JP2839585B2 - Method and apparatus for reducing neptunium - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to an improvement in the extraction efficiency of neptunium in the process of reprocessing spent nuclear fuel. And a method and apparatus suitable for reduction to

[Conventional technology]

Conventionally, as a method for reducing dissolved substances in a nitric acid solution, a semiconductor photocatalyst powder supporting a platinum group element is added to a solution as described in JP-A-63-171641, and the photocatalyst is irradiated with electromagnetic waves. There is a method of reducing dissolved substances by exciting the same. In addition, as a method for reducing transuranium elements using divalent ions of iron as a reduction catalyst, Solvent Extraction and Ion Extraction, Vol. 6, 1988, pp. 247 to 263 (Solv.Extr.Ion Exch. ,
6 , (1988), pp. 247 to 263). Further, neptunium is disclosed in JP-A-54-53609.
The aqueous phase containing a reducing agent for neptunium selected from the group consisting of hydroxylamine salts, hydrazine salts, and a mixture of hydroxylamine salts and hydrazine salts in an organic solvent containing plutonium, uranium, and neptunium as described in It is known that, by contacting at about 30 to 35 ° C., neptunium having a valence of 6 is reduced to pentavalent and the organic phase is transferred to an aqueous phase. Also, Journal of New Clear Science and Technology, Vol. 24 (1987), pp. 724-729 (J. Nu
cl.Sci.Tech., 24 , (1987) p724-729) discusses a method of reducing pentavalent valent neptunium to tetravalent using hydroxylamine hydrochloride as a reducing agent. In order to obtain a practical reduction rate,
It is indicated that the temperature needs to be raised to 70 ° C. or higher.

[Problems to be solved by the invention]

Among the above prior arts, the reduction method using a semiconductor photocatalyst requires a powerful electromagnetic wave irradiation equipment in order to carry out the reaction with an efficiency suitable for practical use, and has practical problems in terms of cost and maintenance. Further, in the method using divalent iron ions as a reducing agent, the amount of metal elements as waste is increased, and the amount of treatment at the time of radioactive waste treatment is increased, resulting in high cost. In the method using a hydroxylamine salt, a hydrazine salt, or a mixture of both, as a reducing agent, at a reaction temperature of 30 to 35 ° C., neptunium having a valence of 6 is reduced only to pentavalent, and an organic solvent is removed from the aqueous phase. It is not possible to obtain tetravalent neptunium which is easily extracted into water and has a great industrial advantage.
In order to reduce the neptunium valence up to tetravalent at a practical rate using hydroxylamine salts, the reaction temperature must be 70
It is necessary to maintain the temperature at not less than ℃, and there is a problem that the corrosive environment of the reactor is deteriorated.

In view of the above, an object of the present invention is to provide a method and an apparatus for reducing neptunium which do not require special equipment such as an electromagnetic wave irradiation equipment and do not deteriorate the corrosive environment of the equipment. It is another object of the present invention to provide a method and an apparatus for reducing neptunium without increasing the amount of metal as radioactive waste.

[Means for solving the problem]

To achieve the above object, hydrazine, hydroxylamine, or a substance that is easily decomposed without metal like hydrazine, hydroxylamine, or their hydrates or salts under mild conditions at atmospheric temperature that does not deteriorate the corrosive environment Is added as a reducing agent and a platinum group element or rhenium having a high proton affinity as a reducing catalyst to the neptunium-containing solution, thereby efficiently reducing the valence of neptunium from pentavalent to tetravalent without performing electromagnetic wave irradiation. It provides a means.

Further, when the present invention is applied to the reprocessing step of spent nuclear fuel, by utilizing the platinum group element in fission products coexisting with neptunium in the solution of nuclear fuel, the platinum group element as a reduction catalyst It is intended to provide a means for reducing the amount of used metal, suppressing an increase in metal waste, and reducing costs.

[Action]

 Hereinafter, the basic principle of the present invention will be described.

In the reprocessing process of spent nuclear fuel, the aqueous nitric acid solution obtained in the cutting and dissolving step of nuclear fuel contains neptunium, plutonium, uranium and fission products,
It contains transuranium elements such as americium and kyurium. These transuranium elements are elements classified into the actinide group, and their ions have a wide valence from +3 to +6, and the ionic form corresponds to each valence when the actinide element is represented by M. _{^{M 3+, M 4+, MO 2}} + and MO ₂
²⁺ . In aqueous solution obtained by reprocessing, uranium mainly hexavalent UO ₂ ^2+, neptunium pentavalent N _P O ₂ ⁺ and hexavalent N _P O ₂ ^2+, plutonium tetravalent Pu ^4+, Americium and Kyurium are trivalent Am ³⁺ and Cm ^{3+, respectively.}
Exists in the valence state of Of these, pentavalent neptunium N _P O ₂ all except ⁺ ions are easily extracted Te cowpea in an organic solvent such as t-butyl phosphate _{(TBP), N P O 2} + various organic solvents, ionic It is difficult to extract by the exchanger and difficult to separate from the aqueous solution. However, it has already been shown that the valence adjustment of pentavalent neptunium to hexavalent or reduction to tetravalent by valence adjustment significantly improves the partition coefficient for organic solvents and facilitates separation from an aqueous solution. Are known. Therefore, a method suitable for adjusting the valency of hexavalent or tetravalent by oxidizing or reducing pentavalent neptunium is used before the extraction step of the reprocessing process of spent nuclear fuel, or from wastewater of the same process. It is effectively used in the neptunium separation step.

