JPS6218486B2 - - Google Patents

Description

Detailed Description of the Invention The present invention is a conversion device for removing nitric acid from a mixed solution of plutonium nitrate and uranyl nitrate (hereinafter referred to as nitric acid solution) produced after processing spent nuclear fuel, and producing a cake of plutonium dioxide and uranium dioxide. Regarding.

Generally, in order to denitrify a nitric acid solution containing nuclear material to produce only nuclear material, a nitric acid solution 1 is poured into a flat container 2 with a low height, and the container is rotated around a rotating shaft 3, as shown in FIG. When the nitric acid solution is heated by irradiating microwaves from the opening 5 inside the enclosure 4, water first evaporates, and then the denitrification reaction begins through an azeotrope between water and nitric acid. If the steam or nitrogen oxides from this denitrification reaction are exhausted, for example, through a microwave inlet, a cake converted from the nitric acid solution will be obtained at the end of this denitrification reaction.

However, the properties of the cake produced by microwave irradiation are not uniform and exhibit the appearance as shown in FIG. That is, the cake 6 converted in the container 2 contains yellow-green enriched uranium crystals 7, yellow semi-solid uranium nitrate 8 in which nitrogen oxides remain undecomposed, and reddish-brown target nuclei. Substance 9 and a black dried substance 10 in a peroxidized state are separated and precipitated.

The first cause of such non-uniform cake properties is uneven heating of the nitric acid solution. That is, because the microwave does not act on the entire nitric acid solution, some parts are locally overheated and some parts are underheated. As is well known, when a liquid is irradiated with microwaves, most of this energy is absorbed in the upper layer of the liquid, and since there is uneven irradiation on the liquid surface due to the shape of the waveguide, etc., the entire solution is It was very difficult to heat uniformly over the area. As a countermeasure for this, the conventional device shown in Fig. 1 rotates the container into which the nitric acid solution is injected to reduce uneven microwave irradiation, and also makes the container itself flat in consideration of the characteristics of microwaves. It is made shallow so that the microwaves can reach the bottom of the solution.

Next, the second cause of uneven cake properties is that the timing of stopping the microwave oscillation was not appropriate, and the portions that were irradiated with microwaves were
Instead of existing as the desired UO ₃ , it ends up as U ₃ O ₈ in the overoxidized state.

In such conventional devices, a shallow flat container is used to limit the liquid thickness, and the container is rotated, which is quite effective, but the conventional drawback of uneven heating has been completely eliminated. Moreover, the rotational motion applied to the container hinders continuous monitoring of the liquid temperature, causing a delay in confirming the end point of the reaction.

The present invention has been made in order to eliminate the above-mentioned drawbacks of the present invention, and an object of the present invention is to provide a conversion device that uniformly heats a nitric acid solution and efficiently produces a homogeneous cake.

Hereinafter, one embodiment of the present invention will be described with reference to the accompanying drawings.

FIG. 3 is a sectional view showing the configuration of the conversion device according to the present invention, in which 11 is a container for injecting nitric acid solution, 12 and 13 are heaters that can be individually energized, and 14 is for preventing the heat of the heater from radiating to the outside. A heat insulating material 15 is an upper enveloping body for insulating it from the outside air, and a heat insulating material 16 is attached to the outer wall of the upper enveloping body, and a packing 27 is inserted between it and the lower enveloping body 23 for sealing. This sealing is accomplished by attaching the flange portion 22 of each enclosure to the bolt nut 26.
This is done using Further, 17 and 18 are thermometers that measure the temperature of the upper and lower layers of the nitric acid solution, respectively, and 19 is a vacuum pump that discharges the gas inside the enclosure to the outside and maintains a constant negative pressure. When exhausting body gases, toxic gases such as nitrogen oxides are treated by the scrubber 21.

Next, 24 is a lifting device for the lower enclosure 23, which is used to loosen the bolts and nuts mentioned above to release the connection with the upper enclosure, and to inject the nitric acid solution into the container placed on the enclosure or after the reaction. 25 is a motor that drives the lifting device 24, and 29 is a motor that is operated when removing nuclear material, and 29 is a motor that is installed on the side that does not face the nitric acid solution in order to prevent nitric acid droplets generated when the nitric acid solution boils from proceeding inside the exhaust pipe 20. Has ventilation holes,
It should be noted that this is a vent that collects the nitric acid droplets that have passed through this ventilation hole at the bottom.

The operation of the converting device according to the present invention constructed as described above will be explained below.

First, the container 11 filled with nitric acid solution is placed on the lower enclosure 23, and after the lower enclosure 23 is engaged with the upper enclosure 15 by the lifting device 24, the bolts 26 are tightened to remove the packing. 27 for sealing. In this state, the upper enclosure 15
Thermometers 17 and 18 passing through the nitric acid solution are set to reliably measure the temperature of the upper and lower parts of the nitric acid solution, respectively.

