Heavy by stariya

VIEWS: 11 PAGES: 8

									Heavy-metal nuclear power: could an unconventional coolant enable
reactors to burn radioactive waste and produce both electric power
and hydrogen?
By Eric P. Loewen | Nov 1, 2004 | 5713 words, 0 images
Thirty years have passed since the last nuclear reactor was ordered in the United States--yet the future
of nuclear power remains a national and international public-policy issue. Early in this year's presidential
campaign, the long-term disposition of nuclear waste once again became a focus of political and legal
debate. Meanwhile the world's energy appetite continues to grow, and many leaders worry over the
environmental and geopolitical costs of relying on fossil fuels for energy. Last year an interdisciplinary
study organized by the Massachusetts Institute of Technology concluded that the "nuclear option" must be
kept on the table and explored, with careful attention given to crucial economic and environmental
questions.
Away from the public spotlight, scientists and engineers have continued to develop new nuclear-power
technologies in the hope that we can build reactors that are safe, efficient, affordable and reliable--and
that resist weapons proliferation and reduce hazardous waste. It's a tall order. My colleagues and I have
studied one of several current technologies that appear to have these attributes and that have been
chosen by the U.S. Department of Energy for continued research and development. Like some others, our
concept is a newer version of a reactor technology that was not widely adopted as electrical utilities and
national power ministries chose designs during the 20th century. Some new science and engineering, and
a new emphasis on the longer-term issues of waste disposal and resource depletion, suggest it may be
time to reconceive the metal-cooled reactor. With a nod, then, to the coming-of-age days of the last
generation of reactor design, we call this technology "heavy metal nuclear power."
Metal-Cooled Reactors: Fast and Hot
Most of us associate tall towers and puffs of steam with nuclear power plants because cooling is an
essential part of nuclear-power generation. Water-cooled reactors, used throughout the U.S., are of a
class called thermal reactors. A reactor can also be cooled with metallic fluid; this is a fast reactor. Back
in the 1950s, the U.S. led the world in the development of reactors that were cooled with liquid metal--
at that time, either with sodium or with a mixture of sodium and potassium that was eutectic, having the
lowest melting temperature possible. Using this approach, the U.S. in the 1950s built the Experimental
Breeder Reactor, the first nuclear power plant to produce usable electricity, and the reactor aboard the
USS Seawolf, the second nuclear-powered submarine in the U.S. Navy.
The U.S. explored the use of lead (a heavy metal) and another alloy, lead-bismuth eutectic or LBE, as the
coolant in the early metal-cooled reactors, but ultimately elected to use sodium because of its superior
ability to transfer heat and compatibility with other materials in a reactor. In those early years, liquid
metal-cooled reactors were anticipated to play an important role in "breeding" nuclear fuel. But as the
supply of uranium increased and its price dropped, interest in liquid metal-cooled reactors in the U.S.
waned. Currently there are no metal-cooled reactors operating in the United States.
The Soviets went further, pioneering LBE-cooled-reactor fission to produce energy. The Soviet Union's
heavy-metal program began in the 1950s with test reactors and culminated with a reactor used in the
titanium-hulled Alpha class of attack submarines, the fastest in the world. During their current economic
transition, the Russians are seeking to adapt this Cold War military technology for commercial
application. Lead-cooled fast reactors, for example, have been mentioned in connection with Russia's
controversial nuclear cooperation with Iran. A new Russian LBE-cooled reactor design called BREST would
have a projected power output of 1,200 megawatts of electrical capacity, comparable to larger reactors
in the U.S. and capable of meeting the electricity demands of more than 900,000 U.S. homes. Russia's
push to bring fast-reactor technology into the marketplace has sparked renewed interest in heavy metal-
cooled reactors in the U.S.
