RU2510540C1 - Radioactive waste disposal method and heat-dissipating capsule for realising said method - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: method involves immersing at least one heat-dissipating capsule into a well formed in geologic formations. A heat-conducting array saturated with radionuclides is placed inside the capsule with an airtight casing. The average density of the capsule with radioactive wastes is higher than the density of the rocks under the capsule. The melting point of the refractory material from which the airtight casing of the capsule is made is higher than that of the rocks under the capsule. The quantitative composition of the mixture of radionuclides inside the casing is selected based on the condition that, the power of volumetric heat release of the radioactive wastes should be higher than the heat power needed to melt the rocks under the capsule. Content of the highly active isotope in the mixture of radionuclides filling the capsule is selected based on the condition: q_min≥1.2 W/cm³, where q_min is the minimum power density of volumetric heat release of the mixture of radionuclides 10 years after immersing the capsule.

EFFECT: enabling relatively fast disposal of the entire range of radioactive wastes, including long-lived radionuclides, irreversible dissolution of radionuclides.

20 cl, 1 dwg

Description

The group of inventions relates to the technology of the disposal of radioactive waste (PAO) of nuclear energy, in particular to methods for the disposal of long-lived radioactive isotopes in the deep layers of the lithosphere, including isotopes of transuranic elements with a low level of activity.

Currently, various methods are known for the disposal of radioactive waste by the method of self-immersion of capsules filled with PAO into the deep layers of the Earth's lithosphere. According to previously developed technical solutions, it was supposed to bury the entire aggregate of PAO produced in the process of irradiating fuel in the core of nuclear reactors. The essence of the PAO self-burial process is to use the intense heat generated by the radioactive decay of the nuclides to melt the surrounding (enclosing) geological rock. A container with capsules or a separate capsule with PAO is immersed in molten geological rock under its own weight and causes further penetration of the geological rocks located under the capsule. When implementing this method, technical problems arise associated with the creation of large containers (capsules), which are necessary to solve two problems simultaneously. First, the maximum possible amount of PAO to be disposed of must be placed in containers (capsules). Secondly, containers (capsules) must provide the necessary volumetric heat dissipation for efficient penetration of geological rocks. At the same time, in the process of rock penetration, the capsule shells must be sealed. Capsule size is selected based on the above conditions.

There is a known method for the disposal of radioactive waste, described in the USSR author's certificate SU 826875 (published on 04/30/1992), according to which radionuclides of a high level of activity are solidified and placed in the internal volume of an airtight capsule. The capsule shell is made of a refractory material with a melting point above 2300 ° C. Due to the heat release of PAO, a temperature in the range of 1900 ° C to 2100 ° C should be provided on the surface of the capsule shell. Capsules are immersed in the rock, which should be melted under the action of heat from the capsules. As a result, the capsules are immersed in the deep layers of the lithosphere to a depth of 30 km or more. According to this method, PAO is buried in the moving part of the Earth's mantle. Capsule immersion in molten rock occurs if the density of each capsule filled with PAO exceeds the density of rocks located under the capsule.

A more specific form of implementing the PAO disposal method is described in patent RU 2115964 (published on July 20, 1998). The method aims to reduce the duration of PAO self-disposal operations by increasing the thermal power released by the capsules. For this, the termite mixture is additionally included in the composition of the capsules filled with PAO. As a thermite mixture, a stoichiometric mixture of granules of aluminum oxides and heavy metal oxides selected from the group consisting of iron, manganese and / chromium is used.

When implementing the known method, flexible mesh containers loaded with steel capsules filled with PAO with a thermite mixture and capsules with flux are dropped into a large borehole. In this case, capsules filled with flux are made with a thin-walled aluminum shell. Self-heating of capsules due to PAO heat release leads to flux melting. As a result, steel fuel capsules with PAO find themselves under a layer of melt interacting with rocks. A further increase in the temperature of the conglomerate of capsules leads to the initiation of an exothermic reaction between the components of the termite mixture. At the same time, due to the total heat release of capsules filled with PAO with a thermite mixture, a temperature of 2000-2200 ° C is ensured. This temperature level exceeds the melting temperature of basaltic rocks.

According to the calculations made by the inventors, the speed of movement (lowering) in the surrounding geological formations of the heavy hot melt, in which the fuel capsules are located, should be 2–3 km per year. However, the need to use a well of sufficiently large diameter to load a conglomerate of capsules (the diameter of the well must exceed the diameter of the mesh container with capsules) and the complexity of manufacturing the container with capsules significantly complicates the process of self-disposal of PAO. In addition, the limited number of components of the thermite mixture placed in capsules does not provide the required heat for a long period of time (more than one year).