The reaction of oxidizing neptunium to adjust the valence to hexavalent has a relatively small activation energy, and the reaction easily proceeds with a normal oxidizing agent. However, when reacting with an aqueous solution of neptunium nitric acid obtained in the reprocessing process, nitric acid generates nitrite by radiolysis at a high radiation dose rate, and the nitrite converts hexavalent neptunium to pentavalent. To reduce. That is, since the reaction is signaled in the opposite direction, which is not preferable, it is necessary to remove nitrous acid. However, nitrous acid has the effect of reducing highly oxidative fission product ions generated by the oxidizing power of nitric acid, and has the effect of suppressing the corrosion of equipment due to these highly oxidizable ions. This leads to deterioration of the corrosive environment of the equipment, which is not preferable.

On the other hand, the reaction for adjusting valence by reducing pentavalent neptunium to tetravalent is represented by the following equation.
The reaction temperature is as high as 100 kJ / mol, and the reaction temperature is extremely slow at room temperature.

_{^{N P O 2 + + 4H +}} + e - = N P 4+ 2H 2 O ... (1) (E 0 = 0.7391V) for reducing the N _P O ₂ ⁺ efficiently N _P ⁴⁺ at room temperature the catalyst or the like Although it is necessary to lower the activation energy by using hydrogen, the present inventor has found that the catalytic action of a hydrogen atom is effective. The mechanism is considered as follows.

^{_{N P O 2 + + H →}} N P OOH + ... (2) N P OOH + + 3H + → N P 4+ + 2H 2 O ... (3) The present inventors, based on these considerations, a strong platinum proton affinity fine powder rows N _P O ₂ ⁺ the reduction experiment to N _P ⁴⁺ a hydroxylammonium nitrate with (platinum black) as the reducing agent Natsuta. As a result, as shown in FIG. 1, when no catalyst was used at room temperature, the reduction reaction proceeded very slowly, but it was confirmed that the reduction reaction proceeded promptly by adding the platinum catalyst. This phenomenon can be considered that the protons adsorbed on the platinum catalyst surface with strong affinity induced the reactions represented by the formulas (2) and (3) on the catalyst surface.

As an effective material other than platinum as N _P O ₂ ⁺ of the reduction catalyst, platinum as well as proton affinity strong iridium, can be cited osmium, palladium, rhodium, platinum group elements and rhenium containing ruthenium.

Further, the shape of the catalyst is desirably one having a large specific surface area such as fine particles or sponge in order to promote efficient reaction.

As the reducing agent, a substance which does not contain a metal in the component and is easily decomposed is preferable from the viewpoint that the disposal amount of radioactive waste is not increased, and an amine compound such as hydrazine and hydroxylamine, or a hydrate or salt thereof. , And mixtures thereof are preferred.

In addition to using a fine-particle or spongy catalyst for efficient catalytic reaction, a fine-powder catalyst suspended in the liquid is captured by a filter, and the reaction solution is circulated through the filter. Various embodiments can be devised, such as a method of contacting, a method of circulating the reaction solution into a particulate or spongy catalyst, or a column packed with a substance carrying the catalyst, etc. is not.

〔Example〕

Hereinafter, an embodiment of the present invention will be described with reference to FIGS.

Example 1 A high-valent neptunium present in an aqueous nitric acid solution was reduced to tetravalent by using hydroxylamine nitrate as a reducing agent and platinum black as a reducing catalyst.
This will be described with reference to FIG. 2 and FIG.

