JPH0552479B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides high-level metal waste (such as spent fuel cladding tubes, upper and lower end members of fuel assemblies, spacers, springs, etc.) after shear melting generated in the pretreatment process of reprocessing facilities. The present invention relates to a volume reduction and stabilization treatment method for (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, this method has a low bulk density (approximately 1.1
g/cm ³ ), there are problems such as the required storage space is enormous and the combination is not stable against unexpected 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. All of these methods have achieved some success in terms of volume reduction, but they have drawbacks in the disposal of waste, such as radioactive gas during melting and radioactivity of refractories in the furnace body.

The constituent materials of the hull etc. are Zircaloy: 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. On the outer surface, zirconium oxide is generated by reaction with the reactor cooling water, and its thickness is approximately 10 to 20 μm in uniformly corroded areas.On the inner surface, zirconium oxide is generated due to the oxygen contained in the nuclear fuel, and the thickness is approximately 10 to 20 μm. is thin, estimated to be less than 10 μm.

In terms of radioactive waste management, transuranic elements (TRU) require the most attention. This is generated by the activation of uranium as a nuclear fuel material in a nuclear reactor, and it adheres to the hull surface when the nuclear fuel is dissolved in nitric acid in the melting process in the reprocessing process, coexisting with the cladding material. do. 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 ZrO ₂ passivation layer is highly corrosion-resistant, and a highly active solution or gas 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 has been 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 treated object such as a spent fuel cladding tube in a thermal cycle of increasing and decreasing temperature in an oxidizing atmosphere to convert the entire body into an oxide, and then press-forms this oxide. It was designed to be disposed of afterwards.

Zirconium forms a wide range of solid solutions with oxygen and has a high affinity for oxygen. A fact that supports this is the standard free energy of formation of zirconium oxide. In other words, ZrO ₂ has a much lower standard free energy value than oxides of Fe, Ni, Cr, etc., which are the main components of stainless steel and Inconel, so it bonds stronger and faster with oxygen. Therefore, when a zirconium oxide layer is present on the surface, it exhibits excellent corrosion resistance against most solutions due to its corrosion resistance. On the other hand, since zirconium inherently has strong reactivity with oxygen, it is significantly oxidized in an oxidizing atmosphere at high temperatures. Oxidizing gases (air), especially nitrogen gas
The oxidation reaction is accelerated therein due to the nitride-mediated oxidation mechanism. On the other hand, stainless steel and Inconel are inherently heat-resistant materials, and because of the standard energy of formation of their constituent oxides, there is no possibility of them being completely oxidized like Zircaloy, and they form in lumps. remains as.

The oxidized zirconium oxide is solidified from powder using the HIP method and becomes a ceramic solidified body enclosed in a metal capsule, which has excellent stability. In addition, even if it is not based on the HIP law,
After adding appropriate additives to lower the sintering temperature and pressure, it is also possible to use hot pressing (uniaxial compression), or room-temperature forming and normal-pressure firing methods, such as those used in the production of ordinary zirconium refractory crucibles. be.

FIG. 1 is a flowchart showing an embodiment of the present invention, in which a workpiece is inserted into an oxidation furnace (step 1),
The atmosphere is adjusted in the oxidation furnace using an atmosphere adjustment system (step 2), and a thermal cycle is applied, as well as vibration and impact (step 3). Tritium and volatile substances generated in the oxidation furnace are discharged from the recovery system (step 4) through the dust recovery system (step 5) while being monitored (step 6). After the prescribed processing,
Unload the workpiece (step 7), remove the stainless steel
Inconel and zirconium oxide are separated (step 8), zirconium is solidified into ceramic by the HIP method (step 9), and other residual metals are treated separately (step 10). The true density of zirconium is 5.68 g/cm ³ , and even considering the increase in volume due to oxygen, a low-density hull with a bulk density of 1.1 g/cm ³ can be reduced to less than a quarter of its volume by the above treatment. It becomes a ceramic solidified body, and a significant volume reduction is achieved.

In order to efficiently proceed with the oxidation, a thermal cycle of heating and cooling in the heating furnace is introduced to pass the zirconium oxide, zircaloy metal phase, and crystal structure transformation point, promoting separation of the oxide layer and the metal phase. Preferably, the supply of oxygen is increased. In order to proceed with oxidation more efficiently, heating may be carried out while applying mechanical vibration shock in a rotary kiln or the like. Stainless steel and Inconel contained in hulls and the like cannot be oxidized until they are turned into powder by heating in such an oxidizing atmosphere, so the powder and lumps may be separated after the fact using a vibrating sieve or the like. After oxidation, the zirconium oxide powder can be processed as is by the HIP method, or it can be hot-pressed after adding appropriate additives to increase sinterability and stability, or it can be cold-formed and then fired. ,
It may be pelletized to obtain a solidified material for disposal.

Example: Zircaloy 2 pipe (simulating a hull, with an oxide layer added to the surface by autoclave treatment)
was heated in the air, subjected to thermal cycles, and changes in the surface oxidation layer were observed.

Sample Zircaloy 2 pipe was autoclaved under the following conditions to give an oxidized layer (equivalent to hull) on the surface.

Processing temperature: 500°C Processing pressure: 105 atm Processing time: 24 hours Processing conditions and results The sample was placed on an alumina boat and heated in an electric furnace in an atmospheric atmosphere. The test was carried out under four conditions shown in FIG. 2, with one sample for each condition. In No. 1, after heating to 750°C and holding for 45 minutes, the temperature was repeatedly raised and lowered between 750 and 1250°C.
This temperature range is a temperature cycle to take advantage of the discontinuity in thermal expansion behavior of zirconium oxide. In No. 2, it was heated to 750°C and held for 45 minutes.
In No. 3, the temperature was repeatedly raised and lowered between 750 and 1250°C. In No. 4, it was heated to 1250°C and held for 15 minutes.
As a result of these treatments, No. 1 and No. 3 were oxidized to the extent that they did not retain their pipe shape at all. In No. 2 and No. 4, no shedding scale was observed, and in No. 4
A thick oxide layer was observed to have grown on the outer surface, especially on the areas directly exposed to the heat radiation from the electric furnace, and could be removed with the fingertips. However, no such removable oxide layer was observed on the inner surface. This is thought to be because the inner surface and the lower part of the sample, which are not directly exposed to thermal radiation (the heating element is located above and on the side), have locally low temperatures and slow reactions.

As explained above, this invention heats spent fuel cladding tubes, etc. in an oxidizing atmosphere by applying a thermal cycle to convert them into oxides, which are then disposed of after volume reduction treatment by pressure molding. This makes it possible to significantly reduce the volume of waste. Also,
The advantage of applying vibration or shock is that oxidation can be carried out quickly. Furthermore, since the waste is a ceramic based on zirconium oxide, it has the advantage of excellent stability.

[Brief explanation of the drawing]

Figure 1 is a flow diagram of the process of implementing this invention;
FIG. 2 is an explanatory diagram showing various thermal cycles. 1... step of inserting the object to be treated into the oxidation furnace,
3... Step of processing in an oxidation furnace, 8... Step of separating waste.

Claims (1)

[Claims] 1. A Zircaloy treated object such as a spent fuel cladding tube is heated in an oxidizing atmosphere through a thermal cycle of increasing and decreasing temperature to convert the entire product into an oxide, and this oxide is pressure-formed and then disposed of. A method for processing spent fuel cladding tubes, etc., characterized by: