JPS5940300A - Method of processing gaseous waste from nuclear fuel reprocessing - 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 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 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. Water containing NOz * K
Exhaust gas containing r*X6 etc. is generated. 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 NOx. %(N
O2 is about 9 degrees], I2220-200pp,
CO2 about 300ppm, I20) race (depending on the operating conditions of the silica gel dehumidification tower, dew point -10 to -15℃)
), trace amounts of radioactive gases such as Kr and Xe (the remainder is air).

One known method for removing I2 is to use zeolite supported with Ag and to recover (2) as silver iodide, but this method is inefficient.

A drawback is that 5 to 10 times the theoretical amount of Ag is required. In addition, “Molecular Sheep 13X” and “Zeolam F
9J, "Molecular Sieve 5AJ," Zeorum A-5"
Type X or Type A zeolites, which are commercially available under trade names such as , can be processed without Ag.

It has also been reported that it has the ability to adsorb

However, most of these zeolites with approximately equal ratios of Si:A, 8 in the main crystal lattice are not durable;
They have a common drawback that 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. The exhaust gas emitted from the silica gel dehumidifying tower mentioned above contains NOx and gas (3), and even though it is dehumidified in the silica gel dehumidifying tower, the dew point is around -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 collapse of the zeolite as an adsorbent, which seriously impedes the actual adsorption operation. 0 This invention was made in order to eliminate the drawbacks of the conventional methods as described above. , focusing on its ability to effectively adsorb and desorb NOx and moisture,
Nuclear fuel reprocessing exhaust gas is passed through a primary adsorption tower using mordenite-based and/or clinoptilolite-based zeolite as an adsorbent, and the water and NOx in this exhaust gas are adsorbed and separated, and the remainder that is not adsorbed is removed. The dry passing gas containing I2 is sent to the secondary adsorption device for removal in the normal process, and the NOx and moisture adsorbed and separated in the primary adsorption tower are desorbed and used in the previous dissolution process (4) and dehumidification. It is an object of the present invention to provide a method for treating nuclear fuel reprocessing exhaust gas that can effectively separate I2 and prevent NOx from being released to the outside by returning it to the process.

Next, an example of the process of the system of the method of this invention will be explained with reference to the drawings. In the figure, reference numeral 1 denotes a dissolution tank containing nitric acid. Sheared fuel rods are dissolved in nitric acid in this dissolution tank 1, and the solution is sent to a predetermined reprocessing step.

In addition, the exhaust gas containing nitric acid mist generated from the dissolution tank 1 is
After passing through a pre-treatment device 2 consisting of a condenser and a mist separator to remove most of the nitric acid mist and water (including tritiated water), it passes through a dehumidification tower 3 filled with a desiccant such as silica gel. Dew point -15℃
It is dehumidified until the temperature reaches about -10°C. The passing gas leaving the dehumidification tower 3 is converted into 02+N2+ and +X6m as described above.
It consists of I2+NO2*NO+H20, etc.

Note that since the desorbed water taken out during regeneration of the dehumidification tower 3 contains tritiated water, it is sent to the tritiated water distillation recovery process.

On the other hand, the gas that has passed through the dehumidification tower 3 is led to an adsorption device 4 to mainly remove NOx and moisture. In this example, the adsorption device 4 has three adsorption units "f 4 a * 4 b *
It consists of 4 c. The gas passing through the dehumidification tower 3 is first introduced into the first adsorption tower 4a from the bottom, and is converted into I2. N01NO2 and I20 are adsorbed.

Mordenite or/and clinoptilolite based zeolite adsorbents have I2
Although it has a low adsorption ability, it does not disintegrate with acids, and NO2 is adsorbed as is, and NO is adsorbed and then adsorbed in the form of oxidized NO2. This oxidation performance is based on mordenite-based and/or clinoptilolite-based zeolite adsorbents, copper, and chromium.

Further improvement can be achieved by supporting elements such as iron, cobalt, nickel, etc. as active security.

