JP3018684B2 - Radioactive waste treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for treating radioactive waste generated by nuclear fission in a nuclear reactor,
In particular, for example, during the reprocessing of spent nuclear fuel,
The present invention relates to a method for treating radioactive waste in the form of nitrates.

[0002]

2. Description of the Related Art Nuclear fuel used in a light water reactor
Spent nuclear fuel) is the highly radioactive fission produced by fission
Unburned besides things²³⁵Nuclear fuel in U and fast breeder reactors
Becomes ²³⁹Contains Pu. Therefore, spent nuclear fuel
Reprocessing to recover these useful components from feed
Have been.

There are a wet method and a dry method for reprocessing spent nuclear fuel, but only a wet method utilizing solvent extraction is currently in practical use, and the Purex method is the most common among them. In the wet method, first, spent nuclear fuel in a state of a fuel rod is cooled as it is to attenuate its radioactivity, then cut into pieces, dissolved in nitric acid, and the insoluble coating is separated. The obtained nitric acid solution is extracted with an extraction solvent such as tributyl phosphate (P
uranium and plutonium are extracted together in tributyl phosphate and the remaining fission products remain in the aqueous phase (nitric acid). The extract containing uranium and plutonium is then subjected to various treatments, including back-extraction, to separate and purify uranium and plutonium for reuse in a nuclear reactor.

On the other hand, elements other than uranium and plutonium
The raffinate (nitric acid solution) containing (fission products) becomes solid nitrate when heated to evaporate water. This solid is a mixture of nitrates of various metals, has a high level of radioactivity, and is to be treated as so-called high level radioactive waste.

[0005] Such nitrate, that is, radioactive waste generated in nuclear fission and existing in the form of nitrate, is hereinafter referred to as "nitrate". The metal element contained in the “nitrate” is mainly an element having an atomic number of 43 or more. Examples of the metal element include lanthanum elements such as neodymium and samarium, and alkaline earth (IIa) metal elements such as strontium and barium. Palladium (Pd), rhodium
(Rh), ruthenium (Ru) and the like are also included in the “nitrate”.

A method that has been conventionally considered in various countries as a method for treating this high-level radioactive waste “nitrate” is to mix and mix 9 parts of glass with 1 part of “nitrate” by volume ratio. The vitrified material is stable for a long period of time, and is cooled and stored at a surface facility for 30 to 50 years to attenuate radioactivity, temporarily stored, and then buried in a deep geological layer below 500 m underground or in rock salt. It is a method of disposing. However, this method has the disadvantage that the volume of the waste is increased by a factor of 10 and that it is not possible to recover expensive useful metals such as the platinum group elements contained in the waste, as well as Cs, There is a problem that long-lived radionuclides such as Sr are contained in waste.

As another method of treating “nitrate”, “nitrate” is first heated from 200 ° C. to 800 ° C. to decompose nitrate to form an oxide (hereinafter, metal oxide produced by thermal decomposition of “nitrate”). A method has been proposed in which this “oxide” is heated to 1800 ° C. to 2000 ° C. to dissolve (melt) it. By this heating and melting, "oxide"
It is reported that Pd, Rh, Ru and other platinum group elements contained in are reduced from oxides to metals and can be recovered as an alloy mainly composed of platinum group elements and molybdenum (M. Hori
e, Transactions of American Nuclear Society, Vol.
62, 1990, pp. 111-113). Hereafter, this "nitrate"
Is referred to as a “high-temperature treatment method”.

[0008] In order to recover the platinum group element as an alloy with molybdenum by this "high temperature treatment method", the temperature is from 1800 ° C to 2000 ° C.
Requires high temperatures such as ° C. However, specific methods for stable high-temperature treatment of “nitrates” and “oxides” and the melting vessels used (for example, crucibles) have not been established, and therefore, platinum group metals have not been recovered. In spite of the advantage that the "high-temperature treatment method" is used, it has not been recognized that it can be put to practical use at present.

[0009]

SUMMARY OF THE INVENTION A first object of the present invention is to aim at practical use of a "high temperature treatment method" of "nitrate".
It is an object of the present invention to provide a dissolving apparatus that can be used for this and a specific processing method thereof.

[0010] Another object of the present invention is to provide a method for easily separating and recovering useful platinum group elements from the remaining waste by high-temperature treatment of "nitrate".

Still another object of the present invention is to separate and remove high-calorific elements (eg, cesium, strontium) in “nitrate” during high-temperature treatment, thereby finally forming a solid. An object of the present invention is to provide a method capable of shortening the storage period of the obtained radioactive waste.

