JPH0723917B2 - Treatment method of recovered nitric acid from nuclear fuel reprocessing step - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improved method for treating recovered nitric acid from a nuclear fuel reprocessing step.

[0002]

2. Description of the Related Art A typical process for reprocessing spent nuclear fuel is to remove iodine in a pretreatment step,
NO _x for Pu oxidation in plutonium purification process
Is used to recover nitric acid in the form of waste liquid.

Since the recovered nitric acid contains radioactive substances at a low level, it is neutralized with caustic soda, converted into sodium nitrate, evaporated and concentrated, and finally stored as a solidified product. There is. Since such a treatment method generates a large amount of solid waste having a low level of radioactivity, improvement has been desired.

The applicant decomposed the recovered nitric acid, particularly electrolyzing it to generate NO _x , and utilizing this in the reprocessing step, the generation of solid waste, which was inevitable in the above-mentioned prior art, was generated. We have invented a system that eliminates this, thereby reducing the amount of waste, and has already disclosed it (Japanese Patent Application No.
231847).

The Applicant has also used a catalyst in case the treatment of the recovered nitric acid has to be carried out independently of the reprocessing step and therefore the NO _x generated by the decomposition of the nitric acid cannot be reused. Establishing a method to reduce the amount of radioactive waste that is sent to the waste treatment process, while making NH ₃ act on NO _x to reduce it and change it to N ₂ to detoxify and release it, I also disclosed it (Japanese Patent Application No. 2)
-231848).

The inventors of the present invention have combined the above new findings to treat the nitric acid recovered from the nuclear fuel recycling process,
Depending on the situation in which the treatment process is carried out, the NO _x that is the product of the treatment is supplied to the retreatment process for reuse, or it is detoxified and released, and finally sent to the waste treatment process. Further research was carried out with the intention of providing a treatment method with a reduced amount of waste that should be in the form of a solidified body, and in particular to provide an established electrolysis technique for recovered nitric acid, and as shown in FIG. Whether the recovered nitric acid from the nuclear fuel reprocessing step is introduced into the electrolytic cell and electrolyzed under the condition that the HNO ₃ concentration in the electrolytic solution does not fall below 5M, and the generated NO _x is supplied to the reprocessing step for reuse. , Or using a catalyst to react with NH ₃ for reduction treatment to detoxify and then release, dilute nitric acid from the electrolytic cell is concentrated by distillation, and part or all of the concentrated nitric acid is circulated to the electrolytic cell. A system consisting of electrolyzing again Stand City, which was also already proposed (Japanese Patent Application No. 2-235023).

[0007]

SUMMARY OF THE INVENTION The first object of the present invention is to improve the current efficiency in the electrolysis of nitric acid carried out by these treatment techniques and to effectively use the energy. The second purpose is to carry out electrolysis with high current efficiency regardless of the amount of nitric acid to be electrolyzed.
The third purpose is to obtain a mixture of NO _x and O ₂ generated by electrolysis in an arbitrary ratio.

[0008]

The method for treating recovered nitric acid according to the present invention, which achieves the first object described above, introduces the recovered nitric acid from the nuclear fuel reprocessing step into an electrolytic cell so that the concentration of HNO ₃ in the electrolytic solution is increased. Electrolysis is performed under the condition of not less than 5M, and the generated NO _x is supplied to the reprocessing step for reuse, or is reacted with NH ₃ using a catalyst for reduction treatment to render it harmless and then released. , Dilute nitric acid from the electrolytic cell is concentrated by distillation, and a part or all of the concentrated nitric acid is circulated to the electrolytic cell to electrolyze again, and the electrolytic solution in the electrolytic cell is fed at a linear velocity of 1 m / h or more. It is characterized in that electrolysis is performed while circulating.

In the method for treating recovered nitric acid of the present invention which achieves the second and third objects, the recovered nitric acid from the nuclear fuel reprocessing step is introduced into the electrolytic cell so that the HNO ₃ concentration in the electrolytic solution is 5%.
NO _x generated by electrolysis under conditions not lower than M
Is supplied to the reprocessing step for reuse, or is reacted with NH ₃ using a catalyst to reduce and detoxify and then release the diluted nitric acid from the electrolytic cell by distillation to concentrate concentrated nitric acid. Part or all of the above is circulated in the electrolytic cell for electrolysis again, and the electrolysis is performed by disposing a cation exchange membrane as a diaphragm between the electrodes.

