Thorium oxide is one of only two naturally occurring fertile materials, uranium oxide being the other, which can be caused to fission and sustain a nuclear reaction, ultimately yielding enormous amounts of energy from comparatively miniscule quantities of matter.
From its inception, the nuclear energy industry has overwhelmingly relied on uranium as its fuel of choice, though thorium fuel was researched as thoroughly as uranium fuel by the leading nuclear powers (USA and Russia) and others. The origins of the decision to make uranium fuel the standard for the industry is rooted in uranium’s ability to make weapons material, and even more significantly, thorium’s inability to make weapons material – a natural choice during the time of the Cold War.
Thorium fuel has many physical characteristics that would make its use in nuclear reactors advantageous to uranium-based fuel. These advantages are listed below, and are described in greater detail subsequently:
i)Thorium fuel generates no new bomb-usable material in the waste profile; the waste consists of the radioisotope Uranium-233, or U233, which is virtually impossible to weaponize;
This is an extraordinary set of advantages over the present-day standard fuels. These advantages are explained below in some greater detail, in order to provide further clarity:
i) Thorium fuel generates no new bomb-usable material in the waste profile; the waste consists of the radioisotope Uranium-233, or U233, which is virtually impossible to weaponize
When Th232 is bombarded (or irradiated) with a thermal, or “slow” neutron, from a nuclear reactor’s moderator, it can absorb said neutron into its nucleus, and transmute into a different atom that is no longer Th232. It becomes Thorium-233, or Th233. Within 22 minutes, Th233 decays naturally into Protactinium-233, or Pa233. After a period of 27 days, Pa233 begins to decay into U233. So, U233 is an entirely man-made material, and does not exist naturally, as Plutonium-239 also does not. U233 is fissile material. U233, when bombarded with a neutron, will split into various lighter atoms, referred to as fission products, various forms of energy are released, and free floating “fast” neutrons, which will collide with other U233 atoms and create more fission reactions. The total energy release of the U233 fission is roughly similar to energy released in the standard uranium-235 fission.
In the standard uranium fuel cycle, the waste products typically are:
§ 94% Irradiated uranium-238
§ 4% Fission products
§ 1% Irradiated uranium-235
§ 1% Plutonium-238, 239, 240, 241 and 242 (50% Pu239)
§ 0.1% Americium, neptunium, curium (referred to as minor actinides)
Plutonium-239, or Pu239, in particular, when separated, is the world’s most effective material for making nuclear bombs. The minor actinides, when separated, can also be used to manufacture bomb-grade material, as can some of the other plutonium isotopes, as well as the U235. The fission products can be recycled, and have value outside of the nuclear energy industry.
In the thorium fuel cycle, the waste products are:
§ 71.2% Irradiated thorium oxide
§ 12% Fission products
§ 16.8% Uranium-233
This is an entirely different waste profile. Noticeably missing are the plutonium and the minor actinides. Noticeably evident is the U233. The Los Alamos National Laboratory (LANL) in the USA did successfully manufacture an experimental nuclear bomb, and in the process, they amply demonstrated that U233 is far more difficult to construct into nuclear bombs than Pu239 or minor actinides. The reason lies chiefly in the uranium-232 (U232) contamination that occurs naturally in all U233. Like most unstable radioactive isotopes, U232 decays into a string of various atoms over time. U232 and some of its daughter decay products emit very strong gamma radiation. Unlike Pu239, U232 emits more gamma over time, and becomes more dangerous. This makes any handling of U232, and by extension any waste material from the thorium fuel cycle, very difficult to handle. To put it into perspective for the reader, a critical mass of plutonium waste product can be handled with a standard glove box, whereas a critical mass of U233 (approximately 5 kilograms) would contain enough U232 content (approximately 1%), that after 10 years, anyone handling it without thick lead shielding would receive a lethal dose within hours or days of such close proximity. The high energy and high rate of gamma rays emitted would also be detrimental to both the explosives and electronics of any weapon and make it highly detectable. In short an extremely poor choice for a nuclear weapons material.
ii) Thorium fuel can be used to safely incinerate the world’s unwanted stockpile of plutonium waste
Unlike uranium, thorium occurs naturally without any resident isotopic material. It is the fissile material, U235, which is resident in naturally occurring uranium, to the extent of 0.711 wt-%, that is concentrated and used to sustain nuclear chain reactions, and ultimately nuclear power. Thorium in nature is 100% thorium-232, with no other isotopes present. Because of this, thorium itself cannot sustain a nuclear chain reaction. It requires some fissile material for this process to take place.
