JPH058400B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention deals with the treatment of high-level metal waste (hereinafter referred to as , Hull, etc.).

In recent years, nuclear energy has been attracting attention and being developed in response to energy shortages centered on petroleum. However, nuclear energy generates a lot of radioactive waste with a long half-life during the energy production process. Establishing technology to safely store waste has become an important issue. In particular, the hulls and the like have not only high radioactivity levels, but also the material (Zircaloy) that they are made of has the risk of spontaneous combustion, which poses problems in handling and storage techniques.

Conventionally, since it is difficult to handle hulls and the like containing such radioactive materials, reprocessing facilities have stored them in drums and submerging them in stainless steel-lined water tanks. However, with this method, the bulk density of the hull etc. is low (1.1g/
cm ³ ), there are problems such as the required storage space being enormous and the combination not being stable against external disturbances.

Therefore, more compact and stable storage formats are being investigated in various fields.For example, in the case of metal waste, there is a method of melting it in a heating furnace and solidifying it to form a dense block. In the case of radioactive incineration ash generated by incinerating combustible waste, a method has been proposed in which radioactive incineration ash is melted and solidified using microwave melting means. Although these methods have achieved some success in terms of volume reduction, they have drawbacks in waste disposal issues such as radioactive gas during melting and radioactivity of refractories in heating furnaces.

The constituent materials of the hull etc. are Galcaloy: 82-92%;
SUS304: 5 to 14%, Inconel: 2 to 4%, and the main component, the Zircaloy cladding tube hull, has an outer diameter of around 10 mm, a wall thickness of around 0.6 mm, and a length of 30 to 50 mm.
mm, and there is deformation due to shearing at the ends. A strong passive oxide layer (ZrO ₂ ) exists on the surface of this cladding tube hull. This layer exists on both the inner and outer surfaces. Among these, zirconium oxide is generated on the outer surface by reaction with reactor cooling water, and its thickness is approximately 10 to 20 μm in the uniformly corroded area.
Zirconium oxide is formed on the inner surface by the oxygen contained in the nuclear fuel, and the thickness is estimated to be less than 10 μm.

Transuranic elements (TRU) require the most attention when managing radioactive waste. This is produced by the radioactivity of uranium as a nuclear fuel material in a nuclear reactor, and is produced on the hull surface when nuclear fuel is dissolved in nitric acid in the coexistence of cladding material during the melting process in the reprocessing process. adhere to. Therefore, the pipe-shaped inner and outer surfaces of the hull are contaminated. It is known that contamination by TRU and the like is limited to the surface of the hull, and substantially all of the contamination is present in the oxide layer.

As expected from the fact that Zircaloy material is used as a corrosion-resistant material, the passivation layer of ZrO ₂ is extremely corrosion-resistant, and highly active solutions or gases must be used to dissolve only this surface oxidized layer. Therefore, post-processing is required, and high reliability is generally required for facilities handling radioactive materials, which is not realistic. Further, it is impossible to apply a polishing method based on mechanical action because the hull has a complicated shape as described above.

The present invention was made against this technical background, and aims to provide a method that can achieve a significant volume reduction effect using a relatively simple method.

That is, this invention heats a Zircaloy workpiece such as a spent fuel cladding tube in an oxidizing atmosphere to thicken the oxide layer, and then rapidly raises the temperature within a temperature range of approximately 800 to 1100°C. Alternatively, the oxide layer and the Zircaloy layer are expanded and contracted by lowering the temperature, and the resulting stress causes them to separate.

Zircaloy metal expands as temperature rises,
Hexagonal (α phase) to cubic (β phase) at 800-900℃
This metamorphosis causes a drop in expansion.
In other words, it contracts in this range, but if it is heated beyond this range, it expands again as the temperature rises.

On the other hand, ZrO ₂ expands as the temperature rises to 1000°C, but contracts when heated above that. 1100
At 800°C, it becomes slightly smaller than its room temperature dimensions, expands again when cooled, and shrinks again below 800°C.

In addition, Zircaloy metal has a very wide solid solution range of oxygen, and the surface oxidation of the hull is due to the supply of oxygen.
The part in contact with ZrO ₂ is an oxygen-saturated zircaloy metal phase. This metallic phase does not transform until 1900°C, so below that temperature it expands continuously as the temperature rises. Therefore, the most effective effect of the difference in thermal expansion behavior is at the interface between the ZrO ₂ layer and the oxygen-saturated zircaloy metal phase. When the temperature rises, oxygen is transported through this interface by diffusion, and when an oxygen source is present, it becomes a growth surface for ZrO ₂ .

Therefore, in order to effectively peel off the oxide layer, it is important to raise and lower the temperature at a sufficient rate so as not to be affected by this oxygen diffusion. Note that in order to effectively utilize the contraction accompanying the transformation of ZrO ₂ , heating from the metal phase by high frequency heating or the like is preferable.