The closest analogues of the group of inventions are the method of disposal of PAO and a fuel capsule intended for disposal of PAO, which are disclosed in patent RU 2152093 (published on June 27, 2000). The method of PAO burial in the deep layers of the lithosphere involves drilling a well and forming a cavity-cavity with a diameter of up to 6 m in rock salt formations. The depth of the rock salt mass in which the well is drilled is selected from 1 to 10 km. A viscous medium is created in the cavity cavity by pumping rock salt solvent into it. The solvent may contain surfactants to enhance convective heat transfer. After this, the cavity is filled with capsules containing PAO of high and medium levels of activity.

The method is based on immersion of capsules in a rock melt as a result of intense heating of the environment during radioactive decay of nuclides inside the capsule shell. Each capsule contains a strong hermetic shell made of refractory and heat-resistant material. The capsule shell is multilayer with corrosion-resistant layers. Capsules are spherical in shape. The outer diameter of the capsule shell is 200 to 300 mm. The internal cavity of the capsule is filled with high and medium activity PAO. The heat release of each capsule is ~ 1 W, which corresponds to the activity of radionuclides 150 ÷ 200 Ci.

When the fuel capsule is in saline (brine), the rock salt is effectively dissolved by intensive heating of the solution. The immersion of fuel capsules occurs when the following conditions are met: the average density of the capsule with the radionuclides filling it must exceed the density of the rocks located under the capsule; the melting point of the refractory material from which the capsule's hermetic shell is made must exceed the melting temperature of the rocks located under the capsule. To ensure a sufficiently high immersion rate, the temperature gradient between the surface of the capsule and a viscous medium (rock salt solution) must be maintained in the range from 3 ° C to 10 ° C.

It is assumed that when implementing the known method for the disposal of PAO using heat-generating capsules with high activity radionuclides, the capsules can be immersed from an initial depth of 1 ÷ 1.5 km to a depth of 5 km for one year. However, despite the rather high predicted rate of capsule immersion, it is impossible to safely bury long-lived PAOs with low activity when implementing this method. PAOs of this kind, including the long-lived isotopes of transuranium elements produced during the irradiation of nuclear fuel at nuclear power plants, are among the most dangerous radiotoxic nuclides.

Based on the radiation safety conditions, the duration of storage of long-lived radionuclides isolated from the environment, which include isotopes of transuranic elements (minors), should be at least ten thousand years. Table 1 shows the half-life of T _1/2 and the life of the main long-lived isotopes of transuranic elements (minors) for one year during a three-year irradiation cycle in a VVER-1000 type nuclear reactor (after holding for six months). It should be noted that all generated long-lived isotopes of transuranium elements are subject to burial, with the exception of plutonium isotopes.

Table number 1 Isotopes of transuranic elements T _1/2 years Operating time, kg per year ²³⁷ np 2.110 ⁶ eleven ²³⁸ Pu 88 3.4 ²³⁹ Pu 2.4 · 10 ⁴ 147 ²⁴⁰ Pu 6.6 · 10 ³ 58 ²⁴¹ Pu 13 40 ²⁴¹ Am 4.6 · 10 ² 1.8 ²⁴³ Am 8 · 10 ³ 3.2 ²⁴⁴ Cm eighteen 1.2 Other isotopes - <0.07

Long-lived radionuclides to be disposed of include the following long-lived fission products resulting from nuclear fuel irradiation: ¹⁵¹ Sm, ⁹⁹ Tc, ^121m Sn, ⁹³ Zr, ¹²⁶ Sn, ⁷⁹ Se, ¹³⁵ Cs, ¹⁰⁷ Pd, ¹²⁹ I, ¹⁶⁶ Ho , ¹⁰⁸ Ag, ¹⁵⁸ Tb, ⁹⁴ Nb. Table No. 2 shows the half-life of T _1/2 for the listed radionuclides.

