Thorium fission and Polywell Fusion
Why they are needed and how to get there
With this fuel cycle in mind, a Thorium-based nuclear fission reactor can realistically achieve following design goals:
◦ Base energy production on a widely available resource with long-term supplies. As the International Atomic Energy Agency's 2005 'Thorium Fuel Cycle' report (IAEA report) summarizes, Thorium is an abundant and easily accessible energy source:
Thorium is 3 to 4 times more abundant than uranium and is widely distributed in nature as an easily exploitable resource in many countries. Unlike natural uranium, which contains ~0.7% ‘fissile’ 235U isotope, natural thorium does not contain any ‘fissile’ material and is made up of the ‘fertile’ 232Th isotope only.
◦ Less radioactive waste production, both in terms of quantity and half life. Traditional fission reactors produce radioactive waste with half-life of several thousand years, while waste from Thorium-based reactor has a more manageable 50 years half-life. Moreover, the Thorium fuel cycle can be designed to reprocess plutonium and depleted uranium waste produced by traditional reactors.
◦ Possibility of reactor design with low pressure fluid reactor core, which can be safely controlled and stopped at any time. One such design is called 'Liquid Fluoride Thorium Reactor', using molten salt to circulate fission elements in the core.
◦ Possibility of arms-proliferation resistant reactor design, which cannot be used to produce weapons-grade uranium or plutonium.
◦ Compact design, with similar energy intensity and costs per kWh output as traditional nuclear fission reactors. The table below comes from a 1969 paper by Oak Ridge National Laboratory, showing that even at 4GW capacity the reactor core fits into a space of 10 meters in diameter. The IAEA report estimates that energy output from Thorium-based reactor is expected to be slightly cheaper - at 4 cents/kWh - than costs per kWh of traditional fission reactors. This estimate is based on detailed analysis of fuel life-cycle and past reactor prototypes.
The IAEA report list a number of possible reactor design options for a Thorium based fission reactor:
◦ Light water reactors (LWR)
◦ Heavy water reactors (HWR)
◦ High temperature gas cooled reactors (HTGR)
◦ Fast reactors (where 'fast' refers to the energy spectrum of fission-inducing neutrons).
◦ Accelerator driven reactor system. This type is studied mostly for disposal of nuclear waste from uranium-based fission reactors and is not relevant at present for energy production.
◦ Molten salt breeder reactors (MSBR), where molten salt fluid is circulating fissile reactor fuel through reactor core.
Hence current research is focused on liquid-core reactor designs for efficient Thorium-based reactor designs. The initial motivation and history of Thorium-based liquid core reactors is summarized by M. W. Rosenthal in his 1969 'Molten Salt Reactors' research paper:
That molten-salt reactors might be attractive for civilian power applications was recognized from the beginning of the American Nuclear Program, and in 1956 H.G. MacPherson formed a group to study the technical characteristics, nuclear performance, and economics of molten-salt converters and breeders. After considering a number of concepts over a period of several years, MacPherson and his associates concluded that graphite-moderated thermal reactors operating on a thorium fuel cycle would be the best molten-salt systems for producing economic power [these are called heterogenous reactor design]. The thorium fuel cycle with recycle of U233 was found to give better performance in a molten-salt thermal reactor than a [depleted] uranium fuel cycle in which U238 is the fertile material and plutonium is produced and recycled.
