home Online services Thorium is the new "battery" in nuclear power. Thorium as a Cure for Nuclear Plague Thorium 232 How is its half-life known

Thorium is the new "battery" in nuclear power. Thorium as a Cure for Nuclear Plague Thorium 232 How is its half-life known

In 1815, the famous Swedish chemist Jens Jacob Berzelius announced the discovery of a new element, which he named thorium in honor of Thor, the god of thunder and the son of the supreme Scandinavian god Odin. However, in 1825 it was discovered that this discovery was a mistake. Nevertheless, the name came in handy - Berzelius gave it to a new element, which he discovered in 1828 in one of the Norwegian minerals (now this mineral is called thorite). This element may have a great future ahead of it, where it can play a role in nuclear energy that is not inferior in importance to the main nuclear fuel - uranium.

	Advantages and disadvantages
+	Thorium on Earth is several times more than uranium
+	No need to separate isotopes
+	Radioactive contamination during the extraction of thorium is significantly less (due to the shorter-lived radon)
+	You can use already existing thermal reactors
+	Thorium has better thermomechanical properties than uranium
+	Thorium is less toxic than uranium
+	When using thorium, minor actinides (long-lived radioactive isotopes) are not formed.
-	In the process of irradiation of thorium, gamma-emitting isotopes are formed, which creates difficulties in the processing of fuel.

Distant relatives of the bomb

Nuclear energy, on which so many hopes are now placed, is a side branch of military programs, the main goals of which were to create atomic weapons(and a little later than reactors for submarines). As a nuclear material for making bombs, one could choose from three possible options: uranium-235, plutonium-239 or uranium-233.

This is what the thorium nuclear cycle looks like, illustrating the transformation of thorium into highly efficient nuclear fuel - uranium-233.

Uranium-235 is found in natural uranium in a very small amount - only 0.7% (the remaining 99.3% is the isotope 238), and it must be isolated, and this is expensive and difficult process. Plutonium-239 does not exist in nature, it must be produced by irradiating uranium-238 with neutrons in a reactor, and then separating it from irradiated uranium. In the same way, uranium-233 can be obtained by irradiating thorium-232 with neutrons.

In the 1960s, it was planned to close the nuclear cycle for uranium and plutonium, using about 50% of nuclear power plants with thermal reactors and 50% with fast ones. But the development of fast reactors has caused difficulties, so that only one such reactor is currently in operation, the BN-600 at the Beloyarsk NPP (and another one, the BN-800, has been built). Therefore, a balanced system can be created from thorium thermal reactors and about 10% of fast reactors, which will make up for the missing fuel for thermal ones.

The first two methods were implemented in the 1940s, but physicists decided not to bother with the third. The fact is that in the process of irradiating thorium-232, in addition to useful uranium-233, a harmful impurity is also formed - uranium-232 with a half-life of 74 years, the decay chain of which leads to the appearance of thallium-208. This isotope emits high-energy (hard) gamma rays, which require thick lead plates to protect against. In addition, hard gamma radiation disables the control electronic circuits, which are indispensable in the design of weapons.

Thorium cycle

However, the torii has not been completely forgotten. Back in the 1940s, Enrico Fermi proposed to produce plutonium in fast neutron reactors (this is more efficient than thermal reactors), which led to the creation of the EBR-1 and EBR-2 reactors. In these reactors, uranium-235 or plutonium-239 is the source of neutrons that convert uranium-238 into plutonium-239. In this case, more plutonium can be formed than is "burned" (by 1.3-1.4 times), therefore such reactors are called "breeders".

Another scientific group led by Eugene Wigner proposed their project of a breeder reactor, but not on fast, but on thermal neutrons, with thorium-232 as the irradiated material. The reproduction rate at the same time decreased, but the design was safer. However, there was one problem. The thorium fuel cycle looks like this. Absorbing a neutron, thorium-232 passes into thorium-233, which quickly turns into protactinium-233, and it already spontaneously decays into uranium-233 with a half-life of 27 days. And during this month, protactinium will absorb neutrons, interfering with the production process. To solve this problem, it would be good to remove protactinium from the reactor, but how to do it? After all, the constant loading and unloading of fuel reduces the operating efficiency to almost zero. Wigner proposed a very ingenious solution - a liquid fuel reactor in the form aqueous solution uranium salts. In 1952, a prototype of such a reactor, the Homogeneous Reactor Experiment (HRE-1), was built at the Oak Ridge National Laboratory under the direction of Wigner's student, Alvin Weinberg. And soon an even more interesting concept appeared, ideally suited for working with thorium: a molten salt reactor, the Molten-Salt Reactor Experiment. Fuel in the form of uranium fluoride was dissolved in a melt of lithium, beryllium and zirconium fluorides. MSRE worked from 1965 to 1969, and although thorium was not used there, the concept itself turned out to be quite workable: the use of liquid fuel increases the operating efficiency and allows harmful decay products to be removed from the core.

