RU147315U1 - ELECTROLYZER FOR PRODUCING METAL ZIRCONIA FROM SOLID RADIOACTIVE WASTE - Google Patents

Abstract

1. An electrolytic cell for producing metal zirconium from solid radioactive waste, comprising a thermally insulated, thermo-regulated, electrically insulated body with a bath filled with electrolyte, a drum cathode in the form of a blind cylinder on a shaft connected to the drive, and an anode characterized in that the anode is made in the form of an anode -cartridges with current supply and porous walls, and a connector is made in the housing for its extraction. 2. The electrolyzer according to claim 1, characterized in that it contains a device for removing the cathode deposit with the receiver of the precipitate. 3. The electrolyzer according to claim 2, characterized in that it contains a gate to shut off the precipitate receiver. The cell according to claim 1, characterized in that it contains a partition separating the gas volumes of the anode and cathode part of the bath. The cell according to claim 4, characterized in that the anode basket is located so that its upper edge is above the lower edge of the partition. The electrolyzer according to claim 1, characterized in that it contains a heating system for the housing and the bath. The electrolyzer according to claim 1, characterized in that it contains a heater for preparing the electrolyte. The electrolyzer according to claim 1, characterized in that it contains a pipe for extracting the electrolyte.

Description

The utility model relates to devices for the electrolytic production of metals and their compounds. It can be used mainly in radiochemical production to obtain metallic zirconium from fragmented solid radioactive waste (SRW), and can also be successfully used in the chemical and metallurgical industries.

It is known that metallic zirconium is the most used structural material in nuclear engineering and is used, in particular, for the manufacture of shells of fuel elements (TVEL) filled with nuclear fuel. The fuel elements equip fuel assemblies (FAs) placed in a specific order in a nuclear power reactor, while the structural elements of the fuel assemblies providing a strictly specified position of the fuel elements in the fuel assemblies and the end parts of the fuel assemblies are made of stainless steel.

The value of metallic zirconium as a structural material for nuclear engineering is determined by the fact that zirconium has a small thermal neutron capture cross section, high corrosion resistance, good mechanical properties, and therefore it is used in large volumes in nuclear energy for the manufacture of fuel elements for thermal nuclear reactors of the type VVER and RBMK nuclear power plants.

Spent fuel assemblies, fulfilled by the technological regulations, together with the fuel rods, are removed from the nuclear reactor, after which they are transported to a special storage and kept there for 5-7 years until it becomes necessary to dispose of them. In practice, the shelf life is long enough, the waste is not processed, which means they are accumulating, and there is a need to remove the radiation hazard from the human environment by processing the waste in order to decontaminate it.

Zirconium containing fragmented SRW is formed as a result of a full cycle of processing spent fuel assemblies stored in the storage facility. Reprocessing has already existed for decades and includes the following main stages: mechanical grinding, namely, chopping of fuel assemblies into metal fragments ~ 30 mm in size with the simultaneous precipitation of nuclear fuel from them, then chemical treatment of the resulting mixture with nitric acid until the fuel is completely dissolved and washed off the metal fragments, after which the dissolved fuel is sent for regeneration, and the metal fragmented waste is stored.

It should be noted that currently metal zirconium is produced from natural ore, and this is a large-scale production, since it requires a large number of zirconium shells for existing nuclear plants, and is expensive, since many technological processes are involved in this production.

The above values and advantages of metallic zirconium and its relevance in nuclear energy are a significant basis for the creation of technology and equipment for the production of deactivated metallic zirconium with a view to its further use in nuclear energy.

The most optimal option, according to the authors of the proposed technical solution, is to obtain from zirconium-containing SRW from deactivated metal zirconium by electrolysis.

Known drum electrolyzer (USSR Author's Certificate No. 619550, IPC C25C 7/00, publ. 08/15/1976), including a housing with end caps and seals, a hollow drum cathode made of metal (titanium or stainless steel) and concentrically installed on the shaft connected to the drive inside the anode, nozzles for supplying and removing electrolyte and a diaphragm. In this case, the anode is equipped with a gas exhaust system and is made porous, and channels are provided inside the cathode shaft for supplying and removing coolant from the cathode cavity. The anode can be made of graphite or a multilayer mesh and covered with a filtering diaphragm.

