Section:
feature article
Uranium enrichment by centrifugation is the basis for the quick and efficient production of nuclear fuel-or nuclear weapons.
The most difficult step in building a nuclear
weapon is the production of fissile material. One can either make
plutonium-239 in a nuclear reactor or enrich uranium to increase the
abundance of its fissile isotope uranium-235. Historically, enrichment
has been the more obscure of the two routes, but the recent spread of
one technology--the gas centrifuge-from the Netherlands
to Pakistan and on to Libya, Iran, and North Korea has brought
enrichment to the forefront of proliferation. That development is
challenging old ideas about how to ensure the peaceful use of nuclear
technology and prevent the further spread of nuclear weapons.
The gas centrifuge is
particularly well suited for acquiring a first nuclear weapon. It is
also the most economically efficient way to enrich uranium for peaceful
power-reactor fuel, and therefore essentially impossible to abandon and
difficult to control by political means. Combined, those aspects lead to
a new kind of control problem that has not been experienced with other
technologies. This article outlines the problem, showing how technical
aspects affect policy options, and discusses some of the solutions
currently under consideration.
The gas centrifuge
The gas centrifuge works much like a classic centrifuge: It is a hollow cylindrical tube that is spun at very high speeds about its axis.( n1)
The centrifugal force is able to separate chemically identical isotopes
because of the variation in isotopic weight. For the separation of
uranium isotopes, the gas fed into the centrifuge is
uranium hexafluoride. Figure 1a shows the feed stream and two withdrawal
streams: the product stream enriched in the desired isotope, 235U, and the tails or waste stream depleted in 235U. Technical details are presented in box 1.
Centrifuges have been
fabricated from a variety of materials, with varying lengths, diameters,
and operating speeds. Figure 1b shows the relative lengths of a number
of centrifuges. The first and second Pakistani centrifuges
were based on early designs by Urenco Ltd, a consortium involving the
UK, Germany, and the Netherlands. Also shown are the more modem Urenco centrifuges. The largest centrifuge was developed in the US and is now called the American Centrifuge. Centrifuges of that design will be used by USEC Inc at a plant in Ohio.
Figure 1. (a) Schematic of the flows into and out of a gas centrifuge
plus the countercurrent flow in the separation chamber. (Adapted from
Sci. Am., August 1978, p. 39.)(b) Comparison of lengths of the first two
Pakistani centrifuges P1 and P2, the range of modern Urenco centrifuges, and the current US centrifuge design. (c) Arrangement of gas centrifuges in a 15-stage ideal cascade.
A single centrifuge cannot simultaneously produce useful enrichment levels and product flow rates. To achieve those, centrifuges are connected in cascades. By connecting the centrifuges
in series, the enrichment level is increased, and by connecting in
parallel, the product flow rate is increased. A cascade schematic is
presented in figure 1c. Each row represents a stage, and the number of
stages in the cascade is determined by the performance of each centrifuge and by the desired enrichment level.
From Charlottesville to Natanz
The separation of isotopes by gas
centrifugation was first suggested by Frederick Lindemann and Francis
Aston in 1919, immediately after the existence of isotopes had been
experimentally confirmed.( n2)
Robert Mulliken in the US, William Harkins in Germany, and Sydney
Chapman in the UK tried unsuccessful experiments for more than a decade.
It wasn't until 1934 that Jesse Beams of the University of Virginia
first reported the successful separation by centrifuge with the isotopes of chlorine. His insight was to place the centrifuge
rotor in a vacuum to thermally isolate it from the environment and
thereby minimize the convective mixing that had foiled earlier attempts.
Figure 2 shows a timeline of the centrifuge's history.
Figure 2. Timeline of centrifuge-related events.
During World War II, Beams and others at the
University of Virginia became involved with the Manhattan Project with
the goal of producing enriched uranium for a nuclear weapon. However,
the technology was not successful during that time because mechanically
reliable ultrahigh-speed bearings had not been perfected. Nonetheless,
development of the gas centrifuge continued after the
war, especially in the USSR, where Austrian prisoner of war Gernot Zippe
introduced a reliable pivot-magnetic bearing combination. In the summer
of 1956, Zippe was released and intercepted by US intelligence agents.
