JPH02115796A - Spent fuel reprocessing plant - Google Patents

Abstract

PURPOSE:To improve the corrosion prevention effect of a material so as to suppress the diffusion of radioactive materials to the outside of a plant by leading the NOX gas produced when nitric acid decomposition is performed in the plant to process equipment used in a highly corrosive environment in the plant. CONSTITUTION:A spent fuel assembly is dissolved in nitric acid after cutting. The nitric acid dissolving the fuel is sent to a primary decontamination process and separated into a U-Pu solution and high-level waste nitric acid solution containing most of fission products. The waste nitric acid solution is condensed by evaporation in a high-level waste solution condensing process to reduce the volume and waste solution discharge quantity is lowered by recovering nitric acid for long-period preservation. The NOX gas produced in the dissolving process is led to a high-level waste solution condensing apparatus after iodine is removed from the gas. Since the NOX gas preserved together with the waste solution reduces highly oxidized ones of the ions existing in the waste nitric acid solution and suppress a corrosion accelerating effect, corrosion of the waste solution condensing apparatus can be reduced. Moreover, since the NOX gas is recycled, the quantity of the processed gas to be discharged into air can be reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a spent fuel reprocessing plant, in particular, where nitric acid is used to dissolve spent fuel and the process equipment is used in a highly corrosive environment. Regarding plant improvement.

[Prior Art] As a corrosion prevention method for equipment that handles nitric acid liquid containing heavy metals and/or their compounds, such as spent fuel reprocessing plants, it is possible to forcibly add NOx gas to nitric acid liquid.
0 Patent Application Publication No. 6, 1997, ``Corrosion Prevention Method for Or Equipment'', which describes the amount of NOx gas injection necessary for corrosion protection, changes the corrosion potential of stainless steel, which is in the underdynamic region, to the passivation region. It has been reported that the amount necessary for migration should be used as a guideline, and that it was experimentally confirmed that the corrosion rate of stainless steel in a nitric acid solution containing heavy metals was significantly reduced by adding NOx gas.

[Problems to be Solved and Used by the Invention] In the above-mentioned prior art, it is further advantageous to use surplus NOx gas in order to reduce the operation of the NOx gas removal device that is generally attached to equipment that handles nitric acid. However, in spent fuel reprocessing plants, the nitric acid contained contains a large amount of radioactive materials, and it is necessary to limit the range of radioactive material contamination of plant equipment and to suppress the release of radioactive materials outside the plant. Therefore, special consideration is required.

The purpose of the present invention is to limit the range of radioactive material contamination of plant equipment by appropriately selecting a NOx gas supply method to process equipment used in a highly corrosive environment and selecting a sampling point for surplus NOx gas. An object of the present invention is to provide a spent fuel reprocessing plant that can suppress the release of radioactive materials to the outside.

[Means for Solving the Problems] The present invention provides a spent fuel reprocessing plant that dissolves spent nuclear fuel in nitric acid and separates nuclear fuel materials from the dissolved solution, in which NOx gas generated during nitric acid decomposition in the plant is used. The main point is that the system is equipped with equipment to guide the process equipment into the process equipment used in the highly corrosive environment within the plant.

[Function] According to the present invention, NOx gas is actively supplied to equipment used in highly corrosive environments, and fuel is dissolved with nitric acid in a fuel reprocessing plant, which was conventionally discharged to the outside. NO generated by the decomposition of nitric acid when
By using x-gas, not only can corrosion of high-level waste liquid concentration equipment be suppressed, but also the dispersion of radioactive materials to the outside of the plant can be suppressed by reducing the amount of process gas released to the outside. . Furthermore, in the present invention, by appropriately selecting the sampling points for NOx gas, it is possible to limit the range of radioactive substance contamination of plant equipment.

In the present invention, the piping connected to the gas phase of the equipment containing NOx gas generated by nitric acid decomposition is opened into the nitric acid solution of the equipment to which the NOx gas is supplied, and the ventilation equipment or depressurization equipment of the NOx gas supply equipment By reducing the pressure in the gas phase, NOx gas was blown into the nitric acid solution of the device.

In addition, by selecting a point with relatively little radioactive material contamination as the NOx gas sampling point, we prevented the spread of radioactive material contamination of the planned equipment and suppressed the release of radioactive material and NOx gas outside the plant.

[Example] An example of the present invention will be described below with reference to FIGS. 1 and 2.

FIG. 1 shows an outline of the process of a spent fuel reprocessing plant to which the present invention is applied. The letters inside the square frames indicate the main operations in each process, the solid lines connecting the square frames mainly indicate the flow of liquid, and the broken lines indicate the flow of gas. Figure 2 shows the high-level waste liquid used in the high-level waste liquid concentration process The structure of the waste liquid evaporator is shown below.

Spent fuel assemblies transported to a reprocessing plant are cut into lengths of several centimeters in a shearing process, and then dissolved in nitric acid in a melting process. In addition to the fuel pellet and the sealed gas, fission product gas is confined within the fuel assembly, and gas containing fission products such as iodine is released during shearing. Also.