Table number 2 Radionuclide T _1/2 years Radionuclide T _1/2 years ¹⁵¹ Sm 90 ¹⁰⁷ pd 6.510 ^{6 99} Tc 2.110 ^{5 129} I 1.610 ^{7 121m} Sn 52 ¹⁶⁶ Ho 1,2 · 10 ^{3 91} Zr 1.5 · 10 ^{6 108} Ag 127 ¹²⁶ Sn May ^{10 158} Tb 180 ⁷⁹ Se 6.510 ^{4 94} Nb 2 · 10 ^{4 135} cs 2,310 ⁶

It should be noted that the known methods for the disposal of PAO do not take into account radiation safety issues associated with the possibility of destruction of the sealed shell of capsules (containers) filled with PAO due to thermoelastic deformations caused by inhomogeneous heating of the fuel capsules. When considering this safety aspect of the burial process of long-lived PAOs, a limitation on the size of the fuel capsules is required.

To ensure reliable burial of long-lived radionuclides, the method of self-immersion of capsules containing PAO into the deep layers of the Earth's lithosphere is theoretically applicable. However, due to the low activity of isotopes of transuranic elements (minors), the heat release of the capsule is insufficient to melt the enclosing geological rocks into which the capsule is immersed. The highest power density of volumetric heat release among the isotopes of transuranium elements shown in table No. 1 is the Curia isotope: ²⁴⁴ Cm. When filling the entire internal volume of a spherical capsule with a radius of R _min = 10 cm with this isotope, the maximum achievable power density of the volumetric heat release will be only 1.2 W / cm ³ . The relative concentration of the ²⁴⁴ Cm isotope in the total number of produced isotopes of transuranic elements is only 0.07. With a sufficiently low level of heat generation of the generated long-lived isotopes and a limited capsule size, it is almost impossible to provide an acceptable rate of immersion of the fuel capsule in the deep layers of the lithosphere: to a depth of at least 10 km for 8 ÷ 10 years.

An increase in the size of the capsule also does not lead to the desired result, since the surface density of the power of the heat release of the capsule is also insufficient for uniform penetration of the host geological rocks in the stationary mode of immersion of the heat-generating capsule in the deep layers of the lithosphere.

It should be noted that the effective implementation of the known method for the disposal of PAO is provided only in a certain form of host rocks, namely, in rock salt formations. All of the analogues described above are characterized by a solution to the technical problem associated with an increase in heat dissipation in capsules placed in the well, but the proposed means for solving this problem are rather complicated in their implementation. At great depths (more than 5 km), it is not possible to control the penetration of host rocks using solvents and thermite mixtures, as well as to predict the behavior of the entire complex system of containers and capsules containing radiotoxic nuclides.

The invention is aimed at providing conditions for the burial of long-lived radionuclides, including long-lived isotopes of transuranic elements, in the deep layers of the Earth's lithosphere at a high speed of immersion of capsules containing PAO in the host geological rocks. At the same time, the task is to eliminate from the composition of the PAO disposal system additional technical (chemical) means, such as thermite mixtures and solvents, which provide additional heat for the melting and dissolution of the host geological rocks.

The solution of the above problems allows the burial of the entire spectrum of PAO accumulated during the irradiation of reactor fuel, including long-lived radionuclides, with relatively high speed. PAO is buried in the mantle of the Earth, where the irreversible dissolution of radiotoxic nuclides takes place. In general, the invention is aimed at reducing the risk of radioactive contamination of the environment and increasing the efficiency of cleaning the biosphere from long-lived PAOs, including long-lived isotopes of transuranic elements.

These technical results are achieved by implementing the method of disposal of PAO, which consists in immersing at least one fuel capsule with a sealed shell, in the cavity of which there are radionuclides, in a well formed in geological formations. Granitoid or basalt massifs, as well as other crystalline rocks, tuffs, salts and clays, can be used as possible media for burial of long-lived PAOs.

To implement the method, a capsule is used, the average density of which with the radionuclides filling it exceeds the density of geological rocks located under the capsule. The melting temperature of the refractory material from which the capsule is sealed must be higher than the melting temperature of the geological rocks located under the capsule. According to a subject of the invention, the capsule-filling radionuclides are a mixture that contains at least one long-lived radionuclide, for example, an isotope of transuranium elements, and at least one of the following highly active isotopes: ⁹⁰ Sr, ¹³⁷ Cs. The last of these conditions suggests the possibility of using a mixture of highly active ⁹⁰ Sr and ¹³⁷ Cs isotopes. The half-life of these isotopes is ~ 30 years. Energy release during one fission event: for the ⁹⁰ Sr isotope - 2.82 MeV, for the ¹³⁷ Cs isotope - 1.176 MeV.