Ultimately, he was persuaded to come to the University of Virginia and
repeat what he had done in the USSR. That work led to a new generation
of advanced centrifuges in the US and the Urenco states. Over time, gas-centrifuge
enrichment plants were built in each of those countries, and eventually
the process became the workhorse of the international enrichment
industry. Today, centrifuges are the primary method of
uranium enrichment, and they will soon replace the two surviving plants
based on the older gaseous-diffusion technology, located in the US and
France.
In 1974 India exploded a nuclear device, which
it called a peaceful explosion. That event spurred the development of
Pakistan's nuclear weapons program. It also incited Pakistani
metallurgist Abdul Qadeer Khan, who was working for Urenco in the
Netherlands, to assist Pakistan by making copies of blueprints for centrifuge
designs. He later returned to Pakistan, where he used the design
information and his contacts in Europe to build an enrichment plant to
produce the fuel for Pakistan's first nuclear bomb.
Once Pakistan had demonstrated that a developing country could make fissile material for nuclear weapons with centrifuges, others followed. In the summer of 1987, Iraq initiated its own covert centrifuge
program. It floundered at first, but with the help of several
disaffected German engineers, Iraq managed to build a modified version
of an old Urenco design and test it in the days just prior to the
invasion in January 1991.
In parallel to Iraq's effort, Khan began to sell old Pakistani centrifuge
parts and blueprints on the black market. Fearing a sting operation,
Iraq declined Khan's offer, but Iran and Libya decided to buy. In 2002
there were reports that North Korea had also been in contact with Khan
and was developing a gas centrifuge of its own. As of
July 2008, traces of highly enriched uranium were reportedly found on
North Korean documents, but no evidence of an enrichment plant has
emerged. The UK and US were successful in convincing Muammar Qaddafi to
dismantle Libya's program, and most of the equipment was shipped to the
US. Iran, however, has continued with its centrifuge
program, including the recent installation of machines in an underground
facility at Natanz. Iran insists that its program is peaceful and has
defied international appeals to suspend the program and fully open it to
inspection.
The Iran story
The controversy sparked by Iran's nuclear
program has done more than any other event in the 60-year history of
nuclear nonproliferation to underscore the challenges related to centrifuge
proliferation. Iran's program was first revealed by non-government
sources in August 2002, and Iran confirmed in February 2003 that it was
constructing two centrifuge plants. By that time the
program had been secretly under way for more than 15 years, according to
information provided by Iran to the International Atomic Energy Agency,
with the first centrifuge blueprints and components received from a foreign source in 1987.( n3)
Eventually, investigations revealed that source to be Khan and his
network of suppliers. (The IAEA Board of Governors has released more
than 20 reports on the status of the Iranian nuclear program since June
2003).
In the months following the revelation of the
Natanz site, the IAEA carried out several inspections there and made
some surprising discoveries. Iran had not declared past enrichment
experiments--activities that they were required to report to the IAEA.
The agency also found documents related to the production of nuclear
weapons and traces of highly enriched uranium, which suggested a foreign
origin for some of the equipment.
Initially, a resolution looked workable. In
November 2003 Iran suspended its enrichment program after it
acknowledged that it had indeed carried out "a limited number of tests,
using small amounts of UF6" in the years 1999 and 2002.
Shortly thereafter, Iran also signed (but did not ratify) the Additional
Protocol and voluntarily complied with its terms, which give the IAEA
broader access to Iran's facilities.
Diplomatic efforts pursued during the
suspension period included attempts to persuade Iran to abandon its
program in return for an incentive package that included fuel supply
assurances and reactor technology. Eventually, all those efforts
collapsed. Iran resumed centrifuge production in June
2004 and enrichment activities in January 2006. In the meantime, Mahmoud
Ahmadinejad was elected president, and the centrifuge
program became a platform for winning domestic political support. In
April 2006 Iran began testing the first complete 164-machine cascade,
shown in figure 3, and reported the successful production of minute
quantities of low-enriched uranium.
