The development of radioactive waste disposal technologies associated with the nuclear fuel cycle is one of the essential conditions of the future of nuclear power engineering. Liquid high-level radioactive waste is the most dangerous. What conservation technologies are the safest? Will the uranium-thorium cycle be of use in mitigating the problem of radioactive waste disposal? Sergei Yudintsev, RAS Corresponding Member, Manager of the Radiogeology and Radioecology Laboratory of the RAS Institute for Geology of Ore Fields, Petrography, Mineralogy and Geochemistry, considered these and other issues in an interview to the Strana Rosatom newspaper. Here's the gist of his interview.

In addition to technologies for insulation of nuclear materials, our laboratory has a variety of problems to address. Most studies provide for a detailed comprehensive research of uranium fields and uranium ore sites by structural geological, mineral petrographic, geochemical, hydrogeochemical and isotopic geochronological techniques. It is necessary to understand how and why uranium fields are formed. We can then use this information to search for deposits of the similar type. Each geological object has common characteristics enabling us to group them for analysis and forecast similar deposits. Our laboratory assists in developing three-dimensional computer models of ore fields with respect to the location of deposits, prospective sites, uranium concentration, etc. Radiogeological surveys begun as early as 1946 became a solid scientific base toward building up this country's uranium resource potential. Today Russia ranks sixth in her uranium reserves.

One of our research groups is working at the Streltsovskoye field (with prospected reserves of uranium estimated at 244,000 tons) discovered in the mid-20th century in the Baikal territory, and still the only field in Russia where large-scale extraction operations are underway. Extraction (about 3,000 tons of uranium daily, or 93 percent of the national and 8 percent of the global production) is carried out by the Priargunsky Mining and Chemical Production Enterprise. The uranium thus mined is supplied to the domestic market.

Another group of our experts is working at the ore field of Khiagda in Buryatia (with inferred reserves at 39,300 tons), where an industrial mining plant was commissioned in 1999. In 2010, the Uranium Holding

ARMZ (Atomredmetzoloto), specializing in the mining of uranium and rare metals, and making part of the ore mining division of the Rosatom State Corporation, produced 135.1 tons of uranium there, and in 2011 it almost doubled the output. By 2019 the company is planning to reach its rated capacity of 1,800 tons of metal a year, in which case Khiagda will become the nation's largest enterprise producing uranium by well-based in situ leaching.

Our laboratory is also studying the global experience of immobilization of highly dangerous liquid waste (over 1 curie per liter) and it has been looking for an optimal disposal technology for over 20 years, that is to insulate radioactive waste in a high resistance matrix and bury it deep underground.

Today vitrification is the most widespread technology of radioactive waste conversion into compact forms. First, wastes are mixed with special borosilicate or phosphate glass; then, the mix is melted and poured into steel containers buried deep underground. However, such matrices are found to be not stable in underground waters, especially if, after inevitable decrystallization, they are exposed to radiogenic heat. This means that radioactive nuclides, including the long-living actinoids

Layout of the underground research laboratory in the Nizhnekansky granitoid solid mass (Krasnoyarsk Territory) for ultimate disposal of high-level radioactive waste.

(Np, Pu, Am, Cm), which are highly dangerous to man, may penetrate the environment.

Ceramic matrixes with a composition and structure similar to natural elements can become a good alternative. Some of them, for instance, those containing uranium and thorium, are from hundreds of thousands to hundreds of millions years old; they are characterized by high physical and chemical stability in deep geological layers. That is why synthesis of artificial analogues of such minerals supplemented with actinides is deemed to be the most promising technology for safe long-term conservation of radioactive nuclides. However, this technology needs more expensive materials (though more reliable), which makes it an issue of a very distant future. The use of ceramic mineral-like matrixes requires mandatory waste fractionation--separation of short-life (Cs, Sr) and long-life actinides together with rare earths. Since this technology has not been yet developed, radioactive waste is commonly disposed of in glass matrices.

