ML20011A569

Analysis of North Anna-1 Quadrant Power Tilt, Interim Rept

ML20011A569
Person / Time
Site:	North Anna
Issue date:	05/31/1981
From:	Carew J, Diamond D, Saphier D BROOKHAVEN NATIONAL LABORATORY
To:	Dunenfeld M Office of Nuclear Reactor Regulation
References
CON-FIN-A-3373 BNL-NUREG-29870, NUDOCS 8110200813
Download: ML20011A569 (79)

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.0RMAL REPORT LIMITED DISTRIBUTICH ANALYSIS OF THE NORTH ANNA-1 QUADRANT POWER TILT STC ,leScarch anc' Technica' .

\ Assistance Report D. SAPHIER, D. J. DIAMOND, J. F. CAREW AND l. D. EISENHART e <o

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_g gh DATE PUBLISHED - MAY 1981 DEPARTMENT OF NUCLEAR ENERGY BROOKHAVEN NAll0NAL LABORATORY UPTON, NEW YORK 11973

,,., _ Prepared for the U.S. Nuclear Regulatory Commission

- Office of Nuclear Reactor Regulation

, y ,' Contract No. DE-ACO2-76CH00016 A. L2!

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i This report was prepared as an account of worL sponuned by the United Sta:en

, Goscinment. Neither the United States nor the United States Nacicar Regulatory j Commiwlon nor any of their employees, nor any of their contras tors, subcontrartors,

{ or their employecs, makes any warranty, espress or implied, or awumes any legal y liability or respornibility for the accuracy, completeness or usefulness of any inforrna-tion, apparatus, prouog or process discloud, or represents that its use would not

infringe prhately owned rights. ,

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f e e IllTERIil REPORT Accession !!o.

Contract Prog' ram or Project

Title:

Licensing Support Calculations f Subject of this Document: Analysis of the North -Anna-1 Quadrant Power Tilt '

Type of Document: Informal Report Author (s): D. Saphier, D. J. Diomond, J. F. Carew and L. D. Eisenhart Date of Document: May 1981 Responsible llRC Individual -

and !!RC Office or Division: Mr. Marvin Dunenfeld Cor.e Performance Branch This document was prepared primarily for prelirainary or internal use. It has not received full review and approval. Since there may be substantive changes, this document should not be considered jfinal.

Brookhaven flational Laboratory Upton, ilY 11973 Associated for theUniversities, m Inc.i!lC leSearCh anC U.S. Department of Energy . ' eC'iDIC .

Prepared for Assistance Report U.S. !!uclear Regulatory Commission Washington, D. C. 20555 Under Interagency Agreement DE-AC02-76CH00016 11RC Fill flo. A-3373 l

IllTERut REPORT

3 BNL-NUR EG -29870 1NFORMAL REPORT LIMITED DISTRIBUTION i -

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ANALYSIS OF THE NORTH ANNA-1

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1 ABSTRACT An analysis of the observed North Anna Quadrant Power tilt has been per formed. Various tilt inducing perturbations including inlet temperature maldistribution, flow blockage, control rod misalignment and fuel assembly misloading were considered. These perturbations all tend to burn-out with exposure and, therefore, cannot reproduce tb North Anna tilt which increas24 with exposure. However, a crud deposition im,;el has been constructed which reproduces the observed Cycle-1 tilt and is_ consistent with observed data.

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TABLE OF CONTENTS Page ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii LIST OF FIGURES ........................... viii L IS T OF TAB L E S . . . . . . . . . . . . . . . . . . . . . . . . . . . . x A C KN 0k t E D GEM E NT S . . . . . . . . . . . . . . . . . . . . . . . . . . . xi

1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2. POSSIBLE ASYMMETRIC MECHANISMS LEADING TO POWER TILTING. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3.

SUMMARY

OF THE CALCULATIONS AND CONCLUSIONS. . . . . . . . . . . . 3

4. THE N0DE-P 3-DIMENSIONAL MODEL FOR POWER DISTRIBUTION AND BUR N-UP C ALCU LAT IONS . . . . . . . . . . . . . . . . . . . . . 4

5. THE POWER TILT AT NORTH ANNA UNIT-1 CYCLE-1. . . . . . . . . . . . 6 5.1 Cyc l e-1 Co re Lo ad i ng . . . . . . . . . . . . . . . . . . . . 6

5. 2 Observed Tilt Variations with Increasing Co re Powe r Level . . . . . . . . . . . . . . . . . . . . . . 7 5.3 Observed Tilt Variation with Increasing Burn-up. . . . . . . 7 5.4 The Feedback Reactivity Coefficients of Cycle-1. . . . . . . 7 5.4.1 The Doppler Coefficient . . . . . . . . . . . . .. . 8

5. 4. 2 The Moderator Temperature Coefficient . . . . . . . . 8

5. 5 Power Tilt Due to Inlet Temperature Ma1 distribution. . . . . 8 i 5. 6 Power Tilt Due to Control Rod Misalignment . . . . . . .. . 10

5. 7 Power Tilt Due to Increased Albedo in the NE Quadrant. . . . 10 5.8 Power Tilt Due to a 0.1% Increa e in the Fuel Enrich-ment of One As s embl y . . . . . . . . . . . . . . . . . . . . 11

5. 9 Power Tilt Due to a Misplaced Type-C Assembly. . . . . . . . 12 5.10 Power Tilt Due to Partial Flow Blockage in Assembly N11 and L13. . . . . . . . . . . . . . . . . . . . . . . . . 12

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l TAl!LE Of CONIENTS (Cont'd) l

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l $.11 Power Ifit Due to a flow Incroato in the NE Quadrant . ......................... 13 l

5.12 Power Tilt Duo to a 17, increase or Decrease in Assembly Reactivtty . . . . . . . . . . . . . . . . . . . 13 S.13 Power itIt Varf ation wt th Iturn-up Result tny l

from a Timo Dependent local Reactivity Change. .. ... .. 13 l

6. Tile POWER TILT AT NORill ANNA UNI T-1 CYCLE-2 . . . . . . . . . . . 45 6.1 Cyclo-2 Coro loading . ................... 45 6.2 Simulation of Cyclo-2 Il0L Power Distribution . . . . . . . . 45 6.3 Power Tilt Due to the Cyclo-2 A*.ymmetric Horn-up Distribution . . . . . . . . . . . . .. . ... .. .. .. 45

6. 4 Power Illt Variation with Changing Power Lovel . . . . . . . 46

6. 5 Power Tilt Variation wi th incronsing llurn-up . . . . . . . . 47

6. 6 A Cycle-2 Iturn-up Dupondent Illt . . .... . . . . . . . 4/

l. IllE CRUD llu l LD-U P MODl:1.. . . . . . . . . . . . . . . . . . . . . . 4/

U. CONCLUS I ONS AND RLCmME NDA T IONS. . . . . . . . . . . . . . . . . . 66 8.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . *;6 H.2 R e c un,m nd o t t o n s . . . . . . . . . . . . . . . . . . . . . . . 56 k lI C R E NC f s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58 APPE ND ICE S l

Ap pend i x 1. North Anna Nuclear Power Plant Hasic Data . . .... 69 Append!x 2. Conorattny ll-Constants for North Anna Cyclo-1 and Cycle-2 fuel Ass.mblies . . . . . . . . . . . . . 60 2a. f! Til-a Program to Produce B-Constantr.

by functional Interpolation and f t ttinq. . . . . 60

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TABLE OF CONTENTS (Cont'd)

Page 2b. B-Constants for Cycl e-l. . . . . . . . . . . . . 63 2c. B-Constants for Cycl e-2. . . . . . . . . . . . . 65 Appendix 3. Modification of the N0DE-P Program to Permit an Assembly-Wise Inlet Temperature Distribution . . . 66 1

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LIST OF FIGURES 4

Figure Title Page 4

. 5.1 tbrth Anna Uni t-1 Cycle-1 Core Loading Map . . . . . . . . . 17

5. 2 North Anna Unit-1 Cycle-1 Control Rod Locations Map. . . . . 18

5. 3 Maximum Quaarant Tilt in the NE Quadrant of North Anna

. Unit-1 Cycle-1 as Reproduced from Table 4.1 of Ref.-l. . . . 19

5. 4 as a i Maximum and Function of BU at Minimum North Anna Power Unit-1 Cycle-1 Tilt at Full Power (1) . . 20....

5. 5 Doppler Temperature Reactivity Coefficient at BOL and E0L for Cycle-1. . . . . . . . . . . . . . . . . . . . . . . 21

5. 6 Doppler Power Coefficient at BOL and E0L of Cycle-1. . . . . 22

5.7 Moderator Feedback Temperature Coefficients for Cycle-1 at BOL for Di f ferent Boron Concentrations. . . . . . . . . . 23 j 5. 8 Moderator Feedback Temperature Coefficient as a Function of Boron Concentration at Different Temperatures at BOL o f Cy c l e - 1 . . . . . . . . . . . . . . . . . . . . . . . . . 24

5. 9 Moderator Feedback Coefficient at Critical Boron Concentration at Full Power Operation During Cycle-1 as a Function of BU . . . . . . . . . . . . . . . . . . . . . . 25 5.10 Reduction in Subassembly Coolant Inlet Temperatures as Compared to the Core Average Inlet Temperatures. . . . . . . 26 5.11 Quadrant Tilt as a Function of Power Level for Cycle-1 Under Di f ferent Operating Conditions . . . . . . . . . . . . 27 5.12 Quadrant Tilt as a Function of BU During Cycle-1. . . . . . 28 5.13 Location of the D Bank Control Rods and the NA-1 Core Location of the Misaligned Cluster at F6 . . . . . . . . . . 29 5.14 NE Quadrant Tilt as a Function of Power Due to Control Rod Mi salignment at North Anna Cycle-1. . . . . . . . . . . 30 5.15 Quadrant Tilt as a Function of BU Due to Control Rod Mi sal ignment Duri ng Cycl e-1. . . . . . . . . . . . . . . . . 31 5.16 SE Quadrant Albedos. . . . . . . . . . . . . . . . . . . . . 32

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d LIST OF FIGURES (Cont'd)

Figure Title Page 5.17 NE Quadrant Tilt as a Function of BU Resulting from a 10% Albedo Increase as shown in Figure 5.16 . . . . . . . . 33 5.18 Location of the Assembly with an Enrichment Increased to 2.7% W/0. The Location of all B-Type BPR Assemblies i s al so i nd i c a t ed . . . . . . . . . . . . . . . . . . . . . . 34 5.19 Change in k. of a C-Type Assembly With and Without BPR as a Function of BU . . . . . . . . . . . . . . . . . . . . . 35 5.20 NE Quadrant Tilt as a Function of BU Resulting from a 0.1% Increase in the Fuel Enrichment of Assembly F3 . . . . .

! 36 5.21 Core Map of North Anna Unit-1 Cycle-1 Showing Locations of C-Type Assemblies and the Location of a Misplaced C-Assembly ......................... 37 5.22 NE Quadrant Power Tilt as a Function of BU Resulting from a Mi splaced Type-C Assembly . . . . . . . . . . . . . . . . . 38 5.23 Quadrant Tilt as a Function of BU in the NE and SW Quadrants Resulting from a Partial Flow Blockage in Channels Located at N11 and L13. . . . . . . . . . . . . . . . . . . . . . . . 39 5.24 Core Map Showing the Locations of the Coolant Channels in wh ic h a 5% Fl ow In c rea se wa s As sumed . . . . . . . . . . . . . 40 5.25 Quadrant Tilt as a Function of BU in the NE Quadrant Re-sulting from a 5% Flow Increase as shown in Figure 5.24 . . . 41 5.26 Quadrant Tilt as a Function of BU . . . . . . . . . . . . . . 42 5.27 Reactivity Perturbation Inserted to Reproduce the Observed Tilt in the NE and SW Quadrants of North Anna Unit-1 Cycle-1. 43 5.28 Calculated and Measured Quadrant Tilts at Full Power as a Fun ct i on o f BU. . . . . . . . . . . . . . . . . . . . . . . . 44 6.1 Loading Map of North Anna Unit-1 Cycle-2. . . . . . . . . . . 51 6.2 Comparison of Measured and Calculated Quadrant Tilts in NIC2 a s a Fun ct ion of Powe r . . . . . . . . . . . . . . . . . 52

6. 3 Cycle-2 Measured and Calculated Quadrant Tilts as Function ofBU............................ 53 6.4 Cycle-2 Measured and Predicted Quadrant Tilts based on the Reactivity Function used to Predict the N1C1 Cycle-1 Tilt . . 54

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LIST OF TABLES Table Title Page 5-1 NE-Quadrant Power Tilt at Different Power Levels . . . . . . 15 5-2 NE-Quadrant Power Tilt as a Function of Power. . . . . . . . 15-5-3 NE-Qu irant Power Tilt at Different Power Levels Resulting from a 0.1% Increase in the Enrichment of Subassembly F3. . . . . . . . . . . . . . . . . . . . . . 16 6-1 Axial BU Distribution at BOL of Cycle-2. . . . . . . . . . . 48

, 6-2 Measured and Calculated Power Distribution Along the Central Line of Fuel Assemblies for NIC2 . . . . . . . . 49 6-3 Comparison of Measured and Calculated Integral Para-1 meters for North Anna Unit-1 Cycle-2 BOL at Different Powe r Le v e l s . . . . . . . . . . . . . . . . . . . . . . . . 50 i 8-1 Comparing " Expected" and Actual E0L Quadrant Power i Tilts Due to Selected Asymmetric Perturbations . . . . . . . 57 i

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s ACKNOWLEDGEMENTS The authors would like to thank M. Chatterton and M. Dunenfeld, of the USNRC and M. Smith of VEPC0 for many useful discussions during the course of this work.