The molten salt reactor allows much more flexible control of the fuel cycle than conventional thermal power plants, and the use of fuel with the greatest efficiency, removing harmful decay products from the core and adding new fuel as needed.

path of least resistance

Nevertheless, molten salt reactors (ZSR) did not become widespread, since conventional uranium thermal reactors turned out to be cheaper. The world's nuclear power industry has taken the simplest and cheapest path, based on proven pressurized water reactors (VVER), the descendants of those designed for submarines, as well as boiling water reactors. Graphite-moderated reactors, such as the RBMK, are another branch of the family tree—they are descended from plutonium-producing reactors. “The main fuel for these reactors is uranium-235, but its reserves, although quite significant, are nevertheless limited,” Stanislav Subbotin, head of the system strategic research department at the Kurchatov Institute Research Center, explains to Popular Mechanics. - This issue began to be considered back in the 1960s, and then the planned solution to this problem was considered to be the introduction of waste uranium-238 into the nuclear fuel cycle, the reserves of which are almost 200 times larger. To do this, it was planned to build a lot of fast neutron reactors that would produce plutonium with a breeding ratio of 1.3-1.4, so that the excess could be used to power thermal reactors. The BN-600 fast reactor was launched at the Beloyarsk NPP, though not in the breeder mode. Recently, another one was built there - BN-800. But to build an effective ecosystem of nuclear energy, about 50% of such reactors are needed.”

All radioactive isotopes that occur in nature under natural conditions belong to one of three families (radioactive series). Each such row is a chain of nuclei connected by successive radioactive decay. The ancestors of the radioactive series are the long-lived isotopes uranium-238 (half-life 4.47 billion years), uranium-235 (704 million years) and thorium-232 (14.1 billion years). The chains end with stable isotopes of lead. There is another series starting with neptunium-237, but its half-life is too short - only 2.14 million years, so it does not occur in nature.

Mighty Thorium

This is where thorium comes into play. “Thorium is often called an alternative to uranium-235, but this is completely wrong,” says Stanislav Subbotin. - Thorium itself, like uranium-238, is not a nuclear fuel at all. However, by placing it in a neutron field in the most ordinary pressurized water reactor, you can get excellent fuel - uranium-233, which is then used for the same reactor. That is, no alterations, no serious changes to the existing infrastructure are needed. Another advantage of thorium is its abundance in nature: its reserves are at least three times higher than those of uranium. In addition, there is no need for isotope separation, since only thorium-232 is found in associated mining along with rare earth elements. Again, when uranium is mined, the surrounding area is contaminated with relatively long-lived (half-life 3.8 days) radon-222 (in the thorium series, radon-220 is short-lived, 55 seconds, and does not have time to spread). In addition, thorium has excellent thermomechanical properties: it is refractory, less prone to cracking, and releases less radioactive gases when the fuel element cladding is damaged. The production of uranium-233 from thorium in thermal reactors is about three times more efficient than plutonium from uranium-235, so that the presence of at least half of such reactors in the nuclear energy ecosystem will close the cycle for uranium and plutonium. True, fast reactors will still be needed, since the breeding ratio for thorium reactors does not exceed one.”

The production of 1 GW during the year requires: 250 tons of natural uranium (containing 1.75 tons of uranium-235) it is required to extract 215 tons of depleted uranium (including 0.6 tons of uranium-235) go to dumps; 35 tons of enriched uranium (of which 1.15 tons of uranium-235) are loaded into the reactor; spent fuel contains 33.4 tons of uranium-238, 0.3 tons of uranium-235, 0.3 tons of plutonium-239, 1 ton of decay products. 1 ton of thorium-232, when loaded into a molten-salt reactor, is completely converted into 1 ton of uranium-233; 1 ton of decay products, of which 83% are short-lived isotopes (decay to stable isotopes in about ten years).

However, thorium has one rather serious disadvantage. Upon neutron irradiation of thorium, uranium-233 becomes contaminated with uranium-232, which undergoes a chain of decays leading to the hard gamma-emitting isotope thallium-208. “This greatly complicates the work of fuel processing,” explains Stanislav Subbotin. “But on the other hand, it makes it easier to detect such material, reducing the risk of theft. In addition, in a closed nuclear cycle and with automated fuel processing, this does not really matter.

