JPS6153598A - Removing system of radioactive iodine - 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 Application of the Invention] The present invention relates to a filter for removing radioactive iodine from exhaust gas emitted from nuclear facilities, and in particular to an iodine removal system suitable for removing high concentrations of iodine in reprocessing plants. Regarding.

[Background of the invention]

At nuclear facilities, various measures are taken to reduce the amount of radioactivity released into the surrounding environment in order to prevent the surrounding residents from being exposed to radiation. Of these, radioactive iodine is selectively absorbed by the human thyroid and increases radiation exposure, so particularly strict measures are taken to reduce the amount of radioactivity released. Measures to reduce exhaust gas include 1-
An iodine removal filter filled with an adsorbent of about 2 mφ is generally installed. Representative examples of nuclear facilities include nuclear power plants and nuclear fuel reprocessing plans. In the former case, impregnated carbon filters have been used for a long time. On the other hand, in the latter nuclear fuel reprocessing plant, a silver-impregnated adsorbent is used as an iodine removal filter. Although they are the same iodine removal filter, the performance required of the two filters is significantly different. In other words, at the former nuclear power plant, the target radioactive iodine has a short half-life of 11
J (half-life 8 days), the concentration of iodine is o, i I) p
b, which is extremely low. On the other hand, in the reprocessing plant, the target radioactive iodine has a long half-life i! s r <half-life 1.
7 x 10' years), and the iodine concentration was 500.000 in the former
It is about twice as high as 5J11. From the above, the following three requirements are required for adsorbents for treating exhaust gas from reprocessing plants. That is, (1) IIJ can be removed with high efficiency in order to reduce the amount of radioactivity released from the plant.

(2) The iodine adsorption capacity of the adsorbent is large and the amount of used adsorbent as waste is small, and (a) 11”I
These three points are that it can be stored semi-permanently as a chemically stable compound.

From this point of view, three methods are currently being researched and developed or put into practical use. Outlines of these three methods are shown in FIGS. 2, 3, and 4.

The system shown in FIG. 2 consists only of an adsorption tower 1 filled with a silver-impregnated adsorbent. This method has already been put into practical use because of its simple system, but it has the problem of using a large amount of expensive silver because the used silver-impregnated adsorbent is directly disposed of. The method in Figure 3 is prc, of D
OE Nuclear Air CleanirgCo
In the method reported in Nferrence (197B), after iodine was adsorbed and removed with a high removal efficiency using a silver-impregnated adsorbent, the silver-impregnated adsorbent was regenerated with 'H2, and the iodine was desorbed from the adsorbent by regeneration. This is an attempt to adsorb iodine to an adsorbent impregnated with lead, which is cheaper than silver. This system consists of two adsorption towers 1 filled with silver-impregnated adsorbent, an adsorption tower 2 filled with lead-impregnated adsorbent, a heater 3 for heating Hz, a cooler 4, and an H3 circulation pump 5. One of the adsorption towers 1 passes the process gas and adsorbs and removes iodine. The other adsorption tower 1 is regenerated with Hz (500C) heated by a heater 3. At this time, iodine is desorbed from the silver-impregnated adsorbent as HI (hydrogen iodide).

The iodine-containing Hz that has passed through the adsorption tower 1 is cooled to 150C by a cooler 4, and sent via a circulation pump 5 to an adsorption tower 3 filled with lead-impregnated adsorbent, where iodine is adsorbed. The adsorption tower 1 that has been regenerated is used again to adsorb and remove iodine from the process gas, so it remains in a standby state, and the lead-added adsorbent that has adsorbed iodine is discarded. Therefore, in this method, the consumption of expensive silver is reduced due to regeneration deterioration.
Since only the silver-adsorbed adsorbent is generated, the amount is reduced to 1/10 to 1/20 compared to the method shown in FIG.

However, since the system becomes complicated, problems arise such as a large initial investment in equipment and complicated operation. The system shown in Figure 4 includes a first stage adsorption tower 1 filled with an adsorbent impregnated with metallic copper or metallic lead as an adsorbent for I8 removal, and a silver compound impregnated to remove the remaining iodine (mainly CHsI). It is composed of a second stage adsorption tower 2 filled with adsorbent. In this method, the I! The amount of adsorption becomes a problem. Among the processing gases, the off-gas from the melting process contains not only 01 but also H2O and NOx. Metallic copper or metallic lead has a problem in that its adsorption performance in the above-mentioned atmosphere decreases.

