JPS61182000A - Method of decontaminating stainless nuclear fuel coated tube - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for efficiently decontaminating stainless steel nuclear fuel cladding tubes discharged in a spent nuclear fuel reprocessing process.

[Prior art] As is well known, plutonium dioxide and other materials used as fuel in fast breeder reactors are formed into pellets and then filled into stainless steel fuel cladding tubes and charged into the reactor. As time passes, fission products such as cesium are generated within the nuclear fuel, reducing combustion efficiency. Therefore, during nuclear reactor operation, it is necessary to periodically remove the fission products generated within the nuclear fuel and replace it with new nuclear fuel.
On the other hand, used nuclear fuel is subjected to reprocessing. In this reprocessing process, the nuclear fuel taken out from the breeder reactor is cut into pieces of a certain length (usually about 30 to 50 m) together with the stainless steel cladding tube, and then immersed in a nitric-fluoric acid mixed aqueous solution and filled into the cladding tube. It melts spent nuclear fuel, leaving cut stainless steel cladding as a melting residue. The material of this cladding tube becomes radioactive when it is irradiated with neutrons, and not only does it emit a considerable amount of β-rays and R-rays, but the inner surface of the tube also contains nuclear fuel (pluton dioxide) that remains as an unexploited product. etc.), which is a major cause of radioactive contamination, so it is necessary to establish reprocessing and storage methods that fully consider safety.

By the way, if the stainless steel material itself is only radioactive, the amount of radiation emitted will attenuate to the safety standard within a short period of time, so it is necessary to secure a storage space for a relatively short period of time and to It is only necessary to carry out safety management, and furthermore, it is easy to perform volume reduction treatment such as compression processing during storage. However, the actual stainless steel cladding tubes subjected to reprocessing are contaminated with transuranium elements, such as plutonium dioxide, which have long half-lives (several thousand to tens of thousands of years), and plutonium dioxide itself is toxic. Due to its high price, storage requires strict management from a safety perspective. As one of these management standards, in the United States, radioactive waste land disposal regulations (10CFRpart) targeting transuranic elements are established.
61) is established.

According to this, alpha-emitting nuclides with a half-life of 5 years or more can be stored even in relatively shallow areas of land if their specific radioactivity is 100 nanocuries/gram or less.
Items with specific radioactivity higher than this require extremely strict management. Therefore, for used stainless steel cladding tubes, we will establish a technology that can remove as much transuranium elements as possible remaining on the inner surface and decontaminate them until the specific radioactivity falls below the above-mentioned standard value. There is a need.

On the other hand, as mentioned above, a nitric-fluoric acid mixed aqueous solution is commonly used to dissolve spent nuclear fuel filled in stainless steel cladding tubes.
% by volume), and this mixed water bath has a stronger dissolving ability than ordinary acids. Moreover, this liquid is heated to a temperature of over 100°C to increase its dissolving ability, and even when immersed in such a strong mixed acid aqueous solution, it completely removes the transuranium elements that remain attached to the inner surface of the cladding tube. It cannot be removed by solution, and the amount of residual transuranium elements on the inner surface of the cladding tube after immersion treatment is usually around 2000 ppm, which cannot meet the above-mentioned standards for specific radioactivity.

For this reason, various studies are being conducted to remove as much transuranium elements as possible remaining on the inner surface of the cladding tube and to reduce the specific radioactivity to below the above-mentioned standard value. In this case, the cladding tube generally has a diameter of 20 m or less and is cut into pieces of 50 mm or less, and since it is sheared during the reprocessing process, the fractured surface is irregular and rough. or that it is necessary;
In consideration of the above, it is conceivable to perform removal using the FA) electrolytic polishing method, the BL pickling method, etc., which have been developed so far.

The above fat electropolishing method is a method in which the cladding tube is immersed in a sulfuric acid or phosphoric acid water bath and subjected to electrolytic treatment to remove deposits on the surface. However, it is not possible to efficiently remove the deposits on the inner surface of the tube (although the pickling method for BL already uses a nitric-fluoric acid water bath solution in the reprocessing process). However, due to the difficulty, attempts have been made to clean the stainless steel substrate itself with the next most powerful acid bath solution, hydrofluoric acid.However, it is said that this acid bath solution removes residual deposits as well as the stainless steel substrate itself. Moreover, it requires the use of a special hydrofluoric acid-resistant material for the processing container, and many practical problems remain unresolved.

[Problem to be solved by the invention J] Under the above-mentioned circumstances, the present invention provides a new method that can efficiently decontaminate stainless steel cladding tubes to which radioactive substances have adhered in a relatively simple manner. This is what we are trying to provide.

[Means for Solving the Problems J] The method for decontaminating a stainless steel nuclear fuel cladding tube according to the present invention is to remove the spent nuclear fuel material filled in the stainless steel nuclear fuel cladding tube by bathing it in an acid water bath. After the reprocessing process, the cladding tube is heated to 400°C or higher in an oxidizing atmosphere to form an oxide film on the surface.Then, radioactive substances on the surface of the stainless steel substrate are removed using an upper furnace or a different acid-free water bath. The main purpose of this method is to remove the oxidized film along with the oxide film.