What is different about fast-reactor technology? These reactors operate at higher temperatures than
water-cooled reactors, and the neutrons involved in the fission reactions inside the reactor are traveling
at relatively high speeds. A reactor running in this way can consume its own waste and also waste from
other reactors. In technical terms, a heavy-metal fission reactor is a sustainable energy source. Nuclear
power's political Achilles heel is the problem of disposing of spent nuclear fuel; the next generation of
nuclear-power technology must incorporate a solution to this problem.
Toward Safer Nuclear Power
All energy on Earth, with some very small exceptions, originates in one of two fundamental reactions--
fusion, where two small atomic nuclei come together (used by stars throughout the galaxies), or fission,
the splitting of large nuclei. Fission takes place in the 103 U.S. nuclear power plants. Electricity,
locomotion and other kinds of useful energy are derived from these atomic reactions. Wind energy, for
example, is possible because of fusion inside the Sun, which heats our atmosphere. In a nuclear power
plant, heat produced by fission turns turbines that generate electricity.
Fission is accomplished by bombarding certain heavy atomic nuclei with neutrons--neutral-charge
particles that come from the nucleus itself--and splitting the nuclei, resulting in the release of more
neutrons, gamma radiation and atom fragments, which all can be converted into heat energy with an end
product of electricity. The by-products of these reactions are short- and long-lived radioactive isotopes,
referred to as fission products, which constitute nuclear waste or spent fuel.
The explosive power of a nuclear bomb comes from a rapidly growing chain reaction. The goal in nuclear
power generation, of course, is quite different: a self-sustaining, controlled fission reaction. To achieve
control we need to regulate the number of neutrons in the system--to constrain the energy and speed of
the neutrons so that they react with the nuclear fuel inside the reactor. Secondary materials interact
with the neutrons to moderate or slow them to a desired spectrum, or range of energy levels. Some
secondary materials, such as control rods, can stop the neutrons by permanently absorbing them.
Because fission is more probable when uranium-235 is bombarded with slow neutrons, water-cooled
reactors use materials of low atomic mass so that the neutrons transfer a significant fraction of their
energy in collisions. The carbon in graphite and the hydrogen atoms in ordinary water meet these
requirements. The kinetic energy gained by the neutrons in fission is gained by the molecules of the
moderator and carried off by water.
A lead-bismuth eutectic is very different from a light-water reactor coolant. Higher specific heat, higher
density, lower neutron absorption and higher scattering, and high boiling point are among its material
properties. These properties give the lead-bismuth eutectic a few safety advantages over pressurized-
water reactors, which helped make "meltdown" part of our popular vocabulary. A heavy-metal liquid's
high boiling point and heat of vaporization dramatically reduces or eliminates the possibility of coolant
loss and subsequent catastrophic core melting.
The attributes of lead--an abundant metal that itself happens to be produced by the natural radioactive
decay of the uranium-actinium-thorium series of elements--are particularly attractive. As everyone
knows, water turns to vapor at 100 degrees Celsius at normal atmospheric pressure. Light-water reactors
are pressurized to 2,000 pounds per square inch, so they can operate at 300 degrees. Under atmospheric
pressure, lead will remain liquid and not boil until it reaches 1,750 degrees. A lead-cooled system can be
operated at atmospheric pressures, preventing common accidents that could take place in a light-water
reactor.
Anyone who has had a dental x ray also knows that lead's density blocks radiation. In a reactor it can both
cool and absorb gamma rays, reducing the need to surround the reactor with additional bulky shielding.
Finally, although lead is toxic to human beings, it is not consumed in the reactor. It is "purified"--its
metallic impurities removed--during operation, and there is no need for refilling or disposal. At the end
of a reactor lifetime--possibly up to 30 years--the lead will freeze around the reactor core inside the
vessel, encapsulating the core to provide resistance to corrosion. There are many lead seals from ancient
Rome that exist today.
Designs for these reactors incorporate many additional safety features. In our design, a passive residual
heat-removal system would limit the maximum temperature of the LBE to about 600 degrees below its
boiling point. In an LBE reactor, the nuclear fuel is highly soluble in the coolant, which in turn has a
density higher than the nuclear fuel; together these attributes can naturally shut down fission reactions.
