EP4038644B1

EP4038644B1 - Container for deep underground deposition of spent nuclear fuel and method of deep underground deposition of spent nuclear fuel

Info

Publication number: EP4038644B1
Application number: EP20743577.7A
Authority: EP
Inventors: Jirí MÁLEK
Original assignee: Individual
Current assignee: Individual
Priority date: 2019-05-31
Filing date: 2020-05-27
Publication date: 2024-01-24
Anticipated expiration: 2040-05-27
Also published as: EP4038644A1

Description

Field of technology

The invention relates particularly to a container for deposition of nuclear waste, namely spent nuclear fuel from nuclear power plants, and a method for deposition nuclear waste. The container according to the invention ensures the safe deposition of radioactive waste by submersion to a great depth (up to 20 km) below the Earth's surface. The energy required for this submersion is drawn in part from the residual heat released by the radioactive decay in the spent fuel and in part from the fresh nuclear fuel, which is added to the waste. When the container is submerged, a very deep well is created at the same time, which can then be used to obtain geothermal energy and to research the Earth's interior.

Background of the invention

According to the WNA (World Nuclear Association), in January 2019, there were 450 nuclear reactors in the world connected to the electrical grid, which have a total capacity of 399 GWe. Another 57 reactors with a capacity of 62 GWe are under construction. One reactor with a capacity of 1 GWe produces 30 to 40 tons of spent fuel per year, which represents a volume of 1.5 to 2.0 m³. The fuel withdrawn from the reactor usually contains 95 % uranium and the remainder is fission products, which are mostly radioactive. This spent fuel must be deposited safely to avoid environmental contamination. The costs of deposition of spent fuel are considerable. The current common practice is to remove the spent fuel from the reactor and transport it below the water surface through a channel to the spent fuel pool in the reactor hall, where they are stored under water for about 3 to 4 years. The water is constantly cooling it, because the radioactive decay still generates heat. In the meantime, the radioactivity drops to about 50 % of the original value. The spent fuel is then placed into containers and taken to an intermediate storage facility. It is stored here for several decades. During this time, considerable heat is still produced from radioactive decay. At present, permanent deposition in deep repositories at a depth of about 0.5 to 1 km is expected for many tens of thousands of years before the radioactivity of the waste ceases. Throughout this long period, it must be ensured that the radioactivity does not overcome the artificial barriers created during waste deposition.
An alternative to the above mentioned procedure is so-called self-burial concept. This concept was formulated as early as in 1970s (Logan, 1974), and is described, for example, by Byalka (1994). It has also been described in patent application RU2012134053/07 . Recent studies include a paper by Chen et al. 2013. The concept lies in that the spent fuel in a special container causes the underlying rock to melt due to the release of residual heat, so that it burns through into great depths, where it remains safely deposited. So far, however, this concept has not been implemented, even though this method would be an order of magnitude cheaper than the current type of permanent repository. The reason is doubts about the safety of this process in the initial phase, when the rock would melt and form magma deposits at shallow depths, and especially the absence of a suitable container. The present invention utilizes the concept of self-burial, wherein the safety of deposition is ensured by the special construction of the container and by diverting the heat by means of a well which arises when the container is submersed.
Geothermal energy is currently one of the sources of so-called "clean" energy without the release of greenhouse gases. The current output of all geothermal power plants in the world is approximately 6 GWe. Most of the power plants are operated in geothermal areas where an anomalous high heat flux exists. The expansion of geothermal power plants to other areas is limited mainly due to the need to drill deep wells, which is very expensive. The container according to the present invention, when storing nuclear waste, forms also a very deep well with a large diameter, which can subsequently be used to obtain geothermal energy.
The deepest well drilled for scientific purposes is a well on the Kola Peninsula with a depth of 12,261 m. A well created with the container according to the invention can overcome this depth and thus provide also valuable scientific information.
The speed of burning through the rock can be accelerated by means of concentrated sun energy, which is directed into the well by means of a system of mirrors and lenses. A similar procedure is used in some types of solar power plants that have been in operation for several years and provide an output of tens to hundreds of MW (for example, the Ivanapah concentration plant in California has a capacity of 392 MW).

