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Unformatted text preview: CHAPTER 1
FIRST HUMAN MADE REACTOR AND BIRTH OF NUCLEAR
© M. Ragheb
“The energy produced by the atom is a very poor kind of thing.
Anyone who expects a source of power from the transformation of these atoms is talking moonshine.”
Lord Ernest Rutherford, 1933.
“The Almighty certainly never intended that people should travel at such breakneck speed.”
About trains travelling at 15 miles per hour.
Martin van Buren, 1830. 1.1 INTRODUCTION
The events that accompanied the birth of the nuclear age are described. The construction
of the Chicago Pile Number 1 (CP-1) as the first human made nuclear reactor and the milestones
in scientific progress that preceded and immediately followed it are considered. The success of
the first man made self-sustained chain reaction was followed by the Manhattan Project, which
culminated into the first use of nuclear weapons in warfare by the USA against Japan.
We discuss the distinction between a nuclear reactor and a nuclear device using an
exponential neutron population growth model.
Humanity's hope lies in using its acquired knowledge in constructive endeavors, and
refraining from its use for destructive actions. This process still continues in the nuclear age and
affects every human in some special way. 1.2 THE CHICAGO PILE NUMBER ONE, CP-1 REACTOR
The following code message was phoned from the Stagg Field at the University of
Chicago to government officials in Washington DC:
“You will be interested to know that the Italian navigator has just landed in
the New World, and the natives are friendly.”
The Italian navigator referred to was not Christopher Columbus (1451-1506), but Enrico
Fermi (1901-1954). “The natives were friendly,” alluded to the success of a major experiment
designated as the Chicago Pile Number 1: CP-1. A painting by Gary Sheahan that reconstructed
the event showing Enrico Fermi's team is shown in Fig. 1. Fig. 1: Painting by Gary Sheahan reconstructing the Chicago Pile Number 1: CP1, first reactor showing Enrico Fermi's team.
Enrico Fermi was an experimental and theoretical physicist, born in Rome, Italy in 1901.
He taught theoretical physics at the University of Rome in Italy. He was the co-inventor with
Leo Szilard of the nuclear pile. He had received a Nobel Prize in Physics for atomic research in
1938: “On the absorption and diffusion of slow neutrons.” He directed the building of the first
reactor at the Metallurgical Laboratory at the University of Chicago. Fermi adhered to the
Catholic faith and was married to a Jewish spouse. He escaped the then prevalent nationalistic
Fascism and Nazism in Europe to teach at Columbia University in the USA, where he became
involved in the field of neutron physics.
Humans initiated a self-sustained nuclear chain reaction and controlled it for the first time
on December 2, 1942. This occurred beneath the West Stands of Stagg Field, Chicago in the
State of Illinois in the USA, at 3:25 in the afternoon. Fig. 2: Enrico Fermi (1901-1954).
In the center of the 30 by 60 feet squash court, where it was constructed, the reactor
consisted of a pile of graphite bricks and wooden timbers. It was square at the bottom and
flattened as a sphere near the top. As an extra safety feature, it was shrouded on all but one side
by a gray balloon cloth envelope, provided by the Goodyear rubber company, to contain any
unexpected radioactivity release. Its sides were straight up to half its height, and the top was
domed like a beehive.
The squash court was situated under the ivy covered stands of the University of Chicago's
Stagg Field, named after baseball's Grand Old Man: Amos Alonzo Stagg.
This experiment was part of an effort that started a few months earlier aimed at releasing
energy from the nuclear fission process. The project was given the code name: “Manhattan
District Project,” in short: “The Manhattan Project.” The objective was to build a fission device.
However, the scientists here were at the preliminary stage of investigating whether a self
sustained release of fission energy could be achieved in the first place.
Starting in April 1942, two test piles were built at the Stagg field in Chicago. By
November 1942, Fermi and his coworkers had constructed a lattice of carbon blocks in the form
of a graphite cube, containing lumps of uranium in the form of spheres.
The uranium was the fuel for the reaction while carbon, in the form of ultra pure
machined graphite, slowed down the neutrons originating from the fission process in the uranium
fuel through collisions with the graphite nuclei from their average fission energy of about 2
Million electron Volts (MeV) down to the thermal equilibrium energy with the surrounding
medium of 0.025 eV. This is a factor of:
= 80 x106
or an 80 million times reduction in energy.
It was then known that fission is likely to occur in the U235 isotope, not the more
abundant U238 isotope of uranium. In addition, the probability of fission of the isotope U235, or
its nuclear cross section, is enhanced if the neutrons are slowed down from their fast 2 MeV
kinetic energy to the slow 0.025 eV thermal equilibrium energy. This is contradictory to
classical physics where a large amount of kinetic energy would be more capable of splitting the
uranium nucleus. However, at these energies, the neutrons behave more like waves than
particles. Under this circumstance, their wave behavior is best described by Quantum
Mechanics, rather than by Classical Mechanics. A simplified heuristic model envisions the
uranium nucleus as a potential well into which a sluggish slow neutron can drop and be readily
absorbed as a wave, distributing its energy among the nucleons in the nucleus and causing it to
fission. Conversely, a fast neutron as a wave would readily pass over the well, jumping over it
without falling in. Fig. 3: Photograph of the layering of the graphite blocks containing the
lumped uranium spheres at the level of the tenth layer, 1942. Graphite block
from CP-1, Photo courtesy: Paul Mikols.
It was also known that to use natural uranium for a chain reaction necessitates the use of
a moderator material, which does not appreciably absorb the neutrons. From that perspective,
carbon as graphite, beryllium or heavy water (HDO or D2O) could be used, but not ordinary
water (H2O). Heavy water occurs in ordinary water, at a ratio of one molecule of heavy water in
6,700 molecules of ordinary water. The deuterium to hydrogen ratio in nature is about: D/H =
150 parts per million (ppm).
Another important consideration was also known: that mixing a moderator with the
natural uranium fuel in a homogeneous manner in the form of a slurry or a salt solution would
not allow the neutrons to slow down to the required thermal energy. A neutron originating from
fission would get absorbed through resonance absorption in the U238 nuclei distributed evenly in
the homogeneous mixture. Distributing separate lumps of the fuel into spheres embedded into
the graphite blocks allows a neutron born through fission in the fuel to exit the fuel lump, find
the nuclei of the moderator to collide with while slowing down in energy, and escaping to a great
extent the process of being absorbed in the U238 nuclear cross section absorption resonances.
Having escaped resonance capture in U238 and slowed down in the moderator, the now slow
neutrons can encounter another lump of fuel and fission some of the U235 isotope nuclei in it.
Blocks of ultra pure graphite numbering about 4,000 and weighing 6 pounds each were
carefully manufactured. Uranium spheres the size of baseballs were positioned into 22,000 holes
that were carefully drilled in the graphite blocks. Figure 3 shows the layering of the graphite
blocks containing the lumped uranium spheres at the level of the tenth layer.
Instrumentation for measuring the neutron flux such as Geiger counters were built and
calibrated to measure the radioactivity arising from the fission process. Fig. 4: Diagram that was part of the patent 2,708,656 application in 1955 for the CP-1
reactor. Source: USA Patent Office.
Control rods consisting of the strong neutron absorption material cadmium (Cd) were
inserted through the pile to control, through neutron absorption, the fission process. For safety
reasons, if the control rods were not able to shut down the chain reaction, buckets containing
boric acid, with boron (B) as a strong neutron absorbing material, were ready to be poured on top
of the pile should the control rods fail in shutting down the chain reaction.
Layer upon layer of the lumped moderator and fuel configuration, up to 56 layers, were
added with the control rods withdrawn with the addition of each new layer. On December 2,
1942 the pile was almost complete, with only the last layer being added.