An interesting possibility is a reactor system that operates totally on natural circulation--no mechanical
pumps to fail! A natural-circulation system provides unique passive safety features and autonomous
operability characteristics for deployment in developing countries. Additionally, in contrast with sodium,
there are no energetic reactions with air, water or concrete, reducing the possibility of fires.
On average, about three neutrons are produced when a large atom is split. These ejected neutrons leave
the fission site at nearly the speed of light. To sustain a fission reaction in, say, a U.S. nuclear-power
plant, the neutrons must be slowed down by 10,000 times--to speeds around 8,000 kilometers per hour.
This neutron-deceleration process, referred to as "moderation" or "thermalizing," is accomplished by
collisions with the hydrogen atoms in water molecules; like billiard balls, the atoms absorb energy from
the neutrons. After about 16 collisions, a neutron is slow enough to react with the next fissile atom. If
you can imagine an ejected neutron rocketing into the Earth's solar orbit at 71 million kilometers per
hour, making an orbital lap in just 13 hours, you can get a sense of how important these collisions are in
applying the brakes.
A fast reactor is said to produce "hard" neutrons, neutrons that maintain a high velocity as they strike the
large atoms of lead. (So you might say that heavy metal is a "hard core" nuclear technology, not so
different from the music named for it.) High-velocity neutrons explode fissile atoms into two major
fragments rather than absorbing a neutron. The result is transmutation: A large radioactive atomic
species is converted to a smaller, less dangerous element or even into stable atoms.
What About Waste?
Back when President Eisenhower presented his postwar "Atoms for Peace" plan, the use of fissionable
material for power generation and other peaceful activities was seen as a way to avert nuclear disaster.
Today, 17 percent of the world's electricity comes from about 440 nuclear power plants.
Current world energy politics focus on concerns about oil reserves and supply, climate change and the
quest for sustainable technologies. At the 2002 World Summit on Sustainable Development in
Johannesburg, South Africa, many participants voiced concerns that nuclear power is not sustainable
because the supply of uranium is finite, and nuclear power generation imposes a large waste-disposal
burden on future generations.
In fact, the longevity of the world's available nuclear-fuel resources, counting only the conventional
geological reserves of uranium, is estimated to be more than 1,000 years at current rates of
consumption. Whether one sees this as a short or a long period is a matter of perspective; in either case,
it would be advantageous to find reactor concepts that extend the resource base and also decrease the
waste burden faced by future generations. Nuclear power itself will always be a high-density energy
source, one that produces a large quantity of energy relative to the resources consumed.
The optimal reactor technology, if it is economically and technically feasible, is one that directly
addresses the waste problem. As the first generation of nuclear power plants come to the end of their
useful lives, the high-level radioactive waste from power generation in the U.S. remains in temporary
storage awaiting a long-term solution. One solution is transmutation.
If transmutation sounds a bit like alchemy, it is. In their attempt to make gold, the alchemists limited
themselves to chemical reactions, which manipulate only the electrons in an atom, not the protons and
neutrons of the nucleus. To transmute or "burn" spent nuclear fuel requires altering the nucleus itself. It
has a track record somewhat better than alchemy's.
Transmutation made its debut shortly after Sir James Chadwick in 1932 discovered the neutron. Striking a
radioactive uranium atom with a neutron transformed the atom into a new element, plutonium, after
two subsequent radioactive decays. The new atom had a shorter half-life than the original atom. In other
types of transmutation, a heavy nucleus explodes in half, or fissions, into two lighter nuclei.
About 95 percent of spent reactor fuel waste consists of uranium, which does not require long-term
permanent storage, since its radioactivity is comparable to that of natural material in the Earth's crust.
