A COMPARISON OF CENTRAL REACTION-RATE RATIO - -

MEASUREMENT TECHNIQUES IN BIZET CORE?

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NEACRP - A -543

R BGlime, KfK, Karlsruhe B L H Burbidge, AEE Winfrith

1. INTRODUCTION

and (33: AEE

In 1976 and 1977 central reaction-rate ratios F8/F5, F5/F9 C8JF9 (F8, F5, F9: fissions per nucleus of U238, U235, Pu239; captures per nucleus of U238) were measured by teams from Winfrith, CEN Mel and KfK Karlsruhe in Zebra'during the BIZET

programme. While the central plate cells and their surroundings were essentially .identical for the measurements, .each team applied its own experimental technique.

This Note evaluates the results and the ass,ociated uncertain- ties on a common basis, derives a set of "best" values and demonstrates that the deviations of the individual results frvm these are consistent with the estimated uncertainties.

2. EXPERIMENTAL TECHNIQUES

The experimental techniques applied by the participants are listed in Table 1 (see also References 1 and 2), and the types of foil used in the activation experiments are given in Table 2: The measurements were made at the centre of the BIZET cores BZA and BZB (3).

3. EVALUATION OF RESULTS

3.1 Method

From the measured data each team deduced cell-averaged reac.tion- rate ratios, which of necessity contained a number of common features. Only the AEEl\' team was able to measure radial var.iations in F5, F8 and C8 and CEN and KfK had to rely on these values. All the participants deduced F9 in the cell from a value measured in- side a stainless steel plate and from a calculated correction factor. The KfK team did the same for F5. Since the measurements were made in more than one cell, it was necessary to correct the results to a common central cell. These corrections were obtained from measurements by the AEEW team. Finally it was necessary to re-normalise the results to ensure that consistent source da.ta were used by each team in'the calibration procedures.

Details of these corrections and~the assoc.i.ated uncertainties are given below.

3.2 ,FS and C8 l?ine Structure Variations ----.. ---.-------

The distributions of F8 and C8 show a pronounced in the unit cell (Figure 1). While the experimental

fine structure technique

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aI!@lied by the ANEW team yi.elds values averaged over the plate thickness only, the CEN and KfK da.ta show good agreement for the relative distributions measured along the central plate axis. The plate averages measured by the AEW team are consistent with these data and .in the subsequent analysis each team has used its own ~values.

To allow for the radial distribution of C8 within the II0 plates, the values measured by each team near the centre of t FT e plates had to be increased by (1:8 L 0.4)s. This important correction was derived from experiments by the AEXW team. So far, it has not been verified by calculation (KAPER currently indicates a 0.4% correction).

On the basis of measurements in previous Zebra cores no correcti~on~ was applied to allow for radial variations in FS, but the uncertainty on the values measured near the centre of the plates was increased by 5 0.5%:

3.3 F9 and Y5 Fine Structure Vari~ations

The correcti.on factors to relate F9 in the steel plate to that in the plutonium plate and, j.n the case of the K:CK measure- ments, to relate F5 in the steel pla.te to that in the 1102 plates were obtained from MURAL (4) calculations. These predickd a 0.4% reduction in F9 and a 0.13% increase in F5.

Correspondi.ng KAPER (5) results for the WJRAL cell model are very similar, i.e. a 0.37% reduction for F9 and a 0.13% increase for F5. The uncertainties in these corrections have been taken as + O.l?h. However , it shoul~d be noted that the calculated corTection for F9 is sensitive to the cell model. I?or a less precise KAPER model, with the steel of the cans smeared with the conten-Is, calculation predicts a reducti.on of O.S4% for lP9 . F5 is less sensitive, the predicted increase being 0.16%.

It was also necessary to apply small corrections to allow for resonance shielding in the foils in the dummy chambers and for the i.nfluence of one type of foil on another. Those corr- ections were generally of the order of 0.1%.

Again on the basis of measurements in previ.ous Zebra cores, no corrections were appli.ed to allow for radi.al varizti~ons in IF5 and F9, but the uncertainties on the values measured near the centre of the plates were increased by + 0.3%. -

3.4 Macroscopic TTariations .-~-

Measurements were made by the AI!XW team to rel,ate the results to a common cell. at the ce~A.i:e of. the core. These correcti~ons reached a maxi~~mum of (O.,s 2 0.2)% for Fs an~d CS at the extremes o.f the measurement regi.on, but were general1.y less than (0.3 2 0.1)s.

