RU2538765C1 - Method of disposing and storing radioactive substances in liquid medium - Google Patents

Abstract

FIELD: physics, atomic power.

SUBSTANCE: invention relates to the technology of disposing and storing spent nuclear fuel and can be used in storages of radioactive substances. The disclosed method comprises disposing radioactive substances (1) in a liquid-filled container (3), wherein the inner cavity of the container (2), which is bounded at the top by a cover (7), is linked to a liquid medium (10) surrounding container by a compensation channel (5) and a gas outlet pipe (8). The volume of the compensation channel is made greater than the volume of the liquid in the container via thermal expansion, and the length of the compensation channel with an outlet opening (6) is selected based on the condition that the time at which radioactive substances dissolved in the liquid diffuse out through the compensation channel is greater than the storage time. The gas outlet pipe has a shape which provides a gas seal (9) therein when immersed in the liquid medium. The compensation channel and the gas outlet pipe are hermetically linked to the inner cavity of the container holding the liquid (4).

EFFECT: preventing radioactive liquid from falling out of the container into the surrounding liquid medium of a pond, removing gases formed in the container during radiolysis and filling the container with liquid without involving personnel and setting up special systems.

7 cl, 4 dwg

Description

The invention relates to the field of nuclear energy, and in particular to a technology for the placement and storage in a liquid medium of radioactive substances, in particular spent nuclear fuel (SNF), and can be used in storages of radioactive substances, mainly in temporary storage basins, including holding pools at the nuclear power plant.

A known method of storing spent nuclear fuel (RF patent No. 2403633, class G21C 19/06, G21F 9/36, publ. 10.11.2010) by placing in the pool with demineralized water canisters, perforated in the upper part, filled with demineralized water. The water level in the canisters and the pool is maintained below the edge of the holes, supplying periodically demineralized water from an autonomous tank to the canisters and the pool. Gases generated as a result of radiolysis of water in the canister exit to the surface of the storage and are vented by the ventilation system.

A device that implements the method consists of an open pencil case (capacity) perforated in the upper part. Cases are equipped with flanges, for which they are suspended on the floor joists of the storage pool and moved by reloading machines.

The method allows to exclude the ingress of radioactive water from the spent fuel canisters into the storage water basin and to ensure the purity of the water in the pool. As a result, the level of ionizing radiation in the serviced premises of the storage facility decreases and the safety of storage of spent nuclear fuel increases.

However, the height of the case should be several meters higher than the height of spent nuclear fuel placed in it, in particular spent fuel assemblies (SFA), to ensure radiation protection of personnel due to the water layer above the spent fuel. Such devices cannot be placed compactly in the pool, for example, in two tiers. SNF reloading from such a canister can only be performed by lifting it out of the water above the end of the canister, which makes it impossible to carry out radiation-safe SNF reloading under water.

In addition, constant monitoring of the water level in the canisters and the pool and maintaining this level below the edge of the holes is required.

A known method of placement and storage of spent fuel assemblies of the VVER-1000 reactor in a liquid medium, implemented in the device "Cover for the placement and storage of spent fuel assemblies of the VVER-1000 reactor" (RF patent No. 2477899, CL G21C 19/06, publ. 20.03. 2013), including the placement of radioactive substances in a liquid-filled container. According to this method, the SFA is placed in the tubes of the case filled with water. The pipes are separated by spacer grids mounted on a central support pipe. The bottoms are hermetically connected to the lower end of the pipes, forming a container, and to the upper ends - flanges. The pipes are hermetically sealed with caps, which are equipped with heads, in the upper part of which ceramic-metal filters are installed. The case is placed in the storage pool.

The method allows for the safe storage of SFAs, including defective SFAs, in the storage pool, since water circulation in the tubes of the cover due to their sealing and the release of radioactive water into the pool is excluded. Gases formed in the inner cavity of the pipes as a result of radiolysis are removed through cermet filters. The device is compact and can be used in a limited space, in particular in holding pools at nuclear power plants, without disturbing the normal operation of the reactor.

In this method, only removal of gases from the internal cavity of the pipes is provided, but the possibility of replenishing the pipes with water is not provided. As a result, it is possible to reduce the water level in the pipes and, as a consequence, overheating of the SFA, which can lead to an emergency.

The inventive method allows to ensure the safety of storage of radioactive substances by eliminating the ingress of radioactive liquid from the tank into the surrounding liquid environment of the pool and the removal of gases generated in the tank during radiolysis, while ensuring the replenishment of the tank with liquid without the participation of personnel and the creation of special systems.

