AECRESEARCHANDDEVELOPMENTREPORT - FAS

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FIEPRODUCTIONCOPY AECRESEARCHANDDEVELOPMENTREPORT

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PLUTONIUM-METAL CRITICAL ASSEMBLIES

— .. .—. ....— -— . . . —.—-. .. .. ------- —---- . .——-— -.-— . . .-

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PIJ8LICLY RELEASABLE

LA-2044REACTORS - RESEARCH ANDTESTING (Distributed accordingto M-3679, 16th ed. )

This ocument consists of 36 pages

No. h 162 copies, Series A

LOS ALAMOS SCIENTIFIC LABORATORYOF THEUNIVERSITYOF CALIFORNIA LOSALAMOS NEW MEXICO

REPORT WRITTEN: May 1956

REPORT DISTRIBUTED:, OGT 171956

PLUTONIUM-METAL CRITICAL ASSEMBLIES

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ABOUT THIS REPORT

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32-4944454647

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626364656667

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75-7679-86

876S8900

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100-101102-103

104105108107108109

110-118117116119120121

122-125126127126129130131132133134

135-144

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ABSTRACT

The two plutonium-metal critical assemblies that have

been studied at Pajarito Site are Jezebel, bare plutonium;

and Popsy, a Plutonium core in a thick normal uranium re-

flector. These assemblies and their properties are described.

UNCLASSIFIED



WiwNCIASSIFIED .O:O , . .●**: ●:8000. ●:.● ✎ ✎ ✎ ✎ ✎ ✚✍



Part I .

Part II .

FIG. 1

FIG. 2

FIG. 3

FIG. 4

FIG. 5

FIG. 6

TABLE OF CONTENTS

Abstract. . . . . . . . . . . .

Introduction . . . . . . . . . .

Jezebel - The Bare Plutonium Critical

Assembly. . . . . . . . . . .

Popsy - Plutonium Core in a Thick Normal

Uranium Reflector . . . . . . . .

References . . . . . . . . . . .

ILLUSTRATIONS

Jezebel - the Bare Plutonium Assembly. . .

The Jezebel Plutonium Components in Disassem-

bled, or “Safe,” Condition. . . . . .

Design of the Active Portion of Jezebel . .

Rossi Alpha vs Number of Mass Adjustment

Buttons in Excess of the Delayed Critical

Configuration . . . . . . . . . .

Positive Pile Period as a Function of Excess

Reactivity in Inches of Linear Control Rod

Excess Reactivity in Inches of Linear Control

Rod vs Excess Reactivity in Cents . . .

.

.

.

.

.

.

.

.

.

.

.

Page—.

3-4

7

8

28

35

9

10

11

14

16

18

. . .



99 *... . . . .

● ☛ ● ● ☛● 0 ●o:●:0 ● ●:0 ●

Page

FIG.

FIG.

I.

II.

III.

IV.

v.

VI.

-- -————ILLUSTRATIONS (Continued)

7 Fission Cross Section of U235 and of PU239 as●

Functions of Energy of Neutron Producing

the Fission . . . . . . . . . . .

8 pOpSy - the Critical Assembly of Plutonium

Metal in Thick Normal Uranium Reflector. .

TABLES

Fission Cross Section Ratios vs Jezebel

Radius . . . . . . . . . . . . .

Comparison of Central Fission Ratios in Various

Assemblies. . . . . . . . . . . .

Jezebel Central Reactivity Contribution Data .

Comparison of Jezebel and Godiva Central Cross

Sections. . . ,,. ., . . . . . . . .

Results of Popsy Calibration. . . . . . .

Cross Section Ratios vs Popsy Radius . . . .

27

29

19

20

22

24

33

34



I

INTRODUCTION

Two plutonium-metal critical assemblies have been stud-

ied at Pajarito Site. Part I of this report describes

.Jezebel, the bare plutonium assembly, and gives its observed

characteristics along with a few comparisons with oralloy

assemblies. Part II covers Popsy, a plutonium core in a

thick normal uranium reflector. As Popsy was relatively

inflexible -- intended only for a preliminary survey -- its

experimental program was much less complete than that of

Jezebel. Comparison of the experimental data with detailed

calculations is a continuing major project, and results are

not sufficiently firm to include in this account.

