Thorium reactors

 Thorium  reactors

Srilanka has a higher potential to use it's natural resources including Thorium compounds to gain it's energy needs through nuclear energy  though construction of nuclear plant is a highly expensive process.Otherwise it's long term profits are very attractive and cheaper in daily operational scale.Srilanka need much more technical support and knowledge related to nuclear science and nuclear engineering field which should be obtained by participating to co-operative workshops with the countries like India whose knowledge may be very useful in both of technical and scientific fields to advance nuclear energy approach for Srilanka.
Indian heavy water reactors (PHWRs) have for a long time used thorium-bearing fuel bundles for power flattening in some fuel channels.There are many other examples all around the world in Germany, US, Norway,France, China as well as Britain for Thorium reactors which of them provide wide range of diversified study for our optimum choice in power plant reactors.The 300 MWe Thorium High Temperature Reactor (THTR) in Germany operated with thorium-HEU fuel between 1983 and 1989. Over half of its 674,000 pebbles contained Th-HEU fuel particles (the rest comprised graphite moderator and some neutron absorbents). These were continuously moved through the reactor as it operated, and on average each fuel pebble passed six times through the core.Here i discussed some of technical and scientific aspects those are significant in Thorium power plant operations which are very important in study for practicle usage for the energy revolution of many developing countries.

Thorium is fertile rather than fissile can only be used as a fuel in conjunction with a fissile material such as recycled plutonium and as a result of this Thorium fuels can be breed fissile Uranium 233 to be used in various kinds of nuclear reactors.Molten salt reactors are well suited to Thorium fuels, as normal fuel fabrication is avoided in them.Thorium is a naturally occurring slightly radioactive metal discovered in 1828 by the Swedish chemist Jons Jacob Berzelius, who named it after Thor the Norse god of thunder.It is found in small amount in most rocks and soils where it is abundant three times more abundant than Uranium .Soil contains six parts per million of Thorium in normal situations.Regarding Srilankan soils and rocks, There are few of researches have been done in estimating relevant Thorium pc in many areas but few of results show that we can keep fair hopes about possible Thorium profitable resources in Srilanka.mineral occurrences have so far not been performed within the area covered by Sri Lankan Mainland.The first survey for the Monazite sands was conducted by Waylands and Cotes during 1910’s  along coastal area centered on Beruwala. During late fifties and early sixties, preliminary radiometric surveys were conducted in several parts of the country, particularly in SOUTH WESTERN sector, by the GSMB (then Geological Survey Department). In these surveys, number of thorium bearing mineral occurrences was identified These include, Thorianite, Thorite, Monazite and Allanite.  Thorium is found in considerable quantities in these minerals. These surveys were conducted with the assistance of Canadian Government under the Colombo Plan program. In 1997, the Geological Survey of Canada conducted a marine geophysical survey in nearshore area off Panadura - Beruwala  in SOUTH WESTWRN of Sri Lanka in order to study off- shore minerals.In 1979, island wide preliminary stream sediment survey was conducted by the GSMB and AEA with the technical assistance of IAEA to identify Uranium Mineralization.  Concentrations of Monazite in beach sands around Beruwala were exploited using a small experimental plant during the period of 1918 to 1922. About 450 tons of Monazite were reported to be recovered from approximately 3000 tons of raw sand during this period.In 1956, a separation plant was fixed at Katukurunda area near Kalutara by the GSMB as a pilot plant to process mineral sands. According to available records, an average of 1000 tons of sand per year with Monazite concentrations  6-8% produced by plant and exported to Japan but due to some difficulties the work of plant has been abandoned.

Here we can see our Thorium contained minerals are equally same to the concentrations of world's ranks that are currently used for power plants.In nature in a single isotopic form – Th-232 – which decays very slowly (its half-life is about three times the age of the Earth). The decay chains of natural thorium and uranium give rise to minute traces of Th-228, Th-230 and Th-234, but the presence of these in mass terms is negligible. It decays eventually to lead-208.The most common source of thorium is the rare earth phosphate mineral, monazite, which contains up to about 12% thorium phosphate, but 6-7% on average. Monazite is found in igneous and other rocks but the richest concentrations are in placer deposits, concentrated by wave and current action with other heavy minerals. World monazite resources are estimated to be about 16 million tonnes, 12 Mt of which are in heavy mineral sands deposits on the south and east coasts of India. There are substantial deposits in several other countries  such as Brazil,Australia,US,Turkey ect. Thorium recovery from monazite usually involves leaching with sodium hydroxide at 140°C followed by a complex process to precipitate pure ThO2. Inferred resources recoverable at a cost of 80 dollar per a killogram according to recent purification costs of Thorium.

Thorium (Th-232) is not itself fissile and so is not directly usable in a thermal neutron reactor. However, it is ‘fertile’ and upon absorbing a neutron will transmute to uranium-233 (U-233) which is an excellent fissile fuel material. In this regard it is similar to uranium-238 (which transmutes to plutonium-239). All thorium fuel concepts therefore require that Th-232 is first irradiated in a reactor to provide the necessary neutron dosing.