At 3:20 pm the nuclear age was born, when after 28 minutes the neutron flux was high
enough to sustain a critical mass of natural uranium in a graphite moderator. The first man made
reactor, CP-1 generated just 1/2 Watt of power from the fission chain reaction using 771,000 lbs
of graphite as a moderator material. As fuel material, it used 80,590 lbs of uranium dioxide
(UO2), and 12,400 lbs of uranium metal (U). It cost $1 million to construct as a flattened
rotational ellipsoid 25 ft wide and 20 ft high.
Patent number 2,708,656 was issued on May 18, 1955 to Enrico Fermi and Leo Szilard
after World War II, for the CP-1 reactor design. Figure 4 shows a diagram that was part of the
patent application. 1.3 THERMAL AND FAST NEUTRONS
The neutrons born from the fission process have an average kinetic energy of 1.99 MeV,
and are designated as “fast neutrons.” They collide with the nuclei of carbon in the form of the
graphite moderator multiple times and slow down in energy until they reach thermal equilibrium
with moderator medium and are then designated as “thermal neutrons.”
The kinetic energy of thermal neutrons, or “kT neutrons,” is given by: Ek = kT
where: (1) k is the Boltzmann constant = 1.38 x 10-16 [erg/K]
T = 273 + oC, is the absolute temperature on the kelvin scale. According to the 13th General Conference on Weights and Measures (CGPM) in 1967, on
the kelvin (K) temperature scale, temperatures are called “kelvins” without capitalization and the
symbol K is not preceded by the degree symbol in contrast to the degree Celsius (oC) and the
degree Fahrenheit (oF) scales.
This can be expressed as: E k kT
= = 1 2
2 from which their speed v can be estimated from:
v= 2E k
m (2) EXAMPLE
We use Eqn. 1 to calculate the energy in eV of a thermal neutron in equilibrium with the
moderating medium at a room temperature of 20 degrees Celsius as: Ek = kT
(273 + 20) K
= 0.025114 eV 0.025 eV = 1.38 x10−16 EXAMPLE
We can then use Eqn. 2 to calculate the speed of the thermal neutrons as: v= = 2E k
1.675x10-24 gm 2x 0.025eVx1.6x10-19 = 2.185 x105 2, 200 cm
sec Thermal neutrons are thus also referred to as “2,200 m/sec neutrons.”
Fast neutrons are close to having relativistic speeds and it may be argued that they
warrant a relativistic treatment.
From the Special Theory of Relativity, for a relativistic particle, the Total Energy is
Total Energy = Kinetic Energy + Rest Mass Energy (3) Expressed in terms of the mass of the particle m and the square of the speed of light, this
equation can be written as; mn c 2 Ek + mn 0c 2
= (4) From this equation the kinetic energy of a relativistic particle is: = mn c 2 − mn 0c 2
= ( mn − mn 0 )c 2 (5) = ∆mc 2
The relativistic mass of the particle depends on the ratio of its velocity v to the speed of light c as β= v
c (6) It can be written as: mn = mn 0
1− c 2 (7) Substituting for the relativistic mass Eqn. 7 into the expression for the kinetic energy Eqn. 5, we
get: mn 0 Ek
= − mn 0 c 2 2 1− v c (8) 1 = − 1 mn 0c 2 2 1− v c The relativistic particle’s speed can be expressed from Eqn. 8 as a fraction of the speed of
light c as:
c Ek 1 + m c2 n0 1 2 1
1 − c
2 1 + Ek 2 mn 0c (9) EXAMPLE
The speed of a fission neutron born at an energy of 2 MeV can be calculated from Eqn. 9
as: 1 2 1
1 − 3x1010 cm
erg 2 x106 eVx1.6 x10−12 eV 1 + 1.675 x10−24 gm(3x1010 cm ) 2 sec 1 2 1
1 − 3x1010 cm
2 2 x1.6 x10−2 sec 1 + 1.675 x 3x 3 1 1 2
= 1 − 3x10 sec 1.00424994541 1
= [ 0.00423195981034] 2 3x1010
= 0.06505 x 3x1010
= 1.95 x109
= 1.95 x107
This amounts to 6.5 percent of the speed of light.
Should the treatment has used Eqn. 2, we would have obtained the similar result: v= = 2E k
1.675x10-24 gm 2x 2x106 eVx1.6x10-19 cm
= 1.95 x107
= 1.95 x109 1.4 ETYMOLOGY OF THE WORD “SCRAM” The etymology of the word Scram, meaning the sudden fast shutdown of a nuclear
reactor, is that it is an acronym reportedly coined by Enrico Fermi, when he placed one of his
colleagues, Norman Hillberry, next to a rope used to raise and lower the control rods into the CP1, equipped with an ax. Hillberry's duty, if called upon, was to chop the rope with a single
swing, immediately dropping the control rods, absorbing the neutrons, hence stopping the fission
The story goes that Hillberry's title in the project was: “Safety Control Rod Ax Man,”
hence the acronym Scram.
There are not any ax men in the control rooms of modern nuclear power plants. There
exist though plenty of red colored Scram switches sometimes labeled: “RX Trip,” RX standing
for “Reactor Scram.” A 45 degree clockwise yank sends the control rods into the core and the
reactor shuts down within seconds. 1.5 SCIENTIFIC MILESTONES
The successful experiment at the CP-1 reactor was preceded by a rapid succession of
events in the study of radioactivity and nuclear processes.
Antoine Henry Becquerel discovered radioactivity in France around 1896.
Pierre and Marie Curie followed by discovering the element radium, and later polonium,
and the chain decays of uranium in 1898. Their work was helped by other discoveries such as
the discovery of the electron through experiments conducted with cathode ray tubes by J. J.
Thomson in 1897.
The energy to mass equivalence equation was introduced by Albert Einstein in 1905.
Ernest Rutherford conducted experiments where he bombarded a thin gold foil with alpha
particles, and inferred the existence of the nucleus at the center of the atom's structure in 1912.
This was followed by the discovery of the neutron by James Chadwick in 1932.
The discovered neutrons were used to induce artificial radioactivity by Irène and Fréderic
Joliot-Curie in the 1930s in France. Enrico Fermi in Italy had conducted experiments producing
new artificial isotopes using neutrons.
Otto Hahn and Fritz Strassmann in Germany bombarded uranium nuclei with neutrons,
and found traces of new nuclei in the middle of the periodic table: this was the discovery of the
fission process. They discovered that when a nucleus of uranium was bombarded by neutrons,
the uranium nucleus splits or, in the parlance of biology, fissions or splits like biological cells
would do. It was later noticed that additional neutrons were emitted in the fission process.
These neutrons become available for inducing further fission reactions with other uranium
nuclei. This fact implied the possibility of a nuclear chain reaction, similar to the fusion nuclear
reactions occurring in the sun and the stars. This chain reaction could be made self sustaining, or
critical, if a sufficient quantity of uranium could be brought together under the proper conditions
that reduce absorptions within the volume and leakage from the surface of the material. If the
system were supercritical, the nuclear energy release would increase exponentially until the
material is no longer in a supercritical configuration. This situation can be the basis of a nuclear
On the other hand, if the release is released in the critical controlled state, this energy can
be harnessed for great benefit in electrical power production, fresh water desalination from the
salty oceans, ship and rocket propulsion, isotopes production for nuclear medicine, harbor and canal excavation, defense against comet and asteroid astral assailants, and other uses requiring
energy densities far exceeding the known sources of chemical energy.
Lise Meitner, who earlier worked with Otto Hahn and corresponded with him while in
England, reported Otto Hahn’s findings and explained its implications to Niels Bohr in Norway,
who fully understood its implications and in turn promptly communicated it to scientists in
England and the USA. Fig. 5: Leo Szilard with Albert Einstein reenacting their writing of a letter to President
Franklin Roosevelt in 1939.
Leo Szilard convinced Albert Einstein, to jointly write a letter to USA President Franklin
Roosevelt in 1939, urging him to initiate work on the possibility of building an atomic weapon.