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PART I .

JEZEBEL - THE BARE PLUTONIUM CRITICAL ASSEMBLY

Design

Design features of Jezebel, the bare plutonium critical

assembly (Figs. 1, 2, and 3), were dictated by the following

requirements. For safety of fabrication, the principal,

nearly spherical mass was constructed as four units, and

these were mounted to provide three-part subdivision for

operational safety. Because of the toxicity of plutonium,

and consequent requirement for completely protective nickel

coatings, tapped or unnecessarily deep holes in the plutonium

were avoided. Demands for the ultimate in reproducibility,

adequate flexibility, and minimum tamping fixed the remaining

features: 1) light but rigid framework and supports; 2) self-

alignment of the three subsections by means of guide wires

(under tension) that also support the floating central sec-

tion, and ball-and-socket support for the upper section;

3) uniform mass adjustment increments to supplement the plu-

tonium control rod; and 4) adjustable air cooling with a

recording thermocouple as the indicator.

The pneumatic system for assembly of Jezebel includes



..” mm*● O:oe:o ::● ● ● *9:*.*. ● *

~ —,, .. .. .

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Fig. 1 Jezebel - the bare plutonium

,.. .

assembly.

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—

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Fig. 2 The .Jezebel plutonium components in disassembled, or“safe, “ condition.



1

r-l-l

.’~e PIANO WIREGUIDES

-CONTROL ROD

-CENTER SECTIONCLAMP 8 GUIDE

LOWER SAFETYBLOC K

Fig. 3 Design of the active portion of Jezebel.



Mp@●●: ●n::: ●

● :● 0 : : :9

● o c.:● oe ● ●:* *

an adjustable dashpot to control the speed of final closure.

As usual, electrical circuits for remote control are supple-

mented by scram systems, automatic and manual, and safety

interlocks to fix the order of assembly of components.

Glen Newby for sound design and Group CMR-11, particu-

larly John Anderson, for careful plutonium fabrication de-

serve much of the credit for the eminently satisfactory per-

formance of Jezebel.

Critical. Mass

The average density of the delta-phase Pu alloy in

Jezebel is 15.82 gm/cm3, the Ga content averages 1.02 w/o,

and the Pu irradiation history is 580 to 600 MWD/T. The ac-

tual Pu alloy mass of Jezebel at critical (with major cavities

filled, control rod fully inserted, and at 29°C) is 16.745 kg.

From reactivity contributions of Pu and Fe (or Ni) at various

radii, and of a Ni-filled gap on one of the parting planes,

it is possible to correct the Jezebel critical mass in detail

for effects of incidental cavities, Ni coating, asphericity,

and tamping of supports. Corrections in kilograms

the surface are:

elimination of cavities (Ni- and air-filled)

reduction to sphere

elimination of 0.005” Ni tamper

elimination of tamping by steel clamps

correction from 29°C to 20°C

of Pu at

-0.398 kg

-0.027 kg

+0.051 kg

+0.084 kg

-0.008 kg



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Thus, for a solid, untamped sphere, the critical mass is

16.45 f 0.05 kg of Jezebel Pu alloy.

Calibration - Rossi Alpha and Positive Periods

of cents.

of Rossi

buttons

Jezebel has effectively two vernier controls of reac-

tivity, the control rod and the temperature adjustment by

remote regulation of the cooling air flow. Both reactivity

controls have been calibrated in terms of mass adjustment

buttons (equal reactivity increments), and in units

The cents scale has been established by measurement

alpha(l) as a function of number of mass adjustment

from delayed critical. As shown by Fig. 4, the value of Rossi

-1alpha at delayed critical is -0.66 + 0.01 x 106 sec . The

alpha data are linear with reactivity and indicate that

5.15 Y 0.10 buttons are equivalent

on relative effectiveness of Pu as

follows that there is an increment

Pu alloy between delayed and prompt critical (132.5 t 4 gm

for the ideal sphere of Jezebel material). From the value

19.42 cents per button, results of calibration may be

summarized :

one linear control rod inch (1 lcri)

= 2.41 buttons (lower) = 46.8 cents,

temperature coefficient of reactivity

= -0.014 lcri/°C = -0.655 cents/°C.

to 100 cents. With data

a function of radius, it

of 135.9 + 4 gm of surface

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I I I I

DELAYEDCRITICALc

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PROMPTCRITICAL

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BUTTONS FROM DELAYED CRITICAL

Fig. 4 Rossi alpha vs numberexcess of the delayed

of mass adjustment buttonscritical configuration.