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1. INTRODUCTION North Anna Unit-1 is operated by the Virginia Electric and Power Company (VEPC0) and started operation in April 1978. During the 18 months since oper-ation began, th9 power plant completed Cycle-1 achieving an average burn-up of 15,892 MWD /MTU.t l ) During-this period, an asymmetric power distribution was observed with the following characteristics:

a. The power per assembly in the NE quadrant increased from BOL to EOL by 1.4% more than the average power level.

b. The power per assembly in the SW quadrant decreased from BOL to E0L by 1.3% relative to average power level.

c. This asymmetric power tilt ircreased significantly when the power was reduced. Also the asymmetric pcver distribution or tilt, re-sulted in asymmetric Cycle-1 core burn-up (BU).

In January 1980, the North Anna Unit-1 began operation with Cycle-2 fuel.

Two-thirds of the fuel was previously used in Cycle-1, having a BU from 9 to 20 GWD/MTV, and 1/3 was fresh fuel located on the core periphery. When oper-ation started, a large positive 9% power tilt was observed in the SW quadrant at zero power, and a comparable negative power tilt was observed in the NE quadrant. When power was increased to 100%, the maximum tilt was reduced to 1%. As the operation of Cycle-2 continued, the positive tilt shifted from the SW quadrant to the NE quadrant and is presently increasing in this quadrant.

At present, the utility does not have an explanation as to what is caus-ing this asymmetric distribution of power in the North Anna reactor.

The technical specification limit for the quadrant power tilt is 2% above average power at full power. The observed tilt did not exceed this value, hence was of no immediate safety concern. The hot channel factors remained within the technical specification limits and, therafore, no operational ac-tion was necessary.

In the present study, an effort was made to understand the asymmetric power tilt and its propagation as a function of power level and burn-up.

Several mechanisms resulting in asymmetric power distribution were investi-gated. A mechanism based on asymmetric crud deposit and its effects on re-activity and flow were proposed as an explanation for the observed phenomena.

Chapter 2 describes the identifying characteristics of the quadrant power tilt phenorena and lists possible phenomena that might result in such a tilt.

Chapter 3 summarizes the calculations performed in the study and presents the conclusions derived from these calculations. Chapter 4 describes the model used to simulate the Norti Anna reactor in a three-dimensional configuration and indicates some of the problems associated with the application of this model.

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Chapter 5 presents the results of the calculations performed and discus-ses the potential of each mechanism to reproduce the tilt phenomena observed.

In each case, the resulting tilt behavior as a function of power and as a l

function of BU is presented.

Chapter 6 presents the observed tilt phenomena of Cycle-2 and also pre-sents a successful BOL simulation of this cycle, using the NODE-P program. It also serves to validate the calculational procedure used in this study. This chapter also shows that the same tilt causing mechanism that was present in Cycle-1 continues to operate during Cycle-2. In Chapter 7 a crud model is l constructed and shown to be capable of reproducing the observed tilt.

Chapter 8 provides the conclusions and recommendations.

2. POSSIBLE ASYMMETRIC MECHANISMS LEADING TO POWER TILTING Reactor construction, fuel loading and operation will cause an asymmetric power distribution. However, these asymmetries are generally small and will disappear with time. In the present study a power tilt with the following characteristics will be investigated:

(1) The quadrant power tilt increases with burn-up (2) The power tilt decreases with inc.reasing core power level l (3) The cause of the tilt is not detectable at the present time and l therefore is probably a second order effect of small amplitude j

l (4) The tilting mechanism is present in Cycle-1 and Cycle-2 of the l North Anna Unit-1 reactor and drives a positive tilt in the NE quadrant and negative tilt in the SW quadrant.

The following asymmetric perturbations leading to a power tilt with some of the above characteristics were considered in this study.

(1) Inlet temperature maldistribution - coolant entering the NE quad-rant is somewhat colder due to a coolant loop returning a colder flow and incomplete mixing in the lower plenum.

(2) A control rod misalignment - it is assumed that a cluster of the D bank located in the NE quadrant is either shorter or has less

reactivity.

(3) Higher albedo in the NE quadrant due to some external structural l

asymmet ry.

(4) An erroneous fuel composition - an assembly containing BPR in the NE quadrant has a 0.1% higher enrichment.

(5) During the fuel loading process a peripheral assembly (enrich-ment 3.1".) has been misplaced in the core central region re-placing a BPR assembly (enrichment 2.6".).

(6) A partial flow blockage (20%) in two assembif es located in the SW quadrant.

(7) A small increase in the flow through the NE quadrant.

(8) An arbitrary 1% increase in the reactivity of two assemblies in the NE quadrant.

(9) An arbitrary 1% decrease of the reactivity of two assembif es in the SW quadrant.

(10) An arbitrary time dependent increase in reactivity in the NE quadrant and a decrease in the SW quadrant to match the BU de-pendent NE and SW tilt observed in North Anna Cycle-1.

3.

SUMMARY

OF THE CALCULATIONS AND CONCLUSIONS NorthThe Annalgmmetric ap ) reactor. perturbations described The N0DE-P program of theabove ARMP were(gli

) systemsimulated was used for the to perfonn the calculations. Various modifications were introduced into the program to facilitate the present study. These are described in detail in Ap-pendix 3, and include the quadrant tilt computation and a core coolant inlet temperature modification.

The actual tilt resulting from any one of the perturbations is very sen-sitive to the reactor " state point," or rather its precise operating condi-tions. This is clearly demonstrated in Section 5.5 and in figure 5.11. For a certain BU and power level, the tilt will vary significantly, according to the control rod position, flow in each channel and the temperature distribu-tion. This presumably is responsible for the large fluctuations in the tilt as a function of burn-up in Figure 5.3.

None of the proposed perturbations leading to asymmetric power tilt matched the necessary conditions. In particular, all the generated tilts dis-appeared with BU.

It can be concluded that the dominant effect in the evolution of the spa-tial power distribution is the burn-up of fuel and the accumulation of fission prodents, which will cause an asymmetry to " burn out" and equalize the core power distribution.

A burn-up dependent reactivity perturbation has been detennined which re-produces almost exactly (see Figure 5.28) the power tilts as observed. Al-though this model provides an indication of the burn-up dependence of the tilt mechanism, it does not provide information concerning the riechanism itself.

Most of the perturbations will result in the tilt decreasing with in-creasing reactor power level. This is mainly due to: (1) the decrease in boron concentration with increasing power and the resulting increase in mod-erator temperature coefficient and (2) the increase in moderator and fuel tem-perature and associated increase in moderator and Doppler feedback. The

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perturbations associated directly with reactivity changes such as fuel enrich-ment error and control rod misalignment will easily produce the observed tilt with a relatively small perturbation change. Others, such as flow, tempera-ture and albedo variation will require a large asymmetric perturbation in or-der to produce the observed tilt. Since the large asymmetric perturbations are easily detected, these can be excluded as single mechanism tilting phenomena.

From the calculations performed, it is also evident that a perturbation occurring in a single quadrant cannot reproduce the positive and negative tilt as observed, hence whatever drives the tilt must involve more than one quadrant.

Finally, it is concluded that the tilt driving force must be a phenomena i increasing with BU in such a manner that it overrides the effect of BU and fission product accumulation which tend to equal he the power distribution.

i That is, the positive and/or negative reactivity must be sufficient to cancel BU effects and also to produce an increasing tilt.

The only phenomena that has been identified at the present time which will produce the necessary effect and which wil) satisfy all fov conditions described in Chapter 2 is crud Jeposition on the fuel pf ns. Prel!minary cal-culations have shown that a small amount of crud (less than 1 mil) deposited asymmetrically in the core, with a resulting flow maldistribution of 1% dis-tributed unevenly in the SW half of the core, can reproduce the tilt observed in North Anna Cycle-1 and Cycle-2.

4 THE N0DE-P 3-DIMENS10NAL MODEL FOR POWER DISTRIBUTION AND BURN-UP CALCULATIONS The NODE-P program is Methodology Program (ARMP)(thq) core neutronics J,4 and provides segment of a three-dimensional the Advanced Recy one-group coarse mesh model for PWR power distribution calculations. Calculational con-vergence is to either critical boron concentration or the target effective multiplication.

A detailed description of the program and its application is given in Reference 4. In what follows only the program sclient features are described to facilitate the understanding of the calculations performed and their linitations.

The code requires as input the description of the fuel assemblies and their location, core and node dimensions, inlet temperatures and power level, nuclear properties of each assembly type and flow characteristics. The nucle-ar properties of each type of assembly are input,as coef ficients (or B con-stants) to fit polynomlais representing k,,, and M2 as functions of coolant temperature, fuel temperature, fuel e<posure, flow, and fits of vt and .cr to f

fuel exposure.

The basic equation solved by NODE-P for each node is the source equation given by 4_

c . _ - . _ - . . - -

k.,1 sm Nm1 y

E 1 k ,1 1- (6-a g) WE,m II}

where k.g is the t-th node infinite multiplication factor S

m is the source of Node-m and the summation is over the six nodes adjacent to the calculated node Wm is the probability of a neutron born in Node-m i being absorbed in Node-1.

A is the effective multiplication factor is an adjustable leakage factor which is zero for og all internal nodes and is an input constant for all the nodes facing a reflector.

The probability of a neutron leaking from Node-m to Node-t is given by M M W = (1_g) m +g m (2)

• ' F 2 2 m,1 r ,,g where g is an adjustable input constant 2

Mm is the effective migration area of Node-m rm,t is the center to center node spacing.

The values of k and M2 are computed by using detailed space enercy calculations (EPRI-CELL and PDQ) under a wide range of operating conditions.

The infinite multiplication factor for each node is given by 4

k* = -km o 1 + aptot* k.,o (3) where U0 tot"00Dop+0Pxenon + # boron+00BU+SPcontrol'00 mod i

(4) t l

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All the variables, such as M2 , k. and all the o's are given as poly-nomials of the reactor state variables and the fuel burn-up. For exampla. the unrodded nigration area is given as a function of coolant temperature only as, 2

M = B y+B2T+B 3

and the reactivity change due to exposure is given by 2 3 APBU " 54+B33 (Eijk + B 34V ijk) +B 35E ijk + B36E ijk (6)

+0 37E ijk where E i jk (Vi jk) is the fuel (BPR) exposure at location ijk.

For each assembly there are about 60 constants to fit the different re-activity values. The albedo constants and leakage constants g are adjusted to i obtain the best fit for either a detailed 3-D calculation or a measured power distribution for the simulated PWR.  !

5. TPE POWER TILT AT NORTH ANNA UNIT-1 CYCLE-1 The North Anna Unit-1 Cycle-1 operation extended from April 5,1978 to September 25, 1979.

duced 15892 MWD /MTU.(gyring 1 these 18 months Power distribution of operation measurement using the power incore andplant ex- pro-core detectors indicated an asymmetric power distribution with a maximum in the North-East quadrant and a minimum in the South-West quadrant. The maxi-mum pcwer tilt was $1.4% above the average quadrant power and the minimum was

$1.3% below the average. Fluctuations of up to N1.8% were also observed toward the end of Cycle-1. This chapter presents the observed measurements and the calculated tilt which resulted from using the different models listed in Chapter 2. It shows how the asymmetric tilt varies with power and with hurn-up under different asymmetric core perturbations. Different methods by which the actually observed tilt can be reproduced are also presented.