Figure
3. The first publicly released picture of the 164-machine cascade
installed at Iran's aboveground Pilot Fuel Enrichment Plant. The photo
was used in a presentation by Mohammad Saeidi of the Atomic Energy
Organization of Iran in September 2005.
By then, the IAEA board of governors had
referred Iran's case to the United Nations Security Council, which
passed Resolution 1696 in July 2006, demanding "that Iran shall suspend
all enrichment-related … activities, including research and development"
to build confidence in the exclusively peaceful purpose of its nuclear
program. Iran continues to defy those resolutions, and it appears
increasingly unlikely that the country will roll back its enrichment
project any time soon, given the project's broad domestic support.
Various international efforts are being made to accommodate the Iranian
technology in a multinational enrichment plant, supplemented by
arrangements and features that would make military use of such a
facility more difficult. See box 2 for more about Iran's program.
A new kind of challenge
Since early in the nuclear age, the IAEA has
been charged with safeguarding nuclear technology to ensure that it is
not used for the production of nuclear weapons. The operating premise of
those safeguards is deterrence through timely detection. Thus it is not
the role of safeguards to prevent proliferation. Rather, safeguards are
meant to detect nonpeaceful activities sufficiently early that they can
be stopped by political intervention. The centrifuge,
however, has properties that make timely detection difficult. One of
those properties is the speed with which any peaceful-use plant can be
converted to nonpeaceful purposes. That so-called rapid breakout enables
the proliferating country to produce nuclear weapons before there is
time for a political response and thus renders safeguards largely
ineffective. A second problem is the potential for clandestine plants.
Compared with nuclear reactors and large gaseous-diffusion plants, a centrifuge
plant uses little electricity and produces little detectable signal, so
it is much easier to hide the plant and evade safeguards altogether.
The rapid-breakout problem. The inventory of UF6 in a centrifuge is limited by the condensation pressure at the wall; the UF6
must remain in gas form, or the rotor will become unbalanced and crash.
For normal operating temperatures, the maximum pressure is on the order
of 0.001 atmosphere, and the corresponding gas inventory is only a few
grams. Typical throughput is on the order of milligrams per second, so
an individual machine (or cascade stage) can be flushed of its UF6 inventory in less than an hour.
In addition, centrifuges
typically achieve separation factors (defined in box 1) of 1.2 to 1.5.
That is high compared with the earlier gaseous-diffusion process, which
is characterized by a separation factor of no more than 1.004. Because
of the larger separation factor, a plant based on centrifuges requires fewer total stages to achieve a given level of enrichment. Even for a first-generation centrifuge,
the gas needs only to pass through a series of 30-40 stages to reach
the high enrichment levels used in nuclear weapons. The combination of
few total, stages with the short equilibrium time per stage means the
overall cascade equilibrium time is also small. Thus a cascade designed
to produce low-enriched uranium for fuel can be re-fed its low-enriched
product and begin converting it to highly enriched uranium suitable for
weapons use in a matter of days--a procedure called batch recycling.
Alternatively, the machines can be reconfigured into a narrower but
longer cascade with more stages, a process that requires additional time
before production of highly enriched uranium can begin but is more
efficient than batch recycling. If the available enrichment capacity is
sufficient, the options give a country the ability to produce weapon
quantities of material before there is time to respond politically. For
an example of a breakout scenario based on Iran's current technology,
see box 3.
The clandestine problem. A country could try to build a clandestine plant in the hope of escaping detection altogether. A clandestine centrifuge plant could be difficult to detect. Centrifuges
can be placed in buildings indistinguishable in appearance from other
industrial facilities. A typical plant uses about 160 W/m², comparable
to an average food services facility; that low consumption makes the centrifuge
plant impossible to detect by IR imaging. (In contrast, the older
gaseous-diffusion plants, which use hundreds or thousands of large
compressors, require 10 000 W/m².) Furthermore, most of the pipes in a centrifuge
plant operate below atmospheric pressure, so little of the process gas
leaks into the atmosphere. Those effluents provide a method of detecting
centrifuge plants, but their exceedingly low level
makes detection impossible at distances of more than a few kilometers,
so it is impractical to detect a covert plant whose location is not
already known.