[0012]

Means for Solving the Problems The inventors of the present invention made use of a crucible-type high-frequency induction furnace having a water-cooled slit to convert "nitrate" to "oxide" and then to a high-temperature treatment. Nitrate can be heated and melted up to about 2000 ° C, and oxides that are not reduced by the force of the pinch effect generated by electromagnetic induction (hereinafter referred to as the electromagnetic pinch force) are placed around the container around the reduced platinum group element. Since only the particles are collected at the center of the container, they have found that they can be easily separated and collected, and have reached the present invention.

Here, the gist of the present invention is to dissolve radioactive waste generated in fission present in the form of nitrate by electromagnetic induction heating in a cooled container having a slit provided with an energizing coil circulating outside. Then, the metal oxide generated by the decomposition of nitrate is accumulated around the container, the reduced white metal element is accumulated in the center of the container by the electromagnetic pinch force, and after cooling and solidification, the formed solid is recovered. This is a method for treating radioactive waste.

Here, the solid radioactive waste generated by nuclear fission in the form of nitrate is the above-mentioned "nitrate". A typical source of “nitrate” is radioactive waste generated in the above-mentioned process of reprocessing spent nuclear fuel, but is not limited thereto. For example, the present invention relates to a fuel cladding tube called a hull. And the like can be applied to the treatment of radioactive waste in the form of any nitrate.

The electromagnetic induction heating of radioactive waste is induced in the initial stage of heating by using other auxiliary heating means in addition to the electromagnetic induction heating, or by electromagnetically heating an electric conductor inserted in the container. be able to.

It is preferable to separate and recover long-lived nuclides such as cesium and strontium volatilized from radioactive waste during electromagnetic induction heating.

[0017]

The structure of the present invention, together with its operation, will be described below with reference to the drawings. FIG. 1 is a schematic half cut perspective view of one example of a dissolution vessel that can be used to batch process "nitrates" according to the method of the present invention. This melting vessel is a cooled vessel provided with an energizing coil circulating outside, that is, a cooling crucible-type high-frequency induction furnace. Hereinafter, the melting vessel by the high-frequency induction heating is referred to as a cooling crucible.

This melting vessel (cooling crucible) is composed of a crucible body (vessel) 1, a water-cooled energizing coil 2 spirally wrapped around the outside of the crucible, and a stopper 4. The current-carrying coil 2 is connected to a high-frequency oscillator 3 and can heat the crucible by electromagnetic induction using a high-frequency current. The inside of the crucible wall is water-cooled by flowing cooling water from the inlet 7 to the outlet 8, and the water-cooled structure can withstand high temperatures. Crucible wall has multiple slits 15 in the vertical direction
Are formed, and are electrically insulated from each other. Crucible is a chamber with adjustable atmosphere
(Not shown), and can be placed in an atmosphere such as Ar or He under pressurized or reduced pressure. Alternatively, the crucible may be provided with a top lid (not shown) to seal the inside of the crucible.

After the addition of "nitrate" into the cooling crucible, nothing happens when a high-frequency current is applied to the current-carrying coil. Because, at normal temperature, "nitrate" is an electrically poor conductor,
This is because induction heating cannot be expected.

Then, after the electric good conductor is charged in the cooling crucible in advance, "nitrate" is added. As the electric good conductor, for example, as shown in FIG. 2, a carbon crucible 9 having a shape inserted into the crucible main body 1, a stainless steel block, or the like can be used. As described above, when a high-frequency current is applied to the coil in a state where the electric good conductor coexists at the initial stage of the heating, the electric good conductor is first heated by induction.
Under the influence of heating, a part of the “nitrate” is heated by heat transfer and melts. "Nitrate", once in a molten state, changes its properties into a good conductor of electricity. Therefore, this time, the “nitrate” itself is induction-heated, and the entire “nitrate” is in a molten state.

As another method for inducing electromagnetic induction heating of "nitrate", a method in which induction heating is performed in combination with auxiliary heating means such as electron beam, laser, or microwave irradiation at the beginning of heating is also possible. The purpose of using the auxiliary heating means together is to generate an induced current instead of melting a part of the “nitrate” and replace it with a good electric conductor, thereby causing self-heating. Therefore, it is not necessary to increase the output of the auxiliary heating means, but it is necessary to increase the heating density.

