JPH01101499A - Treatment of retreatment off-gas - Google Patents

Abstract

PURPOSE:To prevent the corrosion by iodine at the time of starting and stopping of an off-gas treating device by supplying a gas essentially consisting of NO2 to a retreatment off-gas in such a manner that the concn. of the NO2 in the gas attains >=0.5% when the concn. of the NO2 in the gas is low. CONSTITUTION:The respective devices in the system from a pre-condenser 5 to a blower 15 of the off-gas treating device are started so that the treating device is ready to operate. A dissolution tank 4 is heated up to a temp. T1 simultaneously with dissolution of nuclear fuel and further, a nitric acid NO2 generator 19 is heated up to a prescribed temp. T2. The nitric acid vapor, steam and gaseous NO2 from the dissolution tank 4 and the nitric acid NO2 generator 19 are then supplied to the off-gas system. The shearing and dissolution of the fuel are executed by supplying the nitric acid vapor, steam and gaseous NO2 to the off-gas system for the prescribed time t1 so that the concn. of the NO2 in the gas attains >=0.5%.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention relates to a reprocessing off-gas treatment method, and particularly to a method for preventing corrosion caused by iodine in an apparatus for removing iodine and No from reprocessing off-gas. The present invention relates to a suitable reprocessing off-gas treatment method.

[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, off-gas mainly composed of steam, No, etc. is generated, and these are used as carrier gas to remove volatile F, P (fission products) such as iodine in the fuel used. ) is 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 in reprocessing off-gas treatment systems.

A conventional off-gas treatment process has a system shown in FIG. In this treatment process, off-gas 18 generated from the dissolving tank 3 in which spent fuel is dissolved with concentrated nitric acid and the iodine expelling tank 4 from the dissolving liquid first enters the pre-condenser 5, where it is cooled and most of the water vapor is removed. is collected. This condensed water is used as an absorption liquid in the N08 absorption tower 6.

Front capacitor 5'? ! The uncondensed gas is N
o, sent to absorption tower 6.

No, the gas leaving the absorption tower 6 is mixed with the gas coming out from the iodine re-expelling tower 9, then enters the preheater 12 and is preheated to 150°C to lower the relative humidity in the gas, and then the adsorbent is It enters the 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, since a large amount of iodine is mixed in the condensed water from the precondenser 5 and the absorbed acid in the absorption tower 6, after these condensed water and absorbed acid are stored in the absorption acid storage tank 7, , is transferred to an iodine re-expulsion tower 9 by a pump 8. The absorbed acid is 80 to 90 in this iodine re-entering column 9.
The solution is heated to .degree. C., and iodine is expelled from the liquid into the gas using N8 gas. After the absorbed acid from which iodine has been removed is cooled, it is returned to the dissolution tank 3 after being adjusted for reuse as a dissolving solution via the recovery acid storage tank IO and the transfer pump 11.

[Problem that the invention seeks to solve]

In the above conventional technology, since the iodine removal column 13 is installed downstream of the precondenser 5, No. 5, and absorption column 6, highly corrosive iodine is contained in most parts of the off-gas system. become.

The corrosion behavior of equipment components due to iodine in a typical off-gas environment will be described in more detail below.

(ORNL Report No., 0RNL/5u
b-7327/11 (81-11) As a corrosion test of various materials using iodine-containing gas, the test atmosphere was 25°C.
7,4 x 10-'g/mj of iodine in the atmosphere! The results of testing various materials for 400 hours in a gas phase containing 10-'g/ml of water I
loyC-276, [nconel 626, AL-
294 and Hastelloy G) have high resistance to both general corrosion and pitting susceptibility.

However, other ordinary austenitic stainless steels and N1-based alloys with low Mo content have a large amount of general corrosion and a large pitting corrosion depth, and do not show an essentially acceptable amount of corrosion.This means that the equipment configuration If a material with corrosion resistance is selected, the cost of the equipment will be extremely large, and economic efficiency will be ignored.