5,1 Cycle-1 Core Loading The first cycle of North Anna is loaded with three batches of fresh fuel.

Each fuel assembly has 264 fuel pins arranged in a 17 by 17 grid. Twenty-fou r locations are reserved for burnable poison rods (BPR) or have tube guides for control rod assemb'.1es. The central location is reserved for the instrumenta-tion thimble. De' ails of the core loading are.given in Figure 5.1 (from Ref-erence 1) and thf location of the control rod assemblies are given in Figure i 5.2. For a detailed core description, see Chapter 4 of the North Anna FSAR.(2) l Batch A fuel assemblies have an enrichment of 2.1". (w/o U-235) and will l accept the control rod clusters. Batch B has an enrichment of 2.6% and l

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includes BPR. Assemblies A and B are arranged in a checkerboard pattern in the core central region. Batch C has an enrichment of 3.1%. C-Type assem-blies are loaded into the core periphery to achieve power flattening. Some of the Type C assemblies, designated C'. accommodate BPR's.

5.2 Observed Tilt ~ Variations With Increasing Core Power Level From the incore flux map meast.. aments as presented in Referev.e 1, it is obvious that the asymmetric power tilt is mitigated with the increase of power. Some of the measured power tilts are listed in Table 5-1. One should note, however, that not all the measurements, low and high power, were taken on the same date, and the D bank control rods were not always in the same position. Therefore, Table 5-1 does not represent the real tilt variaticn with power, it is rather a general indication of core behavior.

A study by L. Eisenhart(5) indicated that if dropped control fingers were the cause of the power tilt, this tilt would have a strong dependence on the core power level. The variation of the tilt magnitude with power level in the North Anna Unit-1 core, is better observed and documented for Cycle-2 as shown in Section 6.4.

5. 3 Observed Tilt Variation with Increasing Burn-up Reference 1 provides a summary of 44 inccre flux maps. Of these, 33 were measured at power levels higher than 90%. The maximum tilt values (all in the NE quadrant) were plotted in Figure 5.3. The tilt increases with BU from 0%

to about 1.6% as one moves from a BU of 0 to 16 GWD/MTU; roughly 0.1% tilt per 1 GWD. There are, however, significant fluctuations around the average trend of an amplitude of 0.5%. Some of the fluctuations are certainly due to the variations in the conditions under which the measurements were made and the l location of the control banks during the measurements.

Another figure provided by VEPC0(6) from a more limited set of data, Figure 5.4, shows the positive power tilt as function of BU in the North-East quadrant and the negative tilt in the South-West quadrant. The selected points in this figure show smaller fluctuations around the general trend of the curve. The measurements show a rapid initial Increase in the positive tilt to 0.8% from 0 to 2 GWD/MTU, a more or less constant tilt between 2-10 GWD/MTV and then an incrcase toward saturation from 10 GWD to the end of Cycle-1 at about 16 GWD/MTV.

5.4 The Feedback Reactivity Coefficients of Cycle-1 The analysis of any perturbation inserted into the core, whether short range or long range, requires a detailed understanding of all the feedback re-activity coef ficients. For the particular set of calculations performed in this study, the feedback is built into the B constants used with NODE-P.

The reactivity coefficients and their exact vaiues at a set of given con-ditions determine the actual coupling between the neutronics and thermohydrau-lic changes and will determine the magnitude of the local power tilt.

l l

c ,

5.4.1 The Doppler Coefficient The Doppler coefficient is the fastest responding effect to changes in the reactor power level. It is the change of the fuel reactivity as the re-sult of increasing resonance absorption (Doppler broadening of the resonance peaks). It is a negative effect due mainly to the increase of the' parasitic absorption in the large amount of U-238 present in the PWR core. The actual value for Cycle-1 changes with the effective fuel temperature as shown in Fig-ure 5.5. The Doppler effect has the tendency to saturate at higher tempera-tures. For the present study, it is important to note that the Doppler effect is larger at lower temperatures (also lower power) and that the Doppler power coefficient (see Figure 5.6) decreases with BU. The last phenomenon is mainly due to PU buildup in the fuel pins and increased pellet-clad conductivity.

5.4.2 The Moderator Temperature Coefficient The moderator temperature coefficient results from changing the water density. As the water density decreases with increasing temperature, so does the boron density. At very high boron concentrations (above 1000 ppm), this may result in a positive reactivity coefficient. As can be seen from Figure 5.7, however, at full power C 400 F normal operating con-ditions, the coefficient is negative. For the present study one should note, in particular, the moderator coefficient behavior as a function of boron con-centration shown in Figure 5.8. This behavior governs the magnitude of the asymmetric power tilt at different power levels due to different boron concen-tration and different temperatures. Figure 5.9 gives the moderator coeffi-

. cient at full power as a function of BU. The major factor is the reduction of boron concentration from 900 to O ppm. This behavior witn BU will be a major factor in analyzing the various power tilt mechanisms and the magnitude of the tilt as a function of BU.

5. 5 Power Tilt Due to Inlet Temperature Ma1 distribution Small differences of the order of I'F in the coolant return temperature were observed during Cycle-1 operation. It is assumed that if improper mixing in the core inlet plenum exist, there will be small differences in the inlet temperatures of different assemblies. An asymmetric core inlet temperature, as shown in Figure 5.10, was introduced to produce an asymmetric power tilt in the NE core quadrant. The N0DE-P program used in this study was appropriately modified (see Appendix J) to permit the subassembly cnannel inlet temperature to be an input variable. Since the coolant in the NE quadrant is colder than the rest of the core, a positive increment in 4 of the affected assemblies is produced because of both negative Doppler and moderator feedback. The in-crease in power in this quadrant will be balanced by a negative tilt in the other quadrants and because of symmetry conditions will be balanced mainly by the negative tilt in the SE quadrant. D e actual tilts at full power, as cal-culated by the N0DE-P program are

-0.074% l 0.266%

-0.117% l -0.074% +

Tha magnitude of the tilt as a function of power depends strongly on the prevailing flow conditions, the boron concentration and the control rod post-tions. If the flow is kept constant, that is 100% of rated flow at all power -

levels, then the lower the power, the lower the core exit temperature. At "Zero" power the coolant exit temperature is equal to the coolant inlet tem-perature. At this temperature and the critical boron concentration (1332 ppm) and at BOL conditions the. moderator coefficient is either positive or slightly negative as can be seen from Figure 5.7. As a result, the change in k. is minimal and the tilt at low power is much smaller then the tilt at full power.

If flow is adjusted to match the power level to maintain the outlet tempera-tures, then the core average temperature is much higher. As can be seen from Figure 5.7, the negative value of the moderator coefficient increases with temperature. As a result, the low power tilt is higher in this case than in the previous case with 100% flow and lower moderator temperatures.

If boron concentration and temperatures are kept constant during the power plant startup using the control rods to obtain the necessary critical condition, then the tilt at zero power will be much higher than in the pre-vious cases because of the lower critical boron concentration. The previous cases have a zero power boron concentration of about 1300 ppm while in this case the concentration is the same as at full power, that is about 450 ppm.

The magnitude of the power tilt, in both NE and SW quadrants as a function of power, is shown in Figure 5.11.

Figures 5.5 and 5.9 may be used to estimate the magnitude of the power tilt as a function of burn-up and the change in the feedback coefficient as Cycle-1 burns out. The major effect is the reduction in boron concentration and the corresponding increase in the negative value of the moderator fecdback coefficient. It can be expected, therefore, that the tilt will increase with burn-up and will be much larger at EOL than at 80L. Actual calculations at E0L full power conditions with the same coolant inlet temperature maldistribu-tion product a tilt of 0.645% in the NE quadrant. This EOL tilt is 2.5 times larger than the 80L tilt.

The actual tilt versus BU calculations, however, show an actual decrease in the tilt with BU as shown in Figure 5.12. This behavior is due to tilt

" burn out." Whenever a perturbation introduces a positive tilt in the core, the burn-up at this location will be higher than at the rest of the core. The result is that fission product poisoning is stronger at this location and pro-duces an increasing negative r9 activity. If this negative contribution is larger than the increase in the positive effect of the perturbation and the positive effect of Pu buildup, then the net effect will result in a decrease in the observed quadrant tilt.

Comparing the calculated tilt with North Anna Cycle-1 measurements, it is evident that a maldistribution of coolant inlet temperatures is not the source of the tilt (or at least not the only source) since,

1) the predicted tilt increases with power under normal operating conditions, while the measured tilt decreases.

~9-

2) the predicted tilt decreases with BU while the measured tilt increases with BU.

3) the magnitude of the temperature perturbation necessary to pre-dict the observed tilt is large and would have easily been detected.

5.6 Power Tilt Due to Control Rod Misalignment North Anna has four banks (A,B,C and D) of full length control rods, each having 8 clusters, and a partial length control bank of 5 clusters. For power regulation, Bank 0 is generally usee . It is assumed that one cluster of Bank D as shown in Figure 5.13 is not properly aligned. It is assumed that the cluster at location F6 is 13 notches (8 inches) higher than the other clusters of the control bank.

This type of perturbation should result in a much higher power tilt at low power as was indeed observed in the calculations performed, shown in Figure 5.14, in agreement with observations. The tilt decrease with power is the re-sult of stronger feedback at higher power. This feedback being negative will produce a reactivity effect in the opposite direction to the perturbat1on. At zero power there is no feedback opposing the perturbation. Also at full power the high concentration of xenon will have a " shadowing" offect on the perturbation.

The above control rod misalignment when occurring at BOL at 100". power, re-sulted in a positive tilt of 0.56% in the NE quadrant of the core, while at EOL the tilt is 1.34%, that is 2.4 times larger. It can be expected, there-fore, that the tilt magnitude will increase with BU. As seen from Figure 5.15, the power tilt actually decreases with BU, As in the previous case, this is due to the " burn out" of the perturbation. The higher initial power causes increased BU in the NE quadrant. This in turn produces increased fis-sion product poisoning, which has a strong negative effect overcoming the in-creasing positive effect introduced by the control rod misalignment.

Although both the tilt behavior with power and its magnitude are similar to the observed effect, the tilt magnitude as a function of BU is not; there-fore, control misalignment cannot explain the observed phenomena. Also, when all control rods are withdrawn, the tilt should move to the opposite quadrant.

There is no indication of this during Cycle-1. If, however, a mechanism was present by which the worth of one control rod cluster, or a control finger would decrease strongly with time, an increase in tilt with BU could be pro-duced. No such mechanism has yet been identified.

5.7 Power Tilt Due to increased Albedo in the NE Ouadrant An asymnetric reflector (material or geometry) may also produce an asym-metric power distribution in the core. Figure 5.16 shows the albedos used with the N00E-P program to match the power distribution and also indicates the locations at which the albedo was increased by 10%.

As seen from Table 5-2, the tilt resulting from this asymmetric re-flector distribution decreases with increasing power. This is to be expected since at low power there is no feedback mechanism to reduce the flux (power) due to the increased amount of neutrons being returned from the reflector.

The total amount of quadrant tilt resulting from a 10% increase in the albedo is 0.14% at full power BOL. The same increase was observed when the perturbation was inserted at EOL. This indicates that the changing reactor conditions with BU, as calculated with the APMP system, do not change the ef-fect of the albedo.

The effect of BU on this perturbation is a reduction of the tilt due to

the increased BU in the NE quadrant. This is clearly seen in Figure 5.17 show-ing the power tilt as a function of BU.

It is obvious that this mechanism is not responsible for the observed tilt at North Anna Unit-1, since the observed tilt decreases with BU and a large reflector asymmetry is required to produce the tilt of the observed magnitude.

5.8 Power Tilt Due to a 0.1% Increase in the Fuel Enrichment of One Assembly In this section a possible error in the enrichment of one assembly is evaluated. This might be due to either a fabrication error, or a misplaced fuel assembly. The particular assembly considered is a Type B assembly that includes burnable poison rods (BPR) (see Figure 5.1.). The exact location of the assembly as well as other B type assemblies is shown in Figure 5.18.

The more enriched assembly at location F3 has a 6% higher k. than the symmetric assemblies in the other three quadrants. The resulting flux in this and neighboring assemblies will be higher resulting in a positive power tilt in the NE quadrant. The tilt magnitude will decrease with increasing power level since the negative Doppler and moderator feedback will reduce the power.

