Uranium enrichment

Federal
Agency of Higher Education

Tomsk
Polytechnic University

Applied
Physics and Engineering Faculty

“Uranium
enrichment”

Made
by:

Student
of group 0850

Kiselyova
U.V.

.

Tomsk,
2008

1.
Uranium Enrichment – Introduction

There
is one element that occurs in nature that has been the raw material for nuclear
bombs: uranium, chemical symbol U. Uranium occurs in nature as a mixture of
three different isotopes – that is, three different atomic weights that have
virtually the same chemical properties, but different nuclear properties. These
isotopes are U-234, U-235, and U-238. The first is a highly radioactive trace
component found in natural uranium, but it is not useful in any applications;
the second isotope is the only fissile material that occurs in nature in
significant quantities, and the third is the most plentiful isotope (99.284
percent of the weight of a sample of natural uranium is U-238), but it is not
fissile. U-238 can, however, be split by high energy neutrons, releasing large
amounts of energy and is therefore often used to enhance the explosive power of
thermonuclear, or hydrogen, bombs.

Because
of the presence of small quantities of U-235, natural uranium can sustain a
chain reaction under certain conditions, and therefore can be used as a fuel in
certain kinds of reactors (graphite-moderated reactors and heavy water³ reactors,
the latter being sold commercially by Canada). For the most common reactor type
in use around the world today, which uses ordinary water as a coolant and
moderator, the percentage of U-235 in the fuel must be higher than the 0.7
percent found in natural uranium. The set of industrial processes that are used
to increase the percentage of U-235 in a given quantity of uranium go under the
general rubric of “uranium enrichment” – with the term “enrichment” referring
to the increase in the percentage of the fissile isotope U-235. Light water
reactors typically use 3 to 5 percent enriched uranium – that is, the
proportion of U-235 in the fuel is 3 to 5 percent, with almost all the rest
being U-238. Material with this level of U-235 is called “low enriched uranium”
or LEU.

Nuclear
bombs cannot be made from natural or low enriched uranium. The proportion of
U-235, which is the only one of the three isotopes that can sustain a chain
reaction in uranium, is just too small to enable a growing “super-critical”
chain reaction to be sustained. Uranium must have a minimum of 20 percent U-235
in it in order to be useful in making a nuclear bomb. However, a bomb made with
uranium at this minimum level of enrichment would be too huge to deliver,
requiring huge amounts of uranium and even larger amounts of conventional
explosives in order to compress it into a supercritical mass. In practice,
uranium containing at least 90 percent U-235 has been used to make nuclear
weapons. Material with this level of enrichment is called highly enriched
uranium or HEU. The bomb that destroyed Hiroshima on August 6, 1945, was made
with approximately 60 kilograms of HEU. Highly enriched uranium is also used in
research reactors and naval reactors, such as those that power aircraft
carriers and submarines. The HEU fuel meant for research reactors is considered
particularly vulnerable to diversion for use in nuclear weapons.

Thorium-232,
which is also naturally occurring, can be used to make bombs by first
converting it into U-233 in a nuclear reactor. However, uranium fuel for the
reactor, or fuel derived from uranium (such as plutonium) is needed for this
conversion if U-233 is to be produced in quantity from thorium-232.

A
fissile material is one that can be split (or fissioned) by low energy neutrons
and is also capable of sustaining a chain reaction. Only fissile materials may
be used as fuel for nuclear reactors or nuclear weapons. Examples of other
fissile materials, besides uranium-235, are uranium-233 and plutonium-239.

“Heavy
water” is water that contains deuterium in place of the ordinary hydrogen in
regular water (also called light water). Deuterium has one proton and one
neutron in its nucleus as opposed to hydrogen, which has only a single proton.

The
same process and facilities can be used to enrich uranium to fuel commercial
light water reactors – that is to make LEU – as well as to make HEU for nuclear
bombs. Therefore all uranium enrichment technologies are potential sources of
nuclear weapons proliferation. In addition, some approaches to uranium
enrichment are more difficult to detect than others, adding to concerns over
possible clandestine programs.