Therefore, it is important to minimize the amount of silver consumed and to make it easier to reproduce.
! There is a need for a removal system that desorbs iodine without using gas and immobilizes it as a chemically stable iodine compound.

[Object of the invention] The object of the present invention is to provide a reprocessing plant off-gas system that
By eliminating the influence of impurities in the off-gas and effectively removing iodine using an inexpensive adsorbent impregnated with metallic copper or metallic lead, we provide an iodine removal system with a simple system and low silver consumption. It is in.

[Summary of the invention]

The present invention is based on the fact that, through experiments conducted by the inventors, a dehumidifying material used industrially that adsorbs NOx and hydrogen but does not adsorb iodine, and a metal-impregnated adsorbent. This is based on the results of finding effective adsorption conditions for iodine immobilization.

The feature of the present invention is that it does not adsorb I2, and that it does not adsorb NOx and I20.
An iodine concentration tower that is filled with an adsorbent that selectively adsorbs NOx and H2O, and then reversibly adsorbs and removes the remaining iodine mainly through physical adsorption;
and an immobilization tower that immobilizes the regenerated iodine in a stable state as a compound by chemisorption.

A subordinate feature of the invention is that the influence of 02 on the adsorbent packed in the immobilization column is eliminated by using an inert gas during the regeneration of the iodine concentration column.

The present invention was made based on the following experimental results.

Figure 4 shows the iodine (I2
), NOx, and H2O adsorption amounts are shown. This measurement was conducted at a temperature of 50C and a concentration of 200. , concentration t, o
Not of vol*, and H of concentration λ□Vo1%! Measurements were taken by introducing O together with N2 gas until the winding reached equilibrium. Synthetic zeolite 3A, 4A, and 5A are used for narrow diameter, 3, 4, and 5 people, respectively.
It was used. In addition, if the pore size is 10, synthetic zeolite) 1
3X was used. As a result, H2O and NOX were adsorbed by synthetic zeolite with any pore size, but I2 showed a very specific tendency. That is, synthetic zeolite 3A with a pore size of 3 people does not adsorb, and as the pore size increases from 4 people to 5 people to 10 people, ■! The amount of adsorption increased. When looking at silica gel (average pore diameter, about 20 pores) and alumina (average pore diameter, about 50 pores), which have even larger pores, the amount of Is adsorbed decreased.

The above can be understood as follows. The adsorption of Process 2 onto synthetic zeolite is primarily temperature-reversible physical adsorption. Therefore, the smaller the pore diameter, the greater the amount of adsorption due to capillary condensation. However, the maximum length of the molecular diameter of I2 is 5.
Since there were four people, it would be difficult to enter the small pores. Therefore, it is considered that the adsorption amount reached the maximum when the pore diameter was about 10 pores. As a result, the pore size is 3
When we focus on human-made synthetic zeolite, NOx, )(
It can be seen that there is no significant difference in the adsorption of to from synthetic zeolites of other pore sizes, and only for No. 2 there is no adsorption performance.

The fact that I2 is not adsorbed with a pore size of 3 and is adsorbed with a pore size of 5 is qualitatively similar to the known CM, A.

Wah l gren, W, W, Me 1nke,
Nuc Ieonics (1957)),
The adsorption characteristics of Iz cannot be discussed in terms of its molecular diameter, and there is no systematic comparison of the amount of NOx adsorbed for 3, 4, and 5 people. This was discovered for the first time through experiments conducted by the inventors.

Next, Table 1 Vc, 1! The results of measuring the amount of iodine adsorbed in various atmospheres are shown for three metals selected from the elements II. This measurement was carried out under the conditions of a temperature of 150C and an iodine concentration of 200P, using N2 gas, dry air, and NOx 1 v each until iodine adsorption reached equilibrium.
The measurement was conducted by introducing air containing 01% + Hz 01v01% and iodine. The measurement sample was 1 g of activated alumina containing nitrates of Pb, Cu, and Ag on an activated alumina carrier.
H! It was reduced in an atmosphere and attached in a metallic form. As a result,
N without oxygen! In the atmosphere, CuHP b+ A
Iodine is adsorbed in all g, but air atmosphere and NOx
In an air atmosphere containing H2O and Cu, the adsorption amount of the Cu- and Pb-impregnated adsorbent decreased. This tendency is particularly remarkable for PB. The reason for this is that unlike precious metals such as Ag, Cu, Pb
In an oxidizing atmosphere, a chemical reaction with I2 produces CuI and P.
At the same time as bIn is generated, CuO1PbO! This is because the formation of oxides such as oxides cannot be ignored, and the presence of oxides mentioned above has also been confirmed by X-ray diffraction of the product after the reaction. In this way, the above experiment by the inventors is the first time that the metal (, u, pb) has been measured in comparison with the reprocessing plant off-gas atmosphere and its influence has been clarified.