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3.5 Fission Chamber Deposit Calibrations

All tlie teams used a different half-life of Pu239 in their preliminary evaluations. An analysis.of the calibration procedure followed by AEEW and KfK indicated tliat F9 (AEEW) had to be in- creased by 1.0% and F9 (KfK) had to be decreased by 0.2% to be con- sistent with the currently recommended valued of 24llOy used by CEN.

Corrections for fission-produce absorption in the fission chamber deposits were also taken from a common source. For the KfK chambers, which were typical, these were 0.11% for F9, 0.17% for F5 and 1.3% for FS.

A direct comparison of the KfK and AEEW chambers was made, firstly in a thermal column and then, secondly in Zebra (to comment on the U238 content of the natural uranium). In these measurements the chambers were positioned close together and subsequent foil measurements in an identical configuration related fission rates in one chamber body environment to that in the other. The results, see Table 3, show that the measured fiss- ion rate ratios are consistent with the estimated uncertainties in the deposit masses.

3.6 Errors .

The errors associated with the U238/U235 and U235jPu239 fiss- ion ratio measurements are given in Tab1.e 4 and those with the U238 captureJPu239 fission ratio measurements in Table 5. Errors that are systematic to the measurements by the three .teams are suffixed 's' whilst those that are random are suffixed 'r'. This' differs from normal practice, where the systematic and random components quoted are those for a particular technique. This present distinction is necessary at the subsequent stage when the individual ratios are compounded into weighted means, only random errors being used in the weighting process. All the errors quoted here are at the one standard deviation level.

l 4. REACTION-RATE RATIO

The individual reaction-rate ratios, together with the uncertain- ties obtained from Tables 4 and 5 are given in Tablet 6. The weighted mean values, intended to be used as "best" values for BZA/BZB have been formed by weighting the individual results with their respect- ive random errors. It can be seen from Table 6 that individual ratios are in good agreement, the maximum differences being 1.2 standard deviations from the mean in the cases of the KfK F8JF5 ratio and the CEN (X/F9 ratio. Table 7 compares the present results with those obtained in previous experiments where again there was agreement within the estimated errors.

The weighted mean ratios are compared with the calculated values from MURAL and KAPER in Table 8. The C/E (XURAL) values are broadly similar to those of previous-measurements in Zebra.

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5. C0NCLUSI0NS

Reaction-rate ratio measurements have been carried out in the central plate cells of BZA/BZG by AXE:!, CEN and KfJ< teams. The individually measured values are in good agreement with the mean results. .

The major errors in the fission ratio measuremen.ts arise from foil counting uncertainties in the case of AEIw measurements and from fission chamber mass calibrations in the CIZK and KfK measurements. The major errors of (X/F9 measurements arise from the mass calibration of the uranium deposits, including the a-activity of natural uranium in the AI?P! measurements , from thermal flux and cross-section data in the Cl?N measurements and from foils counting in the KfK measurements. The ma:;s uncertain- ties of the KfK plutonium foils also introduce signifi~cant uncertainties. It should be noted that the siqificaM radial C8 fi~ne structure corrections, ,measured by the AXE?! team, has been applied to each set of results. IndeJ>endent verifi~cation of this effect would be of value.

1.

2.

3.

4.

5.

6.

6. RE??ERENCES

SANDJCFS, J. E. et. al., A Review of 7ebra Techni~ques for the fiteasurement of Reactivity ?arametors, ??eacti.on-%a.te Ratios and Spectra. International Symposi.um on Physics of Fast Reactors, Tokyo, 1973.

PIWER, J:i. et. al., Interlaboratory Comparison of Absolute Iission Rate and Uranium 23S Capture Moasurcments i.n the MO I. 1: X . NJ35 Speci.al ?ublication 425, pp. 25%Xl., 1975.

HlX~~~, F. and SANJ>ERS, J. E., An Outline of the UIZJX'f Experi~ments and their Interpreta.ti~on. Fast Reactor ?hysics Symposium, Aix-ell-Provence, 1979.

MACDOUCALL , J . , MURAL13 - A Programme for Calcul~ati ng Neutron Fluxes in Many Groups. AIGW - R 962, :19'77.