The operating principle of molten salt reactors is summarized in the IAEA report as follows:
Molten salt reactors (MSR) use graphite as moderator, molten fluoride salt of high boiling point (≥1400 Celsius) with dissolved ‘fissile’ and ‘fertile’ materials as fuel and primary coolant and operate in an epithermal neutron spectrum. The core of MSR is usually a cylindrical graphite block that acts as moderator, through which holes are bored, in which the molten fluoride salt containing thorium uranium and plutonium circulates. The primary coolant, containing the fuel, flows to a primary heat exchanger, where the heat is transferred to a secondary molten salt coolant and then flows back to the graphite channel of the reactor core. The secondary coolant loop transfers the heat to the power cycle or hydrogen production facility. The operating temperature range of MSRs is between 450 Celsius, the melting point of eutectic fluoride salts to around 800 Celsius. In the secondary molten salt, the temperature is lower than the primary. The reactor and the primary systems are constructed of nickel-based alloys, modified Hastelloy–B and N, inconel or a similar alloy or other promising materials like Nb–Ti alloys for corrosion resistance to the molten salt. Volatile fission products (e.g. Kr and Xe) are continuously removed from the fuel salt. MSRs have a low inventory of fissile materials compared with other reactors because:
(i) these are thermal neutron reactors needing less fissile inventory than fast reactors,
(ii) a low fuel-cycle fissile inventory outside the reactor system,
(iii) little excess reactivity is required to compensate for burnup because of fuel is added on-line,
(iv) direct heat deposition in fuel/coolant that allows high power densities
(v) high absorption fission products such as Xe are continuously removed.
The 8 MWt Experimental molten salt reactor (MSRE), constructed in Oak Ridge National Laboratory (ORNL), USA in the 1960s as part of breeder reactor development programme, is the first and the only thorium-based MSR in the world. The 8 MWt MSRE had a core volume of <2 cubic meters and operated with a molten fuel coolant salt of composition LiF/BeF2/ThF4/UF4, at an outlet temperature of 650 Celsius and demonstrated:
(i) the chemical compatibility of graphite moderator with fluoride salt,
(ii) the removal of Xe and Kr from the fuel
(iii) conversion of 232Th to 233U and in situ fission of the latter. The MSRE generated database on the physical, chemical and corrosion properties of molten salts, worked with different fuels, including 235U, 233U and plutonium during 1965–1969 and paved the way for the conceptual design of a molten salt breeder reactor (MSBR) of 1 000 MW(e) in the mid 1970s. The graphite moderated MSBR–1000 was designed for achieving thermal breeding in 232Th –233U fuel cycle (breeding ratio ~1.06) and generation of electricity using a steam cycle. The proposed fuel core had a volume of 48.7 cubic meters with a molten salt composition of 71.7 mole% Li7F, 16% BeF2, 12% ThF4 and 0.3% UF4 and 233U and Th inventory of ~1 500 kg and 68 100 kg respectively. The MSRE was shutdown in December 1969 and the MSBR–1000 was not constructed.
The discontinued experimental molten salt reactor at Oak Ridge National Laboratory is the only experimental fluid-core reactor even up to the present. The following table from IAEA report lists past research efforts on Thorium-based reactors, including all technology varieties:
Surprisingly, all experimental work involving Thorium-based reactor fuels has been abandoned in both Europe and USA during 1980s - global warming has not yet been a hot discussion point at the time and oil/gas/coal energy options have provided easier political route to soothe public concerns than reactor safety tests and discussions. Recently some renewed research interest has arisen in Europe to use thorium-based reactors for burning of radioactive waste produced by U235 based fission reactors - this focus however most probably results in different reactor design than the one optimized for efficient energy production.
Although the experimental reactor at Oak Ridge National Laboratory has been shut down in 1969, its results highlighted major design issues to be addressed, which have kept researchers busy in subsequent years:
◦ Ensuring that graphite moderators would not have to be replaced too frequently because of radiation damage. This issue can be addressed by proper geometric arrangement of reactor core design.
◦ Ensuring negative temperature coefficient, meaning that reactor output power would decrease when temperature increases. This implies inherent system stability. Although a safety drain valve guarantees safety of liquid core reactors in any case, the negative temperature coefficient is a guarantee of smooth operation for the utility company. This issue is addressed through either appropriate design of molten salt material or through adjustment of neutron energy spectrum.
◦ Avoidance of on-line fuel reprocessing requirement, if possible. This results in major simplification of required chemical processes.
Teams of researchers have worked around the globe, addressing above issues and designing Thorium based molten salt reactor for future energy production.