The above method makes it possible to heat and melt the entire “nitrate”. During heating, “nitrate” is decomposed into “oxide”.
When the temperature is raised to such an extent, nuclides having a long life and thus exothermicity, such as cesium and strontium, are volatilized. By separating highly exothermic nuclides, it is possible to shorten the cooling time when finally disposing of radioactive waste. The nuclide-separated elements are transmuted into short-lived nuclides or non-radioactive nuclides by using a nuclear reactor, so that radioactive waste can be turned into resources and disposal efficiency can be improved.

When the temperature in the cooling crucible is further increased to about 1800 to 2000 ° C. by continuing the electromagnetic induction heating, platinum group elements such as rhodium, ruthenium and palladium in “nitrate” are reduced at such a high temperature. To the metal state from the oxide, while the remaining components, such as barium,
A portion containing zirconium, neptunium, or the like as a main component remains in an oxide state. An electromagnetic force acts on the platinum group element in a metal state, and the electromagnetic pinch force causes the platinum group element to gather at the center of the cooling crucible. On the other hand, the remaining portion composed of the metal element in the oxide state (hereinafter, referred to as
Since no electromagnetic force acts on the ceramics), a part of the ceramics which gathers around the cooling crucible and contacts the inner wall of the cooling crucible is partially solidified by water cooling of the crucible and adheres to the wall. Thus, the platinum group element portion and the ceramic portion can be separated by electromagnetic induction heating using the cooling crucible.

When an electric conductor is inserted before induction heating, the electric conductor made of a metal such as stainless steel is included in the white metal element portion, and carbon is included in the ceramic.

When the application of high-frequency current is stopped after the completion of melting and separation, the mixture is cooled and solidified within a short time, and a platinum group element portion and ceramics are obtained as a solid in a separated state. It is desirable to collect it. The cooling crucible has 20
It withstands a high temperature of 00 ° C. sufficiently, and the treatment of the “nitrate” of the present invention does not cause significant damage to the furnace wall.

The solid obtained from the platinum group element portion is made of an alloy of a platinum group element such as rhodium, ruthenium, palladium and molybdenum, which has a short half-life.
After leaving to attenuate radioactivity, it can be reused for industrial use. In this way, the expensive platinum group element that has been solidified together in the conventional vitrification method can be recovered from radioactive waste and used effectively.

After the solidification, the ceramic part is obtained as a ceramic-like solidified body. This solidified ceramic body is stable for a long period of time and does not elute components, like the solidified ceramic body. Therefore, similarly to the vitrified body, the radioactivity is attenuated to some extent, that is, temporarily stored until cooled, and finally buried in a deep ground layer. However,
According to the present invention, the solidified ceramic body is advantageous in the following points as compared with the conventional solidified glass body.

1) Since the volume of the solidified ceramic is 1/10 that of the solidified ceramic obtained by mixing about 9 times the amount of glass, the storage volume can be reduced. 2) Solidified ceramics generate little heat because elements with long half-life (eg, cesium, strontium) are volatilized and separated during the melting process. Therefore, the cooling period can be shortened, and the storage period can be shortened.

The structure of the cooling crucible for treating radioactive waste is not limited to the batch processing type, and a continuous processing type cooling crucible as shown in FIG. 3 can also be used. This continuous processing type cooling crucible is the same as that shown in FIG. 1 except that it has a cylindrical shape and a structure in which a bottom plate 5 a at the bottom can be moved up and down by a drawing bar 5. In this continuous process, "nitrate" is continuously charged into the crucible, and the bottom plate is lowered by lowering the drawing rod at a speed commensurate with the amount of "nitrate" charged, thereby maintaining the height of the molten liquid level almost constant. I do. This makes it possible to continuously realize melting of the “nitrate” by induction heating in a portion where the coil orbits.

It should be noted that the method of the present invention described above does not require complicated operations, and can be easily carried out using an automated device in a controlled area.

[0031]

EXAMPLES The present invention will be further described below with reference to examples.

Example 1 This example illustrates an example of batch processing of "nitrate" according to the present invention. FIGS. 1, 2 and 4 are schematic half cut perspective views of a batch-processed cooling crucible for high-temperature processing of “nitrate” used in the present embodiment, and a carbon crucible-type electric inserted into the batch-processed cooling crucible. One example of a good conductor container,
The melt flow state inside the crucible is schematically shown.

