High-temperature gas-cooled reactors : Thorium fuel was used in HTRs prior to the successful demonstration reactors described above. This reactor used thorium-HEU fuel elements in a 'breed and feed' mode in which the U formed during operation replaced the consumption of U at about the same rate. The fuel comprised small particles of uranium oxide 1 mm diameter coated with silicon carbide and pyrolytic carbon which proved capable of maintaining a high degree of fission product containment at high temperatures and for high burn-ups.
The particles were consolidated into 45mm long elements, which could be left in the reactor for about six years. About kg of thorium was used in some , pebbles. Light water reactors : The feasibility of using thorium fuels in a PWR was studied in considerable detail during a collaborative project between Germany and Brazil in the s 5. The vision was to design fuel strategies that used materials effectively — recycling of plutonium and U was seen to be logical.
The study showed that appreciable conversion to U could be obtained with various thorium fuels, and that useful uranium savings could be achieved. The program terminated in for non-technical reasons. It did not reach its later stages which would have involved trial irradiations of thorium-plutonium fuels in the Angra-1 PWR in Brazil, although preliminary Th-fuel irradiation experiments were performed in Germany.
Most findings from this study remain relevant today. Thorium-plutonium oxide Th-MOX fuels for LWRs are being developed by Norwegian proponents see above with a view that these are the most readily achievable option for tapping energy from thorium.
This is because such fuel is usable in existing reactors with minimal modification using existing uranium-MOX technology and licensing experience. This reactor platform, designed by Hitachi Ltd and JAEA, should be well suited for achieving high U conversion factors from thorium due to its epithermal neutron spectrum. High levels of actinide destruction may also be achieved in carefully designed thorium fuels in these conditions.
The RBWR is based on the ABWR architecture but has a shorter, flatter pancake-shaped core and a tight hexagonal fuel lattice to ensure sufficient fast neutron leakage and a negative void reactivity coefficient. The central seed portion is demountable from the blanket material which remains in the reactor for nine years f , but the centre seed portion is burned for only three years as in a normal VVER.
Design of the seed fuel rods in the centre portion draws on experience of Russian naval reactors. The fuel contained 2. The experiment was not representative of commercial fuel, however the experiment allowed for fundamental data collection and benchmarking of codes for this fuel material.
The reactor ran over at powers up to 7. There is significant renewed interest in developing thorium-fuelled MSRs.
Safety is achieved with a freeze plug which if power is cut allows the fuel to drain into subcritical geometry in a catch basin. There is also a negative temperature coefficient of reactivity due to expansion of the fuel. A third stream of fast reactors to consume actinides from LWRs is planned. The technical difficulty of using molten salts is significantly lower when they do not have the very high activity levels associated with them bearing the dissolved fuels and wastes.
The experience gained with component design, operation, and maintenance with clean salts makes it much easier then to move on and consider the use of liquid fuels, while gaining several key advantages from the ability to operate reactors at low pressure and deliver higher temperatures. Accelerator-driven reactors : A number of groups have investigated how a thorium-fuelled accelerator-driven reactor ADS may work and appear.
This reactor operates very close to criticality and therefore requires a relatively small proton beam to drive the spallation neutron source. Earlier proposals for ADS reactors required high-energy and high-current proton beams which are energy-intensive to produce, and for which operational reliability is a problem.
Kamini is water cooled with a beryllia neutron reflector. The total mass of U in the core is around grams. Aqueous homogeneous reactor : An aqueous homogenous suspension reactor operated over in the Netherlands at 1 MWth using thorium plus HEU oxide pellets. The thorium-HEU fuel was circulated in solution with continuous reprocessing outside the core to remove fission products, resulting in a high conversion rate to U Thorium fuel cycles offer attractive features, including lower levels of waste generation, less transuranic elements in that waste, and providing a diversification option for nuclear fuel supply.
Also, the use of thorium in most reactor types leads to extra safety margins. Despite these merits, the commercialization of thorium fuels faces some significant hurdles in terms of building an economic case to undertake the necessary development work.
A great deal of testing, analysis and licensing and qualification work is required before any thorium fuel can enter into service. This is expensive and will not eventuate without a clear business case and government support. Also, uranium is abundant and cheap and forms only a small part of the cost of nuclear electricity generation, so there are no real incentives for investment in a new fuel type that may save uranium resources.
Other impediments to the development of thorium fuel cycle are the higher cost of fuel fabrication and the cost of reprocessing to provide the fissile plutonium driver material. The high cost of fuel fabrication for solid fuel is due partly to the high level of radioactivity that builds up in U chemically separated from the irradiated thorium fuel.
Separated U is always contaminated with traces of U which decays with a year half-life to daughter nuclides such as thallium that are high-energy gamma emitters. Although this confers proliferation resistance to the fuel cycle by making U hard to handle and easy to detect, it results in increased costs.