The addition of highly active ⁹⁰ Sr and ¹³⁷ Cs isotopes to a PAO mixture containing long-lived isotopes makes it possible to increase the total power density of the volumetric heat release of PAO contained in the heat-generating capsule by more than an order of magnitude. The quantitative composition of the mixture of radionuclides is selected from the following conditions: the heat dissipation rate of the mixture of radionuclides must exceed the thermal power necessary for the melting of geological rocks located under the capsule.

Additional heat generation, in order to optimize the process of melting of geological rocks at the initial site of immersion, can be achieved by incorporating the highly active ⁶⁰ Co. isotope into the PAO mixture. The half-life of this isotope does not exceed 5 years, but the energy release during one act of fission of the radionuclide is ~ 1.5 MeV, which corresponds to a heat release power of ~ 1.6 · 10 ^-21 W / atom.

As long-lived isotopes of transuranium elements, in particular, the following isotopes can be used: ²³⁷ Np, ²⁴¹ Am, ²⁴³ Am, ²⁴² Cm, ²⁴⁴ Cm. A long-lived radionuclide can also be selected from the following series of isotopes: ¹⁵¹ Sm, ⁹⁹ Tc, ^2lm Sn, ⁹³ Zr, ¹²⁶ Sn, ⁷⁹ Se, ¹³⁵ Cs, ^l07 Pd, ¹²⁹ I, ^l66 Ho, ¹⁰⁸ Ag, ¹⁵⁸ Tb, ⁹⁴ Nb.

The capsule in which the PAO to be buried is placed is preferably spherical in shape. In preferred embodiments, the diameter of the capsule does not exceed 15 cm and is ~ 10 cm.

By introducing additional highly active radionuclides into the composition of low-level radionuclides that satisfy the requirements set forth below, the heat necessary for the penetration of the host geological rocks is provided throughout the process of immersion of the capsule before entering the Earth’s mantle. After this, the radionuclides filling the capsule dissolve in the mantle. By changing the content of highly active nuclides in the PAO mixture, one can pre-set the level of heat release of the capsule and, consequently, the speed of movement (immersion) of the capsule in the layers of the lithosphere. The process of self-immersion of capsules during the implementation of the method becomes easily predictable provided that there is information about the type and nature of geological rocks located along the calculated trajectory of the capsule.

The implementation of the method of disposal of PAO associated with the fulfillment of a number of essential conditions. First, it is necessary that in the process of fission of radionuclides the surface temperature of the capsule exceed the melting point of the environment, i.e. the melting point of geological rocks located under the capsule. This condition is characterized by the following mathematical expression:

$$q{R}^2>3k\Delta T,\left(\mathrm{one}\right)$$

where q is the power density of the volumetric heat release inside the capsule, R is the radius of the capsule of a spherical shape, k is the thermal conductivity of the geological rock, ΔT = T _pl -T ₀ , T _pl is the melting temperature of the geological rock, T ₀ is the initial (before contact with the heat-generating capsule) geological rock temperature.

For a given value of R, this condition is ensured by selecting the composition of the mixture of radionuclides in the shell cavity. An increase in the power density q of volumetric heat release is achieved by increasing the content of highly active radionuclides in the PAO mixture.

Secondly, it is necessary to ensure continuous immersion of the capsule in the melt of geological rocks. The capsule, displacing the melt by gravity, should sink to the bottom of the cavity. For this, the average density of the capsule with the radionuclides filling it must exceed the density of the geological rocks located under the capsule. Capsule immersion speed V under condition (1) can be estimated in accordance with the following ratio:

$$V\cong \frac{qR}{H_0},\left(2\right)$$

where H ₀ = ρ (λ + cΔT) is the amount of energy needed to melt a unit volume of geological rock located under the capsule; ρ, λ, c - density, specific heat of fusion and heat capacity of geological rock, respectively; R is the radius of the spherical capsule.

One of the essential conditions is the condition for ensuring the strength (tightness) of the capsule when it is immersed to a predetermined depth until the PAO dissolves in the Earth’s mantle. This condition characterizes the radiation safety requirement during the implementation of the method. To fulfill this requirement, it is necessary that the thermoelastic stresses resulting from inhomogeneous thermal deformations in the capsule during its heating during the fission of radionuclides do not exceed the value of the tensile strength of the shell.

With uniform heat release in the capsule volume, the temperature difference δT between the central and peripheral parts of the capsule is:

$$\delta T\cong \frac{q{R}^2}{6k_0},\left(3\right)$$

where k ₀ is the thermal conductivity of the capsule core.