FIG. 2 shows a basic configuration of an apparatus used for reducing neptunium in a solution containing neptunium. Neptunium-237 was introduced into a reaction vessel 1 having an internal volume of 10 cm ³ via a neptunium-containing solution inlet 2 at 1 × 10 ⁻³ mol /
3 cm ³ of the contained 3 N nitric acid solution was injected to obtain a reaction solution. Next, the virus tester is passed through the catalyst addition port 3. 0.015 g of platinum black produced according to the above method was added to the reaction solution as a catalyst. The propeller 5 driven by the electric motor 4 is rotated to stir the catalyst in a state of being suspended in the reaction solution, and thereto is added a 40% aqueous solution of hydroxylammonium nitrate 3N nitric acid at 1 cm ³ at room temperature (20 ° C.). It was dropped from the reducing agent inlet 6. After the reduction reaction is performed by continuing stirring for a certain period of time from the end of the dropping, the reaction solution is taken out from the reaction solution outlet 8 through the filter 7 and centrifuged at 3000 rpm for 2 minutes to remove fine platinum black. Thereafter, the concentration of neptunium in the reaction solution was quantitatively analyzed for each valence by absorption spectroscopy. The extinction coefficient used for the measurement of the concentration of neptunium by valence varies depending on the nitric acid concentration, but in the case of a 3N nitric acid solution, 58 mol ^-1 · l · cm ⁻¹ and 62 mol ⁻¹ · cm ⁻¹ , and 3 for a pentavalent neptunium at a wavelength of 981 nm.
It is 00 mol ^-1 · l · cm ^-1 . The absorption wavelength and absorption coefficient of hexavalent neptunium are 1260 nm and 42 mol ^-1, respectively.
L · cm ^-1

Pentavalent Neptunium N _P in the reaction solution after the start of the reaction
(V) The results of measuring the change over time in the concentration are shown in FIG. In a 3N nitric acid solution, neptunium has a valence of 6 and a value of 80.
Although pentavalent accounts for 20%, the concentration of pentavalent neptunium in FIG. 1 is quintuple and tetravalent neptunium in FIG. Expressed as a percentage. The figure also shows the time-dependent measurement of the concentration of pentavalent neptunium when a neptunium-containing solution and hydroxylammonium nitrate were reacted at room temperature (20 ° C.) without a catalyst to clarify the catalytic effect of platinum black. .

In FIG. 1, in the case of no catalyst as a conventional example, the reduction reaction hardly proceeds at room temperature (20 ° C.), and only 2 to 3% of tetravalent neptunium is produced 20 minutes after the start of the reaction. On the other hand, when the platinum black according to the present invention was used as a catalyst, pentavalent neptunium was all reduced to tetravalent in 20 minutes after the start of the reaction, and the effect of adding the catalyst was remarkable. is there.

Example 2 An example of a waste liquid after separation of uranium and plutonium in a process for reprocessing spent nuclear fuel will be described with reference to FIG.

The effluent of the reprocessing process contains fission products and the like, and an example of its concentration is 4.5 g in total per effluent. This content contains about 2% of the platinum group element palladium, about 1% of rhodium and about 5% of ruthenium. The waste liquid has a nitric acid concentration of 2 to 3 normal and contains neptunium at a concentration of about 5 × 10 ⁻³ mol /. An example in which pentavalent neptunium contained in the waste liquid is reduced to tetravalent by using the waste liquid having such properties as a reaction liquid will be described with reference to FIG.

The waste liquid of the reprocessing process is injected into the reaction vessel 1 through the neptunium-containing solution injection port 2. An excess reducing agent is dripped through a reducing agent inlet 6 while rotating the propeller 5 driven by the electric motor 4 and stirring the waste liquid. At this time, the platinum group element in the reaction solution is reduced and precipitates as a fine metal in the reaction solution. Further, the reaction solution is circulated by a circulation pump 9 through a catalyst contactor 10 having a filtration mechanism, so that fine metals deposited in the reaction solution are removed from the catalyst contactor.
Capture on 10 filtration surfaces. The concentration of the platinum group element precipitated in the reaction solution is low and the catalytic effect is not necessarily high when the platinum group element is dispersed in the entire reaction solution. By circulating the reaction solution containing neptunium and excess reducing agent, neptunium can be efficiently reduced to tetravalent.

In this example, in order to improve the reduction efficiency of neptunium, not only a catalyst generated by reduction of a platinum group element present as a fission product in a reaction solution, but also a platinum group metal or rhenium metal was added. Can also be used. Further, even if the catalyst contactor 10 is replaced with a column filled with a substance supporting a platinum group metal or rhenium metal, the effect of the present invention is not lost.

〔The invention's effect〕

According to the method for reducing neptunium according to claim 1 or 3, in the solution containing neptunium,
The pentavalent or higher valent state of neptunium can be converted to a valence of four under normal atmospheric temperature without the need for special equipment such as electromagnetic wave irradiation equipment and without a significant increase in radioactive metal waste. Thus, a simple means capable of rapid reduction can be realized.