Problems of control
The inability of safeguards to adequately deal with centrifuge
plants went largely unnoticed when the technology was held exclusively
by states that already possessed nuclear weapons and by their close
allies. Today, increasing numbers of states possess centrifuges,
including states that are not supporters of the nonproliferation regime
and might willingly transfer the technology to like-minded nations. In
addition to concerns about state-to-state transfer, residual
black-market elements are left over from the Khan network, and qualified
technical people are available for hire. UN Resolution 1540 has been
important in addressing some of those latter problems by requiring
states to put in place stringent export controls and to criminalize
private-party proliferation, but the solutions are neither perfect nor
easily implemented, especially in resource-starved nations.
Some have argued that if controlling the
technology per se is not possible, then it might be possible to set
rules on who can own centrifuges and when. The problems
with that strategy are twofold. First, peaceful-use nuclear energy
provides a legitimate reason to possess centrifuges. States with reactors, or even plans for reactors, can argue that they need to build a national centrifuge enrichment plant to ensure the uninterrupted supply of nuclear fuel for those facilities. Yet a centrifuge
plant built to fuel just one commercial-sized reactor is adequate to
produce highly enriched uranium for dozens of nuclear weapons per year.
Efforts have been made to counter the
energy-security argument by pointing out that it is often cheaper to
purchase enrichment services on the international market than to build a
national plant at home. Although that is technically true, the economic
penalty is not severe. Even if the cost of national enrichment were
triple the market price, it results in less than a 10% increase in the
final cost of nuclear power--a small insurance premium for energy
security. Others have argued that existing market mechanisms have yet to
fail. However, the past shows mainly that the market works when the
enriching and client states are friends; we have yet to see a state
supplying nuclear fuel to one of its enemies. Still others have proposed
various kinds of internationalized fuel-supply assurances.
Paradoxically, those proposals have not received much traction, because
most countries are satisfied with their existing arrangements--and it is
difficult to create a new international system without their support.
The second major problem with attempts to set rules limiting the acquisition of centrifuge
plants is that many states have grown weary of giving up sovereign
rights in the name of nonproliferation. The current nonproliferation
regime was based on a bargain between the nuclear haves and have-nots:
Those without weapons would forgo the right to possess them and subject
themselves to perpetual inspections in exchange for assistance with
peaceful-use nuclear technology and eventual disarmament by the nuclear
weapons states. So far, none of the original nuclear weapons states has
disarmed, "cooperative assistance has been less than forthcoming, and
nuclear energy has not been the panacea it was once thought to be. As a
result, many states oppose nuclear weapons but also oppose what they see
as an inherently unfair nuclear control regime. Some states have even
cast their acquisition of centrifuge technology as a political protest against efforts to cement a permanent state of inequity among nations.
Other incentives to acquire centrifuge technology are also increasing. Because of the Iranian nuclear program and the international attention it attracted, centrifuges
are now seen as a mark of power and prestige in the Middle East.
Although in reality it may be more technically impressive to build any
number of other peaceful-use technologies, the connection to nuclear
weapons, combined with the efforts to prevent the acquisition of the
technology, has rendered the centrifuge a symbol of
power. Governments like those of Pakistan and Iran have successfully
parlayed that symbolism into widespread domestic support for their centrifuge
programs and brought considerable resistance to international efforts
to place those programs into abeyance. What is more worrisome is that
their enthusiasm might be contagious. It is perhaps not mere coincidence
that many Persian Gulf states announced their interest in nuclear power
shortly after Iran's centrifuge program became popular.
Looking ahead
As we have seen, safeguards cannot prevent proliferation, especially in the case of centrifuges.
