FR2697664A1 - Removing of iodine in used fuel soln. - Google Patents

Abstract

Method in which an excessive amt. of an iodate is added to the soln. without blowing NOx into the soln., and the soln. is heated to a temp. between 90 deg.C and the b.pt. while supplying NOx to the soln..

Description

 The present invention relates to a process for removing iodine in a spent fuel solution. The present invention relates more particularly to a process for removing iodine in a spent fuel solution for the reprocessing of spent nuclear fuel in the field of atomic energy.

 It has been thought up to now that the iodine remaining in a spent fuel consists mainly of iodate ions (I03-), and it has been thought that it can be extracted into the gaseous phase effectively in the form of of iodine (I2), by infusing NOx into the heated solution. Thus, such a method of removing iodine by separating it by driving the solution by blowing NOx into the heated solution at about 100 ° C., to reduce an iodate in the form of iodine constituting a single substance, has been preserved until now.

FIG. 1 is a graph illustrating the behavior of iodinated chemical species at the time of separation of iodine by entrainment from the simulation solution (50 ml) heated to 100 ° C. according to the method of the prior art; and
Figure 2 is a graph illustrating the behavior of iodinated chemical species at the time of iodine separation by entrainment from the simulation solution (50 ml) heated to 100 ° C according to the method of the present invention.

 In the figures, No. 1 indicates total iodine, No. 2 indicates colloidal iodine, and No. 3 indicates the addition of 103- (2 mg-1).

 As a result of detailed studies of the iodine-derived chemical species in the spent fuel solution, however, the present inventors have found that the main chemical species is not an iodate but an insoluble iodide colloid such as silver iodide (AgI), palladium iodide (PdI2), etc., and found that it was inefficient to separate the colloidal iodine by entrainment by infusing NOxo accordingly, to remove iodine in a spent fuel solution, it is first necessary to find a method of eliminating colloidal iodine.

The subject of the present invention is therefore a process for eliminating iodine in a spent fuel solution, characterized in that
heats the solution to a temperature of
9 ° C to the boiling point, without blowing
NOx, and then heated to a temperature of 90 ° C to the boiling point by introducing NOx.

 According to other features of the present invention - the amount of iodate is less than the amount of iodine in the spent fuel; and - said NOx is NO, NO2 or a mixture thereof.

As a result of further research, the present inventors have found that by heating in the absence of
NOx, the colloidal iodine is decomposed and the rate of decomposition is increased by adding excess NO 3 -radioactive, and they thought that by adding this process before blowing NOx, the colloidal iodine could be separated by entrainment under the form of 12 in the gaseous phase, and the iodine present in the spent fuel solution, can be almost completely eliminated, and the present inventors have thus realized the method of the present invention of eliminating iodine in a solution spent fuel, in which an iodate is added to the spent fuel solution, and heated to a temperature of 90 ° C to the boiling point, and heated to a temperature of 90 ° C to the point of boiling by introducing NOx
Colloidal iodine is produced by preparing a simulated solution containing uranium and a simulated fission product, by adding iodine (Iodide I ions labeled with 131I radioactive iodine), and heating it. The decomposition conditions of this colloidal iodine are obtained by studying its chemical properties. The effect is confirmed by applying these conditions to the actual fuel solution (spent fuel solution) and by obtaining effective removal of the iodine from the solution.

By heating the colloidal iodine to about 100 ° C in the absence of NOx in a nitric acid solution, it is decomposed into the simple iodine substance (I2) and evaporated from the solution. is increased by previously adding excess non-radioactive iodate to the solution, however since a small amount of iodate (nonvolatile) is produced in combination with 12, NOx is blown into the solution for removal. The iodine present in the solution can be effectively removed by this operation in two steps, i.e. (1) heating in the absence of
NOx and (2) the subsequent blowing of NOx
The two figures 1 and 2 illustrate the behavior of iodized chemical species at the time of iodine separation by entrainment from 50 ml of a simulation solution heated to 100 C, on which is represented an aspect of the heating of iodine. a simulation solution (containing 50 gU / l uranium and an equivalent quantity of a simulated fission product FP) simulating a spent fuel solution with a combustion degree of 40 gigawatts / day (GWJ) / t in y The order corresponds to the quantity of chemical species (g-1/50 ml) in the solution and the abscissa represents the time.

 FIG. 1 illustrates a state after heating at 100 ° C. for 4 hours by injecting NO 2 alone, that is to say 10% NO 2 according to the prior iodine separation method, the separation effect of the iodine by entrainment from the solution being inferior to the case where the colloidal iodine is not removed by NO2.

 FIG. 2 illustrates the effect of the addition of IO3-, wherein, when NO2 is blown for two hours and then excess I03 is added and heated, the colloidal iodine is rapidly decomposed, and when introducing NO 2 in the next step, the iodine content in the solution decreases sharply. Even if I03- is not added, when the solution is heated in the absence of NOx, the colloidal iodine is decomposed. Heating the solution in the absence of NOx is an essential requirement of the present invention.