 The U-233 that is produced can either be chemically separated from the parent thorium fuel and recycled into new fuel, or the U-233 may be usable  in the same fuel form, especially in molten salt reactors (MSR).Thorium fuels therefore need a fissile material as a ‘driver’ so that a chain reaction (and thus supply of surplus neutrons) can be maintained. The only fissile driver options are U-233, U-235 or Pu-239.
It is possible but quite difficult to design thorium fuels that produce more U-233 in thermal reactors than the fissile material they consume.This is referred to as having a fissile conversion ratio of more than 1.0 and is also called breeding.Thermal breeding with thorium requires that the neutron economy in the reactor has to be very good.There must be low neutron loss through escape or parasitic absorption.The possibility to breed fissile material in slow neutron systems is a unique feature for thorium-based fuels and is not possible with uranium fuels.Another distinct option for using thorium is as a ‘fertile matrix’ for fuels containing plutonium that serves as the fissile driver while being consumed and even other transuranic elements like americium.Mixed thorium-plutonium oxide fuel is an analog of current uranium fuel, but no new plutonium is produced from the thorium component, unlike for uranium fuels in U-Pu MOX fuel, and so the level of net consumption of plutonium is high. Production of all actinides is lower than with conventional fuel, and negative reactivity coefficient is enhanced compared with U-Pu MOX fuel.

It is possible – but quite difficult – to design thorium fuels that produce more U-233 in thermal reactors than the fissile material they consume (this is referred to as having a fissile conversion ratio of more than 1.0 and is also called breeding). Thermal breeding with thorium requires that the neutron economy in the reactor has to be very good (ie, there must be low neutron loss through escape or parasitic absorption). The possibility to breed fissile material in slow neutron systems is a unique feature for thorium-based fuels and is not possible with uranium fuels.

In fresh thorium fuel, all of the fission ( power and neutrons) derive from the driver component. As the fuel operates the U-233 content gradually increases and it contributes more and more to the power output of the fuel. The ultimate energy output from U-233 (and hence indirectly thorium) depends on numerous fuel design parameters, including: fuel burn-up attained, fuel arrangement, neutron energy spectrum and neutron flux (affecting the intermediate product protactinium-233, which is a neutron absorb er). The fission of a U-233 nucleus releases about the same amount of energy (200 MeV) as that of U-235.In thorium fission , should be applied very effective principle called heterogeneous fuel arrangement in which  a high fissile (and therefore higher power) fuel zone called the seed region is physically separated from the fertile (low or zero power) thorium part of the fuel often called the blanket. Such an arrangement is far better for supplying surplus neutrons to thorium nuclei so they can convert to fissile U-233, in fact all thermal breeding fuel designs are heterogeneous. This principle applies to all the thorium-capable reactor systems. Th-232 is fissionable with fast neutrons of over 1 MeV energy. It could therefore be used in fast molten salt and other Gen IV reactors with uranium or plutonium fuel to initiate fission. However, Th-232 fast fissions only one tenth as well as U-238, so there is no particular reason for using thorium in fast reactors, given the huge amount of depleted uranium awaiting use.

In some countries like Sweden, Norway, thor- energy is managed by fuels of thor-additive or thorium MOX(with Plutonium) contained fuel rods are irradiated in a certain periods of trial season such as four or five years.Thor Energy and several utilities from North America and Europe are initiating feasibility studies to investigate the use of Th-Add fuel in commercial reactors. This fuel is promoted as a means to improve power profiles within commercial reactors.

There are seven type of reactors in to Thorium can be introduced as a nuclear fuel five of which are operational in service and two of them are still in constructional path.
Heavy Water Reactors (PHWRs)

 These are well suited for thorium fuels due to their combination of


 (i) excellent neutron economy (their low parasitic neutron absorption means more neutrons can be absorbed by thorium to produce useful U-233),


 (ii) slightly faster average neutron energy which favors conversion to U-233,

 (iii) flexible on-line refueling capability.


 Furthermore, heavy water reactors are well established and widely-deployed commercial technology for which there is extensive licensing experience.


Here is potential application to Enhanced  Indian Candu 6 (EC6) and ACR-1000 reactors fueled with 5% plutonium (reactor grade) plus thorium. In the closed fuel cycle, the driver fuel required for starting off is progressively replaced with recycled U-233, so that on reaching equilibrium 80% of the energy comes from thorium. Fissile drive fuel could be LEU, plutonium, or recycled uranium from LWR. Fleets of PHWRs with near-self-sufficient equilibrium thorium fuel cycles could be supported by a few fast breeder reactors to provide plutonium.This type of heavy water reactors are effective in Thorium fuel cycles can be used such a model in Thorium rich country like Srilanka for power generation which of technical and scientific information is largely available in neighboring India.

Next very optimum type of reactor for Thorium is molten salt reactor which is still developing in global scale but be highly profitable of it's low rate of neutron parasitism and pc of Thorium profitability.


Molten Salt Reactors (MSRs)

 These reactors are likely to be very well suited for using thorium as a fuel. The unique fluid fuel can incorporate thorium and uranium (U-233 and/or U-235) fluorides as part of a salt mixture that melts in the range 400-700ºC, and this liquid serves as both heat transfer fluid and the matrix for the fission of  fuel. The fluid circulates through a core region and then through a chemical processing circuit that removes various fission products (poisons) and/or the valuable U-233. The level of moderation is given by the amount of graphite built into the core. Certain MSR structures will be designed specifically for thorium fuels to produce useful amounts of U-233.

Above i discussed two of main reactors which are very effective and efficient in Thorium fuel cycles.Further information and safeguarding guidance can be obtained from International atomic energy agency's guideline for Nuclear energy and safety.As a cheaper energy source atomic energy is much more useful never than now for third world developing countries for their energy needs but some of political and terroist chaos are critical to regard in such type of energy choices.


S.M Anuradha Senarathna  (Bsc(Hon's), PG dip in Sociology--youths for good politics,sera idea blogspot.com