JPS61134697A - Pre-treatment method of spent nuclear fuel - 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 IF1 The present invention relates to a pretreatment method for separating spent nuclear fuel into fuel cladding tubes and fuel pellets, and more specifically, it relates to a pretreatment method for separating spent nuclear fuel into a fuel cladding tube and a fuel pellet. The present invention relates to a pretreatment method for melting fuel cladding tubes and decoating ceramic fuel pellets.

[Conventional technology] In spent nuclear fuel reprocessing Cζ, prior to the main process of separating and recovering unburned fissile material and newly generated fissile material, the fuel cladding tube and its interior are first decladded. It is necessary to separate it from the contained fuel pellets. Conventionally, fj
A subtractive methodological method is used.

As a mechanical decoating method, there is a so-called "shear leach method" in which spent nuclear fuel is cut into several lengths while still being coated, and only the nuclear fuel is leached and dissolved in nitric acid, and this method is widely used.

On the other hand, the chemical decoating method is a method in which the entire spent nuclear fuel is immersed in a solution to dissolve it all, and then separated.

[Problems to be solved by the invention] In the chemical decoating method, in principle, all of the spent nuclear fuel is dissolved in the solution as described above, so a large amount of components of the W pipe to be covered are included. . Therefore, only the components of the cladding tube must be separated after melting, which is very cumbersome.

On the other hand, the mechanical decoating method has the advantage that it reduces loss of nuclear fuel and generates less waste liquid than the chemical decoating method and is economically superior.

However, since the cut fuel is directly chemically dissolved, volatile nuclides are dissolved in the dissolution tank, resulting in various problems. Furthermore, since the gas generated from the dissolution tank is accompanied by acid, it becomes difficult to treat the exhaust gas such as recovering tritium, krypton, and xenon. Furthermore, the cladding tube, which is a dissolution residue, must be treated in a separate process.

As described above, conventional technology involves difficult problems such as spent nuclear fuel decladding, cladding treatment, exhaust gas recovery, etc., and new technology is needed to solve these problems and facilitate chemical dissolution in the main process. At present, development is strongly desired.

The purpose of the present invention is to eliminate the above-mentioned drawbacks of the prior art, to easily perform decladding of spent nuclear fuel, cladding treatment, exhaust gas recovery, etc. in a dry state, and to improve the subsequent main reprocessing process. The object of the present invention is to provide a method for preprocessing spent nuclear fuel that is efficient and practicable, and that can prolong the life of the equipment used in the main process.

[Means for Solving the Problems] The present invention can achieve the above-mentioned objects.When the fuel cladding tube is made of metal and the fuel pellets are ceramic fuel, the melting 1 Focusing on the difference in temperature, spent nuclear fuel is heated to a temperature that can melt the fuel cladding but not the fuel pellets.
This is a pretreatment method in which only the fuel cladding tube is melted and the fuel pellet is decoated while keeping it solid.

The method of the present invention is particularly effective for fast reactor fuels that use stainless steel as the cladding material.

Although the cladding material for light water reactor fuel is Zircaloy, the same technology can be applied.

[Effect]
1. According to the present invention, fuel cladding material and fuel pellets can be easily separated from each other without chemically dissolving them. Furthermore, since a heating step is included, the gas in the fuel can be completely released, and waste gas can be recovered at the same time as decoating.

If the spent nuclear fuel is heated and roasted after decoating and before being dissolved in acid for the reprocessing process, volatile substances and gases mainly caused by nuclear fission contained in the spent nuclear fuel can be completely separated and recovered. , corrosion of equipment in the main process is reduced.

Moreover, this waste gas separation and recovery process is a dry process, so it can be easily performed. By further crushing the obtained fuel pellets, chemical dissolution in the main process can be carried out easily and quickly.

[Example] The present invention will be explained in more detail below.

The present invention is a method of heating spent fuel to melt and de-clad metal fuel cladding tubes. The spent nuclear fuel may be in the form of a fuel assembly or a fuel pin. In any case, the material is directly guided to the melting and heating section without any mechanical cutting or the like. When the material constituting the fuel cladding tube is iron or stainless steel, it starts melting at 1400 to 1500°C, but ceramic fuel pellets such as PuO2 and UO3 start melting at about 2600°C.

Therefore, the heating temperature is set to be exactly in the middle of these melting start temperatures, but in order to reliably prevent the fuel pellets from melting in consideration of the viscosity of the melted fuel cladding material, the heating temperature is usually 1800°C.
It is desirable to carry out the process at a temperature of about ℃. Although the heating method is not particularly limited, the induction heating method uses loss due to eddy current as a heat source, and is extremely convenient because the metal that is the material of the cladding tube directly generates heat.

It is inevitable that some ceramic fuel will be mixed into the molten metal of the fuel cladding tube that has been melted and separated by heating. However, these fuel components can be separated and recovered by electroslag melting, which is currently in practical use as a metal refining method.

Next, a preferred apparatus for carrying out the method of the present invention will be explained with reference to the drawings. FIG. 1 is a schematic diagram showing an example thereof.

The device body is installed in a cell 12 surrounded by a shielding material 10 and the like, and protects radiation and radioactive materials from the outside world.The spent nuclear fuel processed by this device is a spent fuel assembly. A melting separation furnace 14 is installed in the cell]2.The melting separation furnace 14 has an upper part into which the spent fuel assembly to be treated is inserted, a hollow inlet 16 opening, and a bottom part through which molten metal flows out. A ceramic furnace body 20 in which multiple holes 18 are formed, an induction coil 22 provided around the furnace body 20, a container 24 installed below the furnace body 20 to receive the molten metal flowing down, The receiver is composed of an induction coil 26 and the like that heats the container 24. Further, an exhaust gas pipe 28 is attached to the upper part of the furnace body 20 for guiding the gas generated inside to the exhaust gas treatment system.

