JPS63243900A - Reprocessing offgas processor - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a reprocessing off-gas treatment device, and particularly to a reprocessing off-gas treatment device suitable for removing iodine and NOx from off-gas in a nuclear fuel reprocessing facility. .

[Conventional technology]

A nuclear fuel reprocessing plant is a facility whose purpose is to dissolve spent fuel with concentrated nitric acid and recover materials such as uranium and plutonium that can be reused as fuel. In the melting process of dissolving spent fuel with concentrated nitric acid, steam, NO
Off-gas mainly composed of
(fission products) are released into the off-gas process. Iodine is a substance with a long half-life and has a large effect on the human body, so strict environmental release regulations have been established, and its removal is an important component of the reprocessing off-gas treatment system.

A conventional off-gas treatment process has a system shown in FIG. In this treatment process, off-gas generated from a dissolution tank 3 in which spent fuel is dissolved with concentrated nitric acid and a collagen removal tank 4 in which iodine is expelled from the solution first enters a condenser 5, where it is cooled and most of the water vapor is is collected. This condensed water is used as an absorption liquid in the NOx absorption tower 6.

The gas that is not condensed in the condenser 5 is sent to the NOx absorption tower 6, where NOx in the gas is absorbed and removed.

The gas exiting the NOx absorption tower 6 is mixed with the gas exiting from the iodine re-expulsion tower 9, and then enters the preheater 12 and heated to 150°C.
The gas is preheated to reduce the relative humidity in the gas and then enters the adsorbent loaded iodine removal column 13 where most of the iodine entering the dissolved off-gas system is removed. The gas from which iodine has been removed is cooled by a cooler 16, passes through a filter 14, is transferred to an exhaust stack 1 by a blower 15, and is discharged.

On the other hand, condensed water from the condenser 5 and NOx absorption tower 6
Since iodine is mixed into each of the absorbed acids, these condensed water and absorbed acids are stored in a storage tank 7 and then transferred to an iodine re-expulsion tower 9 by a pump 8. The absorbed acid is heated to 80 to 90°C in this iodine re-extraction tower 9, and N2
Iodine is expelled from the liquid into the gas by the gas.

After the absorbed acid (recovered acid) from which iodine has been removed is cooled, it passes through the recovered acid storage tank 10 and the transfer pump 11 and is adjusted for reuse as a dissolving solution.

C Problems to be Solved by the Invention] In the above-mentioned prior art, the iodine re-expelling tower 9 has a high operating temperature of 80 to 90° C. for expelling iodine in the nitric acid solution supplied to the tower; Moreover, the iodine concentration in the nitric acid solution is on the order of 1100 pp, which is higher than other devices in the off-gas treatment system, making it the most severe environment from the viewpoint of corrosion due to iodine.

Figure 6 shows what the inventors have studied, and shows that approximately 50% of iodine is
5US30 under 0pm-containing 0□-N2 gas environment
4L (low C-18Cr-8Ni) is a graph showing the influence of tA on corrosion. In Figure 6, the relative humidity of the gas is 100%, and the form of corrosion is pitting rather than general corrosion, so the vertical axis in Figure 6 is pitting corrosion resistance, which is the reciprocal of the pitting depth. There is. It can be seen from FIG. 6 that pitting corrosion increases as the temperature increases. That is,
The pitting corrosion resistance decreases with increasing temperature, and at the operating temperature in the iodine reextraction tower 9, ordinary austenitic stainless steels such as 5US304L cannot be used at all.

For this reason, Ti, Zr and high NlyiIIMo steels, such as Hastelloy C (56Nl-16Cr-6
Mo) and Inconel 625 (58-Ni-N
It is necessary to use high-grade materials such as i-22Cr-9, which increases the cost of the device and ignores economic efficiency.

From the viewpoint of corrosion resistance, it would be better to lower the operating temperature in the iodine re-purging tower 9, but in this case, as shown in Fig. 7, if the temperature decreases, especially below about 50°C, the iodine re-purging occurs. The iodine expulsion performance, which is the original purpose of the device in the extraction tower 9, is significantly reduced.

Therefore, in the iodine re-expelling tower 9 in the conventional reprocessing off-gas treatment equipment, it was not possible to prevent corrosion due to iodine by using low-grade materials without reducing the iodine expelling performance.

The purpose of the present invention is to solve the problems of the prior art described above,
To provide a reprocessing off-gas treatment device capable of preventing corrosion due to iodine even when using low-grade materials for the device's constituent materials without reducing iodine expulsion performance in an iodine reextraction tower.

[Means for solving problems]

The above object is achieved by installing a pressure reducing device capable of maintaining the inside of the iodine re-expulsion tower under reduced pressure conditions for expelling iodine from the absorbed acid from the NOx absorption tower.

(Operation) The pressure inside the iodine re-extracting tower is reduced by a pressure reducing device that can maintain the inside of the iodine re-extracting tower under reduced pressure conditions.The saturated iodine concentration in the gas phase at normal pressure in the iodine re-extracting tower is the same as that of the iodine re-extracting tower. The temperature at which the iodine concentration is reached is lowered by reducing the pressure compared to normal pressure.