This can be seen from Table 5-3, where the results of the calculated tilt at different power levels is presented.

It is expected that in this case the tilt will increase with BU, mainly because the k of the assembly with BPR increases with BU as the result of the depletion of the burnable poison and the Pu buildup. The behavior of two as-semblies having the same (3.1%) enrichment, with and without burnable poison as a function of BU is shown in Figure 5.19. It is seen that k. increases from about 1.045 at BOL to 1.087 at 17 GWD/MTV, decreasing thereafter. The actual BU calculations, however, have shown (see Figure 5.20) that the tilt decreases with BU. This results from the fact that the negative effect of the accumulating fission prod'_ ts in the surrounding assemblies because of higher power, is stronger than the positive increase in k. as shown in Figure 5.19.

Also the negative moderator feedback increases with time and tends to reduce the power tilt generated by higher enrichment.

It can be concluded, therefore, that although this mechanism satisfies most of the criteria presented in Chapter 2, it cannot account for the in-

c. ease of tilt with BU as observed at NA-1.

5.9 Power Tilt Due to Misplaced Type-C Assembly, As shown in Figure 5.1, the Type-C assemblies have a 3.1% enrichment and are located at the core periphery. The core map in Figure 5.21 shows an as-sembly accidentally misplaced. As expected, this will cause a very large power tilt in the NE quadrant. At BOL this tilt is 2.38% at 100% power and 4.92% at 1% power. Basically the same phenomena as described in the previous section are observed, except that they have a much larger amplitude as seen from Figure 5.22. The tilt also burns out faster. At 6 GWD/MTU the tilt is reduced to zero, becomes negative and then returns to zero again at 15 GWD/MTU. This phenomena clearly cannot represent the observed effect, and is certainly too big to pass undetected.

5.10 Power Tilt Due to Partial Flow Blockage in Assembly N11 and L13 Partial flow blockage in a fuel assembly can occur for a variety of rea-sons. In the present investigation it was assumed that the flow in assem-blies N11 and L13 was reduced by 20%. The direct effect of this reduction in flow is an increase in the coolant temperature in these two channels. Due to the negative feedback present at full power (se'e Figure 5.7 to 5.9), a nega-tive power tilt in the SW quadrant will be generated. This will be balanced by a positive tilt in the NE quadrant.

As the moderator coefficient becomcs more negative with BU the tilt in the SW quadrant should increase. This perturbation when inserted at BOL will cause a tilt of -0.110% while the same perturbation inserted at E0L will cause a tilt of .405%. From Figure 5.23 it can be seen that this is indeed the trend up to about 7 GWD/MTV. Then the increasing tilt is overcome by the BU effect which as seen in all previous cases will equalize the power distribu-tion in a reactor, that is, reduce the size of the tilt. In this case, the flow blockage producing the negative tilt will result in less burn-up in the SW quadrant, which will have an accumulating positive effect and finally bal-ance and overcome the negative temperature effect as shown in Figure 5.23.

Therefore, because of the increased sensitivity of the core to flow in-duced tilts and their apparent slow burnout, it seems plausible that at least part of the tilting effect observed at NA-1 during Cyc'e-1 is due to a flow maldistribution.

5.11 Power Tilt Due to a Flow Increase in the NE Quadrant A symmetric effect to the partial flow blockage in the SW quadrant, de-scribed in the previous section, is a flow increase in the NE quadrant. In this investigation it was assumed, however, that a 5% flow increase takes place in most of the assemblies in the NE quadrant as shown in Figure 5.24.

The tilt at BOL full power is 0.259% while the same perturbation inserted at EOL will cause a tilt of 0.708%. As in the previous case, the EOL increase is due to the much larger moderator coefficient. However, Figure 5.25 does not show any increase in the power tilt with BU. The calculated tilt decreases due to the strong negative effect of the fission product accumulation in the NE quadrant assemblies, but it is not reduced to zero. Rather an equilibrium tilt is maintained as a result of a balance between the accumulation of fis-sion procucts and the increase in the magnitude of the moderator coefficient.

The absence of any increase in the power tilt, as observed in Section 5.9, may also be in part due to the much larger reactivity effect induced by this per-turbation as compared to the previous case.

5.12 Power Tilt Due to a 1% Increase or Decrease in Assembly Reactivity The behavior of the quadrant tilt induced by a 1% increase or decrease of

k. in one assembly is indicative of the process of tilt suppression by BU.

i An increase of 1% in the E3 subassembly gives rise to a positive tilt of 0.383% at BOL in the NE quadrant, while the same perturbation at E0L causes a

~

tilt of 0.464%. Similarly, a negative perturbation of 1% in the symmetric as-sembly of L13 causes a negative tilt of -0.37% at BOL and -0.457% at E0L. In an unperturbed core the normalized power at the E3 assembly, or its symmetric.

counter-part at BOL, is 0.81 while at the E0L, the normalized power is 1.05.

This is due to the power flattening effect of BU which increases the relative power of peripheral assemblies. The same change in k. at BOL and E0L will cause a more significant contribution to the quadrant power tilt at EOL even though the larger negative feedback at EOL tends to reduce this effect. The tilt behavior with increasing BU is shown for both cases in Figure 5.26.

Again the major driving force is the burning out of the tilt affected region, and the reduction of tilt to zero.

It is clear that no asymmetric perturbation which causes a constant change in the k. of one assembly can explain the observed tilt behavior in Cycle-1 of the North Anna reactor.

5.13 Power Tilt Variation with Burn-up Resulting from a Time Dependent Local Reactivity Change l None of the previously discussed asymmetric perturbations can reproduce the quadrant power tilt observed during Cycle-1 at the North Anna reactor and reproduced here in Figure 5.4. Each of the asymmetries was assumed to exist at BOL and the source of the asymmetry was not changed during the cycle. All of the quadrant tilts were either eliminated or reduced in magnitude with BU.

It is possible tn reproduce exactly the mecsured quadrant tilt depen-dence by arbitrarily changing the perturbation with BU in order to match the precise tilt measured.at every calculated BU step.

It is not possible to match the measured tilt by perturbing one quadrant only. The amount of symmetric tilt produced in the opposite quadrants as a result of power sharing will decrease in time until the three unperturbed quadrants will share equally the amount of power tilt produced in the per-turbed quadrant. Therefore, only perturbations increasing with time or BU in-serted in more than one quadrant can reproduce the observed tilt. Such a per-turbation was inserted in the NE and SW quadrants, and is shown in Figure 5.27. The perturbation included the increase of k. in assemblies E3 and C5 in the NE quadrant with a simultaneous decrease in k. in assemblies N11 & L13 in the SW quadrunt. The resulting power tilts superimposed on North Anna mea-surements is shown in Figure 5.28. It is seen that the agreement between the BNL simulated curve and the measurement is good and can be improved to any re-quired precision. However, although the reactivity curve in Figure 5.27 or similarly generated curves does not constitute a physical model, the reactiv-ity curve so obtained might be indicative of the real physical phenomena caus-ing the observed tilt.

4

k Table 5-1 NE quadrant power tilt at different power levels.

BU POWER BANK-D SET DATE MAP # (GWD/MTU) LEVEL (%) TILT (%) STEPS

, 1 10-3-78 N-1-40 4.55 48.0 0.69 178 10-5-78 N-1-42 4.57 73.4 0.51 184 l 10-3-78 N-1-41 4.55 96.5 0.17 202 2 10-6-78 N-1-43 4.60 74.7 0.66 186 10-6-78 N-1-46 4.60 99.5 0.36 206 3 5-31-79 N-1-61 11.7 65.5 2.39 182 5-29-79 N-1-60 11.7 99.9- 1.15- 218 4 9-14-79 N1-77 15.7 34.0 1.72 98-100 j 9-7-79 N1-78 15.5 89.3 1 1.40 225 Table 5-2 NE quadrant power tilt as a function of power.

1 Power D-Bank Flow Boron Tilt 0% 11.5 ft. 100% 1338 ppm 0.27 3% 11.5 ft. 100% 1300 ppm 0.24 100% 11.5 ft. 100% 951 ppm 0.14 t

Table 5-3 NE quadrant power tilt at different power levels resulting from a 0.1% increase in the enrichment of subassembly F3.

Power 0-Bank Boron Fl ow Tilt

% Control ppm  %  %

0 19/228 1201 0 1.12 1 38/228 1174 1 1.02 100 200/226 947 100 0.47 l'

=

R P N M L K J H G F E D C B A C32 C22 C05 1 C13 C36 C40 A18 C39 C07 C48 1(p 2 ggP C30 CIS B17 'A27 B24 A36 B50 C47 C23 12P ]6P 1lP 16P 12P C20 A26 B15 A06 !B51 A10 B33 A05 B37 A51 C38 4 16P 16P 16P 16P C27 C09 B07 A32 B16 !A02 B38 A40 B18 A42 B39 C18 C51 5 12P 16P 16P.

20P 16P 16P 12P C45 B45 A24 B22 A44; B09 A21 B31 A13 B23 A38 B02 C17 6 169 16p 16D 16p 16p 16D C43 C04 A50 B05 A19 B06 A48 B43 A14 B47 A30 B12 A04 C06 C28 7 16P 16P 16P 20P 16P 16P 16P C52 A43 B27 A25 B41 A0d B35 A01 B14 A20 B03 A49 B08 A34 C41 12P 20P 20P 20P 20P 12P

-8 C14 C26 A46 B36 A09 B21 i A12 B30 A3 9 B49 A3/ B32 A03 'C00 G49 16P 16P 16P 20P 9 16P 16P 16P C21 B20 A23 B52 A15 Bll A47 B34 A16 B04 A25 B28 C03 10 16P 16P 16P 16P 16P 16P C42 C16 B48 A53 B1C A33 B25 A45 B01 A07 B42 C3L C12 12P 16P 16P yy 20P 16P 16P 12P C37 A31 B40 A41 13 A52 B26 A17 B29 All C24 12 16P 16P 16P 16P C29 C35 B46 A28 B44 A22 B19 C01 C11 N 12P 16P 12P 16P 12P 13 t SS

!31 NE C44 C08 C46 A35 C33 C19 C10 16P 16P 74 Oued. Cuad. 99 SW SE CO2 C31 C25 15 Quad. Quad. ,

FUEL ASSEMBLY DESIGN PARMIETERS A-Batch B-Batch l C-Batch Initial Enrichment (w/o U235: 2.10 2.60 3.10 Assembly Type 17x17 17x17 17x17 No. of Assemblies 53 52 52 Fuel Rods per Assembly 264 264 264

+ Assembly Identification

+0ne or more of the following:

a. PS - Primary Source Assc=bly

b. SS - Secondary Source Assembly

c. xxP- Burnable Poison Asse=bly (xx - number of reds)

Figure 5.1 North Anna Unit-1 Cycle-1 core loading map.

l

J G F E D C B A R P tl N L K H 1

2 A D A -

3 SA SA -

4 C B PL B C --

SB SB 5 6

A B D C D B A 7

SA SB SB SA B

D PL C PL C PL D 9

SA SB SB SA 10 A B D C D 8 A SB SB II 12 C B PL B C 13 SA SA -

A D A 14 15 Absorber Material: Ag-In-Cd Function flumber of Clusters Controf Tank D a Control Bank C 3 Control Bank B g Control Bank A 8 Shutdown Bank SB 8 Shutdown Bank SA 8 Part length PL 5 Figure 5.2 tiorth Anna Unit-1 Cycle-1 control rod locations map.

3 I I I I I I f I I I I I I I I I i 1.8 -

1.6 -

1.4 -

E p I.2

_i

~

l- I i- 1.0 -

Z I CE o 0.8 - -

D O

O.6 -

0.4 - -

0.2 -

0.0 O 2 4 6 8 10 12 14 16 18 BURNUP (GWD/MTU)

Figure 5.3 Maximum quadrant tilt in the NE quadrant of North Anna Unit-1 Cycle-1 as reproduced from Table 4.1 of Reference 1.

v.  ;> ,

t

/

. t

' I + 1 I i i i i_

l .G i -

NE-QUADRANT A A

A I

A

i. .

1.2 -

A

/

O.8 -

A

- A A i

[

A .- A A A j g 0.4 '

^ _

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O

< -0.4 - * -

a e-

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• e

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e e e _

-l .2 -

e *

- SW- QUADR ANT _

! -1.6 -

, i i i i i i 0 2 4 6 8 10 12 14 16 BURNUP (GWD/MTU) i I

figure 5.4 Maximum and minimum power tilt r.t full power as a function of BU at North Anna Unit-1 Cycle-1.(1) l j J'

, sw - +

i l

l 1

-l.2 ,

u.

