JPS61250586A - Method of processing spent nuclear fuel element - 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] [Field of application of the invention] The present invention relates to a method for processing spent nuclear fuel elements.

In particular, the present invention relates to a method for processing spent nuclear fuel elements suitable for processing spent fuel cladding tubes.

[Background of the invention]

Spent fuel assemblies used in nuclear power plants and transported are stored in storage ponds for several months, and after the radioactivity in the fuel assemblies has attenuated, they are processed.

After cooling for a certain period of time, the spent fuel rods are removed from the fuel assembly, cut into bundles in a processing step, and dissolved with nitric acid. Uranium and plutonium separated from the nitric acid solution are purified and converted, respectively, and reused after t.

On the other hand, the fuel cladding tubes of spent nuclear fuel elements are called hulls and are stored untreated in water tanks as non-flammable radioactive solid waste. However, this method is temporary and requires a huge amount of storage space to store the highly radioactive cladding. Therefore, development of methods for reducing and solidifying radioactive solid waste is underway. For example, ``Atomic Crab Industry'' Vol. 26, No. 8, p. 52 describes methods for reducing the volume of hulls by applying hot hydrostatic pressure to compression mold them to a high density, or by melting and molding them using high-frequency heating or an arc method.
A casting method has been reported.

However, the conventional method is undesirable because radioactive gases such as tritium are released by heating the hull. However, in order to heat or melt the highly radioactive hull, new and special technical problems must be solved.

There are many technical problems in developing a highly reliable device.

[Purpose of the invention]

An object of the present invention was made in view of the above-mentioned problems, and it is an object of the present invention to provide a method for processing spent nuclear fuel elements in which shredded cladding tubes can be safely stored.

[Summary of the invention]

A feature of the present invention is that the spent fuel cladding tube, called a hull, is immersed in a liquid that embrittles it, then compressed and crushed, and then the surface of the zirconium alloy that makes up the hull is electrochemically crushed using the hull as an anode. The purpose is to oxidize and store it.

[Embodiments of the Invention] Examples of the present invention will be described below with reference to FIG. 1st
The figure shows a flowchart of the reprocessing process of this example. A spent nuclear fuel element 10 used in a light water reactor is removed from a transport container [1. The nuclear fuel element is cooled in a storage pond for about half a year, and the radioactivity in the nuclear fuel element is allowed to decay.Then, in the next pretreatment step 12, the nuclear fuel element is mechanically cut and crushed. Thereafter, the fuel pellets present in the cut nuclear fuel elements are melted 13 and the uranium and plutonium are separated 14. according to the Pelex process. Purify 15 times. After separating the nuclear fuel elements,
Approximately 3 Q to one core of a 100,100O light water reactor
It is necessary to treat the cladding tubes generated at a rate of n. Conventionally, these cladding tubes are compressed and sealed and stored as highly radioactive waste.

In the reprocessing method of this example, the cladding tubes (hull) remaining after the melting of the fuel pellets is completed are treated with 1 g of iodine (by weight).
After immersion for about 150 hours at room temperature in a stainless steel container filled with a methyl alcohol solution in which the cladding tubes are immersed for about 150 hours, the cladding tubes are sent to a compressor and crusher 17 for volume reduction.

The hull that has undergone the compression and crushing process is stored in an electrically conductive basket 22 shown in FIG.
immersed in it. The electrolytic solution 25 has a hydration power IJ (KOH)
A 1% aqueous solution was used. A cathode 23 made of stainless steel is immersed in a KOH aqueous solution.

Using the basket 22 as an anode, a DC voltage of 80 V was applied to the power source 19 for 10 hours to anodize the crushed hull 24. Note that the electrolytic cell is a stainless steel container 21.
And the upper part is ft20.

The hull that has undergone the anodizing treatment 18 is dried, sealed in a storage container, and stored as nonflammable solid waste.

By using the processing method of this embodiment configured as described above, the following effects can be obtained compared to the conventional technology. That is, in the prior art, the volume of the hull is reduced by a mechanical compression destruction method. In order to mechanically compress and destroy the hull, an excessive amount of mechanical force is required.1 Usually, liquid nitrogen is used to cool and destroy the hull. A so-called deep cooling method is used. When this deep cooling method is adopted, the system becomes large-scale, and a lot of effort is required to maintain and manage the system. On the other hand, 1
In this example, by immersing the zirconium in a grain boundary embrittlement solution, the bonding strength of the zirconium grain boundaries is significantly reduced, and the zirconium can be compressed and fractured with a small load. In other words, by adding grain boundary embrittlement treatment, the load required for compressive fracture is reduced to about 17,100 yen or less compared to the conventional deep cooling method, making it possible to use a small press and making it easier to maintain and manage the press. It's easy to get used to.

11 When compressing and reducing the volume with a conventional mechanical press, a fresh fracture surface is formed in the zirconium, which is the constituent material of the hull, as the hull is compressed. This was not desirable because there was a high possibility of self-combustion.

In this embodiment, after fresh fractured surfaces are generated in the zirconium material constituting the cladding tube, these fractured surfaces are oxidized in the liquid, so there is no possibility of spontaneous combustion. In order to quantify the effect of anodizing treatment in this example.

Examine the ignition behavior of the zirconium test piece9. That is,
A roughly spherical piece with a diameter of about 0.1 fi was heated in the air from a Zircaloy-2 cladding tube.

The conventional sample, which was mechanically compressed and fractured with a fresh fracture surface, ignited within a few seconds when heated by IK (IK), but the anodized sample of Example 5 did not ignite. From this, it can be seen that the oxidation treatment of the hull in this example has a significant effect on the safety of ignitability during storage of the hull.

Furthermore, as an additional effect, radioactive tritium (tritium) generated in the Atsushi reactor is formed inside the hull, and when the hull was reduced in volume by melting in the conventional plan, this radioactive tritium was released into the facility. However, in the present invention, radioactive tritium is not released from the hull because it is oxidized without raising the hull to high temperatures, which greatly simplifies storage facility management technology. became possible.

In addition, in the above-mentioned example, an iodine/alcohol solution is used as the grain boundary embrittlement liquid, and 9 may be other halogens than iodine, or other organic solvents other than alcohol may be used to obtain the same effect. It will be done. Also, K is used as an electrolyte for anodizing.
Although OH has been described, other hydroxides may be used.

〔Effect of the invention〕

According to the present invention, spent fuel cladding tubes can be easily crushed with a small load, and the fractured surfaces of the fragments of spent fuel cladding tubes are subjected to oxidation treatment, so that safe storage is possible.

[Brief explanation of drawings]

FIG. 1 is a flow diagram showing the processing steps of a preferred embodiment of the present invention, and FIG. 2 is a configuration diagram of an apparatus for carrying out the anodizing treatment shown in FIG. 1. 19...Power supply, 20...Top lid. 21... Container, 22... Basket, 23... Cathode, 24...
HaA/, 25... Electrolyte.

Claims (1)

[Scope of Claims] 1. A step of compressing and reducing the volume of a spent fuel cladding tube made of a zirconium alloy, and a step of electrochemically oxidizing the spent fuel cladding tube in an electrolytic solution after completing the step. A method for processing spent nuclear fuel elements, the method comprising: 2. The compression volume reduction step is a step of compressing the spent fuel cladding after exposing it to a grain boundary embrittlement atmosphere of the spent fuel cladding, and the oxidation step is a step of compressing the spent fuel cladding in an electrolytic solution. 2. A method for treating spent nuclear fuel elements according to claim 1, which is a step of anodizing using a fuel cladding tube as an anode.