2.Uranium
Enrichment technologies

Only
four technologies have been used on a large scale for enriching uranium. Three
of these, gaseous diffusion, gas centrifuges, and jet nozzle / aerodynamic
separation, are based on converting uranium into uranium hexafluoride (UF₆)
gas. The fourth technique, electromagnetic separation, is based on using
ionized uranium gas produced from solid uranium tetrachloride (Ucl₄).

2.1
Gaseous Diffusion

The
gaseous diffusion process has been used to enrich nearly all of the low and
highly enriched uranium that has been produced in the United States. It was
first developed in the 1940s as part of the Manhattan Project and was used to
enrich a portion of the uranium used in the bomb that was dropped on Hiroshima.
All five acknowledged nuclear weapons states within the nuclear
non-proliferation treaty (NPT) regime have operated gaseous diffusion plants at
one time or another, but currently only the United States and France continue
to operate such facilities. The diffusion process requires pumping uranium in a
gaseous form through a large number of porous barriers and, as noted above, is
very energy intensive.

In
order to make the uranium into a gaseous form that can be used in the diffusion
process, the natural uranium is first converted into uranium hexafluoride (UF₆).
The uranium hexafluoride molecules containing U-235 atoms, being slightly
lighter, will diffuse through each barrier with a slightly higher rate than
those containing U-238 atoms. A simple analogy to help visualize this process
is to imagine blowing sand through a series of sieves. The smaller grains of
sand will preferentially pass through each sieve, and thus after each stage
they would represent a slightly higher percentage of the total than they did
before passing through the stage. A schematic representation of one such stage
from a gaseous diffusion plant is shown in Figure 1.

Figure
1: Schematic diagram of a single stage in a gaseous diffusion plant.

The
darker colors represent the UF₆ molecules that contain the heavier
U-238 atoms, while the lighter colors represent gas molecules that contain the
lighter U-235. After each stage the gas to the low pressure side of the barrier
(i.e. the downstream side) has a slightly higher percentage of U-235 than the
stage before.

The
difference in mass, and therefore velocity, between the UF₆ molecules
containing either U-235 or U-238 is very small, and thus thousands of such
stages are needed in order to enrich commercial or military amounts of uranium.
In a gaseous diffusion plant, the stages are arranged into “cascades” that
allow each stage to build on the enrichment achieved by the ones before it and
also to more efficiently make use of the depleted uranium stream. For a sense
of scale, when it was first constructed in the early 1940s the gaseous
diffusion plant at Oak Ridge, Tennessee, was the largest industrial building in
the world. The facility at Oak Ridge is shown in Figure 2 while a picture of
two of the diffusers used in the enrichment process is shown in Figure 3.

Figure
2: Oak Ridge gaseous diffusion plant, built during World War II.

At
the time of its construction this was the largest industrial building in the
world. In part it was decided to locate this plant in Tennessee so that its
large electricity demand could be met by the abundant coal and hydroelectric
plants built by the government run Tennessee Valley Authority. It is now closed
and awaiting decommissioning.

Figure
3: A close up picture of the outside of two of the diffuser stages used at the
Oak Ridge uranium enrichment plant.

The
diffusers contain the porous barriers used to separate the lighter U-235 atoms
from the heavier U-238 atoms. Connected to the diffusers is equipment to compress
the uranium hexafluoride gas and pipe it through the cascade as well as
equipment to remove the large amount of heat generated during the enrichment
process. Each diffuser and compressor are together referred to as a “stage.”

The
most challenging step in building a gas diffusion plant is to manufacture the
permeable barriers required in the diffusers. The material for the barriers
needs to be highly durable and able to maintain a consistent pore diameter for
several years of operation. This is particularly challenging given the highly
corrosive nature of the uranium hexafluoride gas used. Typical barriers are
just 5 millimeters (less than 0.2 inches) thick and have openings that are only
about 30 to 300 times the diameter of a single uranium atom.