L

[ -1.4 t

m" -l.6 -

1 m

EOL

-l.8 -

m_.

2 30L i

w o -2.0 -

u i?

E -2 .2 -

b 5 -2.4 -

d 8

o l l l l I l

-2.6 500 600 700 800 900 1000 1100 1200 EFFECTIVE FUEL TEMPERATURE Teff(F)

Figure 5.5 Doppler tempe;ature reactivity coefficient at BOL and EOL for Cycle-l.

i

-8

$2 o w -10 -

CM EOL b a-SG

=0 W 5 -12 - BOL Et 5!

5-5 4 -14 -

6" o

-16 0 20 40 60 80 100 POWERLEVEL(PERCENTOFFULLPOWER)

Figure 5.6 Doppler power coefficient at BOL and E0L of Cycla-1.

20 2000 PPM C

=g 10 ~

g500 PPN A b

o 0 ~

f G 1000 '""

t goo pea

~~~

.\0

$o o PPB do 3

w .20 '

D

$ _30 ~~~

6 o

3

-40 0 100 200 300 400 500 600 H0DERATORTEMPERATURE(F)

Figure 5.7 Moderator feedback temperature coefficients for Cycle-1 at BOL for different boron concentrations.

i I

5 i

680F p0 5

5.

1

• 4000F 4.

m w

5 -10 -

U C

D 5570F 8

g -15 -

5860F h

5 n.

5 s -20 5

0 5

o 9 -25 -

l l

-30 -

0 500 1000 1500 SOLUBLEBORONCONCENTRATION(PPM) ,

Figure 5.8 Moderator feedback temperature coefficient as a function of bcron concentration at dif ferent temperatures at BOL of Cycle-1.

I

/

I

.i 0

-5 -

O 5

h -10 s.

R.

4 f -15 -

!' 5 G

C b -20 -

S u

$ -25 -

5 i

m -30 -

S

., M w

h -35 -

-40 0 5000 10,000 15.000 20.000 BURNUP(MWD /HTV) d i

I Figure 5.9 Moderator feedback coefficient at critical boron concentration at full power operation during

. Cycle-1 as a function of BU.

1

!! L J H G F E D C B a R P fl K I

NE

-1 -1.7 1

-1 -1.7 -2.4 -2.4 2

3 -1 -1.7 -2.4 -2.4 -2.4 4 -1 -1.7 -2.4 -2.4 -2.4 -2.4 5 -1 -1.7 -2.4 -2.4 -2.4 -2.4 -2.4

6 -1 -1.7 -2.4 -2.4 -2.4 -2.4 -2.4

-1 -1.7 -1.7-1.7 -1.7 -1.7 -1.7 -1.7 7

8 -1 -1 _1 -1 -1 -1 -i 9

10 11 1

12 13 14 15 Figure 5.10 Reduction in subassembly coolant inlet temperatures as compared to the core average inlet temperature.

ilumbers indicate degrees F.

O.7 i , i i NE-QUADRANT O.6 -

CONSTANT BORON CONCENTRATION O.5 -

0.4 -

0.3 - CONSTANT EXIT _

b / TEMPERATURE P O.2 -

F-CONSTANT 100 % FLOW

$ O.I _

m o

CONSTANT 100 % FLOW

/

- 0.1 CONSTANT EXIT TEMP

~

~ ~

CONSTANT BORON CONCENTR ATION

- 0. 3 - -

SW -QUADR A NT

- 0.4 '

O 20 40 60 80 100 POWER (% Rated)

Figure 5.11 Quadrant tilt as function of power level for Cycle-1 under different operating conditions.

l l l l l l l l l l l l l l l l 0.4 -

NE-QUADRANT 3

~

p O.2 N d

g _

$ O.0

( <{

g _

Q

@ -0.2 o _ __

SW-QU A DRANT

- 0. 4 - _

l I I I I I I I I I I I I I I O 2 4 6 8 10 12 14 16 BURNUP (GWD/MTU)

Figure 5.12 Quadrant tilt as a function of BU during Cycle-1.

R P tl M t. K .ll H G F E D C E /,

1 2 0 3

4 5

6 D D

7 8 D D 9

10 11 D D

~

12 13 14 0 D at 200/228 15 D F6 at 213/228 Figure 5,13 Location of the D Bank control rods and the NA-1 core location of the misaligned cluster at F6. .

l 0.8 i i i i NE-QUADRANT 3 0.7 b

r F O.6 - -

, a-M o

<t a

o 0.5 - -

0.4 .

O 20 40 60 80 I00 POWER (% Rated)

Figure 5.14 NE quadrant tilt as a function of power due to control rod misalignment at North Anna Cycle-1.

1 I l l l l l l l l l l l l l e3 0.6 -

NE-QUADRANT _

s

_j _ _

H p O.4 - -

z

<t _ _

O' O

s' <t 0.2 - -

o O _ _

f 0.0 O 2 4 6 8 10 12 14 BURNUP (GWD/MTU)

Figure 5.15 Quadrant til.t as a function of BU due to control rod misalignment during Cycle-1.

(

l F E D C B A R P N M L K J H G

+ + NE 1 NWl

+

2 3

+

4 +

+

5 6 +

7 8

9

.02 10

.04

, 11 12

.14 13 14 ,14 15 SW .02 .04 SE f .02 .02 .02 l Figure 5.16 SE quadrant albedos. Locations indicated by + sign indicate albedo increase of 10%.

I I I I I I I I I I I I I I O.12 NE-QUADRANT l

r p- 0.08 - -

z

<t _ _

Cr O

L <t 0.04 - -

Y D O _ _

O.0 O 2 4 6 8 10 12 14 BURNUP (GWD/MTU)

Figure 5.17 NE quadrant tilt as a function of BU resulting from a 10% albedo increase as shown in Figure 5.16.

i R

) 1 M

. ( J H G E D B A G

1 t

, 2 3 2.6! o 2.7 l

4 e e o e 5 e e , e e e 6 e e e e a 7

e e e o e e e 0

e e e e e a 9 e e e e e e e 10 11 e e e s e 12 , , , ,

13

2. e 2.6 14 e e 15 1

l Figure 5.18 Location of the assembly with an enrichment increased to 2.7% W/0. The location of all B-type BPR assemblies is also indicated.

I I I I I I I I I 1.18 SUBASSEMBLY WITH NO BPR I.16 - -

1.14 1.12 -

9 x

1.10 -

1.08 -

1.06 -

SUBASSEMBLY WITH BPR l ' ' ' ' ' ' ' ' '

1.04 O 2 4 6 8 10 12 14 16 18 20 BURNUP (GWD/MTU)

Figure 5.19 Change in b= of a C-type assembly with and without BPR as a function of BU.

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

l l 1 l l l l l l l l l l l 3 - NE-QUADRANT -

N -

p O.4 -

p _

l-

@ O.2 Z

O

? <C 3-o 0.0 -

I I I I I I I I I I 1 I I I O 2 4 6 8 10 12 14 .

BURNUP (GWD/MTU)

Figure 5.20 NE quadrant tilt as a function of BU resulting from a 0.1% increase in the fuel enrichment of assembly F3.

P fl ft L K J H G F E D C B A R

C C C 2 C C C C C C 3 C C C C C C 4

C C C 5 C _

C C'

6 C C C 7 C C C

8 C C C C C g

10 C C

C C C C 11 C C 12 C C C C 14 C C C C C C 15 Cll C C Figure 5.21 Core map of North Anna Unit-1 Cycle-1 showing locations of C-type assemblies and the location of a misplaced C-assembly. -

I 1

l l l l l l l l l l l l l l n

v g NE - QUADRANT I-2 -

f--

I-Z

<I l o-O L <l

? D O

O N

1 1 1 1 I I I I I I I I I I O 2 4 6 8 10 12 14 BURNUP (GWD/MTU)

Figure 5.22 NE quadrant power tilt as a function of BU resulting from a misplaced Type-C assembly.

l I I I I I I I NE-QUADRANT 5 O. _

I-g F- 0.0 -

$ SW- QUADR ANT m

O

, <t -o" l -

[y N I I I I I I I O 2 4 6 8 10 12 14 BURNUP (GWD/MTU)

Figure 5.23 Quadrant tilt as a function of BU in the NE and SW quadrants resulting from a partial flow blockage in channels located at N11 and L13.

R P fl M L K J H G F E D C B A 1

x l X X X 2

3

~

4 X X X '(

. X 5 X-X X X X X 6 X X X X X X 7

X X X X X X X 8

9 13 11 12 13 14 15 Figure 5.24 Core map showing the locations of the coolant channels in which.a 5% flow increase was assumed.

0.3 , i i i i ,

8 NE-QUADRANT H O.2 - -

_1 F

l-z

<t 0.1 L m 7 o

<t D

I I I I I I O.O

. O 2 4 6 8 10 12 14 BURNUP (GWD/MTU)

Figure 5.25 Quadrant tilt as a function of BU in the NE quadrant resulting from a 5%

flow increase as shown in Figure 5.24.

1

O.4 i i i i i i NE-QUADRANT O.3 - (a) _

O.2 - -

3

~

0.1 - -

h F

i- 0.0 z

<1 Q"

o a

@ -0.1

? o -

-0.2 - -

-0.3 -

(b)

SW-QUADRANT

-0.4 O 2 4 6 8 10 12 14 16 BURNUP (GWD/MTU)

Figure 5.26 Quadrant tilt as a function of BU; (a) a 1%

increase in k at assembly E3, (b) a 1% de-

crease in k, at assembly L13.

_m. mE " _ _ _ _ _ _ _ _ _ _ _ _ _ _

3 Q

L I I I I I I I I I I I I I I I REACTIVITY INSERTED IN NE ASSEMBLIES .

$ E3 and C5 g2 - -

m D

}-- 1 w

Q- V b O ss ^

2 t-

, o

<t L w -1 - -

Y &

D j-2 -

w REACTIVITY INSERTED IN SW ASSEMBLIES

$ NII and Ll3

< l l l l l l l l

_3 l l l l l l l 0 2 4 6 8 10 12 14 16 BURNUP (GWD/MTU)

Figure 5.27 Reactivity perturbation inserted to reproduce the observed tilt in the NE and SW quadrants of North Anna Unit-1 Cycle-1.

i

1 1

l .6 -

,/ _

l.2

/

0.8 - ,' NE-QUADRANT -

^

L s' -*%'m/ / A -

H O.4

_J

/

w

$ O.0 Cr -\ af -

o -

s N--- -

< -0'4 D ~~"~~~s ,

• SW-QUADRANT o _

s _

e N

-0.8 -

N -

_ 's s . _

^* MEASURED \ s

_l~g _

s ' '" ~a _

--- SIMULATED BY VEPCO ~~t -

SIMULATED BY BNL (NODE P)

~

~ ~

~

l l I I I l I O 2 4 6 8 10 12 14 16 BURNUP (GWD/MTU)

Figure 5.28 Calculated and measured quadrant tilts at full .

power as a function of BU.

l 1

44-

6. THE POWER TILT AT NORTH ANNA UNIT-1 CYCLE-2 North Anna Unit-1 completed Cycle-1 operation on September 1979. Batch 1 (fuel elements loaded into theofcore.

Type-A)From was removed andoperational preliminary 52 fresh Type-C(4)fyel data, elements were it is evident that power tilting is cccurring in Cycle-2 and apparently the Cycle-1 tilting mechanism is also present in Cycle-2. In addition, Cycle-2 has an initial i asymmetric exposure distribution producing a large BOL tilt in the SW quadrant.

6.1 Cycle-2 Core Loading The Cycle-2 core loading differs from Cycle-1 in that first, most of the fuel has undergone various degrees of BU (from 9 to 20 GWD/MTU) and second, there are almost no BPR's. The loading map for Cycle-2 is given in Figure 6.1. All type-1(A) fuel elements from Cycle-1 have been removed except the central assembly. All Type-2 (B) fuel elements were moved to locations occu-pied previously by Type-1. The BPR have been removed from these elements and locations provided for control rod insertion. Type-C and C' from Cycle-1 have been moved to the core central region occupying previous Type-B locations.

The new batch has replaced Type-3(C) assemblies.

Data provided by VEPC0(6) only include assembly-wise BU. Since NODE-P is a 3-dimensional code and requires a 3-dimensional distribution of the ini-tial BU, an axial BU distribution based on a N0DE-P average BU calculation for Cycle-1 was assumed. This distribution for the 12 axial Nodes used is given in Table 6-1.

6.2 Simulation of Cycle-2 BOL Power Distribution Several BOL power maps of Cycle-2 were provided by VEPC0(6) at differ-ent power level s. The test of the N00E-P program applicability to the present study is in its ability to reproduce the measured North Anna Unit-1 Cycle-2 (NIC2) power distribution. As a first step, the new B-constants for the dif-ferent assemblies are calculated. This is done by a method of functional in-terpolation as explained in Appendix 2. The measured and calculated power distributions along the central line of fuel assemblies are presented in Table 6-2 for zero and full power. As can be seen, the agreement is good and the average error is about 2.57. for both cases. A total of eight power maps were simulated at different power levels and similar agreement has been obtained.

Some integral parameters of different maps are compared in Table 6-3.

