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The Closed Fuel Cycle is the Future of the Nuclear Fuel Cycle
The nuclear fuel cycle is the cycle through which uranium goes through to produce
nuclear energy as is shown in Fig. 1 [1]. It starts with mining the ore which contains the uranium.
Natural uranium contains only 0.7% U-235 which is fissile (meaning it is able to go through
fission). The other 99.3% is the fertile material U-238 meaning it can capture the neutrons and
become fissile through the breeding process. Once the uranium is gathered, the enrichment
process is able to swiftly proceed. The enrichment of the uranium increases the U-235 amount to
3.5%-5%. After enrichment, it goes through the fabrication process to make the uranium fuel for
the nuclear reactors. The uranium fuel is normally embedded with cylindrically shaped metal
containers, which prevents the radioactive materials from being released into the environment
while being used in a nuclear reactor. Once fabricated, the uranium fuel is transported to a
reactor, where it normally stays in a reactor core for four to six years to produce as much energy
as can be produced from the uranium fuel. All of these steps are considered the “front-end” of
the cycle. The “back-end” of the fuel cycle covers everything after the used fuel is removed from
a reactor [2]. From here, there can be two different methods to deal with the resulting used fuel.
One is the once-through cycle and the other is the closed cycle.
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Figure 1. Illustration of the Nuclear Fuel Cycle [Ref. 1]
When the used fuel is removed from a reactor core, it is extremely hot and profoundly
radioactive. It is then placed into the spent fuel pool at a reactor site for several decades to let it
cool off and for the radioactivity to substantially decrease from its normal amount. The heat and
radioactivity gets down about 1/1000 after 40 years in the spent fuel pool compared to when it
was removed from a reactor [2]. The decrease in radioactivity and heat makes the used fuel
easier to deal with.
In a “once-through cycle”, the used fuel cooled in a spent fuel pool would eventually be
shipped to permanent repository sites for final disposal. It should be pointed out that up to now,
no final disposal facilities had been in any operation yet. The once-through cycle is used in most
of the current nuclear energy generation in which light water reactors (thermal reactors) are
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mostly used. A light water reactor uses water as the coolant to transport the energy out of a
reactor core to generate electricity. The once-through cycle is the method most countries use
except for a few countries in Europe and Asia. Even though it is widely used around the world,
this method has a few obvious disadvantages. One is that we have not yet created a strong
enough container that can hold the radioactive waste for hundreds of thousands of years until the
radioactivity wears off. Another is that if we continue to use this particular cycle, our limited
supply of uranium will run out in the next two hundred years or so. Used fuel typically contains
about 1% U-235, 1% Plutonium, 94% U-238 and the reaming 4% includes fission products and
highly durable radioactive transuranic elements such as americium, neptunium, and californium,
etc. [1]. U-235, U-238, Plutonium and even transuranic elements are valuable energy resources.
The used fuel contains 96% of all these elements combined and they would not be used in any
way by the once-through cycle. Hence, the once-through cycle uses only a small portion of the
potential energy stored in the uranium fuel, totally wasting all the valuable resources available.
Clearly, we have to use some other sort of method that is both effective and pragmatic, which
brings us to the “closed cycle.”
The “closed cycle” goes through all of the front-end steps of the normal nuclear fuel
cycle. The differences only start to show up when it gets to the back-end where instead of just
disposing of the waste, it actually reprocesses the used fuel to extract the potentially useful
elements and separates the nuclear waste, thus utilizing all available resources and being less
negatively, environmentally impactful. The main advantage of the closed cycle is that it utilizes
all the available resources and significantly reduces the amount of nuclear waste to be disposed.
It is both more environmentally friendly and also more practical in its effectiveness.
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The closed cycle isn’t used anywhere except for France and England, and Japan is just
barely starting to utilize it. However, today’s closed fuel cycles are really only partly closed
cycles. This type of methods only reprocesses the used fuel once, extracts the plutonium to
recycle into uranium and plutonium mixed oxide fuel (MOX). [3] Once it recovers the plutonium
and uranium to fabricate it into fresh fuel, the resulting transuranic fuel is sent to the light water
reactor. This is still not using all of the potential stored in the uranium and is also wasting the
resources presented available for use.
