JPS61122599A - Method of melting spent nuclear fuel - 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 (Field of Industrial Application) The present invention relates to a technology for post-processing spent nuclear fuel. More specifically, the present invention relates to a method of dissolving spent nuclear fuel with nitric acid.

(Prior art) In the spent nuclear fuel melting tank, 1
35 U and various fission products generated as a result of nuclear fission! Spent nuclear fuel containing IJ (FP) is dissolved with nitric acid. There are many chemical species of fission products involved, some of which pose difficult problems in dissolving.

For example, dissolved ions with high oxidizing power, such as plutonium, cerium, ruthenium, and iodine, and undissolved residues of noble metals, such as palladium, ruthenium, and rhodium, increase the corrosion potential of the melting tank made of stainless steel mesh. This causes more severe corrosion than expected from the nitric acid concentration. In addition to corrosion problems, undissolved residues also cause blockage problems and emulsion formation in the downstream extraction step of the process, impeding uranium and plutonium extraction operations. Furthermore, fission products that are easily vaporized, such as ruthenium and iodine, are partially or largely transferred into the gas phase, and as a result, great efforts are required to decontaminate the resulting radioactivity.

(Problems to be Solved by the Invention) The present invention aims to suppress corrosion of a stainless steel mesh melting device, reduce undissolved residue, and separate and recover vaporized components when melting spent nuclear fuel using nitric acid. Things like making things easier.

(Means to solve the problem) When dissolving spent nuclear fuel using nitric acid, NOx
This method of melting spent nuclear fuel is characterized by forcibly adding gas.

Spent nuclear fuel as used in the present invention refers to fuel that can be reused by dissolving it with nitric acid or the like.

The higher the concentration of nitric acid, the faster the spent nuclear fuel will dissolve, but the corrosion of the equipment will be more severe.
The pressure inside the device is 2 to 6 normal (hereinafter referred to as N), and more preferably 3 to 4 N. N used in the present invention
Ox gas is No, No2 and Nz of these polymers.
It means O4, NzO3, and even a mixture of these gases has the same effect, and these gases also include O2 and N2.
Alternatively, other inert gases may be mixed.

The required amount of NOx gas depends on the nitric acid concentration, temperature, presence or absence of flow, types and concentrations of coexisting chemical species, their ion valences (these also change depending on the burnup of the nuclear fuel), or the NOx used.
Since it varies depending on the type of gas, etc., it is not possible to specify it in a certain way.

As a result of intensive study on the corrosive environment of stainless steel mesh melting equipment, the inventors of the present invention found that N
It has been found that it is sufficient to forcibly add Ox gas. The corrosion potential of stainless steel is approximately 0.9'+V
(vs, 5cE) or less, the degree of corrosion of stainless steel can be kept below the annual permissible limit of 0.1 mm. (Note that this has the following relationship with the degree of corrosion expressed in g/m"・hr, assuming that the specific gravity of stainless steel is 7.9. That is, 1g7m"-hr=1.1
mm/Y) On the other hand, the upper limit of the amount of NOx gas added is
It is regulated by the capacity of ancillary equipment that processes Ox gas.

As a method for adding NOx gas to the solution, it may be supplied to the gas phase portion of the apparatus under pressure, but it is more preferable to directly blow it into the solution because it is more effective. Generally, when metals are dissolved with nitric acid, NOx is released due to the decomposition of the nitric acid.
Gas is generated, and this is also the case with spent nuclear fuel. Therefore, the addition of NOx gas is
It is not necessary to carry out this procedure when x gas is actively generated.

(Effect) The mechanism by which the corrosion of stainless steel melting equipment is suppressed by the addition of NOx gas cannot be explained unambiguously because many factors are intertwined, but higher-order ions with high oxidizing power Possible methods include reducing the species to lower ionic species, solubilizing and dissolving the residue, and eliminating the high corrosion potential constituted by the undissolved residue.

The insoluble residue is mainly formed from noble metal elements,
It is solubilized by forming a nitrosyl complex with NOx.

The main components that easily vaporize are ruthenium and iodine. Ruthenium has oxidizing ions (Ce”, Cr’) in nitric acid.
h, nitrate ions, etc.) to RuOa.

Ru Oa has a low boiling point of 100°C, and a portion of it vaporizes and scatters into the gas phase, where the Ru Oa is removed by the NOx of the present invention.
The oxidation up to O4 is suppressed, and the oxidation stops at RuO□, which remains on the solution side.

