JP2966066B2 - How to separate fission-generated noble metals - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for separating insoluble residues generated in a reprocessing step of spent nuclear fuel and fission-generated noble metals contained in high-level waste liquid.

[Conventional technology]

Spent nuclear fuel varies depending on fuel composition, burnup, cooling period, etc., but mainly ruthenium (Ru),
It contains fission-producing noble metals such as rhodium (Rh) and palladium (Pb), and its content is up to one-fourth of all fission products. These fission-generated noble metals are contained in both the insoluble residue and the high-level waste liquid after a reprocessing step with nitric acid. A method using lead melting is known as a method for recovering such fission-generated noble metals. First, as a method of recovering from high-level waste liquid, high-level waste liquid is heated and denitrified, then mixed with a reducing agent such as glass forming agent, lead monoxide, and activated carbon, and heated and melted at 1100 to 1200 ° C. Precious metals are recovered in metallic lead reduced by activated carbon, while
Transuranium elements are extracted into glass (for example, Nuclear
r Teclnology Vol.65 May 1984 or JAEA `` 19
89 Autumn Convention "Proceedings, etc.). In addition, as a method of separating the noble metal from the insoluble residue, by mixing a glass forming agent and metallic lead to the insoluble residue and heating and melting at 550 to 1100 ° C,
Noble metals are recovered by lead, and other transuranium elements are extracted by glass (eg, Journal of N
uclear Science and Technology vol.23, [6] Proceedings of the Atomic Energy Society of Japan, 1985 Annual Meeting, or Atomic Energy Society of Japan, 1989 Annual Meeting, etc.) According to the above method, the fission-generated noble metal can be efficiently recovered, and the transuranium element separated by the glass can be treated as it is in a vitrified state. As described above, there are a dry method and a wet method as methods for further separating ruthenium, rhodium, and the like recovered by lead melting. In the former, the noble metal recovered as a lead alloy by lead melting is extracted with a metal such as zinc. Then, mutual separation is performed based on the difference in the distribution ratio at that time, and the noble metal extracted in zinc is recovered by distillation. On the other hand, in the wet method, a noble metal recovered as a lead alloy is dissolved in an acid, and the dissolved noble metal is mutually separated by a solvent extraction method.

[Problems to be solved by the invention]

However, in the conventional method for recovering fission-generated noble metals by lead melting, it is particularly difficult to separate ruthenium, which has a strong radioactivity, by itself, and it must be recovered at the same time as other noble metals. There is a problem that the operation at the time becomes extremely difficult. That is, the mutual separation by the above-mentioned dry method has such advantages that the amount of waste is small and the apparatus is compact, but the mutual separation of noble metals is extremely difficult. In the case of the wet method, when the noble metal recovered as a lead alloy is dissolved in nitric acid, palladium is dissolved.However, most of the most valuable rhodium among the noble metals becomes a residue together with ruthenium, and only the rhodium is separated. It was impossible to do. For this reason, the precious metal had to be dissolved in the solvent extraction in the next step, so that chemically stable rhodium was dissolved in the solution, and at the same time, the operation of mutually separating it from ruthenium could not be substantially performed.

In view of such circumstances, the present invention can efficiently separate ruthenium from other noble metals such as rhodium and palladium, and can form the most valuable rhodium into a solution, and purify the solution by solvent purification or the like. It is an object of the present invention to provide a method for separating each fission-produced noble metal which can be directly processed for the purpose.

[Means for solving the problem]

The method for separating fission-generated noble metal according to the present invention comprises adding bismuth or palladium when recovering the insoluble residue and the noble metal contained in the high-level waste liquid by the lead melting method, and dissolving the obtained lead alloy with nitric acid. It is performed by

In addition, bismuth or palladium added when melting lead is
The weight ratio to rhodium is in the range of 5 times or more and 100 times or less.

Further, the above bismuth or palladium is used in the form of a metal when using lead metal when melting lead, and in the form of a metal or a reducible oxide when using lead oxide when melting lead.

(Operation)

According to the method of the present invention, bismuth or palladium is added when fission-generated noble metal is recovered by lead melting,
Thereby, for example, rhodium among the noble metals forms an intermetallic compound with the added bismuth or the like and is dispersed in lead.
Furthermore, when this is dissolved in nitric acid, the added element dissolves in nitric acid, so that rhodium, which has formed an intermetallic compound, becomes fine particles and is activated to be meltable in nitric acid. on the other hand,
Since ruthenium does not form an intermetallic compound with the added element such as bismuth or lead, the ruthenium remains as a residue.

Further, the bismuth or palladium to be added, by setting the ratio to rhodium in a certain range, by further selecting the form according to the state of the lead used,
Rhodium can be separated most efficiently.