However, mixed with this uranium are short-lived radioactive fission products (isotopes of antimony and
xenon, for example) and long-lived radioactive elements with a higher atomic number than uranium
called transuranics (the first four are isotopes of neptunium, plutonium, americium and curium).
Currently, without separation of the three components, 100 percent of the spent fuel must, by law, be
considered high-level waste. The U.S. nuclear power industry will by 2015 have generated approximately
70,000 tons of this high-level nuclear waste, which will contain about 600 tons of plutonium. This waste,
with its recoverable energy content, currently awaits disposition at Yucca Mountain, a geological
repository not yet open and still entangled in political and legal questions. Fast reactors such as the
heavy-metal kind have the potential to reduce the volume and longevity of waste requiring permanent
storage at Yucca Mountain. Burning neptunium, plutonium, americium and curium (fuel) in a heavy-metal
reactor is possible because of all the fast neutrons careening around the core.
It's not, of course, quite so simple. A treatment facility for spent nuclear fuel would first need to be
established so that the short-lived fission products (non-fuel), uranium and transuranics could be
separated from one another. The fission products and transuranics would then be fabricated into a
metallic fuel (mixed with thorium and zirconium), assembled into fuel bundles and loaded back into the
heavy-metal reactor. A cross section of such a reactor is shown in Figure 5. Inside the core, using fast
neutrons, the transuranics are fissioned. They are consumed to generate energy. The heat from fission is
removed by the flowing heavy metal, which has the viscosity of a cup of coffee. A heat exchanger
submerged inside the reactor vessel transfers this heat to carbon dioxide, which spins the turbines that in
turn produce electricity.
Burning Waste for Energy
Nuclear-waste burning itself is not a simple matter; otherwise it might be going on every day. Instead,
nuclear power generation is now based on a concept called the once-through fuel cycle, which avoids the
costs associated with the reprocessing or recycling of fuel. The costs of partitioning and transmuting fuel,
along with the short-term environmental, safety and proliferation risks, have been considered too high.
My colleagues and I at the Idaho National Engineering and Environmental Laboratory are not economists.
Although we are aware of the complex economic and environmental dimensions of the challenge ahead,
we have looked at the issues surrounding future nuclear technologies from a technical and environmental
standpoint, taking the long view that scientists can.
Traditionally, nuclear reactors employ a large weight fraction of fertile material--material that, when hit
by a neutron, transforms into a nuclear fuel. Examples are thorium, which "breeds" uranium, or uranium,
which becomes plutonium. Isotope numbers explain how: Thorium-232 captures a neutron and decays to
become the fissionable isotope uranium-233. Uranium-238, the predominant component in conventional
nuclear fuel, breeds plutonium-239. These fertile materials compensate for the depletion of fissile fuel
by continuing to feed a reactor; they simplify reactor control and give an operator acceptable feedback.
When the fuel contains fertile isotopes, however, there is a significant reduction in the amount of waste
that can be burned as the reactor is running. This is because as some of the transuranic elements (those
with atomic numbers larger than uranium's) are incinerated, others are produced. If the fertile material
is completely eliminated by, for example, substituting all waste, the performance of the reactor changes
drastically over the fuel's lifespan (usually about 18 months). Reactor control becomes more difficult, and
there are economic penalties, since the reactor fuel is used up rapidly. There is a trade-off, therefore,
between fertile-fuel loading and maximizing waste burning. Fortunately, this trade-off can be optimized
with a heavy-metal reactor.
Figure 6 shows the transuranic isotopes present in today's spent fuel. Four transuranics of interest are
neptunium-237, plutonium-238, americium-241 and curium-242. Isotope number is important: An atom's
half-life and its ability to fission or absorb neutrons can vary by orders of magnitude between isotopes
and depends strongly on the energy of the neutrons used in fission. When a nucleus is hit with and
absorbs a fast neutron, two things can take place: fission or absorption.
