Thorium: Alternative Energy for Future Generations
The
Europeans have a strictly thorium-fueled nuclear reactor for electric power
generation under construction in Belgium. This reactor is called an Accelerator
Driven System, ADS for short. It has
the following outstanding features: 1) it cannot be used for producing the raw
material for atomic bombs, 2) it cannot meltdown under any circumstances, and
3) after 500 years its waste will be no more dangerous than the ashes from a
conventional coal burning power plant.
How can this be true? That is the main subject of this article.
After World War II
the atomic physicists told us that electricity would be too cheap to meter.
Obviously, this did not happen. Accidents at Three Mile Island and Chernobyl
magnified public fear of things nuclear. A decade or so ago I heard that India
was working on nuclear power fueled with thorium because they have domestic
supplies and would have to import uranium.. A year or so ago I googled thorium
to find out what was going on with thorium in India, and found that the
Belgians had the reactor described above under construction (1). The American
public needs a new perception of nuclear energy. Media coverage of these issues, at least in America, has been
inadequate.
Speaking of media inadequacy, is the public aware that
carbon sequestration may be only temporary because of limited sites and
possible reintroduction into the atmosphere? Earthquakes and other unforeseen
subterranean events may release it (google: “carbon sequestration”). We also
know that acidification of the oceans will be a problem from increased
atmospheric carbon dioxide even in the absence of global warming; so injection
of carbon dioxide into the deep oceans is off limits (2). Furthermore, there
has been no thorough, full-scale test of carbon sequestration. So a new look at
energy from the atom must be considered—it is a proven technology, about to be improved, and has much to contribute.
Nuclear
power is in decline as an energy source primarily because of cost and in spite
of recent increases in government subsidies worldwide. No new nuclear power plants are even planned
in the United States let alone any under construction—industry balks at the
cost and the public fears accidents and proliferation of nuclear weapons.
However,
thorium fueled nuclear power may be the energy source of a more remote
future. Like uranium, all of thorium’s
isotopes are radioactive. Unlike uranium only one of thorium’s isotopes has a
half-life long enough to be still present on earth. Th232 has a
half-life of 14 billion years. Fourteen billion years is approximately the age
of the universe, so thorium is toxic as a heavy metal but not because of its
radioactivity—its decay is too slow for it to be dangerous. Thorium is about as
prevalent on earth as lead and is less toxic.
Thorium oxide (ThO2) has the highest melting point of any substance so far tested and makes
dandy mantles for kerosene lanterns because of the brighter light from the
higher temperature obtainable--and with no concern about its miniscule
radioactivity. (Radioactivity was not discovered until decades after this use
of ThO2 was
invented.)
There
are several kinds of thorium-fueled reactors currently under development one or
more of which are the wave of the future if only because of the abundance of
thorium compared to uranium. Thorium by itself cannot be used as nuclear fuel
because it, like U238, is not fissionable, but,
mixed with U235 or Pu239, their neutrons convert some of it to U233, more
fissionable than
U235. This technology is fairly well along toward practical application
and can be used to dispose of excess plutonium from weapons. Two additional advantages: such reactors are
less subject to meltdown as occurred at Chernobyl * and the final waste
products are only dangerous for about
*The accident at Chernobyl was not, strictly speaking, meltdown as
occurred at Three Mile Island, Pennsylvania.
High pressure steam ruptured the cooling water pipes and reacted with
the hot graphite control rods producing hydrogen and carbon monoxide which
produced a chemical explosion that distributed radioactivity much more widely
than meltdown would have. The chemical
explosion and fire would not have occurred with a modern gas cooled reactor or
even with a liquid sodium cooled reactor (personal communication from Kelly
Clifton, radiation biologist at the University of Wisconsin).
500
years—even extra plutonium added to these reactors at the time of fueling is
consumed so that all the waste is still only dangerous for the same 500 years.
Research into this technology is occurring
especially in Russia to get rid of their plutonium and in India because they
have very high grade thorium deposits.
Western
Europe is pursuing an integrated approach to fast breeder reactors called
Generation IV, ready by about 2030. Its integrated fuel reprocessing creates
new fuel only--none of it as metallic plutonium (refined bomb
material), and the same low level
waste of ADS reactors with
the additional benefit of
being a source
of technetium and
stable isotopes of ruthenium,
rhodium, palladium for other industrial uses. Roland Schenkel, European Commission, Joint Research Center, Brussels, Belgium,
stated, “Generation IV reactors are resistant to atomic weapons proliferation
because there is no purified plutonium anywhere in their fuel
cycle and the
plutonium remains mixed with
americium, curium and neptunium, long half life elements that would help to
identify the source of any diverted plutonium.” This is from a symposium entitled Nuclear Reactor Systems of the
Future organized by Aidan Gilligan of the EU Commission at the American
Association for the Advancement of Science annual meeting in Boston on February
16, 2008. The speakers at this symposium
agreed with Roland Schenkel that the existing nonproliferation treaty needs
only a little tweaking (already well on the way to successful
negotiation)—specifically that the current treaty is too permissive about
concentrating U235 as
fuel for power plants and that in the
long run the countries possessing atomic weapons must more actively pursue
disarmament greatly simplifying prevention of proliferation of nuclear
weapons.