Thermonuclear ignition

Experiments on the use of thorium fuel rods in thermal reactors are being carried out in Russia and other countries - Norway, China, India, and the USA. “Now is the time to return to the idea of molten-salt reactors,” says Stanislav Subbotin. — The chemistry of fluorides and fluoride melts is well studied thanks to the production of aluminum. For thorium, molten salt reactors are much more efficient than conventional pressurized water reactors, since they allow flexible loading and removal of decay products from the reactor core. Moreover, they can be used to implement hybrid approaches, using not nuclear fuel as a neutron source, but thermonuclear installations - at least the same tokamaks. In addition, a liquid-salt reactor makes it possible to solve the problem with minor actinides - long-lived isotopes of americium, curium and neptunium (which are formed in irradiated fuel), by "burning out" them in a scavenger reactor. So, in the future, we will not be able to do without thorium in the nuclear power industry.”

1 gram per 28,000 liters. This is the ratio of fuel consumption in automobile engines, if we replace the usual fuel with thorium.

We are talking about the 232nd isotope. It has the longest half-life. 8 grams of thorium is enough to run an engine continuously for 100 years.

Stocks of new fuel are 3 times more than in earth's crust. Laser Power Systems specialists have already begun to develop a new engine.

American company. The operation of the engine will resemble the cycle of a standard power plant. The challenge was developing a suitable laser.

Its task is to heat water, the steam of which launches mini-turbines. While scientists are working out the process, we will learn more about the fuel of the 21st century, and in the future, the entire millennium.

What is thorium?

Thorium metal related to actinides. This family includes radioactive. All of them are located in the 3rd group of the 7th period of the table.

Actinide numbers are from 90 to 103. Thorium comes first. It was discovered first, simultaneously with uranium.

In its pure form, the hero was singled out in 1882 by Lars Nilsson. The radioactivity of the element was not immediately discovered.

So, thorium did not arouse public interest for a long time. Thorium decay proved only in 1907.

Since 1907 thorium isotopes opened one by one. By 2017, there are 30 metal modifications. 9 of them received.

The most stable is the 232nd. Thorium half life in this form lasts 1.4 * 10 10 years. That is why the 232nd isotope is ubiquitous, in the earth's crust it occupies a share of 8 * 10 -4%.

The remaining isotopes are stored for several years, and therefore are of no practical interest and are rarely found in nature. True, the 229th thorium decays in 7,340 years. But, this isotope is "derived" artificially.

Thorium has no completely stable isotopes. In its pure form, the element looks like -, plastic .

It is he who makes the mineral thorite so soft. easy to cut. The mineral was studied by Jens Berzenlius.

The Swedish chemist was able to calculate the unknown in the composition of the stone, but could not isolate it, giving the laurels to Nilson.

Thorium properties

Thorium is an element, whose specific radioactivity is 0.109 microcuries per gram. For uranium 238, for example, the figure is almost 3 times higher.

Accordingly, thorium is weakly radioactive. Several isotopes of thorium, by the way, are a consequence of the decay of uranium. We are talking about the 230th, 231st, 234th and 235th modifications of the 90th element.

The decay of the hero of the article is accompanied by the release of radon. This gas is also called thoron. However, the second name is not commonly used.

Radon is dangerous if inhaled. However, microdoses are contained in mineral waters and have a beneficial effect on the body.

It is the route of entry of thoron into the body that is important. You can drink, absorb - yes, but do not inhale.

In terms of the crystal lattice radioactive thorium appears in only two forms. Up to 1,400 degrees, the structure of the metal is face-centric.

It is based on three-dimensional cubes consisting of 14 atoms. Some of them are in the corners of the figure. The remaining atoms are located in the middle of each.

When heated above 1,400 degrees Celsius, the crystal lattice of thorium becomes body-centered.

The "packing" of such cubes is less dense. The already soft thorium becomes even more loose.

Thorium - chemical an element classified as paramagnetic. Accordingly, the magnetic permeability of the metal is minimal, close to unity.

The substances of the group are also distinguished by the ability to be magnetized in the direction of an external field.

The molar heat capacity of thorium is 27.3 kilojoules. The indicator indicates the thermal capacity of one mole of a substance, hence the name.

It is difficult to continue the list, since the bulk of the properties of the 90th metal depends on the degree of its contamination.

So, the tensile strength of the element varies from 150 to 290 meganewtons per square meter.

Thorium is also unstable. For metal, they give from 450 to 700 kilogram-force.

Standing at the beginning of its group, thorium took over some of the properties from the elements that preceded it. So, the hero of the article is characterized by the 4th degree of oxidation.

In order for thorium to quickly oxidize in air, you need to bring the temperature up to 400 degrees. The metal will instantly be covered with an oxide film.

