ML20065G705

Reactivity Worth of Flooded Isotope Production Element in Core of Univ of Missouri-Rolla Reactor

ML20065G705
Person / Time
Site:	University of Missouri-Rolla
Issue date:	12/31/1988
From:	Straka M MISSOURI, UNIV. OF, ROLLA, MO
To:
References
NUDOCS 9010230024
Download: ML20065G705 (11)

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UMRR/88-1 REACTIVITY WORTH OF THE FLOODED ISOTOPE PRODUCTION ELEMENT IN A CORE OF THE UMR REACTOR Of Missoy 4 1,

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1. EACTOW M. Straka )m Rolla, Fall 1988.

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REACTIVITY IJORTH OF THE FLOODED ISOTOPE PRODUCTION ELEMENT IN A CORE OF THE UMR REACTOR  !

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M. Straka Rolla. Fall 1988-i

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. 16BLE OF CONTENTS ,

Introduction.....................................,,,,,,,,,, 3  ;

i Measurement Method..................................,,,,,,,, 3 Suberitical Exper,iments..................................., 3 i

Conclusions................................................. 6

+ i References................................................ 6 1

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o . j JNTRODUCTION At the UMR Reactor (UMRR) preparations are underwav for a iuel  !

i conversion. The high enriched urantum ( HE U '1 fue4 will be replacea i with a low enriched uranium (LEU) fuel on the similar reometry. One important part of inis work is a theoretical analvsis of Tne new reactor core. Detailed investinations into the reactor phvtics I i

behavior play an important part of this analvsis ilJ.

A set of computer programs has been provided by the Arronne '

National Laboratorv for the theoretical analvess. More cetails on this program package and the analvtical invest irat ions are riven in (21.

It is important to verifv the methods and procedures used 2n-this analysis since no comparable experience is available at this time.

Therefore the techniques suggested for tne conversion analvsis were first applied to the present HEU core. In doing so, the calculational results for a number of important core parameters-couln have been compared with the experimental data, i

In this report, it will be reported on reactivity measurements i

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performed for the isotope production (IP) element inserted in two

. j different positions of the HEU core 67 W. The method will be brieflv  !

discussed, results analyzed, and conclusions dra.un. A comparison with the theoretical results will be made in this report. ~ I MEASUREMENT METHOD The method used to measure a reactivity worth of the IP element

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is bas.ed on the neutron multiplication in a suberit ical react or . The suberitical multiplication factor M is defined as n

r+1 (1)

M : n_E = _1 - k o 1-k I

a

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where n, a nunb0r'of neutrons produced by the cource n,

= number of neutrons produced in the reactor after r 1 l

generations k =

effective multiplication factor (< 1.0)

For a sufficiently long time, eq. (1) simplifies to .

C, 3 N= =

1-k (2) where n, and n, were replaced with C. and C, respectively, which are counts measured with a neutron counting equipment during a certain time interval. They are, of course, proportional t o n, and n.

respectively.

Using eq. (2) for two subsequent channes in the effective

. multiplication factor k: and kg the count r at e C- can be eliminated.

C 2 1 - k)

{ * .1 - k2 (3)

Therefore, when K is known and the count rates C and C are measured the new effective multiplication factor kg can be' calculated. Obviously, the-accuracy with which C

, _ and C were measured and k: determined is very important in this method. The error (or the uncertainty) in the value of k 2 propagates according-to the formula C

ak d 2

• U2 - ak)

(4) where ak t = uncertainty in k Ak t = error in the calculated value of k t (assumine. C and Cs error-free)

The reactivity change between two suberitical states k and ht can be expressed using the following formula 4

2

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1 .1 f=r r2

. 1 es\

an exnmpie is shown t ra Farure_1. T: e react ivi t v wor t n u of th*

UMkh vr 1 c cie v i e r to s.h ou ri at <c a l c u l a a ri u s s n e .- q . . 4?) ane (5). The core muitsplication facitir h, was selecten as the cepence*nt verlabs*

renetne bet ween 0.9497 and 0.99%. The ret io of C,/Ct meature 9 in this suberitical experiment was 1.6. It can be seen, from this example. that an error of about 0.2% in h, leads to a rather laree telativa error in y . Dependine. on the value of h: the error varies between 20 and 80%. This is: rather curpricing, but it can be shown that the absolute reactivity error o' is approximat e1v equal - t o C

a 73-1 ak) i 2 (6 )-

N Therefore, depending on the ratio C /C t . an error of 0.2% in k us11 tive rice t o an aboolut t error of a wide varying magnitude in ibw measureo reactivitv. l It seems that a conclusion can be drawn that the tuberatical method it not very suitable for measurement of small reactivities.

On the other hand, for lar rer react ivit iet . sav 1% and more, thit method can F ive F ood r er.u l t s .  !

i SUBCRITICAL EXPERIMENTS-The method of suberitical multiplication hat been used to i

mearure a reactivity of the Isotope Production.(IP) element in-the i

position-D-L and C-7 of'the HEU core conf i gurat ion 67W.

In'both potitions the IP element war iilled eit her with . air s or unt er and re opec t ive react ivi t iem measured. Tab l e. I shown

• h. procecure -einci the meacured data.

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TCblo 1. IP Elca nt in D-5

, Step ! Reactor  ! Avg. Count Rat e '

k i t

t f e r,ni j '

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! RR 9 24"  !  !  !

