JP3030372B2 - How to separate fission-generated noble metals - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for separating and recovering insoluble residues generated in a reprocessing step of spent nuclear fuel and fission-generated noble metals contained in a high-level waste liquid.

[0002]

2. Description of the Related Art The abundance ratios of various fission-producing noble metals contained in spent nuclear fuel differ depending on the composition, burnup, cooling time, etc. of the nuclear fuel. / 6. These fission-produced noble metals are contained in insoluble residues and high-level effluents in the reprocessing step. As a method for recovering such insoluble residues and noble metals in high-level waste liquid, a method based on lead melting is known. As a method for recovering from high-level waste liquid, a high-level waste liquid heated and denitrated is mixed with a reducing agent such as a glass-forming agent, lead monoxide and activated carbon to obtain 1100 to 12%.
There is a method of heating and melting at 00 ° C (for example, Nuclear Te
chnology vol.65,305 (1984) and Atomic Energy Society of Japan `` 1989
Fall Meeting ”, etc., are described in the proceedings H69 etc.). Further, as a method for recovering from the insoluble residue, there is a method in which a glass forming agent and metallic lead are mixed with the insoluble residue and heated and melted at 550 ° C. or higher (eg, Nuclear Science and Technology)
vol23, 540 (1989) and Atomic Energy Society of Japan, 1989 Annual Meeting
Proceedings L6 etc.). According to these methods, the fission-generated noble metal is efficiently recovered by lead and separated from transuranium elements and the like other than the noble metal, and the transuranium elements and the like are disposed of as a vitrified material as it is. I have.

Further, there are two types of methods for separating fission-generated noble metals (including a plurality of types) recovered from each other by the lead melting method from each other, a so-called dry method and a wet method. In the dry method, noble metal converted into lead metal by lead melting is extracted with a metal such as zinc, and then the metal such as zinc is removed by distillation, whereby the noble metal is recovered. On the other hand, in the wet method, a noble metal recovered as a lead alloy by melting lead is dissolved in an acid, and the dissolved noble metal is separated and recovered from each other by a method such as solvent extraction or ion exchange. In the case of the above wet method, lead dissolved at a high concentration in the solution should be separated from the noble metal as early as possible from the viewpoint of economics such as maintenance of product purity and simplification of equipment. is necessary.

The method of separating lead by such a wet method includes a method of separating and separating lead as hydroxide by neutralization and a method of forming and separating lead precipitate by sulfate ions and other additives. Widely applied to analytical methods.

[0005]

When a lead alloy obtained by recovering a noble metal obtained by melting lead is dissolved in nitric acid, rhodium and palladium move to the solution side, and
Ruthenium moves to the residue side. Therefore, since lead remains at a high concentration in the solution together with rhodium and palladium, most of the noble metals present in the solution accompany the precipitation of lead when the precipitated lead is separated by the above precipitation separation method. However, since the associated precious metal is not recovered, it is unavoidable to end up with a recovery loss, which is extremely uneconomical. on the other hand,
When reusing lead recovered in the form of sulfate, etc., corrosive sulfuric acid gas is generated, and disposal and reuse of such sulfuric acid gas becomes difficult in facilities handling high-level radioactive materials. The quantity of things increases.

[0006] In view of such circumstances, an object of the present invention is to provide a method for separating fission-generated noble metals which can be extremely effectively applied particularly to facilities handling high-level radioactive materials.

[0007]

According to the present invention, there is provided a method for separating fission-generated noble metals, wherein undissolved residues generated in a reprocessing step of spent nuclear fuel and noble metals contained in high-level waste liquid are recovered by melting lead. The lead alloy containing the noble metal obtained by the above is melted with nitric acid, and the lead in the solution is oxidized at an anode set to a potential of +1.4 to 1.7 volts with respect to a silver / silver chloride standard electrode. By doing so, they are removed as lead dioxide.

In the present invention, the potential of the cathode is set so that palladium can be selectively deposited and recovered from the solution of the lead alloy containing a noble metal.

[0009]

According to the present invention, a lead alloy recovered by a method for recovering a fission-generated noble metal obtained by melting lead is dissolved in nitric acid, and a high-concentration of high-concentration metal present together with rhodium and palladium nitrates thus obtained is obtained. From the lead solution, lead can be separated in the form of lead dioxide at the anode by electrolytic oxidation, and at the same time palladium can be recovered selectively at the cathode by reductive precipitation.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the method for separating fission-generated noble metals according to the present invention will be described below. First, here, the recovery of fission-generated noble metal from the insoluble residue was performed using the simulated insoluble residue, but the actual composition of the insoluble residue differs depending on the fuel composition, burnup, and cooling time. For the most part, precious metal elements of ruthenium, rhodium and palladium and alloys consisting of technesium and molybdenum are occupied.
In the case of light water reactor fuel, the average composition ratio of these five elements is about 55: 10: 10: 5: 20 in atomic fraction. Therefore, in the method of the present invention, an alloy composed of ruthenium, rhodium, palladium and molybdenum was assumed as the simulated insoluble residue, and the ratio of these elements was set to 60: 10: 10: 20 in elemental fraction.