I'ISCHJZR, E. A. and URANDL, V,, Status a.nd Validati~on of a the IPast Reactor Latti~ce Code KAPER-,2 for Sl.ab and l?i.n Cel~l~s. IAEA/WA Meeting on Homo~enisa.tion Methods i.n ?kactor Physi.cs, J>ugano, 1978.

I)ARROUZJXT, X. et. al,, Studies of Unit 1~ Lattices i.n Metallic Ura~nium Assembli~es Zebra 311, Sne% 8, Ermine and JJa.rmoni~e UK. International S~~lli~OSiLllll on ?hysics of Fast Reztors, Tokyo, 1973.

August 19S2

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l TABLE I. SJM?JARY OF THE EXPERIIENTAL TECHNIQUES

Parameter Common Features AEE1.J CSN KfK

Fission Rates (F8, F5, F9)

Foil activation; y > 1.28 MeV 0.54 MeVq<O.86 IGV y > 0.70 MeV fission rates from gross fission product y counting with NaI (Tl) detector

Normalisation to via foils in via foils in dummy via foils between reference position dummy fission fission chamber 2 back-to-back

chamber fissiowchambers

Fission rate at measurement with measurement with Measurement prior reference position calibrated fiss?on calibrated fission and during irrad-

chambers prior to chambers prior to iation with cali- irradiation. irradiation., F8: brated fission FS/F5, F5/F9 F5, F9 measured chambers. F8, F5, measured as ratios. absolutely F9 measured

~~~~~~~~~~~~ 27:e27;Lline mmt- il~~il~~~l;~iiF between 99 and lli ing with Ge(Li)

Absolute calibzatio 7

via Am243 source, irradiation in absolutely cali- relative activity thermal neutron field brated Am243 source

: determination of calibrated by Au activity Am and U deposits standards by cx-counting

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XABLE 2 DESCRIPTION OF THE ACTIVATIOX DETECTORS

Type

F9: mterial

Pu23?-cmt@nt,~~

shape

location

?$:material 53 lJ235-content, z

ShElp

location

AEEW

20X PzO2lXi comet, Xi coated

94

disc

l!-netal, Ni coated

rectangular

foils held vertically in nat- ural UO2 piates to smple rez.ction-rate through plate thickness -. Lhwtal, Si coated

0.04.

, F8 as F5; absolute C8 from naturz.1 'JO2 deposits at the horizontal central plane of na?dral IJo?- ps.xtes; related t0 $iate a"erage 3y separate experinent

CEX

Pu-netzl, Al coated

99.1

disc

rear 'norimntal centre plane of W stainless- steel plate

UO2

7

disc.

foiis held horizontally dong central axis of natur2.1 iJo plate

U02

0.04

as F5: C8 plate average as AEZU

1 - -

1

Pu/Al alloy, Al coated

90

disc

near horizmtal centre- plaae of UK stainless- steel plate

uo2 > Ni coated

93

disc

at horizontal central plane of KfK stainless- steel plate

1 uo2 > Ni coated

0.72

discs

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TABLE S COMPARISON OF ZEBRA AND ShJAK FISSION CHAMBERS

&bra Chamber Location .Fission ratios per unit.mass; cabal chamber

- LIZ35 Chamber Pu239 chamber U Nat Chamber

Thermal Neutron Spectrum 1.011 1.017 1.008

Fast lieutron Spectrum 1.009 1.009

Maall 1.010~ o.CYz3 1.017 + 0.006 1.009 + 0.003 -

Note: The errors shown above are those from the comparison measurements alone. They do not include the uncertainties in the mass calibra- tion of the deposits which correspond to + 1.5% for the U235 chambers, t 1.0% for the Pu239 chambers a;d + 2.0% for the natural uranium ch%bers.

