Table Of ContentNUCLEAR
GRAPHITE
Edited by
R.E. NIGHTINGALE
Hanford Laboratories
General Electric Company
Prepared under the auspices of the
Division of Technical Information
United States Atomic Energy Commission
A C A D E M IC P R E S S - 1 9 62
N ew Y o rk a nd L o n d on
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List of Contributors
D. E. BAKER, Hanford Laboratories, General Electric Company, Richland,
Washington
J. C. BOKROS, John Jay Hopkins Laboratory for Pure and Applied Science,
General Atomic Division, General Dynamics Corporation, San Diego,
California
T. J. CLARK, Hanford Laboratories, General Electric Company, Richland,
Washington
R. E. DAHL, Hanford Laboratories, General Electric Company, Richland,
Washington
J. M. DAVIDSON, Hanford Laboratories, General Electric Company, Rich-
land, Washington
D. R. DE HALAS, Hanford Laboratories, General Electric Company, Rich-
land, Washington
W. P. EATHERLY, National Carbon Company, Parma, Ohio
J. C. Fox, Hanford Laboratories, General Electric Company, Richland,
Washington
J. L. JACKSON, Hanford Laboratories, General Electric Company, Richland,
Washington
L. H. JUEL, Great Lakes Carbon Corporation, Niagara Falls, New York
J. KORETZKY, Battelle Memorial Institute, Columbus, Ohio
H. H. W. LOSTY, Research Laboratories, General Electric Company, Ltd.,
Wembley, England
R. A. MEYER, John Jay Hopkins Laboratory for Pure and Applied Science,
General Atomic Division, General Dynamics Corporation, San Diego,
California
P. F. NICHOLS, Hanford Laboratories, General Electric Company, Rich-
land, Washington
R. E. NIGHTINGALE, Hanford Laboratories, General Electric Company,
Richland, Washington
E. L. PIPER, National Carbon Company, Parma, Ohio
W. C. RILEY, Battelle Memorial Institute, Columbus, Ohio
C. A. SWITZER, JR., Great Lakes Carbon Corporation, Niagara Falls, New
York
R. E. WOODLEY, Hanford Laboratories, General Electric Company, Rich-
land, Washington
E. M. WOODRUFF, Hanford Laboratories, General Electric Company, Rich-
land, Washington
H. H. YOSHIKAWA, Hanford Laboratories, General Electric Company, Rich-
land, Washington
v
Preface
Contributions to the literature on nuclear graphite originate from many
technical disciplines. The physicist is concerned with radiation effects and
moderator physics; the chemist studies the gas-graphite kinetics, chemical
reactions, and structure; and the nuclear engineer must be familiar with
the properties of graphite and the changes that occur in the reactor environ-
ment. The boundaries between each of these fields are indistinct. Indeed,
the mutual exchange of ideas resulting from engineers' and scientists' work-
ing as a team on the nuclear applications of graphite has contributed
significantly to the present state of understanding. It is hoped that bringing
together the literature on nuclear graphite within a single volume will
stimulate this cooperative effort to continued growth and achievement.
The graphite technology that has developed since Acheson first de-
scribed the manufacture of graphite in an electric furnace in 1896 was
directly applicable in the early 1940's when the search began for an
extremely pure graphite to serve as the neutron moderator in the first
nuclear reactor. Thus it might be said that the technology of nuclear
graphite is considerably more than 20 years old. However, the effects of
radiation on graphite,, which is a major concern of the modern literature,
were unknown less than 20 years ago, and in this important respect the
technology of nuclear graphite is a young field indeed. The first unclassified
literature began to appear only six years ago following the First United
Nations International Conference on the Peaceful Uses of Atomic Energy,
held in Geneva in 1955. The bulk of the literature still exists in the form
of government reports, many of which are not readily available. One of
the objectives of this book has been to survey the literature and to provide
a comprehensive, selected bibliography of useful references.
The volume of material existing in the field of nuclear graphite can best
be appreciated from the fact that approximately 5000 government reports
and journal publications were located and abstracted in the preparation
of this book. The subject matter is current to January 1961; more recent
material has been included wherever possible.