In
addition to requiring a large amount of electricity during operation, the
compressors in the gas diffusion facilities also generate a great deal of heat
that requires dissipation. In U.S. plants this heat is dissipated through the
use of ozone depleting chlorofluorocarbons (CFCs) such as the coolant CFC-114
(often referred to simply as Freon of Freon-114). The manufacture, import, and
use of CFCs were substantially restricted by the 1987 Montreal Protocol on
Substances That Deplete the Ozone Layer, which the U.S. is implementing through
the 1990 Amendments to the Clean Air Act. As a result of these commitments, the
manufacture of Freon in the U.S. ended in 1995 and its emissions to the air in
the United States from large users fell by nearly 60% between 1991 and 2002.
The emissions from the Paducah gaseous diffusion plant, however, have
remained virtually constant over this time, falling just over 7% between 1989
and 2002. In 2002, the Paducah enrichment plant emitted more than
197.3 metric tons of Freon into the air through leaking pipes and other
equipment. This single facility accounted for more than 55% of all airborne
releases of this ozone depleting CFC from all large users in the entire United
States in 2002. Due to the lack of additional manufacturing of Freon
since 1995, the U.S. Enrichment Corporation is currently looking for a non-CFC
coolant to use. Likely candidates would still have heat trapping potential, and
thus even if they were not as dangerous to the ozone layer, they would still
remain a potential concern in relation to global warming and climate change.

The
high heat signature of gaseous diffusion plants makes it possible that plants
operating significantly in excess of 100 MTSWU per year could be detected.
However, this information would likely only be meaningful as a way of
identifying operations at known plants and not for uncovering clandestine
facilities since there are many industrial processes that generate a great deal
of heat. Thus, while gaseous diffusion plants are perhaps one of the hardest
types of uranium enrichment facility to hide given their size, electricity
needs, and heat signature, it would still be difficult to remotely identify a
facility without access to environmental samples from the surrounding area.

2.2
Gas Centrifuge

Gas
centrifuges are the most commonly used technology today for enriching uranium.
The technology was considered in the U.S. during the Manhattan Project, but
gaseous diffusion and electromagnetic separation were pursued instead for full
scale production. The centrifuge was later developed in Russia by a team lead
by Austrian and German scientists captured during the Second World War. The
head of the experimentation group in Russia was eventually released and took
the centrifuge technology first to the United States and then to Europe where
he sought to develop its use in enriching commercial nuclear fuel.

The
centrifuge is a common technology used routinely in a variety of applications
such as separating blood plasma from the heavier red blood cells. In the
enrichment process, uranium hexafluoride gas is fed into rapidly spinning
cylinders. In order to achieve as much enrichment in each stage as possible,
modern centrifuges can rotate at speeds approaching the speed of sound. It is
this feature that makes the centrifuge process difficult to master, since the
high rate of revolution requires that the centrifuge be sturdy, nearly
perfectly balanced, and capable of operating in such a state for many years
without maintenance. Inside the rotating centrifuge, the heavier molecules
containing U-238 atoms move preferentially towards the outside of the cylinder,
while the lighter molecules containing U-235 remain closer to the central axis.
The gas in this cylinder is then made to circulate bottom to top driving the
depleted uranium near the outer wall towards the top while the gas enriched in
U-235 near the center is driven towards the bottom. These two streams (one
enriched and one depleted) can then be extracted from the centrifuge and fed to
adjoining stages to form a cascade just as was done with the diffusers in the
gas diffusion plants. A schematic diagram of such a centrifuge is shown in
Figure 4 below.

Figure
4: A schematic diagram of the cross section of a single gas centrifuge.

The
rotating cylinder forces the heavier U-238 atoms towards the outside of the
centrifuge while leaving the lighter U-235 more towards the middle. A bottom to
top current allows the enriched and depleted streams to be separated and sent
via pipes to subsequent stages. Like the gas diffusion
process, it requires thousands to tens of thousands of centrifuge stages to
enrich commercially or militarily significant quantities of uranium. In
addition, like the gas diffusion plants, centrifuge plants require the use of
special materials to prevent corrosion by the uranium hexafluoride, which can
react with moisture to form a gas of highly corrosive hydrofluoric acid. One of
the most important advantages to the gas centrifuge over the gas diffusion
process, however, is that it requires 40 to 50 times less energy to achieve the
same level of enrichment. The use of centrifuges also reduces the amount of waste
heat generated in compressing the gaseous UF₆, and thus reduces the
amount of coolants, such as Freon, that would be required. A bank of
centrifuges from an enrichment plant in use in Europe is shown in Figure 5.