This result is achieved by the fact that the method of placement and storage of radioactive substances in a liquid medium comprises placing radioactive substances in a container filled with liquid, wherein the internal cavity of the container is connected to the surrounding liquid container by a compensation channel, at least one, and a gas pipe, at least at least one. The volume of the compensation channel is performed in excess of the increase in the volume of liquid in the tank due to thermal expansion, and the length of the compensation channel is selected from the condition that the exit time is exceeded due to diffusion of the radioactive substances dissolved in the liquid over the storage time taking into account the thermal expansion of the liquid through the compensation channel. The vent pipe is made in a form that provides a gas shutter in it when immersed in a liquid medium. In this case, the compensation channel and the vent pipe are connected to the internal cavity of the tank hermetically.

The compensation channel can be made in the form of a tube, which is mainly laid in the form of a spiral.

The compensation channel can be made in the form of a helical groove, the turns of which are isolated from each other to prevent leaks between them.

The compensation channel and gas pipe can be placed in one housing, while the housing is connected to the internal cavity of the tank hermetically.

The compensation channel may be made covering a gas pipe.

The vent pipe may be connected to the compensating channel by downward outlet ducts.

The container may be provided with a cover with a discharge fitting, the gas outlet pipe connected to the compensation channel with downward outlet channels, and the gas outlet pipe sealed with the container fitting.

The connection of the internal cavity of the container with the liquid medium surrounding the container by the compensation channel and the gas pipe allows you to separate the exit paths from the tank of liquid with the dissolved radioactive substances and gas. Moreover, depending on the design of the tank, the form of execution of the compensation channel and the exhaust pipe, operating conditions and other factors, several compensation channels and / or gas pipes can be used. They can be made independent, or interconnected.

The fulfillment of the volume of the compensation channel in excess of the increase in the volume of liquid in the tank due to thermal expansion and the choice of the length of the compensation channel from the condition that the exit time is exceeded due to diffusion through the compensation channel of the radioactive substances dissolved in the liquid over the storage time, taking into account the thermal expansion of the liquid, will exclude the liquid from it contains radioactive substances into the liquid medium of the storage pool. At the same time, the compensation channel will ensure that the tank is replenished with liquid from the pool, which is guaranteed to exclude a decrease in the liquid level in the tank, and therefore, emergency situations associated with overheating of radioactive substances stored in the tank, in particular, SFA. In addition, ensuring the replenishment of the tank with liquid occurs without the participation of personnel and the creation of special systems.

The execution of the gas outlet tube in a form that provides a gas shutter in it when immersed in a liquid medium, will ensure the exit of gas from the tank through the gas pipe and prevent the escape of liquid from the tank through it. The gas shutter will also exclude the removal of liquid through the vent pipe from the container with gas bubbles. In this case, the gas shutter will be self-healing due to radiolysis of the liquid in the tank and the formation of radiolytic gases. Desalinated water or aqueous solutions containing, for example, anticorrosive additives are used as the liquid in the tank.

The compensation channel and the exhaust pipe are connected tightly to the internal cavity of the tank, thereby eliminating leakage of liquid from the tank into the liquid medium of the storage pool at the junction of the compensation channel and the gas pipe with the internal cavity of the tank.

By performing a compensation channel in the form of a tube, which is mainly laid in the form of a spiral, or by performing a compensation channel in the form of a helical groove, the turns of which are isolated from each other to prevent leaks between them, ensure that no radioactive substances dissolved in the liquid inside the tank are released through the compensation channel due to natural convection.

By placing the compensation channel and the exhaust pipe in one housing and making the compensation channel in the form of a spiral or a helical groove covering the exhaust pipe, the method is realized with a more compact device, which makes it possible to use it in a limited space, in particular in holding pools at nuclear power plants, without violating while the regular operation of the reactor. In some cases, the implementation of the method, for example, for a container, the lid of which has a discharge fitting, the vent pipe can be connected to the nozzle hermetically and connect the vent pipe to the compensation channel by downstream outlet channels. The connection of the vent pipe to the compensating channel with the downflow ducts will only allow gas to escape through the vent pipe and will prevent the gas from escaping through the compensation duct.

The inventive method is illustrated by the schemes presented in figure 1 and figure 2, where 1 is a radioactive substance; 2 - capacity; 3 - storage pool; 4 - liquid in the internal cavity of the tank; 5 - compensation channel; 6 - outlet of the compensation channel; 7 - a cover; 8 - gas outlet pipe; 9 - gas shutter; 10 - liquid medium of the storage pool; 11 - downward branch channels; 12 - fitting.