On the other hand, even Incoloy 825 and Inclinel 671, which have relatively good corrosion resistance, lose 0 per year. Since pitting corrosion of 5 mm occurs, corrosion of 5 mm or more must be taken into account even if the equipment life is 10 years, and deterioration of heat transfer performance due to thickening becomes a problem. As described above, the influence of iodine on corrosion is large, and even a very high concentration of iodine, on the order of ppm, has an adverse effect.

The purpose of the present invention is to solve the problems of the prior art described above,
An object of the present invention is to provide a reprocessing off-gas treatment method that can prevent corrosion caused by iodine in an off-gas treatment system containing highly corrosive iodine.

[Means to solve the problem]

The above purpose is to remove NO from the reprocessed gas containing iodine and NO in the presence of iodine, such as when starting or stopping the equipment. This is achieved by supplying a gas containing NO2 as a main component into the off-gas treatment system so that the concentration of NO2 in the gas is 0.5% or more when the concentration of NO2 is low.

[Effect]

Against corrosion caused by iodine, the coexistence of 0.5% or more of NO□ suppresses the occurrence of pitting corrosion.

That is, this method focuses on the fact that corrosion caused by iodine is suppressed when the NO8 concentration is above a certain level.

Figure 12 shows low C-18Cr-8Ni (SUS304
These are the results of a study on the influence of NO8 concentration on iodine-induced corrosion of L material).

(ORNL Report No., 0RNL/5u
b-7327/l 1 (81-11) As is clear from Figure 11, 5US304 in saturated steam containing iodine
Corrosion of the L material no longer occurs when the NOI concentration is 0.5% or higher.

Although the reason for this is not clear in detail at the moment, the following two reasons can be considered. First, it is thought that NOx forms nitric acid (HNOs) through the reaction shown in the following equation with the thin moisture layer formed on the surface of the test piece, and corrosion is suppressed by the oxidizing power of nitric acid.

2NO + H, O → HN Os + HN O The second reason is considered to be that nitric acid vapor directly adheres to the gas phase metal surface and forms a thin nitric acid liquid film layer.

Therefore, when the NOR concentration in the gas is low in the presence of iodine, such as when starting or stopping the equipment,
If a gas containing NO as a main component is supplied into the off-gas treatment system, and the NO and fi degrees in the gas are made to be 0.5% or more,
It can prevent corrosion caused by iodine.

〔Example〕

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

Figure 1 shows the first reprocessing off-gas treatment method according to the present invention.
It is a system diagram showing an example.

In FIG. 1, an iodine re-extraction tower 9 and a recovered acid storage tank 1 are shown.
A part of the nitric acid produced and recovered at
The device is configured to be able to supply O, nitric acid vapor and NOx gas from the generator 19 to the inlet of the precondenser 5.

Next, an example of a reprocessing off-gas treatment method using the apparatus shown in FIG. 1 will be described with reference to FIG. 2. FIG. 2 is an explanatory diagram showing a start/stop pattern of the reprocessing off-gas treatment device.

First, among the off-gas processing equipment, each device in the system from the pre-condenser 5 to the blower 15 is started, and the melting tank 4 is heated to a temperature T at the same time as nuclear fuel melting. Furthermore, the temperature of the nitric acid/NO* generator 19 is raised to a predetermined temperature Tt, and the melting tank 4 and the nitric acid/N0
8 Nitric acid vapor, water vapor and NO8 from generator 19
Supply gas to the off-gas system. Then, for a predetermined time t, the nitric acid vapor, water vapor, and NO2 are supplied through an off-gas system so that the NO concentration in the gas becomes 0.5% or more1, and the fuel is sheared and dissolved.

The above is the operation pattern at startup.