The Roosevelt Committee for the feasibility of an atomic weapon was established in 1941. In
England, a similar committee was established: the Military Applications of Uranium
Disintegration (MAUD) in 1941. The Army's Manhattan Engineer District Project, or
Manhattan Project in short, was established in the USA under Brig. General Leslie Grove from
the Corps of Engineers in 1940.
As part of this project, work was initiated at the University of Chicago's Metallurgical
Laboratory under Arthur Compton in 1942. The culmination of the effort was the first self
sustained man made fission chain reaction designated as Chicago Pile Number 1: CP-1. 1.6 THE MANHATTAN PROJECT
Upon success of the CP-1 experiment, a nuclear device development effort project was
promptly started in July 1943 with 100,000 employees at three then secret sites at a cost of $2
billion. It was feared that Germany, under the leadership of its leading nuclear scientist Werner Heisenberg, where fission was first discovered under Otto Hahn and Fritz Strassmann, was also
on track for building such a weapon. Heisenberg had suggested the construction of a nuclear
reactor using heavy water (D2O) as a moderator, instead of the graphite used by Enrico Fermi.
The Germans went on an unfruitful track and emphasized the use of thermal neutrons and were
only able to build bulky subcritical assemblies which never achieved criticality. They also never
reached the realization that fast neutrons can be used to construct a compact unmoderated fast
neutron supercritical configuration and hence a weapon.
Brigadier General Leslie Richard Groves, born in Albany, New York in 1896 from the
USA Corps of Engineers (Fig. 6), who had earlier supervised the construction of the Pentagon
building in Washington D. C., was chosen to direct the effort. He enlisted the help of scientists
headed by Robert Oppenheimer (Fig. 7) who was a theoretical physicist, born in New York City
in 1904. Oppenheimer, in turn, sought the help of an international team of scientists such as
Enrico Fermi, Hans Bethe, Leo Szilard. Victor Weisskopf, Niels Bohr, George Kistiakowsky,
and Edward Teller. Fig. 6: Brigadier-General Leslie Groves from the USA Corps of Engineers directed the
Manhattan Project. Fig. 7: Robert Oppenheimer led the team of scientists at Los Alamos National Laboratory. Fig. 8: Cutout through the X-10 pile at Oak Ridge Tennessee, USA. Fig. 9: Face of the X-10 air-cooled graphite pile constructed at the Clinton Engineers
Works at Oak Ridge, Tennessee, USA.
A second pile designated as the Chicago Pile 2 or CP-2, using the uranium from the
dismantled CP-1 pile, was constructed by March 1943 at Argonne National Laboratory near
Chicago. This new pile was surrounded by a five-foot thick concrete shield. It was operated at a
few kilowatts of power, compared with the ½ watt power of CP-1, without an internal cooling
system, producing small samples of plutonium for basic nuclear physics data and chemical
separation experiments. Another pilot experimental graphite moderated and air cooled pile at a design power level
of 1,000 kilowatts or 1 Megawatt, designated as the X-10 pile was constructed at the Clinton
Engineers Works near Oak Ridge, Tennessee in 1943. It was meant to provide enough
plutonium for the chemical separation semi-works. It consisted of a large cube of graphite
blocks surrounded by several feet of high density concrete as a biological shield against gamma
The graphite blocks were pierced by hundreds of horizontal diamond shaped channels, in
which rows of cylindrical uranium slugs were fed horizontally, forming long rods. The pile was
air cooled with circulation through the channels on all sides of the slugs. After a period of
irradiation, new slugs would be fed from one face of the reactor, pushing the irradiated slugs to
fall through a chute off the other side into a bucket immersed in water. After a few weeks of
storage under water to allow the short lived fission products radioisotopes to decay, the buckets
were transferred through an underground canal to a chemical separation plant. A series of cells
with thick concrete walls would contain chemical separation equipment operated remotely.
Large underground tanks were used to store the radioactive waste. The facility was ingeniously
located on a slope to make use of gravity for flow. Figures 8 and 9 show the X-10 air-cooled
pile constructed at the Clinton Engineers Works.
The graphite blocks composing the X-10 pile measured 24 feet on the side and weighed
1,500 tons. It contained 1,243 channels on eight inches centers. The uranium slugs canned in
aluminum jackets were 1.1 inch in diameter and 4.1 inches long. Two boron steel rods, in the
right side of the pile controlled the power level of the chain reaction through inserting them in
and withdrawing them out. Four more rods on the left side of the pile were used to shut down
the pile. A hydraulic system was designed with two suspended weighted pistons, which would
fall and drive the rods into the pile within 5 seconds in the event of a power failure. A second
line of defense consisted of four rods suspended above vertical holes in the pile and dropping in
when the trip mechanism is energized. As a third level of safety, two hoppers were filled with
small boron steel balls to be released into vertical columns in an emergency. Air circulation
traversed the graphite, went under the pile through a filter system to a 200 feet stack besides the
building. A fan house contained a small steam driven air circulator for emergency use, one
50,000 cubic foot per second fan, and another of 30,000 cubic foot per second capacity.
Thermocouples to measure temperatures, a Pitot tube to measure air flow, and ionization
chambers to measure the neutron flux levels were incorporated in the design feeding the
information into an adjacent control room.
The pile reached criticality with 30 tons of uranium, or half of its 60 tons capacity. With
improved cooling capabilities and increased fuel loading to 36 tons, the power level reached 500
kilowatts. Five tons of uranium metal containing just 500 milligrams, or half a gram of
plutonium, was discharged in November of 1943. The power level eventually reached 1,800
kilowatts, double its design level, but the amounts of plutonium produced were only sufficient
for experimental purposes, and could not support a weapon's construction effort.
The real weapons engineering was a massive industrial effort that was carried out at three
geographically dispersed sites. Large industrial complexes were constructed and managed by
different industrial corporations. The DuPont Corporation managed the chemical works on a
cost-plus basis in return for a symbolic payment of just $1. 1.6 SITE W, THE HANFORD SITE, STATE OF WASHINGTON The first site was designated as site W where three large-scale reactors were built at the
Hanford reservation in the state of Washington, to breed plutonium as a possible fissile material
from natural uranium. The reactors' power increased from the 1/2 watt of CP-1 to the 250
Megawatts thermal (MWth) power level. These reactors were water cooled. Each pile needed a
river pump house on the Columbia River, large storage and settling basins, huge motor driven
pumps to deliver water to the faces of the reactors, and facilities for emergency cooling in case of
The first pile area was designated as 100-B. The reactors were designated with letters as
B, D, C, N and F. The tolerances in construction were exceptional. The allowable deviation in
cross section measurements was 0.005 inch; in length, 0.006 inch; in the diameter of longitudinal
holes for the process tubes, 0.003 inch. About 17 percent of the graphite used in the piles, was
tested in a small 30 watts pile constructed at Hanford. The Hanford piles rose more than 120 feet
above the desert floor. Adjacent to them were the water cooling treatment facilities, using the
Columbia River for cooling the reactors.
The irradiated uranium slugs were loaded into heavy shielded casks placed on special
railroad cars operated by remote control, and moved about 5 miles away from the piles. The
buckets were suspended in water inside low concrete structures isolated in the desert. When a
significant amount of the short lived radioactivity has been allowed to decay, the buckets were
moved to the chemical separation plant. Fig. 10: Views of the Hanford piles rising 120 feet above the desert floor with the water
cooling treatment facilities, and the Columbia River. Fig. 11: Assembling the core of the C reactor at Hanford, state of Washington. Fig. 12: Face of the N Reactor, Hanford, Washington. Fig. 13: Spent fuel stored under water at the N reactor, Hanford, Washington.
A bismuth-phosphate chemical separation process carried plutonium from the irradiated
fuel in the separation building. The plutonium would be separated from the phosphate carrier
and other gross impurities in a concentration building. The phosphate carrier was dissolved in
hydrochloric acid and a rare earth fluoride was used to precipitate the plutonium. Disposal of
radioactive gases and wastes from the process building were done in a ventilation building and a
waste storage area.
