AEC DIRIBUTION FOR PART 50 DOCKET MAT'LIAL (TEMPORARY FORM) CONTROL NO: 6291

FTTE FROM: DATE OF DOC: DATE REC'D LTE MEMO 1WI OTHEI Carolina Power & Light Company Raleigh, N. C. 27602 E. E. iUtley / 11-_7-7Py

TO: ORIG SENT AEC _ ___X

Mr. Sk olt CLASS: / (UPROP IFO INPUT NO CYS RECD DOCXT NO:

_______________________4o 5o- p61 DESCRIPTION: ENCLOSURES: Ltr re our 10-3-72 ltr....submitting a report concerning control rod configurations........ W/Attacbments - Tables 1 & 2 & Figures 1 thru 10. NOTE: *PLEASE CIRCULATE-INSUFFICIENT COPIES

RECEIVED FOR FULL DISTRIBUTION D aUt 'uV

PLANT NAMES: H. B. Robinson Unit No. 2

FOR ACTION INFORMATION 11-17-72 AB

BUTLER (L) sCHW CER( L) SCCEMEL( O) NTIGATON(E) W/ copies W/ Copies w/ q Copies W/ copies CLARK(L) STOLZ(L) ZIEMANN(L) YOUNGBI.OOD(E) W/ Copies W/ Copies W/ Copies W/ Copies GOLLER(L) SHASSANE(L) CHITWOOD(FM) IGHAN(E) w/ Copies W/ Copies W/Copies W/ Copies KNIL(L) H. DENTON DICKE )(E) W/ Copies W/ Copies w/ Copies W/ Copies

INTERNAL DISTRIBUTION

REG FILE TECH REVIEW VOLLMER HARLESS WADE E 0-EENDRIE DENTON SHAFER F & M h' OGC, ROOM P-506A SCHROEDER GRIMES F & M BROWN E .. #MUNTZING/STAFF *'* MACCARY * GAMMILL SMILEY G. WILLIAMS E

CASE LANGE(2) KASTNER NUSSBAUMER E. GOULBOURNE L GIAMBUSSO PAWLICKI BALLARD A/T IND BOYD-L(BWR) SHAO SPANGLER LIC ASST. BRAITMA DEYOUNG-L(PWR) KNUTH* ICE L SALTZMAN SKOVHOLT-L STELLO ENVIRO MASON L P. COLLINS MOORE, MULLER WILSON L PLANS

HOUSTON DICKER MAIGRET L MCDONALD REG OPR ' p TEDESCO * KNIGHTON SMITH L DUBE FILE & REGION (2) LONG YOUNGBLOOD GEARIN L MORRIS 0 LAINAS PROJ..GEDER DIGGS L INFO STELLE BENAROYA TEETS L C. MILES

REGAN L

EXTERNAL DISTRIBUTION

d#1-LOCAL PDR Hartville. S. C. P'1-DTIE(ABERNATHY) (1)(5)(9)-NATIONAL LAB'S 1-PDR-SAN/LA/NY

14"l-NSIC(BUCHANAN) 1-R. CARROLL-OC, GT-B227 1-GERALD LELLOUCBE 1-ASL-YORE/SAYRE 1-R. CATLIN, E-256-GT BROOKHAVEN NAT. LAB WOODWARD/H. ST. 1-CONSULANT'S 1-AGMED(WALTER KOESTER,

a 16-CYS ACRS F&W64M SENT TO LIC ASST NEWMARK/BLUNE/AGABIAN Rm c-427, GT) S. TEETS ON 11-17-72 1-RD ... MULLER...F-309GT

y 7' 1972 5 Carolina Power & Light Company

DAIL SECOTION November 10, 1972 C, DOCKETILR CUV N

Mr. D. J. Skovholt Assistant Director Operating Reactors Directorate of Licensing

U. S. Atomic Energy Commission 50 - 2 6 1 Washington, D. C. 20545

H. B. ROBINSON UNIT NO. 2 LICENSE DPR-23

RELATIONSHIPS BETWEEN NUCLEAR HOT CHANNEL FACTORS AND CONTROL AND PART LENGTH CONTROL

ROD CONFIGURATIONS

Dear Mr. Skovholt:

In your letter of October 3, 1972, a request was made for the predicted

maximum fuel power densities for the worst case control rod configurations which

are permissible under the presently allowable control rod insertion limits. The

basic question to be answered was whether CP&L's request for reactor operation with

a lower FN would require revised control rod insertion limits. These limits, as

specified in Figure 3.10-1 of the Technical Specifications, were originally devel

oped assuming FQ equal to 3.13 and a maximum fuel power density of 17.9 kw/ft at

2,200 Mwt. The proposed reduction in FN would reduce the maximum kw/ft from 17.9

to 15.8.