The drawing shows the flow of the treatment method of the present invention, in which the material to be treated is inserted into the pretreatment furnace (step 1), and in the pretreatment furnace 2, the atmosphere is adjusted to an oxidizing atmosphere by the atmosphere adjustment system 3, heated, and the thickness of the oxide layer is adjusted. increase. Tritium generated at this time is recovered by a volatile substance recovery system 4 and released through a dust recovery system 5. The material to be treated is taken out of the pretreatment furnace (step 6), and then inserted into the rapid heating/cooling furnace 7, where the atmosphere is adjusted by the atmosphere adjustment system 3 and the temperature is rapidly raised and lowered to peel off the oxide layer. (Step 8). Volatile substances generated at this time are discharged through the recovery systems 4 and 5. Next, take this out (Step 9)
Vibration is applied using a vibrating screen or the like to separate the zircaloy metal and the oxide (step 10). Then, the metal is subjected to volume reduction treatment (step 12) and non-TRU
Dispose of as waste (Step 13). This volume reduction treatment may be performed using a normal melting and solidification treatment. Also, the oxide is solidified (step 14).
Dispose as TRU waste (Step 15).

According to the above method, most of the hull is zirconium metal from which the oxide layer has been removed, making it non-TRU waste that does not require strict management, while TRU waste (oxide layer)
Solidification results in a significant volume reduction, which greatly reduces disposal costs. In addition,
TRU waste is usually buried several hundred meters underground. Furthermore, since all of the above methods are carried out in a dry manner, a waste liquid treatment system is not required, and secondary waste can be easily treated. Furthermore, since zirconium oxide generated as TRU waste is a stable substance, there is no need for subsequent stabilization treatment, making the process simple. In the above example, an example was shown in which the zircaloy metal and the oxide are separated from each other by increasing and decreasing the temperature and then applying vibration. Separation is also possible. The amount depends on the amount of oxide layer formed in the pretreatment furnace. That is, if a thick film is formed and then peeled off, the amount of peeling will naturally increase. The experimental results regarding this are as follows.

A Zircaloy tube with an outer diameter of 10 mm, a thickness of 0.8 mm, and a length of 50 mm was autoclaved to simulate a hull (spent fuel cladding tube), and the inner and outer surfaces were approximately
A 10μm oxide film was applied (hereinafter referred to as the simulated hull). This simulated hull was placed in an atmospheric furnace and heated to 1000℃.
After heating for 30 minutes, it was taken out of the furnace, cooled in air, and the state of oxide film formation was observed. Although the oxide film had not peeled off, it could be removed with fingers, and when the oxide film on the inner and outer walls was removed, a Zircaloy metal tube of approximately 0.3 mm remained. In other words, an oxide film layer of approximately 0.5 mm was formed.

Next, put the simulated hull of another individual into the atmosphere furnace,
After heating at 1000℃ for 30 minutes while flowing air,
Lower the temperature to 750℃ and further heat to 1100℃ for 15 minutes.
Thereafter, the temperature was immediately lowered to 750°C for 15 minutes, and then taken out of the furnace and observed. The oxide film had peeled off and separated from the metal. The thickness of the metal is about 0.3mm and approx.
This means that the thickness has decreased by 0.5mm. Under the above experimental conditions,
Approximately 1/2 of the original Zircaloy metal is peeled off as an oxide,
It means they are separated. Furthermore, after the oxide film is formed, peeling and separation can be achieved by raising and lowering the temperature once.

Note that the atmosphere adjustment in step 8 is performed by supplying nitrogen gas or argon gas. This is because the purpose here is to peel and separate the oxide layer at the interface with the zircaloy metal, and there is no need to create an oxidizing atmosphere to develop the interface.

As explained above, this invention is to rapidly raise or lower the temperature of the spent fuel cladding tube so that the oxide layer is peeled off due to the difference in thermal expansion, and the treatment can be carried out using a relatively simple method. In addition, it is possible to achieve a significant reduction in the volume of waste.

[Brief explanation of drawings]

The drawings are process flow diagrams showing embodiments of the present invention. 2...Pretreatment furnace, 7...Rapid heating/cooling furnace, 10...
...Steps for separating metals and oxides.

Claims (1)

[Claims] 1. After heating Zircaloy treated objects such as spent fuel cladding tubes in an oxidizing atmosphere to increase the thickness of the oxide layer, the temperature is rapidly raised or lowered within a temperature range of approximately 800 to 1100°C. A method for treating spent fuel cladding tubes, etc., characterized by expanding and contracting an oxide layer and a zircaloy layer, and peeling them off due to the stress generated thereby.