j 6.3 Power Tilt Due to the Cycle-2 Asymmetric Burn-up Distribution 4 The quadrant power tilt ihenomena observed in Cycle-1 resulted in a dis-tribution having the highest BU in the NE quadrant and the lowest BU in the SW quadrant. When Cycle-2 fuel was loaded in the core, assemblies with BU were not moved out of their Cycle-1 quadrants. As a result, at the beginning of Cycle-2, the BU distribution was asymmetric. The Cycle-2 BOL average normal-ized quadrant BU distribution is given by r

NW NE l 1.000107 1.007518 ]

SW SE 0.994726 0.997699 and the percent difference of each quadrant relative to the average core BU is NW NE 0.0107% 0.7518%

SW SE

-0.5274% -0.2351%

This uneven BU distribution produces a positive power tilt in the SW quadrant and a negative power tilt in the NE quadrant. At full power the measured and calculated tilts are NW NE SW SE Measured -0.43 -0.61 0.97 0.07 ,

Calculated -0.05 -2.37 1.99 0.43 Difference - 6 3H T1.76 -T UI -W The agreement between measured and caic.ulated tilt is poor. Better agreement is achieved at lower power levels and higher tilts as can be seen from Table 6-3 and Figure 6.2 6.4 Power Tilt Variation With Changing Power Level As in Cycle-1, the magnitude of the tilt changes with power level. As the power increases, the tilt decreases. Power map measurements at BOL Cycle-2 show a decrease from 9% at zero power to 1% at full power in the SW quad-rant. The simulation of NIC2 at the same power levels shows a decrease from 6% at zero pcwer to 2% at full power. As can be seen in Figure 6.2, the matching of the tilt level as a function of power is good at 32%, 42%, and 83%

power levels, but is worse at zero and full power. The same holds for the NE quadrant negative power tilt.

There are several possible explanations for this discre'pancy at zero and full power.

First, at zero power the tilt is strongly dependent on the boron concan-The boron concentration as given by tratigg)and VEPC0lo forcontrol rodCycle-2 the initial positions.

core loading at zero power is 1348 ppm while the NODE-P calculations achieve criticality at 1239 ppm (see also Table 6-3). The difference of 109 ppm at this level is sufficient to account for the discrepancy in the power tilt. At this level of boron the moderator coef-ficient is close to zero and very sensitive to small variations in the concen-tration. The additional 100 ppm may in fact result in a positive coefficient.

Second, the North Anna measurements at full power are made at 570 MWD /MTV rather than at zero 60 and the tilt can be expected to burnout during J

i 2his exposure. This will not account for the full 1% difference. However, the initial burnout rate is rather fast and the fact that the initial reactor operation was at low power with higher tilt might cause the tilt to burnout much faster for the same exposure than BU at full power.

Finally, as was observed in Section 5.4 (cigure 5.11),-the. precise value of the power tilt is very sensitive to the exact state point of reactor opera-tion. The more important state variables will be the precise power, boron concentration, control rod position, BU, temperature, flow, and xenon concen-tration. Any discrepancy in one or more of these variables between the actual measurement and the simulation might result in different tilt levels.

6.5 Power Tilt Variation with Increasinq BU

, The tilt caused by the asymmetric BU distribution burns out in the same way as did the asymmetric perturbations discussed in Chapter 5. Figure 6.3 presents the calculated tilt as a function of BU in the NW and SE quadrants and the actual measured tilt data from NIC2. According to the calculated tilt '

results the core should achieve equal power distribution somewhere between 8 and 10 GWD/MTV. The measured tilt, however, shows that this is achieved at 3-4 GWD/MTU, whereaf ter the trend is reversed and a positive tilt increasing 1 in magnitude is observed in the NE quadrant and a negative tilt is observed in j the SW quadrant. As seen from Figure 6.3, the discrepancy between the com-puted and measured power tilts is large.

6.6 A Cycle-2 Burn-up Dependent Tilt The data in Figure 6.3 suggests that in addition to the quadrant power .

tilt generated by the asymmetric BU distribution, there is :dso a mechanism which is driving the NE quadrant in the direction of a positive tilt and the SW quadrant in the direction of a negative tilt. When the time dependent reac-tivity function, Figure 5.27, which reproduced the measured tilt in Cycle-1, is introduced in the Cycle-2 BU calculations, the tilt versus BU curve shown in Figure 6.4 is obtained. As can be seen, there is reasonable agreement be-tween the measured and predicted tilt. This indicates that the same process that produced the tilting during Cycle-1 is at work during Cycle-2. Tha reac-tivity function driving Cycle-2 may be, however, somewhat different from the i one used in Cycle-1.

7. THE CRUD BUILD-UP MODEL One of the conclusions based on the computation presented in Chapters 5 ~

and 6 is that whatever physical phenomena drives the tilt, the magnitude must increase with time. This increase with time must be sufficiently strong to overcome the burnout effect of the existing tilt. That is, the positive re-activity in the NE quadrant, or the negetive reactivity in the SW quadrant, or

- both, must increase in absolute value at a much higher rate than the observed t il t.

d 5

. , . , _ _ y -_,._m

Table 6-1 Axial BU distribution at BOL of Cycle-2.

N0DE # NORMALIZED BU 1 0.665

? 0.981 3 1.097 4 1.130 5 1.133 6 1.131 7 1.125 8 1.117 9 1.097 10 1.041 11 0.901 12 0.581 Table 6-2 fleasured and calculated power distribution along the central line of feel assemblies for tilC2.

l MAP-t41C2-18, 100% Power l MIP-N'lC2-01, Zero Power t Til control rods out Control bank D at 213/228 All other rods out.

Esembly fleasured Predicted Calculated c = Meas. Assembly Measured Predicted Calculated c = Meas.

fiODE P - tiODE P% # Power VEPC0 f100E P - NODE P1

1. Power VEPCO til 0. 93 0.97 0.93 0 H1 0.91 0.87 0.90 -1.1

!!2 0.94 1.06 0.94 0 H2 0.97 0.98 0 93 -4.1 l  !!3 1.04 1.18 1.04 0 H3 1.09 1.13 1.07 -1.8 l

In 0,84 0.92 0.84 0 H4 0.95 0.98 0.92 -3.2 35 1.01 1.03 0.97 -4.0 H5 1.09 1.12 1.06 -2.8 0.90 0.90 0.89 -1.1 H6 0.99 1.02 0.99 0.0 L F ii

? t' l.05 1.04 1.06 +1.0 H7 1.15 1.14 1.16 -0.9 bl 0.80 0.78 0.86 +7.5 H8 0.91 0.93 0.96 +5.5 Hs 1.14 1.04 1.12 -1.8 H9 1.17 1.19 1.19 +1.7 H:0 0.97 0.90 0.96 -1.0 H10 1.00 1.02 1.02 +2.0 till 1.12 1.03 1.08 -3.6 till 1.10 1.17 1.11 +0.9 lii 2 0.99 0.92 0.95 -4.0 til2 0.98 0.98 0.96 -2.0 111 3 1.26 1.18 1 19 -5.6 H13 1.10 1.18 1.13 +2.7 112 4 1.11 1.06 1.07 -3.6 111 4 0.98 0.98 0.97 -1.0 111 5 1.04 0.97 1.06 +1.9 HIS 0.86 0.87 0.93 +8.1 a 2.34 5 2.52

,@ 3.22 ,@ 3.10 l

Table 6-3 Cor:parison of measured and calculated integral parameters for North Anna Unit-1 Cycle-2 BOL at different power levels.

Boron Control Date of Concentration Rod BU Quadrant Power Tilt SW SE MAP # Measurement Power PPM Position i1WD/MTU NW NE 1348 ARO 0 -0.3 -9.3 9.1 0.5 NIC2 01 1-16-80 0%

1239 0 -0.32 -6.6 6.04 0.92 CuiPUTED 1349 ARO O -0.2 -8.4 8. 4 1.2 NIC2 03 1-20-80 3%

1207 0 -0.23 -5.30 4.73 0.79 CQiPUTED 1153 D in 0 -1.0 -3.2 3.4 0.8 I

NIC2 04 1-20-80 3%

1031 C at 118/228 0 -0.77 -5.69 5.04 1.42

. C0t1PUTED 1127 D at 99/228 0 -0.3 -5.0 5.0 0.2

? NIC2 06 1-21-80 3%

1138 0 -0.49 -5.23 1.74 1.08 COMPUTED 1070 D at 218/228 50 -1.1 -2.49 2.87 .72 NIC2 10 1-26-80 32%

993 0 -0.08 -3.05 2.61 0.53 COMPUTED 1070-1091 D at 218/228 85 -0.28 -2.0 2.03 0.25 NIC2 11 1-29-80 47%

943 0 -0.07 -2.83 2.40 0.49 Cu1PUTED 953-974 AR0 150 -0.57 -1.31 1.8 0.08 HlC2 14 2-0 4-80 83%

850 0 .05 -2 ',0 2.1 0.45 COMPUTED 810 D at 213/228 570 -0.43 -0.61 0.97 0.07 NIC2 18 2-14-80 100%

611 0 -0.05 -2.37 1.99 0.43 CortPUTED

1 l

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 N M L K J H G F E D C B A R P FUEL 1 3 3 3

0. O. O. -BU 3 3 4 2 4 3 3 2

0. O. O. 17.12 0. O. O.

.# BPR 3 4' 2 2 4' 2 2 4 3 3 0. O. 18.84 19.3 8 9.1819.41 1 9.13 0. O.

4 3 2 3 3 L3 2 3 3 3 2 3

0. 18 .8 5 15.6C 1539I1 8.67' 16.98 13.59 15.63 15.57 18.58 0.0 3 4 3 2 3 ,

2 3 2 3 2 3 4~ 3 5

0. O. 1546 19.06 9.48 18.61 IL68 18.56 9.51 1 8.94 1535 0. O.

3 2 3 3 2 3 2 3 2 3 '3 2 3 6 0 18.85 15.05 9.16 E44 1.0.31 16.57 10A8 16J2 9 .7 15.6 9 18.8 6 0 7 3 4' 2 3 2 3 ; 2 3 2 3 2 3 2 4' 3 00 0 19.06 13.39l 18.41 la 21,18.05 9.26 17. 97 10.47 1835 13.66 19.41 0 0 8 3 2 4' 2 3 2' 3 1 3 2 3 2 4' 2 3 0 2674 9.08 16.95 11.1: 1628 8.88 14.30 9.04 16.5 5 11 52 IS 95 9.1 17. 2 6 0 g 3 4' 2 3 2 3 2 3 2 3 2 3 2 4 3 0 0 18.92 1326 L8. 3 'e 10.5218.02 8.82 17.68 la57 38.4 7 13.61,19.1 6 0 0 3 2 3 3 2 3 2 3 2 3 3 2 3 10 0 18.8 1544 9.41 1626 1047 16.31 10.57 1 6.48 9.42 15 62 18.7 0 3 4' 3 2 3 2 3 2 3 2 3 4' 3 11 0 0 15.25 19.07 9.47 18.0 9 1L33 18.19 9.28 1&75 1559 0 0 3 2 3 3 3 2 3 3 3 2 3 12 0 la51 1537 15 56 13.6 6 17.01 1329 15.60 15.17 18.2 9 r_ ,,

3 4' 2 2 4' 2 2 4' 3 13 0 0 18.4 8 28.7 4 9.12 18.95 18.73 0 0 14 3 3 4' 2 4' 3 3 0 0 0 17. 07 0 0 0 3 3 3 15 0 0 0 Figure 6.1 Loading map of North Anna Unit-1 Cycle-2. Type-1 enrichment is 2.1%; Type-2 enrichment is 2.6%;

Type-3 enrichment is 3.15; Type-4 enrichment is 3.215 and includes BPR.

lO i i i i SW-QUADRANT _

8 -

NIC2 MEASUREMENTS -

--- CALCULATED NODE F ,

1 -

g

\

4 -

\

N N * -

~

b _

I H O z

< v, ct -

Q 4

$ -2 -

~_

y i

-4 -

7

/

-6 -

-8 -

NE-QUADRANT _

I I I I

-10 60 [

O 20 40 8C 10 0 POWER (% Roted) <

Figure 6.2 Comparison of measured and calculated quadrant tilts in NIC2 as a function of power.

2.0 i i , i i i i i i i i i i i

• SW-QUADRANT -

1.6 - -

CALCULATED 1.2

• SW - TILT 0.8 -

,e -

~

• A 0.4 -

A I-g A p -

Ag 8 -

$ O.0 A g _ _

Q A

