JPH06194496A - Reduction method of neptunium in fuel reprocessing solution - Google Patents

Abstract

PURPOSE:To alleviate load in waste liquid treatment by subjecting neptunium in reprocessed waste liquid to valence conditioning of tetravalence for facilitating solution extraction thereby separating neptunium having quite long half-life and toxicity from the waste liquid. CONSTITUTION:Reprocessing high level waste liquid where pentavalent neptunium and trivalent iron ion coexist is fed from a tank 1 to a conditioning tank 3 equipped with a stirrer 2 and a reducing agent for the trivalent iron ion, i.e., aqueous solution of hydroxylammonium nitrate, is added. The waste liquid is fed continuosly at a predetermined flow rate to a column 6 filled with a reduction catalyst, i.e., a platinum black catalyst, by means of a pump 5. When passing through the column 6, trivalent iron ions are reduced to bivalent iron ions which reduce neptunium to tetravalent through strong reducing function. High level waste liquid thus treated is fed to a receiving tank 7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing neptunium suitable for adjusting valence as a pretreatment for extracting and separating neptunium from a radioactive nitric acid waste liquid generated by reprocessing spent nuclear fuel.

[0002]

2. Description of the Related Art Radio Kimia 19 (5) Page 620-
625 (Radiokhimiya, 19 (5), P620-625) shows the reduction rate in the reduction of neptunium pentavalent to tetravalent with hydroxylammonium nitrate. Further, in JP-A-63-17164, as a method of reducing a dissolved substance in a nitric acid solution, a platinum-supported semiconductor photocatalyst is used as a reduction catalyst and is excited by an excited electron generated on platinum by external light irradiation. A method for reducing dissolved substances is described. According to this method, the pentavalent neptunium in the reprocessed high-level waste liquid can be efficiently reduced to tetravalent.

[0003]

The conventional direct reduction of neptunium with hydroxylammonium nitrate is as follows.
It was found that the activation energy of the reaction was large and that reduction did not substantially occur at room temperature. In order to obtain a practical reduction rate, it is necessary to raise the temperature to about 90 ° C. For example, when the waste liquid generated from the spent fuel reprocessing is targeted, the solution is nitric acid and the reaction tank etc. There is concern about the adverse effects of accelerated corrosion. In addition, the second conventional method using a semiconductor photocatalyst requires a powerful light irradiation facility, so that the apparatus becomes large,
It was also found to be problematic in terms of cost and maintenance.

An object of the present invention is to provide a method for reducing neptunium at high speed and with high efficiency in a simple device at room temperature where corrosion of a reduction reaction tank or the like can be ignored.

[0005]

[Means for Solving the Problems] The above object was produced by adding a reducing agent capable of reducing trivalent iron ions coexisting with neptunium and a reducing catalyst to reduce the trivalent iron ions.
This is achieved by reducing neptunium to tetravalent by utilizing the strong reducing action of valent iron ions.

[0006]

[Operation] In the high-level waste liquid generated from the reprocessing equipment,
It includes fission products, corrosion products such as iron, and transuranium elements (TRU) such as neptunium. TRU is front 2
It has an extremely long half-life compared to humans and its toxicity is strong, and the half-life of neptunium exceeds 2 million years. Therefore, it is considered that separation of TRU having a long half-life from the highly radioactive waste liquid can reduce the treatment and disposal of the highly radioactive waste liquid.

It is known that the neptunium in the reprocessing waste liquid is difficult to separate by solvent extraction or ion exchange because the pentavalent valence is very small in the reprocessing waste liquid and the complex formation power is very small. However, reduction to tetravalent makes it possible to separate it from the waste liquid. Pentavalent plus neptunium (NpO)
The reduction of the tetravalent plus neptunium ion with a large amount of activation energy is accompanied by the abstraction of oxygen. Therefore, high temperature conditions are required for hydroxylammonium nitrate having a weak reducing power as in the conventional method. However, divalent iron ions are the most powerful reducing agents for neptunium and reduce neptunium extremely quickly even at room temperature.