A book describing the manufacture, properties, and uses of a material
such as graphite necessarily embraces a wide diversity of topics, and
coherent organization posed a problem. Although some duplication was
unavoidable, it has been minimized by the frequent use of cross references.
A critical review of this volume will reveal variations in the depth and
detail of subject treatment from chapter to chapter. To a large extent this
reflects the varying extent of scientific understanding that currently exists
vii
Vlll PREFACE
in the various topics. Some sections on new or particularly difficult topics
are simply an empirical collation of observations, whereas other topics
have been given a rather rigorous theoretical treatment.
An effort has been made to present the different views of controversial
subjects. When one interpretation seemed clearly preferable, the authors
have so indicated ; in other cases, however, conflicting arguments or experi-
mental observations could not be resolved on the basis of existing informa-
tion, and the authors have reserved judgment.
This book is intended to serve as a reference for those concerned with
the development and use of nuclear graphite. Its organization should also
make it useful as a text to orient the newcomer to the field. The opening
chapter discusses some of the historical aspects of nuclear graphite and
develops the subject matter to the time when the first radiation effects were
observed. Thereafter, the text follows this sequence: manufacture, prop-
erties, radiation effects, and reactor design. The chapters on manufacture,
machining, structure, properties, and gas-graphite reactions contain a
considerable amount of general information and should be most useful to
engineers and scientists concerned with the broad field of graphite tech-
nology. The chapters on theory of radiation effects and irradiation tech-
niques provide the background for subsequent chapters on radiation effects.
The chapters devoted to metal-graphite compatibility and moderator
design will be of special interest to the reactor design engineer.
The editor takes great pleasure in thanking the chapter authors for their
willing help and cooperation in completing this task. All contributed
generously of their own time without any remuneration in the belief
that the result would serve a useful purpose. It was indeed a pleasure to
work with the staff of General Electric's Technical Information Operation
at Hanford; its excellent library and extensive files contributed in large
measure to the preparation of this volume. Mr. B. B. Lane of this staff
deserves special mention for his assistance in searching the literature, refer-
encing, and indexing, and for his helpful editorial work on the manuscript.
The invaluable comments of many technical reviewers in the United
States and the United Kingdom are also gratefully acknowledged. Many
of their suggestions have been incorporated into the final manuscript. The
final review and editing were conducted by members of the Division of
Technical Information of the U. S. Atomic Energy Commission. It is a
pleasure to acknowledge their valuable service.
R. E. NIGHTINGALE
July 1962
CHAPTER ONE
Graphite in the Nuclear Industry
R. E. NIGHTINGALE t
1-1 Early Use of Nuclear Graphite
1-1.1 FIRST NUCLEAR REACTOR
When the group of scientists led by Enrico Fermi decided in 1942 to
attempt to produce a self-sustaining nuclear chain reaction, they chose
graphite as the moderator because it was the only suitable material avail-
able at that time. The first pile, CP-1, was constructed on a squash court
under the West Stands of Stagg Field at the University of Chicago. Earlier
Fermi and his collaborators had assembled the first exponential graphite-
uranium structures at Columbia University for the purpose of determining
the multiplication factor (fc), the ratio of the number of neutrons in any
one generation to the number of corresponding neutrons in the previous
generation. If fc could be made greater than 1, then a nuclear chain reaction
could be produced. The first exponential pile, an 8-ft cube, was assembled
from graphite blocks and contained about 7 tons of uranium oxide in iron
containers; it was completed in July 1941. By the fall of that year, the
multiplication factor (fc«,) for an infinite lattice of this type was known to
be about 0.87. Although it was agreed that this could be increased if purer
graphite and uranium could be produced, it was still not known whether
fcoo could be made greater than 1.