Description of the invention

The container according to the present invention ensures the deposition of spent nuclear fuel in the Earth's crust, by submersion to a depth of up to 20 km. A heat that is released during radioactive decay, during nuclear fission, is used to submerse (self-bury) the container. To speed up the submersion, additional radioactive material (fresh fuel for nuclear power plants or fissile material from old nuclear weapons) and a source of neutrons are added to the waste in the container, which will significantly increase the heat production. The heat produced can also be increased, for example, by electric heating in times of excess electricity, by burning methane or by solar energy directed into a well.
The container for deep underground deposition of spent nuclear fuel comprises a container body, a lid and a casing part as basic parts, wherein the body and the lid form a lower cylindrical part, the casing part forms an upper cylindrical part, the diameter of the upper cylindrical part being smaller than the diameter of the lower cylindrical part. The body of the container is substantially a cylindrical vessel with a cavity for storing spent nuclear fuel, the lower part of the body forms a bottom which is shaped to penetrate the rock, and vertical channels for conducting molten magma transverse the body wall. The lid is a cylindrical body connectable to the upper part of the body, wherein vertical channels for guiding molten rock traverse the lid and adjoin the channels in the body wall. The lid is configured to connect the casing part. The casing part is a cylindrical part connectable to the upper part of the lid, contains a vertical central channel for forming and removing solidified magma, in the lower part contains channels for supplying molten magma to the central channel and contains cooling channels for coolant supply in the wall connected to vertical outlet channels for removal of coolant vapour. Spent fuel elements, or even fresh fuel elements, are stored in the cavity of the container body in a subcritical arrangement, surrounded by a layer of graphite.
The container ensures that the heat produced by the radioactive decay is consumed at the point of contact of the bottom of the container with the rock to reach the temperature of the adjacent rock higher than the melting temperature of the rock. Up to a depth of 20 km, this melting point is usually 600 to 1300 °C (depending on the depth and composition of the rock). The molten rock, the magma, formed below the bottom of the container rises through vertical channels in the wall of the container and in the lid upwards until it reaches the top edge, where it solidifies again. Upon solidification, magma is formed around the casing part to form a casing of the well. The material formed by the solidification of magma has glass-like properties. Excess magma is formed into rods, which are pulled to the surface after solidification.
The container is constructed of materials the melting point of which is higher than the temperature of the surrounding magma. The refractory materials used are of two types:

A) materials with high thermal conductivity, for example carbon steel, stainless steel, tungsten carbide, zirconium, nickel, titanium, tungsten, cobalt, chromium and platinum alloys with other metals or graphite and other carbon materials. When choosing the materials, it is necessary to optimize the high melting point, mechanical resistance and price. In the construction of the container, conductive materials are used mainly in the lower part (bottom), in contact with unmelted rock and possibly for casing the well. Due to the melting point of 1350-1400 °C, steel appears to be a suitable material for the construction of the container body and the casing part. Carbon steel may be preferred, for example A36 type steel containing 0.05 % sulphur has a melting point of 1426-1538 °C. Tungsten (melting point 3422 °C) can be suitably used as a covering (outer) layer of the bottom of the container.
B) materials with low thermal conductivity, for example porcelain or ceramics based on Al₂O₃. These materials are used in the upper part of the container body to slow down the heat flow and thus minimize heat loss. They are also used as an outer layer around the perimeter of the container shell, where they slow down the heat transfer to the surrounding rock and thus direct most of the heat generated to the bottom of the container. Porcelain, due to its heat-insulating properties and melting point of approximately 1680 °C, appears to be a material suitable for the production of a container lid, for external insulation of the container body shell and for insulating some channels in the container body or possibly the casing part.

Preference is given to high-density materials, since the average density of the container with both spent and fresh fuel elements must be higher than the density of magma. In such a case, after the rock has melted under the container, it descends downwards to greater depths.
An estimate of the required heat capacity of the container Q is obtained by calculating the heat Q_T needed to warm the appropriate amount of rock to the melting temperature, the heat Q_R for its subsequent melting, and taking into account the heat losses Q_Z to the surrounding rock. The result is defined by the formula: $Q = Q_{T} + Q_{R} + Q_{Z} = S v ρ t (q_{T} d_{T} + q_{R} + q_{Z})$
Where

Q is total heat that must be generated in the container;
Q_T is heat needed to warm the rock to melting point;
Q_R is melting heat needed to melt warmed rock;
Q_Z is heat loss;
S is cross section of the container;
v is the average velocity of submersing;
ρ is rock density;
t is time for which heat is generated;
q_T is specific heat of the rock;
d_T is temperature difference between the original rock temperature and the melting point;
q_R is specific melting heat of rock melting;
q_Z is heat loss during heating and melting of 1 kg of rock