At present, there is a type of nuclear fuel which uses as its fissile material, not enriched U235, which is the standard fuel, but reprocessed previously-spent plutonium-239, or Pu239. The fuel, commonly called Mixed Oxide fuel, or MOX, uses plutonium that has been separated – from nuclear reactor waste or from dismantled nuclear weapons – and is mixed with reprocessed depleted uranium (obtained from the waste of uranium enrichment facilities) to produce a fuel that acts very similarly to standard fuel. The table below summarizes the differences in the fuel inputs and outputs.
MOX fuel has been successfully produced by French utility company, Electricite de France (EdF) since the 1980’s, in reactors throughout Europe, in Russia, and more recently in Japan. MOX is praised by some for its supposed ability to utilize plutonium material and convert it into energy, rather than having plutonium waste sit in stockpiles where it can be stolen or accidentally released into the local environment. However, what is seldom praised is the fact that the burning of MOX generates new plutonium waste, by dint of U238 being transmuted into plutonium isotopes in the MOX fuel cycle, not all of which fissions, similar to the original fuel cycle. Furthermore, not all the reprocessed plutonium, which constitutes the fissile component, is in fact burned, due to inefficiencies in neutron capture by U238 – also similar to the original fuel cycle. The result is that the MOX cycle leaves more plutonium waste than it begins with, which defeats the very objective of burning MOX in the first place, from a waste profile standpoint. Unfortunately, this plutonium waste is worse than normal plutonium waste, or so-called primary plutonium, because this plutonium waste cannot itself be recycled, due to the low Pu239 content.
MOX fuel that uses thorium as its fertile material, however, instead of uranium does not encounter the same problem. Such a fuel would in essence be a thorium-plutonium mixed oxide fuel. Let us call this new fuel TMOX for simplicity, though this acronym is not used in the industry. As stated above, plutonium remains in the waste profile of the standard MOX fuel cycle for two reasons. Both of these reasons are eliminated when using TMOX:
i) When thorium absorbs a neutron, it transmutes into U233, not plutonium. In order to transmute into plutonium, thorium would have to absorb 6 or more neutrons (and even more neutrons to transmute into minor actinides.) This level of neutron absorption is not possible.
Furthermore, because of thorium’s greater neutron economy, it has been determined that plutonium which has already been used once in a MOX fuel cycle, could be used again in TMOX despite having a lower content of Pu239.
Hence, thorium fuel in a MOX application, as is envisioned in Thorium One’s business plan, provides a far more efficient platform for the incineration of plutonium waste stockpiles than standard MOX.
iii) Thorium fuel cycle waste has a radiotoxicity period of less than 200 years, which compares favourably with the >1,000,000-year radiotoxicity period estimated to exist for uranium fuel cycle waste
There is considerable disagreement on what constitutes adequate containment of spent nuclear reactor waste. It is so contentious that the United States government has grappled with the issue of establishing a central repository for spent nuclear reactor waste for over two decades -- with no resolution. Radioactive waste management company, JAI Corporation, made the following statement at a recent presentation:
The question is frequently raised regarding how long the spent fuel and high level waste must be isolated from the biosphere with a high degree of certainty, and what the limiting radiation dose should be during the isolation period. Many opponents maintain that since some of the contained radioactive and toxic materials have very long lives, that 10,000 or even a million years is not enough, or that the long-term limit on dose received by an exposed individual of the public of 100 mrem/year after 10,000 years is too high. Others argue that it would be unreasonable to expect to isolate the wastes beyond the time that their radiotoxicity had dropped below that of the naturally-occurring uranium ore from which the wastes resulted (uranium ore equivalent). Slide 14 [See Exhibit 3.2 below: Thorium One] shows how the radiotoxicity of spent fuel and high level wastes decreases with time compared to the radiotoxicity of its uranium ore equivalent, which does not change appreciably with time. From the slide it can be seen that the radiotoxicity of spent fuel decays below that of the uranium ore equivalent at somewhat less than 10,000 years after discharge from the reactor, and that the radiotoxicity of high level radioactive waste from reprocessing decays below the uranium ore equivalent less than 1,000 years after discharge.