Figure
5: A section of a typical cascade of centrifuge stages in a European uranium
enrichment plant. The operative power of each centrifuge increases with the
speed of revolution as well as with the height of the centrifuge while in a
cascade each centrifuge also builds on the enrichment achieved in the previous
stages.

Despite
having a larger operative power in each stage compared to the gaseous diffusion
process, the amount of uranium that can pass through each centrifuge stage in a
given time is typically much smaller. Typical modern centrifuges can achieve
approximately 2 to 4 SWU annually, and therefore in order to enrich enough HEU
in one year to manufacture a nuclear weapon like that dropped on Hiroshima
would require between three and seven thousand centrifuges. Such a facility
would consume 580 to 816 thousand kWh of electricity, which could be supplied
by less than a 100 kilowatt power plant. The use of modern weapon designs would
reduce those numbers to just one to three thousand stages and 193 to 340
thousand kWh. More advanced centrifuge designs are expected to achieve up to
ten times the enrichment per stage as current models which would further cut
down on the number necessary for the clandestine production of HEU. The
reported sale of older European based centrifuge technology to countries like
Libya, Iran, and North Korea from the network run by A.Q. Khan, the former head
of the Pakistani nuclear weapons program, highlights the concerns over the
smaller size and power needs of the centrifuge enrichment process from a
proliferation standpoint.

2.3
Electromagnetic Isotope Separation (EMIS)

The
electromagnetic separation technique is a third type of uranium enrichment
process that has been used in the past on a large scale. Developed during the
Manhattan Project at Oak Ridge, Tennessee, the electromagnetic separation plant
was used to both enrich natural uranium as well as to further enrich uranium
that had been initially processed through the gaseous diffusion plant, which
was also located at the Oak Ridge facility. The use of this type of facility,
shown in Figure 6, was discontinued shortly after the war because it was found
to be very expensive and inefficient to operate. Iraq did pursue this technique
in the 1980s as part of their effort to produce HEU, because of its relative
simplicity in construction, but they were only successful in producing small
amounts of medium enriched uranium (just above 20 percent).

Figure
6: The electromagnetic separations plant built at Oak Ridge, Tennessee during
the Manhattan Project. These devices, also referred to as cauldrons, were used
in enriching a part of the uranium for the bomb that was dropped by the United
States on Hiroshima.

The
electromagnetic separations process is based on the fact that a charged
particle moving in a magnetic field will follow a curved path with the radius
of that path dependent on the mass of the particle. The heavier particles will
follow a wider circle than lighter ones assuming they have the same charge and
are traveling at the same speed. In the enrichment process, uranium
tetrachloride is ionized into a uranium plasma (i.e. the solid Ucl₄ is
heated to form a gas and then bombarded with electrons to produce free atoms of
uranium that have lost an electron and are thus positively charged). The
uranium ions are then accelerated and passed through a strong magnetic field.
After traveling along half of a circle (see Figure 6) the beam is split into a
region nearer the outside wall which is depleted and a region nearer the inside
wall which is enriched in U-235. The large amounts of energy required in
maintaining the strong magnetic fields as well as the low recovery rates of the
uranium feed material and slower more inconvenient facility operation make this
an unlikely choice for large scale enrichment plants, particularly in light of
the highly developed gas centrifuge designs that are employed today.