In the present invention, trivalent iron ions coexisting in the reprocessing waste liquid are reduced to produce divalent iron ions, and the divalent iron ions reduce neptunium. Three
The reducing agent for valent iron ions needs to be decomposable so as not to increase the amount of waste, and it is desirable to use hydroxylamine or hydrazine or its nitrate. However, it was found that this reducing agent, like neptunium, is slow in reducing trivalent iron ions. This bottleneck is eliminated by the simultaneous addition of a platinum catalyst. That is, the platinum catalyst is active in the hydrogen generation reaction and adsorbs hydrogen atoms on the platinum catalyst as the reducing agent decomposes. By this hydrogen atom, the trivalent iron ion is rapidly reduced by the reaction shown in Chemical formula 1, and the pentavalent neptunium is extremely rapidly reduced by the reaction of Chemical formula 2.

[0009]

[Chemical 1]

[0010]

[Chemical 2]

[0011]

EXAMPLES Examples of the present invention will be described below.

Example 1 FIG. 1 shows pentavalent neptunium,
1 shows an example of a method and apparatus for reducing neptunium in a high-level reprocessing waste liquid containing trivalent iron ions and fission products to tetravalent. The operation will be described with reference to FIG.

The high-level waste liquid generated from the reprocessing facility and stored in the high-level waste liquid storage tank 1 is sent to a solution preparation tank 3 equipped with a stirrer 2, where a predetermined amount from a hydroxylammonium nitrate aqueous solution storage tank 4 is stored. Aqueous solution of hydroxylammonium nitrate is added. Based on the operation of the pump 5, this solution is continuously sent at a predetermined flow rate to the column 6 filled with the platinum black catalyst. The trivalent iron ions contained during the passage through the column 6 are reduced to divalent, and simultaneously the divalent iron ions produced reduce the neptunium to tetravalent. The catalyst to be packed is preferably a fine particle such as platinum black catalyst in order to increase the catalyst surface area related to the reduction rate, but this is determined in consideration of the column capacity and the like, and the shape is not particularly limited. The high-level waste liquid in which all contained neptunium is adjusted to be tetravalent is sent to the receiving tank 7. This high-level liquid waste is sent to an appropriate liquid waste treatment facility such as an extraction facility for the purpose of separating neptunium from the liquid waste.

According to this embodiment, since the high level waste liquid can be continuously treated, the neptunium in the high level waste liquid can be reduced in a short time.

[0015]

INDUSTRIAL APPLICABILITY According to the present invention, since the valence of neptunium in the reprocessing waste liquid can be easily adjusted by solvent extraction, the valence can be adjusted so that neptunium having a very long half-life and strong toxicity can be separated from the waste liquid. This can reduce the processing load of the waste liquid.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

[Explanation of symbols]

1 ... High-level waste liquid storage tank, 2 ... stirring device, 3 ... solution preparation tank, 4 ... hydroxyammonium nitrate aqueous solution storage tank, 5 ... pump, 6 ... column filled with platinum black catalyst, 7
… Receiving tank.

Claims (4)

[Claims] 1. A trivalent iron ion is reduced by adding a trivalent iron ion reducing agent and a reduction catalyst to a fuel reprocessing solution in which pentavalent or hexavalent neptunium and trivalent iron ions coexist. A method for reducing neptunium in a fuel reprocessing solution, which comprises reducing neptunium to tetravalent by the generated divalent iron ions. 2. A method for reducing neptunium in a fuel reprocessing solution, wherein the reducing agent according to claim 1 is hydroxylamine, hydrazine, salts thereof, or a mixture thereof. 3. A method for reducing neptunium in a fuel reprocessing solution, wherein the reduction catalyst according to claim 1 is a catalyst composed of a platinum group element. 4. A method for reducing neptunium, wherein the fuel reprocessing solution according to claim 1 is a highly radioactive nitric acid waste liquid.