Exhibit 3.2 – Radiotoxicity of Nuclear Waste
Somewhere between less than 1,000 and greater than 1,000,000 years lies the answer to safe storage of uranium fuel cycle waste. Uranium ore is generally not harmful, but is dangerously toxic to humans if ingested, inhaled, or in the event of prolonged contact. Uranium ore cannot be the level of radiotoxicity to which the world strives to reduce nuclear reactor waste before releasing back into the biosphere. Although there is currently no policy on storage anywhere, other than that it be “permanent,” the United States National Academy of Sciences determined that an estimate of one million years of nuclear reactor waste isolation was “fully justified.”
Thorium fuel cycle waste has a much lesser radiotoxicity period comparatively, owing to the fact that there is no so-called transuranic waste (plutonium, neptunium, curium, americium). As above, the waste products of the thorium fuel cycle are U233, Th232, and fission products. U233 itself has a half-life of 160,000 years. However, it is inevitable that U233 will become a recycled waste product. It will be reprocessed and utilized as fissile material for subsequent fuel cycles. This is an inherent property of the thorium fuel cycle. Though the technology does not yet exist to do so, it is generally understood by the nuclear community that the technology will be deployed no later than 2050. Thorium One believes it may occur much sooner.
(The most lethal by-product of the thorium fuel cycle is uranium-232, which is always resident in U233. Uranium-232 (discussed in great detail above) has a half-life of 69 years. The reason for U232’s lethality is its natural daughter decay product, thallium-208, which has a half-life of 3.05 minutes.)
If we consider that U233 itself is not a waste product, then its radiotoxicity period, in practical terms, cannot be considered an environmental health concern. This is a broadly-used rationalization by thorium-based nuclear energy advocates. We then turn to the fission products of the thorium fuel cycle. As explained above, for thorium characteristic number (i) above, fission products are created when the U233 atom splits during the fission event. The fission products typically consists of a combination of two new atoms that are created, which are, in the aggregate, roughly equal to the mass of the original fissile atom – in this case, U233. In many cases, these fission products are unstable radioactive isotopes – owing to their neutron-rich nature. These atoms rapidly decay into longer lived, though still unstable radioactive isotopes, until eventually they decay into stable isotopes. While they are decaying, they emit beta and/or gamma radiation and are therefore toxic. The longest-lived fission products of U233 are statistically estimated to be mostly strontium-89, strontium-90, strontium-91, zirconium-91, zirconium-94; and barium-139, barium-140, lanthanum-141 and cesium-140. Of these long-lived fission fragments, the longest half life is no longer than 90 years. As a comparison, the long-lived decay and fission products generated by the uranium fuel cycle range from 200,000 to 16 million years.
iv) Thorium has superior fuel economy in various respects; thorium generates more energy per unit of mass than uranium by a factor of approximately 30
There are various ways in which the thorium fuel cycle creates energy efficiency vis-à-vis the uranium fuel cycle. The key components are:
i) Thorium does not contain any fissile isotopes which must be concentrated in the manufacture of fuel. Therefore, 100% of thorium mass can be used in the fuel cycle, whereas only approximately 11% of mined uranium mass is used in the uranium fuel cycle. This is explained in greater detail below, for thorium characteristic number (ix);
With these efficiencies, it is possible to generate 20 to 40 times more energy from the same quantity of thorium material as from uranium material. Put another way, whereas reactors today generally require approximately 550,000 pounds of uranium in order to have sufficient fuel to power a reactor for a year of operation, approximately 16,000 to 22,000 pounds of thorium would be required to power the same reactor for the same time, generating an equal amount of power.
