JPH077095B2 - How to stop the offgas treatment equipment - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for stopping an off-gas treatment device, and more particularly to a method for stopping an off-gas treatment device suitable for preventing corrosion of the device by iodine in off-gas.

(Prior art) At a nuclear fuel reprocessing plant, spent fuel is dissolved in concentrated nitric acid, and substances such as uranium and plutonium that can be reused as fuel are recovered. However, a process of dissolving spent fuel in concentrated nitric acid At that time, off-gas containing H ₂ O, NOx, etc. as main components is generated, and further volatile FP (fission products) such as iodine in the spent nuclear fuel is released together with the off-gas.

FIG. 7 is a system diagram of a conventional off-gas processing apparatus of this type. This apparatus includes a dissolution tank 3 for dissolving spent fuel with concentrated nitric acid, an iodine discharge tank 4 for discharging iodine dissolved in the dissolution liquid from the dissolution tank 3, a dissolution tank 3 and an iodine discharge tank. The condenser 5 for condensing most of the water vapor in the off-gas generated from 4, the NOx absorption tower 6 for absorbing and removing the NOx in the off-gas, the condensate from the condenser 5 and the absorption acid solution from the NOx absorption tower 6 are stored. An absorbing acid solution storage tank 7, an iodine re-discharging tower 9 for expelling iodine in the absorbing acid solution storage tank 7, and a recovered acid solution storage tank 10 for storing the acid solution recovered by the iodine re-discharging tower 9. It is composed of an iodine removal tower 13 for adsorbing and removing iodine with an adsorbent.

The NOx absorption tower 6 is of a tray type as shown in the detailed cross-sectional view of the NOx absorption tower of FIG. 6, and has a plurality of trays 50, an absorbing acid liquid downfall pipe 51, and heat generation during NOx absorption. The cooling coil 52 is inserted in the shelf 50 to suppress the temperature rise of the absorption liquid and increase the NOx absorption drop.

In such an apparatus, the offgas 18 generated from the dissolution tank 3 and the iodine discharge tank 4 is sent to the condenser 5 and most of the water vapor is condensed. This condensate is used as the absorption liquid of the NOx absorption tower 6. The gas from which water vapor has been removed by the condenser 5 is supplied to the lower part of the NOx absorption tower 6,
The NOx in the gas is absorbed and removed by the absorption liquid supplied from the upper nozzle of the Ox absorption tower 6. Since a large amount of iodine is mixed in the condensed liquid from the condenser 5 and the absorbed acid liquid from the NOx absorption tower 6, these are stored in the absorbed acid liquid storage tank 7 and then transferred to the iodine re-discharging tower 9. , Heated to 80-90 ℃, N ₂ gas drives out iodine. The absorbed acid solution from which iodine has been expelled is stored in the recovered acid solution storage tank 10, cooled, adjusted as a solution for reuse, and transferred to the dissolution tank 3 by the transfer pump 11.

On the other hand, the gases discharged from the NOx absorption tower 6 and the iodine re-expelling tower 9 are mixed and preheated to 150 ° C. by the preheater 12 so as to reduce the relative humidity in the gas, and the iodine filled with the adsorbent is removed. Most of the iodine sent to the column 13 and flowing into the off-gas system is removed. The gas from which iodine has been removed is cooled by a cooler 16, then passes through a filter 14, and a blower 15
Are transferred to the exhaust stack 1 and discharged.

Next, the results of various studies on the corrosion resistant material used for the NOx absorption tower 6 will be described. Table 1 shows the amount of general corrosion and the pitting depth of various materials in the presence of iodine. The corrosion test was carried out at 25 ° C in the atmosphere with iodine 7.4 × 10 ^-7 g / m 2.
It was carried out for 400 hours under a gas phase atmosphere containing 1 × 10 ⁻⁵ g / ml of water. Table 2 shows the composition of each material used in Table 1 (ORNL Report No. 0RNL / Sub-7327 / 11 '
81-11).