In the USA, the research team at Oak Ridge National Laboratory has consolidated during the 1970s their experimental results into a conceptual industrial-scale reactor design. Further theoretical research has not been funded in the USA, despite several proposals; the latest being the 'Deep-Burn Molten-Salt Reactors' proposal from 2002, jointly prepared by scientists from five research institutes.
In Europe, the French LPSC research institute has a group of scientists working in this design challenge. Their technical research results have provided new insight into the industrially stable design of Thorium-based molten salt reactors (TMSR). Basically their proposal is a reactor design that can run without a moderating graphite rods, with Plutonium as fissile matter on a Thorium basis. They summarize their results as follows:
Within the frame of our studies for nuclear energy production with innovative systems, we have concentrated our efforts on the thorium fuel cycle in molten salt reactors (MSRs). These reactors, based on a liquid fuel circulating in a solid moderator, have been operated successfully in experimental tests done in the 1960s. However, a power reactor project, the Molten Salt Breeder Reactor (MSBR) was discontinued at the time. Although it has been re-evaluated several times during the last decade, the MSBR suffers from several major drawbacks. In particular, the concept aims at obtaining the highest breeding ratio thanks to a high performance and, as a result, constraining, on-line fuel processing system. Today, this fuel reprocessing is deemed non-feasible. Moreover, recent re-evaluations have determined that the MSBR has a slightly positive global temperature coefficient, while, at the time, a negative global temperature coefficient had been announced. With this new finding, the reactor becomes potentially unstable. For these reasons, the MSBR concept, although it is still considered to be one of the reference MSRs, cannot hope to reach industrial status.
...
With these studies, we have gained new understandings of the behavior of MSRs, ranging from very thermalized neutron spectra to fast spectra. Our results represent a split with previous knowledge in this field. The traditional association of the thorium fuel cycle, the MSR, and the (epi)thermal neutron spectrum is now becoming history, since fast spectra lead to very satisfactory results, indeed much better ones. This reopens the issue of starting such a reactor with plutonium. While it generated too many TRUs in the (epi)thermal spectrum, this avenue can no longer be ignored in the case of the fast spectrum MSR.
The problems raised by the MSBR have thus found solutions. The temperature coefficients can be made negative, either by hardening the neutron spectrum, or with a tighter knit moderator network. Breeding can be obtained with fuel processing that is simpler than that considered for the MSBR thanks to the thorium blanket (or even without one in the fast spectrum configuration). Finally, the moderator’s short life-time is an issue that can be solved with the fast spectrum configuration by including no graphite in the core.This work demonstrates that very acceptable reactor configurations can be defined that best respond to each constraint and this is true of all neutron spectra types. Without ignoring the other solutions, we are turning our attention more particularly to the single salt channel configuration both in its conventional version, and in a very high temperature version.
A Russian team of scientists led by Dr R. M. Yakovlev might have arrived at a similar design conclusion as the LPSC research team, although details of their work are not known. Their reactor design plan has been published in a research paper titled 'Homogeneous Molten Metal Reactor on Fast Neutrons (HMMR) with Dispersed Fuel' at the 10th International Seminar on Advanced Nuclear Fuel Cycle for the XXI Century (2007).
