NE-876: N2C8 Core Performance Report Page 28 of 51-

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t l~

NE-876 N2C8 Core Performance Report Page 30 of $1

.. ~.. -. - -. _ -. -. _. _. - _ _ _ _ _ _. -. - -... - _. _... _. -. _., _., _. _. -. _ -. _.. - ~ _ ~

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Figurn 4.2 NORTH ANNA UNIT 2 - CYCLT. 8 ASSEMBLYWISE POWER DISTRIBUTION N2-8-i4 W

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fill

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=

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= -4.532 NL-SM N2C8 Core Performance Report Pa g,e 31 of 51

Figure 4.3 NORTH ANNA UNIT 2 - CYCLE 8 ASSE%LWISE POWER DISTRlbOT10N N2-8*25 R

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NE-876 N2C8 Core Performance Report Pag;c 32 of 51

1 Figure 4.4 NORTil ANNA Unit 2 - CYCLE 8 110T CllANNEL FACTOR NORMALIZED OPERAT1NG ENVELOPE 1.20,

( 6.0.1.C )

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L NE-876 N2C8 Core Performance kcport Page 33 of 51

Figure 4.5 NORT}l ANNA Unit 2 - CYCLE 8 liEAT FLUX 110T CllANNEL FACTOR, F (Z) 9 N2-8 07 2.50--

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TDP NE-876 N2C8 Core Performance Repcrt Page 34 of 51 l

9 Figure 4.6 NORTil ANNA Unit 2 CYCLE 8 ilEAT FLUX ll0T CilANNEL FACTOR, F (Z) q N2-8 14 2.50 I

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TOP NE-676 N2C8 Core Per;ormance Report Pa t;e J5 of 51

Figure 4.7 NORTH ANNA Unit 2 - CYCLE 8 HEAT FLUX HOT CHANNEL FACTOR, F (Z) g N2-8 25 2.50 A6 I

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i NE-8'/6 N2C8 Core Perf ormance Report Page 36 of 51

o Figurt 4.8 NORTH ANNA Unit 2 CYCLE 8 MAXIMUM llEAT FLUX !!OT CilANNEL FACTOR, Fn(Z)/ P, vs. AXI AL POSITION 2.2 N

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10P Of CORE NE-8"6 N2CS Cort-Performance Report Page 37 of 51

Figure 4.9 NORTH ANNA Unit 2 - CYCLE 8 i

MAXIMUM llEAT FLUX ll0T CilANNEL FACTOR, l'q(2), vs. BURNUP

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'S CYCLE BURNUP (GWd/MtU) i NE-876 N2C8 Core Performance Report Page 38 of 51

Figure 4.10 NORTH ANNA 11 nit 2 - CYCLE 8 MAXIMUM ENTHALPY RISE HOT CHANNLL FACTOR, F-delta-ll, vs. BURNUP 1 5 0 r r IILL POWER M

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NE-876 N2CS Core Performaace Rt ort Page 44 of 51

O Section 5 PRIMARY CODI. ANT ACTIVITY i

The specJfic activity levels of radiolodines in the primary coolant are irnportant to core and fuel perforttance as indicators of f ailed fuel and are i rnport ant with respect to offsite dose calculations associated wi*h accident analyses.

Two.aechanisms are responsible f or the presence of radiciodines in the prirra ry coolant.

Radiofodines are always present due to direct fission product recoil from trate fissile materials plated onto core coroponen t s and fuel structured surfaces or trace fissile materials existing as impurities in core structurel materials.

This fissile material in generally referred to as " tramp" material, and the resulting fodines are referred to

.s tramp iodine. Fission products will also diffuse into th-primary coolant if a breach in the cladding (fuel defects) exists.

Fuel detects are generally the pr edom ituint source of rad iciod ines in the primary coolant.

North Anna 2 Technical Specification 3.4.8 limits the radiciodines in the primary coolant to a dose equivalent I-131 value of 1.0 tiCi/gm for

r. odes one through five, inclusive.

Figure 5,1 show:. the dose-equivalent 1-131 act ivity history for Cycle 8.

These data show that the dose equivalent 1-131 activity was substantially below the

1. 0 1.? i / gm limit f or steac'y state power operat ion. The average f ull power equilibrium dose NE-876 N2C8 Core Performance Report Page 45 of 51

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- equivalent 1-131 concentration for the cycle was 2.42 X 10 pC1/gm which corresponds to less than 3% of the Technical Specification limit.

u Correcting the 1-131 concentration for tramp fodine involves calculating the 1-131 activity from tramp fissile sources and subtracting

- this value from the measured 1-131.

The resultant is an estimate of the 1-131 activity resulting directly from defective fuel. The magnitude of the tramp-corrected 1-131 can be used as an indication of the nucher of

' defective - f uel rods.