In the illustrated example, the gas that has passed through the first adsorption tower 4a is supplied to the second adsorption tower 4b, and the gas that has passed through the second adsorption tower 4b is sent to the I2 adsorption tower 5. There is. In the state shown in the figure, the third adsorption tower 4C is being desorbed. In order to perform adsorption continuously, each adsorption tower 4a,
4b and 4c are adsorption 2. Adsorption 1. Switching is performed in a predetermined order so that the three steps of attachment and detachment are carried out sequentially. That is, in the first adsorption step, due to the difference in the adsorption capacity of the adsorbent, as the adsorption progresses, first the oxide flows out, and then the N flows out.

and NO2 sequentially flow out. In addition, in the second adsorption step,
After all, I2 leaks out first, and if you leave it as it is, then KN
O flows out. What is important here is to switch the adsorption tower 4b in the second adsorption step to the adsorption tower that has completed desorption before the NO flows out.
It is possible to prevent o from being mixed in and to avoid troublesome disposal of the discharged gall.

By sequentially passing through the two adsorption towers 4a and 4b, all of the moisture and NOx contained in the gas that has passed through the dehumidification tower 3 and a part of The gas exiting the column 4b (7) and supplied to the I2 adsorption column 5 is 02 I N 2
+ K 1 It becomes a completely dry substance consisting of tXe, I2, etc.

The adsorption tower 5 is filled with known X-type or A-type zeolite, which may or may not support Ag, and adsorbs Ag or A contained in the supplied gas.
Retain as gI. Therefore, the gas passing through the I2 adsorption tower 5 is O2°N2. Contains Kr, Xe, etc.
■, moisture, and NoX are completely removed, and normal K
It is sent to the separation process of r, Xe, etc. This process, adsorption process, and subsequent treatment are the same as normal ones, so a detailed explanation will be omitted. However, since the gas treated in this process does not contain water, the zeolite in the I2 adsorption tower 5 It does not cause alteration, deterioration or disintegration due to acids, and is free from NO
Since x is also not present, the post-treatment process for the released gas is facilitated.

On the other hand, the adsorption tower 4C which has completed the adsorption 2nd step and the adsorption 1st step
Desorption can be carried out by heating the adsorption tower 4c to a temperature of about 200 to 500°C, or by passing a small amount of expelling gas (8) such as superheated steam into the adsorption tower 4c while heating. The desorption gas extracted from the adsorption tower 4C in this way (this is NO2
and a small amount of water and I2) is transferred to the next absorption tank 6.
Inside, it comes into contact with water or dilute nitric acid, which is supplied separately.

In this absorption tank 6, 3NO2 + H20 → 2HNO, +
No...In the reaction of (1), nitric acid and No are about 2=
Generate at a ratio of 1. Of these, the No gas containing moisture is led to the dehumidification tower 3 through the thread 7, and the concentration of the nitric acid aqueous solution is adjusted by means such as distillation in an acid concentration adjustment tank 9 if necessary. It is returned to the dissolution tank 1.

However, if this absorption is carried out efficiently, nitric acid with a concentration of 69%, which is an azeotropic mixing ratio with water, can be obtained, so it can be returned to the dissolution tank 1 as it is. In this way, this system allows a nearly completely closed circuit circulation for NOx. On the other hand, when the caustic soda washing method, which is a normal exhaust gas treatment method, is used, NaNO3 and NaNO2 are generated.
It is impossible to recycle them, and the disadvantage is that they must be stored in isolation along with the highly radioactive elements that accompany the accumulating large amounts of these neutralized salts. Therefore, this method results in a significant volume reduction of radioactive waste.

As described above, according to the present invention, when radioactive iodine is adsorbed and removed by zeolite from exhaust gas generated in a nuclear fuel reprocessing process, an adsorbent made of mordenite-based and/or clinoptilolite-based zeolite, which is difficult to decompose with acids, is used. First, water and NOx are removed by adsorption treatment with a gaseous agent, and iodine in this gas is removed by an X-type or A-type zeolite in the subsequent stage.
Zeolite for 2 adsorption is not degraded by acid, and its iodine adsorption ability can be maximized. In addition, NOx adsorbed by the mordenite-based and/or clinoptilolite-based zeolite adsorbent is returned to the dissolution tank or dehumidification tower, so it only circulates within the system and is not discharged to the outside, making it radioactive. This results in a significant volume reduction in waste treatment, which is extremely advantageous in terms of environmental conservation.

