JPS63304198A - Method for concentrating waste reprocessing liquid - Google Patents

Abstract

PURPOSE:To greatly decrease the corrosion of equipment with the simplified equipment and operation by executing an enriching operation in such a manner as to maintain the concn. of an inorg. salt in a high-level waste liquid at >=5wt.%. CONSTITUTION:0.5wt.% aluminum nitrate is added to a waste nitric acid liquid 4 contg. the radioactive material in reprocessing of nuclear fuel at a high ratio to increase the concn. over the entire part of the inorg. salt in the waste liquid to >=5wt.%. While this waste liquid 4 is continuously supplied from a waste liquid tank 3 to an enriching vessel 5, the liquid is heated by a heater 15 under 50Torr pressure and is enriched by vacuum evaporation. The volume of the liquid in the vessel 5 is maintained constant in this case. Then, the inorg. salt in the waste liquid restrains the hydrated water and, therefore, the hydrated water hardly evaporates and the evaporation rate of the nitric acid eventually increase so that the concn. of the nitric acid of the liquid 6 in the vessel 5 is maintained low. RuO4 having strong oxidation power is prevented. In the figure 8, is a condenser and 13 is an enriched waste liquid tank.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for concentrating nitric acid waste liquid containing a large amount of radioactive materials in the reprocessing of nuclear fuel.

[Conventional technology]

Nitric acid waste liquid (high-level waste liquid) containing a large amount of radioactive materials is usually concentrated by evaporating water and nitric acid to further reduce its volume. Conventional methods for this concentration include: (1) decomposing nitric acid into NOx by adding a reducing substance such as formic acid (Nakamura et al.);

J, Nucl, 5cie, Tech, , 15,76
0 (1978) and (1) evaporating nitric acid under reduced pressure. (2) decomposes nitric acid into NOx, gasifies it, and recovers it, so the concentration of nitric acid in the waste liquid does not increase, and since reducing NOx coexists, corrosion of equipment will not be a big problem. However, there are problems in that NOX recovery equipment is required and it is difficult to control the nitric acid decomposition reaction. ■ is simple vacuum evaporation, which has the advantage of simplifying the equipment and operation, but the nitric acid concentration rises to about 10, and even if the temperature is lowered by vacuum, the corrosive environment is severe, and the inner thickness of the equipment It is necessary to take measures such as making the material thicker, resulting in a heavy and expensive product.

On the other hand, various anticorrosion methods have been proposed to solve these problems. For example, JP-A-60-46380
There is a method of blowing NOx into nitric acid to alleviate the corrosive environment, as described in JP-A-60-33370, and a method of similarly mitigating corrosion by adding reduced hydrazine, etc., as described in JP-A-60-33370. be. However, both of these methods require recovery equipment because NOx gas is generated in the gas phase, and it is difficult to operate as it is necessary to continuously supply a certain amount of additives from the outside in a high radiation storage site. There was a problem.

[Problem that the invention seeks to solve]

These conventional techniques either complicate the operation of the equipment to reduce corrosion or extend the life of the equipment by using heavy equipment.

An object of the present invention is to provide an operating method that can maintain sufficient durability even with simplified equipment and operation.

[Means for solving problems]

The above purpose is to contain a large amount of inorganic salts by 5% by weight in high-level waste liquid.
This is achieved by carrying out the concentration operation in a dissolved state. This can be done, for example, by adding inorganic salts to the high-level waste liquid in advance and concentrating it, or by leaving a part of the concentrated liquid remaining and adding the high-level waste liquid to it for concentration. This is achieved by implementing the following.

The main cause of corrosion in concentrators for high-level waste liquids is the high concentration of nitric acid during concentration operations. This is because the oxidizing power increases as the concentration of nitric acid increases, and among the fission product (FP) nuclides contained in high-level waste liquid, ruthenium in particular has a strong oxidizing power in nitric acid. Conversion to ruthenium oxide (RIJO4) accelerates corrosion of the concentrator. In addition, ruthenium tetroxide is easily volatile, so
Not only the liquid phase of the concentrator but also the gas phase can be subject to significant corrosion. Thus, reducing the nitric acid concentration during the concentration operation can be said to be an extremely useful and fundamental measure to alleviate corrosion. One approach to this end is to add formic acid or hydrazine to decompose nitric acid, but as already mentioned, this method has the problem of being complicated and difficult to operate.

The present invention is based on the discovery that, as a measure to reduce the nitric acid concentration, it is effective to perform a concentration operation by evaporation under reduced pressure while a large amount of inorganic salt is dissolved. Inorganic salts are
Either pre-input or use atomic fission product ions present in high-level waste liquid.

The new experimental facts that led to the discovery of the present invention and the specific effects of the inorganic salt will be explained below.

[Effect]

First, we will discuss vacuum concentration of high-level waste liquid.