When spent fuel is melted, NOx gas is generated by the following reaction, but after these gases are purified by NOx absorption and iodine removal operations, they are conventionally released into the atmosphere from the exhaust stack. (NO,),
+ 2NO+ 4)1. ODo, + 4HNO, →
UO□(No, )2+ 2NO□+28.0 The nitric acid solution in which the fuel is dissolved is sent to the primary contamination process, where an extraction operation using tributyl phosphate removes the uranium/plutonium solution and a high level containing most of the fission products. Separated into nitric acid waste liquid. After uranium and plutonium are separated and purified, they are stored as final products in the reprocessing plant.The high-level nitric acid waste liquid is evaporated and concentrated in the high-level waste liquid concentration process, and the volume is reduced for long-term storage. The amount of waste liquid released will be reduced by recovering nitric acid.

High-level waste liquid contains most of the nuclear fission products contained in spent fuel and is difficult to handle due to its high radiation level. In addition, some of the fission products containing ruthenium, cerium, etc. are removed from structural materials by nitric acid solution. This accelerates corrosion, and chromium, iron, etc. dissolved into the solution due to corrosion further accelerate the corrosion, so there is a problem that the corrosive environment for structural materials is extremely harsh.

The corrosiveness of nitric acid solutions usually increases as the nitric acid concentration and solution temperature rise, so for equipment with particularly harsh corrosive environments, such as high-level waste liquid evaporators and acid recovery evaporators, it is recommended to reduce the nitric acid concentration by using formalin addition nitric acid decomposition method. Alternatively, measures such as reducing the nitric acid temperature using reduced pressure evaporation are implemented, but if we are to consider the rising trend of fission product concentration due to higher burnup and aim to improve the reliability of reprocessing plants, it is necessary to It is necessary to adopt a method of adding NOx gas to suppress the corrosion acceleration effect.

Figure 1 shows the NOx gas generated during the melting process.

The process is configured to remove iodine and then lead to a high-level waste liquid concentrator. The coexistence of NOx is
It has the function of reducing highly oxidized ions present in nitric acid waste liquid and suppressing the effect of promoting corrosion, so coexisting with NOx gas can reduce corrosion of high-level waste liquid concentration equipment. can. Furthermore, since the NOx gas is recycled and used, it is possible to reduce the amount of processed gas released into the atmosphere, and it is possible to suppress as much as possible the transfer of radioactive substances into the environment together with the released gas.

In the embodiment shown in FIG. 1, NOx gas from which iodine has been removed by extraction from the downstream side of the element removal step is used, so that the high-level waste liquid concentration system is not contaminated with iodine.

FIG. 2 is an example in which the corrosion prevention method using NOx of the present invention is applied to a kettle-type evaporator. When concentrating nitric acid waste under normal pressure, the liquid temperature is high and the corrosive environment is harsh, so a technology is used to decompose the nitric acid by adding formalin, etc. to keep the nitric acid concentration low. Gas is generated and the inside of the evaporator becomes a NOx rich environment. Second
In the figure, a partition 1 with an open bottom is provided inside the evaporator,
It has a structure in which a communication pipe 6 is inserted from the gas phase part 5 outside the partition 1 to the liquid phase part 4 inside the partition 1.

At this time, the pipe end 7 of the communication pipe 6 is configured to be above the partition end 8. The evaporator is equipped with ventilation equipment for maintaining negative pressure in the pipe 9 in order to contain the radioactive material contained within the can, but due to the suction effect of this negative pressure, the liquid level 12 inside the partition 1 rises. On the other hand, the liquid level 13 outside of partition 1
is descending. Since the liquid level 14 in the communication pipe 6 coincides with the liquid level 13, by setting the pressure and other conditions so that the pipe end 7 of the communication pipe 6 is above the liquid level 13, the gas phase part 5 is It becomes possible to automatically draw NOx gas into the liquid phase portion 2 through the communication pipe 6. By supplying waste liquid to this liquid phase section 2 via a pipe 11.

NOx gas in the can is mixed in, and a reducing atmosphere can always be maintained throughout the evaporator due to NOx.

If the amount of NOx gas generated during the denitration process is not sufficient to alleviate corrosion of the material, the off-gas generated during the melting process or the gas from the NOx cylinder or NOx generator is passed through the pipe 10 and evaporated by the suction action of negative pressure. Guide it inside the can. However, when using dissolved off-gas as a NOx source, Figure 1
As shown in Figure 3, NOx from which iodine has been removed is used to prevent contamination by iodine from spreading.

Techniques for concentrating nitric acid waste include adding formalin under normal pressure to remove nitrification, as well as attaching decompression equipment to pipe 9 and reducing the pressure inside the evaporator to lower the boiling point of the liquid and alleviate corrosion. . In the case of this reduced pressure evaporation method, since denitration is not performed, the inside of the evaporator does not have a NOx-rich environment. Therefore, in this case too, dissolved off-gas or NOx cylinders or N.