If the orientation values for the container parameters and the values for a typical rock at a depth of 10 km are substituted into the formula, such as S = 1 m², v = 0.0002 ms^-1, ρ = 2.7×10³ kg m^-3, t = 1 s, q_T = 1.4×10³ J kg^-1 K^-1, d_T = 1000 K, q_R = 4.0×10⁵ J kg^-1, q_Z = 5.0×10⁵ J kg^-1, then the heat released in 1s (heat capacity/output) of 1.24 MW is obtained. It should be noted that this calculation provide us with only a very rough estimate, because there is a lot of uncertainty in the input data, for example in the estimation of heat losses q_Z. Furthermore, it should be noted that the heat output of the container gradually decreases during the descending, that the temperature difference d_T decreases with depth, and that the rock characteristics can vary considerably. At the considered average rate of descending v = 0.0002 ms^-1, the container would drop by 17.3 m per day and reach a target depth of 20 km in 3.17 years.
At the same thermal output and the same rock composition, the container would descend more slowly at smaller depths than at greater depths, because at lower depths there is a greater difference between the original rock temperature and the melting temperature. On the other hand, over time, the thermal output of the radioactive material will decrease, so that the descending of the container will slow down. At a certain depth, the container would stop spontaneously if the heat production was equal to the heat loss and the rock was not heated. By heat loss we mean heat that is dissipated to the vicinity of the well or is radiated by the well towards the surface without contributing to the heating of the rock below the bottom of the container. The construction of the container is designed so that a controlled stop occurs before the container would stop spontaneously. To stop the descending, the container is cooled by the water that is fed to the container.
The rate of descending and the target depth are determined before the start of burying and the amount of unspent nuclear fuel to be added to the radioactive waste is determined accordingly. The heat output also depends on the amount of neutrons released into the fuel (see next paragraph) and on the geometric arrangement of the fuel rods. The velocity of descending can be increased by supplying additional energy (in addition to the heat released from the radioactive decay) to the bottom of the container. Additional energy can be supplied, for example, in the form of electricity, heat generated by the combustion of methane or in the form of solar energy.
The heat in the core of the container is generated by (a) spontaneous decay of the radioactive material, (b) stimulated decay of the fissile material by irradiation with neutrons, the source of which is inserted into the container.

(a) The heat output generated by the spontaneous radioactivity of the spent fuel is released in the form of α, β and γ radiation, which is absorbed by the container material or the surrounding rock. Ten days after removal from the reactor, the temperature output of 1 ton of spent fuel is approximately 0.05 MW. If we consider the mass of spent fuel in a container, for example, 20 tons, the heat output is 1 MW, which covers almost the entire output needed to submerse the container. After one year, this heat output will drop to 0.2 MW, after ten years, the output will drop further to approximately 0.02 MW. Spontaneous radioactivity therefore significantly contributes to the overall performance in the first months of the deposition process.
b) Spent fuel contains ²³⁵U and ²³⁹Pu isotopes, which split when stimulated by slow neutrons and thus release a considerable amount of heat. At the same time, additional neutrons are released, the number of which, however, is not sufficient to maintain the fission reaction (under condition of subcritical amount, see the following paragraphs). Consequently, it is necessary to insert a neutron source in the container. A suitable source of neutrons may be an emitter based on californium isotope ²⁵²Cf, which releases neutrons according to the following reaction:

²⁵² ₉₈Cf → ²⁵¹ ₉₈Cf + ¹ ₀n

The half-life of this isotope is 2,645 years. Thus, it will be able to emit neutrons for the entire period of descending of the container, even if the amount of neutrons will decrease over time and the heat output produced as well. The disadvantage of californium is its high price. It is manufactured in special accelerators. In the future, however, it is assumed that its price will decrease if its production increases. The melting point of californium is 900 °C, so its liquid form must be taken into account and it must be enclosed in a refractory capsule. Another possible source of neutrons is ⁹Be. When stimulated by α radiation, a nuclear reaction occurs:

⁴ ₂He + ⁹ ₄Be → ¹³ ₆C → ¹² ₆C + ¹ ₀n
There is strong α radiation in the container, so this reaction will occur intensively when beryllium is inserted.
Beryllium also acts as a neutron flow amplifier based on a nuclear reaction