The graph of Figure 3 and the table of Figure 4 presents the results of calculating the distribution of the activity flow along the length of the compensation channel for different storage periods of the tank.

The method is as follows. Radioactive substances (1) are placed in a container (2), which is installed in the pool (3) and is filled with liquid (4), and is closed with a lid (7). The internal cavity of the tank (2) is connected by a compensation channel (5) having an outlet (6) and a gas outlet pipe (8) with the surrounding liquid medium (10) of the storage pool (3). The gas outlet pipe (8) is made in a form providing a gas shutter (9) in it when immersed in a liquid medium (10). For hermetic connection of the compensation channel (5) and gas pipe (8) with the internal cavity of the tank (2) using known detachable or one-piece connections (not shown in the diagrams). The diagram of Figure 1 shows the implementation of the compensation channel (5) in the form of a tube laid in the form of a spiral and covering the container body (2). In the diagram of FIG. 2, illustrating the implementation of the proposed method, the container (2) is provided with a lid (7) with a discharge fitting (12). The gas outlet pipe (8) and the compensation channel (5) covering it are placed in one housing as a separate device, which is mounted on top of the cover (7) of the container (2), while ensuring a tight connection (connection elements are not shown in the diagram) of the gas outlet pipe (8) with fitting (12). The gas outlet pipe (8) and the compensation channel (5) are connected to each other by downward outlet channels (11). The same diagram shows the implementation of the compensation channel (5) in the form of a helical groove, the turns of which are isolated from each other to prevent leaks between them. The use of several compensation channels and / or gas pipes and their possible connection between themselves is not shown in the diagrams.

Example. SNF in the form of SNF fragments and fuel elements containing fuel pellets were loaded into a 0.03 m ³ capacity vessel immersed in the aqueous medium of the storage pool and filled with water. The volume of water in the tank was 0.024 m ³ . A tube performing the function of the compensation channel and a gas outlet tube that connected the internal cavity of the container with the aqueous medium of the storage pool were hermetically connected to the tank. The cross-sectional area of the compensation channel and gas pipe was chosen equal to ≈10 ^-4 m ² . The vent pipe in the upper part was made curved in the shape of a “gander”, which, when immersed in the pool, ensured the formation of a gas shutter in it. It was supposed to store the SNF tank in the pool for 5 years. The water temperature in the pool during normal storage is from 30 ° C to 40 ° C, in emergency mode, a short-term (several days) rise in water temperature to 100 ° C without boiling is allowed. The absolute pressure inside the vessel (taking hydrostatic pressure into account) ranged from 135 to 205 kPa.

If the temperature of the water in the pool increases from 30 ° C to 100 ° C, the increase in the volume of water in the tank as a result of thermal expansion will be:

$$\Delta V_B={\mathrm{<figure-callout\; id="0"\; label="V\; B"\; filenames="00000007.png"\; state="\{\{state\}\}">V\; </figure-callout>}}_B\cdot \frac{{\mathrm{<figure-callout\; id="2"\; label="\rho "\; filenames="00000006.png"\; state="\{\{state\}\}">\rho </figure-callout>}}_{20}-{\rho }_{60}}{{\mathrm{<figure-callout\; id="2"\; label="\rho "\; filenames="00000006.png"\; state="\{\{state\}\}">\rho </figure-callout>}}_{20}}=0,02\cdot \frac{995,68-958,30}{995,68}\approx 7,5\cdot \mathrm{one}{0}^{-\mathrm{four}}{\mathrm{<figure-callout\; id="3"\; label="m"\; filenames="00000006.png"\; state="\{\{state\}\}">m</figure-callout>}}^3;\begin{array}{ccc}& & \end{array}\left(\mathrm{one}\right)$$

Where

ΔV _B - increase in water volume;

V _B0 is the volume of water in the tank at a temperature of 30 ° C, V _B0 = 0.02 m ³ ;

ρ ₂₀ is the density of water at a temperature of 30 ° C, ρ ₂₀ = 995.68 kg / m ³ ;

ρ ₆₀ is the density of water at a temperature of 100 ° C, ρ ₆₀ = 958.30 kg / m ³ .

The values of the density of water are taken from the reference literature (Chirkin B.C. Thermophysical properties of materials of nuclear technology. Reference book. - Moscow: Atomizdat, 1968).

Thus, with a cross-sectional area of the compensation channel ≈10 ^-4 m ² to compensate for the thermal expansion of water in the tank in all storage modes, its length should be at least 7.5 m.