The cooling crucible body 1 has an inner diameter of 45 mm and a height of
It is 100mm and the material is copper. First, a carbon container 9 is set in a crucible, and about 300 g of “nitrate”, which is radioactive waste obtained as a residue obtained by extracting uranium and plutonium in the process of reprocessing spent nuclear fuel, is placed in the crucible top 16. Was charged from the nitrate supply unit 6 provided in the above. By passing cooling water through the crucible, sealing the top lid 16, and flowing argon from the inert gas supply port 13 to the discharge port 14,
The inside of the crucible was kept in an argon atmosphere. Frequency 30 kHz
z, a high-frequency current of 2000 A coil current was applied to the spiral coil 2 arranged around the crucible.

The electromagnetic induction heating by the high-frequency current caused the carbon container to generate heat and melt red, and then the "nitrate" partially melted from the side close to the crucible wall, and the molten portion eventually expanded to the entire "nitrate". About 5 minutes after the start of operation, strontium and cesium in the exhaust gas increased, and almost all of them (about 20 g in terms of metal) were recovered. After that, when the liquid flow in the crucible is measured with a water-cooled Vive's sensor,
As shown in FIG. 3, in the vertical cross section passing through the central axis, four circulating flows were observed in the platinum group element portion 10 in the central portion, and two circulating flows were observed in the molten ceramic portion 11 in the peripheral portion.

After 10 minutes from the start of operation, energization was stopped, and after cooling and solidification, about 125 g of a solidified platinum group element and about 155 g of a solidified ceramic were separately taken out. The solidified platinum group element contained 17 g of ruthenium, 3.2 g of rhodium, 10.4 g of palladium, and 26 g of molybdenum. The solidified ceramic contained 14 g of barium, 29 g of zirconium, 4 g of neptunium and the like in an oxide state. There was no melting of the cooling crucible.

Example 2 This example shows an example in which "nitrate" is continuously treated by the method of the present invention. 3 and 5 respectively show a half cut perspective view of a cooling crucible used for the continuous treatment of “nitrate” and a schematic view of a melt flow state inside the crucible.

The inner diameter of the cooling crucible is 45 mm and the height is 100 mm
And the material is copper. First, 50 g of stainless steel scrap was charged into a crucible, and after cooling water was passed through the crucible, the crucible was sealed. After applying a high-frequency current under substantially the same conditions as in Example 1, the drawing bar was operated so that the stainless steel scrap was located at a position where the electromagnetic field acts most on it (that is, a middle position of the coil).

Since the scrap was completely melted by the current supply for about 5 minutes, “nitrate” was supplied to the cooling crucible at a rate of 25 g / min from the nitrate supply section 6 provided on the upper lid 16 of the crucible. The withdrawal rod was continuously withdrawn at a corresponding speed and adjusted so that the upper surface, that is, the level of the melt, was almost constant. The melted portion solidified from below, and a solidification interface 12 was formed. Inert gas outlet 14
Volatile metal components (such as cesium) contaminated by cold wrap were recovered from the argon gas discharged from. After a continuous operation for about 20 minutes from the start of the supply of “nitrate” (total input amount: about 500 g), the supply of the high-frequency current was stopped. In addition, when the liquid flow in the melting portion in the crucible being processed is measured in the same manner as in the first embodiment, the flow of the liquid in the first embodiment as shown in FIG.
The same circulating flow was observed.

After cooling, the solidified ingot (about 600 g)
As a result, a solidified platinum group element was deposited at the center of the ingot and a solidified ceramic was deposited at the periphery. Both were easily separated by mechanical destruction. The solidified platinum group element contains 28 g of ruthenium, 5.3 g of rhodium, 17.3 g of palladium and 43 g of molybdenum, and the solidified ceramic contains 23 g of barium, 48 g of zirconium,
6.4 g of neptunium was included as oxide.

Example 3 A cooling crucible having substantially the same structure as that of Example 2 was used except that the bottom plate of the drawn rod was a stainless steel cylinder having an outer diameter of 45 mm and a thickness of 100 mm. Continuous dissolution and coagulation of "nitrate" were performed. When dissolving, before putting `` nitrate '' into the crucible, set the stainless steel cylinder of the bottom plate to a position where the upper part is located in the area where the electromagnetic force of the middle height of the current-carrying coil works effectively. Started. That is, in the present embodiment, this corresponds to disposing an electric good conductor for inducing electromagnetic induction heating of “nitrate” in the crucible from below.

With the upper part of the stainless steel cylinder (the part subjected to the electromagnetic induction heating) heated to about 1800 ° C. to become molten metal, “nitrate” was continuously supplied to the cooling crucible at a rate of 25 g / min. And at the same time, the drawing rod was continuously drawn down at a speed corresponding to the charged amount. "nitrate"
Was heated and melted by the molten stainless steel. Once
When the molten “nitrate” was formed, the molten portion became a good electrical conductor, so that continuous operation by high-frequency heating became possible.
After continuous operation for about 20 minutes from the start of the introduction of "nitrate",
The supply of the high-frequency current was stopped.