Due to inhomogeneous thermal boundary conditions on the surface of the capsule, when the capsule is lowered in the melt of geological rocks, the maximum temperature is shifted relative to the center of the capsule. This phenomenon, in turn, leads to the appearance of an inhomogeneous distribution of thermoelastic tensile stresses in the capsule shell. The value of thermoelastic stresses a is estimated in accordance with the following ratio:

$$\sigma \cong 0.5\cdot \alpha \cdot E\cdot \delta T,\left(\mathrm{four}\right)$$

where α and E are the coefficient of thermal expansion and Young's modulus of the material of the contents of the capsule, respectively.

To ensure the tightness of the capsules during their immersion to the required depth, the stress value σ should not exceed the tensile strength of the shell σ _B. This condition is characterized by the following ratio:

$$q{R}^2\le 6k\delta T_{\max },\left(5\right)$$

where δT _max ≅2σ _B / αE.

Another important condition for the implementation of the method is the selection of the half-life of highly active isotopes placed in a capsule together with long-lived isotopes of transuranic elements. Based on the fact that the capsule with PAO must descend to a depth L for reliable isolation of PAO in the deep layers of the lithosphere, the period T _{1/2 of the} half-life of highly active radionuclides providing the required level of power density q volumetric heat release in the capsule must be at least _t0 lowering capsules to a depth of L. This condition is expressed by the following ratio:

$$T_{\mathrm{one}/2}\ge t_0=L/V,\left(6\right)$$

In addition, it is necessary to take into account additional conditions that enable the implementation of the method. Such conditions, in particular, include the limitation of the duration t _{0 of} the PAO disposal process. This limitation is associated with ensuring radiation safety near the well into which the capsules are immersed. To ensure radiation safety, the capsule shell must have a sufficient margin of corrosion resistance in the active medium of molten geological rocks.

From radiation safety conditions, capsule size limitation also follows. The fuel capsules should be small in size to reduce the release of radionuclides into the environment with possible destruction of the capsule shell. At the same time, the size of the capsules should be limited, taking into account that the creation of a large-diameter well (more than 20 cm) is associated with additional material costs, and the transportation of relatively large-sized fuel capsules to the burial site increases the risk of radiation hazard. On the other hand, if capsules are too small, the heat release intensity, which is determined by the rate of fission of radionuclides and the energy released during one act of fission, may not be sufficient to melt the host geological rock (see condition (1)).

To assess the feasibility of conditions (1) ÷ (6) and additional conditions, the following values of the initial parameters can be accepted: L = 30 km, t ₀ = 30 years, V _min = 1 km / year. Generalized conditions for the selection of highly active isotopes and their content in a PAO mixture filling the capsule volume can be represented as follows:

$$R\le R_{\max }\approx \frac{6\kappa \delta T_{\max }}{H_0{V}_{\min }},\left(7\right)$$

$$q\ge R_{\min }\approx \frac{H_0{V}_{\min }}{R_{\max }},\left(8\right)$$

Relations (7) and (8) determine the conditions for choosing the necessary parameters for the disposal of PAO containing long-lived isotopes of transuranic elements in deep layers of the lithosphere. As a result of the analysis, it was found that the above conditions (7) and (8) are fulfilled when using as a PAO to be buried a mixture of radionuclides, including long-lived isotopes of transuranium elements and at least one of the highly active isotopes: ⁹⁰ Sr or / and ¹³⁷ Cs.

The quantitative content of the selected highly active isotopes in the mixture of radionuclides filling the capsule is selected from the following condition: the volumetric heat release rate of the mixture of radionuclides should exceed the thermal power needed to melt the geological rocks located under the capsule. In particular, the quantitative content of the highly active isotope in the mixture of radionuclides filling the capsule can be selected from the condition: q _min ≥1.2 W / cm ³ , where q _min is the minimum power density of the volumetric heat release of the mixture of radionuclides within 10 years after immersion of the capsule.

It should be noted that highly active isotopes, including ⁹⁰ Sr, ¹³⁷ Cs and ⁶⁰ Co, are produced in the process of nuclear fuel irradiation and, along with long-lived radionuclides, are extremely radiotoxic components of PAO. As a consequence, the highly active isotopes used are also subject to reliable isolation from the biosphere. When using this technical solution, additional technical means are not required to intensify heat generation near the capsule and dissolve the host geological rocks.

