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Nuclear Fuel Cycle with an emphasis on the back-end of the cycle
Introduction
When the first nuclear reactors were built in the 1950s, they were lauded as heralding an
age of cheap and abundant energy resources. But the accidents at Three Mile Island in
Pennsylvania, US, in 1979 and then at Chernobyl, Ukraine, in 1986 were the final straws leading
many countries, including the US, to stop building new nuclear power stations. After years of
stagnation, energy planners have now started showing a favor towards “nuclear renaissance”.
The reason for such a re-thinking has emerged from a heightened global crisis. In contrast to
electricity generated from fossil based fuels, nuclear power does not contribute to greenhouse gas
emissions. Nuclear reactors currently generate around 17% of the world’s electricity, and
renewable energy technologies, such as solar power and wind farms, are simply not yet ready to
take their place. Although the nuclear industry has worked hard to improve the safety of its
reactors, unfortunately, the more intractable problem of how to deal with nuclear waste,
particularly the highly radioactive spent fuel, still remains.
In a typical nuclear reactor, the fuel is made to undergo fission by a beam of neutrons.
The neutrons, upon collision with uranium, split the former into several radioactive fission
fragments. Such a fission process results in the loss of a tiny fraction of mass of uranium which
gets transformed into radioactive heat energy. This heat, generated in the reactor core, is used to
generate electricity. After certain amount of time, the spent fuel is taken out of the reactor and
fresh fuel is loaded in its place.
Fuel Cycle
The journey of uranium from mine to its temporary storage (after its use in a nuclear
reactor) and/or final reprocessing is known as the Nuclear Fuel Cycle (Fig. 1). This fuel cycle
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can broadly be divided into two components. The steps that include from mining to burning in a
nuclear reactor constitute the front end of the nuclear fuel cycle. The back end of the nuclear fuel
cycle consists of three main unit operations: (i) storage (ii) reprocessing of spent fuel and (ii) safe
disposal of radioactive wastes after proper treatment.
Figure 1- Schematic Diagram of a Typical Nuclear Fuel Cycle
The back end of the nuclear fuel cycle is a strategically important activity due to its
significance both in terms of the sensitivity as well as safety. To examine the back-end stages of
the fuel cycle, it is useful to begin with a brief summary of their current status.
Used fuel storage
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All water-cooled reactors store spent nuclear fuel in an underwater pool (Figs. 2A and B).
The reprocessing of cooled fuel is currently being carried out through only a few selected
programs, and disposal of the bulk spent fuel has not yet taken place. Today, as pools are getting
filled up, spent fuel is increasingly being stored in dry storage facilities which, besides having
lower operational costs, can be implemented in a modular fashion.
Figure 2A- Photograph of the Spent fuel Pool of Reactor Unit 4 of Tokyo Electric Power Co.’s
(TEPCO’s) Fukushima Dai’ichi Nuclear Power Station in Fukushima prior to the 11 March
2011 Earthquake and Tsunami.
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Figure 2B – Storage Pond for Used Fuel at a UK Reprocessing Plant
In the late 1970s and early 1980s, the need for alternative storage in the United States
began to grow when pools at many nuclear reactors gradually got filled up with stored spent fuel.
As there was not a national storage facility in operation, utilities began looking at options for
storing spent fuel. The concept of Dry cask storage for spent nuclear fuel then came into being.
These casks are made of steel cylinders and are either welded or bolt-closed (Fig. 3). Inside the
casks, the fuel rods are surrounded by inert gas. Ideally, the steel cylinder provides leak-tight
containment of the spent fuel. Each cylinder is surrounded by additional steel, concrete, or other
material to provide radiation shielding to workers and general public. Spent fuel is currently
being stored in dry cask systems around the world and one such facility is located at the Idaho
National Laboratory.
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Figure 3 – Photograph Showing Dry Cask Storage Area
Reprocessing
In current reprocessing facilities, used fuel is separated into three components: uranium,
plutonium, and fission fragments. The radioactive waste is then processed to produce vitrified
(with glass) blocks. The uranium, recovered from reprocessing, can be reused as fuel. The
plutonium can either be stored or made directly into mixed oxide (MOX) fuel, in which uranium
and plutonium oxides are combined. The vitrified waste is a high-quality standardized product,
well suited for geological disposal. However, there are problems associated with each output
stream. Plutonium and MOX are unstable in storage because of the buildup of another
radioactive material (americium241). MOX fuel is more expensive than fresh UO2 fuel; its
specific decay heat is around twice that of UO2 fuel; and the neutron dose from MOX is about
eighty times that from UO2 fuel. The vitrified waste has a smaller volume than packaged spent
fuel, but it still requires disposal in a deep geological repository. The strongest argument in favor
of reprocessing is that it saves resources. A further positive aspect is that the highly active
vitrified waste, in contrast to spent fuel, presents no proliferation risk. However, the fact that
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current reprocessing technology involves separation of weapons-grade plutonium has led to
some concerns in many countries.