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cc

F8/F5 Error (Yz) F5/F9 Error (Z) source of Error

IX4EA ' CEX KfK XAEA CEX KfK

Cil Fission chamber msses 0.h Z.Gr, 0.2s z.lr,:~o.2.s I.&-, O.!s 2.3r, 0.1s 1.5r, 0.1 (ix1udizg fission zndlor aipha assay) I

(ii) Fission chmber counting rates @.4r o-5r 0.8-c 0.3r 0.b 0.h (includix~g statistics, monitor, 3i2s, dsxl-tixe, reproLuci3ility secondsy isotope correction)

(<ii) Statistics of foil counting 0.2x 0.8~~ 0.3r O.iT 0.7r 0.3r (inc1a!i~g statistics, foil yy:!tioa, dead-time, repro<uc- IaxLity, seco=dzry isotope, genex*i cotinting correc:ions)

(iv) Foil ~~35~s tincertainties 0.17. included in (iii) 0.3r O.Z? included in (iii) lil?c

Cv> ikift durkg foil counting ! 1 I ..ir o.or o.or 1.2r o.or o.or

(vi) Fksonazce shielding in dmmy O.lr 1 0. lr 0. ir O.lr 0. lr 0. jr ch2nber faiis

(vii) l7esomnce shielding interference 0. lr 0.3r O.3r O.lr 0.6r 0.5r

in dumy chanbr2r foiIs

,(viii) ?'er~~arbatior, jxod>jced by the 0.2r 0.2r 0.h

use of foiis in plates 0.2r 1

(ix) Foil in steel plate relative O.ir 0.1s 0.13 0.1s. to flxl

(Contd./...)

Source of Error

(x) IGxroscopic flux corrections

.(xi) Xadial fine structure variation

(xii) Cell averaging procedure (includes random component of axial macroscopic fin correc- tion)

TOTAL

TABLE 4 (contcl.)

FS/FS Error (%)

included in (iii)

I .?r 2.8r 0.6s 0.6s

KfK

0.1s

0.6s

0.5r

2.3~ 0.6s

UKAEA

0.1s 0.4s 0. lr

1.6r 0.4s

F5/F9 Error (%)

CEN

0.1s

0.4s ncluded in (Zi)

2.5r 0.4s

KfK

0.1s

.0.4s:,

0.3r

TABI,E 5 ERRORS IN UZ38 CAPTUW/!?u239 FISSION RATE RfJTIOS -

PJ&.BZB

,z !’

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TAiLE 5 (Contd.)

Error (%) source of Error

IJWA CEN KfK

&viii) Thermal flux determinatimi

(xix) Thermal flux deptiession correction

(xx) Thermal cross-section (including Wescott 'g' etc)

(xxi) Cell average procedure

l.4r

0.2r

0.8r

included included included in (xvi) 'in (xvi) i.n (xvi)

1.3r 2.or 2.5r TOTAL

0.6s 0.6s 0.6s

F8lF5 0.0233? 2 1.7%r, 0.6%~ 0.02350 2 2.8%r, 0.6%~ 0.02249 .t 2.3%x-, 0.6%~ A Sean

0.02315 2 1.2%r,, 0.6%~ + 1.0% 4. 1.5% - 2.8%

\ F5/F9 1

i I.042 2 1.6%~~~ 0.4%~ I.044 2 2.5%r, 0.4%~ 1.034 2 2.1 %r, 0.4%s I.040 2 1.2%r, 0.4%s b $i$? +yg * 0.2% + 0.4% - 0.6%

;S;/F~~< 1 O;!444+z1;l;%r, 0.6%~ 0.1388 I? 2.0%~, 0.6%~ 0.1425 i' 2.5%r, 0.6%~ 0.1427 2 i.O%r, 0.6%~

-.-----.L

- 2.3% - O:l%

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TABLE 7 INTERLABORATORY COMPARISON OF

REACTION-RATE RAT'10 VXASUREMENTS -

-

F8 E

- F5 5 - C8 FT

- C8 F5 -

UK --

1.038 1,014

1.008

1.006 1.004

,1.013

CiN

1.045

1.011

1.01.0 0.986

0.965

1.035

0.974

-- KfK '

1.0 1.0 1.0

1.0 1.0

1.0 1.0 1.0

1.0

l?ocumented

this compari&n Reference 6 Reference .2

this comparison Reference 2

this comparison Reference 6 Reference 2

this compariion

Note: The resulti are normalised to the KfK values.

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+5%

+lo%

0

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FIGURE 1 AXIAL DISTRIBUTION OF.C8 AN!? FS

IN INNER CORE~CELL CCl5-01

zviation of Normalised C8 from Cell Average

--%=-z-AXIS

--=-~-AXIS

Plate-9/4

Deviation of FS Normalised from Cell Average