An electrolyte is fed into the electrolyzer — aqueous solutions of non-ferrous metal salts, voltage is applied to the cathode and anode, the cathode is turned on, and cooling water is supplied to cool the cathode and anode. In the process of electrolysis, metal is deposited in the form of a tape on the cathode, and the spent electrolyte is removed.

However, in aqueous solutions of radioactively contaminated metal, the process of electrorefining (refining) of zirconium-containing SRW is practically impossible to organize due to the chemical nature of zirconium and the activity of SRW.

The closest in technical essence to the claimed electrolysis cell is an electrolyzer for producing metals from molten salts (USSR Author's Certificate No. 387031, IPC C22D 3/02, publ. 03/10/1972), adopted as a prototype.

The cell is made in the form of a heated bath, anode, a hollow cathode located on the shaft, with a drive for rotation, and a device for loading the initial product. In order to prepare the electrolyte, simultaneously with the electrolysis process, a reagent supply tube is mounted inside the cathode, and the cathode cavity is connected on one side to the device for loading the initial product through the hole in the cathode shaft, and on the other hand, to the bath cavity through the holes in the cathode end.

To obtain metals from molten salts in the electrolytic cell under consideration, the required amount of salt is first loaded into the bath, the heater is turned on to turn the salt into a liquid state — an electrolyte (melt). Then, through the nozzle in the cathode cavity serves the initial product in solid form. The cathode rotation drive is turned on and a gas reagent is introduced into the cathode cavity. As a result of vigorous stirring during the rotation of the cathode, the initial solid product dissolves upon feeding the reagent and enters the bath cavity through the holes in the ends of the cathode. An electric current is turned on to carry out the electrolysis process. During electrolysis, a solid product is released on the outer surface of the cathode, which, when the cathode is rotated, is cut off with a knife and enters the receiving container through the tray.

An analysis of the known technical solutions revealed that they are not able to process fragmented zirconium-containing SRW due to the fact that the waste is a mixture of fragments of materials with different electrical properties. Since the technological process will be configured to conduct electrolysis with respect to only one component, zirconium, stainless steel fragments will turn into ballast, which will be a nuisance in the bath, and especially in the drum cathode. Fragments of stainless steel that fall into the gap between the cathode and the anode and accumulate there, can impede the rotation of the cathode, as well as scrape the zirconium deposit from the surface of the cathode, which is contrary to the achievement of the goal.

In addition, the presence of stainless steel fragments in the gap would lead to an increase in the size of the equipment, which is undesirable in radiochemical production.

These disadvantages are eliminated in the proposed technical solution as a utility model, where the stainless steel present in the mixture with zirconium does not interfere with the electrolysis process.

The main objective of the utility model is the practical implementation of the design options for the electrolyzer, providing anodic dissolution of "dirty" metal (electrooxidation) and cathodic separation of pure metal (electrorefining) due to the cathodic reduction of ions to "pure metal" from a chloride salt melt.

In this case, the electrolysis process is carried out without gas evolution in a potentiometric mode and, therefore, the device is not complicated by the introduction of gas collecting and gas pipes from the anode into its design.

The inventive design of the electrolyzer contains a thermally insulated, thermo-regulated, electrically insulated body with a bathtub filled with electrolyte, a drum cathode in the form of a blind cylinder on a shaft connected to the drive, an anode in the form of a porous basket with current supply, for the extraction of which a connector is made in the body.

The electrolyzer may further comprise a device for removing a cathode deposit with a precipitate receiver, as well as a gate for closing the precipitate receiver. To maintain optimal temperature conditions, a heating system for the body and bath can be introduced into the design.

To separate the gas volumes of the bath, a partition is introduced into the device, which allows to increase the purity of the resulting product. In particular, the anode basket is located in the bathtub of the housing so that its upper edge is above the lower edge of the partition.

Preferably, the device comprises a heater for preparing the electrolyte, as well as a pipe for removing it from the bath.

The design of the device is illustrated by drawings, which depict:

- in FIG. 1 is a General view of the electrolyzer in the context,

- in FIG. 2 - section along aa.