As the electrode material of the electrolytic cell, a non-dissolving material, typically platinum, is used as the anode. The cathode is arbitrary as long as it can withstand the electrolytic solution, but graphite is preferable.

[0011]

As described above with respect to the invention, the concentration of nitric acid to be electrolyzed to 5 M or more suppresses the increase in bath voltage and the accompanying generation of N ₂ O and H ₂ , thereby reducing NO _x gas. Is a condition necessary to selectively generate. As described in the previous application, the concentration of the excess nitric acid recovered from the nuclear fuel reprocessing step is about 10N,
This nitric acid is directly electrolyzed until the nitric acid concentration decreases to about 5N, concentrated to about 10N by distillation, and circulated as shown in the flow chart of FIG.

The NO _x gas generated by electrolysis is so-called dry denitration technology, that is, ammonia is added to NO _x , and catalytically reduced at a temperature of 200 to 450 ° C. using a noble metal catalyst to form N ₂ . To make it harmless.

In the distillation apparatus, nitric acid-containing water is distilled out from the top of the column, and concentrated nitric acid is obtained from the bottom of the column.
Since water does not contain radioactive substances, it can be released with the necessary neutralization. The concentrated nitric acid can be circulated in the electrolytic cell and electrolyzed again for treatment, but a part of the concentrated nitric acid must be extracted and discarded. Since this waste liquid is radioactive, it should be treated according to the radioactive waste treatment process.

The cause of the lower current efficiency in the electrolysis of nitric acid is that the reaction (3) is more likely to proceed than the following electrode reactions (1) and (2), and (cathode) before NO ₂ Body + e = NO ₂ ↑ (1) (Anode) O ₂ precursor = O ₂ ↑ + e (2) (In liquid) NO ₂ precursor + O ₂ precursor = H + NO ₃ ~
(3) It is conceivable that NO _x or an intermediate product (not identified), which is an electrolytic product on the cathode side, diffuses in the electrolytic solution and is oxidized on the anode side.

Therefore, it is effective to prevent the cathode reaction product from being reoxidized at the anode by circulating the electrolyte and moving it at a flow rate higher than a certain limit.
When a membrane that can prevent the movement of ions such as a cation exchange membrane or a membrane that can suppress the diffusion of NO _x produced by reduction is used, the effect of preventing reoxidation does not depend on the flow of the electrolytic solution. can get. That is, using the same equipment, it is possible to always achieve high current efficiency regardless of the amount of liquid to be treated.

Since the use of the diaphragm makes it possible to separately take out NO _x generated at the cathode and O ₂ generated at the anode, in recycling NO _{x to} the nuclear fuel reprocessing step,
NO _x / O ₂ mixture in any proportion can be supplied.

[0017]

EXAMPLE An experimental apparatus having the flow shown in FIG. 2 was assembled.
A direct current is supplied to the electrolytic cell (1) from a constant current / constant voltage power supply (2), and the anode (11) is made of platinum and the cathode (12).
The black nitric acid was used as the material of (1) to electrolyze the aqueous nitric acid solution at a current density of 0.05 A / cm ² . A sulfonic acid type cation exchange membrane is used as the material of the diaphragm (13) to divide the electrolytic cell into two chambers, and the electrolytic solution is circulated by the circulation pumps (3) and (4).
It was circulated between the electrolyzer and tanks (5) and (6).

Nitrogen was supplied as a carrier gas into the tank from the constant flow valves (14) and (15), and the generated gas was taken out. The gas is cooled by the coolers (16) and (1
After removing the mist in 7), a part of the mist was taken out through the constant flow valve (18), and the composition and concentration were measured by the NO _x meter (7). The rest of the gas is the ozone generator (8)
After the total amount was oxidized by the ozone generated in 1., it was absorbed by an alkali trap (9) containing an aqueous solution of caustic soda, and the absorption liquid was titrated to determine the NO _x amount.

A circulating flow rate was changed within a range of LV (linear velocity) of 0.2 to 2.5 m / h with an aqueous solution having a nitric acid concentration of 8 M as an object of electrolysis. The total amount of NO _x generated was calculated both with and without the diaphragm.