In the nitric acid waste solution after uranium and plutonium are separated in the reprocessing process, atomic fission products such as noble metal elements, rare earth elements, alkali metal elements, alkaline earth metal elements, etc. are converted into ions and weigh approximately 0.5% by weight. percent dissolved. This waste liquid is further concentrated in a concentrator in the 'a condensation step to concentrate the atomic fission products approximately 20 times, and the supplied liquid a is treated as waste.
Recovered nitric acid will be reused. FIG. 2 shows the relationship between the degree of concentration and the concentration of nitric acid in the concentrator when this concentration operation was carried out using a simulated waste liquid. The simulated waste liquid was continuously evaporated under reduced pressure at a constant supply rate of 0.2 Q/h, a constant retention rate of 10.5 Q/h in one condenser, and a pressure of 50 Torr. According to the results shown in FIG. 2, the nitric acid concentration initially increases by 9 to 10 normal, and then gradually decreases as the concentration increases.

When a solution containing only nitric acid is used, the concentration remains constant at 9 to 10 normal. The present invention focuses on the decrease in nitric acid concentration as the degree of concentration increases when this simulated waste liquid is concentrated. A theoretical vapor-liquid equilibrium analysis revealed that this decrease in nitric acid concentration was due to the salting-out effect due to atomic fission product ions. In other words, atomic fission product ions are chemically dissolved as ordinary metal or inorganic cations, which bind about 10 water molecules in the waste liquid as hydration water, and this hydration water evaporates less easily than free water. Therefore, as the degree of concentration increases, the proportion of hydration water in the can liquid increases, making it difficult for water to evaporate as a whole.

As a result, the amount of evaporation of nitric acid increases, so the concentration of nitric acid in the can liquid decreases.

In view of these experiments and theoretical considerations, it is clear that the increase in nitric acid concentration can be suppressed if a large amount of inorganic cations are present in the can liquid in advance during the concentration operation. The cation is more effective if it can bind as many water molecules as possible, that is, if it has a large hydration number, and in practice, it is sufficient to add an inorganic salt that is soluble in nitric acid. In addition, as a method to achieve this effect without adding new inorganic salts, there is the following method6: As shown in Figure 2, the atomic fission product ions in the concentrated can liquid after the concentration operation are It is about 20 times more concentrated in comparison, and the concentration reaches 10 weight percent. This concentrate is usually
It is called high-level concentrated waste liquid, and after being removed from the concentrate can and stored in a special tank, it is solidified. In the present invention, the next concentration operation is carried out by supplying nitric acid waste liquid while leaving a part of the concentrated can liquid in the can without taking out all of it. In this way, a large amount of inorganic cations are already present in the concentrate can, and the same effect as adding a new inorganic salt can be obtained, making operation extremely easy. Next, the effectiveness of the present invention will be described based on Examples.

〔Example〕

An embodiment of the present invention will be described below with reference to FIG. Embodiment 1 FIG. 1 shows a concentration system for high-level nitric acid waste liquid using a vacuum evaporation method. The high level nitric acid waste liquid 4 is continuously supplied from the high level nitric acid waste liquid 3 to the concentrator 5. At this time, the water in step 2 is not supplied to the concentrator in step 5. The pressure inside the concentrator 5 was reduced to about 50 Torr by the pressure reducing device 9.
The boiling state is maintained using 15 heaters. pressure 50
Torr is 50-60°C. The high-level acid waste liquid 4 supplied to the concentrator 5 is concentrated by evaporation under reduced pressure. The gasified water vapor and nitric acid pass through the mist separator 7 and are liquefied under the condenser 8 and enter the recovery acid tank 12 as recovered acid 11. The 10 non-condensable gases are sent to the exhaust gas treatment system within the reprocessing plant.

The operation of the concentrator 5 was continued while continuously receiving the high-level nitric acid waste liquid 4 until the concentration of fission products in the concentrator liquid 6 reached about 20 times that of the supplied liquid. During this time, the 15 heaters are controlled so that the amount of liquid in the can remains constant. The concentration of atomic fission products in the liquid in the tank of No. 6 is higher than that of the feed liquid.
When it reaches 0 times, the supply of the high-level nitric acid waste liquid No. 4 is stopped, and this time, the water No. 2 in the water tank No. 1 is supplied to the concentrator No. 5, and the same operation is carried out. In this case, the operation is stopped when the concentration of nitric acid in the liquid in the concentrated can No. 5 reaches 4N, and the liquid in the can No. 6 is transferred to the high-level waste liquid tank No. 13. This one
The liquid No. 4 is a high-level concentrated waste liquid, and after being stored for a while, it solidifies. In this system, a simulated waste liquid (atomic fission product concentration 0.5 wj%) was used.

The results of treatment with nitric acid concentration (3N) are shown in Figure 2.
The concentration of nitric acid in the cotton can increases to a maximum of 10N, and as a result, some of the ruthenium ions contained in the liquid change to highly oxidizing ruthenium tetroxide (RuO,t), causing corrosion of the concentration can. Along with the significant acceleration, ruthenium tetroxide also volatilized into the gas phase. On the other hand, it has also been revealed that ruthenium tetroxide is generated when the nitric acid concentration of the liquid in the can is 8N or more.