There were three of these plants designated as T, U and B. The separation buildings,
called canyons, were 800 feet long, 65 feet wide and 80 high. They resembled a large aircraft
carrier floating on a sagebrush sea in the desert. In each one, a row of forty concrete cells,
fifteen feet square and twenty feet deep, ran the length of the building. Each cell was covered by
concrete blocks six feet thick, and was separated from its neighbors by six feet of concrete. The
35 tons concrete lids had to be poured maintaining a 1/8 of an inch tolerance to provide adequate
shielding. The chemical separation plants were so large that they were called canyons. Along one side of the cell row, and behind seven feet of concrete, were the operating
galleries on three levels. The lowest level was for the electrical controls. The intermediate level
was for piping and remote lubrication equipment. The upper level was for operating control
boards. The area above the cells was enclosed by a single gallery sixty feet high and running the
length of the building. Its five foot concrete walls and three foot roof slabs were designed to
prevent the escape of radiation when the cell covers were removed. Even with all the covers in
place, radiation levels in the gallery would be so high that unprotected personnel could not be
present. Once operation started, this huge gallery, or canyon as it became to be called, would
become a silent concrete no-man's land isolated from the outside world. Fig. 14: Purex process chemical separation canyons at the Hanford site. Fig. 15: Interior view of the H reprocessing canyon. Fig. 16: Canyon tank overhead view. Fig. 17: Reprocessing canyon F control room.
Radiation needs remote control equipment. This mandated simplicity of design,
mechanical perfection, maintenance free operation, and interchangeability of parts. Steam jets
were used to transfer process materials from one tank to another, to avoid servicing pumps and
valves. Centrifuges, being more reliable than filters, were developed for separating materials.
Liquid level and density meters were used to follow the progress of the operation.
Once the plant was operating, the only access to the cells would be by means of large
bridge cranes, which traveled the length of the building. From the heavily shielded cab behind a
concrete parapet above the gallery, operators used periscopes and video monitors to view the inside of the gallery. They could use a seventy-ton hook to lift off the cell covers and lighter
equipment to work within the cell. With impact wrenches and special tools, they could remove
connecting piping, lift the damaged pieces of equipment, and isolate it in a storage cell. They
would then lower another new piece of equipment into the operating position and reconnect the
process piping. All of this was done at sixty feet or more without direct vision, and with the
requirement of extreme accuracy in the dimensions of cell components. Equipment and
connections were standardized. Distinctive color codes were used on all units. All concrete
faces were coated with paint that is easily washable, corrosion resistant and adequately adherent
to the concrete. Because the chemicals were highly corrosive, a special grade of niobium
(columbium) stainless steel was used.
The final stage in plutonium recovery involved a peroxide method. This process was
based on the fact that all nitrates, except those of uranium, thorium and plutonium, are soluble in
hydrogen peroxide. The plutonium could be isolated, by separating it from the lanthanumfluoride carrier, converting it into a nitrate, and adding peroxide. The product would be pure
plutonium nitrate, which was then sent to the Los Alamos laboratory in New Mexico for
reduction to metal. The great value and high toxicity of the product required specialized
laboratory techniques. 1.8 SITE X, OAK RIDGE, TENNESSEE Fig. 18: The Alpha I racetrack Calutron (left) and the Beta track (right) used in the Y12
electromagnetic separation process at Oak Ridge, Tennessee.
The second site was designated as site X where scientists were isotopically separating the
fissile U235 isotope from natural uranium using electromagnetic separation in 184 inch cyclotrons
called Calutrons deriving their name from: “California cyclotrons.” Copper for manufacturing of
the magnet coils was in short supply and was allocated to the war effort. An amount of 14,700
tons from the strategic metals silver stockpile at Fort Knox was used instead of copper, and was
returned back at the end of the war. About 200 grams of the U235 isotope were produced for
The Electromagnetic separation process was a monumental white elephant designated as
the Y-12 plant. It produced only a few grams of impure U235 and had to be closed after
repetitively breaking down. Figure 18 shows a racetrack used in the electromagnetic process at Oak Ridge, Tennessee. The protruding ribs are the silver-wound magnet coils. The box-like
cover on top contains a solid silver bus-bar. Two types of tracks were used: the Alpha I track for
an initial enrichment, and the Beta rectilinear racetrack to reach higher enrichments and allowing
a smaller scale of the equipment.
Other enrichment processes such as thermal diffusion and centrifugation were tried at this
time, without producing significant amounts of the U235 isotope.
The main success in the production of U235 was carried out in the K-25 Gaseous
Diffusion enrichment plant using 4,000 separation cells and uranium hexafluoride (UF6),
consuming as much electricity as New York City. The electricity needs were provided by Coal
and Hydroelectric plants operated by the Tennessee Valley Authority (TVA) government
agency. Fig. 19: The K-25 gaseous diffusion plant, at Oak Ridge, Tennessee, was closed in 1987.
Centrifugal pumps designs, with appropriate seals, operating in the highly corrosive
environment of uranium hexafluoride had to be developed. Nickel plating and nickel steel alloys
resisted corrosion by the gas, and were used. This was the basis of the development of stainless
steel as a spinoff industrial product. A barrier material capable of maintaining a separation
capability over a long period of time without being clogged had to be chosen. It had to be
submicroscopic, but not susceptible to clogging. It had to be porous, but solid enough to be
manufactured. Nickel barriers involving a complex ten steps manufacturing process were
initially used. Later development of a nickel powder barrier, possibly sintered powder, replaced
the initial metallic barrier. Figure 19 shows the K-25 gaseous diffusion plant. In the center lie
different service buildings. Fig. 20: Uranium hexafluoride UF6 storage cylinders. Fig. 21: Depleted Uranium (DU) storage cylinders.
About 50 kilograms of highly enriched uranium were produced at Oak Ridge over a
year's time for the Little Boy device, which was dropped on Hiroshima in 1945. The East
Tennessee Technology Park nowadays is a former K-25 uranium-enrichment site.
Oak Ridge is currently the USA Department of Energy's largest science and energy
laboratory. Between 1942 and 1945, as part of the Manhattan Project, it turned the rural
countryside about 20 miles west of Knoxville, Tennessee into a secret city inhabited by 75,000
people. 1.9 SITE Y, LOS ALAMOS, NEW MEXICO
The third site Y was situated at Los Alamos in New Mexico. Its objective was to create a
critical mass using the fissile isotopes manufactured at the two other sites. There existed an
uncertainty about which of the two fissile isotopes would be suitable for constructing a weapon.
There was concern about the process of spontaneous fission generating a neutron source in the
different plutonium isotopes, and whether a device could be built from it. Spontaneous fission would initiate the fission reaction before a supercritical mass is fully assembled and such a
device would prematurely fizzle releasing a minor amount of energy. The assembly of such a
device would have to proceed at a much faster speed than in a gun barrel, necessitating an
inverse rocket process designated as implosion. The production of both the isotopes was thus
pursued. The group working on one isotope did not know about the existence of the other group.
Ultimately both approaches were successful, culminating in, not just one, but two fission
device designs. The first design that was developed was a gun barrel type design using U235.
The second design was an implosion type device using Pu239. Fig. 22: Inert gas plutonium glove box. Fig. 23: Finished plutonium puck or button (left) and enriched uranium ingot (right).
The intended target for these weapons was unequivocally Germany, but the war with
Germany had already ended with its surrender. The Manhattan project was initiated out of fear
that Germany was developing a fission weapon. The Germans under Werner Heisenberg had a
modest primarily theoretical effort directed at building a subcritical reactor using heavy water as
a moderator material. The production of heavy water available to Germany and its separation from ordinary water required the use of electricity produced from hydroelectric power in
Norway. Aerial bombardment and a commando raid sunk the shipments of D2O to Germany and
disabled the heavy water plant twice. This concerted effort, whose purpose was not clear to
Germany, but whose implications were fully understood by the Allies, brought a halt to the
German effort. An investigative team, called the Alsos team, investigated the assumed German
program at the end of the war in Europe in 1944. The team discovered that the much feared
German weapon project just did not exist.