However, safeguards can be extended to nuclear materials so as to make
the breakout and clandestine loopholes less attractive. In a breakout
scenario, speed is the critical factor, and breakout can be made about
three times faster if the state uses preenriched UF6 instead
of natural uranium to feed its cascades. Thus it would be sensible to
require that all enriched uranium be stored offsite and in a chemical
form, such as uranium oxide, that is not suitable for direct
reenrichment. That requirement would minimize the amount of low-enriched
uranium that can be readily fed back into the centrifuge
cascade, extend the breakout timeline, and allow more time for
political intervention. However, the solution works only for small-scale
facilities; large facilities could enrich uranium fast enough to break
out using unenriched uranium feed.
Safeguards might also address the covert-facility problem by safeguarding flows of unenriched UF6, starting at the facilities where the UF6 is produced. Traditionally, that material has received relatively little attention. Monitoring unenriched UF6
more carefully can make its diversion to a covert plant more difficult.
Thus, although direct detection 'of covert plants may not be possible,
safeguards can make it more difficult to operate those plants with
undeclared feed.
With material controls helping to close the loopholes, the application of safeguards to the overall centrifuge
complex becomes important again, with a focus especially on uranium
flows in the plant. Existing safeguards do not adequately address many
of the strategies for centrifuge misuse. Upgrades
directed toward better monitoring of enrichment levels and flows are
needed both in and around the plant. New technologies, such as RF
identification tags, can automate and facilitate the tracking of UF6
containers. Online monitors can report throughput and enrichment levels
in real time. It is important that any new measures be put in place
quickly because several large-scale facilities are under construction or
planned for Iran, Brazil, France, Russia, and the US; it will be far
more difficult to retrofit those plants later, given the delicate nature
of centrifuges and their propensity for failure during
spin-up and spin-down. Those facilities are likely to set a de facto
standard for new plants in other countries, so there is now a unique
opportunity to define a new baseline for best practices and safeguards
by design.
Safeguards will not, however, be a complete
solution. Breakout is still possible with a plant of sufficient size,
and covert plants are possible, especially when combined with covert UF6 production. Owing to the lack of good technical solutions, the centrifuge challenge might be better addressed in the political domain, with arrangements to limit the number of states owning centrifuges or to raise the barriers to using them for weapons purposes.
One proposal now receiving increasing support
is the criteria-based approach, which aims to set minimally politicized
criteria for the acquisition of a national enrichment capability.
Proposed criteria have included the acceptance of certain voluntary
safeguard measures, a minimum infrastructure requirement to justify
domestic enrichment, and a requirement that the installation of the centrifuge
plant not be regionally destabilizing. There may yet be hope for the
international fuel-supply assurances discussed earlier, including
multinational ownership of facilities, but that depends on whether
nations can develop a fair fuel-supply framework that is robust enough
to persuade existing nuclear states to give up their right to operate
national enrichment plants.
Barring solutions, the problem is likely to
grow, especially if there is an expansion in the total number of
countries using nuclear energy, which might--or might not--happen in the
coming decades. And even if proposed technological and institutional
fixes are put in place, they cannot entirely solve the problem;
incentives to acquire centrifuge enrichment as a
nuclear weapons hedge will remain. Solutions to those problems must
involve a country's national security-not just its energy security.
Box 1. How the gas centrifuge works
The gas in the Centrifuge
settles into a dynamic equilibrium, balancing the centrifugal force that
presses the gas against the wall of the rotor and the diffusive force
that seeks to distribute the gas equally throughout the volume of the
rotor. For a binary mixture and no internal flow, the resulting
distribution holds independently for each species. An equilibrium
separation factor α0 representing the difference in the concentrations of the species at the wall of the rotor is given by
( 1) α0 = exp[(M2- M1)va, sup 2[/2RT,
where Va is the peripheral speed of the rotor, M1 and M2
are the molecular weights of the two species, R is the universal gas
constant, and T is the gas temperature. Normally, a c0untercurrent flow
is established as depicted in figure 1, and that convective flow carries
the lighter isotope to the top of the centrifuge and
the heavier isotope to the bottom. That results in an axial separation
factor that tends to be much larger than the radial separation factor
given by equation 1. The overall separation factor for the centrifuge is defined as
( 2) α = X]sub p]/1 - Xp/ xW/1 - xW,
where xp and xw are the concentrations of uranium-235 in the product and waste streams, respectively.
