JPH0141238B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for safely processing exhaust gas containing radioactive materials generated when spent nuclear fuel is reprocessed in a reactor of a nuclear power plant. In order to reprocess spent nuclear fuel, a wet method is used in which fuel rods are sheared and dissolved in nitric acid.In this dissolution process, along with nitric acid vapor, iodine containing radioactive iodine and tritium water are released. Exhaust gas containing water, NO _x , Kr, X _e , etc. is generated. Of this, most of the nitric acid mist and moisture (including tritiated water) are captured by the mist catcher and silica gel dehumidification tower, but _the exhaust gas still contains _{approximately} 1 vol.% of NO %
degree), _I2 20-200ppm, _CO2 approximately 300ppm, _H2O
Trace (depending on the operating conditions of the silica gel dehumidification tower, dew point is about -10 to -15°C), trace amounts of radioactive gases such as X _r and X _e (the remainder is air) are included. One known method for removing I ₂ is to use Ag-supported zeolite and recover I ₂ as silver iodide, but it is inefficient and produces 5 to 10 times the theoretical amount of Ag. The disadvantage is that it requires Also "Molecular Sieve 13X", Zeorum
F9”, “Molecular Sieve 5A”, “Zeolum A-5”
It has also been reported that type X or type A zeolites, which are commercially available under trade names such as zeolites, have the ability to adsorb I ₂ even without Ag. However, most of these zeolites with a nearly equal ratio of Si:Al in the main crystal lattice are not durable, and in the presence of acid, the crystal structure itself easily collapses and the adsorption effect disappears, making it impossible to use it as an adsorbent. They have a common drawback. The exhaust gas emitted from the silica gel dehumidification tower mentioned above contains _NOx and _I2 , and even if it is dehumidified in the silica gel dehumidification tower, it will contain moisture with a dew point of about -10 to -15℃ as mentioned above. When this exhaust gas enters an adsorption tower filled with zeolite, it generates acid in the zeolite, leading to the disintegration of the zeolite as an adsorbent, which seriously impedes the actual adsorption operation. This invention was made in order to eliminate the drawbacks of the conventional method as described above. Silica gel has the ability to adsorb NO ₂ as well as water (NO ₂
found that the modified natural tuff obtained by hydrogenating natural tuff in a conventional manner has good _{I 2 adsorption} and acid resistance. , a step of removing moisture and NO ₂ in the exhaust gas with silica gel, a step of removing I ₂ with the above modified natural tuff, and a step of removing the remaining _NO
The purpose of the present invention is to provide a treatment method that can effectively adsorb and remove I ₂ and _NO It is said that Although silica gel is generally thought to exhibit adsorption properties only to water, the present inventors have discovered that in the presence of moisture, silica gel exhibits strong adsorption properties, particularly to NO ₂ in gases. In other words, when the mist is removed from reprocessed exhaust gas and then brought into contact with silica gel, it is possible to adsorb and remove a considerable portion of NO ₂ along with the moisture contained in this gas, which was previously impossible. This was something that was not known. For example, NO ₂ 0.84% by volume, I ₂ 80ppm,
A gas mixture of 0.78% H ₂ O and 1 l/min of air was passed through 27 g of silica gel for about 30 hours. Gas flow rate is approximately 1800l
The weight increase of the silica gel layer was 15.83g. When this silica gel was heated to 550°C in a N ₂ stream to desorb it, 3.309 g of HNO ₃ , 0.036 g of I ₂ ,
A total of 15.26 g of desorbed material was obtained, including 0.2008 g of NO ₂ and 11.71 g of H ₂ O. This result shows that I ₂ is hardly adsorbed, but H ₂ O, HNO ₃ and NO ₂ are well adsorbed. The modified natural tuff used in this invention is obtained by hydrogenating and activating mordenite-based or clinoptilolite-based natural tuff, which is naturally occurring in the Tohoku region, Chugoku region, Kyushu region, etc., using an appropriate method. This is what was obtained. This hydrogenation is
Particles of a suitable size obtained by crushing raw stones are washed with water, air-dried, and then treated with, for example, 1N-HCl at 100℃.
This can be done by refluxing for 30 hours, followed by washing with water and air drying. In this specification, the term "hydrogenation" means replacing the Na component of the target natural tuff with hydrogen ions, as shown in the formula below. Ze−Na ⁺ +H ⁺ →Ze−H ⁺ +Na ⁺ This hydrogenation reaction may be performed by directly applying HCl to the target natural tuff, or after ion exchange with NH ₄ Cl etc. It can also be hydrogenated by heating to drive off _NH3 . These reactions are as follows. [Direct method using HCl] Ze−Na ⁺ +HCl→ZeH ⁺ +Na ⁺ Cl [Indirect method using NH ₄ Cl] Ze−Na ⁺ +NH ₄ ⁺ Cl→Ze−NH ₄ ⁺ +Na ⁺ Cl Ze−NH ₄ ⁺ heating―― → ZeH ⁺ +NH ₃ ↑ The natural tuff hydrogenated in this way is not only free from adsorbed water but also before use.
It is desirable to carry out activation which consists of removing chemically bound water of crystallization. This activation process consists of natural tuff crushed to an appropriate grain size.
This can be done by holding the temperature at 550°C for about 1 hour. Natural tuff modified in this way was found to have better adsorption properties for _I2 than for _NOX . The graph in the figure shows the results of a test to examine changes in I ₂ adsorption due to modification. In this test, each natural tuff adsorbent was packed into an appropriate column to form an adsorption layer with a bed height of 12.7 cm, and 242 g of test gas was applied from the top to the bottom of the column at a linear velocity of 2.3.