2.4
Jet Nozzle / Aerodynamic Separation

The
final type of uranium enrichment process that has been used on a large scale is
aerodynamic separation. This technology was developed first in Germany and
employed by the apartheid South African government in a facility which was
supposedly built to supply low enriched uranium to their commercial nuclear
power plants as well as some quantity of highly enriched uranium for a research
reactor. In reality, the enrichment plant also supplied an estimated 400 kg of
uranium enriched to greater than 80% for military use. In early
1990, President de Klerk ordered the end of all military nuclear activities and
the destruction of all existing bombs. This was completed roughly a year and a
half later, just after South Africa joined the NPT regime and just before
submitting to inspections and safeguards by the International Atomic Energy
Agency.

The
aerodynamic isotope separation (which includes the jet nozzle and helicon
processes) achieves enrichment in a manner similar to that employed with gas centrifuges
in the sense that gas is forced along a curved path which moves the heavier
molecules containing U-238 towards the outer wall while the lighter molecules
remain closer to the inside track. In the jet nozzle plants, uranium
hexafluoride gas is pressurized with either helium or hydrogen gas in order to
increase the velocity of the gas stream and the mixture is then sent through a
large number of small circular pipes which separate the inner enriched stream
from the outer depleted stream. This process is one of the least economical
enrichment techniques of those that have been pursued, given the technical
difficulties in manufacturing the separation nozzles and the large energy
requirements to compress the UF₆ and carrier gas mixture. As with
gaseous diffusion plants, there is a large amount of heat generated during
operation of an aerodynamic separations plant which requires large amounts of
coolants such as Freon.

2.5
Other Technologies

There
are a number of other uranium enrichment technologies such as atomic vapor
laser isotope separation (AVLIS), molecular laser isotope separation (MLIS),
chemical reaction by isotope selective laser activation (CRISLA), and chemical
and ion exchange enrichment that have been developed as well, but they are mostly
still in the experimental or demonstration stage and have not yet been used to
enrich commercial or military quantities of uranium. The AVLIS, CRISLA, and
MLIS processes make use of the slight difference in atomic properties of U-235
and U-238 to allow powerful lasers to preferentially excite or ionize one
isotope over the other. AVLIS makes use of uranium metal as a feed material and
electric fields to separate the positively charged U-235 ions from the neutral
U-238 atoms. MLIS and CRISLA on the other hand use uranium hexafluoride mixed
with other process gases as a feed material and use two different lasers to
excite and then chemically alter the uranium hexafluoride molecules containing
U-235, which can then be separated from those molecules containing U-238 that
remained unaffected by the lasers. AVLIS was pursued for commercial use by the
U.S. Enrichment Corporation, but was abandoned in the late 1990s as being
unprofitable while other countries have also abandoned all known AVLIS and MLIS
production programs as well. The chemical and ion exchange enrichment processes
were developed by the French and the Japanese. These techniques make use of the
very slight differences in the reaction chemistry of the U-235 and U-238 atoms.
Through the use of appropriate solvents, the uranium can be separated into an
enriched section (contained in one solvent stream) and a depleted stream
(contained in a different solvent that does not mix with the first in the same
way that oil and water do not mix). This enrichment technique was also pursued
by Iraq. Currently all known programs involving this technique have been closed
since at least the early 1990s. All of these technologies have been
demonstrated on the small scale and some, like AVLIS, have gone further along
in the development process that would be necessary to scale up to production
level facilities. This would be particularly true if the profitability of the
plant was not an issue and it was only meant to enrich the reasonably modest
quantities of HEU necessary for one to two bombs per year. Currently, however,
the gas centrifuge appears to be the primary technology of choice for both
future commercial enrichment as well as for potential nuclear weapons
proliferation.

Reference
List

1.David
Albright, Frans Berkhout and William Walker. “Plutonium and Highly Enriched
Uranium 1996”. Stockholm, 1997.

2.Laughter,
Mark D. (2007) “Profile of World Uranium Enrichment Programs – 2007”. ORNL/TM –
2007/193

3.David
Albright “Irag’s Programs toMake Highly Enriched Uranium and Plutonium for
Nuclear Weapons Prior to the Gulf War”, 2002

4.Nuclear
Engineering International. 2004 World Nuclear Industry Handbook. Wilmington
Pub. Co., 2004