In our country, active search is on for geological formations suitable for long-term underground storage of nuclear materials. For instance, the Nizhnekansky granitoid solid mass (Krasnoyarsk Territory) is considered optimal for conservation purposes. It is planned to launch an underground research laboratory there by 2020 in compliance with radioactive waste insulation safety. It will be built at a depth of about 500 m and take about 1 km². It will operate as a mining complex with chambers to store containers filled with waste, horizontal and vertical ventilation wells, freight wells, as well as other auxiliary elements. After disposal of radioactive waste in the chambers, they will be filled with a solid concrete-bentonite mix. Vertical wells of 6 m in diameter and 508 m deep will be equipped with lifts to transport people and freight in the course of prospecting operations, construction and operation of the laboratory.

Another argument for this site is that here the Mining Chemical Plant for weapon-grade plutonium production has been operating since 1950 and its basic facilities are located underground in the rock. Long-term field studies of geomechanical, physical and chemical processes in terms of the continuous heat exposure (produced by heat exchangers of reactor systems) showed that this rock mass is suitable for geological disposal of radioactive waste.

At the same time, nuclear engineers are attacking yet another global environmental problem--that of developing an actinide waste-free fuel cycle.

In the wild, there are two radioactive elements useful for nuclear power engineering: ²³⁵U fissionable under exposure to slow neutrons (artificial ²³⁹Pu "reproduces" the same way) and ²³²Th that can be converted

through a number of subsequent reactions to ²³³U--a "nuclear match" capable of sparking a chain fission. This very radioactive nuclide is a key element of the nuclear uranium-thorium fuel cycle.

The interest in thorium power engineering has been constantly growing worldwide what with the global reserves of this low-radioactive metal exceeding those of uranium by 4-5 times. As for Russia, the prospected uranium reserves will be depleted in 20 years, while thorium deposits in the area of Novokuznetsk and Tomsk (the Tuganskoye field) are quite big.

Experts say, this element has some advantages in generation of fissionable nuclides: it is a refractory metal above all. Its crystal lattice starts undergoing phase changes only at 1,400-1,500 °C. This makes it possible to operate thorium reactors at higher temperatures. Thorium radioactive waste is less dangerous than uranium waste (and produced in a smaller volume). Finally, a thorium "pile" is not reactive and not capable of triggering an uncontrolled chain reaction.

Attempts have been made to adapt nuclear reactors of almost all types to the thorium fuel. High-temperature coolant gas reactors are most promising. In addition to electric power, these reactors generate high-grade heat. By comparison, the coolant in thermal reactors heats up to 300 °C, in fast-neutron reactors (BN-600)--to 600 °C (liquid metal coolant), in helium gas reactors--to 900-1,000 °C, with the efficiency factor increasing. At a modern nuclear power station, one third of the generated power is transformed into electricity but two thirds are lost. In gas reactors, the efficiency factor is above 40 percent due to a different mechanism of transfer and transformation of nuclear reaction energy into electric power. Each percent of the efficiency factor is a highly important cost efficiency element. High-grade heat can be used in oil chemistry to gasify biomass and produce hydrogen from water, to obtain synthetic fuel, glass or concrete and for some other purposes, where temperatures as high as 800-1,000 °C are needed, Dr. Yudintsev stressed.

In Russia, he went on, the main studies in the field of thorium power engineering are carried out at the Leipunsky Institute of Physics and Power Engineering (Obninsk, Moscow Region) and at the National Research Center "Kurchatov Institute" (Moscow). So far our scientists and engineers have not been able to offer an adequate technology of thorium or thorium-uranium fuel production (chiefly due to the underfinancing of alternative projects).

Today, China and India have made the best progress in the thorium fuel cycle. In China, an experimental 2MW reactor is under construction. By 2020, a 100 MW gas reactor is to be commissioned in South Africa, with all works done by a private company.

Half a century ago thorium power engineering could not compete with uranium-based power production since thorium did not produce plutonium needed for nuclear weapons. Today, however, this low-radioactive element can turn the tables in atoms for peace.

I. Morgunov, Thorium vs. Uranium, "Strana Rosatom" newspaper, No. 31, 2013

Illustrations from Web sources

Prepared by Marina KHALIZEVA