When the off-gas treatment device is stopped, both the dissolution tank 4 and the nitric acid/NO8 generator 19 continue to operate for a period of t3 even after fuel dissolution is stopped, and the iodine remaining in the off-gas treatment device is treated with No. 8 in the gas. Set the concentration to 0. When it is 5% or more, iodine is removed in the coexistence of nitric acid vapor and NO8, and a stop pattern is 0 or more. In this case, since the removal of iodine is easier at higher temperatures, the pre-condenser 5 and the NOx
The temperature of the absorption tower 6 and the like may be increased.

The nitric acid/NO1 generator 19 forms nitric acid vapor using a heating method, and when the nitric acid concentration is high and the temperature is high, the following reaction occurs due to thermal decomposition of nitric acid,
A large amount of NOo is generated.

4HNOx→ 2Ht O+4NO+03 This reaction has strong temperature dependence, and the operating temperature T of the nitric acid/NO8 generator 19 is preferably to heat the nitric acid to 100°C or higher. In this case, the time t until the fuel dissolves , can be shortened.

However, even if only the low-temperature nitric acid vapor is used, the time t until the fuel melts can be lengthened.The nitric acid vapor, water vapor, and NO8 from the nitric acid/N08 generator 19 are removed by the blower 15 installed on the downstream side of the off-gas system. Although it is supplied to the off-gas system, it is more effective to use a carrier gas such as N1 gas at this time.

In this way, before the fuel is dissolved, that is, before iodine is contained in the off-gas, nitric acid vapor, water vapor, and NOx are passed through the surface of the device in the gas phase, which is a severely corrosive environment, and an aqueous nitric acid solution is applied to the surface of the device in the gas phase in advance. By forming a thin liquid phase consisting of the composition, corrosion caused by iodine can be prevented.

In addition, the concentration of iodine generated by dissolving fuel during operation of the off-gas treatment equipment is diluted with nitric acid vapor, water vapor, and NOx gas from the nitric acid/No□ generator 19, so that it is introduced into the off-gas treatment system. The iodine concentration in the off-gas decreases, which is effective in terms of corrosion resistance. In addition, an increase in nitric acid concentration in the liquid phase can be expected, which is effective in terms of corrosion resistance.

FIG. 3 shows the second reprocessing off-gas treatment method according to the present invention.
FIG. 3 is an explanatory diagram of a start/stop pattern to show an example.

In the second embodiment, when starting/stopping, the temperature of the melting tank 4 is set to T2 as shown in FIG.
This is to increase the concentration.

That is, in the second embodiment, the dissolution tank 4 is made to function in the same manner as the nitric acid/N08 generator 19 in the first embodiment by changing the operating conditions of the dissolution tank 4 during startup/stop. .

Therefore, in the second embodiment, corrosion due to iodine can be prevented without newly installing the nitric acid/N08 generator 19 as shown in FIG.

FIG. 4 shows the third example of the reprocessing off-gas treatment method according to the present invention.
It is a system diagram showing an example. In the third embodiment, NO! is applied to the inlet of the iodine removal column 13! A monitor 20 is installed. In this case, the NOx concentration in the gas from the NO1 removal tower 6 and the iodine reextraction tower 9 is determined by monitor 20
nitric acid/based on the Now'a degree measurement value.
The temperature of the NO8 generator 19 is controlled, and the amount of nitric acid vapor, water vapor, and N2 introduced from the nitric acid/NO8 generator 19 into the off-gas system is controlled.

FIG. 5 shows the fourth reprocessing off-gas treatment method according to the present invention.
It is a system diagram showing an example.

In the fourth embodiment, an iodine monitor 21 is installed at the entrance of the iodine removal column 13. In case 2, the start/stop pattern is the same as that shown in FIG. The t1 time in FIG. 2 is set depending on the presence or absence of residual iodine. That is, if the iodine monitor 21 determines that no iodine remains in the gas, the equipment will not be corroded by iodine and chlorine, so nitric acid/NO!
The supply of nitric acid vapor, water vapor and NOl to the off-gas system from generator 19 is stopped.

FIG. 6 shows the fifth example of the reprocessing off-gas treatment method according to the present invention.
It is a system diagram showing an example.