Therefore, a study was undertaken to determine if there existed any com

bination of full length and part length rods which would result in a peak kw/ft

greater than 15.8. The fact that any axial offset greater than +26% would result

in a scram and limit the maximum kw/ft was also considered in this analysis (Tech

nical Specification 2.3.1.2.d.l).

METHODS AND ASSUMPTIONS USED

A three dimensional nodal analysis was used to simulate the core power

distribution and control rod interactions using 18 axial nodes per assembly. This

model is continually being compared with actual operating data from H. B. Robinson

and the agreement to date is excellent. For example, Figure 1 compares the calcu

lated and measured assembly relative power at 9,000 MWD/NTU. Except for a few

peripheral assemblies, the agreement is within +3%. Agreement between calculated

and measured values has been at least this good throughout Cycle 1.

The calculations for the present study were performed at a core average

exposure of 14,000 MWD/MTU. The conclusions drawn from calculations at this expo

sure are valid through the end of Cycle 1, since the current exposure is approxi

mately 13,000 MWD/MTU and the end of cycle is projected to be at 15,000 MWD/MTU.

The validity of the insertion limits was not checked for Cycle 2. It is intended

to provide this information after the core loading pattern for Cycle 2 has been

firmly established.

336 Fayetteville Street o P. 0. Box 1551 c Raleigh, N. C. 27602

e 0 Mr. D. J. Skovholt - 2 - November 10, 1972

In addition, this analysis was directed at investigating normal and antic

ipated transients. Abnormal occurrences such as those identified in Technical

Specification 3.10.2 are not considered, since the requirement is to determine the

peaking factors for all such occurrences with the moveable incore detectors. Due to

the fact that only transient situations were being investigated, the xenon distribu

tion used in the calculations is consistent with the power distribution during our

normal operation. It was assumed that the xenon did not have time to redistribute

during the time required to move the control rods. Prolonged operation with control

rods inserted would result in xenon redistribution; however, this mode of operation

is not anticipated. Furthermore, this assumption is conservative. Examples will

also be shown where the axial distribution of xenon is oscillatory just prior to

control rod insertion.

Due to the use of a nodal analysis in the calculations, there are two

additional factors which must be applied to the calculated F. First, there is an

assembly local peaking factor which must be applied, since the intra-assembly peak

ing is not considered in a nodal calculation. The assembly local peaking factor was

assumed to be a conservative 1.10 in this analysis. A Westinghouse quarter core, rod

by rod PDQ calculation was the basis for this assumption. The second factor consid

ered was the possibility of underestimating the axial peaking factor due to the

pointwise calculation being applied over eight inches of core when 18 axial nodes are

used. Figure 2 illustrates this point. After plotting the axial peaking factors for

a number of cases, it was found that a value of 0.05 added to the calculated FN would

cover the worst possible case of underestimating the peaking.

An additional assumption employed in these calculations is the use of only

4 of the 8 part length control rods. The location of these rods is shown in Figure 3.

These rods were chosen because the start-up tests were performed with these rods.

CP&L has been granted a change in our Technical Specifications to permit movement of

part length rods in groups of four. With the use of an 8 part length control rod

bank, rod motion must be operationally restricted due to the large axial offset pro

duced by that mode of operation.

RESULTS AT 100% POWER

Figure 4 is a reproduction of Figure 3.10-1 given in the Technical Specifi

cations. This figure shows that control bank D is permitted to range from 228 steps

(all out condition) to 107 steps at 100% power. Power distributions were calculated

for four positions of bank D in this range and for six positions of a 4 part length

rod group at each of these four bank D positions. Table 1 shows the 24 combinations

of full length and part length rod configurations which were considered. Also shown

are the axial offset and peak kw/ft at each of the 24 combinations. These data were

used to construct Figures 5 and 6.

It can be seen from Figure 5 that the maximum fuel power density for the

worst combination of full length and part length rods using present Technical Speci

fications limits is less than 13 kw/ft. The figure also shows a trend of increasing

kw/ft with insertion of bank D. When part length rods are inserted, holding D bank

at a constant insertion, the peak kw/ft increases to a maximum value around 125 to

150 steps and then rapidly decreases. More importantly, however, Figure 5 shows that

there is no combination of full length and part length rods which would result in the

peak kw/ft exceeding 15.8 at 100% power. Moreover, there is a factor of at least 1.2

between the calculated maximum kw/ft and the allowed maximum of 15.8 kw/ft.