In the spring of 1942, the work on experimental piles under Fermi was
transferred to the University of Chicago. The first pile1 constructed there
of U 0 and AGX graphite^ yielded a fc«, of 0.944. Several lattice structures
3 8
using different lots of graphite and uranium were used in the assembly of a
number of experimental piles. Experiments on 29 of these are mentioned in
early reports.2 In July 1942 A. H. Compton estimated that a fc«, of 1.04 to
1.05 could be obtained in a pile of highly purified graphite and uranium
oxide if the air were removed to reduce neutron absorption. A value of fc«,
greater than 1 (actually 1.007) was obtained experimentally for the first
time for Experimental Pile 9.
Enough data were collected and enough pure materials soon became
available to make possible an attempt to produce a self-sustaining nuclear
t Hanford Laboratories, General Electric Company, Richland, Wash.
t AGX graphite was made from petroleum coke and coal-tar pitch by the National
Carbon Company, using one pitch impregnation. It was graphitized by the Acheson
process.
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1. GRAPHITE IN NUCLEAR INDUSTRY 3
reaction, and the assembly of a pile was begun in October 1942. By this
time the certainty of producing a chain reaction in an infinitely large system
had virtually been established.3 The values of fc«, were estimated at 1.04
and 1.07 for uranium oxide-graphite and uranium-graphite piles, respec-
tively, with an accuracy sufficient to make a chain reaction in an infinite
system almost certain. The immediate questions remaining were: Could a
self-sustaining reaction be produced in a structure of practical size? Would
it be thermally stable, or would the reaction rate increase dangerously with
temperature? Would it be controllable? The answer to all these questions,
although not known with certainty, was believed to be "yes."
The production of the first nuclear chain reaction in CP-1 on Dec. 2,
1942, was, of course, a major milestone in the eventual utilization of nuclear
energy for economic production of power. In his monthly report of the
Physics Division of the Metallurgical Project, Fermi described this historic
event very simply :4
Experimental Production of a Chain Reaction. The activity of the Physics
Division in the past month has been devoted primarily to the experimental
production of a divergent chain reaction. The chain reacting structure has
been completed on December 2 and has been in operation since then in a
satisfactory way. A program of tests on the operating conditions of the
chain reacting unit and experiments for the investigation of the various
radiations inside and outside the pile is in progress. The results will be
reported as soon as possible.
The metal lattice at the center of CP-1 and two other major lattices
making up most of the volume had been studied separately in Exponential
Piles 18, 27, and 29. Because only a relatively small amount of metal
(about 6 tons) was available and because several types of graphite of
different purity were used, the pile was assembled in an approximately
spherical shape with the purest materials in the center. The pile was sup-
ported on a wooden structure, with the lowest point of the graphite resting
on the floor. No concrete or other heavy shielding was provided. The pile
was surrounded by a tent of rubberized balloon fabric so that neutron-
absorbing air might be evacuated. To complete the 26-ft-diameter sphere
as planned would have required about 75 layers of the 4%-in. graphite
bricks. Criticality was achieved at layer 57 without evacuating the air,
and assembly of the sphere was discontinued about one layer above the
critical dimensions (Fig. 1.1).
CP-1 was operated for several days at about 0.5 watt. It was found that
not only was precise control accomplished by manual regulation of the
cadmium strips, but also stable automatic control was possible. On Decem-
ber 12 a high-intensity test was carried out at about 200 watts, and measure-
ments were made of the radiation intensities in and around the pile. Because
of the danger of radiation at higher power, CP-1 was dismantled early in
1943 and reconstructed from the same materials at what is now the Argonne
4 R. E. NIGHTINGALE
National Laboratory. The addition of concrete shielding to the recon-
structed pile, designated CP-2, allowed an increase in power to about
100 kw.
It is interesting to note that according to Fursov 5 work in Germany led
to the conclusion that graphite could not be used with natural uranium to
produce a nuclear chain reaction and no further effort was made to build
such a pile. Calculations by physicists in the U.S.S.R. indicated that crit-
icality could be achieved with 25 to 50 tons of uranium and a few hundred
tons of graphite, and they proceeded to test this experimentally. Their
experience was quite similar to that in the United States. Assembly of the
pile with a spherical core and a 7.87-in. graphite-uranium lattice was
started. The corners were eventually partially filled, and the pile became
critical with 54 layers. Initial tests were at 10 watts, with a few short runs
at several kilowatts. A negative reactivity temperature coefficient was found.