The electrolyzer contains a housing 17 with a bath 7 inside the lid 8, filled with an electrolyte 2, preferably a chloride melt, a blind cylinder cathode 12, located in the bath 7 on a shaft rotating by means of the drive 18, and the anode 4 with the starting material 5 above which is made a sealed connector 10. The housing 17 has an electrical insulation 19 and a thermal insulation 6, thermostatically controlled in individual sections, its heating is carried out by the heating system 16.

The anode 4 is made in the form of a porous basket (anode basket) with a current lead 11. An additional partition 3 is inserted into the device, recessed under the mirror of the electrolyte 2, which separates the anode and cathode gas volumes of the bath 7 and determines geometrically the levels of immersion of the anode basket 4 and cathode 12, and the upper edge of the anode basket 4 is located above the lower edge of the partition 3. This eliminates the intensity of diffusion-convection flows in the electrolyte 2.

To remove the cathode deposit, a device 13, a receiver 15, which moves the removed precipitate, and also a gate 14 using which periodic overlapping of the receiver 15 can be used, are used.

The preparation of electrolyte 2 from a molten salt (chloride) is carried out under the influence of a heater 1, and the extraction of electrolyte 2 from a bath 7 through a pipe 9.

The device operates as follows.

Before starting the electrolyzer, the operability of heat supply systems, process control and control is checked, the cathode 12 is preliminarily scrolled.

Salt is loaded into the open sealed connector 10 for the anode basket 4 to obtain melt 2. The heater 1 is turned on, as well as the heating system 16 of the housing 17 and bath 7.

After the salts melt, the anode basket 4 is immersed in them with the starting material 5 loaded into it, for example, fragmented particles of the coated material and the cut spacer grids. The source material 5 is loaded into the anode basket 4 outside the device, which excludes its distribution throughout the electrolyte 2 during immersion and during electrolysis.

Connector 10 is sealed. The electrolysis current is turned on for a given potentiometric voltage parameter. The drive 18 starts the rotation of the cathode 12. The precipitate deposited on the surface of the cathode 12 is cut off by a special device 13 and, through the receiver 15, which can be blocked by the gate 14, is sent to the next technological stage.

At the end of the electrolysis process, the connector 10 for the anode basket 4 is depressurized and the last is removed from the bath 7 with the sludge remaining in it, pieces of fragmented spacer TVEL grids, which in the initial state are compactly removed together with the anode basket 4. At the same time, the porous walls of the anode - baskets 4 exclude the ingress of thin, dispersed colloidal particles into the electrolyte 2, which ensures the purity of the resulting metal.

Next, another anode basket 4 can be loaded into the connector with a new portion of the source material 5, subjected to anode processing in the melt 2. The connector 10 is again sealed.

If necessary, part of the spent melt 2 is discharged through the pipe 9, and salt is introduced into the bath 7 in an amount equivalent to the extracted mass of the melt 2.

A complete stop of the device is performed in the following order. The connector 10 is depressurized, the anode basket 4 is removed, the electrolysis current is turned off, the electrolyte 2 is extracted through the pipe 9, the rotation of the cathode 12 is stopped.

Thus, the use of the claimed utility model makes it possible to obtain metallic zirconium from fragmented SRW by electrolysis, and high product purity is achieved while simplifying the design of the device.

Claims (8)

1. An electrolyzer for producing metallic zirconium from solid radioactive waste, comprising a thermally insulated, thermo-regulated, electrically insulated body with a bath filled with electrolyte, a drum cathode in the form of a blind cylinder on a shaft connected to the drive, and an anode, characterized in that the anode is made in the form of an anode -cartridges with current lead and porous walls, and a connector is made in the housing for its extraction. 2. The electrolyzer according to claim 1, characterized in that it comprises a device for removing a cathode deposit with a deposit receiver. 3. The electrolyzer according to claim 2, characterized in that it contains a gate for shutting off the precipitate receiver. 4. The cell according to claim 1, characterized in that it contains a partition separating the gas volumes of the anode and cathode part of the bath. 5. The electrolyzer according to claim 4, characterized in that the anode basket is located so that its upper edge is above the lower edge of the partition. 6. The electrolyzer according to claim 1, characterized in that it contains a heating system for the housing and the bath. 7. The electrolyzer according to claim 1, characterized in that it contains a heater for preparing the electrolyte. 8. The electrolyzer according to claim 1, characterized in that it contains a pipe for extracting the electrolyte.

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