The war with Japan was still ongoing, and the use of the developed weapon against Japan
was promptly considered. Japan, realizing that it will eventually lose the war, fearing the entry
into the war by Russia and its occupation of its northern islands, and preferring to reach an
accommodation with the USA, now that the war in Europe had come to an end, was sending
feelers and trying to negotiate an end to it.
There was some opposition against the use of the atomic bomb, with suggestions of
demonstrating it as a warning to Japan before its actual use. Questioning the need for using the
atomic bomb in the war against Japan, and fully aware of its vast destructive potential, James
Conant and Vannevar Bush, at the urging of scientists working on the Manhattan Project,
addressed a letter on September 30, 1944, to Secretary of war Stimson. They urged a
demonstration of the weapon’s capability before its use against Japan. Arthur Compton in May
1945, wrote a letter to superiors where he raised the “… question of mass slaughter for the first
time in history.” Niels Bohr in July 1944 sent a warning to the USA president that: “… any
advantage the atomic bomb might seem to possess would be outweighed by a perpetual menace
to human security.” He had correctly predicted the nuclear arms race that would follow later.
James Frank on June 12, 1945 led seven atomic scientists from the University of Chicago in
presenting the Chicago Petition or the Frank Report. Farrington Daniels, director of the
Metallurgical Laboratory at the University of Chicago, on July 12, 1945, polled 150 scientists
working on the weapon project. A majority favored some form of demonstration of the device.
Sixty eight scientists at Oak Ridge recommended demonstration of the weapon.
In the diary of then President Harry Truman, he made the case that atomic devices are
purely military weapons:
“We have discovered the most terrible bomb in the history of the world. It
may be the fire destruction prophesied in the Euphrates Valley Era, after Noah
and his fabulous Ark. This weapon is to be used against Japan. [We] will use it
so that military objectives and soldiers and sailors are the target and not women
and children. Even if the Japs are savages, ruthless, merciless and fanatic, we as
the leader of the world for the common welfare cannot drop that terrible bomb on
the old capital or the new. The target will be a purely military one.”
Leo Szilard who ironically had earlier written a letter with Albert Einstein to President
Franklin Roosevelt, urging the start of the bomb development project, found himself with 69
other scientists writing a petition to President Harry Truman asking him to first demonstrate the
use of the atomic bomb to Japan before using it. Fig. 24: Declassified Leo Szilard petition to President Harry Truman with 69 other
scientists on July 17, 1945. Source: USA National Archives. Fig. 25: Assumed schematic of the Trinity plutonium device designated as “The Gadget”
surrounded by the chemical explosive lenses initiators. Source: Gamow, “The Curve of
From May to July 1945, a committee designated as the “Interim Committee’ studied the
implications of using the atomic bomb. It was composed of the following members: Henry
Stimson, George Harrison, James Byrnes, Ralph Bard, William Clayton, Vannevar Bush, James
Conant, Arthur Compton, Enrico Fermi, Ernest O. Lawrence, and J. Robert Oppenheimer.
The Interim Committee, after deliberation, recommended the use of the bomb on Japan
on a dual target without prior warning. Navy Under-Secretary Ralph Bard dissented and
withdrew his agreement. Fig. 26: Mockups of the Trinity device showing the core, tamper and surrounding explosive
lenses and their initiators. .. Fig. 27: Photographs of the Trinity device on top and at bottom of its test tower showing
the initiators wiring. Fig. 28: Energy museum model of the Trinity device at Albuquerque, New Mexico and
assembly of Fat Man device.. Fig. 29: A photograph taken of the fireball and the horizontal Mach stem generated by the
Trinity test at 0.016 second into the explosion. Fig. 30: Mushroom cloud from the Trinity test on July 16, 1945 at 5:29:45 am.
Because of uncertainties about the behavior of the implosion type device, it was tested for
the first time at the Trinity Test Site on July 16, 1945 at 5:29:45 am. The test was conducted at
Jornada del Muerto (In Spanish: Journey of Death), 100 miles south of Los Alamos in the
Alamogordo desert of southern New Mexico. It was a plutonium implosion device placed at an
optimal height on a 100 foot tall tower, to maximize its shock wave or blast effects through the
reinforcing interference of an incident and the reflected shock waves into the ground as a
horizontally propagating Mach stem blast wave. The plutonium fission core was surrounded by
an array of explosive lenses, in turn enclosed in a metal casing. Figure 25 shows a schematic,
Fig. 26 shows a mockup, and Fig. 27 shows a photograph of the Trinity plutonium device,
designated as “The Gadget” surrounded by the chemical explosive lenses' initiators. 1.10 NUCLEAR ENERGY RELEASE
The yield of the Trinity test was about 19 kilotons (kT) equivalent of the high explosive
Tri-Nitro-Toluene (TNT). The site is now a historical site open to the public only once a year on
the first Saturday of October each year. Figure 29 is a photograph taken of the fireball generated
by the Trinity test, and its Mach stem at 0.016 second into the explosion.
An average value of the energy release per fission event in a fissile isotope such as U235 is
200 million electron volts (MeV). The MeV energy unit is equivalent to 1.6 x 10-6 ergs or 1.6 x
10-13 joules. The proportion in which this energy is distributed in a fission reaction is shown in
Only a part of the energy is available in a nuclear explosion. This includes the kinetic
energy in the fission products, most of the energy of the prompt gamma rays, which is converted
into other forms of energy within the exploding weapon primarily ionization and x rays, and
most of the neutron kinetic energy, but only a small fraction of the decay energy of the fission
products. There is some contribution from the energy released in reactions in which neutrons are
captured in the device's fragments. If we exclude the antineutrinos, whose 10 MeV are
unavaible, and the delayed gamma rays (6 MeV) and the beta particles decay energy (7 MeV), and allow about 3 MeV for the neutron gamma reactions with the device fragments, about 200 10 – 6 -7 + 3= 180 MeV are immediately available per fission event.
Table 1: Apportionment of Energy Release from fission. Distribution of Fission Energy
Kinetic energy of fission fragments
Prompt gamma rays energy
Kinetic energy of fission neutrons
Beta particles from fission products
Delayed gamma rays from fission products
Anti neutrinos from fission products
Energy release per fission event
Neutron- gamma reactions with device fragments
Unavaible antineutrinos energy
Unavailable fission products beta particles
Unavailable fission products gamma rays
Energy available in fission device Energy (MeV)
165 ± 5
5 ± 0.5
200 ± 6
180 There are 6.02 x 1023 nuclei or Avogadro's number in a gram molecular weight or mole
or 235 grams of U235, or 239 grams of Pu239. The explosive energy released by Tri-NitroToluene C7H5N3O6 or TNT ranges from 900 to 1,100 calories per gram.
The energy release from a molecule of TNT yields: C7 H 5 N 3O6 → 6 CO + 5
H 2 + N 2 + C + 608,000 J
2 Some of the released carbon oxidizes into CO2 yielding 393,510 J per mole of C. The
hydrogen oxidizes into water releasing an extra 241,826 J per mole of H2. The carbon monoxide
oxidizes into CO2 yielding 282,980 J per mole of CO. If all the carbon burns, and the nitrogen
does not, this adds up to: 5
608,800 + 6 x 282,980 + 241,826 + 393,510 =
608,800 + 1,697,880 + 604,565 + 590, 265 =
The molecular weight of TNT is: 7 x12.011 + 5 x1.0079 + 3x14.0067 + 6 x15.994 =
84.077 + 5.0395 + 42.0201 + 95.964 =
The reported value of the heat of combustion for TNT is 3,305,000 J/mol, or:
1 mol 9 gm
mol 227.1 gm
kT 4.184 J
3,305,000 A complication arises in that to optimize the energy of a blast, ammonium nitrate
NH4NO3 (80 gm/mol) is added as an oxidizer to TNT actually yielding an explosive mixture
designated as “amatol.” An optimal mixture is 21.3 percent TNT and 78.7 percent ammonium
nitrate by weight yielding the reaction;
2C7 H 5 N 3O6 + 21 NH 4 NO3 → 14CO2 + 47 H 2O + 24 N 2 + 9,088,600 J
For practical dynamic considerations a mixture for the amatol is 20 percent ammonium
nitrate and 80 percent TNT.