The duet of thorium with oxygen, by the way, is the most refractory of terrestrial oxides, softens only at 3,200 degrees Celsius.

At the same time, the compound is also chemically stable. Pure metal reacts with

Any radioactive isotope thorium interacts with it even at room temperature.

The remaining reactions with the hero of the article take place at elevated temperatures. At 200 degrees, there is a reaction with.

Powdered hydrides are formed. Nitrides are obtained when thorium is heated in the atmosphere.

A temperature of 800 degrees Celsius is required. But, first you need to get the reagent. Let's find out how they do it.

Mining and deposits of thorium

$350,000,000. Approximately the same amount is allocated annually for the development of thorium energy. There are a lot of deposits of the 232nd isotope in the country.

This is alarming, which risks losing its leadership in fuel if the 90th element becomes the main energy resource in the world.

There are reserves in the country. Millions of tons of metal, for example, are located near Novokuznetsk.

However, it is necessary to defend the priority right to use thorium, and for them the world is fighting. Everyone understands what the future is.

Usually, thorium is found in the form of shiny sand. This is the mineral monazite. The beaches from it are often included in the resort areas.

On the coast of the Sea of Azov, for example, it is worth considering not only solar radiation, but also that which comes from the earth. Veined thorium is found only in South Africa. The ore deposits there are called Steenkasmkraal.

If you extract thorium from ores, then it is easier to get an element along the way with. It remains to be seen where thorium could be useful, apart from the car engines of the future.

Application of thorium

Insofar as thorium nucleus unstable, natural use of the element in nuclear energy. For its needs, fluoride and thorium oxide are purchased.

Remember the temperature that the oxide of the 90th metal can withstand? Only such a compound will work in molten-salt reactors.

Thorium oxide also comes in handy in the aviation industry. There, the 90th metal serves as a hardener. The service of thorium is also in the body.

About 3 milligrams of a radioactive element comes in daily with food. It is involved in the regulation of system processes, absorbed mainly by the liver.

Thorium is also bought by metallurgists, but not for food. Pure metal is used as, that is, an additive that improves the quality, in particular, magnesium. With a ligature, they become heat-resistant and better resist tearing.

Finally, we will add information about the new car engine. The thorium in it is not nuclear fuel, but only the raw material for it.

By itself, the 90th element is not capable of providing energy. Everything is changed by the neutron environment and the water reactor.

With them, thorium is converted into uranium 233. Here it is - efficient fuel. How much do they pay for raw materials for it? Let's try to find out.

Thorium price

Thorium price differentiates into pure metal and its compounds. This is common phrase from . Of the particulars - only the price tag per kilo of thorium oxide is about 7,500.

This concludes the open requests. Sellers are asked to clarify the cost, since they sell a radioactive element.

There are no offers of pure thorium on the Internet, just as there is no data on per gram of the metal. Meanwhile, those interested in a new type of automotive fuel are haunted by the question, just as they are haunted by whether the requests for the 90th element will jump in case of its widespread use.

Initially, for the sake of ousting gasoline engines from the market, thorium will be made as profitable as possible. But what will happen later, when a return to the past is already unlikely?

There are many questions. There are few specifics, however, as in everything new, unknown, which seems like a gamble in the first couple.

Although, the first versions of the thorium engine are already ready. They weigh about 200 kilograms. Such a device can easily be placed under a medium-sized hood.

THORIUM

Thorium is a naturally occurring weakly radioactive metal discovered in 1828 by the Swedish chemist Jens Berzelius, who named it after Thor, the Scandinavian god of war. In not large quantities it is present in many rocks and soils, where its content is almost three times higher than that of uranium. Soil contains approximately six parts per million of thorium.

Thorium is found in many minerals, the most common of which is a rare earth mineral - thorium phosphate - monazite, which contains up to 12% thorium oxide. There are deposits of this mineral in several countries. Thorium-232 decays very slowly (its half-life is almost three times the age of the Earth), but other isotopes of thorium are found in it and in the decay chains of uranium. Most of them are short-lived elements, and therefore they are much more radioactive than Th-232, although their mass content is negligible.

World reserves of thorium (available for mining)

The country	Stocks (in tons)
Australia	300000
India	290000
Norway	170000
USA	160000
Canada	100000
South Africa	35000
Brazil	16000
Other countries	95000
Total	1200000

(Source - US Geological Survey, Mineral Reserves, January 1999)

Thorium as nuclear fuel

Thorium, like uranium, can be used as a nuclear fuel. By itself, Th-232, which is not a fissile material, absorbs slow neutrons and forms fissile uranium-233. Like U-2238, thorium-232 is a fuel feedstock.