In Japan, a group of researchers, with participants from multiple institutions and led by Dr Kazuo Furukawa, has chosen the strategy of physically separating the fissile breeding process, which turns thorium into fissile U233, and the energy production process. The fissile breeding part would be accomplished by an Accelerator Molten-Salt Breeder (AMSB) device, which accelerates a high energy proton beam into the thorium fuel. The energy product part would be accomplished by the 'FUJI' molten salt reactor, whose triple core design concept enables running it over 30 years without replacement of graphite control rods. Dr Furukawa summarizes their results the following way in a research paper:
For global survival, we need to launch a rapid regeneration of the nuclear power industry. The replacement of the present fossil fuel industry requires a doubling time for alternative energy sources of 5–7 years and only nuclear energy has the capability to achieve this. ... The Thorium Molten-Salt Nuclear Energy Synergetic System [THORIMS-NES], described here is a symbiotic system, based on the thorium–uranium-233 cycle. The production of trans-uranium elements is essentially absent in Th–U system, which simplifies the issue of nuclear waste management. The use of 233U contaminated with 232U as fissile material, instead of plutonium/235U makes this system nuclear proliferation resistant. The energy is produced in molten-salt reactors (FUJI) and fissile 233U is produced by spallation in Accelerator Molten-Salt Breeders (AMSB). This system uses the multi-functional “single-phase molten-fluoride” circulation system for all operations. There are no difficulties relating to “radiation- damage”, “heat-removal” and “chemical processing” owing to the simple “idealistic ionic liquid” character of the fuel. FUJI is size-flexible, and can use all kinds of fissile material achieving a nearly fuel self-sustaining condition without continuous chemical processing of fuel salt and without core-graphite replacement for the life of the reactor. The AMSB is based on a single-fluid molten-salt target/blanket concept. Several AMSBs can be accommodated in regional centers for the production of fissile 233U, with batch chemical processing including radio-waste management. FUJI reactor and the AMSB can also be used for the transmutation of long-lived radioactive elements in the wastes and has a high potential for producing hydrogen-fuel in molten-salt reactors. The development and launching of THORIMS-NES requires the following three programs during the next three decades: (A) pilot-plant: miniFUJI (7–10 MWe), (B) small power reactor: FUJI-Pu (100–300 MWe), (C) fissile producer: AMSB for globally deploying THORIMS-NES.
Note: values in parenthesis on reactor core diagram are fuel/graphite volume ratios.
The way from these very promising conceptual results to building the first commercial thorium based fission reactor leads through a number of pre-commercial experimental validation projects. It is important to understand the contrast between present scenario and the circumstances within which technology of U235 based fission reactors were commercialized.
At the time when first nuclear fission reactors were started up, industrial countries were in a nuclear arms race. Divided into two blocks, they raced to build up a vast amount of nuclear arsenal as credible deterrent against attack by the other side. Nuclear fission reactors were needed for production of weapons-grade uranium and plutonium material. By their reason for existence, these plants were rather military objects than civilian ones. Costs and technology risks did not matter in the public-financed effort for their development. To the public this effort has been of course presented as investment into sustainable energy for the atomic era - which is a true statement, but hides the primary objective. It needs to be emphasized that no investor in the market economy would find such investment to be justified - as long as traditional energy sources remain abundant.
Let's contrast that to current environment, which shapes nuclear industry:
◦ There is no military need for the development of thorium based fission energy
◦ Electric utilities would not venture into building power plants from concept designs of 'unproven technology' - at least not without an applied political pressure. The risk of investment loss of be too high, caused by some unexpected malfunction.
◦ From investment point of view, as long as traditional energy sources supply demand, it is too risky to spend billions on building an experimental reactor or thorium mining capacity, when this amount could be profitably and at low risk invested into traditionally fueled power plants. Demand for new power plants reduces to or below replacement level only at the point of peaking resource supply capacity, described in previous chapter.
◦ The favored governmental practice is to channel taxpayer money into support of solar power, wind power, and biofuel installations, as 'sustainability policy'. It is natural for politics to gravitate towards supporting such installations which look good on press photos, have low implementation risk, and can be assigned under the management of some major companies. These projects enable a positive press coverage and public relations feedback already before next elections. Which politician would choose then to support a multi-year thorium fission experimentation, when general public prefers solar to nuclear and would have a difficult time distinguishing between U235 based and Thorium based nuclear fission?
◦ Thorium fission related research activities are almost non-existent at universities and research institutes, with the mentioned LPSC institute being the only one in the EU with a research group working on this subject. I presume it follows from above points and the history of shutting down earlier thorium fission research plants that researchers feel that any scientific results in this area would be shelved during their lifetime.