$ -0.4 ,A -

o _

-0.8 - -

CALCULATED

-l.2 -

N E - TILT -

-1.6 -

ANE-QUADRANT

_ MEASUREMENT _

' ' ' ' ' ' ' ' ' i I ' '

-2.0 O 2 4 6 8 10 12 14 BURNUP (GWD/MTU)

Figure 6.3 Cycle-2 measured and calculated quadrant tilts as a function of BU.

2.0 i i i i i i i i i i i i i .

~

eSW-QUADRANT 1.6 MEASUREMENT _

~

~

CALCULATED 1.2 . SW - Tl LT -

O.8 -

A o4 _

l- A g

A, * -

F- 0.0 A z

y _ _

Q A 34 -0.4 ,A o _

A

• A i - 0.8 -

l I -

CALCULATED

- 1.2 - NE- TILT _

-1.6 - ANE-QUADRANT _

MEASUREMENT _

1

. 2.0 O 2 4 6 8 10 12 !4 BURNUP (GWD/MTU)

Figure 6.4 Cycle-2 measured and predicted quadrant tilts based on the reactivity function used to predict the NIC1 Cycle-1 tilt.

l

Since flow maldistribution has the highest potential gain in terms of reactivity, it may be part of the tilt mechanism. However, flow by itself cannot acccount for the observed tilt since the absolute effect is small. In order to provide the observed tilt, the amount of flow maldistribution would be large and easily detected. At present, the only phenomenon that has been identified on the fuelwhich rods.could Previous have the required (time observations gr BU 7,8,9) dependance in other reactorsishave crudshown buildup that crud accumulates asymmetrically in the upper core.

The details of crud deposition are not well known and it is difficult to quantify the effect of crud deposit on fuel pins, however, it is clear that crud will produce the following effects:

1) Crud deposit causes a decrease in k, of the affected assembly at an approximate rate of 0.2% ak per mil of crud.

2) The crud deposit will increase friction in the coolant channel and re-duce the flow rate. Reduced flow rate will cause coolant temperature to increase and induce a negative reactivity effect.

3) Since a constant flow throughout the reactor is maintained, the flow in the unaffected channels will increase, increasing appropriately the reactivity in these channels.

4) Crud deposit will reduce heat transfer and increase fuel temperature thereby further reducing the reactivity in the affected channels.

5) Boron can be trapped in the crud, further reducing the reactivity in the affected channels.

A preliminary calculation affecting about half the core (essentially the same region over which the tilt has been observed) with an uneven crud buildup of 0.05 to 0.10 mil (ak=-0.0001 to -0.0002) and a flow decrease of 0.2% to 0.4% in the channels affected by the crud was introduced. An appropriate flow increase in the unaffected channels was also simulated. This model resulted in a positive tilt of 0.286% in the NE quadrant and a -0.284% negative tilt in the SW quadrant at a BU of 1 GWD/MTU.

A gradual increase (as in Figure 5.27 for example) to about five times the preceding values, will produce a tilt similar to the tilt observed in North Anna Cycle-1. Five times the above values is a reactivity decrease of about 0.05% to 0.1%, a crud deposit of about 0.25 to 0.5 mil and a flow decrease of about 1 to 2%. The necessary change in reactivity alone to repro-duce the observed tilt might be 2 or 3 times higher which represents only about 1 to 1.5 mil of crud.

The above values of crud and flow maldistribution would be difficult to detect by any of the standard monitoring systems, especially since the crud is believed to disappear during reactor depressurization.

8.

CONCLUSIONS AND RECOMMENDATIONS 8.1 Conclusions A major conclusion of these calculations is that any asymmetric perturba-tion present in the core causing an asymmetric power tilt will " burn out" in time. The dominant effect being the BU of the fuel and accumulation of fis-sior, products. Both of these effects reduce the local reactivity and result in a strong tendency of the power distribution to equalize.

A related phenomena is the flattening of the power distribution as it reaches E0L conditions. Also, higher local power results in a more rapid burnout of the asymmetry. Table 8-1 compares " expected" EOL tilts calculated s by introducing the perturbation at EOL for each of the physical phenomena con-sidered in this study. Also presented are the actual EOL tilts resulting from BU calculations. It can be noted from Table 8-1 that all perturbations are more effective at E0L (i.e., if inserted at E0L only) and will cause a much However, BOL perturbations all burn-out and larger power tilt than at BOL.

are significantly reduced at EOL except for the flow blockage perturbation which because of the high gain with BU overcomes the BU poisoning effect ano remains approximately constant during Cycle-l. The higher tilt caused by in-serting a moderator related perturbation into a symmetric core at EOL is due ,

mainly to the much higher moderator reactivity coefficient at E0L. Except for the increasing BU dependent reactivity perturbation, none of the tilt inducing phenomena considered was capable of producing the observed tilt.

The observed tilt may be explained by the deposition of crud on the as-sembly fuel' rods. A crud deposition rate was determined as a function of burn-up which will reproduce the North Anna-1 tilt BU dependence and is con-sistent with all known observations. Although only qualitative, this study suggests that crud is responsible for the observed North Anna quadrant tilt. -

8.2 Recommendations The results of this analysis suggest the following recommendations:

1. A review of the plant operating data and loop water chenhary, with respect to the projected crud deposition rate should be performed to I determine if crud buildup is indicated. I

2. If the results of this review indicate crud buildup, a detailed simu-lation of crud buildup during Cycle-1 and 2 should be made. The ac-tual crud deposition rate may be estimated by adjusting the crud thickness to reproduce available North Anna power maps. In order to represent accurately the effect of crud on the fuel temperature and on local flow, this simulation will require modifications to the N0DE-P program.

Table 8-1 Comparing " expected" and actual EOL quadrant power tilts due to selected asymmetric perturtstions.

Expected Calculat'd Description of the BOL E0L E0L No rmalized Normalized Perturbation Tilt % Tilt % Tilt % Prediction Calculated Temperature maldistribution 0.27 0.645 0.125 2.39 0.46 Control rod misalignment 0.56 1.34 0.067 2.39 0.12 10% increase in Al bedo 0.136 0.136 0.008 1.00 0.06 Partial flow blockage -0.110 -0.405 0.110 3.68 1.0 5% flow increase 0.259 0.708 0.175 1.73 0.68 e 1% increase in Assembly k 0.383 0.464 0. 0 1.21 0.0 -

1% decrease in Assembly k -0.370 -0.457 0.005 1.24 0.01 Measured Cycle 1 0.18 1.38 7.7

REFERENCES

1. J. R. Tu, D. M. Kajuschinsky, " North Anna Unit-1, Cycle-1 Core Perfor-mance Report," VE?-FRD-34, Virginia Electric & Power Company, Richmond, Virginia, (December 1979).

2. " North Anna Power Station, Units 1 and 2, Final Safety Analysis Report,"

Virginia Electric and Power Co. , Richmond, Va. , (1973)

Docket-50338-62.

3. Advanced Recycle Methodology Program EPRI CCM-3 Part 11.4, prepared by Nuclear Associates International Corooration 6003, Executive Boulevard, Rockville, MD. 20852.

4. B. M. Rothleder et al. "PWR Core Modeling Procedures for Advanced Recycle Methodology Program," Draft. Prepared for EPRI by NAI project 976-1, (1979).

l 5 L. Eisenhart, " Core Power Tilts," Internal Memorandum 10-29-79, I Brookhaven National Laboratory.

6. Private communication - Virginia Electric & Power Company, Richmond, Va.

(July, August,1980).

7. Y. Solomon, J. Rosmer, " Measurement of Fuel Element Crud Deposits in Pressurized Water Reactors," Nucl. Tech., 29,, (1976) pp 166.

8. J. H. Hicks, "Recent Concerns with Reactor Coolant Chemistry Technology In Pressurized Water Reactors," Nuclear Technology, 29, (1976) pp. 146,

9. R. Riess " Chemistry Experience in the Primary :at Transport Circuits of Kraftwerk Union Pressurized Water Reactors," Nucl. Tech., -29 (1976) pp. 153.

10. J. W. Herczeg, T. Lai, M. Todosow, D. J. Diamond, " Simulation of a PWR First Cycle with the ARMP System," BNL-NUREG-25607, Brookr even National Laboratory (1979).

APPENDIX 1. NORTH ANNA NUCLEAR POWER PLANT BASIC DATA The data used in tha present study are listed below. The data were col-lected from References 1 and 2.

Reactor Thermal Power 2775 MW Coolant Inlet Temperature Sa6.80F Coolant Outlet Temperature 613.70F Coolant Inlet Pressure 2300 psi Coolant Exit Pressure 2250 psi Core Diameter (equivalent) 119.7 i n Core Height 144 in Fuel Assembly (157) Cycle-1 Cycl e-2 Batch A B C A B C Cl D Initia': Enrichment 2.1 2.6 3.1 2.1 2.6 3.1 3.1 3.21 (w/o 235) ,

Assembly Type 17x17 17x17 17x17 17x17 17x17 17x17 17x17 17x17 No. of Assemblies 53 l 52 52 1 52 12 40 5?

Fuel Rod per Assembly 264 264 264 264 264 264 264 264 BU of BOC (MWD /MTU) 0 0 0 14.3 18.1 14.1 11.6 0

1. of loops- 3 Total flow rate- 105.2 x 10 61b/hr e

APPEllDIX 2. GENERATING B-C0f!STANTS FOR fl0RTH ANNA CYCLE-1 ,

AND CYCLE-2 FUEL ASSBiBLIES '

The proper way to calculate B-constants is described in Reference 3.

This includes unit-cell calculations for the fuel pin and assembly calcula-tions with PDQ. This process, however, is rather long, and was not feasible for the present study. Instead a method of functional interpolation was de-vised by which B-constants were calculated from constants calculated for a typical three-loop BWR (see also Reference 10). The program used to calcu-late the B-constants for this Study is given below.

2a. FITB-A Program to Produce B-Constants by Functional Interpolation and Fitting Constants that require functional interpolation were evaluated by FITB.

The following polynomials were first produced for exposure values, E, from 0 to 16 GWD/MTV:

ap/Nb oron=B21(1 + B22

• E + B38 *E) 2 (A-1)

KIf = B23 + B24

• E + B25 *E 2, (A-2) vrf = B26 + B27 *E + B28 *E2 (A-3)

Ap(E) = B33(E+B34*V) + B35 *E2 +B36

• E3+B37*E4 . (A-4)

Af ter the polynomial was reconstructed, the system subroutine POLFIT was used to produce the fit to the calculated values, and system subroutine PCOEF was used to calculat.e the coefficients.

The listing of the program is given below.

q 2

PROGRAM FITB (INPUT,0UTPUT)

C A PROGRAM TO CALCULATE B 33,35,36,37 CONSTANTS FOR NODEP C**** IF ENX>0 B CONSTANTS FOR ASSEMBLIES WITH N0 BPR CALCULATED.

C**** IF ENX<0 B CONSTANTS FOR ASSEMBLIES WITH BPR (N0-CR) ARE CALCULATED.

DIMENSION B33(4), B35(4).B36(4), B37(4),B(5),838(4)

DIMENSION B21(4),82%(4),B23(4),824(4),B25(4),B26(4),827(4),B28(4)

DIMENSION ENR(4),R0(16),BR(16),SF(16),SK(16)

DATA B21/0.27374E+5, 0.17777E+5, 0.19956E+5, 0.17006E+5/

DATA B22/-0.02166, -0.0031813, -0.0049715. -0.75375E-3/

DATA B23/15.663, 24.885, 21.845, 26.165/

DATA B24/0.39943, 0.19884, 0.32419, 0.25258/

DATA B25/ ;0098178, .0083050, .0097776, .0093027/

OATA B26/-0.18785. 0.30022. 0.26300, 0.31569/

DATA B27/0.0069443, 0.0042426, 0.0060833, 00051153/

DATA B28/ .15497E-3, .12902E-3, .15378E-3, .14505E-3/

DATA B33/0.82549E-2, 0.79257E-2, -0.88972E-2, -0.4599E-2/

DATA B35/0.1647E-3, -0.97408E-6, 0.90493E-3, 0.52101E-3/

DATA B36/-0.61459E-5, 048355E-6, -0.20120E-4, -0.91602E-5/

DATA B37/0.56757E-7, -0.58664E-8, 0.15927E-6, 0.58378E-7/

DATA B38/0.001329, 0.00049689, 0.0005158, 0.00032652/

DATA ENR/1.85, 3.10, 2.55, 3.10/

PRINT 100 READ 110,ENX ENX=-2.7 11=1 S I2=2 IF(ENX.GT.0.0) GOTO 5 11=3 S 12=4 $ ENX=-ENX 5 DENR=(ENX-ENR(11))/(ENR(12)-ENR(II))

100 FORMAT (* TYPE ENRICHMENT *)

00 10 I=1.16 X=I E1=X*(B33(II)+X*(B35(II)+X*(B36(II)+X*B37(11) ) ) )

E2=X*(B33(12)+X*(B35(I2)+X*(B36(I2)+X*B37(12) ) ) )

R0(I)=El+(E2-E1)*DENR PRINT 120, x,El,E2,R0(I)

BR1=B21(II)*(1.0+B22(II)*X+B38(11)*X*X)

BR2=B21(12)*(1.0+B22(I2)*X+B38(12)*X*X)

BR(I)=BR1 + (BR2-BR1)*DENR SFl=B23(II)+X*(B24(11)+B25(II)*X)

SF2=B23(12)+X*(B24(12)+B25(12)*X)

SF(I)=SF1+(SF2-SF1)*DENR SK 1= B26 ( II ) +X* ( 827 ( II ) +B 28 ( I I ) *X )

SK2=B26(12)+X*(B27(12)+B28(12)*X)

SK( I ) =SKl+( SK 2-SK1) *DENR 4 10 CONTINUE CALL FIT 4(R0,4,B)

CALL FIT 4(BR,2,8)

CALL FIT 4(SF,2,B)

CALL FIT 4(SK,2,B) 110 FORMAT (F7.3) 120 FORMAT (

• EXP=* ,FS.1,* R01+*,E12.4,* RC2= * ,E12.4,* R03= * ,E12.4)

STOP END l

(

The subroutine FIT 4 to perform curve fitting and calculate the fitted poly-nonial coefficients is as follows C

C SUBROUTINE FIT 4(RO,MAXORD,TC)

C SUBROUTINE TO FIT COSTANTS TO FOURTH ORDER POLINOMI AL.

DIMENSION R0(16),X(16),11(16), A(100),R(100),TC( 5)

DATA A/100*0.0/,R/100*0.0/

DATA N/16/

00 5 I=1.5 5 TC(!)=0.0 00 10 I=1,16 X(I) =I W(I)=1.0/R0(I)**2 10 CONTINUE EPS=0.0 CALL POLFIT(N,X,R0,W,MAXORD,NORD,EPS,R,IERR,A)

PRINT 110, (X(I),R0(I),W(I),R(I),I=1,N)

L=NORD PRINT 100,IERR,NORD,EPS C=0.0 CALL PROEF(L,C,TC,A)

PRINT 200,TC RETURN 100 FORMAT (

• IERR=* ,13,* HORD=* ,13,* EPS=*.E12.5) 110 FORMAT (

• X=*F5.1,* R0=* ,E10.4,* W=* ,E10.4,* R=* ,E10.4) 200 FORMAT (* TC= *,8E12.5)

END

2b. B-Constants for Cycle-1 Type-A assemblies have an enrichment of 2.1% (w/o U235) and have channels that can accommodate the reactor control rods. The 60 B-constants for this assembly are listed in Table A-1.

Table A-1 B-constants for Type-A assembly.

230.13 -737.77 750.38 .45964 .72873

.99782 3.1432 1.6668 .9949 -3.4478

-1.7745 -1.0127 .8999 .1429E+8 .23356E+7 0.0 .001247 575.4 1237. 0.0 25455. .019079 17.507 .35931 .0095152

.2103 .006404 .14978-3 0.0 0.0 1.5187 42.965 .0081891 0.0 .13157E-3

.48200E-5 .44232E-7 .0012128 0. O.

O. O. O. 1.0 0. ,

0. 0. O. O. 0.

O. O. O. O. O.

O. O. O. O. O.

Type-B assemblies have an e'irichment of 2.6% (w/o U235) and contain burn-l able poison rods. The 60 B-constants for this assembly are listed in Table A-2, l Table A-2 B-;onstants for Type-B assemblies.

1

.24361E+03 .77648E+03 .78118E+03 0. O.

.81804E+00 0. O. .15894E+01 0,

0. .16725E+01 0. .11529E+08 .23440E+07

0. 0.17162E-02 .57540E+03 .12370E+04 0.

.19688E+05 .46402E-02 .22238E+02 .31768E+00 .97344E-02

.26779E+00 .59953E-02 .15299E-03 0. O.

O. .10000E+01 .85065E-02 0. .87003E-03

.19124E-04 .15010E-06 .50093E-03 0. O.

O. O. O. 1.0 0.

O. O. O. O. O.

O. O. O. O. O.

0 0. O. O. O.

I

Type-C assemblies have an enrichnent of 3.1% (w/o U235) and have holes to accommodate control rods. The 60 B-constants for this assembly are listed in Table A-3.

Table A 3 B-constants for Type-C assemblies.

.24380E+03 .78263E+03 .78660E+03 .48929E+00 .77146E+00

.10536E+01 .36203E+01 .19478E+01 .11711E+01 .39057E+01

.20557E+01 .11965E+01 .90424E+00 .10274E+08 .22660E+07 0, 0.10392E-02 .57540E+03 .12370E+04 0.

.17777E+05 .31813E-02 .24885E+02 .19884E+00 .83050E-02

.30022E+00 .42426E-02 .12902E-03 0. O.

.17556E+01 .43902E+02 .79257E-02 0. .97408E-06

.48355E-06 .58664E-08 .49689E-03 0. O.

O. O. O. 1.0 0.

O. O. O. O. O.

O. O. O. O. O.

l

0. O. G. O. O.

l Type-C' assemblies have the same enrichment as Type-C, 3.1%, but include burnable poison rods instead of holes for control rods. The 60 B-constants for this assembly are listed in Table A-4.

Table A-4 B-constants for Type-C' assemblies.

.24367E+03 .77658E+03 . 78184E+-0 3 0. O.

.83434E+00 0. O. .17252E+01 0.

O. .18004E+01 0. .98047E+07 .22660E+07

0. 0.15907E-02 .57540E+03 .12370E+04 0.

.17006E+05 .75375E-03 .26165E+02 .25258E+00 .93027E-02

.31569E+00 .51153E-02 .14505E-03 0. O.

O. .10000E+01 .45990E-02 0. .52101E-03

.91602E-05 .58378E-07 .32652E-03 0. O.

O. O. O. 1.0 0.

O. O. O. O. O.

O. O. O. O. O.

O. O. O. O. O.

l

l 2c. B-Constants for Cycle-2 The loading map of Cycle-2 is given in Figure 6.1. As can be seen, there is only one assembly of Type-A (designated 1), at the center of the core. The Type-B (designated 2) were moved to the locations previously occupied by Type-A assemblies. The burnable poison has been removed from this assembly and they can accommodate control rods instead of BPR. All the assemblies of Type-C and C' (designated 3) with BPR removed (except in 4 assemblies) were moved from the core periphery to replace the B-type assemblies. A new batch of fresh fuel, Type-D, was inclJded to replace Type-C assemblies. This batch has an enrichment of 3.21%, however, the constants used p'evicusly for Type-C (3.1%) assembly were used for this fuel. Also, since the content of the BPR was very low, it was assumed in the calculations that the assemblies of Type-C' and D' have no BPR.

The new B-constants for Type-B assemblies without BPR are listed in Table A-5.

Table A-5 B-constants for Type-B assemblies with no BPR.

.23696E+03 .75970E+03 .76859E+03 .47447E+00 .75010E+00

.10257E+01 .34117E+01 .18073E+01 .10691E+01 .36767E+01

.19182E+01 .11048E+01 .90206E+00 .12282E+08 .23008E+07

0. 0.11430E-02 .57540E+03 .12370E+04 0.

89101E-02

.21616E+05 .12542E-01 .21196E+02 .27908E+00

.25527E+00 .53233E-02 .13940E-03 0. O.

.16372E+01 .43434E+02 .80574E-02 0. .65296E-04

.21682E-05 .19183E-07 .91339E-03 0. O.

O. O. O. 1.0 0.

O. O. O. O. O.

O. O. O. O. O.

O. O. O. O. O.

t

APPENDIX 3. MODIFICATION OF THE N0DE-P PROGRAM TO PERMIT AN ASSEMBLY-WISE INLET TEMPERATURE DISTRIBUTION Since N0DE-P was developed from the FLARE code which was a BWR simulation code, the core inlet temperature is given in degrees of succooling. The values accepted by the program are SUBC1 and SUBC2 where SUBC = Hsat-Hin and is given in BTU /lb. The actual subcooling is made power dependent by the relation, SUBC=SUBC1 + SUBC2 * (Fraction of Rated Power). (A-5)

The code was modified so that instead of a single subcooling value for the coolant entering the core, each assenbly can now have its own subcooling value. The format of the new input data is the same as for data types 06 to

08. The new data type is 18 and should be inserted in the following way.

Type Column Content Remark 18 1-10 Any user identification 11-12 OU 13-14 blank 15-16 I Row designation 17-18 SBCL(I,J) Degree of subcooling of coolant entering the core for eaca channel in the I-th row where J=1, . . . . .JMAX, a nd the units are BTV/lb.

The changes to the N0DE-P program permitting channel-wise coolant inlet tem-perature variation are listed below using the CDC UPDATE source file mainte-nance program.

• IDENT LDE001

• INSERT EDT1.238 IF(ISYM.EQ.-1) CALL TILT ( AAA,IMAX,JMAX,NAME)

• hSERT BNLC.2 COMMON / DAVE / SBCL(30,30)

• INSERT BNLC.32 COMMON / DAVE / SBCL(30,30)

• INSERT BNLC.13 COMMON / DAVE / SBCL(30,30)

• INSERT FLAREB.83 DIMENSION QIND(30,30)

• INSERT FLAREB.185 QIND(I.J)=-SBCL(I,J)/HFG

• DELETE FLAREB.186 Q(1)=QIND(I,J)+QX*S(I,J,1)

• DELETE BDV1.199 2222 G0TO(23,26,34,44,45,46,47,48,61,62,63,72,75,80,90,100,110,85),LOLD

• INSERT RDV1.266 GOTO 49.

. 85 ASSIGN 84 TG MA

• DELETE 061974.80 50 GOTO MA (51,53,55,57,59,84)

• INSERT RDV1.280 GOTO 60 84 SBCL(N,J)=X(J)

• DELETE RDV1.428 1 186,190,192),L ,

' INSERT ROV 1.511

• CARD TYPE 18. SUBC00 LING BY SUBASSEMBLY SBCL(I,J) 192 WRITE (ITAP0T,193) l'? FORMAT (*0 KD*27X,+SUBCCOLING OF SUBA5SEMBLY INLET FLOW. SBCL(I,J) 1)=HSAT-HIN.*)

GOTO 16 k

e DISTRIBUTION U.S. Nuclear Regulatory Commission H. Denton D. Ross M. Dunenfeld (2) L. Rubenstein S. Fabic D. Fieno (7) Director, Office of Standards R. Minogue Devel opment Technical Assistant, Executive Director's Office Public Document Room Bethesda Technical Library Advisory Committee Reactor Safeguards (16)

Brookhaven National Laboratory Core Performance Group 'N(

DNE Associate and Deputy Chairmen Nuclear Safety Group Leaders Ex,ternal T. Anderson, W G. Sherwood, GE G. Lellouche, EPRI M. Smith, VEPC0 R. Mills, C-E J. Tayl or, B&W C. Owsely, ENC B. Zolotar, EPRI e

d

,