Furthermore, this pretreatment device includes an aggregate gripper 30, a lifting device 32 thereof, and the like. The aggregate gripper 30 has a rotating or moving function, and can be quietly raised or lowered by the lifting device [32].

Now, the spent fuel assemblies 34 to be pretreated are gripped by the assembly gripper 30 and hauled from the assembly storage area. The spent fuel assembly 34 is then slowly fed from the upper end inlet portion 16 of the furnace main body 2o by the lifting device 32.

The fuel assembly 34 is heated to about 1800° C. by the induction coil 22 inside the furnace body 20. This causes the fuel cladding and the like to melt, flow down through the hole 18 formed in the bottom, and collect in the container 24 as molten cladding material 36. On the other hand, ceramic fuel pellets 38
Since it does not melt, it remains in a solid state at the bottom of the furnace body 20. The waste gas generated during the melting process (released from the fuel pellet by heating) is sent to the exhaust gas treatment system through the exhaust gas pipe 28 and is treated by a normal radioactive nuclide treatment process, etc. Become.

FIG. 2 shows a suitable example for carrying out the method of the present invention.
FIG. 2 is an explanatory diagram showing an example of a device of i. A high frequency induction heating furnace 42 is installed in a sealed box 40. The box 40 is similar to the previous embodiment in that an exhaust gas pipe 44 is provided at its upper end and is connected to a gas recovery device. The high frequency induction heating furnace 42 has an induction coil 4 around it.
6 is attached and installed in an inclined state, and a melting separation tube 48 is passed through the axial center of the furnace.

The melting separation tube 48 is a tubular body made of a material to which molten metal is difficult to adhere and easily peeled off, or whose inner surface has been treated to be made of such a material.
It is rotatably supported by a rotating H1 mechanism consisting of a transmission gear stand 2 and the like. Further, this melting separation tube 48 is provided with a stopper member 53 as shown in an enlarged view inside the heating portion thereof. In the same figure, melting amount #
l! The upper right end of the tube 48 serves as an inlet, and the lower left end serves as an outlet. Containers 54 and 56 are installed below the outlet for receiving fuel pellets and metal cladding material, respectively.

The spent nuclear fuel processed by this device is spent fuel pins. The fuel pin 58 is inserted from the inlet of the melt separation tube 48 . At this time, since the tip of the fuel pin 58 is supported by the stopper member 53 in the heating section, it cannot move further downward as long as it maintains its original shape as a fuel pin. When the induction coil 46 is energized and the fuel pin 58 is heated to about 1800° C., only the fuel cladding tube melts or decomposes. The motor 50 is driven to melt the separation tube 4.
8, the molten fuel cladding material 60 flows down and the fuel pellet 62 falls in a solid state. The specific gravity of metals such as iron and stainless steel, which are cladding materials, is 7.
~8, whereas the specific gravity of ceramic fuel pellets such as PuO2/UO2 is around 10~11. The corresponding receivers can be separated into containers 54 and 56.

The unmelted fuel pin portion remains locked by the stopper member 53 and is held in the heating section until it is melted. It is also possible to continuously melt and separate the fuel pins by adjusting the heating temperature, the rotation speed of the melting separation tube, etc.

Volatile substances generated during the melting process are stored in a closed box 4.
0 and the exhaust gas pipe 44 to be sent to the exhaust gas treatment system for treatment. Of course, it is also possible to connect an exhaust gas pipe directly to the melting separation pipe 48 and recover the volatile substances generated.

Thus, the method of the present invention allows for extremely easy decoating in a dry state. Therefore, if the obtained nuclear fuel pellets are heated and roasted before being sent to the main process such as stage separation, volatile substances and gases mainly caused by nuclear fission can be completely separated and recovered in a dry process, and the exhaust gas Not only is the treatment easier, but since the waste gas and the like can be removed in advance, corrosion of equipment during the main process is reduced. Furthermore, if the decoated fuel pellets are finely pulverized in advance, chemical dissolution in the main process can be carried out easily and quickly.

[Effects of the Invention] The present invention is a spent nuclear fuel pretreatment method configured as described above, and does not involve chemically dissolving the fuel, but only melting and removing the cladding by heating. ,
This will have the effect of making it easier to uncoat and to process the cladding.

The present invention also has the effect that exhaust gas treatment can be performed relatively easily because exhaust gas containing volatile components can be recovered in a neutral and dry state.

In other words, unlike the conventional shear leach method or chemical uncoating method, which uses chemicals for dissolution, it is different from the so-called wet method, which generates less waste and can significantly reduce pretreatment costs. .

In addition, since the decoated fuel pellets are in a solid state, they can be roasted afterwards to separate volatile components and gases, or they can be crushed to an appropriate particle size and used for reprocessing. The present invention has many advantages, such as easier chemical dissolution.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram showing an example of a spent nuclear fuel pretreatment device suitable for implementing the method of the present invention, and FIG. 2 is an explanatory diagram showing another example of a device suitable for implementing the method of the present invention. be. 14... Melting separation furnace, 20... Furnace body, 22... Induction coil, 34... Spent fuel assembly, 36... Melting cladding tube material, 38... Fuel pellets, 42... Heating induction furnace,
48... Melting separation tube, 58... Fuel pin, 60...
- Cladding tube material, 62...Fuel pellet.

Claims (1)

[Claims] 1. Spent nuclear fuel is heated to a temperature where the metal fuel cladding tube can be melted but the ceramic fuel pellets inside cannot be melted, and the fuel cladding tube is melted and decoated. Pretreatment method for spent nuclear fuel.