In one of the essential indicators, U-233 is superior to uranium-235 and plutonium-239, having a higher yield of neutrons per absorbed neutron. If you start the reaction with another fissile material (U-235 or Pu-239), you can implement a fissile material production cycle similar to, but more efficient than the cycle for U-238 and plutonium in slow neutron reactors. Th-232 absorbs a neutron and is converted to Th-233, which decays into Pa-233 and then into U-233. The irradiated fuel can be unloaded from the reactor, the U-233 separated from the thorium and loaded into another reactor as part of a closed fuel cycle.

Over the past 30 years, there has been interest in thorium as a nuclear fuel, since its reserves in the earth's crust are three times those of uranium. In addition, all mined thorium can be used in reactors, as opposed to 0.7% of the U-235 isotope from natural uranium.

The primary option in PWRs would be fuel assemblies mounted such that a blanket of mostly thorium covers a highly enriched seed cell containing U-235 that produces neutrons for the subcritical blanket. Since U-233 is produced in a blanket, it burns there as well. Here we are talking about a light water breeder reactor, which successfully passed demonstration tests in the USA in the 1970s.

Research and development

The possibility of implementing thorium fuel cycles has been studied for about 30 years, but much less intensively than uranium or uranium-plutonium cycles. The main research and development work was carried out in Germany, India, Japan, Russia, Great Britain and the USA. Trial irradiation of thorium fuel in reactors was also carried out until high level burnout. Several experimental reactors were fully or partially loaded with thorium fuel.

Noteworthy experiments on the thorium cycle include the following (the first three were carried out in high-temperature gas-cooled reactors):

Between 1967 and 1988, an experimental bulk blanket AVR reactor at a power of 15 megawatts was operated in Germany for more than 750 weeks. 95% of the entire period of operation of the reactor was the work on thorium fuel. The fuel consisted of 100,000 fuel cells in the form of balls. The total weight of the thorium fuel was 1360 kg; thorium was used mixed with highly enriched uranium. The maximum burnup depth was 150,000 MW day/ton.
Thorium fuel rods, consisting of thorium and uranium in a ratio of 10:1, were irradiated for 741 days in the Dragon reactor with a power of 20 megawatts in English city Winfit. The Dragon reactor was operated as part of a joint project in which, along with the UK, Austria, Denmark, Sweden, Norway and Switzerland participated from 1964 to 1973. Thorium-uranium fuel was used to produce U-233, which replaced the consumed U-235 in about the same ratio. The fuel could work in the reactor for six years.
In 1967-1974, the Peach Bottom high-temperature uranium-thorium fuel reactor with a capacity of 110 megawatts manufactured by General Atomic operated in the USA.
In India, in 1996, a 30 kW Kamini experimental research reactor was launched at Kalpakkam as a neutron source, operating on U-233 obtained by irradiating ThO 2 in another reactor. The reactor was built near a 40 megawatt fast neutron breeder reactor, in which ThO 2 was irradiated.
In the Netherlands, a 1 megawatt homogeneous water-mixed reactor was operated for three years. The reactor used fuel in the form of a solution of highly enriched uranium and thorium; in order to remove fission products, processing was continuously carried out, as a result of which, with high efficiency. U-233 was produced.
A number of experiments were carried out with fast neutron reactors.

Power reactors

On the basis of the AVR reactor in Germany, a 300 MW THTR reactor was developed, which operated from 1983 to 1989; the reactor operated on a bulk blanket of 674,000 elements, of which more than half were uranium-thorium fuel, and the rest were graphite moderator and neutron absorbers. The fuel elements were continuously renewed during loading, and on average passed through the reactor six times. Fuel production was put on an industrial basis.
The Fort St Vrain reactor was the only commercial thorium-fueled reactor in the USA; this reactor was also designed on the basis of the German AVR and operated from 1976 to 1989. It was a graphite-moderated, helium-cooled high-temperature reactor (1300°C) with a design power of 842 megawatts (330 megawatts electrical). The fuel elements were fabricated from thorium carbide and Th/U-235 carbide in the form of microspheres to contain fission products, coated with silicon dioxide and pyrocarbon. The fuel rods were in the form of hexagonal columns ("prisms"). The reactor used almost 25 tons of thorium; the burnup depth was 170,000 MW day/t.
Studies of thorium fuel for PWR reactors were carried out at the US Shippingport reactor; U-235 and plutonium were used as initial fissile fuel material. It was concluded that thorium would not seriously affect the operating modes and lifetime of the core. Here, from 1977 to 1982, a light-water breeder reactor of seed-blank type was successfully tested on thorium-uranium fuel coated with a zirconium alloy.
The 60 MW Lingen BWR reactor in Germany used Th/Pu fuel rods.