⁹ ₄Be+ n → 2 ⁴ ₂He+ 2 n
Thus, it can emit more neutrons than it absorbs.
Consequently, a suitable neutron source may be constructed by a combination of californium and beryllium. It has the shape of a rod with a diameter of several cm, in which the individual layers are concentrically assembled. The inner core is formed by molten californium (melting point is about 900 °C). Another thin layer consists of zirconium, which remains in a solid state (melting point 1855 °C) and, in addition, has the advantageous property that neutrons pass well through it. Another layer consists of beryllium, which is mostly in the solid state (melting point is 1287 °C), but it can also melt. Therefore, it is enclosed in another layer of zirconium.
The neutrons in the container are decelerated (moderated) using a suitable moderator (e.g. graphite) and then participate in the nuclear fission of ²³⁵U and ²³⁹Pu. This process is quite similar to nuclear reactor process. A multiplication factor was introduced as the probability that neutrons released during the fission of one nucleus will cause the fission of another nucleus. The essential difference between the described container and the nuclear reactor is that the multiplication factor is less than one in the container, while it is kept equal to one in the reactor. This ensures that nuclear fission in the container is completely safe, as the chain reaction quickly ceases without a source of neutrons. For the same reason, the chain reaction does not have to be controlled by control rods, as it is in the case of nuclear reactor. The fuel inside the container must be configured so that the multiplication factor is as close as possible to one, but always less than 1. This is achieved by placing the neutron source in the middle of the container, with unspent fuel around it and nuclear waste around it further from the centre. Even further from the centre of the container is a layer with a moderator and a neutron reflector. This layer is made of graphite (as it is in some nuclear power plants).
This maximizes heat output. The required heat output of the container (typically MW units) represents only a few per mille (%o) of the output of the nuclear reactor from which the nuclear waste originates, and therefore the consumption of nuclear fuel during deposition is very small.
The typical composition of fresh nuclear fuel is (according to data from the Temelin nuclear power plant) as follows: 1000 kg of the fuel contains 967 kg ²³⁸U and 33 kg ²³⁵U. After removing the fuel elements from the reactor, 1000 kg of spent fuel contains 943 kg ²³⁸U, 8 kg ²³⁵U, 8.9 kg of various plutonium isotopes, 4.6 kg ²³⁶U, 0.5 kg ²³⁶Np, and 35 kg of other fission products. Most of these products are radioactive and produce radioactive heat. However, the main part of the heat output is obtained by fission of the ²³⁵U and ²³⁹Pu nuclei.
At the target depth, a controlled deceleration of the descending of the container can begin, i.e. an intensive cooling of the container by means of water, which changes into water vapour. Consequently, the temperature of the container drops below the melting temperature of the rock and descending of the container will stop. The radioactive material thus remains safely deposited at this target depth. If the possibility of controlled deceleration is not used, the descending of the container to greater depths will continue spontaneously, until the heat production of the container falls below a level sufficient to melt the rock.
In an alternative solution, the well can be used for a long time, therefore its durability must be ensured by means of another casing, for example by casing made of steel. Such a casing may be attached to the upper part of the container, i.e. to the top of the casing part. When descending the container, the casing is pulled behind the container and is extended at the top by connecting its other parts. This procedure is similar to the routine steel casing of drilled wells and is currently well known in the art. The walls of the casing are thicker at the bottom to prevent the destruction of the well due to the lateral pressure of the rocks. The inner diameter of the casing is the same everywhere, but the outer diameter of the casing changes. This reduces friction when moving the casing down. The casing can advantageously comprise the channels for cooling the container and the light guide for additional solar energy, or electric cables or fuel supply, if descending is accelerated by means of electric energy or by burning methane, as described below in Example 2.
The container remains in a vertical position during submersing process. This is achieved by the fact that the lower part of the container (where nuclear fuel consisting mainly of uranium and plutonium is concentrated) has a higher density than the upper part consisting mainly of porcelain and steel. The container is to some extent surrounded by liquid (molten magma) and the container thus behaves like a body immersed in a liquid, where the part with a higher density tends to descent faster and thus always remains at the bottom.
The deposition of spent fuel by means of a container according to the present invention offers considerable advantages over the solution by means of a permanent repository at a depth of 500 to 1000 m below the surface, which is now considered to be a standard. Spent fuel can be deposited immediately after being withdrawn from the reactor in the vicinity of the power plant site. This completely eliminates storage in the spent fuel pool, in the intermediate repository and in the permanent repository. There is also no need to transport radioactive waste over long distances. The deposition will take place over a period of several months to years, and there is no need to ensure the long-term sustainability of the repository. An extraneous benefit of this solution is the possibility of scientific research into the earth's interior. Once the target depth has been reached and the container has cooled, scientific instruments can be installed at the bottom of the well. It is possible to measure a number of parameters that are of considerable importance for scientific knowledge, such as stress tensor, chemical composition of rocks or speed of seismic waves. It is also possible to place a seismograph into the well, which will be placed in an environment with a much lower level of seismic noise than on the surface. It is necessary to use special constructions of such devices, which can work at high temperature (for example 300 °C).
The main advantage of the present solution is that the deposition of spent radioactive fuel is absolutely safe. Even if for some reason the well is destroyed (as a result of a terrorist act, war conflict or a strong earthquake) and the connection to the container is interrupted, the descending of the container to greater depths will continue spontaneously, until the heat production of the container falls below level sufficient to melt the rock.
In other alternative solutions, it is possible to accelerate the descending of the container by supplying additional thermal energy. From the technical point of view, the easiest way to speed up the descending of a container is to use electric heating. In such a case, an electric wire is gradually sunk into the well. The wires must be made of alloys that can withstand high temperatures (for example, an alloy of iron and nickel). At the lower edge of the container there is a heating spiral made for example from tungsten. The heat released increases the thermal output of the container. Economically, this solution makes sense if electricity from the distribution system is used in times of its surplus.
Another option is to use the combustion of methane, or natural gas or other fuels. In such a case, methane is led to the container through one pipe, then oxygen is supplied through another pipe. At the lower edge of the container, the two gases are mixed and burned.
The descending of the container can also be accelerated by concentrating sun energy. When using this principle to accelerate the descending of a spent fuel container, slightly curved parabolic mirrors are placed around the well head, reflecting and concentrating the sun rays to the top of the tower, which is built above the well head. There, these concentrated rays are reflected by other (smaller) mirrors into the well head. Furthermore, the rays are guided through the well like through a light guide to the container, where they are absorbed and warm the container. Due to the changing position of the Sun in the sky, the mirrors must be directionally adjustable so that the rays can always be directed to the well head.
In another alternative solution, the container according to the present invention can also be used for digging deep wells. In such a case, higher proportion of fresh, unspent nuclear fuel is used to achieve higher thermal output. Advantageously, acceleration of container descending by solar energy, electric heating or methane combustion can be used to reduce the amount of nuclear fuel required. If the well is intended for the subsequent use of geothermal energy, additional resources may be retained after the completion of the well to increase the output of the geothermal power plant. This creates a new type of power plant that can combine several energy sources.
The present invention therefore provides a container for deep underground deposition of spent nuclear fuel as defined in claim 1. Further advantageous embodiments of the container are defined in the dependent claims 2 - 10. Another object of the present invention is a method for the deep underground deposition of spent nuclear fuel as defined in the claims 11 - 13.

