KR102094481B1 - Method for reducing spent nuclear fuel and apparatus for reducing spent nuclear fuel - Google Patents

Abstract

The present invention relates to a spent fuel reduction method and a reducing apparatus, and more particularly, to a spent fuel reduction method and a reducing apparatus capable of efficiently processing large-capacity spent fuel.
The present invention proposes a new process for reducing a large-capacity oxide spent fuel corresponding to one or more aggregates to metal, thereby simplifying the structure of each device, so that a large-capacity process can be easily performed in a hot cell environment. In particular, since there are no restrictions on the batch capacity and shape, there is an advantage in improving the safety measures of spent fuel and linkage with post-war processes.

Description

Method and reduction device for spent nuclear fuel {METHOD FOR REDUCING SPENT NUCLEAR FUEL AND APPARATUS FOR REDUCING SPENT NUCLEAR FUEL}

The present invention relates to a spent fuel reduction method and a reduction device, and more particularly, to a spent fuel reduction method and a reduction device capable of efficiently processing large-capacity spent fuel.

The reduction process of converting oxide spent fuel to metal fuel is one of the core technologies of pyroprocessing, and many countries are conducting related studies. The Korea Atomic Energy Research Institute has attempted a chemical reduction process using Li metal, but has introduced an electrolytic reduction process that solves this due to the difficulty of separation of Li ₂ O generated after the reduction process. The electrolytic reduction process is a process of converting spent nuclear fuel to metal using Li generated by decomposing Li ₂ O into Li and O ₂ by adding electricity after adding Li ₂ O to the LiCl salt. At this time, since the generated Li is used for the conversion of the spent fuel with oxide, and at the same time performs an equilibrium reaction with Li ₂ O, the produced Li must not be close to the reduction reaction until the distance between the cathode electrode where Li is produced and the spent fuel with oxide is close Mainly consumed and can be prevented from being consumed by an equilibrium reaction with Li ₂ O. Therefore, as the size of the basket containing oxide spent fuel increases, the utilization of Li decreases. To solve this, the spent fuel must be divided into small baskets and electrodes must be connected. That is, an anode electrode should be disposed between a plurality of basket cathodes as in the previously published patent (application number 10-2013-7016171), and a shroud structure (application number 10-2013) to prevent corrosion of the device from generated oxygen -7016169) is required. In addition, due to the safety of the anode electrode, expensive platinum is used, which is a major cause of cost increase. Therefore, the current technology requires an expensive and complex electrolytic reduction device for the treatment of large-scale oxide spent fuel, which is desirable in terms of durability and ease of operation when considering remote operation of a hot cell due to high radioactivity of spent fuel. Can't.

A method using a carbon anode as a new electrolytic reduction process has been proposed (J. Radioanal. Nucl. Chem., 310, 463 (2016) & Nucl. Eng. Technol. 49, 1451 (2017)). When a carbon anode is used, LiCl is electrolyzed in the absence of Li ₂ O to produce Li and Cl ₂ , and Li is not consumed in the equilibrium reaction with Li ₂ O, but is used to reduce the fuel after the use of oxide to increase efficiency. have. However, the existing documents still employ a reactor structure using a platinum anode, and the structural complexity of the apparatus for processing large-scale oxide spent fuel described above has not been solved.

Republic of Korea Patent Publication No. 10-2013-0143612

The present invention provides a new method of reducing spent fuel and a reduction device of a new method used to reduce a large-capacity oxide spent fuel corresponding to one or more aggregates to metal.

The present invention, the step of charging the spent fuel loaded basket in the reactor (step a); Removing the basket after mounting a first cover to the reactor and performing a spent fuel reduction process (step b); And performing a lithium oxide electrolytic process (step c) after replacing the first cover with a second cover, wherein the first cover is a salt evaporation prevention cover, and the second cover is an electrode for electrolysis. It provides a method for reducing spent fuel, characterized in that it comprises.

In addition, the present invention, using a plurality of reactors as a method for reducing spent fuel performed in a multi-stage operation, comprising the steps of charging the spent fuel loaded basket in the first reactor; Mounting the first cover on the reactor and performing a spent fuel reduction process, and then moving the basket to the second reactor; And performing a lithium oxide electrolytic process after replacing the first cover with the second cover in the first reactor, and performing a spent fuel reduction process by mounting the first cover in the second reactor. The first cover is a salt evaporation prevention cover, and the second cover provides a spent fuel reduction method comprising an electrode for electrolysis.