J  ! all SS 9 19" l 277.9  : 0.857997 ! -16.S5

! no IP .

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! RR 9 24"  !  : 1 2  ! all SS 9 19"

405.6  ! 0.902705 ! -10.78 IP u/ air  !  !  !

! RR 9 24"  !  !  !

3 all SS 9 39"  ! 402.7 1 0.902005 ! -10.86

! IP w/ water!  !  !

From Table 1. one can see that the reactivity worth of the air-filled element in the pocition D-5 is about 5.8%.

Floooinn with water will decrease its worth bv at.out 0.1%.

Incidental 1v. this value is of the same order of magnitude as the meacurement error. For the reactivity one-sirma deviation was calculated to be 0.07%.

Measur ernen t data and results. for the position C-7 are shown in Table II.

Table II. IP Element in C-7 Step ! Reactor l Avg. Count Rate ! k  !

! (comi  ! l-1  ! (%)

! RR 9 24"  :  ! i 1  ! all SS 9 18"  ! 6246.  ! 0.993267

? no IP  ! -0.678  ;

1 t  !

l  !  !  !

! RR 9 24"  !  !  !

2  ! all SS 9 18" 12399.1 t IP w/ air 1

! 0.996608  ! -0.340 i  !  ?

!  ! j

! RR 9 24"  !  !  !

3  ! all SS 9 18" l 25708.7  ! 0.998364 1 -0.164 1 1P w/ water! ' '

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e The rocet ivs tv worth of tho IP olom:nt in the p:ripherv nosition C-7 sc about 0.34%.

Floodine of thit. element wail increase itt worin bv about 0.18%

AccordinF to the analvsis advanced in the previous Section the relative measurement error can be larte cince cmall reactivity valuet are mescured. It was, therefore, of interect to compare results of thit work with the retulto obtained bv the criticality methods and by the computer code 2DB-VM [21. Table III chows thic comparison. The Table Ill. Keactivity Measured in C-7 bv Suberitteel and Critical Experiment. and Calculated f%) i Method IP Element  :

Suberitica11tv ? Criticalitv ! ' Calculation w/ air 1 0.338 ^ >At '

O.421 WLWhlit '

O.514 '

O.u?t

• O.u79

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('oid oniv '

-0.176  ?

-0.160  !

-0.006 results of all three n.ethods are consistent in that a vot e. r eac t ivi t v at the UMRR peripherv it alwave nerative.

The error of determining the reactivity worth of the IP element is quite larse, varyins between 20 and 60% depending upon what value in accepted as the true one. The worth of void is, however.

i remarhably close, at least for both experimental methode. One thould note, that while a large error can be attached to the reactivity of both the air-filled and water-filled IP element due to an incorrect value of k: used in the suberatical calculation, this becomes rather inconsequential when both reactivities are compared between i themselvet. Therefore, the reactivity worth of the " void oniv" mentured by this method i r e>:pec t ed t o aeree ciii t e well with the one measured by the criticality methnd. This is. of course, confirmec in i

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the cbovo Tcblo.

nnNc'st$1rg$

The method of subcritical multiplicat2or. tan be used to measure the reactivity of an unknown ob,iect olaced at or into the* reactor provided that its magnitude is larger than 1%. For ob.iects whose reactivity worth is less than 1% this method can be used t oo, but the accuraev will suffer, if the value of k: is not known well. The method seems to be verv well suitable (even f or unall react ivit ies )

if oniv a comparison between two different samnies (or states of a sample) is southt. Then the value of h 3 does not seem to be relevant because only the difference in reactivity of the two samples is the variable of interest.

It was shown in this work that flooding of the isotope production element positioned in the HEU core center will have only netligible reactivity effect. At the core peripherv. flooding will rive rise to a small positive reectivity change of about 0.2%. A similar behavior is expected for the proposed LEU core.

i RFFERENCES l [11 Straha M. nnd Covington L., "Studv of Neutron Physics:

Conversion of the Universitv of Missouri-Rolla React or t o Low Enriched Fuel". Ir6A b_AE. . 4V_q) u S_qqa . Vol. 55. November 1987 f21 Covington L., "Neutronies Studv of the Conversion of the i

Universit v of Missour t -Rolla React or t o Low Enriched Uranium Fuel". MS Thesis . Universiiv of Missouri-Folla. 1989.

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Void Reactivity vs. Core Multiplication -

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O.9897 0.9917 0.9936 0.9956 0.9976 0.9996 core multiplication factor Figure 1.

a , _ __ . a _ -. e- ' = - , - +-r ce -

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• KEY TO PREFIXES 4  !  !  !  !

B !_  !  !  !  ! S  !  !  ! '

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F - Standard Elements

!. C  !  !

!HR-1!F-14!F-1 !C-4 !  !  !

C - Control Elements

! HF - Half Front Element D  ! !F-8 !C-1 !F-16tF-9 !F-4 !F-10!  ! HR - Half Rear Element i  ! l

'  ! S - Source Holder I E l-  !

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!F-6 !C-2 !F-19!C-3'

!F-12!F-11!  ;

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!BRT !F-17!F-15!F-7 !CRT !  !  ;

1 2 3 4 5 6 7

! 8 9 BRIDGE SIDE i

UMRR CORE STATUS .

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Other BRT - Bare Rabbit Tube 1

CRT - Cadaium Rabbit Tube Figure 2. UMRR Core 67 I

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