〔Example〕

Hereinafter, a first embodiment of the method for separating fission-generated noble metals according to the present invention will be described. First, the composition of the insoluble residue from which fission-generated noble metals should be recovered depends on the composition and burnup of nuclear fuel, but in the case of residues generated in light water reactors, most of them are precious metals, molybdenum (Mo) and technetium. (T
It is an alloy having a pentagonal hexagonal close-packed structure including c). An example of the ratio (%) of the atoms of the elements molybdenum, technetium, ruthenium, rhodium and palladium in these alloys is roughly Mo: Tc: Ru: Rh: Pb ≒ 20: 5: 55: 10: 10 (atomic% However, in this embodiment, a quaternary hexagonal close-packed structure alloy of molybdenum, ruthenium, rhodium and palladium is used as a simulated insoluble residue, and their ratio (atomic%) is Mo: Ru. : Rh: Pb = 20: 60: 10: 10

The lead dissolution conditions were as follows: the glass forming agent (sodium borate) was added 10 times and the metallic lead was added 50 times to the simulated insoluble residue,
Also, palladium was added to the rhodium in the insoluble residue in a weight ratio of 5 to 100 times. And
These were heated and melted at a temperature of 750 to 1100 ° C., and the obtained lead alloy was dissolved with nitric acid (3N). The results of measurement of the concentration of each noble metal element by an ICP analyzer are shown in Table 1 below.

As is clear from Table 1, the dissolution rate of rhodium was significantly higher than that in the case where palladium was not added, and most of the added palladium was dissolved. On the other hand, ruthenium is below the lower limit of detection (0.1 ppm) of the ICP analyzer regardless of the amount of palladium added, and its separation factor is a value of 10 ° or more.

Next, in the second embodiment, bismuth was added in place of the palladium of the first embodiment so that the weight ratio was 5 to 100 times that of rhodium. Table 2 below shows the measurement results obtained by the ICP analyzer of the second embodiment.

Also in the case of the second embodiment, the dissolution rate of rhodium in dissolving nitric acid was extremely high, and all ruthenium was below the lower limit of the ICP analyzer. And the separation factor of ruthenium is
The value was 10 ⁴ or more, and similar results were always obtained even when the melting temperature was changed in the range of 750 to 1100 ° C.

In the above embodiment, the element to be added at the time of melting the lead is not limited to bismuth and the like, and may be any element that does not alloy with ruthenium but alloys with rhodium or palladium. Bismuth or palladium is most suitable. In order to promote the reaction, it is preferable to mix well with an insoluble residue or a high-level waste liquid denitration material having a small particle size in order to enhance the effect.
The form of bismuth or the like to be added can be sufficiently performed in an air atmosphere or an inert atmosphere such as argon or nitrogen in the case where metallic lead is used at the time of melting the lead. When lead oxide is used, it is preferably used in the form of a metal or a reducible oxide. Furthermore, the heating temperature when melting lead is 600
Within the range of -1200 ° C, the same effects as in the above embodiment can be obtained.

〔The invention's effect〕

As described above, according to the method of the present invention, in the recovery of fission-generated noble metals in spent nuclear fuel, conventionally, various noble metals, particularly ruthenium, which could not be separated from each other, can be separately separated from other noble metals, and With the ruthenium as residue, other noble metals can be separated in a molten form with a high separation factor prior to a purification process such as solvent extraction. Therefore, the safety against radioactivity in the subsequent purification step is improved, and the purification step itself can be greatly simplified. In addition, it is necessary to convert a noble metal into a solution as a pretreatment in the purification step. However, according to the method of the present invention, there is an advantage that a solution of rhodium and palladium can be formed simultaneously with separation of ruthenium.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Masatoshi Kitagawa 547-12 Funashikawa, Tokai-mura, Naka-gun, Ibaraki Prefecture (72) Inventor Hiroshi Okada 1873-2, Toyooka, Tokai-mura, Naka-gun, Ibaraki Prefecture

Claims (3)

(57) [Claims] (1) When recovering the insoluble residue generated in the reprocessing step of spent nuclear fuel and the noble metal contained in the high-level waste liquid by the lead melting method, bismuth or palladium is added and the obtained lead alloy is treated with nitric acid. A method for separating fission-generated noble metals in which ruthenium is separated into a residue and ruthenium is separated by dissolving a noble metal such as palladium or rhodium into a solution. 2. The fission according to claim 1, wherein the bismuth or palladium to be added at the time of melting the lead has a weight ratio of the fission-generated noble metal to rhodium in the range of 5 to 100 times. Separation method of generated noble metal. 3. The bismuth or palladium to be added at the time of melting the lead is in the form of a metal when using metallic lead at the time of melting lead, or a metal or reducible when using lead oxide at the time of melting lead. The method for separating a fission-generated noble metal according to claim 1, wherein the method is used in the form of a simple oxide.