Next, the vacuum pump 19 is activated to exhaust the air inside the enclosure to the outside, and when the degree of vacuum reaches 10 torr, the heaters 12 and 13 are energized. In this case, since the nitric acid solution exists in an atmosphere extremely close to a vacuum state, its boiling point drops and boiling has already begun before electricity is applied to the heater. In addition to this, if electricity is applied to a heater having a capacity greater than the heat of vaporization, water will evaporate extremely efficiently. The nitric acid solution in this process is sufficiently agitated by convection and maintained at a substantially uniform temperature throughout. Here, a heater 12 provided on the side of the container
Of course, it promotes the evaporation of water, but it also aims to keep the sides of the container warm, where the proportion of heat radiation is large, and the liquid temperature rises to about 100°C at both the top and bottom, and water evaporation ends. When a large amount of heat of vaporization is no longer required, electricity is cut off. After that, the heater 13
Heating to generate nuclear material is carried out by the heat exchanger, and at a temperature of 350° C. at which the denitrification reaction is completed, the power supply to the heater 13 is cut off by a switch built in the thermometer 18.

Water vapor generated in this process and nitrogen oxides during the denitrification reaction are treated by the scrubber 21, but even after the heater 13 is turned off, the vacuum pump 19
is operated continuously until the product gradually cools down to 40°C.

Figure 4 shows a conventional microwave heating device,
The reaction progress of the apparatus using the heater according to the present invention is shown by solid lines and broken lines, respectively. In the conventional apparatus, the liquid temperature rises from time T ₀ when microwaves are applied, reaches the boiling point, and the water vapor evaporates. The temperature is maintained at 100°C until the end of the reaction, _T1 . Then about 120
℃, an azeotropic phenomenon of water and nitric acid occurs, and at time _T2 when this ends, a denitrification reaction begins and the desired nuclear material is produced. This denitrification reaction raises the temperature of the product to approximately 350°C, and if this is done incorrectly, substances in a peroxidized state are likely to be produced.

On the other hand, in the device of the present invention, since the enclosure is maintained at a high vacuum, water starts to evaporate at approximately 10°C, and as the heat from the heater acts on it, the water evaporates in an extremely short time. . That is, in a reduced pressure state, the azeotropic phenomenon during the reaction disappears, and the reaction gradually shifts to the denitrification reaction. Therefore, the reaction completion time shifts from a in the conventional apparatus to a' in the apparatus according to the present invention, and the reaction can be completed in a short time.

FIG. 5 is a diagram showing another shape of the container constituting the conversion device according to the present invention.
In other words, in order to keep the liquid thickness constant, the drain pipe 3
0, the injection level of the nitric acid solution is kept constant, and the solution overflowing from this drain pipe is guided to a place where it will not be heated by microwaves or a heater. In b, the container is made double in order to store the solution overflowing from the drain pipe in the same container and guide it to the bottom.The heat insulating material is also thick so that the microwave and heater do not act directly on it. By employing this container, it is possible to homogenize the produced substances and also to easily manage the nitric acid solution overflowing from the drain pipe.

As is clear from the above explanation, according to the conversion device of the present invention, since the inside of the solution is heated while maintaining it at a uniform temperature, it is possible to prevent the generation of heterogeneous nuclear material due to uneven heating. Since there is no need to rotate the reactor, temperature control is easy and the transition of the generated nuclear material to a peroxidized state can be prevented. Furthermore, since the reaction time is significantly shortened, the production capacity of nuclear materials can be improved, and the complexity of the equipment and the difficulty of maintenance, such as those found in equipment using microwaves, can be eliminated.

[Brief explanation of the drawing]

FIG. 1 is a sectional view showing the configuration of a conventional conversion device;
Fig. 2 is a diagram showing the properties of nuclear material produced using the same device, Fig. 3 is a sectional view showing the configuration of an embodiment of the conversion device according to the present invention, and Fig. 4 explains the operation of the same embodiment. Figures 5a and 5b are diagrams showing other shapes of the container constituting the same embodiment. 1... Nitric acid solution, 2... Container, 3... Rotating shaft,
4... Enclosure, 5... Opening, 11... Container, 1
2, 13... Heater, 14... Insulating material, 15...
Upper enclosure, 16...Insulating material, 17, 18...Thermometer, 19...Vacuum pump, 20...Exhaust pipe, 2
1...Scrubber, 22...Flange, 23...
Lower enclosure, 24... Lifting device, 25... Motor, 26... Bolt nut, 27... Packing.

Claims (1)

[Claims] 1 A container equipped with a heater for injecting a nitric acid solution containing nuclear material and heating this solution, an enclosure for isolating the evaporation space of the nitric acid solution from the outside air, and a container for heating the nitric acid solution. and a pump that absorbs the evaporated reaction gas and at the same time maintains the internal pressure of the enclosure below atmospheric pressure, lowers the boiling point of the nitric acid solution, promotes the evaporation of water by the heater, and provides convection to the nitric acid solution. A conversion device configured to denitrify and generate homogeneous nuclear material while causing