Next, add 0% aluminum nitrate to the high-level nitric acid waste solution in Step 4.
．． When tested with a solution containing 5 wt%, the nitric acid concentration in the can reached a maximum of 7.5 normal, and no ruthenium tetroxide was generated and no acceleration of corrosion was observed. This is because the nitric acid concentration decreased due to the salting out effect in which the aluminum in the added aluminum nitrate becomes a cation and binds water.

This salting-out effect is stronger for cations with a large hydration number, so in addition to aluminum nitrate, alkali metals such as cesium nitrate, alkaline earth metals such as barium nitrate, rare earths such as cesium nitrate, palladium nitrate, etc. Any inorganic salt such as a noble metal may be used. However, it is best to avoid halogen salts, which are highly corrosive on the anion side.

By the way, the reprocessing waste liquid originally contains B a HCe,
Pd, Ru, Cs, Y, Rh, and about 30 other metal ions of atomic fission products exist as salts dissolved in nitric acid, and there is a method of utilizing this. An example of this is shown below.

Example 2 The entire amount of the atomic fission product liquid 6 in the can was not transferred to the high-level concentrated waste liquid tank 13, and the high-level nitric acid waste liquid 4 was supplied to the concentrator 5 with a portion remaining, and the operation was started. It is to be implemented. This allows evaporation and concentration in a state where a large amount of atomic fission product ions already exist. Figure 3 shows the relationship between the maximum concentration of nitric acid in the can and the residual rate when the residual rate was varied. The maximum concentration of nitric acid decreases as the residual rate increases. In particular, the residual rate was set to 50 (vo1%
) or less, the nitric acid concentration will be 8N or less. At that time, the concentration of atomic fission products in the concentrated can liquid is 5 (w%) or more, and more generally, by operating at this concentration or more, the nitric acid concentration can be reduced to 8N or less due to the salting out effect mentioned above. It became possible to do so.

Although all of the concentration systems in the above examples are carried out under reduced pressure, the pressure is preferably below 100 Torr. This means that the boiling point of nitric acid at 100 Torr is 60.
℃, and if the temperature rises above this temperature, ruthenium tetroxide will be generated even if the nitric acid is 8N or less. Therefore, under a pressure of 100 Torr or less,
Corrosion of one piece of equipment can be particularly significantly suppressed by adding an inorganic salt or by leaving a portion of the concentrated waste liquid.

〔Effect of the invention〕

According to the present invention, the process of concentrating high-level nitric acid waste liquid can be performed in a simple manner and the increase in nitric acid concentration can be suppressed, so that corrosion of the concentrator and its connected equipment can be significantly reduced.

[Brief explanation of drawings]

FIG. 1 is a system flow diagram in an embodiment of the present invention.
FIG. 2 shows the relationship between the concentration of atomic fission products and the concentration of nitric acid in the concentrated bottom liquid, and FIG. 3 shows the relationship between the residual rate of the concentrated bottom liquid and the maximum concentration of nitric acid in the concentrated bottom liquid. 1...Water tank, 2...Water, 3...High level nitric acid waste liquid tank, 4...High level nitric acid waste liquid, 5...Concentrator can,
6... Concentrated can liquid, 7... Mist separator, 8...
Condenser, 9... Pressure reducing device, 10... Non-condensable gas, 11
Condensate, 12... Recovery nitric acid tank, 13... High level concentrated waste liquid Diagram Z

Claims (1)

[Claims] 1. In a method for batchwise concentration of nitric acid waste liquid containing atomic fission products in nuclear fuel reprocessing, the concentration of all inorganic salts contained in the waste liquid to be treated is maintained at 5% by weight or more. A method for concentrating reprocessed waste liquid, characterized by carrying out a concentration operation in this manner. 2. Claim 1, wherein the inorganic salt is at least one metal salt selected from alkali metal salts, alkaline earth metal salts, rare earth metal salts, noble metal salts, and transition metal salts. The described method for concentrating reprocessed waste liquid. 3. The method for concentrating reprocessed waste liquid according to claim 1 or 2, wherein the concentration operation is carried out under reduced pressure of 100 Torr or less. 4. In the batch method of concentrating nitric acid waste liquid containing nuclear fission products in nuclear fuel reprocessing, a part of the concentrated waste liquid is left in or returned to the waste liquid to be concentrated in the next step and mixed. A method for concentrating a reprocessed waste liquid, which comprises carrying out the next step of concentration operation while maintaining the concentration of all inorganic salts contained in the waste liquid to be treated at 5% by weight or more. 5. Claim 4, wherein the inorganic salt is at least one metal salt selected from alkali metal salts, alkaline earth metal salts, rare earth metal salts, noble metal salts, and transition metal salts. The described method for concentrating reprocessed waste liquid. 6. The method for concentrating reprocessed waste liquid according to claim 4 or 5, wherein the concentration operation is carried out under reduced pressure of 100 Torr or less.