Disposal
Today, it is widely accepted that the only feasible method to ensure very long term safety
for high-level waste or spent nuclear fuel is its isolation in a stable, deep geological repository. In
fact, such a concept was formally adopted, as a final goal, in many countries, including the
United States, Canada, Sweden, Finland, Belgium, Switzerland, France, Spain, South Korea, the
United Kingdom, and Japan. For example, in the United States, the Waste Isolation Pilot Plant
(WIPP) in New Mexico, as a deep repository for transuranic wastes, has been operating
successfully for last ten years. In Finland, Sweden, and France, deep repository programs are in
very advanced stages. Most other countries are trying to implement, the combined technical and
societal approaches, employed by Sweden and Finland. In 2008, when the U.S. Department of
Energy chose Yucca Mountain as the site for a geological repository, the U.S. program was
perceived as being one of the most advanced. However, with the mid-2009 declaration, by the
Obama administration, that Yucca Mountain is “not an option,” the timescales to the
implementation of an appropriate geological repository may have been set back, perhaps by
decades.
“Once-through” and “Closed” Fuel Cycle Options
If in a nuclear reactor, the fuel (such as UO2) is used just once and the spent fuel is then
destined for storage in a geologic disposition without reprocessing, such a fuel cycle is termed as
a “Once-through” fuel cycle. Contrary to this, the closed fuel cycle uses the option of re-use of
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the uranium and plutonium, recovered from the spent fuel through a suitable reprocessing
scheme (Fig. 4).
Figure 4. Schematic Representation of a Closed Fuel Cycle Option
What does it mean to have a closed cycle ?
Closed fuel cycle option (i) doubles the amount of energy being recovered from the fuel
and (ii) removes most of the long-lived radioisotopes from the spent fuel. Countries with large
nuclear power program (including France, Japan and the UK) employ closed nuclear fuel cycle,
where spent fuel is recycled to recover uranium, plutonium and long-lived radioisotopes.
However, the opponents argue that the closed cycle option is cost-prohibitive, particularly for
those countries which have good uranium reserves, such as the US. Some also argue that a closed
cycle option shall give rise to proliferation concerns with respect to the management of
plutonium.
Why does the U.S. currently use a once-through approach?
The current US practice for a once-through fuel cycle approach is based upon the fact
that there is going to be no short supply of uranium, in the foreseeable future, to fuel light water
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reactors. This fuel cycle is the simplest and the most economic fuel cycle today. Also, the US
perhaps believes that scientifically sound methods exist to manage spent nuclear fuel. According to
the estimates, the costs of SNF storage are small because the total quantities of SNF (~2000
tons/year in the United States requiring a total of 5 acres/year if placed in dry-cask storage) are
small. Moreover, licenses for dry-cask SNF storage have been granted for 60 years at some plants.
Managed storage is believed to be safe for a century although degradation of the spent fuel and
storage casks occurs over time due to its heat load, radioactivity and external environmental
conditions. In my opinion, clearly there is a need to re-think on the current US practices.
What are the disadvantages of a once-though cycle in terms of energy utilization
and waste production?
Once-through fuel cycle approach is highly inefficient as only about 1% of uranium is
used in the production of energy leaving the vast majority as spent fuel for storage. Also, such a
process is not at all attractive from the standpoint of nuclear waste generation and its subsequent
management. In a once-through fuel cycle, more than 98 percent of the expected long-term
radiotoxicity is caused by the resulting neptunium 237 and plutonium 242 (with half-lives of
2.14 million and 387,000 years, respectively). Environmentalists have pointed out that
controlling the long-term effects of a repository becomes simpler if these long-lived actinides are
also separated from the waste and recycled.
What are some elements of the U.S. Department of Energy’s fuel cycle research
program?
The mission of U.S. Department of Energy’s Fuel Cycle Research Development Program
(FCRD) is to help develop sustainable fuel cycles in order to improve uranium resource
utilization, maximize energy generation, minimize waste generation, improve safety, and limit
proliferation risk. The FCRD program also aims at developing suitable options to manage used
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fuels from nuclear reactors. The overall goal is to demonstrate the technologies necessary to
allow commercial deployment of solutions for the sustainable management of used nuclear fuel
that is safe, economic, secure, and widely acceptable to American society by 2050. The fuel
cycle strategies intend to examine three fuel cycles options, in particular: (i) once-through (ii)
modified open, and (iii) full recycling. Once-through approach intends to develop fuels that
increase the efficient use of uranium resources and reduce the amount of used fuel requiring
direct disposal for each megawatt-hour of electricity produced. Modified open investigates fuel
forms and reactors that would increase fuel resource utilization and reduce the quantity of longlived radiotoxic elements in the used fuel. Full recycling option shall develop techniques to
enable the long-lived actinide elements to be repeatedly recycled rather than disposed. The
ultimate goal is to develop a cost-effective and low-proliferation-risk approach that will decrease
the long-term risk, posed by the waste and, reducing uncertainties associated with its disposal.