The implementation of the method of disposal of PAO with the achievement of the above technical results is provided by means of a fuel capsule filled with PAO, which are to be disposed of. The sealed capsule shell is made of a refractory material (alloy of metals or ceramics). According to the invention, a mixture containing at least one long-lived radionuclide, for example, a long-lived isotope of transuranic elements, and at least one of the following highly active isotopes: ⁹⁰ Sr, ¹³⁷ Cs, are used as radionuclides filling the shell cavity.

The heat-generating capsule may include a heat-conducting matrix, which is located in the cavity of the sealed enclosure. The matrix is filled with radionuclides. A mixture of radionuclides can include at least one of the following long-lived isotopes of transuranic elements: ²³⁷ Np, ²⁴¹ Am, ²⁴³ Am, ²⁴² Cm, ²⁴⁴ Cm. The following fission products of nuclear fuel can also be used as long-lived radionuclides: ¹⁵¹ Sm, ⁹⁹ Tc, ^121m Sn, ⁹³ Zr, ¹²⁶ Sn, ⁷⁹ Se, ¹³⁵ Cs, ¹⁰⁷ Pd, ¹²⁹ I, ¹⁶⁶ Ho, ¹⁰⁸ Ag, ¹⁵⁸ Tb, ⁹⁴ Nb. The mixture of radionuclides may include the highly active isotope ⁶⁰ Co.

The heat-conducting matrix can be made of foam corundum. In this case, the mass content of the highly active isotope in the mixture of radionuclides is at least 75%. In another embodiment of the capsule, the heat-conducting matrix is made of porous stainless steel. The mass content of the highly active isotope in the mixture of radionuclides in this embodiment of the capsule is at least 25%.

The fuel capsule in preferred embodiments has a sphere shape whose diameter does not exceed 15 cm.

Further, the group of inventions is illustrated by a description of specific examples of the implementation of the method of disposal of PAO using fuel capsules intended for the implementation of the method. In the explanatory drawing (figure 1) schematically shows a section of a fuel capsule.

The spherical capsule contains an airtight shell 1 made of a refractory tungsten alloy, the melting temperature of which exceeds the melting temperature of basalt rocks (1500 ÷ 1700 K). In the cavity of the shell 1 is a heat-conducting matrix 2 filled with a mixture of radionuclides. The composition of the mixture includes long-lived low-activity radionuclides, which include the following isotopes of transuranic elements: ²³⁷ Np, ²⁴¹ Am, ²⁴³ Am, ²⁴² Cm, ²⁴⁴ Cm. The following nuclear fission products can also be used as long-lived radionuclides: ¹⁵¹ Sm, ⁹⁹ Tc, ^121m Sn, ⁹³ Zr, ¹²⁶ Sn, ⁷⁹ Se, ¹³⁵ Cs, ¹⁰⁷ Pd, ¹²⁹ I, ¹⁶⁶ Ho, ¹⁰⁸ Ag, ¹⁵⁸ Tb, ⁹⁴ Nb.

The mixture of radionuclides also contains at least one highly active isotope: ⁹⁰ Sr or ¹³⁷ Cs. These isotopes provide a high power density of the volumetric heat during the time of immersion of the capsule in the mantle of the Earth (up to 30 years). To optimize the process of penetration of geological rocks at the initial site of capsule immersion, a ⁶⁰ Co isotope is included in the mixture of radionuclides, which provides a high level of heat release during the first five years of capsule immersion.

The average density of the capsule exceeds 3 g / cm ³ . The temperature on the surface of the capsule is at least 2200 K. The quantitative composition of the radionuclide mixture is selected from the following condition: the volumetric heat release power of the radionuclide mixture must exceed the thermal power required for the melting of geological rocks located under the capsule.

The heat-release capsule is placed in a well drilled in basalt rocks to a depth of 1,500 m. Due to the high total level of heat release in the capsule, which is ensured by the inclusion of highly active ⁹⁰ Sr and / or ¹³⁷ Cs isotopes in the mixture, a high rate of immersion of the capsule into geological formations. The depth of immersion of the fuel capsule during the first ten years may be more than 10 km relative to the initial level of immersion in the capsule well.

Example No. 1

As a heat-conducting matrix of the capsule, a matrix made of porous stainless steel was used. The saturation of the matrix with radionuclides was carried out to a density of 3 g / cm ³ . For the selected type of basaltic rocks, the amount of energy Н ₀ , which is necessary for melting a unit volume of geological rock located under the capsule, is ~ 6 · 10 ⁹ J / m ³ . The minimum immersion speed V _min of the fuel capsule in the host geological formations is chosen equal to 1 km / year. The permissible maximum temperature difference δT _max between the central and peripheral parts of the capsule was selected from the condition of ensuring the strength and tightness of the capsule during immersion to a given depth: δT _max = 200 K. The calculation was carried out with the following initial data: α≅10 ^-5 K ^-1 , σ _B / E≅10 ^-3 . The thermal conductivity of the steel-oxide-radionuclide system was 20 W / m · K.