In addition, the present invention, a reactor in which the spent fuel reduction process and the lithium oxide electrolytic process are sequentially performed; And a first cover and a second cover of the reactor, wherein the cover is provided in a replaceable form, and the first cover is used in the process of reducing the spent fuel to prevent salt evaporation, The second cover includes an electrode for electrolysis, and provides a spent fuel reduction device, which is used in a lithium oxide electrolysis process.

 The present invention proposes a new process for reducing a large-capacity oxide spent fuel corresponding to one or more aggregates to a metal, thereby simplifying the structure of each device, so that a large-capacity process can be easily performed in a hot cell environment. In particular, since there are no restrictions on the batch capacity and shape, there is an advantage in improving the safety measures of spent fuel and linkage with post-war processes.

More specifically, the present invention can improve process efficiency by sequentially performing a reduction of spent nuclear fuel and a lithium oxide electrolytic process. In addition, by providing a plurality of reactors and performing a multi-stage operation reduction process, a large-capacity spent fuel can be reduced more efficiently. In addition, by providing a plurality of reactors, when one or more of the reactors need to be repaired due to failure or malfunction, the effect on the entire process is minimized. In addition, by replacing the cover of the reactor, it is possible to sequentially perform the reduction of used fuel and the lithium oxide electrolysis process in one reactor, thereby simplifying the structure of the apparatus.

1 is a multi-stage operation reduction process flow diagram according to an embodiment of the present invention.
2 is a configuration diagram of a reducing device during a spent fuel reduction process according to an embodiment of the present invention.
3 is a configuration diagram of a reduction device in a lithium oxide electrolytic process according to an embodiment of the present invention.
4 is a graph showing changes in the reduction rate for each reduction batch according to the lithium (Li) concentration.

Hereinafter, the present invention will be described in more detail through drawings and examples. The objects, features, and advantages of the present invention will be readily understood through the following examples. The present invention is not limited to the embodiments described herein, and may be embodied in other forms. The embodiments introduced herein are provided to sufficiently convey the spirit of the present invention to those skilled in the art to which the present invention pertains. Therefore, the present invention should not be limited by the following examples.

The present invention, the step of charging the spent fuel loaded basket in the reactor (step a); Removing the basket after mounting a first cover to the reactor and performing a spent fuel reduction process (step b); And performing a lithium oxide electrolytic process (step c) after replacing the first cover with a second cover.

The first cover is a cover for preventing salt evaporation, and the second cover may include an electrode for electrolysis. The electrode includes a carbon-based anode, and the anode may be provided with a shroud structure surrounding the anode.

The spent fuel reduction method may be performed in a multi-stage operation using a plurality of reactors, and steps a to c may be performed in each reactor. The plurality of reactors may be 2 to 4 reactors, but is not limited thereto. In addition, in step a, a molten salt containing LiCl may be supported in the reactor, and the number of reaction cycles may be controlled by controlling the concentration of Li and the number of reactors in the molten salt supported in the reactor.

In the spent fuel reduction method, the spent fuel reduction process and the lithium oxide electrolytic process may be sequentially performed in the same reactor.

The method of reducing the spent fuel is a multi-stage operation of the spent fuel reduction (ex. UO ₂ + 4Li-> U + 2Li ₂ O), lithium oxide electrolytic process (ex. 2Li ₂ O-> 4Li + O ₂ ). Through this, it is possible to efficiently process large-capacity spent fuel.

In addition, the present invention is a method for reducing spent fuel performed in a multi-stage operation using a plurality of reactors, comprising: charging a basket in which the spent fuel is supported in a first reactor; Mounting the first cover on the reactor and performing a spent fuel reduction process, and then moving the basket to the second reactor; And performing a lithium oxide electrolytic process after replacing the first cover with the second cover in the first reactor, and performing a spent fuel reduction process by mounting the first cover in the second reactor. The first cover is a salt evaporation prevention cover, and the second cover provides a spent fuel reduction method comprising an electrode for electrolysis. After the spent fuel reduction process is performed in the second reactor, it may include the step of reloading the basket on which the spent fuel is loaded into the first reactor.