From these tables, a normal austenitic stainless steel and Ni alloy with a small Mo content have large values of general corrosion and pitting corrosion depth, but do not have practically acceptable corrosion resistance,
It is understood that Ti, Zr and Hastelloy C-276, Inconel 625, AL-294 and Hastelloy G containing Mo in high Cr or high Ni have high resistance to general corrosion and pitting corrosion susceptibility. However, the corrosion resistant Ti and Z
The use of materials such as r and Hastelloy C-276 as materials for the off-gas treatment device causes an extremely large equipment cost and is economically problematic. In addition, since Incoloy 825 and Inconel 671 which have relatively good corrosion resistance also cause pitting corrosion of 0.5 mm per year, it is necessary to consider a corrosion amount of 5 mm or more even for a 10-year equipment life, which makes the equipment thicker and thicker. The deterioration of heat conduction performance due to fleshing becomes a problem.

In the prior art, in order to reduce the corrosion of the device due to such iodine, at the time of starting the operation of the NOx absorption tower 6,
It is necessary to fill the tower with absorption liquid to a specified level in advance, and when the operation is stopped, the absorption liquid containing iodine is replaced with NOx.
It was necessary to extract from the absorption tower 6 and to purge off-gas containing iodine in the tower.

(Problems to be Solved by the Invention) An object of the present invention is to solve the above-mentioned problems of the prior art, to prevent the corrosion of iodine by a simple method when stopping the off-gas treatment device, and to remove the absorbing liquid. An object of the present invention is to provide a method for stopping an offgas processing device that does not require an offgas purging operation.

(Means for Solving Problems) The above object is achieved by maintaining the NOx concentration in the NOx absorption tower of the offgas treatment device at 0.5 or more during the shutdown period.

That is, the present invention, when stopping the off-gas treatment apparatus consisting of NOx absorption tower, iodine removal tower, etc.
The NOx concentration in the gas in the NOx absorption tower is maintained at 0.5% or more.

(Operation) FIG. 2 is a diagram showing the amount of corrosion with respect to the change in NOx concentration in the presence of iodine. From the figure, the corrosion in the presence of iodine is remarkable when the NOx concentration is 0.5% or less, and the NOx concentration is 0.5%.
From the above, it can be seen that excellent corrosion resistance is exhibited. The reason for this is not clear, corrosion reaction of metals in the case where NOx is not present, the cathode reaction ^{_{1 / 2I 2 → I - +}} e - coexist while a, NOx is more than 0.5% ^- anode reaction M → M ^{+ +} e Then, the cathode reaction will remain the iodometric reaction or anode reaction is changed to NOx → NO _2, or, passive film on the metal surface is excellent stable coating film in corrosion resistance by the presence of NOx It is estimated to be because.

In order to maintain the NOx concentration at 0.5% or higher during the shutdown period of the off-gas treatment device, it is preferable that the NO concentration in the gas in the NOx absorption tower be 0.45% or higher at the time of shutdown. For that purpose, it is preferable to measure the NO concentration of the outlet gas of the NOx absorption tower with an NO detector and close the valve provided on the gas upstream side of the NOx absorption tower when the NO concentration becomes 0.45% or more.

FIG. 3 is a diagram showing a change with time of the NOx concentration when the NOx concentration is set to 0.7% and the circulating liquid in the NOx absorption tower is stopped. From the figure, it can be seen that the NOx concentration in the gas gradually decreases from 0.7%, and after 24 hours the NOx concentration is 0.2%. This is because NOx in the gas was absorbed by the absorbing acid solution accumulated in the NOx absorption tower.

Fig. 4 shows the initial NO concentration in NOx (NO concentration when operation is stopped).
It is a figure which shows the time-dependent change of NOx density | concentration by change, and FIG. 5 is a figure which shows NOx density 50 hours after operation stop with respect to initial NO density change. It can be seen from FIG. 4 that when the initial NO concentration is 0.45% or more, the NOx concentration does not become 0.5% or less even after 50 hours have passed, and from FIG. 5, the initial NO concentration is
It can be seen that when it is 0.45% or more, the NOx concentration after 50 hours does not become 0.5% or less.

