One of the basic principles of the modern environmental movement is the
simple mantra to “reduce, reuse, and recycle”. It is my intention to
show in this essay that the technology of the liquid-fluoride reactor,
coupled with the energy source thorium, make it possible to achieve
these goals to a far greater degree than other nuclear energy
technologies.
Introduction
Liquid-fluoride reactors are based upon the use of dissolved actinide
fluoride salts in a carrier medium of low-absorption fluoride salt
solvents. The most common formulations that have been considered and
demonstrated for this mission are solvents based around low-melting
point mixtures of beryllium fluoride (BeF2) and lithium fluoride (LiF)
isotopically enhanced in the more-abundant component lithium-7. The
actinide fluorides most commonly employed are thorium tetrafluoride
(ThF4) and uranium tetrafluoride (UF4). LiF-BeF2 salt mixtures have
very low neutron absorption properties, excellent heat capacity,
stability under intense radiation, and the ability to dissolve
appreciable amounts of thorium or uranium tetrafluoride.
Liquid and Solid Fluoride Salt Mixtures
Despite providing some degree of neutron moderation, LiF-BeF2 mixtures
are not terribly good neutron moderators, thus liquid-fluoride reactors
generally employ solid moderating materials in order to moderate
neutrons to thermal energies. Graphite is most commonly employed, being
abundant, relatively inexpensive, and chemically compatible with the
salt. Graphite is not “wetted” by the fluoride salt and can be sealed
in ways that limit the intrusion of fission product gases (especially
xenon) into the structure of the graphite.
Thorium as a nuclear fuel is not as well-known as uranium, but has
properties that have special merit for nuclear use. Thorium also has a
number of drawbacks for its use as a common nuclear fuel, but
fortunately, by using thorium in fluoride form, nearly all of these
drawbacks can be eliminated or strongly mitigated.
Thorium is common in the Earth’s crust, consisting of about 10 parts per
million of common continental crust, approximately three to four times
more common than uranium. Thorium is not fissile and consists of a
single natural isotope (232) but thorium can be converted to a fissile
fuel by the absorption of a neutron followed by a short period of beta
decay. After absorbing a neutron, thorium-232 is transmuted into
thorium-233, which then beta-decays with a half-life of 22 minutes into
protactinium-233, which is chemically distinct from the parent thorium.
Protactinium-233 has a half-life of about 27 days, after which is
beta-decays to uranium-233, which is fissile and has impressive
properties. Uranium-233 produces enough neutrons from fission by a
thermal neutron to sustain the continued conversion of thorium to
energy, even accounting for normal losses, provided that the reactor is
neutronically efficient.
Reducing the Production of Transuranic Nuclear Waste
One of the biggest concerns about today’s approach to nuclear power
generation concerns our use of low-enrichment uranium (LEU) in
solid-uranium-oxide-fueled light-water reactors. In these reactors, LEU
fuel is irradiated by thermal neutrons and a significant amount of
plutonium is produced from the uranium-238 that makes up 95-97% of the
original fuel. Some of this plutonium is consumed as the solid-oxide
fuel rod is further irradiated, but from the plutonium other isotopes of
plutonium are formed by neutron capture, and then higher actinides like
americium and curium are produced. From the small fraction of U-235
present in the fuel even some long-lived neptunium-237 is produced.
After an irradiation period of 3-4 years, the fuel rod can no longer
sustain addition irradiation and is removed and placed in a spent fuel
pool for cooling as high-heating decay products move inevitably towards
stability.
In our current approach to civilian nuclear power, these irradiated
uranium oxide fuel rods are not reprocessed to separate and partition
their different chemical components, but are instead bound for disposal
in a deep geological repository in Nevada. There after several hundred
years the transuranic actinides still present in the spent nuclear fuel
will generate the bulk of the heating that dictates their spacing in the
repository and its ultimate capacity. Furthermore, the transuranic
actinides carry the vast majority of the radiotoxicity that repository
licensers must deal with as they plan for the performance of the
repository over the next ten thousand years.
Reducing the amount of transuranic waste that will be sent to any
future repository would therefore be an important goal of a future
approach to civilian nuclear power generation, and this is eminently
doable by using thorium in a liquid-fluoride reactor. Transuranic waste
production can be drastically reduced by a clever combination of the
inherent properties of the thorium fuel approach and by the flexibility
of the liquid-fluoride fuel form.