The gas from the X generator is guided through the tube 10 into the evaporator by the suction action of reduced pressure. When using dissolved off-gas as a NOx source, it is used after removing iodine.

Figure 3 shows that the waste liquid supplied to the evaporator is treated with NOx in a separate container in advance, and the NOx remaining in the container is drawn into the liquid transferred into the evaporator to create a reducing atmosphere inside the evaporator. This figure shows the mechanism for The waste liquid to be concentrated is once placed in the pre-tank 15 and then supplied to the inside of the evaporator by a steam jet 16. Ventilation equipment or decompression equipment for maintaining negative pressure is attached to the evaporator via a pipe 19, and concentration operation is performed. If the pressure difference between the gas phase part 20 of the pre-tank 15 and the gas phase part 22 inside the evaporator is set appropriately, the gas will automatically pass through the pipe 24 and enter the liquid phase part 23 of the evaporator due to the suction action of negative pressure. can be drawn in. At the same time, gas can be drawn into the liquid phase portion 21 of the pre-tank 15 from the outside through the pipe 26. Liquid phase part 21
The amount of gas that cannot be completely sucked moves to the gas phase section 20, but is eventually drawn into the liquid phase section 23 of the evaporator through the pipe 24. If the pipe 26 is configured to supply NOx gas as a dissolved off-gas, the waste liquid before concentration is mixed with NOx in the pre-tank 15, and even after being transferred to the evaporator by the steam jet 16, it continues to contain N.

Since the NOx is mixed with the gas in the gas phase section 20 of the pre-tank 15 containing X, a reducing atmosphere can be maintained inside the evaporator due to NOx, and corrosion of the material is alleviated. In this case as well, when the dissolved off-gas is used as a NOx source after removing iodine, NOx after removing iodine is used, and care is taken to prevent the spread of contamination by iodine.

In the above example, iodine contamination was mentioned when the dissolved off-gas is used as NOx gas, but the same applies to tritium and ruthenium.

When the evaporator has a low tritium concentration or a low ruthenium concentration, the NOx gas used as a corrosion mitigation measure should be one containing low lid or ruthenium to prevent tritium contamination of the plant equipment.

or suppress ruthenium contamination.

[Effects of the Invention] According to the present invention, NOx gas generated by nitric acid decomposition or NOx gas generated by fuel dissolution can be drawn into the nitric acid solution by the suction action of the ventilation equipment or decompression equipment for maintaining negative pressure. Therefore, it works to enhance the anti-corrosion effect of the material.

Also, NOx gas will be reused.

It is possible to reduce the amount of processed gas released into the atmosphere, and it is possible to suppress as much as possible the transfer of radioactive substances into the environment together with the released gas.

By using NOx downstream of the process of removing iodine, tritium, or ruthenium, equipment to which NOx is supplied can be prevented from being contaminated with these substances.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing an overview of the process flow of an embodiment of the present invention, FIG. 2 is a longitudinal cross-sectional view of an evaporator according to an embodiment of the present invention, and FIG. 3 is a diagram showing an outline of the process flow of an embodiment of the present invention. FIG. 3 is a longitudinal cross-sectional view of the can. 1... Partition, 2... Liquid phase part, 3... Liquid phase part, 4
... Gas phase part, 5... Gas phase part, 6... Communication pipe, 7.
...Tube end, 8...Partition end, 9...Tube, 10...Tube, 11...Tube, 12...Liquid level, 13...Liquid level, 1
4...Liquid level

Claims (1)

[Claims] 1. In a spent fuel reprocessing plant that dissolves spent nuclear fuel in nitric acid and separates nuclear fuel materials from the solution, NOx gas generated during nitric acid decomposition within the plant is A spent fuel reprocessing plant characterized by being equipped with equipment for guiding it into process equipment used in a highly corrosive environment. 2. The spent fuel reprocessing plant according to claim 1, wherein the point at which NOx is collected to be supplied to the process equipment used in the highly corrosive environment is the ventilation system of the plant. 3. In the inventions of claims 1 and 2, the collection of NOx gas generated when melting spent fuel is used to supply it to the process equipment used in the highly corrosive environment. A spent fuel reprocessing plant characterized in that the point is downstream of the iodine removal equipment in the dissolution tank off-gas treatment process. 4. A spent fuel reprocessing plant according to any of claims 1 to 3, characterized in that negative pressure within the process equipment is used to guide NOx gas into the process equipment used in the corrosive environment. 5. The spent fuel reprocessing plant according to claim 1, wherein the NOx gas supplied to the process equipment used in the highly corrosive environment has a low iodine concentration. 6. The spent fuel reprocessing plant according to claim 1, wherein the NOx gas supplied into the process equipment used in the highly corrosive environment has a low tritium concentration. 7. The spent fuel reprocessing plant according to claim 1, wherein the NOx gas supplied into the process equipment used in the highly corrosive environment has a low ruthenium concentration.