Mr. D. J. Skovholt - 3 - November 10, 1972

The axial power shape for cases A3, B3 and C3 are shown in Figures 6, 7 and 8. These figures show the core average FN versus axial node, and in addition, F is plotted for that assembly which has the peak F. Locations of the full and part length rods are indicated. The assembly local peaking factor and the nodal correction factor are not included in the plotted values of FN, since the purpose of these figures is to merely show the shape of the axial power.

An additional safety margin, which limits the maximum fuel power density, is provided by the axial offset scram signal contained in the over-power and overtemperatureaT protection circuits. Figure 9 shows the calculated axial offset as

a function of D bank and part length bank position. The axial offset tends to vary with D bank and part length bank insertion in the same manner as the maximum kw/ft. That is, the axial offset increases with D bank insertion and peaks with the part length rods in the range of 125 to 150 steps. It is noted that the -26% offset

limit greatly reduces the permissible range of control configurations where the

largest values of kw/ft occur. This fact can be seen in Figure 7 where the -26%

offset limit has been superimposed on the graph of kw/ft versus control configura

tion. When this limit is considered, the worst control configuration permitted by

our present Technical Specifications will result in only a 12.0 kw/ft peak. In this

case, there is a factor of 1.3 between the calculated maximum kw/ft and the allowed

maximum of 15.8 kw/ft.

RESULTS AT 70% POWER

The previous results were based on rod insertion limits at 100% power.

A similar calculation was also made at 70% power. In this instance, the maximum

insertion allowed under current Technical Specifications limits was D bank at 59

steps and C bank at 189 steps. Since the 100% power results indicated the worst

peaking at maximum full length rod insertion, only the corresponding 70% power maxi

mum full length rod insertion cases were considered. The results of these calcula

tions are shown in Table 2.

As expected, the trend in axial offset and kw/ft versus part length

insertion is similar to the 100% power case. The peak FQ is indeed higher with

the greater full length rod insertion; however, the kw/ft falls off more rapidly

than F increases. The net result is that the 70% power condition is less limitQ ing than the 100% power condition.

RESULTS AT LESS THAN 70% POWER

Since the H. B. Robinson core is very close to the end of cycle, the ex

cess reactivity is very close to being depleted. Calculations show that at 70%

power with control rods inserted at the maximum allowable limits, the core could

remain critical only with all soluble boron removed. Therefore, calculations of

FQ were not made at power levels less than 70% since criticality could not be main

tained even with all soluble boron removed.

EFFECT OF OSCILLATORY XENON

The previous calculations have assumed that the axial xenon distribution

was in equilibrium with the axial power distribution just prior to control rod in

sertion. This condition is not always true, since it is a normal occurrence to

have small oscillations in the axial power which produces oscillations in the axial

Mr. D. J. Skovholt - 4 - November 10, 1972

xenon concentration. The effect of non-equilibrium xenon on control rod insertion limits was considered by examining two cases where the xenon was redistributed either toward the top or bottom of the core prior to control rod insertion.

The magnitude of the xenon redistribution approximated the effect of +20% and a -20% axial power offset. Since the magnitude of the oscillations in xenon concentrations is inversely proportional to the magnitude of the axial power oscillations, the offset in xenon was -20% and +20% respectively for the two cases mentioned above. These conditions approximate a worst case condition because it is extremely unlikely that the reactor operators would ever let the axial offset get larger than +20% before taking corrective action. A more typical value of axial offset during normal operation is +5%.

Case A3, which had the largest N was re-examined with the non-equilibrium xenon. The results are shown in Table 3 . With the xenon peaked at the top, F in

creases from 2.26 in the equilibrium case to 2.55. The axial offset becomes even more negative, so that a scram would occur before the peaking factor reached 2.55.

For xenon peaked at the bottom of the core, F is reduced to 1.54. Therefore, it is concluded that the effects of oscillatory xenon will not invalidate the current

control rod insertion limits.

CONCLUSIONS:

Results of these analyses demonstrate that during the remaining of Cycle 1

there are no permissible normal configurations of control and part length rods which will result in the nuclear hot channel factor exceeding 2.75 or 15.8 kw/ft at a rated

power of 2,200 Mwt. Additionally, it is further shown that control and part length

configurations are considerably more restricted by permissible axial offsets than by

the minimum rod insertion limits.