An ABS
thorium-fueled reactor, mentioned in my opening paragraph, cannot produce
enough neutrons for continuous fission and must be kept fissile with an
external proton beam from a small linear accelerator integrated with the
reactor. The proton beam emits neutrons from a lead target. Physicists call the
process spallation. Again, the
neutrons create U233 from
Thorium232. The U233
fissions promptly with an additional neutron, but the mixture remains
sub-critical. This means that power production must be perpetuated by more
neutrons from the spallation, therefore no possibility of a chain reaction—shut
off the linear accelerator and
the action stops
immediately. This makes
it totally immune
to meltdown and, also important, it cannot produce material for
bombs. No full sized facility has yet
been constructed, but the nuclear reactions are well known and any new
engineering required will be quite standard. A prototype was to have been
completed in Belgium in 2011, but will be delayed until 2020, perhaps even
totally abandoned because of cost overruns.
Possibly,
further research on ADS reactors could wait until inexpensive uranium sources
are exhausted and plutonium from scrapped atomic weapons has all been converted
to electricity, but meanwhile using some ADS reactors would permit heat and
electric power production in densely populated areas because of no danger of
meltdown (and similar accidents, see footnote above). Large increases in efficient energy production for heat-requiring
industrial processes are occurring using cogeneration—currently salvaging waste
heat from coal-powered plants. Research
to permit earlier use of cogeneration using thorium powered ADS reactors might
be money well spent because it would greatly reduce carbon dioxide release into
the atmosphere by expanding the use of atomic energy for more than electricity
generation.
Here are
some further facts to consider in long term planning for stable energy supplies
without too much carbon dioxide pollution of our atmosphere. The United States
under Jimmy Carter chose to dispose of the waste without reprocessing because
of concerns about diversion to make nuclear weapons by rogue states. As a result, all of our nuclear power plants
are fueled by new U235, the fissionable isotope of natural uranium, so our nuclear waste contains long-lived radioactivity from
“unburned” fuel. .If we rely on uranium nuclear
fuel without reprocessing spent fuel to obtain the plutonium, supplies of U235,
which is only 0.7% of uranium as found in all its ores, would be exhausted in a
few hundred years. Even transmuting U238
to plutonium would last us only 20,000 years or so. Thorium is three times more abundant than uranium in the earth’s
crust. These facts alone would justify some continuing development of ADS
reactors.
So the solutions are political and are needed
for many reasons besides nuclear proliferation. Start with educating the
public and reforming the UN’s efficiency—some world government
will be needed
at least for disarmament and
inspections. It seems obvious that
there will be no world government beside the UN any time soon—so hats off to
innovative thinking like Ted Turner’s in paying the United States’ back dues to
the UN ($800 million). This paragraph
is intended to get our thinking off dead center, not to make political
suggestions. I hope I have provided
some facts for the thinking process.
A salient point: we should increase research
in ADS technology so that ultimately we will be less likely to feel compelled
to build more conventional nuclear power plants--they are susceptible to
meltdown, produce more long-lived waste, and can be used to produce pure Pu239,
a raw material for bombs. Thorium fueled nuclear power does look very promising
in the long term. After thorough demonstration of no danger of meltdown,
thorium power could even be incorporated in cogeneration projects—it would
become safe enough for densely populated areas where the incidentally produced
high-pressure steam could be readily utilized. This advantage will invite
government subsidies in the future because of permitting nuclear power for
industrial heat processing—safe enough for siting in a congested area and
almost no carbon dioxide discharge (doubtless a small amount incidental to
construction).
The
bottom line: nuclear power, especially that involving thorium, can be justified
on scientific and political grounds. Unfortunately, costs are now much higher
than the alternatives--renewable sources of energy and, above all, avoiding
wasting of energy (cogeneration is obviously only one example). I close with a quotation from John Maynard
Keynes: “The difficulty lies not in the new ideas but in escaping the old ones,
which ramify, for those of us brought up as most of us have been, into every
corner of our minds.”
Appendix: a crash course in scientific literacy—what the general public needs to
know about nuclear power for generating electricity and for high-pressure steam
for industry. Most of the public already know that atomic energy, unlike
burning fossil fuels (coal, petroleum, natural gas), produces no greenhouse
gases, in this case carbon dioxide.