In addition, the present invention, a reactor in which the spent fuel reduction process and the lithium oxide electrolytic process are sequentially performed; And a first cover and a second cover of the reactor. The cover may be provided in a replaceable form, the first cover is used in the process of reducing the spent fuel to prevent salt evaporation, and the second cover is configured to include an electrode for electrolysis, lithium oxide electrolysis Can be used during processing. The reactor may be provided in plural. More specifically, 2 to 4 reactors may be provided, but are not limited thereto. The reduction device of the present invention can perform both the spent fuel reduction process and the lithium oxide electrolysis process in the same reactor by replacing the cover of the reactor, and since there is no restriction on the batch capacity and shape, safety measures and before and after the spent fuel Connectivity to the process can be improved.

Hereinafter, the present invention will be described in more detail with reference to the drawings.

The system according to an embodiment of the present invention uses a reduction device having a plurality of identical structures, and each reactor has two types of lids. 1 shows an example of the configuration and flow chart of the present system. Since spent nuclear fuel is disassembled in units of aggregate, it is advantageous to manage the process based on this. Therefore, the aggregated spent fuel for one light water reactor weighing about 500 kg is fully loaded into one basket. After the spent fuel basket is reacted for a certain period of time in the reduction device-1, it is taken out and further reacted in the reduction device-2. Thereafter, the metal conversion rate of the oxide is gradually increased while moving to the reduction device-3. Before the oxide spent fuel basket reacts, an electrochemical reaction cover is placed on each reduction device to produce excess Li. After this, when the reaction of the spent fuel is over and the basket moves to the next reduction device, an electrochemical reaction cover is mounted again to electrolyze Li ₂ O and LiCl to produce excess Li, and a new oxide spent fuel basket will be loaded. The preparation is finished. If the reduction device-3 does not obtain a sufficient reduction rate, the basket moves to the reduction device-1 again and repeats the above-described process to gradually increase the reduction rate. The configuration of this system can be variously changed to the number of reduction devices to suit the operating environment.

Hereinafter, the reduction device will be described in detail with reference to FIGS. 2 and 3 showing schematic views of the reduction device. Each reduction device alternately performs two types of processes: a spent fuel lithium reduction process (see FIG. 2) and a lithium oxide electrolysis process (see FIG. 3). In each process, different covers (140, 150) are designed to be used. In the spent fuel lithium reduction process, a simple structure cover (140) that prevents evaporation of salt is used. In the lithium oxide electrolytic process, an electrochemical reaction is performed. Use the cover 150 is provided with an electrode for. In the process of reducing lithium for spent fuel, a basket 130 containing spent fuel corresponding to one aggregate in molten salt 120 in the Li-LiCl state is loaded into the reducing device to be immersed in salt. After this, the reduction reaction (UO ₂ + 4Li → U + 2Li ₂ O) is maintained for a certain period of time, and then the spent fuel basket is taken out and delivered to the next reduction device. When the reduction reaction is completed, Li ₂ O is generated in the reduction device to be changed into a salt of Li ₂ O-Li-LiCl. At this time, a cover for a lithium oxide electrolytic process is installed to convert to a Li-LiCl salt. Li ₂ O and LiCl are electrolyzed at a voltage of 4 V or more using a metal cathode (such as iron) and a carbon anode for the lid for the Li manufacturing process. At this time, Li metal is generated at the cathode, and gases such as O ₂ , Cl ₂ and CO ₂ are generated at the anode. The generated gas is removed through the gas discharge pipe 172. The reduction device must be maintained at a melting point of LiCl of 610 ° C or higher because the salt must be kept in a molten state when the spent fuel lithium reduction process and lithium oxide electrolysis process are performed. In addition, since there is a limit to the metal conversion ratio of spent nuclear fuel in one reduction process, the reduction rate is increased while sequentially moving several devices. When a Li film is formed on the surface due to excessive generation of Li during the Li manufacturing process, a current may occur between the cathode and the anode, so a ceramic shroud is applied to prevent this (see FIG. 3). In addition, the anode is designed to be replaceable in preparation for the consumption of the carbon anode.