Thorium, with an atomic mass of 232, begins the nuclear energy
generation process at least five neutron absorptions removed from the
first transuranic isotope that could be generated. As previously
mentioned, thorium-232 absorbs a neutron, transmuting to
protactinium-233 and then uranium-233, which is fissile. In a thermal
neutron spectrum, uranium-233 tends to fission 90% of the time it
absorbs a thermal neutron. The other 10% of the time is converts to
uranium-234. Another neutron absorption in uranium-234 leads to
conversion to uranium-235, which is also fissile and represents another
opportunity for destruction through fission. Uranium-235 fissions in a
thermal neutron spectrum approximately 85% of the time, and the other
15% of the time is converted to uranium-236. Uranium-236 has a rather
low neutron absorption cross-section, and only after absorbing a neutron
is the first transuranic isotope of this approach produced:
neptunium-237. Neptunium can be removed from the fluoride salt mixture
readily by fluorination from NpF4, which is in solution to NpF6 which is
gaseous. Thus, unlike our current approach to nuclear power where the
majority of the fuel (97% U-238) is a single neutron absorption away
from the production of the first transuranic isotope (Pu-239), in the
thorium-based approach, the fuel is five neutron absorptions away from
the production of a transuranic isotope, and in the course of those
absorptions roughly 98.5% of the original fuel is removed by fission.
Thus, by using thorium in the fluoride reactor rather than uranium in
the solid-oxide reactor, it is possible to REDUCE the amount of
transuranic material generated by a very large factor.
Reusing Nuclear Fuel
As previously mentioned, today’s approach to nuclear fuel employs
low-enrichment uranium is solid-oxide form in zirconium cladding,
cooling and moderated by ordinary water. As an oxide, uranium is quite
chemically stable and able to achieve high temperatures without melting
down. Unfortunately, as an oxide, uranium is also subject to the low
thermal conductivities common to most all oxides, and therefore high
temperatures at the centerline of the solid fuel element become an
inevitable consequence of heat transfer out the surface of the fuel
element. In fact, the centerline fuel temperature of a uranium oxide
fuel element, relative to the melting temperature of uranium oxide, is
one of the key geometrical constraints.
As uranium oxide fuel is irradiated, fission products and transuranics
accumulate in the ceramic oxide matrix. Intense radiation from the
fission process and the decay of fission products also damages the fuel
structure, causing dislocations and swelling in the crystalline matrix.
Especially damaging to the fuel element are in the in-growth of gaseous
fission products such as xenon and krypton, which further distend and
crack the fuel structure. One of the isotopes of xenon (135) has a huge
appetite for thermal neutrons and causes control transients during the
changing of power settings within the reactor.
After a period of time the uranium oxide fuel element has been
depleted of fuel, swollen, cracked, distended, inflated, and compromised
by the fission process and must be removed before cladding failure
leads to the loss of fission products and other radioactive isotopes to
the water loop of the reactor system. Spent solid-oxide fuel rods must
be replaced by new fuel rods and are sent to a cooling pond where decay
heat can be removed. Although the spent fuel still contains large
amounts of unused fuel in the form of both uranium and other actinides,
that fuel cannot be accessed until a reprocessing program takes place
that involves chemically changing the solid uranium oxide into a liquid
uranium nitrate fuel form through the application of strong nitric acid.
Then a combination of chemical processes in aqueous and hydrocarbon
solvents takes place to separate gaseous fission products, other fission
products, transuranics, and uranium from one another. The resulting
waste streams from these processes can be utilized productively, but the
cost is significant due to the aggressive chemical steps involved and
the chemical intensiveness of the new forms.
Many, many recycles of the fuel would be needed to “burn-down” the
uranium-238 present in the original spent fuel to energy (through
fission) and the costs involved in reprocessing dictate that spent
nuclear fuel is rarely subjected to more than one or two recycles before
it is disposed.
Thorium and the fluoride reactor present an entirely different
approach to fuel management that makes repeated recycling not only easy
but economically advantageous. That is because nuclear fuel in the
liquid fluoride form rather than in the solid oxide form has distinct
advantages. It is already in a chemically stable form as a fluoride.
There is no reagent to treat the fuel that will be favored over its
current state. Thus it is protected from chemical attack, combustion,
burning, or corrosion. But more importantly, as a fluid is it in a form
where chemical processes can be employed directly to remove fission
products or to add new fuel to compensate for burnup. Additionally, the
ionic nature of liquid-fluoride salt renders the fuel essentially
impervious to radiation damage. Despite the passage of large amounts of
gamma radiation, neutron radiation, alpha radiation, etc. the fuel
remains chemically unaltered and with a complete retention of its
physical properties.