Though the partly closed cycle decreases the environmental impact by extracting the
plutonium from the spent fuel, it does not significantly increase the uranium utilization. In order
to truly reduce the waste and dramatically increase the uranium utilization, we must turn to the
fully closed cycle which would include the deployment of innovative fuel separation
technologies and fast spectrum reactor technologies. One major difference between thermal
reactors and fast reactors is the material they use as coolants. Thermal reactors use water as the
coolant, which slows down the neutrons. Fast reactors use sodium which does not slow down the
neutrons, hence the name. Fast spectrum reactors can breed fertile U-238 elements into fissile
plutonium elements. In the fully closed cycles, the spent nuclear fuel would be reprocessed
many times to recover the fissile plutonium and transuranic elements and put them back into fast
reactors. Because of this, truly closed cycles are also called self-sustained cycles. In truly closed
fuel cycles, the energy produced per kilogram uranium mined is fifty to sixty times more than the
once-through cycle. However, the biggest concern linked with the closed cycle is the fear of
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terrorists retrieving the plutonium produced from the nuclear fuel cycle and using it to create
weapons.
Therefore, in my view, one vitally important aspect of the research and development
being done now for the closed cycle should be to develop the reprocessing technology which
would not separate plutonium from the other radioactive elements. Another important aspect is to
research and develop new ways to lower the costs of utilizing the closed fuel cycle, including the
costs of fast reactors. The future of nuclear energy clearly lies with fast reactors and the closed
cycle. In the current status quo, the closed cycle used isn’t what the closed cycle should be. In
the future, the closed cycle should be a self-sustained fuel cycle, with no need to continuously
mine for uranium then waste it. No new uranium would have to be sought after until the used
uranium fuel has exhausted all of the energy contained in it. This will substantially lower the
wasted uranium and resources available.
The U.S. Department of Energy’s fuel cycle research program [4] conducts research to
develop new technologies for integrated system studies, fuel cycle options and fast reactor
technologies. The Idaho National Laboratory (INL) plays a crucial role for the Department of
Energy in conducting fuel cycle research. INL has a long history in developing fuel cycle and
fast reactor technologies. The technologies have been proven by Experimental Breeder Reactor I
(EBR-I) and Experimental Breeder Reactor II (EBR-II). I myself had visited the EBR-1 twice
and learned that EBR-1 was the first nuclear power plant in the world. The four light bulbs
EBR-I first illuminated made a very deep impression on me. I also had the opportunity to tour
the INL’s Materials and Fuels Complex during INL’s latest Open House in September of 2011.
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During the tour I was able to see the silver dome for EBR-II and I also was able to see the
demonstration for EBR-II fuel design and the control room for EBR-II. I have also toured the
very impressive fuel cycle facilities such as the Hot Fuels Examination facility and the Fuel
Conditioning Facility. EBR-II had operated successfully for 30 years with a priceless wealth of
knowledge of fuel cycles and valuable data generated. It served as the prototype fast reactor for
the Integral Fast Reactor program [6].
However as what has been pointed by the article [7] I found from Wikipedia (use caution
in relying on this source) that the fast reactors are not economical. This had impeded the
commercialization of the fast reactor technologies, and negatively impacted the closed fuel cycle
development. The main reason for the economic problems is that sodium is very reactive with
water and air, which requires extra protection measures. Further research and development
investment is required to improve the safety and economics of fast reactors.
The closed fuel cycle and the fast reactors still require much more research as how to
make them more economically practical. Even with all the challenges it faces, from all of the
evidence and information gathered, the future of nuclear as an alternative energy source is
clearly pointing towards the fully closed cycle.
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References
1. http://www.world-nuclear.org/info/inf03.html
2. http://uraniumsa.org/fuel_cycle/back_end_nfc.htm
3. http://www.world-nuclear.org/info/inf98.html
4. http://www.ne.doe.gov/FuelCycle/neFuelCycle.html
5. C. E. Till, Y.I. Chang, W.H. Hannum, “The Integral Fast Reactor – An Overview”, Progress
in Nuclear Energy, Vol. 31 issue 1-2, 1997 http://en.wikipedia.org/wiki/Fast-neutron_reactor
6. http://en.wikipedia.org/wiki/Fast-neutron_reactor
7. http://en.wikipedia.org/wiki/Experimental_Breeder_Reactor_II