JPH085781A - Preprocessing method for spent nuclear fuel - Google Patents

Abstract

PURPOSE:To more quickly and strongly eliminate oxide film of a cladding in a method in which the fuel rod cladding is degraded by hydrogen before finely cutting the fuel rod in reprocessing. CONSTITUTION:A fuel rod is heated in vacuum at the temperature at least the beta-transformation point of zirconium alloy constituting the cladding, the fuel rod is cooled in hydrogen flow and the oxide film on the clad is eliminated by making use of quick oxygen diffusion velocity in beta metal mother phase.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a reprocessing of spent nuclear fuel, and more particularly to a pretreatment method for embrittlement of a fuel rod cladding tube with hydrogen as a preprocess for shredding fuel rods. is there.

[0002]

2. Description of the Related Art Spent nuclear fuel is subjected to a predetermined reprocessing in order to recover components such as uranium and plutonium therein.

The conventional reprocessing is a method called the Purex method. This method cuts spent fuel rods in the state of an assembly into a length of several cm by a cutting device and immerses them in nitric acid. In this way, the internal pellets are dissolved, and then the uranium, plutonium and other elements are recovered by solvent extraction.

By the way, the surface of the cladding of the fuel rod is a few μm.
Since a hard ZrO ₂ layer having a thickness of m to several tens of μm is formed, it is necessary to replace the cutting blade in the above cutting step because the cutting edge of the cutting device is often damaged and the radiation level is high. There is a problem that it is difficult.

Further, since the inner surface of the ZrO ₂ layer is a Zr compound metal, the cutting surface is likely to be deformed, and there is a high possibility that the end portion of the cladding tube will be blocked. In this case, it is difficult for the chemical solution to penetrate into the cutting tube during dissolution. When the pellets made of ceramic are bitten by the impact during cutting, it becomes difficult to dissolve the pellets.

Further, since the pellets and the cladding tube of the burned fuel are in strong contact with each other, it is difficult for the chemical solution to permeate the inside of the cladding tube from the cut portion even when the pellets are cut, and thus the pellets are hardly dissolved. There is a problem.

[0007]

Therefore, in order to overcome the above problems, it has been proposed to embrittle the cladding tube with hydrogen. When this cladding tube is hydrogenated, the ZrO _{2 on the} surface of the cladding tube becomes a barrier this time and hinders the hydrogenation. Heating is suggested.

However, the burnup of spent fuel rods for high burnup is 30 GWd / t to 60 GWd / t.
Since the above oxide film becomes as thick as about 100 μm, it was found that the above-mentioned normal vacuum heating is not sufficient.

The present invention addresses the above-mentioned actual situation, and particularly aims to remove the oxide film even in the above high burnup fuel rod by utilizing the β transformation of the zirconium alloy. Is.

[0010]

That is, the feature of the pretreatment method for spent nuclear fuel of the present invention which meets the above-mentioned object is that the fuel rod cladding tube with hydrogen is used as a pre-step of the fuel rod shredding as described above. In the method of embrittlement, the fuel rod is vacuum heated at a temperature not lower than the β transformation point of the zirconium alloy forming the cladding tube, and then the fuel rod is cooled in a hydrogen stream.

In the method of the present invention, the temperature of the vacuum heating is set to about 870 to 1000 ° C., the holding time of this temperature is set to about 0.5 to 3 hours, or in the cooling in the hydrogen stream, It is also preferable to cool the fuel rods more rapidly after the end of the temperature retention point where the temperature of the cladding tube rises or becomes constant in a certain region during cooling.

[0012]

When the zirconium alloy having the oxide film on the surface is heated, oxygen in the oxide film diffuses into the zirconium metal phase, and the metal phase changes to the α-Zr phase containing oxygen.

On the other hand, zirconium alloy is generally 870 ° C.
Although it changes from the α phase to the β phase before and after, the present invention has found that the diffusion rate of oxygen is about 10 times faster in the β phase than in the α phase.

That is, the β phase is more advantageous for diffusing oxygen in the surface oxide film into the metal matrix phase. Therefore, according to the present invention, the oxide film of the fuel rod cladding tube is utilized by utilizing such an action. Can be eliminated faster and more powerfully.

The vacuum heating temperature is about 870-100.
Keeping the temperature at 0 ° C and the temperature holding time at about 0.5 to 3 hours is to consider the pressurized gas in the fuel rod, and quenching from the temperature retention point at the time of cooling is sufficient hydrogen as quickly as necessary. This is a consideration for efficient absorption.

[0016]

EXAMPLES Examples of the present invention will be further described below.

When the fuel rod cladding tube is embrittled with hydrogen as a pre-step of shredding the fuel rod of the spent PWR fuel assembly, the β transformation point of the zirconium alloy constituting the cladding tube of the fuel rod is about 870 ° C. or more. Vacuum heating the fuel rod at a temperature of
After that, the fuel rods were cooled in a hydrogen stream.