Brief description of the figures

FIG. 1: Scheme of a container for deep underground deposition of spent nuclear fuel: A) side view, B) top view, C) vertical section through the centre of the container, D) horizontal section through the container.
FIG. 2: Scheme of horizontal section of the casing.

Exemplary embodiments of the invention

EXAMPLE 1

Container for deep underground deposition of spent nuclear fuel

The example describes a container intended for deposition of spent fuel when the created well will be closed after the fuel is deposited. The well is sheeted only with solidified magma. It is assumed that within a few years the well will deform due to horizontal stress, which is not the same in all directions. The target deposition depth is 10 km, which is a safe depth in most tectonic areas to ensure that radioactive material is not carried to the surface. It is assumed that the geological conditions at the deposition site correspond to the average conditions in the continental crust, i.e. that the temperature of rocks at a depth of 10 km reaches 300 °C, the melting temperature of rocks at this depth is 1200 °C, the average rock density between 0 and 20 km is 2 700 kg m^-3 and the lithostatic pressure at a depth of 10 km is therefore 270 MPa. The person skilled in the art is able to adjust the container for other parameters if the geological situation requires it.
The container (see Figs. 1A, 1C) has an elongated shape of two superimposed coaxial cylinders, where the upper cylindrical part has a smaller (about a half) diameter than the lower part and has a greater (about twice) height than the lower cylindrical part. The ratios of diameters and heights of the upper and lower parts of the container can be chosen differently in other embodiments of the container, also as depending on the rock. The lower cylindrical part with a height of 10 m and a diameter of 1 m forms the body 1 of the container and the lid 2. The body 1 is a cylindrical vessel containing a cavity for storing nuclear fuel surrounded by a 10 cm thick steel (carbon steel) wall which is provided on the outside with porcelain layer of 2 cm thickness. The porcelain layer here acts as a thermal insulation and at the same time as a protective layer with a higher melting point than the steel wall. Vertical channels 9 for the discharge of molten magma 8 transverse the wall of the body 1. These channels 9 also continue through the lid 2.
The bottom 4 of the container is rounded (spherical cap shape), it is also made of carbon steel and is provided with a steel rim 7 over its entire circumference, extending beyond the circumference of the bottom 4 of the container. This rim 7 ensures the collection of molten magma 8 and its collection by the channels 9 in the wall of the body 1 and the lid 2. The channels 9 widen slightly upwards and thus allow pass crystals which have not been able to melt at the bottom 4 of the container. Thus, the temperature of the magma 8 may be lower than the melting point of the most resistant crystals contained in the rock. The channels 9 have an oval cross-section and thus withstand radial pressure. The bottom 4 is covered with a protective layer on its outer surface (towards the rock) - a thin layer (1 cm thick) of tungsten, which has great mechanical and thermal resistance.
The container body 1 is provided with a lid 2 at the upper end, the lid 2 being 1 m thick (high) and is made of porcelain. The lid 2 is fixed to the body 1 by means of steel screws which pass through the porcelain and are drilled into the steel part. The screws are covered with porcelain plugs. In the lid 2 there is a channel 22 with a diameter of a few cm passing through the whole lid 2, which serves to insert the neutron source 13 to initiate the fission reaction.
Upwards (above the lid 2) the container continues with the casing part 3, which is an upper cylindrical part made of stainless steel. It has a diameter of 0.5 m and a length of 20 m, the wall thickness is 14 to 16 cm. After inserting the fuel rods 11, 12, the lid 2 is sealed and firmly connected to the body 1, and after inserting the neutron source 13 and closing the channel 22, the casing part 3 is fixed to the lid 2 by suitable connecting means - flanges and screws.
Both spent fuel rods 11 and fresh fuel rods 12 are stored in the body 1 of the container. The fuel rods 11, 12 are located in the container in a subcritical arrangement known to the person skilled in the art (multiplication factor is less than 1). The dimensions of the container are adapted to allow the storage of several tens of tons of spent fuel in one container. For example, fuel rods removed from one reactor at the Temelin nuclear power plant during fuel change weigh 23 tons, which corresponds to one quarter of all fuel rods in the reactor. Several tons of fresh nuclear fuel is added to this. The arrangement (see Fig. 1D) is such that a neutron source 13 is located in the vertical axis of the container, which is surrounded by rods 12 with fresh fissile material. The spent fuel rods 11 are located further from the axis of the container. The fuel rods 11 are surrounded around the entire circumference of the cavity (see Fig. 1D) by a layer 10 of graphite, which serves both as a moderator and as a neutron reflector.