As a result of our frequent (at least monthly during operations) in-core

power measurement program, and our demonstrated ability to accurately predict these

measurements analytically, the results of these analyses assure that Carolina Power &

Light can operate the H. B. Robinson Unit No. 2 limiting the peak kw/ft to 15.8

within the present structure of the Technical Specification for the remainder of

Cycle 1 operations.

Very truly yours,

E. .tley

Vice President Bulk Power Supply

WMS:jb

CC: Mr. C. D. Barham Mr. N. B. Bessac Mr. B. J. Furr Mr. S. P. Grant

TABLE 1

RESULTS AT 100% POWER

Power D Bank C Bank PL A.0. FN Case _ (Steps) (Steps) (Steps) (%) Kw/ft

Al 100 107 217 225 -35.1 2.05 11.5 A2 100 107 217 167 -55.5 2.21 12.4 A3 100 107 217 110 -44.2 2.26 12.7 A4 100 107 217 81 -37.6 2.22 12.4 A5 100 107 217 52 -24.2 2.06 11.5 A6 100 107 217 0 -19.0 1.76 9.9

B1 100 150 228 225 -25.5 1.84 10.3 B2 100 150 228 167 -34.8 2.00 11.2 B3 100 150 228 110 -35.8 2.06 11.5 B4 100 150 228 81 -28.5 2.01 B5 100 150 228 52 -18.4 1.84 10.3 B6 100 150 228 0 -18.5 1.58 8.9

Cl 100 190 228 225 -13.1 1.65 9.2 C2 100 190 228 167 -24.9 1.83 10.3 C3 100 190 228 110 -23.8 1.87 10.5 C4 100 190 228 81 -19.2 1.78 10.0 C5 100 190 228 52 -05.2 1.61 9.0 C6 100 190 228 0 +01.3 1.49 8.3

Dl 100 228 228 225 -05.6 1.46 8.2 D2 100 228 228 167 -14.9 1.68 9.4 D3 100 228 228 110 -09.3 1.65 9.2 D4 100 228 228 81 -00.8 1.57 8.8 D5 100 228 228 52 +01.8 1.66 9.3 D6 100 228 228 0 +11.5 1.71 9.6

TABLE 2

RESULTS AT 70% POWER

Power D Bank C Bank PL A.O. FN Case _(% (Steps) (Steps) (Steps) (%) Q Kw/ft

El 70 59 189 225 -41.5 2.34 9.2 E2 70 59 189 167 -48.4 2.46 9.6 E3 70 59 189 110 -51.8 2.57 10.1 E4 70 59 189 81 -46.5 2.54 9.9 E5 70 59 189 52 -40.0 2.38 9.3 E6 70 59 189 0 -21.7 2.00 7.8

TABLE 3

EFFECT OF OSCILLATORY XENON

D Bank C Bank PL A.O. FN

Xenon Distribution (Steps) (Steps) (Steps) (%) Kw/ft

Equilibrium 107 217 110 -44.2 2.26 12.7

Peaked at top 107 217 110 -63.5 2.55 14.3

Peaked at bottom 107 217 110 -07.8 1.54 8.6

FIGURE 1

RELATIVE ASSEMBLY POWER

1.087

1.084

1.188 1.120

1.173 1.089

1.117 1.194-- 1.117

1.092 1.181 1.096 .

1.178 1.106 1.170 1.077

1.177 1.092 1.175 1.070

1.053 1.146 1.075 1.073 0.894

1.071 1.154 1.059 1.093 0.867

1.104 1.046 1.061 0.979' 0.698

1.095 1.016 1.067 1.034 0.700

0.943 0.997 0.907 0.642 MEASURED

0.902 1.016 0.913 0.692-e-- CALCULATED

0.743 0.594

0.28 0.618

NOJLoq

________~~~~.. ......~ .. 2 2 .V1 ~ .

. .t .--- +- - . .. .. 7 1

_4 !Z- 21_1 L

--- --- --...

IL- 1. ____ -__- ____ -- -77,

:4-: t

00

0r- 0*

77T _ __ _ __ d0 IL

'-74

FIGURE 3

CONTROL & PART LENGTH ROD BANK LOCATIONS

C P & L - H. B. ROBINSON UNIT NO. 2

N-41 2

3

-65

C, P/u L_

43

N-440 N-42 14

15

00

FIGURE 4

CONTROL GROUP INSERTION LIMITS FOR 3 LOOP OR 2 LOOP OPERATION

100

Ij 4 llji.;fP .4ll -' il! i 200 i1 171

80~f I .' .i.. i, 4 { .200-. . ~ "~ 1 jJ. .. 44jj f . -4 ., it

.....llIl lIi 'i H~i i V ~{J l.'

