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OCR for page 49
8
A Renaissance for Nuclear Power?
Patricia A. Baisden,
Lawrence Livermore National Laboratory
There are many reasons why nuclear power might expand in the future.
Energy resource availability, climate change, air quality, energy security, and
independence from other nations to maintain our standard of living will likely
have a combined impact on the allocation and use of a variety of energy sources.
The questions in this scenario are what role nuclear power will play and whether
there will be the political will and technical work force present to allow nuclear
power to occupy an increasing percentage of future energy needs.
Currently, about 16 percent of the world's electricity is generated from 435
nuclear reactors in 31 countries. Most of these are thermal reactors, which use
neutrons at very low energies to induce the fission reaction. This is in contrast to
fast reactors, which use neutrons that are orders of magnitude more energetic.
Over 80 percent of commercial thermal reactors are light water reactors (LWRs)
that use normal hydrogen in water for neutron moderation. During neutron
moderation, neutrons with energies on the order of several million electron volts
lose their energy as a result of numerous scattering reactions with a variety of
target nuclei. After a number of collisions with nuclei, the neutrons have energies
the thermal range (~0.025 eV).
There are many advantages to nuclear power. In a fission event, 200 million eV
(10-~i J/atom) are released per atom that undergoes fission. In contrast, the
breaking of chemical bonds in fossil fuels produces approximately 10-~9 J/atom.
Therefore, nuclear energy produces 108 times more energy per atom than fossil
fuels. Additionally, nuclear fuel has a limited use to mankind, while fossil fuels
have a wide variety of applications, including pharmaceuticals, plastics, and
petroleum. Nuclear power does not produce greenhouse gases like fossil fuel
burning plants.
49
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so
ENERGY AND TRANSPORTATION
For full public acceptance of nuclear power, a number of issues must be
addressed, including waste disposal, reactor safety, economics, and nonprolifera-
tion. All of these issues depend on the fuel cycle that is used, but for any fuel
cycle a geological repository will be needed for high-level waste storage. What
will differ are the nature, hazard, half-life, and volume of the waste.
Of a 1000-kg block of uranium oxide fuel going into an LWR, only about 3.5
to 4.0 percent is fissile material (i.e., 3.5 to 4 percent 235U or 35 to 40 kg) and the
remaining mass is 238U. After about 2 to 3 years, the fuel is spent and must be
replaced. Of the original 235U, about 75 percent has reacted by capturing a neutron
and fissioning to produce a wide variety of fission products with half-lives that
range from very short (days to tens of years as in the case of 90Sr and 137CS) to
very long (hundreds to millions of years as seen with 99Tc and i29I). Products
heavier than uranium are also made by the capture of neutrons that do not lead to
fission. In general, the unreacted uranium isotopes represent about 95 percent of
the LWR spent fuel; the fission products, about 3.4 percent; all of the plutonium
isotopes, about 1 percent; and the minor actinides (primarily neptunium, ameri-
cium, and curium), about 0.6 percent. If the spent fuel is chemically processed to
recover the unused uranium and plutonium, only about 4 percent of the spent fuel
(the minor actinides and the fission products) needs to be managed as high-level
waste.
There are three major fuel-cycle options that are being studied worldwide.
The first is the once-through fuel cycle. In this option the fuel in an LWR is used
for 2 to 3 years. It is then successively put into wet storage, dry storage, and
eventually into a geological repository. No material is recovered or recycled;
therefore, in the short term this power production method is the most economical
since no costly chemical processing plant is required. It is also the safest regard-
ing proliferation potential because the fissile material the fuel is "protected
against diversion" by an intense radiation field. However, in the long term this
option yields a very large amount of waste destined for the repository. In addi-
tion, after 250 to 500 years, a radiation field will no longer protect the fissile
material because the fission products will have mostly decayed away. Because
this option does not remove uranium and plutonium prior to geologic disposal,
criticality issues must be addressed in the design of the repository, and the need
for multiple repositories over time must be considered.
There are many challenges to the chemical sciences with the once-through
fuel cycle. Scientists need to provide a scientific basis for understanding the
performance of the geological repository. People must be convinced that the risk
to the public is low and acceptable. Finally, chemists must determine the relevant
physical and chemical processes occurring over the time the material is in storage.
These characteristics and processes then need to be measured and used to validate
models that assess the performance of repositories over thousands of years.