There is no question that thorium fuel is more energy efficient than any uranium-based fuel.
v) Thorium is three to four times more abundant in nature than uranium, and is widely distributed throughout the Earth’s crust
Thorium occurs naturally as thorium oxide, or ThO2. Thorium oxide is three to four times more abundant in nature than uranium, and approximately as common as lead. Because of thorium oxide’s insolubility, it tends not to spread homogeneously throughout the biosphere, but rather concentrate into large deposits, particularly in alluvial sands. The highest concentration of thorium is in mineral beach sand deposits, which are alluvial placer deposits that have accumulated on the edge of existing or ancient oceans, and most commonly remain at surface. The distribution of these deposits, however is very broad. Significant deposits are reported in Australia, southern Africa, India, southeast Asia, Scandinavia, Brazil, Mexico, USA, northern Canada and elsewhere. In short, large concentrations are known to exist in every continent, and nearly every corner of the world, granting easy access to this material to virtually every sovereign nation.
Conversely, uranium, though it was until recently believed to be available at supply levels adequate to sustain the international nuclear energy industry for an indefinite period, is in imminent danger of creating a supply crisis for the nuclear industry. Though there is ample uranium found in nature, it scarcely occurs in economic concentrations (at recent prices). Furthermore, there is insufficient mining developments to increase the production levels to meet the annual reactor requirements, nor are there sufficient mining developments for the foreseeable future, due to the fact that uranium prices are too low to justify them. Uranium mining is costly and very time-consuming. From the time of a potentially economic discovery, many years of regulatory approvals processes are required before any new mine can begin production. It is not uncommon for a new uranium mine to have a 10- to 15- year gestation from discovery to production. Thorium material, due to its geologic nature, is less costly and less environmentally obtrusive to mine. Thorium mining will likely require far less regulatory scrutiny and hence, will be mined much quicker. From the beginning of thorium mine development to production may take 2 to 3 years, rather than the challenging 10 to 15 years of a uranium mine development. Therefore, not only is thorium more abundant naturally, it is also far more likely than uranium to be mined in quantities that will be sufficient to sustain the international atomic industry.
vi) Thorium can be mined with relatively low-cost, environmentally-unobtrusive mining methods, because it exists in high concentrations, in high-grade, surface, alluvial material
Thorium is hosted most significantly in the minerals: monazite, thorite, uranothorite, thorianite, and within larger mineral structures such as, carbonatite, bastnaesite, allanite and others. The majority of monazite is found in mineral beach sands, as discussed above, in relatively low grade, but in very large tonnages. These mineral sand deposits are typically on the order of billions of tons. Monazite is typically very high grade in thorium and rare earth elements; on average, monazite material grades 6% to 7% thorium. (For further details of the natural occurrences of thorium, please go to Natural Occurrences of Thorium.) Owing to the high concentrations and the high-grade nature of these deposits, and the fact that they occur at surface, the costs of mining are consequently low and environmental impact is minimal. The ore material, which is sand, can simply be skimmed off the surface by way of dredge mining. The thorium oxide is liberated from the monazite by way of gravitational, magnetic and electrostatic circuit, employing a minimum of chemicals.
This compares to uranium, which is always mined from underground ores, and which must always be mined by either: traditional underground mining methods, open pit mining, or the least obtrusive in-situ leach method. Each of these three methods are comparatively very expensive, and create considerable environmental impact, particularly the former two. The uranium ores must then be milled, to liberate the uranium oxide from the rock, employing the use of many potentially dangerous acids and other chemicals.