An atom that absorbs a neutron remains the same element but becomes a different isotope; it has the
same number of protons but one more neutron. The newly formed isotope is usually less stable, thanks to
the extra neutron in the nucleus, and subsequently decays into a more stable element by beta decay, the
emission of an electron, a gamma ray and a particle called a neutrino. (In this particular type of beta
decay the neutrino is actually an "antiparticle" called an antineutrino, but this does not matter for our
purposes.) It may alternatively decay by alpha emission (releasing an alpha particle: two protons and two
neutrons stuck together), becoming an element two atomic numbers below the original element.
The fast neutrons used in a heavy-metal reactor greatly enhance the probability of fission over
absorption, increasing the production of fission products--a net destruction of transuranics. Thus the
transmutation of high-level waste produces a type of waste dominated by fission products, which are
more radioactive but have shorter half-lives. This waste requires safeguarding for 300 years, compared
with the 100,000-plus years now required for the waste from light-water reactors. Whether or not we
believe we can build structures that will last for 100,000 years, the human race can have some
confidence that our structures will stand 300 years, given that the landscape is full of human structures
built with 17th-century materials and still standing.
Neutron Physics
By now you may be curious about just how a heavy-metal reactor would "burn" transuranic elements. The
physics of the process varies slightly from one element to another, so I'll provide a quick overview of four
important transuranics.
Neptunium (abbreviated Np) is element 93, the first transuranic element. When neptunium-237, with a
half-life of about 2 million years, absorbs a neutron, it becomes [.sup.238]Np. [.sup.238]Np has a short
half-life of about 2 days. The [.sup.238]Np decays into a plutonium isotope, [.sup.238]Pu. That is, a
neutron in the [.sup.238]Np nucleus turns into a proton by beta decay, ejecting an electron, a gamma ray
and a neutrino. The newly formed [.sup.238]Pu then absorbs a neutron to become [.sup.239]Pu, which
easily fissions when hit by yet another neutron. A good rule of thumb is that isotopes having odd numbers
of neutrons are 10 to 20 times more likely to fission, rather than absorbing a thermal neutron, than are
even-numbered isotopes of the same element. This is how neptunium-237, with a 2-million-year half-life,
is burned in a heavy-metal reactor and transmuted into very short-lived radioactive elements such as
cesium, iodine and krypton, with half-lives of 10 to 30 years.
Plutonium. The next pesky element of concern in high-level waste, well known and irrationally feared, is
element 94, plutonium, of which [.sup.238]Pu is an isotope. The plutonium produced in a light-water
reactor contains many different isotopes. Plutonium-239 is an excellent reactor fuel for making
electricity. The odd-numbered isotopes of plutonium fission easily into smaller, very short-lived nuclei.
The even-numbered isotopes absorb a neutron to become odd-numbered and then fission. That's how
radioactive plutonium-238, with a half-life of 88 years, can be burned up while producing energy to
power our society.
Americium. The next element, with atomic number 95, is americium, widely used in home smoke
detectors. In nuclear waste the [.sup.241]Am comes from the beta decay of another plutonium isotope,
[.sup.241]Pu. Spent fuel from light water reactors contains an appreciable fraction of [.sup.241]Pu; it is
about 20 percent of the plutonium, which makes up about 1 percent of spent fuel. The [.sup.241]Pu has a
half-life of 14 years. Assuming the first heavy-metal reactor might come on line in 2025, the spent fuel
stocks in the U.S. would have an appreciable content of [.sup.241]Am as a result of [.sup.241]Pu decay
before the transmutation process is implemented.
When americium-241 is hit with a neutron, 80 percent turns into [.sup.242]Am and 20 percent into
[.sup.242m]Am, where the "m" stands for metastable. Metastable americium-242 has a long half-life (140
years) and a tendency to build up inside a reactor core. However, this is the most fissionable isotope we
have. If it is hit with a neutron, [.sup.242m]Am will break in half, yielding a significant increase in
reactor power. In addition, a small fraction of [.sup.242m]Am decays into [.sup.238]Np and is eventually
fissioned as explained above. What this all means is that loading a core with a high content of what
society considers nuclear waste (some of which is [.sup.241]Am) results in better reactor performance--
the ability to produce energy over a longer core lifetime. As an analogy, it would be similar to filling your
tank with a gasoline whose fuel efficiency increases as you drive, allowing you to motor farther before
the tank is exhausted.