Iodine, which existed as a +03- ion, is reduced to I2 by NOx, moves into the gas phase, and ? No residue remains on the 8M liquid side. Therefore, iodine is collected in the gas phase and subjected to treatment.

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited thereto.

Examples 1 and 2, Comparative Examples 1 and 2 The degree of corrosion of stainless steel mesh (31oNb mesh) under boiling 3N nitric acid mixed with various metal ions in a dissolution tank environment was
A comparison was made with and without the addition of NOx gas. Among the test conditions here, the concentration of cerium ion and ruthenium ion (added as trosylruthenium ion) was determined from the Origen code value under the following conditions.

Nuclear fuel PWR fuel (UO□ 3.3%) burnup
40.000 MwD/l-U neutron density 4.
07 x 13"n/cm"・sec Cooling period after use: 160 days Dissolution tank liquid volume: 800 It/B.

U preparation amount: 400kg/B.

The 3 1 ONb net used here is an example of a stainless steel net constituting the melting device, and its chemical composition (each, weight %)
is C: 0.010. Si: 0.26Mn:0
．． 67, P: 0.019. S: 0.001Cr: 2
4.85. Ni: 20.27°Nb: 0.24
The sample was subjected to sensitization heat treatment at 650°C for 2 hours. In the test, a test piece was placed in the liquid phase and gas phase parts of a corrosion testing apparatus equipped with a reflux condenser shown in Fig. 1, and 1 patch was immersed in boiling water for 7 hours, and the degree of corrosion was determined from the change in weight before and after the test. The specific liquid volume in this test was 25
It is cc/cm2. The test results are shown in Table 1.

From this, the blowing of NO□ gas is performed using stainless steel Tjr4.
(310N bw4) ノFllt-*J < Suppression t
It can be seen that it is effective in preventing not only Rutake but also Ru from scattering into the gas phase. Ru coexists with oxidizing ions (C
oxidized to RuO4 by boiling nitric acid). Since the boiling point of this is low (100℃), a part of it vaporizes at the boiling point of nitric acid (3N) (104℃) and is reduced to RuO□ on the wall of the gas phase container, resulting in a black color. Generates a kimono. The blowing of NOx gas is R.
It has the effect of effectively preventing oxidation to ub. Also,
It can be seen that the corrosion here is significant when the corrosion potential exceeds about 0.9V, whereas corrosion is prevented when the corrosion potential is reduced to about 0.9V or less by blowing NOz gas.

Examples 3 to 6, Comparative Examples 3 to 6 Table 2 shows the results of a further study of the environment simulation experiment for the dissolved layer. The selection of environmental conditions followed the same procedure as in Example 1, but here, in addition to Ce and Ru, 10j- ions were used as test objects, and R was used as a noble metal-based insoluble residue.
In addition, ys- was added to the h metal powder to examine the corrosivity of Pu ions (Pu'').As a result of examining the corrosion of stainless steel mesh in nitric acid where oxidizing ions coexist, it was found that the corrosion here As shown in Fig. 2, we found that the corrosion potential of the stainless steel mesh can be used to determine the corrosion potential of the stainless steel mesh, regardless of the types of coexisting chemical species.
, vS゛/ with the value closest to the redox potential of 3+
This is a study of the corrosivity of P in the y4- system.

The test equipment was the same as in Example 1, but the test was conducted for 8 hrs.
The test was repeated in 5 batches. 8 hrs. During the test, it takes 40 minutes to raise the temperature of the test solution, so the boiling time is 7 hrs.
0m1n. In addition, the specific amount was tested under conditions as small as 4 cc 1 cm, assuming the actual machine conditions with criticality limits.The test pieces were the same as in Example 1, but here, solution heat treated materials were used for the test. The test results are shown in Table 2.

From this result, it is clear that noble metal-based insoluble, IO3- or Pu
It can be seen that even in a more complex system in which simulated ions coexist, the effect of NOx, that is, the reduction of corrosion and the prevention of Ru scattering into the gas phase, can be achieved. Furthermore, in Example 3-6, it was also shown that IO3-, which had been dissolved during the night, disappeared from the liquid by adding NOx gas. This means that 103- ions are converted to 12 by nitrous acid, which is a dissolved product of NOx gas.
It is thought that this is because the iodine was reduced to and transferred to the gas phase under boiling, but being able to concentrate all the iodine, which is a source of radioactivity, in the gas phase has the effect of making downstream iodine countermeasures easier. .