The molecular weight of ammonia is:
2 x14.0067 + 4 x1.0079 + 3x15.994 =
28.013 + 4.0316 + 47.982 =
In the case of the amatol mixture, the energy yield becomes: J
mol 9 gm
mol (2 x 227.1) + (21x80.003) gm
mol 9 gm
mol (454.2 + 1680.063) gm
mol 9 gm
mol 2,134.263 gm
kT 4.184 J
9,088,600 The kiloton could be considered as the short kiloton corresponding to 2 x 106 pounds, the
metric kiloton would be equal to 2.205 x 106 pounds, or the long kiloton equal to 2.24 x 106
To avoid ambiguity, the energy released from one thousand tons or 1 kiloton (kT) of TNT
is defined to be 1012 calories. This is equivalent to 1 short kT of TNT if the energy release is
1,102 calories per gram, and to 1 long kT if the energy release is 984 calories per gram of TNT.
This resulted in the published energy equivalences shown in Table 2.
Table 2: Energy equivalence of 1 kt of TNT.
Device Energy Release
1 kT of TNT =
2.6 x 1025 MeV
4.184 x 1019 ergs
4.184 x 1012 joules
1.16 x 106 kW.hrs
3.97 x 109 BTUs
fission of 56.8 grams of fissile nuclei
fission of 2.00 ounces of fissile nuclei
Fission of 1.45x1023 fissile nuclei
The number of fissions that would release the equivalent of 1 kT of TNT can be
1 kT TNT=1012 calories x 4.184 Joule
MeV 1 fission
calorie 1.60207x10 Joule
eV 180 MeV =1.45x1023 nuclei where we used the average extractable energy of 200 – 10 – 6 -7 + 3 = 180 MeV per fission
event, excluding the antineutrinos and the delayed beta gamma rays whose energies cannot be
extracted, but allowing for about 3 MeV for the (n, gamma) capture reactions with the device
debris.. Using Avogadro’s law: N= g
M (10) where:g is the mass in grams,
M is the atomic mass in amus,
A v = 0.6 x1024
the mass of fissile material resulting from the fissioning of N fissile nuclei is: g= N .M
Av (11) EXAMPLE
The mass of U235 nuclei leading to the release of 1 kT of equivalent TNT excluding the
antineutrinos and delayed gamma rays is:
g= N .M
Av 1.45 x1023 x 235
= 56.8 gm
= 56.8 gm x 0.3527
= 2.00 ounces
= 1.11 DISTINCTION BETWEEN A NUCLEAR REACTOR AND AN
MULTIPLICATION FACTOR AND CRITICALITY
We try to identify and clarify the difference between a nuclear reactor and an explosive
device in terms of the energy release as a function of the number of neutron generations in the
fission process. Let us define:
: the average number of neutrons released per fission event,
: the average number of neutrons lost by absorption in the active material per generation,
: the average number of leakage neutrons released from the active geometry per generation. We can define a multiplication factor k as:
k=ν -a- (12) Three situations present themselves according to the value of the multiplication factor k: k=1 ⇒ a critical system,
k>1 ⇒ a supercritical system,
k<1 ⇒ a subcritical system. (13) EXPONENTIAL NEUTRON GENERATIONS GROWTH MODEL
The number of fissionned nuclei in 1 kT of TNT equivalent to the fission of 56.8 gms of
U235 from Table 2 and Eqn. 11 is: N= 56.8
0.6x1024 = 1.45 x1023 nuclei. .
235 If N is the number of neutrons present at any instant, from Eqn. 12, the number of
neutrons in the next generation will be: Nk=N(ν -a-)
The increase in the number of neutrons from one generation to the next becomes: dN=Nk-N
=N(ν -a- − 1)
Let the generation time be denoted by τ. Thus we can write the rate of neutrons increase
as: dN N ( k − 1)
τ (14) Separating the variables followed by limit integration, assuming that k and τ are constant,
yields: N (t ) ∫ N0 ln dN (t ) ( k − 1)
t N (t ) ( k − 1)
τ N (t ) = N 0e + ( k −1) τ (15) t by taking the exponential of the natural logarithm function.
This shows that the neutron generation undergoes an exponential growth as a function of
We can express the number of neutrons as a function of the number of generations as: n= t (16) τ Substituting from Eqn. 16 into Eqn. 15, results in a change of variables from t to n as: N(n)=N 0e + ( k −1) n (17) EXAMPLE
Adopting the numerical values: ν = 2.5, a = 0.5, = 0, k = 2.5-0.5-0.0 = 2, then: N(n)=N 0e + n (18) This assumes no leakage of the neutrons. The number of generations in which 0.1 kT of
TNT equivalent of energy corresponds to the fissions of 0.1 x 1.45 x 1023 = 1.45 x 1022 nuclei,
and the release of 100 kT of TNT equivalent corresponds to the fissions of 100 x 1.45 x 1023 =
1.45 x 1025 nuclei. If we further assume that the chain reaction is initiated by a single neutron,
then N0 = 1, and thus: N = e+ n (19) Taking the natural logarithm of both sides we get: ln N ln e n n= n
= = ln e
We can thus construct Table 3 relating the total energy release to number of neutron
Table 3: Energy release as a function of the number of neutron generations
Energy Release Number of fissions Number of generations kT TNT
1.45x1025 n = ln N
57.93 Hence the release of 1 kT of TNT equivalent requires the confinement of the fission
reaction for at least 53 neutron generations, thus in nuclear devices the reaction must be confined
by use of a tamper.
In a nuclear reactor no tamper is used, hence it is impossible to confine a supercritical
system for more than a few neutrons generations, since the heat release would disassemble,
through melting followed by vaporization, the configuration into a subcritical one shutting down
the chain reaction.
The following observations can be made:
1. There is a need for a very large number of 51 neutron generations to release a mere 0.1 kT of
2. Most of the energy release: 100 − 0.1 99.9
= = 0.999,
or 99.9 percent is released in the last 58 – 51 = 7 generations in a short period of approximately 7
x 10-8 sec = 0.07 μsec, or less than a tenth of a microsecond.
3. Nuclear explosives are “tamped” inertially with a high strength high-density “blanket”
material such as Be, BeO, W, Pb, Ta, Au or U to increase the inertial confinement time and
obtain fissions and energy release from the latest number of generations.
4. Nuclear reactors are not tamped. The heat release will thus melt then vaporize any
supercritical configuration and disperse it. Dispersion restores subcriticality of the configuration
with stoppage of the energy release.
5. Thermal reactors cannot explode like nuclear explosive devices, since they are constructed
6. The worst that can happen in a fast reactor accident is compaction of the core to
supercriticality with a substantial nuclear energy release, but not a complete explosion, since
dispersal and subcriticality would occur quite rapidly.
OTHER TECHNICAL DIFFERENCES
To initiate a nuclear explosion in a nuclear device, the fissile materials such as U235 or
Pu , or a combination of them in a dual core configuration, are brought together very rapidly
and then kept together for a long enough time, such that a significant fraction of their nuclei
undergo the fission process. The fissile components of the device are driven together with
explosive force producing rapid increases in the reactivity of the system way beyond the
neutrons prompt critical state. This necessitates the use of Pu239 and/or U235 with a high
enrichment in the fissile isotopes because the other isotopes act as a sink for the fast neutrons
needed for the process. Under these circumstances, the fissions are due to fast neutrons, the time
239 between the neutron generations is very short, no delayed neutrons exist, high enrichments are
used, and massive amounts of energy are released before the material has time to blow itself
apart, overcoming the inertial resistance to its expansion by the surrounding tamper, terminating
the fission reaction.