6

in



~ifw●*9m. ● am 9.* 9.* . .

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The ratio of the effective delayed neutron fraction of

Jezebel, (yf)J, to that of Godiva, the bare oralloy assembly,

(yf)G, may be obtained by means of the following relation

from G. E. Hansen (unpublished):

(~f)J (&n/mc )~= 1.033

(’yf)G (Am/mc)G’

where Am is the surface mass increment between delayed crit-

ical and prompt critical, mc is the critical mass, and,

again, J and G refer to Jezebel and Godiva. Using data for

idealized spheres (2) :

(Tf)J 0.1325 52.04—= 1.033 ——= 0.34.(yf)G 16.45 1.27

This leads to (Tf)J = 0.0023, if (yf)G = 0.0068 as determined

by Hansen in Report LA-1525.(3) Furthermore, from

adc = -0.66 + 0.01 x 106 see-l for Jezebel at delayed crit-

ical, we have

Auadc -1—= -— = 2.9 x 108 sec ,

AK (yf)J

Positive pile periods are shown in Fig. 5 for various

linear control rod increments above delayed critical. From

each period, a value of excess reactivity in cents was com-

puted by means of the PU239 delayed neutron data of Keepin

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FROM ROSS I - a DATA

1 Icri =46.8 cents

I

Fig. 5

\

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I I I I 1 1 I 12 0.4 0.6 0.8 I .0

EXCESS REACTIVITY - LINEAR CONTROL ROD

Positive pile period as a functiontivity in-inches of linear

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90● 00: :16.; ;● * 9

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control

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of excessrod.



I and Wimett .(4)

Results (Fig. 6) lead to the value 1 lcri = 51

cents. The reason for disagreement between this value and

the presumably more precise number from alpha measurements is

presently unknown. It may be associated with differences be-

tween delayed neutrons from Pu240

and PU239, or differences

in effectiveness of various groups of delayed neutrons in the

Pu assembly.

Spectral Indices

Response ratios of fissionable elements, which charac-

terize neutron spectra, have been measured at various posi-

tions in Jezebel. The measurements were made using small

235quadruple fission chambers containing foils coated with U ,

U236,U

238 237, and Np , permitting direct comparisons of the

fission responses without the need for an additional monitor.

Other central values were obtained by J. A. Grundl using good

resolution spiral fission chambers.

Results are shown in Table I. The fission cross section

ratios are quite flat throughout the interior of the Jezebel

sphere, but are depressed near the surface due to the expected

hardening of the flux in this region.

The Jezebel central fission cross section ratios are com-

pared in Table II with the corresponding values for Topsy and

Godiva.

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I I I I I I i I I I I I I10 20 30 40 50 60

EXCESS REACTIVITY IN CENTS

6 Excess reactivity in inches of linear control rod vsexcess reactivity in cents (from positive periods andKeepin-Wimett delayed neutron data).

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b ●*O ● ●O* 000 ;e-

TABLE I .

FISSION CROSS SECTION RATIOS VS JEZEBEL RADIUS

Normalized Normalizedto 7.39 for to 4.56 for

Topsy Central Topsy Central

235Zf (u )

237Ff(Np )

[ 4236

Radiuszf (u )

(in. ) Zf (U238, Zf (U238, t7f(NP ) rel

o 5.13*

0.5 ‘ 5.12

1.0 5.10

1.5 5.08

2.0 4.94

*Calibrated spiral chambers

235Zf (u, )/d’f(U238) = 4.56 +

gave

4.04* 1.000

4.05 0.999

4.05 0.996

4.05 0.998

4.00 1.000

for these values:

4% and 6f(Np 237)/Ff (U238) = 4.76 A

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TABLE II.