The 80 percent of [.sup.242]Am produced from [.sup.241]Am has a half-life of just 16 hours and beta-
decays into curium-242. Curium-242, with a 163-day half-life, forms lighter plutonium-238 by alpha
decay. [.sup.238]Pu is three times better at fissioning than [.sup.242]Cm and also, as mentioned above,
can absorb a neutron to become [.sup.239]Pu with eight times the ability to fission. Thus, bombarding
[.sup.241]Am produces a different atom that is almost 10 times easier to fission. Returning to the gas-
tank analogy, this is comparable to raising the fuel's efficiency by a factor of ten. If the reactor is
designed to accommodate this, the [.sup.241]Am can contribute to increasing the performance of the
reactor.
Curium. The final major transuranic element group is curium. In spent fuel, most of this element occurs
as [.sup.242]Cm with a 163-day half-life, decaying into one isotope of plutonium or another. However,
through neutron absorption the curium acquires a higher isotopic number. Isotopes such as curium-243, -
244 or -245 can either decay or release energy through fission. The longest-lived curium isotope has a
half-life of 29 years.
A heavy-metal reactor swarms with energetic neutrons, using these three major transmutation pathways
to consume what is now considered waste in the spent fuel from light-water reactors. In this vision spent
nuclear fuel becomes a source of high-quality "recycled" fuel for the production of useful energy,
meanwhile converting a waste stream with 10,000- to 100,000-year half-lives into a waste stream with
10- to 100-year half-lives. Our work has shown that 660 kilograms of transuranics would be burned
annually in one core producing 1,800 megawatts of thermal power. (In conventional nuclear power
production, about one-third of the thermal power produced in the core is turned into usable electricity;
using that rule, the power production of such a plant would be about 600 megawatts electric.)
Reduce, Reuse, Recycle--Safely
Just as household recycling requires separating paper from plastic from glass, recycling of spent nuclear
fuel requires partitioning the types of waste so that uranium, transuranics and short-lived fission products
can be dealt with separately. Also as with household recycling, this is not a trivial matter, and it is
important to keep the partitioning process safe and secure. Yet waste partitioning has been safely
demonstrated in the U.S., France and Britain.
Once the transuranics are isolated, they can be burned in a once-through fuel cycle using a heavy-metal
reactor. Another approach would involve a multiple-pass fuel cycle in which the heavy metal reactor's
spent fuel is additionally recycled. In the latter scheme, additional "burnable" material is obtained,
namely the long-lived transuranics that are generated in situ during heavy-metal reactor operation.
A once-through cycle provides a significant reduction in the spent fuel's transuranics inventory, though
not complete elimination, and a dilution of the plutonium isotopes. This is an important feature in
preventing nuclear-weapons proliferation, because such transmutation would make it extremely difficult
to obtain or isolate weapons-grade materials. This option, however, does not decrease the radiotoxicity
or decay heat in the final package that initially exits the plant en route to disposal. The long-lived
radioactivity has been converted into high-energy short-lived waste. From an environmental-impact
perspective, there remains room for improvement.
In a multi-pass fuel-cycle scheme, however, it is possible to achieve a 99.9-percent reduction in the
overall long-lived transuranics-waste inventory that would require permanent disposition and storage in a
repository. Thus the burden on the planned Yucca Mountain repository would be greatly reduced.