India

In India, 500 kg of thorium fuel was loaded into units 1 and 2 of the atomic power plant at Kakrapar after launch to improve efficiency. The 1st unit of the AES was the first reactor in the world in which thorium rather than depleted uranium was used to equalize the power in the core. Working on thorium fuel, the 1st block reached its full capacity in 300 days, and the 2nd block - in 100 days. Thorium fuel is planned to be used in units 1 and 2 of the Kaiga APP and units 3 and 4 of the APP in Rajasthan, which are under construction.

With thorium reserves six times greater than those of uranium, India has set the task of introducing the thorium cycle as the main task of industrial energy production, which will be solved in three stages:

CANDU heavy water reactors fueled by natural uranium will be used to produce plutonium;
fast breeder reactors (FBRs) based on recovered plutonium will produce U-233 from thorium;
promising heavy water reactors will run on U-233 and thorium, getting 75% of their energy from thorium.

The spent fuel will then be reprocessed to recover fissile materials and reprocess them;

As another possibility for the third stage, subcritical accelerator complexes (ADS) are considered.

Development of advanced reactors

Design solutions for advanced thorium-fuelled reactors include:

The use of thorium in complexes with accelerators (ADS)

In complexes with accelerators, high-energy neutrons are produced due to the fission reaction of nuclei by high-energy accelerator protons colliding with heavy target nuclei (lead, lead-bismuth or other elements). These neutrons can be sent to a subcritical reactor containing thorium, where the neutrons produce U-233 and cause it to fission. It is possible to provide a self-sustaining fission reaction that can be directed either to energy production or to the transmutation of actinides resulting from the U/Pu fuel cycle. The use of thorium instead of uranium means that fewer actinides will be produced in the ADS reactor itself.

Development of the thorium fuel cycle

The problems associated with solving this problem boil down to the high cost of producing fuel, partly due to the high radioactivity of U-233, which always contains U-232; similar problems apply to the processing of thorium due to the high radioactivity of Th-228, a certain risk of distribution of U-233 as a weapon material, as well as a number of technical problems of processing (not yet properly resolved). Much work remains to be done before the thorium cycle is commercialized, but as long as large quantities of uranium can be mined, such work seems unlikely.

Nevertheless, the thorium cycle, with its potential for reproduction without the use of fast neutron reactors, will remain promising for a long time to come. This cycle is a determining factor in the development of self-sufficient nuclear power.

Thorium fuel cycle is a nuclear fuel cycle using Thorium-232 isotopes as nuclear feedstock. Thorium-232 during the separation reaction in the reactor transfers transmutation into the artificial isotope Uranium-233, which is used as nuclear fuel. Unlike natural uranium, natural thorium contains only very small fractions of fissile material (for example, Thorium-231), which is not enough to start a nuclear chain reaction. To start the fuel cycle, it is necessary to have an additional fissile material or another source of neutrons. In a thorium reactor, Thorium-232 absorbs neutrons to eventually produce Uranium-233. Depending on the design of the reactor and the fuel cycle, the created uranium-233 isotope can be fissioned in the reactor itself or chemically separated from spent nuclear fuel and remelted into new nuclear fuel.

The thorium fuel cycle has several potential advantages over the uranium fuel cycle, including greater abundance, better physical and nuclear properties not found in plutonium and other actinides, and better proliferation resistance. nuclear weapons, which is associated with the use of light water reactors rather than molten salt reactors.

History of the study of thorium

The only source of thorium is yellow translucent grains of monazite (cerium phosphate)

Controversy over the world's limited uranium reserves led to initial interest in the thorium fuel cycle. It became obvious that uranium reserves are exhaustible, and thorium can replace uranium as a nuclear fuel feedstock. However, most countries have relatively rich uranium deposits and research into the thorium fuel cycle is extremely slow. A major exception is India and its three-stage nuclear program. In the 21st century, thorium's potential to resist nuclear proliferation and the characteristics of spent fuel feedstock have led to renewed interest in the thorium fuel cycle.

Oak Ridge National Laboratory used the Molten Salt Experimental Reactor using Uranium-233 as the fissile material in the 1960s to experiment and demonstrate the operation of the Molten Salt Breeder Reactor operating on the thorium cycle. Experiments with the Reactor on the Molten Salts of the possibility of thorium, using thorium fluoride (IV) dissolved in the molten salt. This reduced the need for fuel cell production. The PPC program was terminated in 1976 after the dismissal of its curator, Alvin Weinberg.