After cooling, when the solidified ingot was taken out, a solidified platinum group element was deposited at the center of the ingot and a solidified ceramic was deposited at the periphery, and both were easily separated as in Example 2. . The solidified platinum group element contained 28 g of ruthenium, 5.3 g of rhodium, 17.3 g of palladium, and 43 g of molybdenum, and the solidified ceramic contained 23 g of barium, 48 g of zirconium, and 6.7 g of neptunium as oxides.

[0043]

When the radioactive waste "nitrate" is treated at a high temperature by the method of the present invention, the following effects can be obtained.

1) Since a highly heat-producing element having a long half-life and a high heat generation can be selectively volatilized and separated from the radioactive waste during the high-temperature treatment, the storage period for cooling the radioactive waste to be finally disposed is reduced. Be shortened. The separated long-lived elements can be effectively used by nuclear conversion.

2) Since the electromagnetic pinch force acts only on the platinum group elements reduced to the metallic state, they accumulate in the center of the container. Therefore, the solidified platinum group element and the solidified ceramic can be easily separated by cooling and solidifying the radioactive waste after the high-temperature treatment.

3) Since the platinum group elements separated and recovered from radioactive waste have a short half-life, they can be reused industrially after cooling, and are disposed of together by the conventional vitrification method. The expensive platinum group elements can be recovered and used effectively.

4) The remaining solidified ceramic from which the platinum group element has been separated is a solidified body that can be disposed of as it is. There is no need to mix and solidify with a large amount of glass, and the volume decreases during high-temperature treatment, so the amount of radioactive waste to be disposed of is almost one-tenth that of conventional vitrified waste. Volume can be reduced.

5) The cooling crucible used in the method for treating radioactive waste according to the present invention can be used semipermanently and does not generate secondary waste.

Therefore, according to the present invention, an industrially applicable high-temperature treatment method for “nitrate” has been established. In addition, this method can significantly reduce the volume of radioactive waste and shorten the storage period as compared with the conventional vitrification method, and can recover useful resources.

[Brief description of the drawings]

FIG. 1 is a schematic half cut perspective view of a batch-processed cooling crucible that can be used in the method of the present invention.

FIG. 2 shows an example of an electric good conductor that can be inserted into the cooling crucible shown in FIG.

FIG. 3 is a schematic half cut perspective view of a continuous processing type cooling crucible that can be used in the method of the present invention.

FIG. 4 is a schematic diagram showing a flow state inside a batch processing type cooling crucible.

FIG. 5 is a schematic diagram showing a flow state inside the continuous processing type cooling crucible.

[Explanation of symbols]

1: cooling crucible (container) 2: energizing coil
3: High frequency oscillator 4: Plug 5: Pull rod 5
a: Drawer bottom plate 6: Nitrate supply unit 7: Cooling water inlet
8: Cooling water outlet 9: Good electric conductor 10: Platinum group element part 1
1: Ceramic part 12: Solidification interface 13: Inert gas supply port 1
4: Inert gas outlet 15: Slit 16: Crucible top lid

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G21F 9/30 G21F 9/32 G21C 19/46

Claims (5)

(57) [Claims] 1. Radioactive waste generated in fission present in the form of nitrate is dissolved by electromagnetic induction heating in a cooled vessel having a slit provided with an energizing coil circulating outside, and the nitrate is decomposed. Radioactive waste treatment consisting of collecting the generated metal oxide around the container, accumulating the reduced white metal element in the center of the container by electromagnetic pinch force, and then collecting the solidified product after cooling and solidification. Method. 2. The solidified body according to claim 1, wherein the solidified body containing the platinum group element collected in the central part of the container is separated from the solidified ceramic material in the peripheral part of the container. Method. 3. The method according to claim 1, wherein the electromagnetic induction heating of the radioactive waste is induced by using other auxiliary heating means in addition to the electromagnetic induction heating at an early stage of the heating. 4. The method according to claim 1, wherein an electric conductor is charged at an early stage of the heating, and the electromagnetic conductor is heated by electromagnetic induction to induce electromagnetic induction heating of the radioactive waste. 5. The method according to claim 1, wherein the long-lived nuclide volatilized from the radioactive waste during electromagnetic induction heating is separated and recovered.