This is because the chemical form of iodine is HI, and the adsorption of iodine by the lead-impregnated adsorbent in the method shown in FIG.
As shown in the table, it is different from I2, which is the target of the present invention, and the atmosphere is H! The atmosphere was different from the atmosphere conditions in Table 1. However, in the 1950s, experiments were conducted using adsorbents impregnated with (, u, pb) or metal foils, but these were aimed at nuclear power plants, so it was difficult to remove iodine. Area with low adsorption amount (1 in Table 1)
/100 or less). In such a region where the amount of adsorption is small, there is a phenomenon in which iodine is adsorbed without chemically bonding with Cu, PB, etc., and the amount of adsorption varies depending on the experimental conditions.

Therefore, this fact does not detract from the novelty of the present invention.

Based on the above experimental results, specific embodiments will be described below.

[Embodiments of the invention]

Preferred embodiments for carrying out the present invention will be described in detail below with reference to the drawings.

FIG. 1 shows the basic flow of a preferred embodiment of the present invention. This example uses an adsorption tower 7 filled with a dehumidifying material with a pore size of at least 4 Å or less, and a dehumidifying material with a pore size larger than 5 Å and smaller than 20 Å. It consists of an iodine concentration column 8, a heater 3, a cooler 4, a circulator 5, and an immobilization group 9 filled with a copper metal-impregnated adsorbent. Two towers are installed, one tower is for NOx.

Concentration operations are performed by adsorbing H2O or iodine, and the other tower is capable of regeneration operations by heating, etc. These component devices are connected by piping and the like as shown in FIG. 5, and each device is isolated by valves and the like. Iz, NOx,
The air containing H2O is introduced into the adsorption tower 7, where NO
x, N20 is removed. Iodine (mainly I2) is then removed in an iodine concentrating column 8. Some of the iodine not removed in the iodine concentrator 8 (mainly up to 5
% CH3I) is completely removed in a downstream adsorption tower (not shown) packed with a silver-impregnated adsorbent.

Therefore, the silver consumption is 1/20μ of the conventional technology shown in Figure 1.
Become below. Adsorption of N Ox and Hz O is usually carried out at a temperature ranging from room temperature to 100C. The iodine adsorption temperature in the iodine concentrator 8 is preferably from room temperature to 50 tZ''. Figure 6 shows the results of a dynamic experiment on the adsorption of iodine in the iodine concentrator 8. An adsorbent of aluminum impregnated with silver nitrate, a silver compound conventionally used as an adsorbent, and a synthetic zeolite (Molecular Sheep 13x) suitable for use in the present invention.
), each adsorbent with a particle size of 1 to 2 mm was packed into the iodine reduction tower 8, and in the case of silver-alumina adsorbent, 150 tons
In the case of synthetic zeolite, air containing Step 2 was passed through at a temperature of 50C and a linear velocity of 5 cm/8 for a predetermined period of time. ■! The amount of adsorption is not much different between the two, and is saturated at the entrance of the adsorbent layer. There is an unsaturated part behind it, and the length of this unsaturated part is generally called the adsorption zone length.
Although the length of the portion showing an adsorption amount of 5% to 95% of the saturated adsorption amount is longer in the case of synthetic zeolite than that of alumina impregnated with silver nitrate, this does not pose a problem in terms of packed bed design. Recognize. Moreover, from FIG. 6, it can be seen that even with synthetic zeolite, the removal efficiency is 99% or more.

Next, the iodine removed in the iodine concentrator is heated to a temperature of 150
C or higher, the inert gas, here N2 gas, is desorbed by passing it through the iodine concentrating tower with the function of a circulator, and the following chemical reaction is used to remove the inert gas, with an immobilization group filled with a copper metal-impregnated adsorbent. , immobilize in a chemically stable state.