COMPARISON OF CENTRAL FISSION RATIOS IN VARIOUS ASSEMBLIES

235T* (u ) ~f (NP237) iff(U233) =f (Pu239)

AssembliesCompared 238, ~f(u238) 235

zf (u Zf (u ) Ff(uw

God -IvaVs 0.894 0.989 0.998 1.017

ToPsy(Quadruple Chamber)

JezebelVs

ToPsy(Quadruple Chamber)

GodivaVs

ToPsy(Spiral Chamber)

JezebelVs

ToPsy(Spiral Chamber)

0.694

0.891

0.685

0.885

0.994

0.908

0.988 1.065

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Reactivity Contributions of Various Materials

Determinations have been made of reactivity changes in-

curred when a void in Jezebel is filled with samples of var-

ious elements. Results of central measurements with 1/2”

cylindrical samples are listed in Table III, both as reac-

tivity contributions in cents per gram-atom, and as apparent

absorption cross sections (aa) in barns, which include changes

in effectiveness of neutrons in addition to actual absorption.

The latter are obtained by normalizing to oa(Oy-93.7) = -1.86

barns for a 1/2” sample. For some of the isotonically simple

nonfissionable elements, capture cross sections in the Jezebel

spectrum (ac) have been estimated as 6% greater than Hughes’

235 (5)values for the U fission spectrum. The difference be-

tween capture and effective absorption cross sections then is

attributed to the effect of energy degradation of neutrons by

scattering; that is, if Z is not small, Uc - ua = @ oin,

where A-y is the change of effectiveness per neutron to which

the inelastic scattering cross section ~in is applicable.

Table IV compares for Jezebel and Godiva, the bare oralloy

assembly, (6) values of aaj %’ ~in> and ~ corresponding to

Beysterls values of ain for scattering out of the fission

238 (7)spectrum to below the U threshold.

The characteristically negative values of ~ ~in for

Jezebel as compared with predominantly positive values for

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Godiva are understandable in terms of fission cross section

curves (Fig. 7). The minimum in fission cross section of

~239below about 2 Mev is sufficiently broad to cover most

inelastically scattered neutrons of interest. On the other

hand, t~e effect of the much narrower region of reduced fis-

235sion cross section for U would generally be overshadowed

by the large rise in cross section below about 0.4 Mev.

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PART II .

POPSY - PLUTONIUM CORE IN A THICK

NORMAL URANIUM REFLECTOR

Descri~tion

The Fopsy assembly derived its name from the substitu-

tion of a spherical plutonium core for the oralloy core in

the Topsy critical assembly machine, and was set up only

temporarily for a cursory survey of its more significant

parameters. Topsy, described in detail in Report LA-1579,(9)

needed only minor reflector modifications to adapt it for the

plutonium sphere. Figure 8 presents a schematic drawing of

Popsy showing the layout of essential components when assem-

bled for critical operation.

The core consisted of two concentric, nesting, spherical

shells (each divided at an equatorial plane), 3.508” O.D. by

0.840” I.D., and 0.810” O.D. by 0.410” I.D. The material was

delta-phase Pu alloy having a Ga content of *1.O w/o, and an

average density of 15.79 gm/cm3. The Pu had an irradiation

history of about 200 MWD/T. All Pu surfaces were coated with

Ni averaging 6-1/2 roils in thickness.

As modified to accommodate the Pu ball, Topsy provided

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MASSADJUSTMENT

BUTTONS

REFLECTOR

SAFETY BLOCK

-.. ,

<>

.

Fig. 8 Popsy - the critical assembly of plutonium metal inthick normal uranium reflector.

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9-1/2” of normal uranium reflector around the core. The

innermost part of the reflector immediately adjacent to the

Pu consisted of a spherical shell of uranium 1/8” thick that

had sixteen 3/4” diameter holes through it. Into these holes

could be put matching (spherical radius) buttons of Pu (about

12-112 gm each) or uranium (about 15 gm each) for reactivity

adjustments greater than that available from the two uranium

reflector control rods.