In the centre of the casing part 3 of the container (see Fig. 1B) there is a cylindrical cavity 19 in which rods of excess magma are formed, which penetrate into the cavity 19 through four channels 15 located at the base of the casing part 3. The channels 15 may be substantially horizontal or sloping, when their outlet in the cavity 19 is higher than their inlet openings on the circumference of the casing part 3, which facilitates the flow of magma into the cavity 19. The diameter of the cavity 19 is 19 cm at the lower edge and 21 cm at the upper edge. This upward expansion ensures a smooth upward movement of the solidified magma through the cavity 19. The solidified magma rods are gradually pulled to the surface. Two vertical cooling channels 5 pass through the casing part 3. Cooling water is supplied from the surface through a pipe 17 (made of stainless steel) and enters the cooling channels 5. At the lower edge of the casing part 3, the cooling channels 5 are connected to two outlet channels 6. By passing through the channels 5, the water changes into vapour, which escapes through the outlet channels 6 and further through a second pipe 16 (stainless steel) upwards to the well head, where it is cooled and condenses again to water. The cooling channels 5 and the outlet channels 6 are arranged in a square when viewed from above (see Fig. 1B). The channels 5 are in two adjacent corners and are connected to the channels 6, which are located in the remaining two corners of the square. The cooling serves to solidify the magma faster. As the container is submersed, the cooling is maintained only at such intensity that is sufficient to solidify the magma 18, which forms the casing of the well before it reaches the upper edge of the container. After reaching the target depth, intensive cooling is started, which causes the entire container to cool and stop descending. The cooling of the container takes place until the heat production of the container decreases to such an extent that the surrounding rocks are able to dissipate this heat and the rock no longer melts.
The volume of gradually melted magma 8 is greater than the amount of magma 18 used to sheet the well. In this example, where the diameter of the container is 1 m and a well with a diameter of 0.5 m remains behind the container, the material used for casing represents 75 % of the original volume. Excess 25 % of the magma is led through horizontal (or sloping) channels 15 to the central channel 19, where it solidifies in the form of rods with a diameter of 0.2 m. The rods are then lifted to the surface by means of a steel rope 20. A steel grip 21 is used, which is lowered from the surface on the rope 20. When the grip 21 comes into contact with the upper part of the solidified magma rod, the grip 21 is clamped. First, the magma rod is pulled out by 2 m. This separates the rod at the point where the magma changes to a solid state. After a few minutes waiting the lower part of the drawn magma rod completely solidifies and then the rod is pulled to the surface. The material that forms when the magma re-solidifies is similar to igneous rocks that form during rapid solidification near the Earth's surface (such as obsidian or other glass-like rocks). This material has excellent short-term strength. However, because it is an amorphous material, it plastically deforms over a long period of time.
To start descending of the container, procedure is as follows: A well 10 m deep and approximately 1.1 m in diameter is excavated. The body 1 of the container without the porcelain lid 2 is inserted into it. Graphite moderator 10, the rods 11 of spent fuel end the rods 12 of unspent fuel are inserted into the container. An empty space is left in the centre to insert the neutron source 13. Then the porcelain lid 2 is closed and screwed to the body 1 of the container with steel screws. The holes above the screws are covered with porcelain plugs. Through the channel 22 in the lid 2, a neutron source 13 is inserted into the container, thus initiating fission reactions and intensive heating of the container. The channel 22 in the lid 2 is closed with a porcelain stopper. Then the casing part 3 is fastened above the lid 2 by means of a flange and steel screws. The container begins to sink and the magma 8 begins to flow through the channels 9. In order to prevent the magma flow around the well, the well head around the narrowed part is closed by a steel annulus. Thus, from the beginning, the excess magma is formed into the shape of rods in the channel 19 in the centre of the casing part 3. This starts the process of burying the container and creating a well.