From the above mathematical dependencies (1) ÷ (8), it follows that with a maximum radius of a spherical capsule R _max = 12 cm, the minimum power density of volumetric heat release q _{min of a} mixture of radionuclides within 10 years after immersion of the capsule in the well should be ~ 1.7 W / cm ³ . This level of volumetric heat release power density is ensured by the next choice of the quantitative composition of the mixture of radionuclides. The total mass content in the PAO mixture of long-lived isotopes of transuranic elements: ²³⁷ Np, ²⁴¹ Am, ²⁴³ Am, ²⁴² Cm, ²⁴⁴ Cm in a proportion corresponding to the production of these isotopes during irradiation of nuclear fuel in the reactor should be 75%. The mass content of the highly active ⁹⁰ Sr isotope in the PAO mixture should be 25%.

Thus, with the selected size of the heat-generating capsule, the material of the heat-conducting matrix, and the quantitative content of long-lived low-activity radionuclides and highly active radionuclides in the PAO mixture, the rate of immersion of the heat-generating capsule in the well will be at least 1 km / year. At this capsule immersion speed, reliable and safe disposal of highly toxic PAO in the Earth's mantle is ensured for no more than 30 years.

Example No. 2

As a heat-conducting matrix 2, a matrix made of foam corundum was used. Using the same initial data taking into account the selected matrix material, it was established as a result of the calculation that, with a maximum radius of a spherical capsule R _max = 4 cm, the minimum power density of volumetric heat release q _{min of a} mixture of radionuclides within 10 years after immersion of the capsule in the well should be: q _min ≅5 W / cm ³ .

This level of volumetric heat release power density is ensured by the following choice of the quantitative composition of the radionuclide mixture: total mass content of long-lived transuranium elements isotopes in the PAO mixture: ²³⁷ Np, ²⁴¹ Am, ²⁴³ Am, ²⁴² Cm, ²⁴⁴ Cm in the proportion corresponding to the production of these isotopes during irradiation nuclear fuel in the reactor is equal to 25%; the mass content of the highly active ⁹⁰ Sr isotope in the PAO mixture is chosen equal to 75%.

In this example embodiment, the immersion rate of the fuel capsule in the well is also at least 1 km / year. (with the selected size of the heat-generating capsule, the material of the heat-conducting matrix and the quantitative content of long-lived low-activity radionuclides and highly-active radionuclides in the PAO mixture) With this speed of immersion and the required strength characteristics of the capsule, reliable and safe disposal of highly toxic radionuclides in the Earth's mantle is carried out. The PAO burial process takes no more than 30 years.

The performed calculations confirm the possibility of increasing the penetration rate of the host rocks and, accordingly, the rate of immersion of the fuel capsule in the Earth's lithosphere, provided that the capsule is leakproof. It should be noted that for the implementation of the method of burial of long-lived PAO, the use of additional means that ensure the melting of the host rocks is not required. The invention allows for the disposal of the entire spectrum of PAO accumulated during the irradiation of reactor fuel, including long-lived radionuclides. In this case, the risk of radioactive contamination of the environment is reduced and the efficiency of cleaning the biosphere from long-lived PAOs, including long-lived isotopes of transuranic elements, is increased.

The above-described embodiment of the invention is based on the use of specific materials from which the fuel capsule is made, radionuclides that are part of the RAW, which are to be disposed of. However, you can use other materials that satisfy the essential conditions included in the claims. As long-lived radionuclides to be disposed of, various radioactive isotopes produced during the irradiation of nuclear fuel and other fissile materials can be used. A mixture of ⁹⁰ Sr and ¹³⁷ Cs isotopes can be used as highly active radionuclides.

The invention may find application for the disposal of a wide range of PAOs formed during the irradiation of nuclear fuel and other fissile materials in various devices and devices, primarily in nuclear power reactors, as well as in medical devices and devices.