[Brief explanation of drawings]

The figure is a flow sheet showing an example of the steps of the method of this invention. 1... Dissolution tank, 2... Pretreatment device, 3... Dehumidification tower, 4... Adsorption device, 4a, 4b, 4c... Adsorption tower,
5... I2 adsorption tower, 6... Absorption tank, 7, 8... Yarn path, 9... Acid concentration adjustment tank. Patent Applicant Industrial Development Research Institute 1, Incident Indication 1981 Poetry Permit No. 151277 2° Invention'
cl) Name Kh Nuclear fuel reprocessing exhaust gas treatment method 3,
Relationship with the case of the person making the amendment Patent applicant Address: Industrial Development Research Institute 4, Agent Address: 2-11-1 Yoyogi, Shibuya-ku, Tokyo 151
Contents of the amendments to the appendix on the 7th floor of Honshi Building No. 2 (1) The scope of claims in the specification will be amended to the appendix. (2) An object of the present invention is to provide a method for treating nuclear fuel reprocessing exhaust gas that can effectively remove I2 and NOx by replacing "while sending" with "while sending" in line 19 of page 4 of the specification. It is corrected to ``furthermore''. (3) In the 5th page, lines 2 to 4, "Do not emit...It is intended for music purpose." is corrected to "It is possible to process it so that it does not emit." (4) “NOx is...” on page 10, lines 16-17.
. . . "In the system because it is returned" is corrected to "If the desorbed gas that has desorbed NOx is returned to the dissolution tank or dehumidification tank, NOx will be in the system." r2) Amended Claims (1) A method for treating exhaust gas containing NOx generated when dissolving spent nuclear fuel in nitric acid for reprocessing, which comprises introducing the exhaust gas into a dehumidification tower. The process of dehumidifying
A step in which the gas that has passed through the dehumidification tower is introduced into an adsorption tower filled with an adsorbent made of mordenite zeolite and/or clinoptilolite zeolite to mainly adsorb residual moisture and NOx; A step in which the dry gas that has passed through the tower is introduced into a second adsorption tower filled with X-type or A-type zeolite to adsorb and retain iodine;
Treatment of nuclear fuel reprocessed exhaust gas, characterized by extracting desorbed gas mainly consisting of water and NO2 produced by the desorption operation of the adsorption tower filled with an adsorbent made of clinoptilolite-based zeolite and/or mordenite-based zeolite. Method. (2) A method for treating exhaust gas containing NOx generated when dissolving spent nuclear fuel in nitric acid for reprocessing, which includes a step of introducing the exhaust gas into a dehumidification tower to dehumidify it; A step of introducing the gas that has passed through the wet tower into an adsorption tower filled with an adsorbent made of mordenite zeolite or/and clinoptilolite zeolite to mainly adsorb residual moisture and NOx; The passed dry gas is introduced into the adsorption tower filled with X-type or A-type zeolite to adsorb and retain iodine, and the adsorbent made of clinoptilolite-based zeolite or/and mordenite-based zeolite is filled. The desorption gas mainly composed of water and N02 produced during the desorption operation of the adsorption tower is brought into contact with separately supplied water or dilute nitric acid to remove the NO2 contained in the desorption gas.
A step in which nitric acid and NO gas are generated by absorbing the nitric acid into water, and the nitric acid generated in this step is used as nitric acid for the reprocessing, and the gas containing NO is returned to the dehumidification tower. A method for treating nuclear fuel reprocessing exhaust gas, comprising a step of circulating NOxK in a closed cycle.

Claims (1)

[Claims] In a method for treating exhaust gas containing NOx generated when dissolving spent nuclear fuel in nitric acid for reprocessing,
Introducing the above exhaust gas into a dehumidification tower to dehumidify it, and introducing the gas that has passed through the dehumidification tower into an adsorption tower filled with an adsorbent made of mordenite-based zeolite and/or clinoptilolite-based zeolite. and remaining moisture and N
A step of mainly adsorbing Ox, and a step of transferring the dry gas that has passed through the adsorption tower to I2 filled with X-type or A-type zeolite.
Desorption consisting mainly of water and N02 produced by the step of introducing iodine into an adsorption tower and adsorbing and holding iodine, and the desorption operation of the adsorption tower filled with an adsorbent made of clinoptilolite zeolite or/and mordenite zeolite. A step of generating nitric acid and No gas by bringing the gas into contact with separately supplied water or dilute nitric acid to absorb N02 contained in the desorption gas into the water;
The nitric acid produced in this step is used as nitric acid for the above-mentioned reprocessing, and the NOx-containing gas is returned to the dehumidification tower to circulate the NOx in an almost closed cycle. A method for treating nuclear fuel reprocessing exhaust gas.