Additionally, multi-pass recycling of the transuranics present in both light-water and heavy-metal
reactors' spent fuel would reduce the radiotoxicity of the consolidated final waste stream to a level
similar to what a comparable amount of uranium ore would emit in 300 to 600 years. Therefore, the
multi-recycling option is a preferable choice if society does not want to build additional repositories.
My colleagues and I at the Idaho National Laboratory collaborated with nuclear engineers at the
Massachusetts Institute of Technology in investigating four design concepts for heavy-metal reactors, one
for producing electricity with a once-through fuel cycle and three for waste burning.
We studied the once-through fuel cycle concept first. Of the alternatives, this cycle produced cheaper
electricity because there are no costs associated with fuel reprocessing and recycling. Today's reactors
operate this way partly because it is most economical for power generation. This reactor design has a
harder (faster) neutron spectrum that affords some in-core breeding and excellent safety characteristics.
Next, we looked at a fertile-free transuranics burner. The goal of this reactor concept is to achieve
maximum burning of transuranic waste. The fuel, made of transuranics and the mineral zirconium, is
burned in the core, then reprocessed to remove fission products. The left-over transuranics are combined
in new fuel with new transuranics and placed back in the reactor core. In our model system, the
recycling of fuel is usually done in 18-month increments but can be extended to longer residence times in
a heavy-metal reactor. This concept has a security advantage: The residual plutonium being recycled in
the spent fuel has a higher number of neutron-absorbing even-numbered isotopes, thus making it
virtually impossible to produce fissile material for weapons.
A third concept is the fertile-free minor transuranics burner. A heavy-metal reactor would be designed to
maximize the rate at which minor transuranics (curium-242, americium-241, and neptunium-237) are
destroyed, without destroying any plutonium. The plutonium from spent light-water-reactor fuel is
separated and burned in a light-water reactor while the minor transuranics are burned in the heavy-
metal reactor. Because of its slower neutrons, a light-water reactor can burn plutonium but not minor
transuranics. In this hybrid concept, fewer heavy-metal reactors would be needed to burn existing and
future minor transuranics. In light-water-reactor spent fuel, 0.8 percent is plutonium and only 0.1
percent is minor transuranics.
Finally, we can imagine a fertile-transuranic-burning reactor with a dual mission: to produce economical
electricity and to burn transuranics from spent light-water-reactor fuel. As I mentioned above,
conventional reactors use fertile elements; so does this concept, which would employ thorium. Although
thorium exists mainly as isotope 232, which does not fission well, when hit with a neutron this isotope
decays to uranium (specifically the isotope [.sup.233]U, which fissions as well as the commonly used
[.sup.235]U).
The addition of fertile thorium creates supplemental fuel and improves reactor performance and
operational stability. Furthermore, as Mujid S. Kazimi noted in these pages ("Thorium Fuel for Nuclear
Energy," September-October 2003), there exist large reserves of thorium, which is three times more
abundant than uranium in the Earth's crust. The drawback in our concept is that the thorium takes up
room where the transuranics reside, so that more heavy-metal reactors would need to be built to burn
the same amount of transuranics.
Of the concepts we have analyzed, the two fertile-free reactors have the best potential for burning old
and existing radioactive waste; for example, a 700-megawatt-thermal modular reactor could burn 0.2
metric tons of transuranics per year. This is two-thirds the annual waste output of a large 3,000-
megawatt (thermal) light-water reactor.
How many heavy-metal reactors would be needed to break down the transuranics produced up to now?
Depending on which concept is employed, the job would take 35 to 50 of these small reactors running for
40 years, admittedly an ambitious undertaking.
In all four heavy-metal reactor schemes we evaluated, the plutonium content in the spent fuel becomes
rich in the plutonium isotopes [.sup.238]Pu and [.sup.240]Pu and lean in [.sup.239]Pu, the plutonium
isotope most useful in making bombs. As mentioned above, even-numbered isotopes of plutonium don't
fission very well and produce more internal heat when decaying to uranium, making it more difficult to
extract fissionable material for weapons. The higher the weight percentage of [.sup.238]Pu and
[.sup.240]Pu in spent reactor fuel, the more undesirable it is as a source for weapons material.