There are similar problems in recycling thorium itself due to highly radioactive Th an alpha emitter with two-year half life present. Some of these problems are overcome in the LFTR or other molten salt reactor and fuel cycle designs, rather than solid fuel. Particularly in a molten salt reactor, the equilibrium fuel cycle is expected to have relatively low radiotoxicity, being fission products only plus short-lived Pa, without transuranics. These are continually removed in on-line reprocessing, though this is more complex than for the uranium-plutonium fuel cycle.
Nevertheless, the thorium fuel cycle offers energy security benefits in the long-term — due to its potential for being a self-sustaining fuel without the need for fast neutron reactors. It is therefore an important and potentially viable technology that seems able to contribute to building credible, long-term nuclear energy scenarios. With huge resources of easily-accessible thorium and relatively little uranium, India has made utilization of thorium for large-scale energy production a major goal in its nuclear power programme, utilising a three-stage concept first proposed at the University of Chicago in In all of these stages, used fuel needs to be reprocessed to recover fissile materials for recycling.
India is focusing and prioritizing the construction and commissioning of its fleet of MWe sodium-cooled fast reactors in which it will breed the required plutonium which is the key to unlocking the energy potential of thorium in its advanced heavy water reactors.
As such, to make reactor fuel we have to expend considerable energy enriching yellowcake, to boost its proportion of U Once in the reactor, U starts splitting and releasing high-energy neutrons.
The U does not just sit idly by, however; it transmutes into other fissile elements. When an atom of U absorbs a neutron, it transmutes into short-lived U, which rapidly decays into neptunium and then into plutonium, that lovely, weaponizable byproduct. When the U content burns down to 0. This waste fuel is highly radioactive and the culprits — these high-mass isotopes — have half-lives of many thousands of years.
As such, the waste has to be housed for up to 10, years, cloistered from the environment and from anyone who might want to get at the plutonium for nefarious reasons. Thorium's advantages start from the moment it is mined and purified, in that all but a trace of naturally occurring thorium is Th, the isotope useful in nuclear reactors.
Then there's the safety side of thorium reactions. Unlike U, thorium is not fissile. That means no matter how many thorium nuclei you pack together, they will not on their own start splitting apart and exploding.
If you want to make thorium nuclei split apart, though, it's easy: you simply start throwing neutrons at them.
Then, when you need the reaction to stop, simply turn off the source of neutrons and the whole process shuts down, simple as pie. Here's how it works. When Th absorbs a neutron it becomes Th, which is unstable and decays into protactinium and then into U That's the same uranium isotope we use in reactors now as a nuclear fuel, the one that is fissile all on its own.
Thankfully, it is also relatively long lived, which means at this point in the cycle the irradiated fuel can be unloaded from the reactor and the U separated from the remaining thorium.
The uranium is then fed into another reactor all on its own, to generate energy. The U does its thing, splitting apart and releasing high-energy neutrons. But there isn't a pile of U sitting by. Remember, with uranium reactors it's the U, turned into U by absorbing some of those high-flying neutrons, that produces all the highly radioactive waste products.
With thorium, the U is isolated and the result is far fewer highly radioactive, long-lived byproducts. But if the number of reactors increases, we could reach a situation where supply would no longer keep up, and using thorium can drastically reduce the need for uranium. That makes it a potentially more sustainable option," Sylvain David explained. According to supporters of thorium, it would also a "greener" solution. Unlike the uranium currently used in nuclear power plants, burning thorium does not create plutonium, a highly toxic chemical element, Nature pointed out.
With so many positives on their side, why are molten salts and thorium only being used now? The other two must be bombarded with neutrons for the material to become fissile able to undergo nuclear fission and be used by a reactor: a possible but more complex process. Once that is done on thorium, it produces uranium , the fissile material needed for nuclear power generation. That then becomes another problem with thorium: "The radiation emitted by uranium is stronger than that of the other isotopes, so you have to be more careful," Francesco D'Auria warned.
The feasibility of molten-salt reactors is also questionable as it creates further technical problems. Add in actinides such as protactinium half life: 33, years and it soon becomes apparent that thorium's superficial cleanliness will still depend on digging some pretty deep holes to bury the highly radioactive waste. With billions of pounds already spent on nuclear research, reactor construction and decommissioning costs — dwarfing commitments to renewables — and proposed reform of the UK electricity markets apparently hiding subsidies to the nuclear industry, the thorium dream is considered by many to be a dangerous diversion.
Energy consultant and former Friends of the Earth anti-nuclear campaigner Neil Crumpton says the government would be better deferring all decisions about its new nuclear building plans and fuel reprocessing until the early s: 'By that time much more will be known about Generation IV technologies including LFTRs and their waste-consuming capability. In the meantime, says Jean McSorley, senior consultant for Greenpeace's nuclear campaign, the pressing issue is to reduce energy demand and implement a major renewables programme in the UK and internationally — after all, even conventional nuclear reactors will not deliver what the world needs in terms of safe, affordable electricity, let alone a whole raft of new ones.
The technology is not tried and tested, and none of the main players is interested. Thorium reactors are no more than a distraction. Don't believe the spin on thorium being a greener nuclear option. Ecologist : It produces less radioactive waste and more power but it remains unproven on a commercial scale. Reuse this content.
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