In Examples 3 to 6, the above-mentioned effects can be obtained both with NO and with NO2.
It is also seen that 02 and N2O4 are equally effective, and the effect remains the same even when 02 coexists.

Example 7, Comparative Example 7 Lupanic corrosion of stainless steel when it came into contact with noble metal-based insoluble residue was investigated. The test was conducted using 31 ONb test piece (same test piece as in Example 1, 65 ONb test piece) as shown in FIG.
0°C x 2 hrs, sensitized heat treated (sample)
d plate was brought into contact with Cr'+ 500 rsW/(l) at 3 N, under boiling nitric acid for 68 hrs.
, ■Hatch test was conducted. The specific liquid amount is 25 cc/d.

The test results are shown in Table 3.

From Table 3, 31 ONb stainless steel is corroded by precious metals (
The corrosion of Pd was accelerated by 1.8 times due to contact with Pd, ), but when NO2 gas was injected, the corrosion of Pd increased unexpectedly.The degree of corrosion of Pd increased by about 400
3 1 ONb that was in contact with this
Corrosion of stainless steel was completely prevented. P here
The corrosion degree of d' is rather a corrosion rate that should be called dissolution,
It can be seen that blowing NOx gas greatly helps dissolve Pd, which is one of the undissolved residues.

Example 8, Comparative Example 8.9 The relationship between the temporal change in NOx generation during dissolution and corrosion was investigated. The test equipment and test piece were the same as in Example 1, but the test piece was used as a solution heat treated material. The dissolution of spent fuel occurs violently in the early stages of dissolution, and subsides in the middle and late stages, and the generation of NOx gas decreases accordingly, eventually ending. Since the solution at the time when NOx gas generation has stopped has the highest concentration of various ions and undissolved residues, how the corrosivity changes during this time is important for the corrosion of the equipment here. It is. For this reason, 1 batch 2
The corrosion test time of 0 hrs was divided into an initial period of 4 hrs, and a middle and late period of 164 hrs, and the corrosion occurring during each period was divided and evaluated. The test results are shown in Table 4.

The results show that the corrosion that was reduced by the initial injection of NOx gas increases again when the injection of NOx gas is stopped. The corrosion potential of stainless steel also exceeded the limit value of approximately 0.9V and returned to the dangerous corrosion potential. Here, even if NOx injection is stopped, the initially blown NOx is thought to remain saturated in the solution, and despite this, the corrosion has been restored. This indicates that there is a non-negligible risk of corrosion at the final stage of dissolution, when the occurrence of corrosion stops. This corrosion problem is solved by forcibly adding NOx gas in the latter half of the dissolution reaction as shown in Example 8.

As shown in the above examples, the present invention prevents corrosion of stainless steel equipment in a melting tank, promotes dissolution of undissolved precious metal residues, and prevents vaporization and scattering of Ru by a simple method of blowing NOx gas. This can be expected to have the ultimate effect of simplifying (unifying) radioactivity countermeasures by preventing iodine from remaining in the liquid. Furthermore, a noteworthy feature of the present invention is that the substance used (NOx gas) is not a foreign substance to the dissolution tank environment, and therefore there is no need to make major changes to the reaction within the system or the treatment system for excess NOx gas. There is no.

[Brief explanation of the drawing]

Figure 1 shows the corrosion test equipment. The numbers in the figure indicate the following. 1 ---------1・---------N Ox gas blowing port 2--・--・--1--Test piece (gas phase) 3 ・～・- 1--------...Test piece (liquid phase) 4---------1--Glass hook 5 ・-----1----Condenser 6-
・・・−・−１−−−・−・500mA! flask 7
-------・-m-------・-・Mantle heater Figure 2 shows the influence of coexisting chemical species on the corrosion of 31ONb steel in boiling 3N nitric acid, organized by potential. be. Figure 3 shows a galvanic corrosion test piece. The numbers in the figure indicate the following. +11-...-----------------N b Stainless steel mesh (25 mm length x 20 mm width x 2 mm thickness) (21----------P d Metal plate (20 mm length x lOmm (width
Figure 3

Claims (1)

[Claims] When dissolving spent nuclear fuel using nitric acid, NO_
A method for melting spent nuclear fuel, characterized by forcibly adding x gas.