The second option is the reprocessing fuel cycle. Unlike the once-through
option, with reprocessing the fuel cycle is actually closed since the unused fissile
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A RENAISSANCE FOR NUCLEAR POWER?
51
materials 239Pu and 235U are recovered and recycled back through a reactor to
produce more energy. Before 235U can be reused as a fuel and sustain criticality in
an LWR, it has to be enriched back to the level of 3.5 to 4.0 percent. Plutonium,
on the other hand, can be recycled by mixing it with either natural or depleted
uranium to make mixed oxide fuel and burned in either a thermal or a fast reactor
(a reactor that uses no moderator and induces fission with neutrons at energies at
or near where they were created, ~2 MeV). Because the used 235U is recovered,
reprocessing not only allows a higher fraction of the energy content of uranium to
be utilized but also reduces enrichment costs. Reprocessing also offers a conve-
nient method of separating out the wastes (fission products and minor actinide
elements) and placing them into a form acceptable for a waste repository. Cur-
rently, these waste materials are vitrified, that is, put into a boron-silicate glass.
The industry standard for reprocessing is the PUREX process (Plutonium
Uranium Redox EXtraction), which yields uranium and plutonium in forms suit-
able for making new fuel. PUREX is an aqueous-based solvent extraction process
that, unfortunately, creates a large amount of high-level liquid waste that contains
the majority of the fission products and the minor actinides. This waste has to
undergo denitrification and calcinations before it can be vitrified. Additionally,
other waste streams are created during the process because solvents and reagents
undergo radiolysis and degrade as a result of the intense radiation. Chemists and
chemical engineers are challenged to improve the PUREX process in modern
plants, to improve the selectivity of reagents and optimize the reaction kinetics, to
minimize solvent degradation and improve the recycle of solvents and reagents,
and generally to reduce the number and volume of secondary waste streams.
In the reprocessing fuel cycle, the initial motivation for the fast reactor cycle
was to "breed" plutonium from uranium to maximize uranium fuel utilization.
This is because less than 1 percent of the energy content of mined uranium is
realized in burning uranium in conventional thermal LWRs. In fast reactors,
239Pu is made by placing a blanket of uranium around the core, where 238U captures
a neutron to produce the short-lived 239U (tin = 23.5 minutes). 239U then under-
goes beta minus decay to another short-lived isotope, 239Np (tin = 2.35 days),
which again undergoes beta minus decay to finally become 239Pu. Fast "breeder"
reactors are configured to produce more 239Pu than they consume. As a result,
since the natural abundance Of 238U is 99.3 percent and the conversion efficiency
through breeding is on the order of 70 percent, an almost infinite supply of fissile
fuel for future generation could be produced using fast reactor technology.
Because the expected shortage of uranium never materialized and uranium
remained abundant and cheap in the mid- 1 980s, the need for fast reactors to breed
plutonium was no longer compelling or economical. This, along with mounting
concerns about the potential proliferation of plutonium, caused interest in fast
reactor technology development to further wane. Recently, fast reactors are gain-
ing interest as an efficient means of destroying long-lived actinide elements in
more advanced fuel cycles.
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52
ENERGY AND TRANSPORTATION
The last and most futuristic fuel-cycle option is an extension of the repro-
cessing fuel cycle. In this option, in addition to recovering uranium and pluto-
nium to produce more energy, the long-lived minor actinides and some of the
long-lived fission products are chemically separated (partitioned) from the high-
level waste stream and then converted into less radiotoxic or otherwise stable
isotopes by a process called transmutation. Transmutation uses nuclear reaction,
or a process to transform a radioisotope into one that is stable or has a shorter
lifetime than the original isotope. Transmutation can be accomplished through a
combination of LWRs, fast reactors that are configured to burn plutonium rather
than breed it, and subcritical accelerator-based transmutation devices. The waste
stream remaining after partitioning would contain only relatively short-lived
fission products that would go to a repository and decay to the background level
of high-grade uranium ore in about 250 years.
The partitioning of neptunium, iodine, and technetium is already possible
through modifications to the PUREX process. This is not the case for americium
and curium. Other aqueous separation schemes involving more selective
extractants are needed to separate americium and curium, because they are similar
in charge and size (ionic radius) to the lanthanide elements. Also, the lanthanide
elements constitute about one-third of the total mass of all of the fission products
in the PUREX high level waste stream. For such separations to be economically
viable, new methods cannot generate more secondary waste than is generated
using currently accepted processes. Further, new reagents used in the separation
must be synthesized in sufficient quantity at reasonable cost to be used on an
industrial scale. Reagents and solvents must also be compatible with the solvent
extraction and other process equipment being used with PUREX.