2Cu+Iz42CuI In this case, before passing the Nz gas containing I2 through the immobilization group, it is necessary to use a cooler to bring the N2 gas to a temperature of 100 to 150 C, which is the optimum temperature for Cu to react with Iz3. The N-maro gas that has passed through the immobilization group passes through a circulator and a heater,
After being heated to 200C or higher, which is the optimum temperature for regeneration, it is sent to an iodine concession tower. With the above operation, more than 99% of the iodine removed in the iodine concentrator is desorbed, and several tens of P are removed in the treated gas.
Since the iodine contained in the N2 gas contains a high concentration of iodine of several orders of magnitude, the reaction with the Cu metal-impregnated adsorbent is accelerated and can be almost completely immobilized as CuI. At this time, N! There is NOx in the gas.

The I20 impurity is 1/100 or less compared to the concentration of 1 to 2 thiords in the processing gas, and
Does not inhibit metal reactions. Also O in N2 gas! Since the concentration is low, Cu can be utilized to the maximum extent, and as a result, the amount of waste generated can be reduced.

In the example shown in FIG. 5 above, two adsorption towers and two iodine concentration towers are shown, but the number is not limited to two towers, and it is sufficient to install multiple towers or more, and they can be switched and operated using known technology. Also good. In adsorption towers intended for the removal of NOx and H2O, for example, a hygrometer such as a dew point meter or a NOx meter is installed behind the tower, and by detecting the breakthrough of I20 and NOx and performing switching operation, It is possible to prevent NOx and HtO from flowing into the iodine concentration column on the latter stage.

In addition, the used adsorption tower is regenerated at a temperature of 200 to 400 C, which is usually used industrially, and is reused. Further, the operation of the iodine concentration tower is similar to that of the adsorption tower.

In this embodiment, the adsorbent to be filled in the immobilization tower 9 has been described using a copper-impregnated adsorbent as an example, but a lead-impregnated adsorbent may also be used. Further, although the copper and lead attached need to be metals, an alloy containing at least one of these may be used. Activated alumina, silica gel, and crystalline aluminosilicate can be used as the carrier to which these metals are attached.

〔Effect of the invention〕

According to the present invention, adsorbents impregnated with copper and lead metals, which are easily affected by atmospheric gases, can be used as immobilizing materials with a relatively simple system, and silver consumption can be reduced by filling the adsorbents impregnated with silver. It has the effect of reducing the amount by about 11/20 compared to the adsorption tower independent removal method. Furthermore, the adsorption temperature, which was conventionally 150C, can be lowered to 50C, and the heated l-metal capacity of the processing gas can be reduced by 1/3.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the basic flow of a preferred embodiment for carrying out the present invention, and Fig. 2 is a diagram showing the flow of a conventional iodine removal method using an adsorption tower alone filled with a silver-impregnated adsorbent. , Fig. 3 is a diagram showing the flow of a method of repeatedly using a conventional silver-impregnated adsorbent, and Fig. 4 is a flowchart of a method of using a metal-impregnated adsorbent such as copper or lead in the first stage adsorption tower. Figure 5 shows the influence of pores on the adsorption of I2, NOx, and H2O, and Figure 6 shows an example of the iodine adsorption distribution in an adsorption column filled with synthetic zeolite. FIG. 1... Adsorption tower filled with silver-impregnated adsorbent, 2... Adsorption tower filled with lead-impregnated adsorbent, 3... Heater, 4...
Cooler, 5... Circulation pump, 6... Adsorption tower filled with copper-based groove adsorbent, 7... Dehumidifying material (pore diameter 4 Å or less)
adsorption tower filled with
9. Immobilization tower filled with copper-impregnated adsorbent.

Claims (1)

[Scope of Claims] 1. A system for removing iodine from gas, including an adsorption tower in the first stage filled with a dehumidifying material that selectively adsorbs NOx and H_2O contained in the gas, and a system that reversibly removes iodine. A radioactive substance comprising a subsequent iodine concentrating column filled with a dehumidifying material for adsorption and desorption, and an immobilization column filled with an adsorbent for adsorbing iodine that is desorbed by regeneration of the iodine concentrating column. Iodine removal system. 2. In the radioactive iodine removal system according to claim 1, the pore size of the dehumidifying material filled in the adsorption tower is 4.
Å, the pore diameter of the dehumidifying material packed in the iodine concentration tower is in the range of 5 to 20 Å, and the adsorbent packed in the immobilization tower contains at least a metal containing either metallic copper or metallic lead. A radioactive iodine removal system characterized by an attached adsorbent. 3. The radioactive iodine removal system according to claim 2, characterized in that the iodine concentrator is used to regenerate iodine with an inert gas.