The operation of the system according to an embodiment of the present invention proceeds as follows:

1) Preparation of Li-LiCl state salt by performing lithium oxide electrolytic process in a reduction device

 2) After loading the spent fuel into the reduction device-1, the spent fuel lithium reduction process is performed for a certain period of time.

3) The spent fuel charging basket moves to the reduction device-2, and after charging, the reduction process is performed for a certain period of time.

4) Simultaneously with 3), the reduction device-1 converts the Li ₂ O-Li-LiCl salt back to a Li-LiCl salt by performing the lithium oxide electrolytic process of 1).

5) The spent fuel transfer basket moves to the reduction device-3 to proceed with the reduction process.

6) Simultaneously with 5), the reduction device-2 performs a lithium oxide electrolytic process (same as process 4))

7) After the spent fuel charging basket is reloaded into the reduction device-1, the process from (2) is repeated).

In this system, the reduction rate in each reduction device can be adjusted according to the capacity of the molten salt in the reduction device, the ratio of Li, and the reaction time. The reduction rate in each reduction device can be calculated using the Li-Li ₂ O equilibrium constant at the process conditions and the respective activity coefficient. In FIG. 4, the reduction rate that varies with each reduction device when the concentration of Li is 1 and 2 wt% in 500 kg of UO ₂ at 1000 kg LiCl salt at 650 ° C. was calculated. When the concentration of Li in the salt increased, it was confirmed that the reduction occurred more rapidly, and theoretically, the number of batches of the reduction device required to obtain a reduction rate of 100% was confirmed to be 7 batches at 1 wt% and 4 batches at 2 wt%. In actual operation, considering the engineering margin, multiplying by 1.5 times is expected to result in 11 batches at 1 wt% and 6 batches at 2 wt%. These conditions vary depending on the amount of LiCl salt, that is, the size of the batch and the concentration of Li, and the number of batches is further reduced when the Li concentration is increased. In the case of the operation using the three reduction devices described above as an example, the reduction reaction cycles of the reduction devices -1 to -3 are performed 4 times (1 wt% Li) or 2 times (2 wt% Li), respectively, to obtain a 100% reduction rate. You can.

100: configuration diagram of the reduction device during the spent fuel reduction process
100 ′: configuration diagram of a reduction device during a lithium oxide electrolytic process
110: reduction device electrolytic cell
120: molten salt
130: spent fuel charging basket
140: used fuel reduction process cover
150: Lithium oxide electrolytic process cover
160: cathode electrode
170: anode electrode
171: Shroud
172: gas discharge pipe
180: power supply

Claims (10)

As a spent fuel reduction method performed in a multi-stage operation using a plurality of reactors,
Charging a basket in which the spent fuel is loaded into the first reactor;
Mounting the first cover on the reactor and performing a spent fuel reduction process, and then moving the basket to the second reactor; And
And replacing the first cover with the second cover in the first reactor to perform a lithium oxide electrolysis process, and mounting the first cover in the second reactor to perform a spent fuel reduction process,
The first cover is a salt evaporation prevention cover, the second cover is a spent fuel reduction method comprising an electrode for electrolysis.
The method according to claim 1,
A method for reducing spent fuel, comprising the step of reloading a basket in which the spent fuel is loaded into the first reactor after the spent fuel reduction process is performed in the second reactor.
The method according to claim 1,
The plurality of reactors is a spent fuel reduction method, characterized in that 2 to 4 reactors.
The method according to claim 1,
The spent fuel reduction method is a spent fuel reduction method characterized in that the spent fuel reduction process and the lithium oxide electrolytic process are sequentially performed in the same reactor.
The method according to claim 1,
Method for reducing spent nuclear fuel, characterized in that the first reactor is loaded with molten salt containing LiCl.
The method according to claim 1,
The electrode comprises a carbon-based anode, the anode is a spent fuel reduction method, characterized in that provided with a shroud structure surrounding the anode.
The method according to claim 6,
The anode is a spent fuel reduction method, characterized in that provided to be replaceable.
The method according to claim 1,
When the reduction process and the lithium oxide electrolytic process is performed in the reactor, the spent fuel reduction method characterized in that the salt is maintained in a molten state.
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