[Operation J] In the present invention, the cladding tube that has undergone the reprocessing step is drained and then heated in an oxidizing atmosphere to form an oxide film on the surface,
If the cladding tube is then washed again with an acid water bath, the residual nuclear fuel material firmly attached to the inner surface of the cladding tube will be easily washed out or peeled off together with the oxide film, and the specific radioactivity of the cladding tube will be drastically reduced. can. The reason why the degree of decontamination is dramatically improved by the pickling treatment after the primary oxidation treatment can be considered as follows. In other words, in the first acid bathing process, residual radioactive substances such as plutonium dioxide, which adhere to the stainless steel surface of the inner surface of the cladding tube, are not eluted and removed, but once the cladding tube is oxidized, it , the stainless steel base itself is oxidized, and an oxide film of several tens of micrometers or more is formed. As a result, when the residual radioactive substances that were firmly attached to the metal layer as a stainless steel base were pickled, the oxide layer became much more corrosion resistant than the corrosion resistant stainless steel metal. For example, the pickling solution easily penetrates through the oxide film that expands and cracks due to oxidation, and this causes the oxide film to peel off from the surface of the base material. It is thought that the removal efficiency of radioactive substances will also be improved.

Incidentally, the decontamination efficiency according to the present invention is significantly affected by the conditions for forming the oxide film, and as will be clear from the examples below, if the amount of the oxide film produced is insufficient, a satisfactory decontamination effect cannot be obtained.

The degree of formation of the oxide film is determined by the oxidation treatment conditions, and the results of experiments have shown that a satisfactory decontamination effect can be ensured by performing heat treatment at 400° C. or higher in an oxidizing atmosphere.

That is, if the heat treatment temperature is less than 400°C, it will not be possible to form an oxide film with a thickness sufficient to achieve the purpose of decontamination in a short time.
The residual radioactive material cannot be sufficiently removed by the subsequent pickling treatment. However, when heated to +OO'C or above in an oxidizing atmosphere, a sufficiently thick oxide film can be formed in about 15 minutes, and by removing or peeling off this oxide film by pickling, it is possible to form a sufficiently thick oxide film inside the cladding pipe. Radioactive substances remaining on surfaces can be almost completely removed.

There is no upper limit to the heat treatment temperature for oxidation as long as it is 400°C or higher, but it is not thermoeconomically advisable to heat it too high, and furthermore, heating to a temperature higher than 600°C makes the oxide film a little dense. Therefore, the most preferable is 40%.
The range was set as 0 to 600'C. The type of acid bath solution is also not particularly limited, and any acid bath solution used for pickling stainless steel may be used, but the most preferred are nitric-fluoric acid mixed bath solution and Examples include a hydrochloric acid-hydrogen peroxide mixed water bath solution.

[Actual example J: 5US316 stainless steel pipe simulating a used nuclear fuel cladding tube (inner diameter 11 mφ, outer diameter 14 m++6, length 35
wa) in the presence of air in a heater using an infrared lamp as the heat source from 200℃ to 1000℃.
The heating temperature was changed at 0° C. intervals, and oxidation treatment was performed by heating at each temperature for 15 minutes. The color of the oxide film was observed as an indicator of the state of the oxide film formation.

An experiment was conducted in which each sample tube that had undergone oxidation treatment was washed using the following two types of acid water bath solutions to remove the oxide film.

<Acid water bath A> Nitric acid 2 46% by volume Hydrofluoric acid; 4... Water: 50% by volume Citric acid water bath solution B> Salt Acid: 5% by volume Hydrogen peroxide: 2ml/1 Water: 95% by volume Pickling is performed by immersing the oxidized sample tube in each acid water bath solution for 3 minutes, then soaking and drying. The operation was repeated and the amount of radioactive material remaining after the oxide film was removed was examined.

The results are shown in @1.2 Table 1.

These experiments confirmed the following.

First, in forming the oxide film, a temperature of 400° C. or higher should be employed; if the temperature is lower than 400° C., the object of the present invention cannot be achieved. That is, at temperatures below 200°C, almost no oxide film is formed, and at 3ω°C, the oxide film formed is too thin, so even if the oxide film is completely removed, the remaining radioactive substances adhering to the inner surface of the tube cannot be sufficiently removed. cannot be removed.

However, heating to a temperature of 400°C or higher can form an oxide film thick enough to meet the purpose of decontamination, and by removing this completely with pickling, the residual radioactive material attached to the inner surface of the tube can be almost completely removed. be able to. In addition, there are considerable differences in pickling efficiency depending on the type of pickling solution.
Oxide films formed at temperatures of 400 to 600°C and 1000 to 1200°C are relatively easy to remove by pickling, whereas oxide films formed at temperatures of 700 to 900°C are dense and difficult to remove by pickling. It is somewhat difficult. Therefore, if the thermoeconomic efficiency and pickling efficiency at the time of oxide film formation are considered comprehensively, it is considered that the optimum temperature for forming the oxide film is in the range of 400 to 6θ0°C. However, even an oxide film formed at a temperature of 700 to 900° C. can be sufficiently removed by immersing it in a pickling solution for 60 minutes, and at the same time, the purpose of decontamination can be fully achieved.

〔Effect of the invention〕

The present invention is constructed as described above, and after the spent nuclear fuel material has been removed by bathing, the stainless steel cladding tube is subjected to a predetermined oxide film formation treatment, and then the film is removed by pickling. It is possible to efficiently remove residual radioactive substances strongly adhered to the inner surface of the M tube, and its specific radioactivity can be reduced to 10CF.
It is possible to reliably reduce the amount below the safety standard value as stipulated by R-Part 51, and the waste treatment can significantly accumulate rIR. Also, by using this technology,
It is also possible to decontaminate waste in which β and α contaminants such as cobalt-60 adhere to the inside of stainless steel pipes.

Claims (1)

[Claims] After the spent nuclear fuel material filled in the stainless steel nuclear fuel cladding tube is eluted and removed with an aqueous acid solution, the cladding tube is heated to 400°C or higher in an oxidizing atmosphere to form an oxide film on the surface, and then the above-mentioned process is carried out. A method for decontaminating a stainless steel nuclear fuel cladding tube, characterized in that radioactive substances on the surface of the stainless steel substrate are removed together with the oxide film using a water bath solution of the same or different acids.