As described above, the method of the present invention performs lead melting to recover the fission-generated noble metal, and this lead melting involves adding 10 parts of the glass forming agent to the weight of the simulated insoluble residue.
This is carried out at a temperature of 1100 ° C. by adding metal lead 50 times and metal lead 50 times. By dissolving the obtained lead alloy with 3N nitric acid, a solution of noble metal and lead is obtained. In the electrolytic cell for this nitric acid solution, platinum is used for the anode, carbon is used for the cathode, and glass is used. A filter diaphragm was used in combination.

First, in order to recover the noble metal in the nitric acid solution, the lead is separated by oxidizing lead as lead dioxide at the anode. The potential of the anode is + 1.times. With respect to the standard silver / silver chloride electrode. 4 to +1.7 volts. Here, when the potential of the anode is lower than +1.4 volts with respect to the silver / silver chloride standard electrode, the deposition rate of lead dioxide becomes extremely slow, and when the potential is higher than +1.7 volts, Since the generation of oxygen at the anode is remarkably increased and the efficiency is reduced, the optimal condition is that the potential of the anode is in the range of +1.4 to +1.7 volts as described above. On the other hand, at the same time as the precipitation of lead at the anode, reductive precipitation of palladium is performed at the cathode, and the deposition potential is +0.1 volt or less with respect to palladium. Therefore, by setting the potential of the cathode to a range of +0.1 volt or less with respect to the silver / silver chloride standard electrode, only palladium can be selectively electrolytically deposited.

Next, the potential of the anode is set to +1.4 to +1.7 volts with respect to the silver / silver chloride standard electrode, and the potential of the cathode is set to +0.1 volts or less, and electrolysis is performed. Table 1 shows the results of the measurement.

[0014]

[Table 1]

As is apparent from Table 1, lead was recovered at the anode at a current efficiency of 95% or more, and palladium was selectively recovered at the cathode. As apparent from Table 1, if the potential of the anode is in the range of +1.4 to +1.7 volts with respect to the silver / silver chloride standard electrode, the concentration of lead in the solution on the anode side is It will be low enough.

In the above embodiment, the lead dioxide recovered at the anode can be reused as it is as a raw material in the lead melting step. As the electrode material of the anode and the cathode, a general electrode material such as a platinum plate and a carbon material can be used, and a diaphragm for isolating the anode and the cathode is a general diaphragm such as a glass filter. Materials may be used. In addition, such a diaphragm is necessary for preventing the lead dioxide precipitated at the anode from being dissolved in the solution again at the cathode when the lead dioxide is separated from the electrode plate and dispersed in the solution, thereby lowering the recovery efficiency. .

[0017]

As described above, according to the method of the present invention,
In particular, the lead alloy obtained by the lead melting method for recovering the undissolved residue generated in the wet reprocessing facility and the fission-generated noble metal contained in the high-level waste liquid is mixed with the noble metal in the solution obtained by dissolving with nitric acid. Although lead is separated and recovered by electrolysis, particularly, lead can be recovered as an oxide that can be recycled to the lead melting step at the anode. Thereby, there is an advantage that generation of waste can be significantly reduced when fission-generated noble metal is recovered. Further, the separation of palladium, which is a fission-generated noble metal, can be performed at the same time, so that the process can be simplified. Therefore, the recovery of the fission-generated noble metal can be made extremely economical.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Hiroshi Nakahira 4-5-14 Mizukino, Moriya-machi, Kitasoma, Ibaraki Prefecture Person Hiroshi Okada 1873-2 Toyooka, Tokai-mura, Naka-gun, Ibaraki Prefecture (58) Field surveyed (Int. Cl. ⁷ , DB name) G21F 9/06 G21C 19/46 JICST file (JOIS)

Claims (2)

(57) [Claims] An insoluble residue generated in a reprocessing step of spent nuclear fuel and a noble metal contained in a high-level waste liquid are recovered by lead melting, and a noble metal-containing lead alloy obtained by the recovery is melted with nitric acid. A fission-generating noble metal which is removed as lead dioxide by oxidizing the lead in the solution at an anode set at a potential of +1.4 to 1.7 volts with respect to a silver / silver chloride standard electrode. Separation method. 2. The separation of a fission-generated noble metal, wherein a potential of a cathode is set so that palladium can be selectively electrodeposited and recovered from a solution of the lead alloy containing a noble metal according to claim 1. Method.