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                                                

References

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)