We found other reasons to give these concepts good marks for safety and security in our evaluation. For
starters, when a fertile isotope is used in the core of one of these reactors, it does not breed plutonium.
When plutonium isotopes are produced from other transuranics during operation, they are even-
numbered isotopes unsuitable for weapons production. Also, longer operating cycles (making reactor fuel
in situ) and on-site fuel reprocessing and fabrication significantly minimize fuel movements and access to
the core, eliminating some scenarios under which radioactive material could be stolen.
The safest reactors are those that shut down by themselves. In a heavy-metal reactor incorporating
thorium (with a bit of uranium to denature the [.sup.233]U produced) in the fuel as the fertile material,
operation is stable and transuranics are destroyed at a high rate relative to power generation. Thanks to
the fuel mixture and shape, as well as to a passive-decay heat-removal design, all transients result in
inherent shutdown without exceeding safe fuel and structural temperature limits. This is an extra degree
of defense the current fleet of reactors can't claim.
H, Too? Oh!
It's important to note that a heavy-metal reactor is one of six concepts being studied as the U.S.
Department of Energy considers the next generation of nuclear technologies. The DOE's Generation IV
Reactor Program aims to develop and demonstrate by 2015 a reactor that has a high degree of
sustainability, safety, reliability, economy and proliferation resistance. It focuses on energy systems that
accomplish electricity production, waste management, hydrogen production and fissile material creation.
Could a heavy-metal reactor produce hydrogen for automobile fuel? The prospect is intriguing. Since
elemental hydrogen does not exist in nature, hydrogen itself is not a fundamental energy source. Nuclear
power emerges as a high-density energy source that can be utilized to produce vast amounts of hydrogen.
The hydrogen-production mission is best accomplished by designs that can achieve high reactor outlet
temperatures (700 to 900 degrees Celsius, depending on the process). Such a plant can either drive high-
temperature electrolysis or utilize process heat directly. Heavy metal-cooled fast reactors are ideally
suited to meet this mission since they operate at high temperatures but at very low pressure. LBE's
boiling point is 1,670 degrees, almost twice that of the most commonly used current metal coolant,
sodium, at 883 degrees. As noted above, the temperature of the pressurized water exiting a conventional
nuclear-power plant is about 300 degrees.
Do heavy metal reactors have a future? As this article is being edited for publication, athletes are
competing in the Olympic Games, and in many events the winner is far from certain. The six contestants
(chosen by DOE from more than 100 entries) in the Generation IV program are in the early stages of their
preparation for a final showdown. Test reactors will likely have to be small; the U.S. has lost the
capability to produce a sufficiently fast neutron flux for large-volume fuel and materials testing.
Fortunately our concept can be tested utilizing existing off-the-shelf reactor technologies in a test design
of about 30 megawatts of thermal power. It is worth noting here that the high heat-transfer capability of
a heavy-metal reactor enables compact cores with high power density (smaller and more economical
cores) compared with our current fleet of light-water reactors, so that small reactors might end up being
the norm rather than the exception if this technology is adopted.
We can take some lessons from the Russian experience, and it's possible that even as heavy-metal-reactor
research continues in the U.S., the BREST project will serve as a full-scale commercial demonstration of
the technology. The Alpha-class nuclear submarines were the fastest and deepest-diving subs in the
Soviet fleet. That experience showed that material corrosion and oxygen control in the liquid lead were
important issues in this kind of reactor. Pilot testing can attempt solutions to such known problems.
I'll conclude by offering this thought: We may need to accomplish nuclear-reactor design differently in
the future by engaging the entire technical community. First-of-a-kind systems should be built for and by
the testing community (our nation's universities and the national laboratories) working together. All we
need is to round up our heavy-metal nuclear physicists and have a little fast-neutron jam session.

								
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