In a thermal reactor, on the other hand, the situation is totally different. The fuel is low
enriched uranium with a large proportion of U238, which absorbs neutrons, and a small proportion
of U235 and Pu239 at only 3-5 percent. The fuel is also surrounded by a moderator that slows
down the energy of the neutrons produced. In a power surge, even if a prompt neutrons critical
state is attained, the presence of the U238 damps down the increase in reactivity by absorbing the
neutrons as they are slowed down through the nuclear negative feedback mechanism of Doppler
resonance broadening. This inherent negative Doppler coefficient of reactivity is a safety
characteristic of all commercial nuclear plants and research reactors and describes the fact that as
the fuel gets hotter it gets less effective at using neutrons to induce fission and produce power
causing the reactor’s reactivity to fall. The nuclear reactors’ Doppler coefficient is always
negative, causing its reactivity to decrease as its temperature increases.
In addition, the majority of fissions in nuclear reactors are caused by slow neutrons for
which the time between successive fissions is longer than in the case with fast neutrons. The
effective neutron lifetime and hence the reactor period is increased even more by the presence of
the delayed neutrons released from the decay of the fission products. The end result is that the
energy released by a power surge will disrupt the fuel and terminate the chain reaction long
before the reactivity reaches the very high levels needed in a nuclear device. The energy
released in a power surge, including fast reactors, would be far lower: by as much as 100 million
times less by volume than in a nuclear device. 1.12 NUCLEAR DEVICES USE IN WAR
The decision was reached to use the atomic bomb against Japan on two targets without
prior demonstration or warning. The argument that won the debate, concerning prior
demonstration, was that it would save soldiers lives on both sides, as well as civilians on the
Japanese side, who would have otherwise to incur heavy losses as collateral damage in a
conventional invasion of the Japanese islands. The large human losses on both the American and
Japanese sides in the invasion of the Japanese island of Okinawa supported this fear. There were
opposing rumblings and accusations to the effect that the decision was based on the political
need to use the weapons to justify the effort and expenses of building them.
What is remarkable is that the two approaches that were followed to construct the bomb,
the uranium235 enrichment approach as well as the plutonium239 breeding approach, both
succeeded. Figure 32 shows a replica of the gun barrel design using U235 as a fissile material,
designated as Little Boy. Figure 34 shows the actual Fat Man Implosion device employing Pu239
as a fissile material. Gun barrel casing Tamper Chemical propellant Accelerated two hemispheres configuration Chemical propellant Tamper Gun barrel casing Single accelerated hemisphere configuration Tamper
Chemical propellant Gun barrel casing Accelerated plug configuration Fig. 31: Gun barrel configurations could contain one to two critical masses of highly
enriched U235. Fig. 32: A replica of the Little Boy large metallic gun barrel device dropped on Hiroshima.
It was 304 cm long and 71 cm in diameter. It was exploded at a height of 570 meters at
08:15 hour on August 6, 1945 generating about 15 kT of yield. It had a low efficiency and
is reported to have fissioned only 1.5 lbs of its uranium content. The overall device weighed
4-5 metric tonnes. It was made of 64 kgs of uranium with an enrichment of 80 percent or
51 kgs of U235. At 50 kgs as critical mass, this corresponds to 7,000 kgs equivalent of
natural uranium. This corresponds to the critical mass of an uncompressed bare
unreflected sphere of U235. Sophisticated methods of uranium storage and controlled
compression have reduced the critical mass by a factor of 2-3. The hazard of uncontrolled
explosions limits the amount of U235 content of a modern warhead with 5-10 times the yield
of Little Boy.
lenses Tamper Solid core implosion Casing Explosive
lenses Tamper Spherical shell implosion Casing Explosive
lenses Tamper Porous core implosion Fig. 33: Implosion configurations using solid, spherical shell or porous cores. With
compression and a tamper, modern weapons are reported to use 15-20 kgs of Weapons
Grade Uranium (WGU) instead of the about 50 kgs bare critical mass of U235. Fig. 34: The actual Fat Man Implosion device dropped on Nagasaki, being loaded on a
bomber for delivery to its target. It was exploded at 503 meters height at 10:58 hour on
August 9, 1945. It was 325 cm long and 154 cm in diameter. It generated 22 kT of yield
mostly from plutonium. It weighed 4-5 metric tonnes of which the nonfissioning assembly
material constituted a thick blanket surrounding the fission core. Colonel Paul Tibbets lead the 509th Composite Bomb group and prepared at Wendover
air base in Utah for the bomb dropping. The bombers had to train on bomb dropping procedures,
plane maneuvers and tactics that would allow the bombers to mitigate the effects of the released
gamma rays traveling at the speed of light on the bomber crew, followed by the neutrons, and
escape the blast wave from the explosion.
On August 1, 1945, B-29 bombers arrived on the coral island of Tinian in the Pacific
Ocean, 1,500 miles south of Japan. The USS Indianapolis heavy cruiser had delivered the bomb
components. The war was still continuing, the Indianapolis was later sunk by a Japanese
submarine on its trip back from its secret mission.
On August 6, 1945, the Enola Gay B-29 bomber, on exhibit at the Smithsonian Museum
in Washington D.C., dropped a gun barrel bomb design using U235, designated as Little Boy, on
Hiroshima, Japan at 8:16:02 am. It measured 28 inches in diameter and 129 inches in length. It
weighed 9,000 pounds. From 80,000 to 140,000 persons perished and 100,000 more were
injured. The radiation dose resulted primarily from fast neutrons. The yield of the weapon was
12.5 kT of TNT equivalent. Fig. 35: The mushroom cloud that rose from the Little Boy Hiroshima device's fireball on
August 6, 1945, to the left, and the one over Nagasaki, August 9, 1945 to the right. Fig. 36: Survivor burn victim from the Hiroshima explosion. Burns are more prominent
where the x rays were preferentially absorbed in the dark clothing areas. Fig. 37: The effects of the Hiroshima weapon 4,000 feet from “ground zero.” A fire station
and its equipment are shown. Fig. 38: The effects of the Hiroshima weapon showing a concrete movie theatre remaining
structure. Fig. 39: The B-29 bomber Bockscar dropped the Nagasaki plutonium Fat Man device. Fig. 40: Aftermath of the bombing and ensuing fire at Nagasaki. Fig. 41: Catholic cathedral explosion remnant at Nagasaki. Fig. 42: Nagasaki ground blast effects.
On August 9, 1945, the Bockscar B-29 bomber dropped an implosion device using Pu239,
designated as Fat Man on Nagasaki, Japan. The latter measured 60 inches in diameter, and was
128 inches long. It weighed about 10,000 pounds. About 74,000 persons perished and 75,000
were injured out of the 286,000 inhabitants of the city.
A photograph of a survivor burn victim in Fig. 36 shows the effect of x-ray absorption at
the dark parts of the clothing. The hilly nature of the landscape at Nagasaki, compared with the
flat one at Hiroshima, resulted in a lower number of victims despite the larger released explosive
yield. The radiation dose was primarily from x and gamma rays. The weapon's yield was 22 kT
of TNT equivalent.
Figure 35 shows the mushroom clouds that resulted from the Hiroshima and Nagasaki
devices fireballs, with the buildings of the cities appearing at the bottom of the photograph.
Figures 37, 38 show its effects at “ground zero” directly under the explosion, and Figs. 40-42
show the blast effects at the second bomb explosion at Nagasaki.