Gaseous fission products, including the important fission product
poison xenon-135, are effortlessly easy to remove from liquid-fluoride
salt. They simply come out of solution in the pump bowl during the
pumping of the fluid through the loop. This has the additional benefit
of keeping pressures low and allowing the reactor to change power states
rapidly without concern for the effect of xenon on power changes.
In a modern incarnation of the liquid-fluoride reactor, there are two
separate fluoride salts in action in the reactor core: the “fuel salt”
and the “blanket salt”. The fuel salt is a mixture of uranium
tetrafluoride in the lithium-beryllium fluoride carrier solvent. The
uranium consists predominantly of uranium-233 but also contains U-234
and U-236 at equilibrium levels of concentration. Depending on the
reprocessing approach it also contains fission products in the form of
fluorides. The blanket salt is a mixture of thorium tetrafluoride in
the lithium-beryllium fluoride carrier solvent. The blanket salt
geometrically surrounds the fuel salt with a graphite barrier between
them. Fission in the fuel salt produces neutrons, roughly half of which
end up in the blanket salt, transmuting thorium to uranium by neutron
absorption followed by beta decay. The uranium formed in the blanket is
removed by the simple process of fluorination, whereby uranium as a
tetrafluoride in solution is converted to a hexafluoride that is
gaseous. Since thorium has no gaseous hexafluoride, it is left behind
while uranium is removed in this simple, one-step process. Then the
fuel salt is “refueled” by this same stream of fresh uranium
hexafluoride by converting it from hexafluoride back into tetrafluoride
through contact with hydrogen gas. Thus freshly generated uranium is
continuously removed from the blanket salt and added to the core salt,
where it subsequently undergoes fission that continues the process all
over again.
By keeping fissile materials out of the blanket by continuous
reprocessing, the blanket fluid can be kept relatively free of fission
products. The fuel salt, on the other hand, will accumulate fission
products as uranium fission continues. The most troublesome fission
product, xenon, is effortlessly removed by pumping action, but other
fission products will become of increasing concern. Samarium,
neodymium, and other lanthanides are fission products whose neutron
absorption cross-sections are significant enough to merit attention. In
order to purify the fuel salt, the first step is to remove the uranium
fuel by fluorination. Then the carrier salt (LiF-BeF2) can be distilled
from fission product fluorides in a high-temperature still. The
remaining fission product fluorides constitute the equivalent of
“high-level waste” from fluoride reactor reprocessing. The extracted
LiF-BeF2 is recombined with the uranium and reinserted into the reactor
core for another cycle of power generation.
The fluid nature of the reactor fluids allow them to be used over and
over again, removing only the products that have been generated during
operation (uranium in the blanket, fission products in the fuel salt).
This ability to continually REUSE the reactor nuclear fuels represents a
profound advantage over the solid-fueled uranium approach.
Recycling the “Wastes” of Fission
Fission processes inevitably generate a variety of fission product
elements and a large number of isotopes, most of which are neutron-rich
and radioactive. The familiar double-humped distribution of fission
products reflects the physical reality that each fission event results
in two fission products, a “heavy” one and a “light” one. As each of
these fission products tends to have many more neutrons than is needed
for nuclear stability at its new “station” in life, rapid beta decay
generally follows fission and most fission products assume a stable form
quite quickly.
When all of the isotopes of an element reach stability it can
logically be asked whether or not they are worth chemical extraction and
recycling to other, non-nuclear uses.
Consider the case of xenon. Xenon is a noble gas and fission product
that accounts for a fair fraction of the mass of fission products from
uranium fission. Xenon has a variety of isotopes but the longest lived
one (133) has only a half-life of 5.2 days. Therefore, proceeding on
the rule-of-thumb that “ten half-lives and you’re gone” after 50 days of
storage the xenon remaining from fission would be essentially
non-radioactive. In a conventional solid-core reactor the xenon is
bound up in the solid-oxide fuel rod and can only be extracted by
chopping up and dissolving the fuel element, but in a fluoride reactor
it is very easy to extract xenon. In fact, it will come out of solution
with essentially no effort at all. Since xenon is a valuable gas,
rather than vent the xenon to the atmosphere it can be separated from
the krypton by cryogenic distillation and sold. NASA and commercial
satellite operators, for instance, use xenon for ion engines for
spacecraft. Future NASA missions to Mars that have considered using
xenon have had to seriously consider whether the world supply of xenon
was sufficient to make such missions possible. Xenon recovered from
fission might increase xenon supply.