Example
A spent fuel rod from a boiling water editor (PWR) with a burn-up degree of 21-39 watts.day / t (GXJ / t) was cut into lengths of 3-5 mm (approx. each) and were respectively introduced into a vial with a volume of 100 ml. 30 ml of 4 M nitric acid was added and heated to 100 ° C to dissolve. After separation into a solution and insoluble residues, an attempt was made to eliminate (blow-off) the iodine remaining in the solution. The results of the comparison of the NOx insufflation according to the prior art, and the method of the present invention are mentioned in Tables 1 and 2.

Table 1
Volatilization of iodine according to the process
iodine drive in solution
by blowing
(reduction of IO3- in I2 by NO and addition
of 103- in excess as a vehicle,
and again introduction of NO)

<tb> Type <SEP> of <SEP> fuel <SEP> Quantity <SEP> of iodine <SEP> 2 <SEP> x <SEP> 100 <SEP>%
<tb><SEP> degree of <SEP> combustion) <SEP> volatilized <SEP> Bq / g-U0 <SEP> 1 <SEP> x <SEP> 2
<tb> (sample <SEP> n ") <SEP> 1) <SEP> NO <SEP> (2h) <SEP> 2) <SEP> I03
<tb><SEP> + <SEP> NO <SEP> (2h)
<tb> U02 <SEP>
<tb> (30.8 <SEP> GWJ / t) <SEP> 15.8 <SEP> 7.9 <SEP> 33.3
<tb> (89G08)
<tb> U02
<tb> (39.7 <SEP> GWJ / t) <SEP> 8.5 <SEP> 3.2 <SEP> 27.4
<tb> (87 <SEP> H02) <SEP>
<tb> Cd203-02
<tb> (21.4GWD / t) <SEP> 18.9 <SEP> 15.9 <SEP> 45.7
<tb> (89G01)
<tb> Cd203-02 <SEP>
<tb> (25.3 <SEP> GWJ / t) <SEP> 35.7 <SEP> 28.2 <SEP> 44.1
<tb> (89H06) <SEP>
<Tb>
Table 2
Volatilization of iodine according to the process
separation of iodine in solution
by blowing (decomposition as AgI, colloidal PdI2,
with I03- in excess, and introduction of NO)

<tb> Type <SEP> of <SEP> fuel <SEP> Quantity <SEP> of iodine <SEP> 2 <SEP> x <SEP> 100 <SEP>%
<tb><SEP> degree of <SEP> combustion) <SEP> volatilized <SEP> Bq / g-U0 <SEP> 1 <SEP> x <SEP> 2
<tb> (sample <SEP> n) <SEP> 1) <SEP> NO <SEP> (2h) <SEP> 2) <SEP> NO <SEP> (2h)
<tb> U02
<tb> (30.8 <SEP> GWJ / t) <SEP> 9.6 <SEP> ND <SEP> 0 <SEP>
<tb> (89G08) <SEP>
<tb> U02
<tb> (39.7GWJ / t) <SEP> 2.1 <SEP> ND <SEP> 0 <SEP>
<tb> (87 <SEP> HO2) <SEP>
<tb> Cd203-02 <SEP>
<tb> (21.9 <SEP> SWJ / t) <SEP> 24.0 <SEP> ND <SEP> O
<tb> (89G01) <SEP>
<Tb>
ND: undetectable; U0: initial uranium.

 The case in which NOx (NO) was first blown in according to the method of the prior art is shown in Table 1.

 The iodine evaporated, but to test whether iodine was still remaining, after adding excess iodate (I03-), the solution was heated to 100 C, and again blown in. NO. As a result, iodine volatilized further, and its amount reached 27-46% based on the total amount of iodine in the solution. It was therefore found that the elimination of iodine in the solution was not sufficient by blowing in only NOx.

In the case where the remaining iodine in the solution is in the form of colloidal iodine, in particular AgI,
PdI2, etc., it is effective to heat it in the absence of NOx to break it down. When an excess non-radioactive iodate is added, the rate of decomposition is further increased. Table 2 illustrates a real proof of the effect. A method of adding iodate in excess and heating and then injecting NOx (NO) was carried out. As a result, almost 10% of the iodine present in the solution was removed in the first step, and the amount of iodine volatilized by NOx insufflation was less than the detection sensitivity.

 It was found that the iodine removal rate according to the prior method of simply blowing NOx was 54 to 73%, while almost 100% of the iodine was removed according to the method of the present invention. Thus, the action and effect of the present invention have been effectively proven.

Although in the example of the remaining iodine has not been detected, since a portion of the iodine is oxidized to iodate remaining in the solution in the first step, it is necessary to then introduce NOx to reduce it again iodine (I2) to separate it by entrainment.

Claims (4)

 A process for removing iodine in a spent fuel solution, characterized in that the solution is heated to a temperature of 90 ° C. up to the boiling point without injecting NOx therein, and then heated to a temperature of 9O0C up to the boiling point by introducing NOx.  A process for removing iodine in a spent fuel solution according to claim 1, characterized in that excess iodate is added to the spent fuel solution prior to heating it.  A process for removing iodine in a spent fuel solution according to claim 2, characterized in that the amount of said iodate is less than the amount of iodine in the spent fuel.  The process for removing iodine in a spent fuel solution according to claim 1, characterized in that said NOx is NO, NO2 or a mixture thereof.