Altogether, above contrast points out why it seems improbable that importance of thorium utilization would be automatically recognized before an eventual energy crisis. Researchers working on the theory of thorium-based fission have tried to initiate experimental programs, mostly unsuccessfully. The Japanese research groups report: 'Having proposed developmental projects for governmental supports without success'. In the USA, the Department of Energy-led 'Generation IV' initiative for nuclear technology has assigned about 40 000 USD budget for the study of molten salt reactor technology - no further comment is needed on that program.
The most promising news is from France, where an intention to start building an experimental thorium based reactor about 9 years from now has been communicated. Quoting from the 2007 newsletter of Nuclear Energy Agency:
Besides France, the only European country which has currently shown slight interest in Thorium utilization is Norway. The Norwegian Ministry of Oil and Energy has commissioned the above mentioned 'Thorium as Energy Source' report. Following are the relevant practical recommendations of the report:The project of research program for the MSR was updated during the year 2007. In accordance with the general development plan for this system, the phase of exploration and selection will continue until 2011. At this stage, the parameters for the salts (choice, properties and compatibility with other materials) will be known. The design of reference will have been selected in 2018, when the project reaches the stage of the execution.
◦ 'Testing of thorium fuel in the Halden Reactor should be encouraged, taking benefit of the well recognized nuclear fuel competence in Halden.'
◦ 'Any new nuclear activities in Norway, e.g. thorium fuel cycles, would need strong international pooling of human resources, and in the case of thorium, a strong long-term commitment in university education and basic science. All these should be included in the country level strategy aiming to develop new sustainable energy sources. '
While these recommendations point to the right direction, they are well short of an experiment reactor design for efficient utilization of Thorium energy. In plain language, this report just recommends to strengthen theoretical know-how and to see if thorium can be used somehow in an existing fission plant.
In the view of an overall peaking of energy capacity in less than 20 years, the French plan is way too little too late, and the passivity of other countries reflects serious irresponsibility. A reasonable planning for energy security would include following steps:
1. Urgent selection of reference design for industrial-scale experimental thorium-based fission reactor, by comparison of available reference designs and theoretical results.
2. Building of the industrial-scale experimental reactor, and validating whether all design goals have been achieved.
3. Establishing the industrial process for factory manufacturing of validated thorium-based fission reactor design. This step is required for efficient mitigation of energy scarcity. It has never been done for traditional nuclear reactors, which are all custom-built today. Taking the analogy of cars, it is easy to see the difference in costs and delivery times between custom-built and factory-built cars.
Consequent to observed governmental passivity and outlined logic of disinterest in thorium utilization, activism of informed citizens is very much needed to bring about a realistic solution to energy security before the eventual downturn! As presented in this chapter, there is sufficient evidence on both theoretical and experimental levels, that the abundant potential of thorium-based fission fuels can be unlocked in safe and reliable manner. Each informed person may choose to contribute according to his or her capabilities or may choose to hope passively for 'avoiding the tsunami'.
History books will one day describe the nuclear energy of 20th century with the following words:
Would you like the next of this history textbook to say the same about early 21st century before describing the energy crisis, or to report some sign of collective wisdom finally emerging?The use of nuclear fission power has begun in the mid-20th century. The selected fission process has utilized just 0.7% of uranium, namely the U235 isotope content, and even that has been only partly burned. The remaining 99% of fertile U238 has been treated as waste product, except for its occasional military use in projectiles, which persisted despite reported radioactive exposures in affected areas. This wasteful practice has not been rationalized during 20th century, not even after safety flaws of military-optimized reactor designs resulted in some reactor accidents. Instead, the use of nuclear fission has been supressed altogether, and most countries prioritized returning to an earlier practice of coal burning. This trend has culminated at the end of 20th century with the commissioning of more than one new coal-fired power plant every single week. People seem to have voluntarily agreed to this backwards transition, despite the resulting collective shortening of life expectancy. Records indicate that such course of events resulted not from a lack of scientific knowledge, as several more economical and safe reactor designs were known from the very beginning of nuclear era. Anthropologists are currently searching for the resolution of this paradox.