1-1.2 GRAPHITE IN CP-1
By the time a divergent nuclear chain reaction had been produced,
information had already been gathered on the nuclear purity of several
graphites by testing in special "sigma (σ) piles." These were constructed
to measure the thermal-neutron-capture cross section of several grades of
graphite available at that time. Accurate determination of the nuclear
capture cross section of the graphite was necessary for two reasons: (1) if
there were too much absorption in the graphite, a chain reaction would be
impossible or might require that the dimensions of the pile be unpractically
large and (2) since the effect of absorption on reactivity is proportional to
the square of the neutron density, the use of low-cross-section material near
the center would reduce the size of the structure. For these reasons cross
sections were measured on each grade of graphite used in CP-1, with the
results shown in Table 1.1.
Table 1.1 — GRAPHITE2 IN CP-1
Average thermal
Desig-
Source Amount, tons absorption cross
nation
section, mb
AGOT National Carbon Company 255 4.97
Speer Speer Carbon Co. 72.5 5.51
US United States Graphite Co. 16 6.38
AGX National Carbon Company 30 6.68
AGX National Carbon Company) . (Control-rod
Speer Speer Carbon Company / support pier)
385.5
The assembly of σ piles was a sensitive method capable of differentiating
between the purest of the graphites then available. The graphite used in
CP-1 was less pure than that now commercially available, but, because
1. GRAPHITE IN NUCLEAR INDUSTRY 5
methods of determining absorption cross sections have been changed and
improved since these early measurements, these values are not precisely
comparable to values quoted for graphites today (Sec. 4-3).
1-1.3 WIGNER EFFECT
During 1943 the Theoretical Physics Group of the Metallurgical Project,
led by E. P. Wigner, was busy developing the information necessary to
construct a large plutonium-producing pile. At that time the effect of
intense heavy-particle radiation on pile materials was relatively unknown.
In the same monthly report4 in which the nuclear chain reaction was re-
ported, Wigner first called attention to the possible effects of fast particles
on solids. He suggested that fast particles would act as projectiles and
displace atoms from equilibrium lattice sites. Early calculations6 indicated
that one 2-Mev neutron produced in fission would possess enough energy
to displace about 2000 carbon atoms. The displacement of atoms from
lattice sites by momentum transfer (which was soon to be confirmed experi-
mentally) has become known as the "Wigner effect." The possible effects
on strength, thermal conductivity, and dimensions of the graphite aroused
much concern. The early investigators were faced with a problem that has
arisen many times since in radiation-damage studies, i.e., that of attempting
to learn or somehow predict what radiation-induced property changes will
occur during the lifetime of a new higher flux reactor.
When construction was started on the first power-producing reactors
(the X-10 Reactor in early 1943 and the first Hanford reactor in mid-1943),
the only fast-neutron sources then available, i.e., the cyclotrons at Washing-
ton University and at the University of California, were wholly inadequate
for a satisfactory prediction of what might happen to the graphite in a
nuclear reactor. Cyclotron irradiations of several months were estimated to
be equivalent in damaging effects to approximately one day in a Hanford
reactor.7 Based upon the first radiation results from the X-10 Reactor in the
spring of 1944 and the assumptions necessary at that time,8 it was stated:
"W (Hanford Pile) could operate at least nine days without failure." Thus,
by mid-1944 when it became apparent that there were no radiation sources
of sufficient intensity to be used in predicting property changes of the
graphite in the Hanford reactors beyond a few days operation, plans were
made to construct several water-cooled test holes in the Hanford reactors so
that changes in properties might be determined at frequent intervals.
The displacement of carbon atoms by fast neutrons was expected to
produce changes in most of the properties of graphite, including a decrease
in thermal and electrical conductivity, changes in dimensions and strength,
and a storage of internal energy (Szilard complication). The possibility was
suggested that the graphite bars might fuse together from the opening and
closing of valence bonds at the surfaces of the bars.9 This question could not
be dismissed lightly since calculations showed that during operation of the