Claims (20)

1. A method for the disposal of radioactive waste, including immersing at least one fuel capsule with a sealed shell, in the cavity of which there are radionuclides, in a well formed in geological formations, using a capsule, the average density of which with its filling radionuclides exceeds the density of geological rocks located under the capsule, the melting temperature of the refractory material of which the capsule is sealed exceeds the melting temperature of geological genus arranged under the capsule, characterized in that as radionuclides, filling a capsule, a mixture containing at least one long-lived radionuclide and at least one of the following high-isotopes: ⁹⁰ Sr, ¹³⁷ Cs, and quantitative composition radionuclide mixtures are selected such that the volumetric heat release power of the radionuclide mixture exceeds the thermal power needed to melt the geological rocks located under the capsule. 2. The method according to claim 1, characterized in that the quantitative content of a highly active isotope in the mixture of radionuclides filling the capsule is selected from the condition q _min ≥1.2 W / cm ³ , where q _min is the minimum power density of volumetric heat release of the mixture of radionuclides during 10 years after immersion of the capsule in the well. 3. The method according to claim 1, characterized in that a long-lived isotope of transuranic elements is used as a long-lived radionuclide. 4. The method according to claim 3, characterized in that they use a long-lived isotope of transuranic elements selected from the following series: ²³⁷ Np, ²⁴¹ Am, ²⁴³ Am, ²⁴² Cm, ²⁴⁴ Cm. 5. The method according to claim 1, characterized in that as a long-lived radionuclide use an isotope selected from the following series: ¹⁵¹ Sm, ⁹⁹ Tc, ^121m Sn, ⁹³ Zr, ¹²⁶ Sn, ⁷⁹ Se, ¹³⁵ Cs, ¹⁰⁷ Pd, ¹²⁹ I , ¹⁶⁶ Ho, ¹⁰⁸ Ag, ¹⁵⁸ Tb, ⁹⁴ Nb. 6. The method according to claim 1, characterized in that the mixture of radionuclides further includes a highly active isotope ⁶⁰ Co. 7. The method according to claim 1, characterized in that they use a capsule with a heat-conducting matrix in which radionuclides are placed. 8. The method according to claim 7, characterized in that the heat-conducting matrix is made of foam corundum, while the mass content of the highly active isotope in the mixture of radionuclides is at least 75%. 9. The method according to claim 7, characterized in that the heat-conducting matrix is made of porous stainless steel, while the mass content of the highly active isotope in the mixture of radionuclides is at least 25%. 10. The method according to claim 1, characterized in that they use a capsule of a spherical shape, the diameter of which does not exceed 15 cm 11. The method according to claim 1, characterized in that the immersion of the fuel capsules is produced in a well formed in geological formations, including at least one of the following types of geological rocks: crystalline rocks, tuffs, salts, clay. 12. A fuel capsule for the disposal of radioactive waste containing an airtight shell made of refractory material, while the cavity of the shell is filled with radionuclides, characterized in that as a radionuclide filling the cavity of the shell, a mixture containing at least one long-lived radionuclide and at least one of the following highly active isotopes: ⁹⁰ Sr, ¹³⁷ Cs. 13. The capsule of claim 12, wherein a long-lived isotope of transuranic elements is selected as a long-lived radionuclide. 14. The capsule of claim 13, wherein the long-lived isotope of transuranic elements is selected from the following series: ²³⁷ Np, ²⁴¹ Am, ²⁴³ Am, ²⁴² Cm, ²⁴⁴ Cm. 15. The capsule according to claim 12, characterized in that an isotope from the following series is selected as a long-lived radionuclide: ¹⁵¹ Sm, ⁹⁹ Tc, ¹²¹ mSn, ⁹³ Zr, ¹²⁶ Sn, ⁷⁹ Se, ¹³⁵ Cs, ¹⁰⁷ Pd, ¹²⁹ I, ¹⁶⁶ Ho, ¹⁰⁸ Ag, ¹⁵⁸ Tb, ⁹⁴ Nb. 16. The capsule of claim 12, wherein the radionuclide mixture further comprises a highly active isotope ⁶⁰ Co. 17. The capsule according to item 12, characterized in that it includes a heat-conducting matrix located in the cavity of the hermetic shell and filled with radionuclides. 18. The capsule according to claim 17, characterized in that the heat-conducting matrix is made of foam corundum, while the mass content of the highly active isotope in the mixture of radionuclides is at least 75%. 19. The capsule according to claim 17, characterized in that the heat-conducting matrix is made of porous stainless steel, while the mass content of the highly active isotope in the mixture of radionuclides is at least 25%. 20. The capsule according to item 12, characterized in that it is made in the form of a sphere, the diameter of which does not exceed 15 cm