The cycle average tramp corrected iodine-131 concentration was 1.57 X 10-2 pCi/gm with an average demineralizer flow rate -of approximately 77 gpm during power operation.

This magnit0de of tramp corrected 1-131 typically indicates the presence of defective fuel rods.

Another positivn indication of defective fuel is the presence of spikes in radiofodine during large or rapid power transients.

Several lodine spikes can be seen on Figure 5.1.

r The. ratio of the specific activities of I-131 to 1-133 is used to characterize the type (size) of fuel failure or failures which may have occurred in the reactor core. Use of the ratio for this determination is feasible because I-133 has a short half-life (approximately 21 hours2.430556e-4 days <br />0.00583 hours <br />3.472222e-5 weeks <br />7.9905e-6 months <br />) compared to that of I-131 (approximately. eight days).

For pinhole defects, where.the diffusion time through the defect is on the order of days, the I-133 decays ! caving the 1-131 dominant in activity, thereby-causing the ratio to be roughly 0.5 or more. In the case of larger leaks and tramp material', where the diffusion mechanism is negligible, the l

I-131/1-133 ratio will generally be less than J.1.

The use of these l,

NE-876 N2C8 Core Performance Report.

Page 46' of 31

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ratios with regard to defect size is empirically determined and generally used f.hroughouc the commercial nuclear power industry.

-rigure 5.2 shows the 1-131/I-133 ratio data for North Anna 2 Cycle 8.

Aside from the large increases in the ratio during the time when the defects occurred, the 1-131/I-133 ratio settled out below a ratio of 0.5 toward the middle and end of cycle.

This indicates that the defects in-the cladding were likely to be myierately sized.

Fuel ultrasonic testing was performed during the Cycle 8 to Cycle 9 refueling outage. Eight fuel rods in five fuct asser.blies were confirmed to be defective.

The five fuel assemblics are X49, Y39, Y40, Y42, Y47.

i Assembly X49 was used for two cycles, Visual confirmation of the defective rod showed a t hrough-wa ll def ect. on a corner rod below the bottom grid.

This defect. appears to be externally generated, but it is not clear whether debtis or some other external mechanism induced the primary defect.

Extensive hydriding was also observed toward the upper spans of this rod.

Assemblics Y39, Y40, Y42,-and Y47 are all from the i

new fuel batch for Cycle 8.

No evidence of debris induced failures was found, during the visual examination of these assemb!!cs There was evidence of hydriding of the fuel cladding just above the bottom grid on the failed fuel rods that were visible.

Possible failure mechanisms are currently being evaluated with the fuel vendor (Westinghouse). These fuel assemblies will be restricted f rom further use pending any repair projects to replace the defective fuel rods.

1 NE-876~ N2C8' Core Performance Report Page 47 of 51 g

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DATE NE-876 N2C8 Core Performance Report Page J.9 of 51

i 9

Section 6 CONCLUSIONS P

The North Anna 2, Cycle 8 core has completed operation.

Throughout this cycle, all core performance indicaters compared favorably with the design predictions and the core redated Technical Specifications limits were met. with significant margin.

No significant abnormalities in reactivity or burnup accumulatien were detected.

Kadiofodine enalysis indicated that there were apparr.nt fuct rod defects during Cycle 8.

During ultrasonic tunting of the fuel, eight fuel rods in five fuel assemblics were determined to be defective.

One of the five fuel assemblies was tand for two cycles of ope ra t ion.

The remining four asemblies woro e sed only one cycle.

.These five assemblics will be

. reatricted from further use pending repair.

4 6

NE-876 N2C8 Core Performance Report Page 50 of 51

l t-Section 7 REFERENCES 1)

E. A. Hoffmaa, "Noith Anna Un'.

.vcle 8 Startup Phys ics Test Report," NE-817..icr

+f, 1991.

2) North Anna Power duo. ion Jnit 2 Technical Specifications, Sections 3/4.1, 3/4.2 and 3/4.4.8.

3)

T. K. Ross, "NEWTOTE Code", VEPCO NFO-CCR-6 Rev. 9, April, 1984 4)

R. D. Klatt, W. D. Leggett, ill, and L. D. Eisenhart.

" FOLLOW Code," WCAP-7482, February, 1970.

5)

W. D. Leggett, III and L. D. Eisenhart, "INCORf. Code,"

WCAP-7149, December, 1967.

6) MemGr;t ud um from R. G. McAndrew to J. R. Hayes " Core Operating Limits Report (COLR) Tech Spec Amendment 146/130", July 5, 1991.

7)

T. T. Nguyen, " North Anna 2, Cycle 8 FULOW Input and Calculations",

PM-363, rev. O. Addendum B, May 1992.

.-2C8 Core Performance Report Page 51 of $1 la-876 4

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