Almost all of us also know about the unsolved problem of disposing of
atomic waste because of its radioactivity. Radioactivity is the spontaneous
transmutation of an element to another element with the release of heat energy
and ionizing radiation. The spent
atomic fuel remains highly radioactive for tens of thousands to hundreds of
thousands of years because of elements, newly formed in the reactor, with long
half-lives (especially plutonium). Half-life is the time required for half of
the atoms to decay to other elements by its radioactivity. Radioactivity falls
to acceptable levels after 10 to 20 half-lives have passed.
Rather
elementary chemistry can separate “unburned” uranium and plutonium for recycling
as new fuel for reactors. This is
expensive because of the cost of remote manipulation of the process occasioned
by the high level of radioactivity, but it is cost-effective not only because
of the value of the recovered fuel but also because the radioactivity of the
remaining waste, while high initially, remains dangerously radioactive for only
a few hundred years—orders of magnitude less than the untreated waste. France and England have recycled their fuel
since early in the atomic age because of these advantages—low cost new fuel and
greatly reduced radioactivity of the residual waste.
We have
purified, stockpiled, and manufactured much plutonium into bombs. In our case decommissioning of nuclear
weapons as part of the ongoing disarmament agreements therefore involves
treating this purified plutonium as waste even though it could be used for
nuclear fuel—
all this
because of our decision, previously mentioned, to use only U235 for
commercial reactor fuel. Storing this purified plutonium, a byproduct of
disarmament, as waste is not only costly, but also dangerous if only because of
its long half-life. The Russians are using their plutonium for electric power
production.
Separation of U235 from U238, the common isotope of uranium, cannot be done chemically because they are the same element and chemically identical. This difficult separation is accomplished by making a gaseous compound of uranium (UF6, uranium hexafluoride). The slight difference in the weight of the molecules containing the different isotopes of uranium is utilized for the separation either by diffusion through many orifices in series (with much pumping) or by fantastically powerful centrifuges to effect the separation by the enormous artificial gravity. The great complexity of these processes reduces the danger of proliferation of weapons because of the difficulty of obtaining sufficient quantities of sufficiently pure metal for a bomb. The critical mass for a bomb is only a few pounds of either U235 or Pu239. This quantity in one hunk undergoes a spontaneous chain reaction of fission into atoms of elements that are about half the atomic weight of the uranium or plutonium such as cesium137 and iodine131, all with much shorter half lives than Pu239 (24,000 years), U235 (700 million years), or U238 (4 billion years). The bomb is triggered by suddenly propelling together several pieces smaller than the critical mass of the metal but totaling more than its critical mass. The pieces have to be accurately machined to fit closely, as if they were one piece. (Don’t worry about this description getting out—all the “competent” terrorists know this part already.)
Finally, here is the physics of the enormous power of atomic fission and even more powerful atomic fusion. Elements lighter than iron and nickel release energy on being fused into heavier elements and those heavier yield energy on being split into several lighter ones. The packing fraction of an isotope is the difference between the weight of its ingredient protons, neutrons compared to the weight of the protons and neutrons of iron and nickel. All isotopes of a given element have the same number of protons and differ in the number of neutrons. When a radioactive isotope decays or a U235 atom fissions to two atoms about half its weight (the total of protons and neutrons remaining the same) the energy released is the difference in the weight of the initial nucleus (isotope) and the weight of all the resulting isotopes. The actual energy is given by Einstein’s famous formula: energy equals mass times the velocity of light squared: e=mc2. The velocity of light is an enormous number even before it is squared, so the loss of a tiny amount of mass releases enormous energy—this tiny difference in mass is what you multiply by the speed of light twice (!) to get the exact, enormous energy.
Stars, including our sun,
release their enormous amounts of energy continuously for billions of years by
fusing hydrogen and helium to heavier elements with attendant loss of mass but
never to elements heavier than iron and nickel. The lesser amounts of stored energy represented by the packing
fractions of isotopes (and elements) heavier than iron and nickel is stored in
these large atoms from the energy of supernova explosions and from no other
sources, not even the “big bang.” In a supernova explosion much of the star’s
mass is expelled into interstellar space to await addition to a cloud of
hydrogen that is going to form new stars and perhaps planets. Nickel and iron predominate in this expelled
material because they are stable through all these vicissitudes. This is why planets, asteroids, and many
meteorites have cores of iron and nickel.
(Science is a challenging hobby at times)
John A. Frantz, MD, NASW
July 7, 2008
1) Klapish R. Accelerator driven systems: an application of proton accelerators to nuclear power industry. Europhysics News (2000) Vol. 31 No. 6
or on line: www.europhysicsnews.com/full/06/article8/article8.html
2) Riebesell U. Acid test for marine biodiversity. Nature 454: 46-7 (2008)