The performance of a gas centrifuge
is measured in separative work units per Unit time, which has units of
kgU/yr. The separative work ΔU is not a measure of energy, but it is
nonetheless a measure of the effort expended by the centrifuge. A function of flows into and out of the centrifuge and the concentrations of the streams, it is calculated by the formula
( 3) ΔU = PV(xp) + WV(xW) - FV(xF),
where P, W, and F are product, waste, and feed mass flows, respectively, xF is the concentration of 235U in the feed, and V(x) is the value function derived by Paul Dirac and is given by
( 4) V(x) = (2x- 1)In[x/( 1-x)].
The expression for the maximum theoretical performance of a gas centrifuge was also derived by Dirac and given by
( 5) ΔU(max) = π/2 L ρD (ΔM va, sup 2/2RT)².
Dirac's work was published as part of a book by Karl Cohen.( n4)
In equation 5, L is the length of the centrifuge,
ρD is the binary diffusion coefficient, and ΔM is the difference in
molecular weights. The actual, or achievable, performance has some
efficiency factors related to the shape of the flow profile and the
strength of the countercurrent flow. Equation 5 shows that the
performance has a fourth-power dependence on the peripheral speed of the
rotor Va and is directly proportional to the length. In practice, the dependence on speed is closer to Va, sup 2, but that is still a strong dependence and emphasizes the importance of rotor speed.
Controlling the countercurrent flow optimizes both the flow profile efficiency and the separative work produced by a single gas centrifuge.
Solving the fluid-dynamics equations of motion allows the flow pattern
to be optimized, That has been done by directly solving the equations
numerically and by obtaining exact solutions. In the US program, a
theory group led by Lars Onsager addressed the problem in the 1960s.
Onsager used a minimum principle to obtain a single sixth-order partial
differential equation, which he solved by eigen-function methods. An
analysis of the mathematical details can be found in reference 5.
Making long centrifuges spin
at high speeds requires consideration of the materials of construction
and dynamics of the rotor. To first order, the maximum peripheral speed
is given by Va = √ σ/ρ where σ is the tensile strength and ρ
is the density of the rotor. There is thus a need for strong,
lightweight materials.
Long rotors spinning at high speeds have natural bending frequencies, which should not coincide with the operating frequency. A centrifuge operating above the lowest bending frequency is called a supercritical centrifuge,
otherwise subcritical. One way to traverse the resonant speeds is to
connect a number of shorter rotor segments together with flexible
bellows, which provide damping to help the rotor accelerate past the
resonances.
Box 2. Is Iran pursuing a nuclear weapon?
Many questions relating to the scope and
nature of Iran's nuclear program have been addressed over the past few
years. However, as of May 2008, the International Atomic Energy Agency
remains unable to certify that Iran's program is for entirely peaceful
purposes. From other states, the IAEA obtained evidence that points to
weaponization efforts: alleged studies on converting uranium to UF4
(a precursor of uranium metal), testing of special firing equipment and
detonators used in nuclear weapons, and the design of a special missile
reentry vehicle suitable for nuclear warheads. Iran maintains that
those allegations are baseless and all related documents fabricated.
In addition, early in the investigation, a
15-page document, was found in Iran describing the process of converting
uranium into metal form and machining it into hemispheres, a step
related to the production of weapons. Iran has reiterated that it
obtained the document through the Abdul Qadeer Khan network in 1987
along with centrifuge documentation, but that it had
not requested that information. To date, the IAEA still seeks to confirm
with contacts in Pakistan the circumstances of the delivery of that
document.