Example 2

Container for deposition of spent nuclear fuel and / or digging a well for a geothermal power plant and for scientific purposes
In this example, the container is used to reach a depth of 20 km. This would be the greatest depth in history. Unlike the previous example, the well created by submersing the container should be stable for a long time. Therefore, stainless steel casing is used along the entire length of the well, which further strengthens the solidified magma casing 18. The casing (see Fig. 2) is a tubular body which is fixed to the top of the casing part 3 of the container and descends together with the container. Due to construction reasons, the section of casing has a certain limited length, e.g. 10-20 m, therefore when the container is submersing at the wellhead, further sections are gradually connected to the casing. The thickness of the walls 23 of the steel casing varies to withstand the increasing pressure. At the earth surface, the walls 23 of the casing are the thinnest and they are the strongest at the bottom close to the container. The steel casing has a diameter of about 2 cm smaller than a well sheeted with solidified magma, so the casing is easily sunk. Channels of several types are built into the casing. The cooling water channels 27 are insulated with porcelain so that the water no longer turns into vapour on the way down. Superheated vapour is led upwards through other channels 28. These channels 28 are also thermally insulated with porcelain, but only from the surface to a depth of approximately 10 km (where the temperature is below 300 °C, which is the expected temperature of the rising vapour). There may be other channels, e.g. channels 25 for locating a metal wiring for electric heating insulated with porcelain. Finally, some channels 26 may serve as light guides for solar energy. The central channel 24 of the casing remains empty, through it the rods of solidified magma are pulled out of the channel 19 of the casing part 3.
The process of submerging the container is started similarly as in Example 1, with the only difference that the prepared first part of the steel casing is attached to the casing part 3 immediately after lowering its upper part to the ground level.
The well drilled in this way must be created relatively quickly in order to avoid significant deformation of the solidified magma casing due to unequal horizontal stress, which could prevent further descending of the container. Therefore, a high heat output must be achieved. In addition, it is advantageous to choose a smaller container diameter so that there is no need to melt so much rock, and in addition, a smaller well diameter is more resistant to horizontal stress. Plutonium or enriched uranium, the amount of which is slightly subcritical, is preferably introduced into the container. If plutonium is used, this amount is relatively small. Pure metallic plutonium 239 has a critical mass of only 10.5 kg, with the use of a neutron reflector it can even be only 2.5 kg. If the container is used only for digging the well, without storing a large volume of spent fuel, the body 1 of the container can be very short, for example 1 m. As in Example 1, a layer 10 of graphite is used as a moderator and a neutron reflector and californium 252 and beryllium 9 as neutron source 13. The thermal output of nuclear fuel is planned to 2 MW.
The heat output can be further increased by a further 2 MW by concentrating the sun energy in sunny weather or by electric heating. In total, we work with an average heat output of 4 MW. The diameter of the cylindrical container is in this case 0.4 m, the diameter of the well behind the container 0.2 m. The thickness of the steel casing at the container level (at a final depth of 20 km) is 3 cm, at the wellhead 1 cm. The outer diameter of the casing thus decreases towards the surface, which contributes to easier movement of the casing downwards. By modifying the energy balance calculation mentioned in the description of the invention, an average rate of descending of 0.005 m^s-1 can be achieved. At this speed, the target depth of 20 km is reached in approximately 50 days. The well can then be used as an energy source. Immediately after reaching the target depth and starting intensive cooling, the container has a radioactive heat output of 2 MW. Another 2 MW can then be supplied by the solar power plant and another several MW can be obtained thanks to geothermal energy. Over time, however, the power of the radioactive source decreases, so after a few years the power decreases by 2MW. The power plant can then operate for many years with an output that is a combination of geothermal energy and concentrated solar radiation. It is possible to drill many such wells over a relatively small area, thus multiplying the power plant's output.
Because the wells created in this way are cased, they are stable for many decades and can be used to place scientific instruments. The wells are cooled, at a depth of 20 km the temperature after cooling is around 300 °C. Some specially adapted electronic measuring instruments can operate at such a temperature.

References

Byalko, A.V. (1994): Nuclear waste disposal: Geophysical Safety. CRC Press. ISBN 0-8493-4469-7
Logan S.E. (1974) Deep Self-Burial of Radioactive Wastes by Rock-Melting Capsules, Nuclear Technology, 21:2, 111-124, DOl: 10.13182/NT74-A31367
Chen W., Jianli Hao J., Chen Z. (2013): A Study of Self-Burial of a Radioactive Waste Container by Deep Rock Melting. Hindawi Publishing Corporation, Science and Technology of Nuclear Installations, Volume 2013,
Aruyjunjan R.V., Bolshov L.A., Kondratenko P.S., Matveev L.V. (2012): Radioactive waste disposal method and heat-dissipating capsule for realising said method. Application: 2012134053/07 , 9.8.2012.