In the fifth embodiment, nitric acid/No. gas from the generator 19 is supplied to the inlet of the precondenser 5 and also supplied to the iodine re-entering column 9 in operation to be used as a carrier gas for re-expelling iodine. This is what I did.

In this case, there is no need to use a new carrier gas for re-expelling iodine, and the effect of re-expelling iodine is improved compared to the conventionally used N□ gas.

FIG. 7 shows the sixth reprocessing off-gas treatment method according to the present invention.
It is a system diagram shown in an example.

In the sixth embodiment, nitric acid vapor, water vapor, and NO7 are introduced from the nitric acid/N2Ox generator 19 into the upper portions of the NOx absorption tower 6 and the iodine re-expulsion tower 9, respectively.

Since the NO8 concentration decreases in the upper part of each of the NOx absorption tower 6 and the iodine re-extraction tower 9, nitric acid vapor, water vapor, and NOx from the nitric acid/NO0 generator 19 are introduced into this region to remove the gas. By setting the NO1 concentration therein to be 0.5% or more, corrosion inside the NOx absorption tower 6 and the iodine reextraction tower 9 can be prevented.

FIG. 8 shows the seventh example of the reprocessing off-gas treatment method according to the present invention.
It is a system diagram showing an example.

In the seventh embodiment, two systems of off-gas processing equipment are installed in parallel, and the recovered acid storage tank 10 in each system is installed in parallel.
and a nitrogen Nt/NOz generator 19 are connected. Also, nitric acid/No! The nitric acid vapor, water vapor and NOt from the generator 19 are sent to the precondenser 5 of each system.
It can be introduced at the entrance of the

Normally, in off-gas treatment equipment, when the treatment system is on standby, iodine elements, which are harmful to corrosion, adhere to the surface of each member of the equipment, and at the same time, the water remaining inside the members evaporates, and the temperature during standby is around room temperature, so it is relative. Humidity increases to about 300% or more, and the corrosion rate and pitting depth of each structural member increases.

In the seventh embodiment, by introducing nitric acid vapor, water vapor, and NOx from the nitric acid/No3 generator 19 into the inlet of the precondenser 5 on the side of the processing system on standby, the inside of the processing equipment on standby is reduced to N08. Corrosion caused by iodine can be prevented even if iodine remains in the processing equipment on standby.

FIG. 9 shows the eighth example of the reprocessing off-gas treatment method according to the present invention.
It is a system diagram shown in an example.

In the eighth embodiment, a NOx supply device 22 is installed together with the nitric acid/NOx generator 19, and a gas supply system is arranged at the inlet from each device to the precondenser 5.

In the eighth embodiment, when the amount of nitric acid vapor, water vapor, and NO8 from the nitric acid/NOx generator 19 is less than the amount necessary to prevent corrosion by iodine, Nag, P/O is supplied from the NO8 supply device 22. ' is introduced into the inlet of the precondenser 5.

FIG. 10 is a system diagram showing a ninth embodiment of the reprocessing off-gas treatment method according to the present invention.

In the ninth embodiment, a nitrous acid supply device 23 for supplying nitrous acid to the nitric acid/No. generator 19 is installed. In the ninth embodiment, when the amount of nitric acid vapor, water vapor, and NO8 from the nitric acid/NO2 generator 19 is less than the amount necessary to prevent corrosion caused by iodine, the nitrous acid supply device f23
Nitric acid is supplied to the nitric acid/No2 generator 19 from the nitrous acid/No2 generator 19.

The present invention provides NO! in a gas containing iodine! Concentration is 0.5%
This was achieved as a result of the knowledge that corrosion of structural members does not occur in the above cases.
When iodine, which is harmful to corrosion, is present and the NO8 concentration in the gas is low, such as during a shutdown, the NO8 concentration in the gas may be 0.
NO! in equipment where corrosion is likely to progress to 5% or more!
This introduces a gas containing Therefore, the above-mentioned embodiment is merely an example of the present invention, and in the above-mentioned state, NO! The configuration of the device for introducing gas containing gas into the device can be changed in various ways.

