JPH0149000B2 - - 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 nuclear power plant reactor. 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 , K _r , 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 _x (NO ₂ is 90
%), I ₂ 20-200ppm, CO ₂ 300ppm,
H ₂ O trace (depending on the operating conditions of the silica gel dehumidification tower, dew point around -10 to -15°C), trace amounts of K _r ,
Contains radioactive gases such as X _e (the remainder is air). 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 In addition, “Molecular Sieve 13X” and “Zeolam
F9”, “Molecular Sieve 5A”, “Zeolami 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 is lost, making it impossible to use it as an adsorbent. They have common drawbacks. The exhaust gas emitted from the silica gel dehumidification tower mentioned above contains NO _x and I ₂ , 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°C 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 most of HNO3 exists as _HNO3 , and certain synthetic zeolites have good _I2 adsorption and acid resistance, and the process of removing moisture and _NO2 in flue gas with silica gel and certain By combining the process of removing I ₂ with synthetic zeolite and the process of removing remaining NO _x with natural coagulated gray, I ₂ and NO The purpose of this invention is to provide a treatment method that allows _x to be adsorbed and removed. Although silica gel is generally thought to exhibit adsorption properties only for water, the present inventors have discovered that in the presence of moisture, silica gel exhibits strong adsorption properties for gases, particularly NO ₂ . 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. Furthermore, the synthetic zeolite used for I ₂ removal in this invention is a Faujasite-based synthetic zeolite hydrogenated using a conventional method, and compared to other zeolites and molecular sieves, it has Na ₂ O content is about 3% (regular one is about 7%)
~8%), and the SiO ₂ /Al ₂ O ₃ ratio is approximately 1.3.
(Regular ones are 1.5 to 5.0) and small. In this specification, the term "hydrogenation" means replacing the Na component of the target fusiasite-based synthetic zeolite with hydrogen ions, as shown in the formula below. Ze−Na ⁺ +H ⁺ →Ze−H ⁺ +Na ⁺ This hydrogenation reaction can be carried out by directly applying HCl to the target zeolite. For hydrogenation, it is desirable to perform ion exchange with NH ₄ Cl or the like, then heat to drive off NH ₃ . The reaction is as follows. Ze−Na ⁺ +NH ₄ ⁺ Cl→Ze−NH ₄ ⁺ +Na ⁺ Cl Ze−NH ₄ ⁺ Heating――→ ZeH ⁺ +NH ₃ ↑ Furthermore, natural tuff for removing NO _x is found in the Tohoku region and Chugoku region, for example. Mordenite and clinobutyrolite natural tuff naturally occurring in the Kyushu region and the like can be crushed to a suitable particle size (for example, 8 to 30 mesh) and then activated. This activation is carried out, as is generally known, by heating the natural tuff particles in an inert gas such as _N2 . 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 is subjected to pretreatment such as dehumidification. There's no need. This exhaust gas is sufficient to simply remove the mist and is then sent to the first adsorbent layer of silica gel. This first adsorbent layer mainly adsorbs and removes NO ₂ as well as removes moisture from the inflow gas (most of the NO ₂ is present in the form of HNO ₃ ). The gas leaving the 11th adsorbent layer has almost completely removed moisture, and then passes through the second adsorbent layer made of the above-mentioned synthetic zeolite, where mainly I ₂ in the gas is adsorbed. The remaining NO _x is adsorbed and removed by passing through a third adsorbent layer made of natural tuff. This third
In the adsorbent layer, NO ₂ in the gas is adsorbed as is, but after being adsorbed, NO is retained in the form of oxidized NO ₂ . This oxidation performance can be further improved by supporting a metal such as copper, chromium, iron, cobalt, or nickel as an active center on a mordenite-based or cucrinobutyrolite-based natural tuff adsorbent. 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 is 25g of silica gel (manufactured by Union Carbide, trade name "Purasiv-N301")
The second adsorbent layer is made of fusiasite-based synthetic zeolite 10 hydrogenated by the NH ₄ Cl method.
Consists of g. In addition, the third adsorbent layer is made by crushing mordenite-based natural tuff (from Itado) and then heating it to 550°C.
The material is activated by firing. A gas to be treated (1800 ml) was flowed into this adsorption tower from the top to the bottom at a linear velocity of about 7 cm/sec over a period of 30 hours. The gas to be treated is air (moisture 0.78%) as a carrier gas, _I2 80ppm, _NO2 0.84% by volume,
It was adjusted by adding 0.10% by volume of NO.
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 types of adsorbents are used: silica gel hydrogenated fusiasite-based synthetic zeolite and natural tuff.
By combining two basic adsorption processes, I ₂ and NO _x can be effectively adsorbed and removed and recovered without being affected by moisture, no matter how much water the exhaust gas contains. becomes. Therefore, the pretreatment process can be simplified, and the adsorption process can be easily managed.

Claims (1)

[Claims] 1 In a method for treating exhaust gas containing I ₂ and _{NO 2}
The first adsorption step adsorbs and removes I2, and the second adsorbent layer is made of an adsorbent made by hydrogenating fusiasite-based synthetic zeolite, which mainly adsorbs _I2 in the gas that has passed through the first adsorbent layer. A second adsorption step for removing _NO A method for treating nuclear fuel reprocessing exhaust gas, comprising a third adsorption step for oxidation removal.