Probably the most inviting and challenging R&D area impacting waste man-
agement for the advanced fuel cycle is the development of nonaqueous processes.
Examples of these include separations based on differences in volatility, liquid-
liquid extractions involving molten salts or liquid metals, and electrochemical
methods (electrorefining) that use a cell potential to selectively remove a metal
by reduction on to a cathode. Because of the higher fuel burnup needed for
transmutation and the need for multirecycling with minimum cooling time
between fuel discharge and reprocessing, nonaqueous processes offer several
important advantages. Nonaqueous processes are much more radiation resistant
compared with aqueous processes and thus can be used to reprocess spent fuel
after considerably shorter cooling periods. Other advantages of nonaqueous
processes include the potential to produce small volumes of secondary waste,
waste that is more suitable for long-term disposal, ease of reagent recycle,
compactness of equipment, and reduced costs due to a smaller required plant
footprint. At the present time, major drawbacks are smaller separation factors
requiring multiple stages to achieve the required level of decontamination, limited
throughput because non-aqueous processes are usually operated as a batch rather
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A RENAISSANCE FOR NUCLEAR POWER?
53
than a continuous process, and the need for highly controlled atmospheres to
avoid unwanted side reactions such as hydrolysis and precipitation reactions.
When compared with fast reactor technology, accelerated-driven systems
(ADS) compared with fast reactor technology are in their infancy. An ADS con-
sists of a high-power proton accelerator, a subcritical target that produces a high
neutron flux upon bombardment with high-energy protons, a blanket system that
utilizes this intense source-driven neutron flux to fission the actinides and trans-
mute some of the long-lived fission products, and a supporting chemical process-
ing plant. The heart of the system is the subcritical target, where an intense (>100
mA), high-energy (1 to 2 GeV) proton beam impinges on a high Z target to pro-
duce neutrons in a process called spallation. Accelerator systems producing 40 or
more energetic neutrons per incident proton are being proposed. The waste mate-
rial is loaded into the blanket where it is "incinerated" by capturing one of the
neutrons, causing it either to fission or to be converted to a short-lived or stable
isotope. This process therefore reduces the radiotoxic inventory of the waste
material. The heat produced from the transmutes (the combination of the target
and blanket) can then be used to generate power, ~10 percent of which can be
used to run the accelerator; the remaining 90 percent can be put on the power
grid. Unlike fast reactors that operate on the criticality principle, where the
nuclear reaction is sustained, ADS are subcritical. When the proton beam is off
in an ADS, no neutrons are created and no nuclear reaction occurs.
Some of the challenges to the chemical sciences (also metallurgy and materi-
als science) related to the ADS involve the design and operation of the neutron
source as well as the form and chemical composition of the blanket. Nuclear
chemistry and chemical and nuclear engineering expertise is of great importance
for developing and implementing the processes needed in the supporting chemi-
cal plant. In the chemical processing plant, separation processes are used to first
partition the spent fuel to obtain the waste material that is introduced into the
ADS and then later to support multiple recycles, fuel fabrication, and finally pro-
duction of the final waste forms acceptable to a geological repository.
The training situation is dire in nuclear chemistry, radiochemistry, and
nuclear engineering. There is great concern that our nation will neither have the
right expertise nor enough expertise to meet future demands. Therefore, it is
necessary to immediately reinvest in the education system, particularly for the
training of nuclear scientists. Otherwise, the United States could lose on many
fronts its leadership in nuclear science and technology, its ability to influence
Third World or emerging nations, and its ability to safely manage the existing
nuclear enterprise. Without a properly trained work force, in the future the United
States will not be able to preserve its options involving nuclear technology and
nuclear energy.
Issues related to the expansion of nuclear power range from waste manage-
ment to nonproliferation. There are some technical solutions, but these solutions
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54
ENERGY AND TRANSPORTATION
need to be improved and new ones need to be developed. The greatest challenge
may simply be to educate the public and increase their confidence in nuclear
technology. Regardless of public opinion, the United States has long-term nuclear
issues that require expertise in nuclear science, and chemists and chemical engi-
neers need to ensure that we have that expertise.
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A RENAISSANCE FOR NUCLEAR POWER?
Representative terms from entire chapter:
fuel cycle