Critical Mass

Without any corrections for the influence of Ni coatings,

the critical mass of the two split concentric shells (with

0.410” diameter cavity) in 9-1/2” of uranium (control rods

fully inserted) is 5.837 t 0.020 kg of Pu alloy. Again

making no Ni corrections, the critical mass of the larger

split spherical shell (with 0.840” diameter cavity) for the

same reflector and control rod configurations would be

5.914 t 0.020 kg of alloy, Finally, the critical mass (mc)

of the outer shell was corrected to give mc for a solid

sphere of alloy as follows:

mc for outer split shell with Ni 5.914 kg

substitution of Pu for Ni atparting planes -0.019 kg surface mass

filling 0.840t’ diameter cavitywith alloy -0.103 kg surface mass

mc for solid sphere 5.791 + 0.020” kg

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The substitution of 6-1/2 roils of uranium for Ni

side reduces mc by at most 1 gm of surface mass.

on the out-

These cor-

rections were based on 1) observed increments of reciprocal

multiplication when one or both inner hemispherical shells

were added, and 2) the results of Topsy material replacement

measurements (6) scaled to give reactivity contributions of

active material and Ni vs radius in Popsy. A crude estimate

of the change in reciprocal multiplication when the reflector

thickness was increased to about 12” on four sides indicated

a decrease in m of approximately 12 gm of surface mass, show-C

ing that the 9-1/2” reflector was not quite infinitely thick.

Rossi Alpha and Reactivity Calibrations

The value of alpha, the prompt neutron chain decay con-

stant, was measured by the Rossi method (1)at delayed criti-

cal, and at reduced reactivities of two and three mass ad-

justment buttons below delayed critical. Also, positive

periods were observed for reactivity increments produced both

by adding buttons and also by adding control rod in the linear

portion of effectiveness. These data, together with the PU239 ‘

delayed neutron data of Keepin and Wimett, (4)permit the cal-

culation of AK/button in cents, &/AK in sec -1 cents-1

, and

control rod effectiveness in cents per linear control rod

inch (lcri). By taking

at delayed critical was

100 x As/AK another value for alpha

obtained. Finally, from AK$/button

./ .,!.- .j;i.~



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and a correction for the effectiveness of button material

vs surface material, the increment of surface mass between

delayed and prompt critical was determined. These results

are summarized in Table V. The discrepancy between the two

values of alpha is thought to be due most likely to the fact

that the fission chamber had to be placed in the reflector

rather than in

whose absolute

the core, 238and hence was influenced by U ,

yield of delayed neutrons is some 6-1/2 times

that of PU239.

Spectral Data

A few spectral indices were obtained to permit a crude

comparison of Popsy with other assemblies. They were ob-

tained entirely from radiochemical analyses (by J. Sattizahn

and co-workers of Group J-n) of samples irradiated at various

positions in the assembly. The results

Table VI as cross section ratios, since

of isotope were obtained in each case.

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REFERENCES

(1) J. D. Orndoff and C. W. Johnstone, Time Scale Measure-

ments by the Rossi Method, Los Alamos Scientific

Laboratory Report LA-744, November 1949.

(2) R. E. Peterson, LADY GODIVA; An Unreflected U-235 Crit-

ical Assembly, Los Alamos Scientific Laboratory Report

LA-1614, September 1953.

(3) Clifford Maier and Gordon E. Hansen, Material Replace-

ment Experiments: Theory and Measurements for the Lady

Godiva Assembly, Los Alamos Scientific Laboratory Report

LA-1525, April 1953. .

(4) G. R. Keepin, Delayed Neutrons - A Review as of October

1955, Los Alamos Scientific Laboratory Report LA-1970,

October 1955.

(5) D. J. Hughes, R. C. Garth, and J. S. Levin, Fast Neutron

Cross Sections and Nuclear Level Density, Phys. Rev. 91,

1423 (1953).—

(6) L. B. Engle,’G. E. Hansen, and H. CO pa~ton, Material

Replacement Measurements in Topsy and Godiva Assemblies,

Los Alamos Scientific Laboratory Report LA-1708, July

1954.

(7) H. A. Bethe, J. R. Beyster, and R. E. carter, Inelastic

CrOSS Sections for Fission Spectrum Neutrons, Los Alamos

Scientific Laboratory Report LA-1429, January 1955.

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REFERENCES (Cent inued )

(8) M. D. Goldberg, J. A. Harvey, D. J. H~ghe~, and v. E,

Pilcher, Neutron Cross Sections, Brookhaven National

Laboratory Report BNL-250, August 1954.

(9) R. H. White, Topsy, A Remotely Controlled Machine for

the Study of Critical Assemblies, Los Alamos Scientific

Laboratory Report LA-1579, June 1953.



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———- ——



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