Thorium can also be mined from other mineral hosts, such as carbonatite, as described above. This mining would be similar to traditional mining with corresponding costs and environmental impact, however there is such an abundance of monazite in the mineral sands that it is not likely that any source for thorium beyond monazite will ever be necessary. Furthermore, monazite-hosted material contains a wealth of other valuable by-products so as to make monazite mining more economic. There should be no desire, on the part of any economic concern, to seek thorium elsewhere besides monazite.
vii) Thorium fuel can be used in 90% of the world’s existing reactors with no differential infrastructure or differential costs; thorium fuel can be fabricated at existing mixed oxide (MOX) fuel fabrication facilities with no differential infrastructure or differential costs
Mixed oxide fuel (MOX) is widely used throughout Europe and Russia, and is more recently being adopted in Japan. As of 2010, there are 75 nuclear reactors in the world (out of the total 440 installations), which are licensed to burn MOX in one-third or more of their reactor cores. Generally speaking, all currently operating reactors have the technical capability of burning up to 50% of their cores with MOX without the need for any significant infrastructural changes. The fact that only 75 reactors are licensed for MOX is a regulatory restriction, and not a technical one. All new reactors (such as the Areva’s EPR, Westinghouse’s AP1000, GE-Hitachi’s ESBWR, etc) are designed to burn 100% of their cores with MOX.
MOX fuel fabrication facilities exist in France, Russia and the UK, where they have operated successfully for decades. Most recently, new facilities have begun construction in Japan, China, India and the USA, and will be operational within the decade. Thorium-Plutonium MOX (TMOX) can be fabricated using these facilities with only minor calibrations to any one of the production lines.
viii) Thorium MOX can be burned in any reactor that is licensed to use MOX; as of 2010, there are 75 reactors in the world with such a license, and this number is rapidly expanding; all new reactor designed are capable of burning MOX in 100% of their cores
As of 2008, only 30 reactors were licensed to burn MOX in 30% of their reactor cores. As of 2010, there are 75 reactors licensed to burn MOX in 30% or more of their cores. The number is expected to continue climbing, as all reactors technically do not lack the capability of burning MOX, but only require the licensing from the relevant nuclear regulatory body. As above, all major new reactors are designed to burn 100% of their cores with MOX.
ix) Unlike uranium, thorium does not possess any fissile isotopes in its naturally occurring form; consequently, there is no material that can be enriched to weaponizable levels, and the conversion and enrichment phases of the fuel cycle become obviated
As above, in thorium characteristic (iv), thorium is considered a more fuel efficient fuel, in part, because it does not require conversion or enrichment as uranium does. The reason for this is that thorium occurs naturally with no isotopic contamination. Natural thorium consists 100% of the stable fertile isotope Th232. Natural uranium consists of approximately 99.284% stable isotope U238, approximately 0.711% fissile isotope U235, and approximately 0.0055% non-fissile U234. In order to create viable uranium-based fuel, the fissile component – in this case, U235 – must be concentrated, or enriched, to at least 3% of the fuel mass. The average level of enrichment is 4.2%.
There are numerous difficulties with the processing of uranium raw material into fuel. As above, the naturally-occurring fissile content must be enriched, which is an expensive and energy-intensive process. In effect, fissile material is being concentrated into a form that is better suited for bomb production. In Iran, there has been a great deal of controversy surrounding Iran’s alleged proprietary capability of enriching uranium. This capability in itself is not harmful if the level to which the fissile material is held to 3% to 5%, as it should be for fuel manufacture. However, the danger exists that enrichment to a much higher concentration, such as 85% to 100% U235, will take place, as is suspected in Iran. U235 concentrated to these levels constitutes bomb-grade material. (The material used in the bomb dropped on Hiroshima, Japan during World War II was highly-enriched uranium [50 kilograms enriched to 89% U235 plus 14 kilograms enriched to 50% U235, for an average enrichment of 80%]). Therefore, it is undesirable for sovereign nations who are not party to the Non-Proliferation Treaty to have the proprietary capability of uranium enrichment. However, uranium enrichment is required for the production of nuclear fuel, and sovereign nations certainly have the right to generate electric power from nuclear energy.
Another difficulty is the conversion process. In order for uranium to be able to undergo the enrichment process, uranium oxide must first be converted into fluoride material. This fluorination process is known as “conversion.” Conversion is also an expensive and energy-intensive process. At each of these early stages of the nuclear fuel cycle, besides the expense, there also exists the risk of theft, as the uranium must undergo various stages of transport:
§ uranium mine to conversion facility
§ conversion facility to enrichment facility
§ enrichment facility to fuel fabrication facility
§ fuel fabrication facility to reactor
The transport between each of these phases poses a threat of possible theft of concentrated fissile material, which may be further concentrated, or used as it is, for a so-called “dirty bomb”. As a consequence of these risks, transport in between these phases, and the facilities themselves must have adequate security to prevent a potential theft or attack. This added security comes at no little expense.