〔Effect of the invention〕

As described above, according to the present invention, gas containing NO□ can be introduced into the off-gas treatment device even when corrosion of structural members due to iodine is likely to proceed □, such as when starting/stopping the off-gas treatment device. This creates an environment in which the gas phase surface within the off-gas treatment device has corrosion resistance, and corrosion caused by iodine can be prevented.

[Brief explanation of the drawing]

Figure 1 shows the first reprocessing off-gas treatment method according to the present invention.
A system diagram showing an embodiment, FIG. 2 is an explanatory diagram showing an operation pattern in the first embodiment, and FIG. 3 is an explanatory diagram of an operation pattern to show the second embodiment of the reprocessing off-gas treatment method according to the present invention. , FIG. 4 is a system diagram showing a third embodiment of the reprocessing off-gas treatment method according to the present invention, FIG. 5 is a system diagram showing a fourth embodiment of the reprocessing off-gas treatment method according to the present invention, and FIG. 7 is a system diagram showing the fifth embodiment of the reprocessing off-gas treatment method according to the present invention, FIG. 7 is a system diagram showing the sixth embodiment of the reprocessing off-gas treatment method according to the present invention, and FIG. FIG. 9 is a system diagram showing the seventh embodiment of the reprocessing off-gas treatment method according to the present invention, FIG. 9 is a system diagram showing the eighth embodiment of the reprocessing off-gas treatment method according to the present invention, and FIG. FIG. 11 is a system diagram showing a ninth embodiment of the reprocessing off-gas treatment method, FIG. 11 is a system diagram showing a conventional reprocessing off-gas treatment method, and FIG. 12 is a graph showing the influence of NO8 concentration on corrosion by iodine. 3...Dissolution tank, 4...Iodine expulsion tank
, 5... Front capacitor, 6... No
, absorption tower, 7... absorption acid storage tank, 9...
Iodine re-extraction tower, lO...Recovery acid storage tank, 13.
...Iodine removal tower, 14...Filter,
16... Cooler, 19... Nitric acid/N
08 generator, 20...NO2 monitor, 21.
...Iodine monitor, 22...NO2
Supply device, 23... Nitrous acid supply device. Agent: Patent attorney Masaru Nishimoto - Nimuni Ward, Fukuro Hinoki ■；Shunt

Claims (6)

[Claims] (1) When gas containing at least iodine is present in the off-gas treatment system, such as when starting or stopping a reprocessing off-gas treatment equipment equipped with an NO_x absorption tower, an iodine removal tower, etc., NO_2 is the main component. A reprocessing off-gas treatment method, characterized in that the gas is supplied to the off-gas treatment system so that the NO_2 concentration in the off-gas treatment system is 0.5% or more. (2) The reprocessing off-gas treatment method according to claim (1), wherein the gas containing NO_2 as a main component consists of NO_2 nitric acid vapor and water vapor. (3) The gas containing NO_2 as a main component is
x Pre-condenser installed upstream of the absorption tower, NO_
Claim (1), characterized in that: The reprocessing off-gas treatment method described. (4) The gas consisting of NO_2, nitric acid vapor, and water vapor is obtained by heating the recovered acid from the iodine re-extraction tower. Reprocessing off-gas treatment method. (5) The gas containing NO_2 as a main component is
The reprocessing off-gas treatment method according to claim (1), which is obtained by controlling the temperature of a dissolution tank as an off-gas supply source to the x-absorption tower. (6) Measuring the NO_2 concentration in the gas at the inlet of the iodine removal tower and measuring the presence or absence of iodine in the gas;
Claim 1, characterized in that the gas containing NO_2 as a main component is supplied into the off-gas treatment system when iodine exists in the gas and the NO_2 concentration in the gas is 0.5% or less. ) to (5).