All the documents suggesting weapons-related
activities date to before the year 2004. That is consistent with the
November 2007 US National Intelligence Estimate, which judged with high
confidence that in fall 2003, Teheran halted its nuclear weapons
program," primarily in response to international pressure. Iran
maintains that it never pursued a nuclear weapons option or program.
Even if Iran has terminated specific weapons-related activities for the time being, the remaining centrifuge
plant represents the most significant step in acquiring weapons; it can
be readily converted to weapons purposes and the other details worked
out quickly. It is that fact, combined with the lack of transparency,
past infractions, and the possible sublimated interest in nuclear
weapons, that continues to fuel tensions between the West and Iran.
Box 3. How credible and fast is a breakout scenario?
Represented in the figure are two plants, one
with 12 and one with 36 164-machine cascades (1968 and 5904 machines,
respectively), all based on P1-type centrifuges (see
figure l b). Two different strategies can be pursued for breakout:
simple batch recycling, in which product material is fed back into the
original cascades, and cascade interconnection, which involves
reconfiguration of the cascades. In each scenario, the material to be
used for breakout may be either natural uranium (0.72% 235U) or a stock of low-enriched uranium (3.5% 235U). The objective is the production of weapons-grade highly enriched uranium (90% 235U or more).( n6)
Breakout using natural uranium feed is less
credible because most of the required separative work goes into
enriching the uranium to LEU levels-an activity that could plausibly be
carried out under safeguards prior to breakout. However, if it were done
using the full 36-cascade plant, about 40 kg of HEU could be produced
in one year by batch recycling; the process is much more efficient if
about 12 of the 36 cascades, or about 2000 P1 centrifuges,
are reconfigured as dedicated LEU-to-HEU cascades. More than 90 kg/yr
of HEU can be obtained that way, but the reconfiguration requires
replacement of the complex cascade pipework, which could add several
weeks or months up front.
Breakout becomes more credible when
preenriched feedstock is available. Then the 12-cascade plant can
produce 90 kg of HEU per year, and the 36'cascade plant can yield three
times that amount. One concern is that the cascades designed for
LEU-to-HEU production may be located at an undeclared site, which would
avoid the need to reconfigure the safeguarded centrifuge
plant. The covert plant could be contained in a building as small as
500 m² and would be impossible to detect using satellite imagery alone.
With a second covert plant, LEU from the declared facility, still in the
form of uranium hexafluoride, could be transferred to the undeclared
site, and HEU production could commence without further delay. There is
still a risk of detection if the diverted LEU is subject to safeguards.
However, existing safeguards might be unable to detect the production of
excess LEU via certain covert arrangements, and that excess could serve
as an unsafeguarded source of LEU for an undeclared facility.
Twelve
164-machine cascades can produce 90 kg/yr or more of HEU when supplied
with low-enriched feed by the remaining 24-cascasdes
(n1.) S. Villani, ed., Uranium Enrichment, Springer, New York (1979).
(n2.) R. S. Kemp, Sci. Global Sec. (in press).
(n3.)
International Atomic Energy Agency Director General, Implementation of
the NPT Safeguards Agreement in the Islamic Republic of Iran,
GOV/2004/83, IAEA Board of Governors, Vienna, Austria (15 November
2004).
(n4.) K. Cohen, The Theory of Isotope Separation as Applied to the LargeScale Production of U235, McGraw-Hill, New York (1951).
(n5.) H. G. Wood, J. B. Morton, J. Fluid Mech. 101, 1 (1980).
(n6.) A. Glaser, Sci. Global Sec. (in press).
~~~~~~~~
By Houston G. Wood; Alexander Glaser and R. Scott Kemp
Houston Wood is a professor of mechanical
and aerospace engineering at the University of Virginia in
Charlottesville. Alexander Glaser is an associate research scholar and
Scott Kemp is a PhD candidate in the program on science and global
security at, Princeton University in Princeton, New Jersey.
Copyright
of Physics Today is the property of American Institute of Physics and
its content may not be copied or emailed to multiple sites or posted to a
listserv without the copyright holder's express written permission.
However, users may print, download, or email articles for individual
use.