Table 4 gives estimates of the casualties caused by the atomic blasts at Hiroshima and
Nagasaki compared with those from the fire storm bombing of Tokyo with conventional and
incendiary explosives on March 9, 1945, and with the average of 93 aerial attacks on other
Japanese cities. The fire bombing evidently caused higher casualties. However, the higher
mortality and casualty rates per square mile destroyed by the atomic devices is also clearly
Table 4: Casualties comparison between conventional and nuclear explosives.
Hiroshima Population per square mile
Square miles destroyed
Killed and missing
Injured Nagasaki Tokyo
1,667 tons TNT
and incendiary 35,000
102,000 Average of
1,129 tons TNT
1,830 Mortality per square mile
Casualities per square mile
destroyed 15,000 20,000 5,200 1,000 30,000 42,000 11,800 2,000 As a comparison, the “Dresden Commission of Historians for the Ascertainment of the
Number of Victims of the Air Raids on the City of Dresden on 13-14 February 1945” has
provisionally estimated the likely death toll at around 18,000 and definitely no more than 25,000.
Dresden was a German city of 3/4 of a million people, its population further swollen by hordes
of anonymous refugees from the Eastern Front. Its historic heart was destroyed in one
apocalyptic night by aircraft armed with more than 4,500 tons of high explosive and incendiary
bombs. The devastated area amounted to around 13 square miles or34 square kilometers. 1.13 NAVAL TRAGEDY OF THE USS INDIANAPOLIS CRUISER
The USS Portland class heavy cruiser Indianapolis (CA-35) had sailed 5,000 miles across
the Pacific Ocean in just 10 days to deliver the atomic bomb components to the Tinian Island on
July 27, 1945. It had been repaired in a dry dock after being attacked and suffering a near fatal
hit on March 31, 1945 by a Japanese army Oscar airplane on a Kamikaze, Divine Wind, mission
on its naval strafing mission, prior to the invasion of the Japanese island of Okinawa, 400 miles
from the Japanese mainland.
Being on a secret mission, the crew did not know the nature of its cargo. It sailed alone
and was not escorted by ships equipped to detect and fight the Japanese submarines. At the time
of her sinking the ship’s commanding officer Captain Charles McVay III was blamed for her loss
and the loss in its crew. However, information was withheld from Captain McVay about
submarine activity in the area on its way to the Philippines and especially about how the Navy
policies at the time precluded the navy from even realizing that the ship was lost for days.
On July 30, 1945, it was intercepted and hit by two torpedoes at 12:05 am, one blowing
off most of her bow, where the officers slept. About 900 of the 1,196 sailors on board initially
survived its sinking, which lasted 12 minutes. The rest were trapped inside the sinking hull.
However, only 316 or 316 / 1196 = 0.264 or 26 percent of its crew would ultimately survive the
horrors of hallucination from dehydration, heat and terrorizing attacks by dozens of sharks. An
account of their tragic ordeal over 4 ½ days and 5 nights was described in the Hollywood movie:
The Indianapolis was sunk by the Japanese submarine I-58 under Lt. Cdr. Mochitsura
Hashimoto, who was later promoted to Commander for the same attack, in the Philippine Sea.
The survivors were not missed until August 2, 1945; three days later. It was planned that the
Indianapolis would return to Leyte on July 31, 1945. A reconnaissance aircraft on a routine
flight accidentally spotted the survivors in the water on August 2, 1945. Rescue operations did
not complete until August 8, fully 9 days after the sinking.
The ship’s commanding officer, Capt. McVay survived, only to commit suicide in 1968
being upset about receiving hate mail. Fig. 43: The Portland class heavy cruiser USS Indianapolis, or The Indy, off Mare Island
shipyard on July 10, 1945, after its repair from a Kamikaze attack off the island of
Okinawa. It was lost on July 30, 1945.
Reports of the Indianapolis sinking were buried by the news of the Japanese surrender
and the end of World War II. Captain Charles McVay III, the commander of the Indianapolis
was court-martialed supposedly for not adopting a zigzag sailing course to evade submarine
detection. His men thought he was made a scapegoat. In the year 2000, thirty two years after he
committed suicide, an act of the USA Congress cleared his name. 1.14 DISCUSSION
Japan was fearful of Russia’s entry into the war with its territorial claims in the northern
Japanese islands, accepted the Potsdam agreement and the Japanese general Nazami signed the
documents with American general Westmoreland and other Allies representatives on the
Missouri battleship in Tokyo bay, thus ending the ordeal of World War II on August 15, 1945.
The Missouri is kept as a floating museum, open to the public, at the Pearl Harbor Naval Base in
Hawaii. Fig. 44: Monument at the ground zero of the Trinity site. The site is now a historical site
open to the public only once a year on the first Saturday of the month of October.
The nuclear age was born in the building of the first reactor, followed by the Manhattan
Project, the first nuclear test at the Trinity site and the first use of nuclear devices against Japan
ending the Second World War. The forces and knowledge unleashed in these events is sure to
affect human life forever. The genie was out of the bottle. The Nuclear Age was born. EXERCISES
1. Calculate the speed in meters per second of thermal or kT neutrons with an energy of 0.025
2. If a single fission reaction produces about 200 MeV of energy, use Avogadro’s law to
calculate the number of grams of U235, U233 and of Pu239 that would be fissioned to release 1 kT
of TNT equivalent of energy. Assume that all the energy release is available, except for the
energy carried away by the antineutrinos, and the delayed gamma rays and beta particles in the
fission products, which is not fully recoverable.
3. The yield from the Hiroshima device was 12.5 kT of TNT equivalent, and the yield from the
Nagasaki device was 22 kT of TNT. Assuming that one critical mass of lead reflected U235
Oralloy at about 30 kgs, and one critical mass of Pu239 at about 10 kgs were used to generate
these yields, compare the energy release efficiencies of the two devices as the fraction or
percentage of the fissile material converted into energy in the case of the gun barrel versus the
implosion process. 4. Using the exponential growth model to calculate the number of generations needed for the
release of 1 kT and 100 kT of TNT equivalent, in what neutron generations are the last 99
percent of the energy released? Use the following data: ν = 3.0, a = 0.4, = 0.1.
5. Compare the power in MeV/sec and in Watts of:
a) The experimental CP-1 pile.
b) The pilot X-10 pile.
c) The Hanford plutonium production reactors.
d). A modern electrical nuclear power plant.
-6 -13 Note: The MeV/sec power unit is equivalent to 1.6 x 10 ergs/sec or 1.6 x 10 Joules/sec or
6. Calculate the speed in meters per second of neutrons possessing the following
a. Fast neutron from fission at 2 MeV,
b. Intermediate energy neutron at 10 keV,
c. Thermal energy neutrons at 0.025 eV.
7. Plot the neutron population N as a function of the generation number n using the
exponential model for the following values of the neutron multiplication factor k:
a) k=1.0, critical system,
b) k=0.5, subcritical system,
c) k=2.0, supercritical system. REFERENCES
1. S. Glasstone, Ed., “The Effects of Nuclear Weapons,” U. S. Atomic Energy Commission,
2. S. M. Ulam, “Adventures of a Mathematician,” Charles Scribner’s Sons, New York, 1976.
3. A. C. Brown, and B. MacDonald, “The Secret History of the Atomic Bomb,” Dell Publishing
Co., New York, 1977.
4. R. G. Hewlett and O. E. Anderson, Jr., “The New World, A History of the United States
Atomic Energy Commission,” Volume 1, 1939/1946, U. S. Atomic Energy Commission,
5. D. S. Saxon et al., “The Effects of Nuclear War,” Office of Technology Assessment,
Congress of the United States, Washington DC, May 1979.
6. William E. Loewe, “New Radiation Dosimetry Estimates for Hiroshima and
Nagasaki,” Energy and Technology Review, Lawrence Livermore National
Laboratory, pp. 25-36, November 1984. ...
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This note was uploaded on 06/16/2010 for the course NPRE 402 taught by Professor Ragheb during the Spring '08 term at University of Illinois at Urbana–Champaign.
- Spring '08