From the above, during the operation of the NOx absorption tower, by the presence of NOx at a concentration of 0.5% or more, and when the operation of the NOx absorption tower is stopped, by making the NO concentration at the time of stop 0.45% or more, It has been found possible to prevent corrosion of the device by iodine. Therefore, at the time of starting and stopping the operation of the NOx absorption tower as before, it is possible to prevent the corrosion of the equipment by iodine without supplying the absorption liquid in the NOx absorption tower, withdrawing the absorption liquid, and purging the gas in the tower. can do.

(Example) Hereinafter, the present invention will be described in detail based on examples.

FIG. 1 is a system diagram of an off-gas treatment device according to the present invention. In FIG. 1, the same parts as those in FIG. 7 are designated by the same reference numerals and the description thereof will be omitted. In the figure, the difference from the conventional device of FIG. 7 is that a NO detector 35 is provided at the gas outlet of the NOx absorption tower 6, and the gas upstream side of the NOx absorption tower 6 (condenser 5
The dampers 31 and 32 are provided at the inlet gas line) and the gas downstream side (preheater 12 inlet gas line), and the gas line 41 is further provided at the rear of the damper 32. In such a configuration, when the operation of the offgas treatment device is stopped, NOx
The NO detector 35 provided at the gas outlet of the absorption tower 6 measures the NO concentration in the gas, and when the NO concentration becomes 0.45% or more, the damper 31 and the damper 32 are closed. Further, the N ₂ gas and the inlet gas of the exhaust stack 1 are ventilated by the gas line 41 provided at the rear portion of the damper 32 in an amount that replaces the off gas contained in the pipe and the reactor.

By such a method, the initial NO concentration in the NOx absorption tower 6 is 0.4
When the operation was stopped at 5% or more, NO in the NOx absorption tower
The x concentration could be kept above 0.5% for 10 days.

(Effects of the Invention) According to the present invention, it is possible to prevent corrosion due to iodine in off-gas by a simple method, and it is possible to greatly extend the life of the apparatus material, and to simplify the start and stop operations of operation and Economical because it can shorten the time,
It is advantageous in terms of safety.

[Brief description of drawings]

FIG. 1 is a system diagram of an off-gas treatment device according to the present invention,
FIG. 2 is a diagram showing the amount of corrosion with respect to changes in NOx concentration in the presence of iodine, and FIG. 3 is a diagram showing changes with time in NOx concentration when the circulating fluid is stopped with the NOx concentration being 0.7%.
The figure shows the change over time in the NOx concentration due to changes in the initial NO concentration in NOx (about NO when the engine is stopped).
NOx concentration 50 hours after operation stop due to O concentration, Fig. 6 shows NO
FIG. 7 is a detailed cross-sectional view of the x absorption tower, which is a system diagram of a conventional off-gas treatment device. 1 ... Exhaust stack, 3 ... Dissolution tank, 4 ... Iodine ejection tank, 5
...... Condenser, 6 …… NOx absorption tower, 7 …… Absorbing acid liquid storage tank, 8 …… Circulation pump, 9 …… Iodine re-discharging tower, 10 ・ ・ ・
… Recovered acid solution storage tank, 11 …… Transfer pump, 12 …… Preheater, 13
…… Iodine removal tower, 14 …… Filter, 15 …… Blower, 16
…… Cooler, 31 …… Damper, 32 …… Damper, 35 …… NO detector, 41 …… Glass line, 50 …… Shelves, 51 …… Absorbing acid downcomer pipe, 52 …… Cooling coil.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location B01D 53/77 G21F 9/02 511 D 7381-2G

Claims (3)

[Claims] 1. An offgas characterized by maintaining the NOx concentration in the gas in the NOx absorption tower at 0.5% or more during the shutdown period when stopping the offgas treatment apparatus comprising the NOx absorption tower, iodine removal tower, etc. How to stop the processing equipment. 2. The method for stopping an off-gas treatment device according to claim 1, wherein the NO concentration in the gas in the NOx absorption tower at the time of stop is 0.45% or more. 3. The NO concentration in the gas inside the NOx absorption tower is measured by the NO detector provided at the outlet of the NOx absorption tower according to claim 1 or 2, and the NO concentration becomes 0.45% or more. A method for stopping an off-gas treatment device, which comprises closing valves provided on the gas upstream side and the gas downstream side of the NOx absorption tower at the time of opening.