This was done by flowing at cm/sec. The test gas was air (dew point -55 to -60°C) used as a carrier gas, to which 0.004% by weight of _I2 was added. From this result, it is clear that the modified natural tuff adsorbent has excellent I ₂ adsorption properties that are not observed in the unmodified natural tuff adsorbent that has undergone only activation (in the figure, 〇 indicates modified, ・mark indicates non-modified). It is not clear why natural tuff exhibits I ₂ adsorption properties when hydrogenated, but the pore size of unhydrogenated mordenitic natural tuff is 4-6 Å, and that of clinoptilolite-based tuff is 3-3 Å. 5 Å, whereas in the hydrogenated version the pore size increases to approximately 10 Å,
One reason is thought to be that the property of adsorbing I ₂ was significantly improved due to this increase in pore size. Hydrogenation of zeolite-based adsorbents is well known and the technology is common, but the purpose of hydrogenation in this case is to improve acid resistance, and even if ordinary zeolite is hydrogenated, Adsorption to I ₂ hardly improves. The gas that has passed through the adsorption layer made of the above-mentioned modified natural tuff adsorbent contains almost no I ₂ due to the good I ₂ adsorption properties of this adsorbent, but it is contained in the nuclear fuel reprocessing exhaust gas. Since there remains a significant amount of NO _x remaining, a third adsorption step to remove NO _x is required following the second adsorption step to remove I ₂ . This third adsorption step can be carried out using conventional adsorbents that have adsorption properties for NO _x , but the best results are obtained by applying activation treatment to natural mordenite or clinoptilolite tuffs. Obtained when using an adsorbent treated with In this unmodified natural tuff adsorbent, NO ₂ is adsorbed as is, but after being adsorbed, NO is retained in the form of oxidized NO ₂ . This oxidation state is caused by copper and copper in the above natural tuff.
Further improvement can be achieved by supporting metals such as chromium, iron, cobalt, and nickel as active centers. The gas to be treated that is supplied to the first adsorbent layer made of silica gel is the exhaust gas generated when the sheared fuel rods are melted in the melting tank, and this exhaust gas has been subjected to pretreatment such as dehumidification. It is not necessary, and it is enough to remove the mist. This exhaust gas is sent to a first adsorbent layer made of silica gel.
In this first adsorbent layer, in addition to removing moisture from the inflow gas, NO ₂ is mainly adsorbed and removed (most of the NO ₂ is present in the form of HNO ₃ ). 1st
The gas leaving the adsorbent layer has almost completely removed moisture, and then passes through the second adsorbent layer made of the above-mentioned modified natural tuff, where mainly I ₂ in the gas is adsorbed. and a third layer made of natural tuff.
The remaining NO _x by passing through the adsorbent layer of
is removed by adsorption. In addition, the silica gel column in the first adsorption step, which adsorbs and removes H ₂ O and NO ₂ , can be used in a so-called push-up method using two or more columns to remove moisture more completely. Detach one tower of
It is desirable to use a continuous method for regeneration. The second adsorption step and the third adsorption step can also be performed in a continuous manner. Example An adsorption tower was prepared in which first to third adsorbent layers were sequentially formed through glass wool from the upper end to the lower end in a column having a diameter of 17.5 mm and a length of 250 mm. The first adsorbent layer was made of 27.05 g of silica gel (manufactured by Union Carbide, trade name "Purasiv-N301").
The second adsorbent layer consists of 9.50 g of hydrogenated mordenite-based natural tuff (from Itado).
The third adsorbent layer is made of crushed mordenite-based natural tuff (from Itado) and activated by firing at 550°C. Into this adsorption tower, 1.785 liters of the gas to be treated was flowed from the top to the bottom at a linear velocity of about 7 cm/sec over a period of 29.75 hours. The gas to be treated is air as a carrier gas (moisture 0.78
%) to _I2 80ppm, _NO2 0.84% by volume, NO0.10% by volume
It was prepared by adding. Table 1 shows the composition (volume ratio) of the processing gas flowing out from each adsorbent layer.

[Table] As described above, according to the method of this invention, three basic types of adsorption are possible using three types of adsorbents: silica gel, hydrogenated mordenite-based or clinoptilolite-based modified natural tuff, and natural tuff. By combining the steps, it is possible to effectively adsorb and remove I ₂ and NO _X without being affected by the moisture, no matter how much moisture the exhaust gas to be treated contains. Therefore, the pretreatment process can be simplified, and the adsorption process can be easily managed.

[Brief explanation of drawings]

The figure is a graph showing the I ₂ adsorption characteristics of modified natural tuff and unmodified natural tuff.

Claims (1)

[Claims] 1 In a method for treating exhaust gas containing I ₂ and _{NO 2}
a first adsorption step for adsorbing and removing the above-mentioned first
The second adsorption step mainly adsorbs and removes I ₂ in the gas that has passed through the adsorbent layer, and the third adsorbent layer is made of mordenite-based or (and) clinoptilolite-based natural tuff. A method for treating nuclear fuel reprocessing exhaust gas, comprising: a third adsorption step for adsorbing and removing _NOx remaining in the gas that has passed through the adsorbent layer.