60 _F1 'ft1;: 1iJ 4Uj' '' jf i t

44 0 .

4, I E '-4 ii 4- :~ I i

1 - .14 -Mi 1

4 ,4 4 1441 14j

POWE 5L0PRCN

FIGURE 5

CONTROL BANK D POSITION VS.

P/L BANK POSITION 100%. POWER

OUT> .. T.

PEAK 1KW/FT 4

!fli ELI.

14 4 . .Tl H !I m 11K

~~~~~i -:3: :rI: ii 4i:L ' i ' II ; T C S E

i 1H 'I 4

A1 4.1 iT 7,f: IN f 0 414 "N4ii

P/l BIN POIIN SES

FIGURE 6

CASE A 3

20 .~::i 7 ! IT''44j.~'t

t!Y1 14 1 .

I:1t .u...4,: 1 ..

I I I ::- 1A J 4 :

I~~~ 44~cI ~ 14- I I

q I i I I , i v

I~T 1 . I .

1 . T~ 1- 11- 4M~KI I

ill N ;lli ILE 7"j, HI IITTL I I4ii m

1 I or Aerg

1 8 , 1 611 :_,. - .,.: 1 4 1 1 0 8 6- T AXALND

FIGURE 7

CASE B 3

D Bank T__ __ _ _ 1.

... .... .. QI I

1.6 'IPart Length lH 7j,. 'i : -- i-I ..

4; ~ ~ ~ ~ ~ ~ ~ : T7,jp.1 .... a4 H itH 4 Qj~ iI -4. . L~~ 2;. iir.aa. j ~ J-a L.. a '

:r. al7, 11-X.

411 ii,.

L "T a 4 1 ! 4 ii~

-Ia a7 il ,, . . . .. .

1.0 ~ , a -fl I

.. .. , . a .. ~ .a . a ... . .aal.

7-ii

.6 71 Ja I FQssml 1. t1- ~ t 7. T T

-L i ,ji,~ Ia 4'.. T -q. Moi Ave ag ..t.

I I a I a I. . 8 16 14 12108

JJ- ~Aia Node,.77r

FIGURE 8

CASE C 3

1.8.

Bank JWTK.i!~ 4 f'4'.

16 :77 J, i4 j Pr e hi

.:I*'. H4'411. S. , 4 ; f 4f , : 1 '' .

1.6 Part 4+itTh

1.2 i .1 I.T:

4, '-4 1 7. J1. t44i

1.41. 8 j1

~~~~~Z I I ,~ek Q Asm l

it 11 1i Ll j.. j182 161J1 08 i

4AIA NOD E

FIGURE 9

CONTROL BANK D POSITION

P/L BANK POSITION FOR CONSTANT

AXIAL OFFSET VALUES

OUT ,......i;j ~~;'~~

u ~~AXIAL OFFSET i.}l...ij

L:. . .I;

t ! .. I]: L ) .j

I.l I~~j I I j4;

4

z itiI 71

II L Kt 7j.. v fjj/. 1 10/0 * ( 67i

FITT ~injIMUM TEC. SPEC. II I l

fl. 11~11 FI ORII m-, 4 100/ POWER ll .J

IN4 50A 100 I5-00 P/L ANK OSITON (TEPS

FIGURE 10

CONTROL BANK D POSITION VS.

P/L BANK POSITION

100%~ POWER

OUT :477_

if ii ITi- 'i4 II

IT 44~42.., ,A

PEAK KW/FT~ 42 '' .I. iLiIiK IrI' -7]

200 A A O',,..i,' ,I'

t-1 = T, Ii f. II ll

4i~ tK-Tt- 7 "'AI~ ''.i:~V-*

if T442:4l4~11.2:yAA4112~ 4 r ~A' 1~' 1 ~ ~ ~ i14 7 '

4 I7 yx]- -I* .- ,:4 t, A''' 1 444,1 '''~' '

(4A I~ ]J[ tf' i11, 111IZ ;---4UiJ i- :1PO E

II 01 . '' t4LK+'. 4., .

IN0 50! 10 520T

P/L~~~~ ~ J BAKPSTOlSES