In 2006, Carlo Rubbia proposed the concept of an energy booster or "controlled accelerator", which he saw as an innovative and safe way to produce nuclear energy using existing energy acceleration technologies. Rubbia's idea offers the possibility to burn highly radioactive nuclear waste and produce energy from natural thorium and depleted uranium.

Kirk Sorensen, a former NASA scientist and Chief Nuclear Officer of Teledyne Brown Engineering, has long promoted the idea of a thorium fuel cycle, in particular Liquid Thorium Fluoride Reactors (LFRs). He pioneered research into thorium reactors while at NASA, when he was evaluating various power plant concepts for lunar colonies. In 2006, Sorensen founded the website "Energyfromthorium.com" to inform and promote this technology.

In 2011, the Massachusetts Institute of Technology concluded that, despite few barriers to the thorium fuel cycle, the current state of light water reactors provides little incentive for such a cycle to enter the market. It follows that the chance of the thorium cycle displacing the traditional uranium cycle in the current nuclear power market is extremely small, despite the potential benefits.

Nuclear reactions with thorium

During the thorium cycle Thorium-232 captures neutrons (this occurs in both fast and thermal reactors) to be converted into Thorium-233. This usually leads to the emission of electrons and antineutrinos during?-decay and the appearance of Protactinium-233. Then, during the second?-decay and re-emission of electrons and antineutrinos, Uranium-233 is formed, which is used as fuel.

Waste from fission products

Nuclear fission produces radioactive decay products that can have half-lives ranging from a few days to over 200,000 years. According to some toxicology studies, the thorium cycle can completely process actinide waste and only emit waste after fission products, and only after a few centuries the thorium reactor waste will become less toxic than uranium ores, which can be used to produce depleted uranium fuel for a light water reactor of a similar nature. power.

actinide waste

In a reactor where neutrons hit a fissile atom (for example, certain uranium isotopes), both nuclear fission and neutron capture and atom transmutation can occur. In the case of Uranium-233, transmutation leads to the production of useful nuclear fuel, as well as transuranium waste. When Uranium-233 absorbs a neutron, a fission reaction or conversion to Uranium-234 can occur. The chance of splitting or absorbing a thermal neutron is approximately 92%, while the ratio of the capture cross section to the neutron fission cross section in the case of Uranium-233 is approximately 1:12. This figure is larger than the corresponding ratios of Uranus-235 (about 1:6), Pluto-239 or Pluto-241 (both have ratios of about 1:3). The result is less transuranium waste than in a traditional uranium-plutonium fuel cycle reactor.

Uranium-233, like most actinides with a different number of neutrons, does not fissile, but when neutrons are “captured”, the fissile isotope Uranium-235 appears. If no fission or neutron capture reaction occurs in the fissile isotope, Uranium-236, Neptunium-237, Plutonium-238, and eventually the fissile isotope Plutonium-239 and heavier isotopes of plutonium appear. Neptunium-237 can be removed and stored as waste, or preserved and transmuted into plutonium, which is better fissile, while the remainder turns into Plutonium-242, then americium and curium. These, in turn, can be disposed of as waste, or returned to reactors for further transmutation and fission.

However, Protactinium-231, with a half-life of 32,700 years, is formed through reactions with Thorium-232, despite not being a transuranium waste, is the main cause of long-lived radioactive waste.

Infection with Uranium-232

Uranium-232 also appears during the reaction between fast neutrons and Uranium-233, Protactinium-233 and Thorium-232.

Uranium-232 has a relatively short half-life (68.9 years) and some of the decay products emit high-energy gamma rays, as do Radon-224, Bismuth-212, and partially Thallium-208.

The thorium cycle produces harsh gamma radiation that damages electronics, limiting its use as a trigger for nuclear bombs. Uranium-232 cannot be chemically separated from Uranium-233 found in spent nuclear fuel. However, the chemical separation of thorium from uranium removes the decay products of thorium-228 and radiation from the rest of the half-life chain, which gradually leads to the re-accumulation of thorium-228. Contamination can also be prevented by using a Molten Salt Breeder Reactor and separating Protactinium-233 before it decays to Uranium-233. Hard gamma rays can also create a radiobiological hazard requiring telepresence operation.

Nuclear fuel

As a nuclear fuel, thorium is similar to Uranium-238, which makes up most of the natural and depleted uranium. The index of the nuclear cross section of the absorbed thermal neutron and the resonance integral (the average number of the nuclear cross section of neutrons with intermediate energy) for Thorium-232 is approximately equal to three, and is one third of the corresponding index of Uranium-238.