Another valuable material from fission is neodymium. Within the last
20 years, the discovery of a neodymium-iron-boron alloy that can be
used to make super-strong, super-light magnets has caused neodymium
demand to increase tremendously. Ironically, one of the markets that is
in greatest demand for neodymium is the wind turbine market. They need
large electrical generators due to the diffuse nature of the wind
energy source, and they need these electrical generators to be as
lightweight as possible so that they can be mounted on top of large
towers. Neodymium magnets are particularly suited to this demanding
application.
Neodymium is the third-most-common element generated from fission (by
mass) and also achieves nuclear stability relatively quickly; its
longest-lived isotope (147) has a half-life of 10.9 days. By aging the
high-level waste from the distillation process in fluoride reactors
appropriately, one could extract the neodymium trifluoride from the
other fluorides and convert it to a metallic form through electrolysis
or metallic reduction. The neodymium would then be available to sell to
the burgeoning market.
Xenon and neodymium represent two recycling opportunities where a
period of “aging” is needed before the isotopes stabilize and
partitioning and marketing is possible. But there are other isotopes in
the “waste” stream of a fluoride reactor where the radioactive form of
the isotope is the desirable and economic product. An example of this
case is the life-saving medical isotope molybdenum-99. Currently,
molybdenum-99 is generated in specially-designed medical isotope
production reactors in Canada and rushed to medical facilities across
North America. Mo-99 decays to technetium-99m, which is then extracted
and introduced into human patients in order to facilitate diagnostic
procedures. The market for Mo-99 is quite large, but in solid-fueled
reactors, the Mo-99 produced by fission is not accessible until the fuel
is reprocessed. Since that is an infrequent event in solid-fueled
reactors, the overwhelming majority of the Mo-99 produced in such
reactors is never productively utilized; rather it simply follows its
decay chain to Tc-99. In a fluoride reactor, on the other hand, the
fluid nature of the reactor makes it possible to continuously extract
Mo-99 along with the other isotopes of molybdenum. Molybdenum forms a
volatile hexafluoride much like uranium does, and when the fuel salt is
fluorinated, U, Mo, and several other elements come out of solution as
gaseous hexafluorides. These can then be separated on from another by
distillation at different temperatures, much like crude oil is refined.
The molybdenum could then be shipped to medical facilities, where the
Mo-99 would decay to Tc-99m that could be chemically extracted and given
to patients who need it.
Xenon, molybdenum, and neodymium are three of the most common fission
products but many others have value too. The fluid nature of the
fluoride reactor makes RECYCLING of the so-called waste quite likely to
be economically attractive in many circumstances.
Summary: Reduce, Reuse, Recycle
The environmental dictum of “reduce, reuse, recycle” has been
considered in terms of the thorium-fueled, liquid-fluoride reactor and
found to be a simple and unifying theme for the options that this
technology makes available. Relative to a conventional, solid-fueled
uranium reactor, one can drastically REDUCE the generation of
transuranic actinides, REUSE the thorium and uranium fuel is a way that
allows for complete consumption of the energy resources, and RECYCLE
three of the most common fission products into economically useful and
even life-saving applications. The thorium-fueled liquid fluoride
reactor is worthy of significant further attention, investigation, and
funding based on these and many other merits.
I was born in Scotland, and chose to become an American.
I was raised a Presbyterian, and am now an atheist.
But it boggles my mind that so many of my now fellow-Americans imagine that Christianity is compatible with outrageous wealth and the destruction of the environment, a.k.a. the rest of God’s Creation!
But I disagree with Robert Carroll if he thinks that the first priority should be shutting down the nuclear industry.
We ‘d be far better shutting down the entire fossil carbon industry, AND those damned ugly wind turbines, with nuclear power, preferably owned by the people’s Government, like the BPA, the TVA, the BBC, and until the EU forced them to privatize, France’s EDF .
Technology, if good, doesn’t take huge manpower to maintain it. So if we solve these problems, we must also solve the problem of redistributing incomes and leisure fairly, and curtail the horrific power of the Employer to ruin the Employee, whether by malice or the stupidity of people like Lehman Bros.