The results are shown in FIG. When a diaphragm is not used, a high current effect can be obtained in the flow velocity range of LV = 1.0 m / h or more, but in the flow rate range of LV = 1.0 m / h or more, the current efficiency decreases as the flow rate decreases. On the other hand, when the diaphragm is used, high current efficiency is obtained regardless of the circulation flow rate.

[0021]

According to the method of the present invention, the effect of the present invention proposed previously, i.e. to generate NO _x is decomposed to recover nitric acid from nuclear fuel reprocessing step by electrolytic reprocessing in accordance with this situation It can be supplied to the process for reuse, be harmless by catalytic reduction and be released into the atmosphere, or can be arbitrarily selected, and dilute nitric acid produced by electrolysis can be concentrated by distillation and circulated to an electrolytic cell for re-electrolysis. By targeting, it is possible to enjoy the advantage that the amount of radioactive waste to be finally solidified can be significantly reduced, and it is possible to meet the demands for energy saving and treatment cost reduction by realizing high current efficiency.

According to the method of the present invention in which a diaphragm is used, the flow rate of the electrolytic solution can be arbitrarily set, so that the same equipment can be used regardless of the amount of recovered nitric acid to be treated. This simplifies the equipment and reduces the equipment cost. Further, as a result of separating the cathode and the anode, the respective generated gases can be recovered separately, and by adjusting the NO / O ₂ ratio, the ratio of NO / NO ₂ to be returned to the nuclear fuel reprocessing facility and reused is adjusted. be able to.

[Brief description of drawings]

FIG. 1 is a flowchart showing steps of a processing method of the present invention.

FIG. 2 is a diagram showing the configuration of an apparatus for which the method of the present invention was tested.

FIG. 3 is data of electrolysis of nitric acid according to an embodiment of the present invention, showing NO _{x with} respect to a linear velocity of an electrolytic solution flow in an electrolytic cell.
The graph which shows the generation amount and cathode current efficiency.

[Explanation of symbols]

1 electrolytic cell 2 constant-voltage constant-current power supply 3 and 4 circulating pump 5,6 tank 7 NO _x eight ozone generator 9 lye 11 anode 12 cathode 13 diaphragm 16, 17 mist separator 14, 15 and 18 constant flow valve

 ─────────────────────────────────────────────────── --Continued from the front page (56) References JP-A-4-110795 (JP, A) JP-A-4-110796 (JP, A) JP-A-4-115197 (JP, A) JP-B Sho-60- 38999 (JP, B2)

Claims (3)

[Claims] 1. The recovered nitric acid from the nuclear fuel reprocessing step is introduced into an electrolytic cell, electrolysis is performed under conditions where the HNO ₃ concentration in the electrolytic solution does not fall below 5 M, and the generated NO _x is supplied to the reprocessing step. It is reused as a catalyst or is reacted with NH ₃ using a catalyst to reduce the detoxification after detoxification and then released. The diluted nitric acid from the electrolytic cell is concentrated by distillation, and part or all of the concentrated nitric acid is electrolyzed. A method for treating recovered nitric acid from a nuclear fuel reprocessing step, characterized in that the electrolytic solution is circulated in a tank and electrolyzed again, and electrolysis is performed while circulating an electrolytic solution in the electrolytic cell at a linear velocity of 1 m / h or more. . 2. The nitric acid recovered from the nuclear fuel reprocessing step is introduced into an electrolytic cell, electrolysis is performed under conditions where the HNO ₃ concentration in the electrolytic solution does not fall below 5 M, and the generated NO _x is supplied to the reprocessing step. It is reused as a catalyst or is reacted with NH ₃ using a catalyst to reduce the detoxification after detoxification and then released. The diluted nitric acid from the electrolytic cell is concentrated by distillation, and part or all of the concentrated nitric acid is electrolyzed. A method for treating recovered nitric acid from a nuclear fuel reprocessing step, which comprises circulating in a tank to perform electrolysis again and disposing a cation exchange membrane as a diaphragm between electrodes to perform electrolysis. 3. The treatment method according to claim 1 or 2, which is carried out by using platinum as the anode material and graphite as the cathode material of the electrolytic cell.