Therefore, by reducing the pressure, a predetermined iodine expulsion efficiency can be maintained even at low temperatures. For this reason, operating conditions (low-temperature conditions) that can prevent corrosion caused by iodine, which tends to occur at high temperatures.
can maintain high iodine expulsion efficiency.

[Embodiments of the invention]

Embodiments of the present invention will be described below based on the drawings.

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

In FIG. 1, a pressure reducing device 18 is installed on the off-gas line from the iodine re-expelling tower 9, and a pressure regulating valve 19 is installed in the middle of the piping connecting the NOx absorption tower 6 and the iodine re-extracting tower 9.
is installed.

Further, a check valve 20 is provided on the outlet side of the recovered acid storage tank 10 that stores the recovered acid from the iodine re-extraction tower 19.

The N2 gas line 17A that supplies Nt gas into the iodine re-expelling tower 19 has a flow rate adjustment valve 21A that adjusts the amount of N2 gas.
are installed, as well as an iodine re-expelling tower 9 and a pressure reducing device 1.
A pressure gauge 22 is installed between the pressure gauge 8 and the pressure gauge 8.

In FIG. 1, components other than those described above are the same or equivalent components to those of the conventional reprocessing off-gas treatment apparatus shown in FIG.

Next, the functions and effects of the reprocessing off-gas treatment apparatus configured as described above will be explained.

The nitric acid solution containing iodine coming out of the NOx absorption tower 6 is
The iodine is introduced into the iodine re-expulsion tower 9 through the pressure regulating valve 19. This iodine re-expelling tower 9 has a flow rate of N! adjusted by a flow rate regulating valve 21A. A gas is supplied, and the iodine in the nitric acid solution is driven out of the solution by the N2 gas. The N2 gas and the iodine gas expelled from the nitric acid solution pass through the pressure reducing device 18 and join with the outlet gas of the NOx absorption tower 6.

On the other hand, the nitric acid solution from which iodine has been removed passes through the recovered acid storage tank 10 and the check valve 20 and is returned to the dissolution tank by the transfer pump 11 for reuse. In such an operation, the pressure within the iodine re-expulsion tower 9 is monitored by a pressure gauge 22.

In the case of this embodiment, the pressure reducing device 18 is installed at the outlet gas section of the iodine re-expelling tower 9 to reduce the pressure inside the iodine re-extracting tower 9. The recovered acid storage tank 10 is of a liquid level type that does not contain a gas phase.

Next, Figure 4 shows the relationship between the saturated concentration of iodine in gas and temperature using pressure as a parameter. As explained in FIG. 7 above, the iodine expelling efficiency begins to decrease when the operating temperature of the iodine reexpelling tower 9 becomes approximately 50°C or lower, and the initial iodine concentration at that time is 90°C.
Since the iodine concentration is 90 pmm, the vertical axis of FIG. 4 also shows the iodine concentration when saturated iodine is diluted to 90 pmm under conditions of normal pressure and 50°C. From Figure 4, the iodine concentration is 90p under the conditions of normal pressure (latm) and 50℃.
It is shown that the temperature at which the iodine concentration is the same as in 1 is lowered by reducing the pressure. In FIG. 4, for example, as a condition for the iodine concentration to be 90p+ui, 1at
What was 50℃ when e was 40℃ at 0.5at++
℃, and if the pressure is reduced to 0.2 atm, the temperature becomes 30℃.

Moreover, FIG. 5 shows the state at normal pressure (latlI+) and at 0. 2at
This is a comparison of iodine expulsion efficiency when the pressure is reduced to s+. As is clear from FIG. 5, it is possible to maintain a high level of iodine expulsion efficiency even at low temperatures by reducing the pressure.

Therefore, as is clear from a comparison between FIG. 7 and FIG. 5 described above, sufficient iodine expulsion efficiency can be maintained even at low temperatures by maintaining the inside of the iodine reexpelling tower 9 at a predetermined reduced pressure condition. Because of this, it is possible to reduce the temperature without impairing the iodine expelling performance and avoid the high temperature conditions that tend to cause corrosion due to iodine. Therefore, without using high-grade materials,
Ordinary austenitic stainless steel such as 5US304L can be used as the material for composing the iodine reexpelling tower 9 and the like.

FIG. 2 is a schematic configuration diagram showing another example of the configuration of the iodine re-expulsion tower peripheral portion in the reprocessing off-gas treatment apparatus of the present invention.

In FIG. 2, the recovered acid storage tank 10 is connected to the iodine re-extraction tower 9.
The recovered acid storage tank 10 is arranged at almost the same level as the tank IO, and has a gas phase section above the iodine re-expelling tower 9 and the upper gas phase section of the recovered acid storage tank 10 by a pressure reducing device 18. At the same time, the pressure is reduced. Further, a flow rate adjustment valve 21B for adjusting the supply amount of N8 gas is provided in the N2 gas line 17B for the recovered acid storage tank 10 as well.