Thorium fuel is an entirely different proposition, and presents a greatly simplified fuel cycle. Once again, natural thorium consists only of its stable isotope Th232. There is no isotopic contamination as with uranium. Th232 is not fissile material, and cannot by itself begin or sustain a chain reaction. Th232 is fertile material, however, and when added to an extraneous quantity of fissile material, such as Pu239, can then be transmuted into U233 and then undergo fission. The process for adding this fissile material (Pu239) is the exact same process that is used to fabricate standard MOX fuel. The key here is that two steps in the nuclear fuel cycle are obviated. Conversion and enrichment are not only unnecessary, but impossible with thorium. Therefore, the thorium only undergoes two transport segments, rather than the four transport segments necessary with uranium fuel. Namely, the thorium (with no resident fissile material whatsoever) is transported from mine to fuel fabrication facility, and from the fuel facility directly to the reactor. In reality, the security for the first transport segment can be greatly reduced because there is no danger of any fissile material falling into the wrong hands, because there is no fissile material.
These differences pose great savings and risk mitigating factors for the nuclear fuel cycle.
x) Thorium fuel cycle waste can be reprocessed and used as fissile material in a closed fuel cycle, meaning that eventually no new fissile material will be required to power reactors; however the reprocessing technology (to separate U233) does not yet exist
The present design for Thorium-Plutonium MOX fuel (TMOX) considers adding plutonium recycled from nuclear reactor waste (or weapons) and mixing it with thorium in order to have the required fissile material necessary to begin and sustain a chain reaction, which is essential in the nuclear energy process. It was explained above that the waste product of such a fuel would be devoid of any dangerous plutonium or minor actinides, and that the waste profile would in fact consist only of Th232 and U233.
Uranium-233 is one of the rare isotopes in the physical universe which is fissile. It does not occur naturally, but can only be created as a by-product of the thorium fuel cycle. The technology does not exist at present for the U233 waste from a thorium fuel cycle to be safely and/or economically separated, reprocessed and recycled into new fuel. However, development is currently underway, and is a solution is expected to be inevitable. A fuel cycle which creates a waste product that can be recycled for reuse is referred to as a “closed cycle.” The implication is that no new fissile material need be introduced into the fuel cycle, and the nuclear power station self-sufficiently generates power for an indefinite period.
The fuel campaign at the Shippingport nuclear reactor in Pennsylvania, USA burned a fuel made of Th232/U233. The U233 was manufactured at a major US national laboratory at an unknown cost. There is no question that U233 can be used as fissile materials in a thorium fuel. The only question which remains is how to use it economically and safely. As above, what makes the thorium fuel cycle so proliferation-resistant is the fact that U233 is naturally contaminated by trace amounts of U232, whose decay products emit lethal doses of gamma radiation. The non-proliferative feature, then, of the thorium fuel cycle, also becomes the difficulty in reprocessing the waste for a closed fuel cycle. The concept of the closed cycle is so powerful, particularly for its ramifications for a sustainable power source. There can be no more sustainable or renewable energy source than one that never actually requires any new fuel.
As a final note to this section, it has been noted extensively in the literature that reactors which use thorium-based fuel cannot melt down. This is in fact true, but it is misleading in the sense that all Generation III reactors are built with so-called “passive safety” features. This means that the laws of physics themselves, rather than human intervention, are used to prevent any runaway reactions or leaks, or any other possible reactor malfunctions. It is true that the properties of thorium are even better suited to passive safety, but it is not a unique quality of thorium fuel, nor is it the whole truth to proclaim, that thorium fuel is “meltdown-proof.”
“Thorium” World Nuclear Association, Oct 1, 2009 (http://www.world-nuclear.org/info/default.aspx?id=448&terms=thorium)