EP0352594B1 - Electropolishing process for decontamination purposes - Google Patents

Description

The invention relates to an electropolishing process for the purpose of decontamination of parts of a nuclear power plant, in particular a nuclear power plant.

When operating nuclear plants, contamination of parts of the plant that come into contact with radioactive substances, such as pipelines, containers, shafts and the like, can usually not be avoided. The contamination is based mainly on the deposition of radioactive nuclides on the surfaces of these parts of the plant. Electropolishing processes, which are known for example from DE-A-33 43 396 and DE-A-33 45 278, have so far proven successful for decontamination of the system parts. In most of these electropolishing processes, the part of the system whose surface is to be decontaminated is switched as the anode and a sponge electrode as the cathode. The conductive connection between the cathode and the anode is produced by deionized water, to which an electrolyte is added to ensure sufficient conductivity. In most cases, dilute sulfuric acid or dilute phosphoric acid is used. The sponge electrode is used to wipe the surface to be decontaminated when the voltage is switched on. In this process, a very thin surface layer with the surface-attached impurities is removed and the removal is washed away with the electrolyte solution of deionized water and electrolyte. Experience has shown that the radioactivity of contaminated surfaces can easily be reduced by more than a power of ten.

The acid used in decontamination contains radioactive residue from the material removal after decontamination and must therefore be disposed of at great expense. The disposal effort is considerable both in terms of costs and in terms of personal protection. The acidic solutions that remain after decontamination are collected in special containers and transported to processing plants. Special shielding is required in the factory and during transport in order to reduce the radioactive radiation into the environment to the permissible level.

At the same time, decontamination must ensure that leaks are largely avoided in order to avoid contamination of neighboring plant areas. Entire pipe sections must be carefully sealed, particularly when decontaminating primary power plant circuits. Unavoidable leaks into a pipe connected to the component to be decontaminated require careful suction of the leaked liquid. This additional work leads to an increase in the radiation exposure of the personnel and at the same time also increases the technical risk and the cost burden.

The object of the invention is to reduce the radiation exposure of the personnel during the decontamination of system parts on the one hand and the technical and financial outlay of such measures on the other.

This object is achieved according to the invention in that deionate is used for electropolishing, which is enriched to ensure electrical conductivity with an electrolyte which is present in the primary coolant anyway, and in that an electrolyte present in a remaining solution is worked up in a factory water treatment system .

Due to the use of deionized water, which is enriched with electrolytes that can be processed by the factory water treatment system, the solution that remains after decontamination no longer needs to be transported in shielded containers, but can be removed by the factory water purification or Processing plant to be refurbished. The resulting salts can be filtered out. In addition, if the component to be decontaminated leaks into other system parts, for example into a connected pipeline, it is possible to dispense with a special suction of the leaked liquid. From there, it gets to the factory water treatment plant without further measures, where it is treated. This significantly reduces the radiation exposure of the staff. By using an electrolyte that is already present in the primary coolant, material damage due to electrolyte residues remaining in the adjacent system parts is reliably avoided because the materials used are designed for these electrolytes.

In the case of nuclear power plants, it is particularly advantageous to add boric acid or lithium hydroxide to the deionate.

For example, the temperature of the deionate enriched with an electrolyte is increased above the ambient temperature. This increases its conductivity, which improves the result of the decontamination.

According to another example, a sponge electrode is used whose distance from the surface of a component to be treated is less than 10 mm. A largely complete decontamination is thus achieved.

Further details of the invention are explained using an exemplary embodiment.

For example, the inner wall of a pressure line in the primary circuit of a nuclear reactor is intended for the purpose of implementation be decontaminated from maintenance work, for example an electropolishing device can be used, as disclosed in German Offenlegungsschrift 33 45 278. According to the method according to the invention, however, instead of the electrolyte solution used there - mostly dilute sulfuric acid - deionate is used, to which boric acid, for example, has previously been added. Since the electrical conductivity that can be achieved with boric acid is significantly lower than the conductivity that can be achieved with sulfuric acid, the current flow and thus the erosion per unit of time will also be lower. In order to compensate for this underperformance, the deionate enriched with boric acid supplied to the sponge electrodes can be warmed up before being introduced into the sponge electrode. The limit temperature to be maintained is limited by the temperature resistance of the sponge and the other components of the sponge electrode and by the formation of steam. Temperatures of the deionate enriched with boric acid around 75 ° C are realistic when using temperature-resistant sponges. Furthermore, in order to increase the removal per unit of time, the thickness of the sponge used, ie the distance of the metallic part of the sponge electrode from the surface to be decontaminated, can be reduced. Sponge thicknesses of 10 mm and below, preferably 5 mm, can be used well.

The operation of the electropolishing device can be carried out in a conventional manner, as is known from DE-OS 33 45 278 or DE-OS 33 43 396. On the plant side, however, it is expedient to connect a heater for heating the electrolyte solution to the supply line to the sponge electrode. After the completion of the electropolishing work, the boron acid-enriched deionate with the removed surface substances, including the radioactive material originally attached to the surface, is located in a collecting container for the electrolyte solution. In addition, the interior of the tube or container treated in this way is left with small amounts of Electrolyte solution contaminated. Since these remaining electrolyte solution quantities essentially consist of the deionate and boric acid already in the primary circuit and their quantity is negligible compared to the quantity of deionate that can be put into operation, the nuclear power plant can be put into operation again after the decontamination without cumbersome extraction of the remaining electrolyte solution quantity. The resulting slight increase in the boric acid content in the coolant can easily be worked up by the factory water treatment system. This saves the human effort that would otherwise be required with the extraction. The amount of electrolyte solution in the collecting container can also be gradually fed to the factory water treatment system after filtering off the removed material, processed there and fed to the primary coolant.

It is a great advantage of this type of decontamination to see that the laborious and laborious extraction of liquid escaping from unavoidable leaks can be avoided and that there is also no need to transport the used electrolyte solution quantities outside the power plant. Finally, it is a further advantage of this method that the electrolyte solution can be worked up after decontamination using the factory water treatment system. It should not be forgotten that after the solution has been worked up, most of it, namely the deionized material, can be reused and only a very small sludge-like residue has to be disposed of, as is done from time to time with the residues from the operational water purification process.

Claims (5)

1. Electropolishing process for decontaminating component parts in a nuclear installation, in particular a nuclear power station, characterised in that deionised water is used for electropolishing which, so as to guarantee electrical conductivity, is enriched with an electrolyte already present in the primary coolant, and in that an electrolyte present in a remaining solution is processed in the plant's own water-processing installation.
2. Process according to claim 1, characterised in that boric acid is added to the deionised water.
3. Process according to one of claims 1 or 2, characterised in that lithium hydroxide is added to the deoionised water.
4. Process according to one of the preceding claims, characterised in that the temperature of the deionised water enriched with an electrolyte is increased past the ambient temperature in order to increase its conductivity.
5. Process according to one of the preceding claims, characterised in that a sponge electrode is used where the spacing from the electrode to the surface to be treated is less than 10 mm.