In the embodiment shown in FIG. 2, the pressure is reduced in the recovered acid storage tank 10 as well, and N2 gas is introduced, so that the iodine contained in the recovered acid in the recovered acid storage tank 10 is also expelled, reducing the iodine expulsion efficiency. can be increased.

FIG. 3 is a schematic configuration diagram showing still another example of the configuration of the peripheral portion of the iodine re-expelling column in the reprocessing off-gas treatment apparatus of the present invention.

In the apparatus shown in FIG. 3, in addition to the device 18 for reducing the pressure inside the iodine re-extraction tower 9, a decompression device 18A for reducing the pressure in the gas phase of the recovered acid storage tank 10 is installed. The expulsion rate does not need to be as high as the iodine expulsion rate in the iodine re-expulsion tower 9, and the temperature of the recovered acid storage tank 10 is originally lower than the operating temperature in the iodine re-expulsion tower 9, so the reduced pressure conditions are It can be lower than that of the iodine re-expelling tower 9. Therefore, the pressure reducing device 18A separate from the pressure reducing device 18A for the iodine reexpelling tower 9 may have a capacity sufficient to maintain the reduced pressure condition corresponding to the recovered acid storage tank 10.

In FIGS. 1 to 3, the pressure reduction conditions in the iodine re-extracting tower 9 are different because the temperature dependence of corrosion resistance shown in FIG. Determined by the material used for 9.

In addition, in FIGS. 1 to 3, the iodine re-expulsion tower 9 (and the recovered acid storage tank 10 ) is maintained in a reduced pressure state, but since the amount of Nt gas introduced into the iodine re-expulsion tower 9 affects the pressure inside the iodine re-expulsion tower 9, the pressure reduction device 18
needs to be set to have a capacity sufficient to cover the amount of N2 gas introduced into the iodine re-expelling tower 9.

In the present invention, the pressure inside the iodine reexpelling tower 9 is reduced by the pressure reducing device 18, but any means for reducing the pressure may be a vacuum pump, a vacuum blower, or an ejector using steam, water, gas, etc. .

The iodine re-extracting tower 9 has a structure in which iodine is removed while spraying the iodine-containing nitric acid solution from the NOx absorption tower 6 at the upper part of the iodine re-extracting tower 9, or the iodine re-extracting tower 9 It is also possible to have a structure in which iodine is expelled from the iodine-containing nitric acid solution by gas-liquid contact using a packed type, and the structure of the iodine re-expulsion tower 9 itself is not particularly restricted.

〔Effect of the invention〕

As described above, according to the present invention, the iodine expelling efficiency can be maintained at a high level at a relatively low temperature by reducing the pressure of the iodine reexpelling tower. For this reason, it is possible to maintain iodine expulsion efficiency and at the same time improve the corrosion resistance of equipment components such as the iodine re-extraction tower, eliminating the need to use high-grade materials as equipment components, and significantly extending the life of the equipment. can be extended to

[Brief explanation of drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of the reprocessing off-gas treatment apparatus of the present invention, and FIG. 2 is a schematic diagram showing another example of the peripheral area of the iodine re-expulsion tower in the reprocessing off-gas treatment apparatus of the present invention. Fig. 3 is a schematic block diagram showing still another example of the peripheral area of the iodine re-expelling tower in the reprocessing off-gas treatment equipment of the present invention, and Fig. 4 shows the temperature and pressure of the iodine concentration in the gas phase. Figure 5 is a graph showing the effect of pressure on iodine expulsion efficiency, Figure 6 is a graph showing the influence of temperature on corrosion resistance in an iodine-containing gas phase, and Figure 7 is a graph showing the influence of pressure on iodine expulsion efficiency. FIG. 8, a graph showing the influence of temperature on performance, is a schematic diagram of a conventional reprocessing off-gas treatment apparatus. 3...Dissolution tank, 4...Iodine expulsion tank,
5...Condenser, 6...NOx absorption tower, 7...Absorption acid storage tank, 9I...Iodine re-expulsion tower, 10... Recovery acid storage tank, 12...
...Preheater, 13...Iodine removal tower, 14.
...Filter, 17.17A, 17B...
・N2 gas supply line, 18.18A... pressure reducing device, 19... pressure regulating valve, 20...
Check valve, 21A121B...Flow Ill valve, 2
2...Pressure gauge agent Patent attorney Masaru Nishimoto - Figure 2 Figure 3 Fees Figure 4 Figure 5

Claims (1)

[Claims] (1) A NOx absorption tower for absorbing and removing NOx in off-gas from the reprocessing facility, an iodine re-expulsion tower for expelling iodine contained in the absorbed acid from the NOx absorption tower, and at least an iodine A reprocessing off-gas treatment device equipped with an iodine removal tower for removing iodine in gas from the re-expulsion tower, characterized in that a decompression device capable of maintaining the interior of the iodine re-expulsion tower under reduced pressure conditions is installed. Reprocessing off-gas treatment equipment.