How is the Idaho National Laboratory involved in this research program?
Research and Development work pertaining to (i) development of new generation
reactors with better and more advanced fuel cycle options and (ii) reprocessing of spent nuclear
fuels are being currently pursued at Idaho National Laboratory. INL is engaged in developing
novel reactor technologies that could operate truly closed fuel-cycles, in which all the actinides
in the spent fuel shall be recycled back into the reactor, a process known as burning. Much of
this work is being done under the aegis of the Generation IV Nuclear Energy Systems Initiative,
whereby ten countries, including the US, the UK and Japan, are combining their efforts. This
initiative has already identified six promising reactor technologies, five of which can operate
closed fuel cycles. This work, being at an early stage, requires the development of cooling
systems that utilize novel materials, such as liquid lead, and novel fuel formulations, such as a
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liquid mixture of sodium, zirconium and uranium fluorides. ‘This is high risk, high payoff”, says
Todd Allen, a nuclear engineer from Idaho National Laboratory. “If you can get these reactors to
work it will be a great improvement, but there’s a bunch of technical issues to be tackled”, he
added.
Researchers at Idaho National Laboratories are also working on an advanced technology,
known as pyroprocessing to process the spent fuel. This electrometallurgical processing
technique, developed by the Argonne National Laboratory (Fig. 5), appears to have great
potential for application in the treatment of spent nuclear fuel for their ultimate disposal. This
process is applicable to over 90% of the DOE spent fuel inventory and offers the advantages of
being a simple, compact system that is both economical and technically sound. INL is a world
leader in the field of pyroprocessing and currently the laboratory is carrying out pioneering
research and testing work to make pyroprocessing a viable alternative to process the spent fuel.
Recently, South Korea has joined the Idaho National Laboratory in this effort. INL is also
working to develop effective strategies to process the light water reactor spent fuel. This
technology is popularly known as “Oxide Reduction” (Fig. 6). INL is also working hard to make
all safe guard issues relevant and proliferation resistant. Figure 7 sums up various INL activities,
listed under an integrated Fuel Cycle Research program.
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Figure 5 –Various Components of EBR II Spent Fuel Treatment Project
Figure 6 – Schematic Diagram of an Oxide Reduction Electrolytic Cell
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Figure 7 – DOE Advanced Fuel Cycle Initiative
Conclusions
Being complex in nature, a great amount of care and skill is required to harness this
seemingly inexhaustible energy source in a safe manner. It seems to me that perhaps the best
option would be to share the cost, associated with the safe generation of nuclear energy and the
subsequent management of nuclear waste, on a global basis. For example, several countries can
store spent fuel in jointly owned storage facilities, Similarly, countries can share the cost to
process the spent nuclear fuel and thereby encourage substantial reduction in nuclear wastes for
storage in geological repositories. Several countries can share the permanent geological
repositories in each other’s facilities. Perhaps, we should learn from Europe where countries,
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both from within and outside Euro zone, share the cost to develop Fusion energy (TOKAMAK
project).
I can foresee a greater role being played by our country, in general, and Idaho National
Laboratory, in particular, in the growth of nuclear industry in the years to come.
Literatures Cited
1. A.M.Macfarlane, “The Overlooked Back End of the Nuclear Fuel Cycle”, Science, Vol.
333, issue 6047 (September 2011), pp. 1225-6.
2. Jon Evans, “Nuclear Comeback”, Chemistry and Industry (27 August 2007), 22-24.
3. Charles McCombie and Thomas Isaacs, “The Key Role of the Back-End in the Nuclear
Fuel Cycle” in “Multinational Approaches to the Nuclear Fuel Cycle”, authored by
Charles McCombie etal, American Academy of Arts and Sciences, 2010, pp. 14.
4. The Future of the Nuclear Fuel Cycle, An Interdisciplinary MIT Study, 2010.
5. S. I. Stepanov etal, “Concept of Spent Nuclear Fuel Reprocessing”, Doklady Chemistry,
2008, Vol. 423, Part 1, pp. 276–278.
6. http://www.ne.doe.gov/pdf files/fact sheets/2012_FCRD_Factsheet_final.pdf, December
04, 2011.
7. http://inlportal.inl.gov/portal/server.pt/community/nuclear_energy/277/afci_homepage,
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