On the other hand, since the fuel rod for PWR, especially the fuel rod for PWR, has a high initial pressurization of about 30 kg / cm ² , there is a risk of burst destruction when heated at high temperature for a long time. Therefore, a PWR fuel rod having an oxide film of about 50 μm in the outer layer (initial pressure of about 3
0 kg / cm ² ) at 800 ° C, 900 ° C, 1000
When heated at 1100C for 2 hours in vacuum, the results shown in Table 1 below were obtained.

[0019]

[Table 1]

From this result, it is found that the value exceeds 90 which exceeds the β transformation point.
At a temperature of 0 ° C. or higher, oxygen diffuses quickly and the oxide film almost disappears. Further, in these fuel rods B to C, numerous cracks were observed on the surface of the cladding tube due to the oxide film being destroyed by the expansion of the cladding tube. Further, the gap between the coating tube and the pellets can be enlarged by the expansion of the coating tube due to the heating at the above high temperature, which facilitates the separation of the pellets during shredding and the penetration of the chemical solution into the fine fragments during dissolution. I was able to.

The oxide film of the actual spent fuel is several 10 μm
~ 100 μm, and there are many parts over 50 μm,
Since the expansion causes countless cracks and a new metal layer appears on the surface, it is considered that the purpose can be achieved by heating at about 900 ° C.

On the other hand, when the sample as described above is gradually cooled from the heated state in a hydrogen atmosphere, a calorific value Q is generated as in the following chemical formula 1.

[0023]

[Chemical formula 1] Zr + H ₂ → ZrH ₂ + Q

That is, by the exothermic reaction of the above chemical formula,
There is a region where the cooling is canceled and the temperature is kept constant, there is a temperature retention point, and it can be judged that the absorption of hydrogen has almost ended here, so after that, by rapidly cooling,
It is possible to efficiently absorb a sufficient amount of hydrogen in the cladding tube and to eliminate extra time.

The cooling rate until the end of the temperature retention point is appropriately 1 ° C./min to 20 ° C./min, and the cooling rate after the completion is suitably 20 ° C./min to 100 ° C./min.

The vacuum heating temperature is about 870 to 1
A holding time of 000 ° C and this temperature of about 0.5 to 3 hours is suitable.

(Experimental Example) UO ₂ was placed in a Zircaloy-4 cladding tube having a length of about 20 cm and a surface oxide film thickness of about 50 μm.
The pellets of about 30 kg / cm ² together with He,
The heating was performed in a vacuum furnace capable of hydrogen substitution. FIG. 1 shows the temperature pattern. The procedure of the experiment is

The sample is sealed in a furnace and a vacuum is applied. The temperature is raised from room temperature to 950 ° C. in about 30 minutes, and kept for about 1 hour. After holding for about 1 hour, Ar is introduced into the furnace, and then hydrogen is introduced. Cool at 10 ° C / min. The temperature of the sample gradually decreased from around 550 ° C., and after 30 minutes, when it reached about 400 ° C., the increase in cooling rate was detected and the heater was automatically turned off.

The elongation rate of the outer diameter of the sample after taking out is about 60.
%, The oxide film disappeared, and numerous cracks were generated on the surface. In the inspection after the experiment, the amount of hydrogen in the sample was about 1 wt%.

[0030]

As described above, according to the method for pretreatment of spent fuel of the present invention, the fuel rod is vacuum heated at a temperature not lower than the β transformation point of the zirconium alloy forming the cladding of the fuel rod, and then The fuel rod is cooled in a hydrogen stream, and the oxide film of the cladding tube can be eliminated more quickly and strongly by utilizing the high oxygen diffusion rate of the β metal matrix. The expansion of the coating tube by heating at the above high temperature can increase the gap between the coating tube and the pellets, thereby facilitating the separation of the pellets during shredding and the penetration of the chemical solution into the fine fragments during dissolution. Is played.

During the cooling in the hydrogen stream, the fuel rods are cooled more rapidly after the temperature retention point of the cladding tube is finished, so that the cladding tube can efficiently absorb necessary and sufficient hydrogen. In addition to that, it is possible to automatically detect the extra time and omit it.

[Brief description of drawings]

FIG. 1 is a graph showing a temperature change of a fuel rod treated by the method of the embodiment of the present invention.

Claims (3)

[Claims] 1. In a pretreatment method for spent nuclear fuel, which comprises embrittlement of a fuel rod cladding tube with hydrogen as a pre-step of shredding fuel rods in reprocessing, a β transformation point of a zirconium alloy constituting the cladding tube. A method for pretreatment of spent nuclear fuel, comprising heating the fuel rod in a vacuum at the above temperature and then cooling the fuel rod in a hydrogen stream. 2. The vacuum heating temperature is about 870-100.
The pretreatment method for spent nuclear fuel according to claim 1, wherein the temperature is maintained at 0 ° C and the temperature is maintained for about 0.5 to 3 hours. 3. The method for pretreatment of spent nuclear fuel according to claim 1, wherein the cooling of the fuel rod is further rapidly performed after the end of the temperature retention point of the cladding tube during the cooling in the hydrogen stream. .