Advantages

Thorium is estimated to be three to four times more common in the earth's crust than uranium, although in reality data on its reserves are extremely limited. Current demand for thorium is met by secondary rare earth products mined from monazite sands.

Although the fissile thermal neutron cross section of Uranium-233 is comparable to Uranium-235 and Plutonium-239, it has a much lower capture neutron cross section than the latter two isotopes, resulting in fewer absorbed non-fissile neutrons and an increase in the neutron balance. . After all, the ratio of released and absorbed neutrons in Uranium-233 is more than two in a wide range of energies, including thermal. As a result, thorium-based fuel can become the main component of a thermal breeder reactor. A breeder reactor with a uranium-plutonium cycle is forced to use the fast neutron spectrum, since in the thermal spectrum one neutron is absorbed by Plutonium-239, and on average 2 neutrons disappear during the reaction.

Thorium-based fuel also exhibits excellent physical and Chemical properties, which allows to improve the technical data of the reactor and the repository. Compared to uranium dioxide, the predominant reactor fuel, thorium dioxide has a higher influence temperature, thermal conductivity, and a lower coefficient of thermal expansion. Thorium dioxide also shows better chemical stability and, unlike uranium dioxide, is not capable of further oxidation.

Because the uranium-233 produced in thorium fuel is heavily contaminated with uranium-232 in proposed reactor concepts, thorium spent fuel is resistant to weapons proliferation. Uranium-232 cannot be chemically separated from Uranium-233 and has several decay products that emit high-energy gamma rays. These high energy protons carry radioactive hazard, which necessitates remote work with separated uranium and nuclear detection of such substances.

Substances based on uranium spent fuel with a long half-life (from 1,000 to 1,000,000 years) carry a radioactive hazard due to the presence of plutonium and other minor actinides, after which long-lived fission products reappear. One neutron captured by Uranium-238 is enough to create transuranium elements, while five such "captures" are needed for a similar process with Thorium-232. 98-99% of the thorium nuclear cycle results in the fission of Uranium-233 or Uranium-235, so fewer long-lived transuranium elements are produced. Because of this, thorium appears to be a potentially attractive alternative to uranium in mixed oxide fuels to minimize the production of transuranium substances and maximize the amount of decayed plutonium.

disadvantages

There are several obstacles to the use of thorium as a nuclear fuel, in particular for solid fuel reactors.

Unlike uranium, naturally occurring thorium is generally single-nuclear and contains no fissile isotopes. Fissile material, typically Uranium-233, Uranium-235, or plutonium, must be added to achieve criticality. Together with high temperature sintering required for thorium dioxide, this complicates the production of fuel. Oak Ridge National Laboratory conducted experiments on thorium tetrafluoride as a fuel for a molten salt reactor in 1964-1969. It was expected that the production process would be facilitated and the separation of substances from pollutants to slow down or stop chain reaction.

In a single fuel cycle (for example, Uranium-233 processing in the reactor itself), more severe burnup is needed to achieve the desired neutron balance. Although thorium dioxide is capable of generating 150,000-170,000 megawatt-days/ton at the Fort St. Raine and Jülich Experimental Nuclear Power Plants, there are serious challenges to achieve such performance in light water reactors, which constitute the vast majority of existing reactors.

In a single thorium fuel cycle, the remaining uranium-233 remains in the spent fuel as a long-lived isotope.

Another hurdle is that the thorium fuel cycle takes comparatively longer to convert Thorium-232 into Uranium-233. The half-life of Protactinium-233 is approximately 27 days, which is much longer than the half-life of Neptunium-239. As a result, the main ingredient in thorium fuel is the strong Protactinium-239. Protactinium-239 is a strong neutron absorber, and although conversion to fissile Uranium-235 can occur, it takes twice as many neutrons to be absorbed, which destroys the neutron balance and increases the likelihood of transuranium production.

On the other hand, if solid thorium is used in a closed fuel cycle where uranium-233 is processed, remote interaction is necessary to produce the fuel due to the high levels of radiation provoked by the decay products of uranium-232. This is also true when it comes to recycled thorium due to the presence of thorium-228 being part of the decay chain. Moreover, unlike the proven technology for reprocessing uranium fuel, the technology for reprocessing thorium is now only developing.

Although the presence of Uranium-232 complicates matters, there are published documents showing that Uranium-233 was used in nuclear tests. The US tested a sophisticated bomb containing uranium-233 and plutonium in the core during Operation Teapot in 1955, although a much lower TNT equivalent was achieved.

Although thorium-based fuels produce much less transuranium than uranium-based counterparts, a certain amount of long-lived actinides with a long radioactive background, in particular Protactinium-231, can sometimes be produced.