Neptunium having a valence of 4 is stably present in a solution due to a high activation energy of an oxidation reaction from tetravalent to pentavalent, and its partition coefficient to an organic solvent is about 100 times larger than that of pentavalent neptunium. Therefore, by applying the present invention to the stage preceding the neptunium extraction step in the spent nuclear fuel reprocessing process, the extraction efficiency is dramatically improved. Also, in the step of washing the waste liquid of the reprocessing process with an organic solvent to separate and remove trace amounts of transuranium elements, the present invention is applied to the preceding stage of the step, so that conventional separation and removal are difficult. In addition, it is possible to efficiently separate and remove the neptunium, which eliminates the necessity of long-term management of waste liquid due to the remaining neptunium having a long half-life, which is advantageous in terms of both economy and safety.

Further, according to the method for reducing neptunium according to claim 3, in a solution in which neptunium and a fission product coexist like a solution of spent nuclear fuel, a platinum group element present in the solution as a fission product is reduced. By implementing the means for using as a catalyst, the cost required for the catalyst is reduced, and the cost of the reduction step is reduced, which is economically advantageous. The neptunium reduction device according to claim 6 is suitable for implementing the neptunium reduction method according to claim 1, and provides a neptunium reduction device advantageous in both economical efficiency and safety.

An apparatus for reducing neptunium having a bypass passage for circulating a reaction solution having a catalyst contactor according to claim 7, which realizes a local high concentration of the catalyst in the reaction solution, and therefore, the catalyst which is extremely extremely thin on average. It is possible to carry out a reduction reaction with a reaction solution having a concentration, and particularly to provide an apparatus suitable for carrying out a reduction reaction using a platinum group element in a fission product as a catalyst, Great economic effect.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the change over time of the concentration of pentavalent neptunium in a reaction solution when a reaction for reducing pentavalent neptunium to tetravalent was carried out by the method described in Example 1 of the present invention. FIG. 2, FIG. 2 and FIG. 3 are diagrams each showing the configuration of the reaction apparatus described in one embodiment of the present invention. 1 ... reaction vessel, 2 ... neptunium-containing solution inlet,
3 ... catalyst addition port, 4 ... motor, 5 ... propeller, 6
...... Reducing agent inlet, 7 ... Filter, 8 ... Outlet, 9
… Circulation pump, 10… Catalyst contactor.

Continuation of the front page (56) References JP-A-48-38297 (JP, A) JP-A-49-58020 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) G21C 19 / 46 C22B 60/00

Claims (11)

(57) [Claims] 1. A method for reducing neptunium in a solution containing neptunium, wherein a substance having high proton affinity is used as a catalyst and hydrazine, hydroxylamine, a hydrate or salt thereof, or a mixture thereof is used as a reducing agent. A method for reducing neptunium, comprising: 2. The substance having a high proton affinity includes platinum, iridium, osmium, palladium, rhodium,
2. The method for reducing neptunium according to claim 1, wherein a platinum group element or rhenium comprising one or a combination of plural elements selected from ruthenium is used. 3. A spent nuclear fuel aqueous solution in which neptunium and a fission product coexist in reducing neptunium by using a platinum group element present as a fission product in the aqueous solution as a catalyst, comprising hydrazine, hydroxylamine,
A method for reducing neptunium, comprising using a hydrate or salt thereof or a mixture thereof as a reducing agent. 4. An aqueous solution in which neptunium and a fission product coexist as ions, hydrazine, hydroxylamine,
4. The method according to claim 3, wherein the platinum group element contained in the fission product is precipitated as a metal by causing the hydrate or salt thereof, or a mixture thereof to act as a reducing agent, and used as a catalyst. Neptunium reduction method. 5. The catalyst according to claim 3, wherein a platinum group element of a fission product contained in the spent nuclear fuel and a platinum group element other than the fission product are mixed and used as a catalyst.
The method for reducing neptunium according to the above. 6. A neptunium reduction apparatus characterized in that a catalyst addition port is provided in a neptunium reduction reaction vessel. 7. The apparatus for reducing neptunium according to claim 1, further comprising a bypass passage for circulating the reaction solution in the reaction vessel through the outside of the reaction vessel into the reaction vessel, and a catalyst contactor is provided in the middle of the passage. And a reduction device. 8. The reduction device according to claim 7, wherein a packing of a carrier holding at least one metal of a platinum group element and rhenium is used as the catalyst contactor. 9. The reduction device according to claim 7, wherein a packing of a particulate matter comprising at least one metal of a platinum group element and rhenium is used as the catalyst contactor. 10. The reduction device according to claim 7, wherein a filling of a spongy substance comprising at least one metal of the platinum group element and rhenium is used as the catalyst contactor. 11. A catalyst contactor having a filtration mechanism, wherein the reaction solution is circulated and passed through the filtration surface to remove fine particles of at least one metal of the platinum group element and rhenium present in the reaction solution. The reduction device according to claim 7, wherein a filter that has been collected is used.