To estimate the release of radioactive substances from the canister due to diffusion through the compensation channel, we use Fick's law, replacing the concentration gradient with the gradient of volumetric activity of water:

Where

j is the activity flux density, Bq / (m ² s);

a - volumetric activity of water (the sum of all isotopes), Bq / m ³ ;

x is the distance from the inlet of the compensation channel, m;

D is the diffusion coefficient of fission products in water, m ² / s.

The distribution of the volumetric activity of water along the length of the compensation channel x after time t can be found from the solution of the non-stationary problem:

Initial and boundary conditions:

a (t≤0, x> 0) = a _BV - at the initial moment, the compensation channel is filled with pool water with activity a _BV ,

a (t, x = 0) = a _P - the activity of water at the entrance to the compensation channel is equal to the activity of water in the tank a _P ,

a (t, x = ∞) = a _BV ;

The solution of this problem is similar to the solution of the non-stationary problem of heating a semi-infinite layer (Bird R., Stuart V., Lightfoot E., Transport Phenomena. Transl. From English NN Kulova. - M .: "Chemistry", 1974) and for the activity flow J through the cross-section S of the compensation channel with the coordinate x in time t after sealing the container has the form:

$$J\left(t,x\right)=j\left(t,x\right)\cdot S=\left(a_P-a_{B\mathrm{AT}}\right)\cdot S\cdot \sqrt{\frac{D}{\pi t}}\cdot exp\left(-\frac{x^2}{\mathrm{four}Dt}\right).\begin{array}{ccc}& & \end{array}\left(\mathrm{four}\right)$$

When calculating J (t, x), the diffusion coefficient for all radionuclides is assumed to be 10 ^-9 m ² / s (conservative value). The volumetric activity of water in the tank was determined by the limiting solubility of uranium 1.62 · 10 ^-2 g / l taking into account the half-lives of isotopes.

The graph of Figure 3 and the table of Figure 4 presents the results of calculating the distribution of the activity flow along the length of the compensation channel for different storage periods of the tank.

Taking into account (1) and the calculation results presented in the table of Figure 4, the length of the compensation channel should be at least 10.25 m: 7.5 m to compensate for the thermal expansion of water in the tank and 2.75 m to prevent the release of radioactive substances into the pool water due to diffusion through the compensation channel. The value of 10.5 m was conservatively chosen.

During 5 years of storage of the tank in the pool, periodic measurements of the local volumetric activity of water at the outlet of the compensation channel were carried out, which did not show an excess over the average activity of water in the pool, which confirms the absence of the release of radioactive substances dissolved in the liquid from the tank. The removal of gases exiting through the vent pipe was carried out by a regular blow-off system from the pool water surface. At the same time, during storage, constant replenishment of the tank with liquid was provided.

Thus, the tests confirmed the feasibility of the method and ensuring the safety of storage of radioactive substances, that is, the achievement of the claimed effect.

Claims (7)

1. The method of placement and storage of radioactive substances in a liquid medium, comprising placing radioactive substances in a tank filled with liquid, characterized in that the internal cavity of the tank is connected to the surrounding liquid tank by a compensation channel, at least one, and a gas pipe, at least , one, while the volume of the compensation channel is performed in excess of the increase in the volume of liquid in the tank due to thermal expansion, and the length of the compensation channel is selected from the condition that the output time exceeds ode due to diffusion through the compensation channel of the radioactive substances dissolved in the liquid over the storage time, taking into account the thermal expansion of the liquid, the exhaust pipe is made in a form that provides a gas shutter in it when immersed in a liquid medium, while the compensation channel and the exhaust pipe are connected to the internal cavity of the tank hermetically sealed. 2. The method according to claim 1, characterized in that the compensation channel is in the form of a tube, which is mainly laid in the form of a spiral. 3. The method according to claim 1, characterized in that the compensation channel is in the form of a helical groove, the turns of which are isolated from each other to prevent leaks between them. 4. The method according to claim 1, characterized in that the compensation channel and the vent pipe are placed in one housing, while the housing is connected to the internal cavity of the tank hermetically. 5. The method according to claim 1, characterized in that the compensation channel is performed covering the gas pipe. 6. The method according to claim 1, characterized in that the vent pipe is connected to the compensation channel by downward outlet channels. 7. The method according to claim 1, characterized in that the tank is provided with a lid with a discharge fitting, the gas outlet pipe is connected to the compensation channel by downward outlet channels, and the gas outlet tube is sealed to the union.

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