List of reference numerals

1 container body
2 container lid
3 casing part
4 bottom with tungsten layer
5 cooling channels
6 output channels
7 rim for prevention of magma leakage
8 molten magma at the bottom edge of the container
9 vertical channels for passing molten magma
10 graphite layer (neutron moderator and reflector)
11 spent nuclear fuel rods
12 fresh nuclear fuel rods
13 neutron source
14 solidifying magma
15 horizontal channels for removal of excess of magma
16 pipes for water vapour exhausting
17 pipes for cooling water supplying
18 solidified magma forming the casing of the well
19 central channel in the casing part
20 rope for extracting rods of solidified magma
21 grip for gripping a solidified magma rod
22 channel for inserting a neutron source
23 casing wall
24 channel for drawing solidified magma rods
25 electrical wire in porcelain insulator
26 light guides
27 water inlet channels in porcelain insulator
28 output channels for superheated steam

Claims

A container for deep underground deposition of spent nuclear fuel, comprising
a container body (1), a lid (2) and a casing part (3), characterized in that the body (1) and the lid (2) form a lower cylindrical part of first diameter and the casing part (3) forms an upper cylindrical part with a second diameter, where the second diameter is smaller than the first diameter, and where the body (1) with lid (2) and casing part (3) form a structure of two coaxial cylindrical parts after assembling the container, whereas

the body (1) is a cylindrical vessel with a cavity for storing nuclear fuel, the lower part of the body (1) forms a bottom (4) configured to penetrate the rock, and vertical channels (9) for conducting molten magma (8) traverse the wall of the body (1);

the lid (2) is a cylindrical body connectable to the upper part of the container body (1), through which the vertical channels (9) for conducting molten magma traverse and adjoin the channels (9) in the wall of the body (1), and is adapted to connect the casing part (3) ; the casing part (3) is connectable to the upper part of the lid (2), contains a vertical central channel (19) for forming and removing solidified magma, comprises channels (15) for feeding molten magma (8) into the central channel (19) in its lower part and further comprises in its wall vertical cooling channels (5) for the supply of cooling liquid connected to vertical outlet channels (6) for discharging the cooling liquid vapour.
The container for deep underground deposition of spent nuclear fuel according to claim 1, characterized in that the bottom (4) of the container is rounded, preferably in the shape of a hemisphere or spherical cap, or is sharply or bluntly pointed, preferably in the shape of a cone or a rotating paraboloid.
The container for deep underground deposition of spent nuclear fuel according to claim 1 or 2, characterized in that the wall of the container body (1) and the bottom (4) are made of steel, preferably high melting carbon steel, the wall of the body (1) is provided with an outer thermally insulating jacket of ceramic or porcelain and the bottom (4) is provided with a cover layer of a thermally conductive material with a high melting point, preferably of tungsten.
The container for deep underground deposition of spent nuclear fuel according to any one of the preceding claims, characterized in that the lid (2) is made of porcelain and the channel (22) is configured to insert a neutron source (13) into a cavity in the container body (1).
The container for deep underground deposition of spent nuclear fuel according to any one of the preceding claims, characterized in that the bottom (4) is provided with a rim (7) around its entire circumference to ensure the collection of molten magma (8) and its discharge through the channels (9).
The container for deep underground deposition of spent nuclear fuel according to any one of the preceding claims, characterized in that the vertical channels (9) located in the wall of the body (1) have a circular or elliptical cross-section.
The container for deep underground deposition of spent nuclear fuel according to any one of the preceding claims, characterized in that both spent fuel rods (11) and fresh fuel rods (12) in a subcritical arrangement surrounded by a graphite layer (10) are stored in the cavity of the container body (1).
The container for deep underground deposition of spent nuclear fuel according to claim 7, characterized in that the spent fuel rods (11) contain radioactive isotopes and the fresh fuel rods (12) contain ²³⁸U and ²³⁵U and plutonium ²³⁹Pu,
The container for deep underground deposition of spent nuclear fuel according to claim 7 or 8, characterized in that a source (13) of neutrons, which is preferably californium ²⁵²Cf or beryllium ⁹Be or a combination thereof, is accommodated in the cavity of the container body (1).
The container for deep underground deposition of spent nuclear fuel according to any one of the preceding claims, characterized in that a casing, preferably made of steel, is attached to the upper end of the casing part (3), where the casing comprises a wall (23), a central channel (24), a cooling channel ( 27) and a vapour channel (28).
A method of deep underground deposition of spent nuclear fuel, characterized in that the container according to any one of the preceding claims is used, the method comprising the steps of
a) excavating a well with a depth corresponding to the height of the container body (1) and placing the container body (1) into the well,

b) introducing a layer of graphite moderator (10), spent fuel rods (11) and fresh fuel rods (12) into a cavity in the container body (1),

c) connecting the lid (2) to the container body (1) and its sealing and fastening,

d) inserting the neutron source (13) into the cavity in the body (1) of the container through the channel (22) in the lid (2) and sealing and closing the channel (22), and

e) connecting the casing part (3) to the lid (2).
The method for deep underground storage of spent nuclear fuel according to claim 11, characterized in that the casing of the well during submersion of the container into the rock is formed by solidified magma (18) in such a way that molten magma (8) under the bottom (4) of the container flows through channels (9) above the lid (2) and is gradually cooled by the action of water in the cooling channels (5) and changes into the solidifying magma (14) and then into the solidified magma (18) around the circumference of the casing part (3).
The method for deep underground deposition of spent nuclear fuel according to claim 11 or 12, characterized in that it further comprises the step
f) connecting a casing to the top of casing part (3) of the container.