aecresearchanddevelopmentreport - fas · 2016. 10. 21. · an adjustable dashpot to control the...
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I UNCLASSIFIEDa-~~J!&---CIC-14 REPORT COUECTIO~
FIEPRODUCTIONCOPY AECRESEARCHANDDEVELOPMENTREPORT
LOS ALAMOS SCIENTIFIC LABORATORYOF THE UNIVERSITY OF CALIFORNIAo LOS ALAMOS NEWMEXICO
PLUTONIUM-METAL CRITICAL ASSEMBLIES
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LEGAL NOTICE
This report was prepsred as an account of Govern-ment sponsored work. Neither the United States, nor theCommission, nor any person acting on behalf of the Com-mission:
A. Makes any warranty or representation, expressor implied, with respect to the accuracy, completeness,or usefulness of the information contained in this report,or that the use of any information, apparatus, method, orprocess disclosed in this report may not infringe privatelyowned rights; or
B. Assumes any liabilities with respect to the useof, or for damages resulting from the use of any infor-mation, apparatus, method, or process disclosed in thisreport.
As used in the above, “person acting on behalf of theCommission” includes any employee or contractor of theCommission to the extent that such employee or contrac-tor prepares, handles or distributes, or provides accessto, any information pursuant to his employment or con-tract with the Commission.
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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
Work done by Report written by
Members of Group N-2 G. A. JarvisG. A. LinenbergerH. C. Paxton
Contract W-7405 -ENG. 36 with the U. S. Atomic Energy Commission
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REACTORS-RESEARCH AND TESTfNGLA-2044
Los Aismos Report LibraryAF Plant Representative, BaltimoreAF Plant Representative, BurbankAF Piant Representative, MariettaAF Piant Representative, Santa MonicaAF Plant RepresentStiye, Seattie
iw#%%!l!!%*’tR%Ri”eAlbuquerque Operations OfficeAlto Pruducts, Inc.Argome National LaboratoryArmed Forces Speciai Weapons Project, SandiaArmed Forces Special Weapons Project, WashingtonArmy Chemical CenterAssistant Secretary of the Air Force, R&DAtomic Energy Commission, WashingtonBatteile Memorial InstituteBettis Plant (WAPD)Brookhaven National LaboratoryBureau of ShipsChicago Operations OfficeChicago Patent GroupChief of Naval ResearchCombustion Engineering, Inc. (CERD)Contfnentsl Army Commanddupont Company, AikenduPont Company, WilmingtonEngineer Research and Development LaboratoriesFoster Wheeler CorporationGeneral Electric Company (ANPD)General Electric Company, RichlandHanford Operations OfficeHartford Area OfficeHeadquarters, Air Force Special Weapons CenterIowa State ColiegeRnoils Atomic Power LatvmatoryMound LaboratoryNational Advisory Committee for Aeronautics, CleveiandNational Advisory Committee for Aeromutics, WashingtonNational Lead Company of OhioNavsi Air Development and Materiai CenterNaval Research LaboratoryNew York Operations OfficeNorth American Aviation, Inc.North American Aviation, Inc. (Aerophy.eics Division)Nuclear Development Corporation of AmericaNuclear Metals, Inc.Office of the Quartermaster GeneralPatent Branch, WashingtonPhillips Petroleum Company (NRTS)Pratt and Whitney Aircraft Division (Fox Project)Public Health ServiceSan Francisco Operations OfficeSpecial Devices CenterSylvania Eiectric Products, Inc.Union Carbide Nuclear Company (ORNL)USAF Project RANDU. S. Navai Ordnance LaborstoryU. S. Navai Postgraduate SchoolU. S. Naval Radiological Defense LaborstoryUCLA Medicai Research LaboratoryUniversity of California Radmtion Laboratory, BerkeleyUniversity of California Radiation Laboratory, LivermoreVitro Engineering DivisionVitro Laboratori&3Wright Air Development Center (WCOSI-3)Glenn L. Martin CompanyRichard G. Rowe
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47146-162
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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
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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
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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
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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
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Fig. 1 Jezebel - the bare plutonium
,.. .
assembly.
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Fig. 2 The .Jezebel plutonium components in disassembled, or“safe, “ condition.
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1
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.’~e PIANO WIREGUIDES
-CONTROL ROD
-CENTER SECTIONCLAMP 8 GUIDE
LOWER SAFETYBLOC K
Fig. 3 Design of the active portion of Jezebel.
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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
● ☛☛be ., 13 :. : :
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DELAYEDCRITICALc
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BUTTONS FROM DELAYED CRITICAL
Fig. 4 Rossi alpha vs numberexcess of the delayed
of mass adjustment buttonscritical configuration.
6
in
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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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1 Icri =46.8 cents
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Fig. 5
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EXCESS REACTIVITY - LINEAR CONTROL ROD
Positive pile period as a functiontivity in-inches of linear
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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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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
mm●*● ee ● ● ● 9*
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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
-.. ,
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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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I
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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are presented in
reactions per atom
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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.
.** ● 98* ● *. ● .●-b ● . . . w .
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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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